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i'/^

ANIMAL PHYSIOLOGY.

WILLIAM B. CAEPENTEE, M.D.

F.R.S. F.G.S. F.L.S.

KEGISTR.IR OF THE UNIVERSITY OF LONDON.

Ufto (lEbition,

THOROUGHLY REVISED, AND PARTLY RE-WRITTEN.

LONDON :
H. G. BOHN, YORK STREET, COVENT CxARDEN.

1859.

^^f^mfifi^ii

33576^'^'^^

'4r

Of

1947

o

%0|yAL MU^

London:
r. clay, pninter, bread street hill.

TO

SIR JAMES CLARK, BART. M.D. F.R.S.

Sw PHYSICIAN IN ORDINARY TO THE QUEEN AND TO PRINCE ALBERT,

ETC. ETC.

My dear Sir James,

I I cannot more approj^riately inscribe this Treatise,

having for its object the general diffusion of sound Physio-
logical knowledge, than to one whose Professional eminence

' is foiuided on his enlightened application of it to the pre-
vention and cure of Disease, and who has ever been the
consistent advocate of Liberal Education.

The grateful sense I entertain of many acts of personal
kindness, makes me feel additional pleasure in paying this
' humble tribute.

Believe me to remain.

My dear Sir James,
Your obliged Friend and Servant,

WILLIAM L. CAPvPENTEK.

University Hall, London,
January, 1859.

Ik

PREFACE.

The issue of the present Volume may be considered as an
attempt to supply what the Author has long considered to be
a deficiency in the literature of this countrj'', — that, namely,
of an Educational Treatise on Animal Physiology, which
should at the same tmie communicate to its readers the facts
of gi-eatest importance as regards theii' practical bearing,
and present these in such a form as to place the learner
in possession of the essential j^rinciples of Physiological
Science.

The Author has followed the general plan of the Treatise on
Animal Physiology contributed by Professor Milne-Edwards,
one of the most eminent Naturalists in France (in which
country it is not thought beneath the dignity of men of the
highest scientific reputation to write elementary books for
the instruction of the beginner), to the " Coui^s Elementaire
d'Histoii^e Xaturelle" adopted by the French Government as
the text-book of instruction in the Colleges connected with
the University of Paris, which requires from every Candidate
for its Degree of " Bachelor of Sciences " a competent know-
ledge both of Animal and of Vegetable Physiology. He has
also had at his disposal the admirable series of Illustrations
prepared for that work, which, as a whole, are unsurpassed
either in beauty or in exactness.

In carrying-out this plan, however, the Author has entirely
followed his own judgment ; and has made so much more use

b

VI PREFACE.

of his own materials than of those siippUed by the treatise of
Professor Milne-Edwards, that the work may be regarded
as almost entirely original. The present Edition, too, has
undergone very considerable modifications ; the first chapter,
which now contains a complete outline of the Elementary
Tissues of the Animal Body, and the last, in which a com-
prehensive sketch is given of the principal ])lienomena
of Reproduction and Development throughout the Animal
Kingdom, having been entirely re-written and illustrated with
numerous additional figures. In order to make room for the
large amount of new matter now introduced (not less than
one-fifth of the entire volume), the second chapter, contain-
ing a General View of the Animal Kingdom, has been much
abridged ; — a change the Author has the less regretted ])eing
obliged to make, since there are now before the public several
excellent Elementary Treatises on Zoology, which had no
existence at the time this volume originally appeared.

Everyone Avho desires to see the study of Physiology duly
appreciated as a branch of General Education, must feel
gratified at the progress which has been made of late years in
the public recognition of its value. The University of London
led the way, by the introduction of Animal Physiology into
the programme of study to which all Candidates for its Degree
of Bachelor of Arts are required to conform. The Universi-
ties of Oxford and Cambridge have since admitted it as one
of the subjects which Candidates may select for their Bachelor
of Arts Examination, and in which they may obtain Honours.
And in many of the large Public Educational Institutions
with Mdiicli this country is now so abundantly furnished, it
forms a 2:>art of the regular course of instruction.

It has been the Author's steady aim, not merely to adapt
his treatise to the Avants of those who wish to acquire a
general knowdedge of the principal facts and doctrines of
Physiological Science, but also to render it suitable to that

PREFACE. Vll

which he considers a far more important purpose of the study,
— namely, the culture and discipline of the Mind itself. Having
been satisfied, by no inconsiderable experience of ditterent
modes of Education, that Natural Science, if judiciously
taught, is second in value to no other subject as an educational
means, and that it may be made to call forth a more varied
and wholesome exercise of the mental powers than almost any
other taken singly, — he has kept this purpose constantly in
view ; and he trusts that the experience of intelligent In-
structors will be found so far to concur with his own, that the
study of Physiology may be still more generally introduced
into Popular Education. It can only be by the general diffu-
sion of sound information on this subject, that the Public
Mind can be led to understand the difference between
Rational Medicine, and that Empiricism which now presents
itself under so many different forms ; that it can appreciate
the true value of measures of Sanitary Eeform, the efficiency
of which must depend upon the amount of support they
receive from an intelligent public opinion ; and that it can
be preserved from those Epidemic Delusions, whose preva-
lence, from time to time, is not less injurious to the minds
of which they lay hold, than is that of Epidemic Diseases to
the bodies of those who suffer from them.

He has only further to add that, whilst keeping in view
the most important practical applications of the Science of
Physiology, he has not thought it desirable to pursue these
too far; since they constitute the details of the Art of pre-
serving Health, which is founded upon it, and which may be
much better studied in a distinct form, when this outline of
the Science has been mastered. And, for the same reason,
he has adverted but slightly to those inferences respecting the
Infinite Power, Wisdom, and Goodness, of the Great First
Cause, which are more obvious, although, perhaps, not really
more clear and valid^ in this Science, than in any other.

Vlll PREFACE.

Believing, as he does, that such inferences are more satisfac-
torily based upon the general manifestations of Law and
Order, than upon individual instances of Design, he has
thought it the legitimate object of this treatise to lay the
foundation for them, by developing, so far as might be, the
Principles of Physiology, — leaving it to special treatises on
Natural Theology, to build-up the applications.

University Hall, London,
Jan. 1859.

CONTENTS.

TAGE

INTRODUCTION 1

CHAPTER I.
On the Vital Operations of Animals, and the Instruments

BY which they are PERFORMED 17

CHEMICAL CONSTITUTION OF THE ANIMAL BODY ... 31

STRUCTURE OF THE PRIMARY TISSUES 36

CHAPTER II.

General View of the Animal Kingdom 84

Vertebrata 84

mammals 89

BIRDS 02

reptiles 93

batrachdv 97

fishes 100

Abticulata 102

insects 104

arachnida 105

crustacea 106

cirrhifeda 109

myriapoda 110

annelida ?v>.

ENTOZOA Ill

MOLLUSCA 112

CEPHALOPODA 116

PTEROPODA 117

GASTEROPODA 118

CONCHIFERA ib.

TUNICATA 121

POLYZOA 122

CONTENTS.

CHAPTER 11.— Continued.

PAGE

Radiata 123

echinodepmata 125

ACALEPH^ 128

polypifera 129

Protozoa 185

rhizopoda 136

infusoria 139

porifera 140

CHAPTER III.

Nature and Sources of Animal Food 142

CHAPTER IV.

Digestion and Absorption 162

prehension of food 163

mastication 166

insaliyation 176

deglutition 178

digestive apparatus 181

gastric digestion : — CHyMIFICATION 188

intestinal digestion :— chylification 193

defecation 195

absorption of nutritive MATERIAL 196

SANGUIFICATION 199

CHAPTER V.

Of the Blood, and its Circulation 202

properties of the blood 203

circulation of the blood 216

circulating apparatus of THE HIGHER ANIMALS . . 222

FORCES THAT MOVE THE BLOOD 232

COURSE OF THE BLOOD IN THE DIFFERENT CLASSES OF

ANIMALS 240

CONTEXTS.

CHAPTER VI

PAGE

Of Respiration 258

nature of the changes essentially constituting

respiration 259

structure and actions of the respiratory apparatus 265

CHAPTER VII.

Of Excretion and Secretion 292

general purposes op the excreting processes . . ib.
nature of the secreting process.— structure of the

secreting organs 298

characters of particular secretions 304

CHAPTER VIII.
General Review of the Nutritive Operations. — Formation

OF the Tissues 316

GENERAL REVIEW OF THE NUTRITIVE OPERATIONS , . ih.

FORMATION OF THE TISSUES 317

REPAIR OF INJURIES 323

CHAPTER IX.

On THE Evolution of Light, Heat, and Electricity by

Animals 327

animal luminousness ih.

ANIMAL heat 332

animal electricity 340

CHAPTER X.
Functions of the Nervous System 345

structure AND ACTIONS OF THE NERVOUS SY'STEM IN THE

PRINCIPAL CLASSES OF ANIMALS 350

FUNCTIONS OF THE SPINAL CORD. — REFLEX ACTION . . 374

FUNCTKJNS OF THE GANGLIA OF SPECIAL SKNSK. — CON-
SENSUAL ACTIONS 380

FUNCTION OF THE CEREBELLUM. — COMBINATION OF MUS-
CULAR ACTIONS 384

FUNCTION OP THE CEREBRUM. INTELLIGEXCI': AND WILL . 385

Xll CONTENTS.

CHAPTER XI.

PAGE

On Sensation, and the Organs of the Senses 387

sense of touch 390

sense op taste 395

sense of smell 398

sense of hearing 401

sense of sight 413

CHAPTER XII.

Op Animal Motion, and its Instruments 443

contractile tissues.— muscular contractility . . 444

applications of muscular power. — bones and joints . 453

motor apparatus op man. — skeleton and muscles , 464
of the attitudes of the body, and the various kinds

of locomotion . 489

CHAPTER XIII.

Of THE Production of Sounds : Voice and Speech .... 513

CHAPTER XIV.

Of Instinct and Intelligence 525

manifestations of intelligence 546

CHAPTER XV.

Of Reproduction 552

gemmiparous or non-sexual reproduction .... 553

sexual reproduction, or generation 557

ANIMAL PHYSIOLOGY.

inteoductiojn-.

The importance of tho study of Animal Physiology, as a
branch of General Education, can scarcely be over-estimated ;
and it is remarkable that it is not more generally appreciated.
It might have been supposed that curiosity alone would have
led the mind of Man to the eager study of those wonderful
actions by which his body is constructed and maintained ;
and that a knowledge of those laws, the observance of which
is necessary for the due performance of these actions, — in other
words, for the maintenance of his health, — would have been
an object of universal pursuit. That it has not hitherto been
so, may be attributed to several caasea The very familiarity
of the occurrences is one of these. We are much more apt
to seek for explanations of phenomena that rarely present
themselves, than of those which we daily witness. The Comet
excites the world's curiosity, whilst the movements of the
sun, moon, and planets are regarded as things of course. AYe
almost daily see vast numbers of animals of different tribes,
in active life around us ; their origin, growth, movements,
decline, death, and reproduction, are continually taking place
under our eyes ; and there seems to common apprehension
nothing to explain, where everything is so apparent. And of
Man too, the ordinary vital actions are so famiUar, that the
study of their conditions appears superfluous. To be born,
to grow, to be subject to occasional disease, to decline, to die,
is his lot in common with other animals ; and what know-
ledge can avail (it may be asked) to avert the doom imposed
on him by his Creator ]

B

2

INTRODUCTION.

In reply to this it is sufficient to state, tliat millions annually
perish from a neglect of the conditions which Divine wisdom
has appointed as requisite for the preservation of the hody from
fatal disease ; and that millions more are constantly suffering
various degrees of pain and weakness, that might have been
prevented by a simple attention to those principles which it is
the province of Physiology to unfold. From the moment of
his birth, the infant is so completely subjected to tlie in-
fluence of the circumstances in which he is placed, that the
future development of his frame may be said to be governed
by them ; and thus it depends, in great part, npon the care
with wdiich he is tended, and the knowledge by which that
care is guided, whether he shall grow up in health and vigour
of body and mind ; or shall become weakly, fretful, and self-
willed, a source of constant discomfort to himself and to
others ; or shall form one of that vast proportion, whose lot
it is to be removed from this world before infancy has ex-
panded into childhood. The due supply of warmth, food, and
air are the principal points then to be attended to ; and on
every one of these the greatest errors of management prevail.
Thousands and tens of thousands of infants annually perish
durmg the few first days of infanc}'', from exposure to cold,
which their feeble frames are not yet able to resist ; and at
a later period, when the infant has greater power of sustain-
ing its own temperature, and is consequently not so liable to
sutler from this canse, the seeds of future disease are sown,
b}'- inattention to the simple physiological principles, which
should regulate its clothing in accordance with the cold or
lieat of the atmosphere around. ISTor is less injury done by
inattention to the due regulation of the diet, as to the quan-
tity and quality of the food, and the times at which it should
be given ; the rules for which, simple and easy as they are,
are continually transgressed through ignorance or carelessness.
And, lastly, one of the most fertile sources of infantile dis-
ease, is the want of a due supply of pure and wholesome air ;
the effects of which are sure to manifest themselves in some
way or other, though often obscurely and at a remote period.
It is physiologically impossible for human beings to grow np
in a sound and healthy state of body and mind, in the midst
of a close, ill-ventilated atmosphere. Those that arc least
able to resist its baneful influence, are carried ofl" by the dis-

INTRODUCTION. 6

eases of infancy and cliildliood ; and those whose native
vigour of constitution enables them to struggle through these,
become the victims, in later years, of diseases which cut short
their term of hfe, or deprive them of a large part of that
enjoyment which health alone can bring.

Kor is the effect of these injurious causes confined to
infancy, though most strikingly manifested at that period,
" The child is father to the man," in body as well as in mind;
but the vigorous health of the adult is too often wasted and
destro3'ed by excesses, whether in sensual indulgence, in
boddy labour, or in mental exertion, to which the very feeling
of buoyancy and energy often acts as the incentive ; and the
strength which, carefully husbanded and sustained, might
have kept the body and mind in activity and enjoyment to
the full amount of its allotted period of "threescore years
and ten," is too frequently dissipated in early manhood. Or,
again, the want of the necessary conditions for the support of
life, — the warmth, food, and air, on wliich the body depends
for its continued sustenance, no less than for its early deve-
lopment, — may cause its early dissolution, even where the
individual is gmiltless of having impaired its vigour by his
own transgressions.

These statements are not theoretical merely : they are based
upon facts drawn from observations carried oh upon the most
extensive scale. Wherever we find those conditions, which the
Physiologist asserts to be most favourable to the preservation
of the health of the body, most completely fulfilled, there do
sickness and mortality least prevail. A few facts will place
this subject in a striking light. " The average mortality of
infants among rich and poor in this country (and with little
variation throughout Europe) is about one in every four and
a-half before the end of the first year of existence. So directly,
however, is infant life influenced by good or bad management,
that, about a century ago, the workhouses of London presented
the astounding result of twenty-three deaths in every twenty-
four infants under the age of one year. For a long time this
frightful devastation was allowed to go on, iis beyond the reach
of human remedy. But when at last an improved system of
management was adopted in consequence of a parliamentary
inquiry having taken place, the proportion of deaths was
speedily reduced from 2, GOO to 450 in a year. Here, then,.

B 2

4 INTRODUCTIOX.

was a total of 2,150 instances of loss of life, occnrring yearly
in a single institution, cliargeable, not against any unalterable
decrees of Providence, as some are disposed to contend as an
excuse for tlieii' own negligence ; but against tlie ignorance,
indiiference, or cruelty of man. And what a lesson of vigi-
lance and inquiry' ought not such occurrences to convey,
when, even now, with all our boasted improvements, every
tenth infant still perishes within a month of its birth /" ^

The effect of attention to cleanliness and ventilation in the
reduction of an excessive infantile mortality, has been equally
shown in the experience of the Dublin I.ying-in Hospital.
At the conclusion of 1782, it was found that out of 17,6.50
inflmts born alive, no fewer than 2,944, or one in every six,
had died within the lirst fortnight. By the more cihcient
ventilation of the wards, the proportion of deaths during the
lirst fortnight was at once reduced to 419 out of 8,033, or
but little more than one in twenty; and it has subsequently
been still further dimmished.

In the island of St. Ivilda, the most northern of the Heb-
rides, according to the statement of a gentleman who visited
it in 1838, as many as eiyht ont of tvery ten children die
between the eighth and twelfth day of their existence ; in
consequence of which terrible mortality, the population of the
island is diminishing rather than increasing. This is due,
not to anything injurious in the position or atmosphere of the
island ; for its ** au' is good, and the water excellent : " but
to the " tilth in which the inhabitants hve, and the noxious
effluvia which pervade their houses." The huts are small, low-
roofed, and without windows ; and are used during the winter
as stores for the collection of manure, which is carefully laid
out upon the floor, and trodden under foot, till it accumulates
to the depth of several feet. The clerg}-man, who lives
exactly as those around him do^ in every respect, except as
regards the condition of his house, has reared a famil}-of four
children, all of whom are well and healthy ; whereas, accord-
ing to the average mortality arouud him, at least three out of
the four would have been dead within the lirst fortnight.

It is not a little remarkable that a recent sanitary inquiry
carried ov.t by order of the Danish govermnent, into the con-

^ Dr. A. Combe on the Physiological and Moral Management of
luiauej.

INTRODUCTION.

dition of the Icelandic population, should have disclosed the
existence of almost precisely similar hahits of life among
them, with almost precisely the same results. The dwellings
of the great bulk of the peasantry seem as if constructed for
the express purpose of poisoning the air which they contain.
They are small and low, without any direct provision for
ventilation, the door serving alike as window and chimney ;
the walls and roof let in the rain, which the floor, chiefly
composed of hardened sheep's-dung, sucks up ; the same
room generally serves for all the uses of the whole 'family,
and not only for the human part of it, but frequently also for
the sheep, which are thus housed during the severest part of
the winter. The fuel employed in this country chiefly con-
sists of cow-dung and sheep's-dung, caked and dried ; and
near the sea-coast, of the bones and refuse of fish and sea-
fowl ; producing a stench, which to those unaccustomed to it
is completely insupportable. In addition to this, the people
are noted for their extreme want of personal cleanliness ; the
same garments (chiefly of black flannel) being worn for
months without having even been taken oil at night. Although
the Icelanders enjoy an ahnost complete exemption from
many diseases (such as consumption) which are very fatal
elsewhere, and the number of births is fully equal to the
usual average, the population of the island does not increase,
and in some parts actually diminishes. This result is in great
measure due, as at St. ELilda, to the very high rate of infiantile
mortality ; a large proportion of . all the infants born being
carried olf before they are a fortnight old. It is in the httle
island of AVestmannoe, and the opposite parts of the coast of
Iceland, v/here the bird-fuel is used all the year round, instead
of (as elsewhere) during a few months only, that the rate is
the highest; the average mortality for many years having
been sixty-four out of every hundred, or nearly two out of
three, of all the infants born in these locaUties,

But it is yet more remarkable that the immediate cause of
the high rate of infantile mortality should have been pre-
cisely the same in the Workhouses of London, the Lying-in
Hospital of Dublin, and the close filthy huts of the peasantry
of Iceland and St. Kilda ; for it w^as almost entirely referrible
to one single disease, " Trismus nascentium," or, " Lock-jaw
of the ISTew-born ; " and this disease has diminished in exact

6 INTRODUCTION.

proportion to the improvement of the places it previously
infested, in respect to ventilation and cleanliness. Thus, it is
80 rare for a case of it now to occur in London, that many
practitioners of large experience have never seen the disease.
In the Dublin Lying-in Hospital, the number of deaths from
it has been reduced to three or four yearly. And there can-
not be a reasonable' doubt, that, by due attention to the same
conditions, it might be exterminated from Iceland and from
St. Kilda. There is scarcely, in fact, a disease incident to
humanity, which is more completely preventible than tliis ;
and yet the annual sacrifice of life which it formerly caused
in our own country alone, might have been reckoned by tens
of thousands.

Although the peculiar susceptibiltty of the constitution of
children, gives to foul air and other causes of disease a much
more destructive influence over them, than the like causes
have over persons more advanced in life, yet it is now well
ascertained that the rate of mortality among different classes
of the community varies in a degree which bears a very close
relation to the nature of the conditions under which they live.
Thus, whilst the annual average number of deaths in the whole
of England and Wales is about. 22 out of every thousand
persons living, there are localities in which the annual
average exceeds 50 in a thousand, and others in wliich it falls
as lov/ as 11 in a thousand. And it is not a little remarkable,
that the difference is almost entirely referrible to the mortality
produced by Fevers and allied diseases, which, as experience
has now fully demonstrated, are absolutely preventible by due
attention to the ordmary conditions of health.

As the population of England and Wales may at present be
estimated at about twenty millions, and its actual mortality at
about 440,000, what maybe termed its inevitable mortality —
arising from diseases that would not be directly affected by
sanitary improvements — would be only one half, or 220,000 ;
so that the same number of lives may be considered to be
annually sacrificed by the public neglect of the means of pre-
serving them, — the deaths from typhus alone being no fewer
than 50,000. But as it is scarcely to be supposed that every
part of our population could be placed in conditions as favour-
able as those which prevail where the rate of morttility is
the lowest, v/e may take 13 per thousand as the average to

INTRODUCTION. <

whicli it may bo safely affirmed, on the basis of actual expe-
rience, that the annual mortality may be reduced, by such
efficient sanitary measures as render the dwellings of the mass
of the population fit for human habitation ; this would give
an annual mortality for England and Wales of 260,000,
sho^\ing a saving of 180,000 lives annually in that one por-
tion of the British empire. And it must be remembered
that this amount of mortahty represents a vastly greater
amount of sickness, since, for every death, there are numerous
cases of severe illness ; so that it would be scarcely too much
to affirm that at least a million out of the whole number of
such cases annually occurring, are preventible, hke the
180,000 deaths, by adequate provisions for the supply of pure
air and water, and by efficient sewerage for the removal of
decomposing matters. It cannot be doubted that, even in
a mere pecuniary point of view, the expense of such arrange-
ments would be amply compensated by the prevention of a
vast amount of .that loss of productive labour of various
kinds, wjiich is at present due to disease ; and, considered on
the large scale, as a question of social economy, the import-
ance of sanitary legislation can scarcely be over-rated. But
much cannot be expected to be done in this direction, until
such an inteUlgeut puhlic opinion shall have been created, by
the general dihusion of sound physiological information, as
shall be sufficiently forcible to bear down the self-interested
opposition of those, who do not see that the value of their
pro^Derty will be permanently increased at least in proportion
to the amount of money judiciously expended upon it.

A more remarkable illustration of what is to be effected by
sanitary improvements can scarcely be adduced, than that
which is presented by the comparison between the locality
termed "the Potteries," in the immediate vicinity of Ken-
sington, and the " Model Lodging-houses," which have been
erected in various parts of the Metropolis. The site of the
group of dwellings constituting the former is far from being
insalubrious in itself, and rows of handsome houses are rising
up in its unmediate neighbourhood ; but the condition of
these dwellings is most hltliy. A few years ago, as many as
3,000 pigs were kept in this locality (the number has since
been somewhat dimmished) ; and the boiling of fat and other
offal, which is carried on by some of the pig-feeders, some-

8 INTRODUCTION.

times taints the air for a mile round. Very few of tlie tene-
ments have any water-supply ; the wells are useless, or worse
than useless, through the contamination of their water with
putrescent liquid which filters down into them; and the
drainage of the dwellmgs both for men and pigs is almost
entirely superficial, being chiefly discharged into a stagnant
piece of water called the " Ocean," which is covered with
a filthy slime and bubbles with poisonous gases, and very
commonly has dead dogs or cats floating on its surface. It is
difficult to conceive anything more horribly offensive than
the rears of some of the houses, whose yards are filled with
ordure and other filth collected for manure, which is here
stored for weeks, or even months, until an opportunity occurs
for selling it. And even the public ways are generally
covered with black putrescent mire. ISTow, during ten
months of the year 1852, when no epidemic prevailed, as
many as forty deaths occurred in the Potteries, out of a
population of about one thousand, — the mortality being
thus at the rate of 48 per thousand annually ; and no
fewer i\iMi four-fifths of these deaths occurred at, or beneath,
five years of age. In the first ten months of 1849, when
cholera was prevalent, the number of deaths was fifty, or
about one in twenty of the whole population, twenty-one of
these being due to cholera and diarrhoea, and twenty-nine to
typhus and other diseases. — On the other hand, in the whole
population of the " Model Lodging-houses," amounting to
1,343, only seven deaths took place in the whole twelve
months of 1852, or at the rate of scarcely more than 5 per
thousand; and although they contain a large proportion of
children, yet only half the number of deaths occurred below
ten years old. Duruig the prevalence of the cholera- ej)idemic,
no cases of that disease occurred among them, although it was
raging in their various neighbourhoods ; and from the time
that their drainage has been rendered thoroughl}^ efficient, no
case of fever has presented itself among their inmates.

The experience of Cholera-epidemics is peculiarly valuable,
on account of the marked tendency of this disease to search
out and expose defects, which have continued to produce
other diseases year after year, without having been suspected
as the causes of them. The greatest severity in each visita-
tion has shown itseK in identical localities, provided those

INTRODUCTION. 9

remained in the same foul state as at first ; whilst new loca-
lities have been affected, jnst in proportion to the degree in
which they have participated in the same conditions ; and those
originally attacked have escaped, wherever they had adopted
the requisite means of purification. Thus, at JSTewcastle-on-
Tyne and Gateshead, the first outbreak occurred in the very
same streets, and even in the same houses, in the three visi-
tations of 1831, 1848, and 1853. An outbreak which
occurred in 1853, at Luton, in Bedfordshire, — ^\;rhere, out of
a population of 126 persons, inhabiting twenty-five houses,
no fewer than fifty-four attacks of choleraic disease, fifteen of
them fatal, took place within three weeks,' — was most dis-
tinctly traceable to defect of sewerage, which had been pre-
viously manifesting its malign influence on the general health
of the town. And the fearful pestilence which devastated
the neighbourhood of Golden Square (London) in the autumn
of 1854, was no less distinctly traceable to the contamination
of the pump-water by the bursting of a sewer into the well.
On the other hand, Exeter and ]N"ottingham, which suffered
severely in the first epidemic, escaped comparatively un-
harmecl in the subsequent visitations ; and this result is
plainly clue to the sanitary improvements which had been
made in the interval. In 1832 there perished of the epide-
mic in Exeter, as many as 402, out of a population of
28,000, or no fewer than one in seventy ; and a vast amount of
suffering, with a heavy expense, was entailed upon the town.
In 1848-9, on the other hand, out of a population of about
32,600, there were but 44 deaths, or less than one in seven
hundred ; and upwards of one-half of these occurred in
a single parish, that lies very low, and in the midst of putrid
exhalations from the city drains. In Nottingham, with a
population of 50,000, there were 296 fatal cases of cholera in
1832, nearly all of these being in the lower part of the town,
which was ill-drained, extremely filthy, and densely popu-
lated ; but in 1848-9, though the population had increased to
58,000, the number of deaths from cholera was no more than
18, all of these occurring in locaHties, which, in spite of ^hat
had been done, retained much of their previous filth.

The foregoing are only samples of a vast number of c^eses
which might be adduced, in proof of the absolute preventi-
bihty of Cholera, and of other diseases of the same class. It

10 INTRODUCTION.

may be well to siil^join a few additional facts, derived from
the cliolera-exiDerience of 1848-9, which, from its general
diffusion, tested, in a very remarkable degree, the relative
healthfulness of different provincial towns, and of different
metropolitan districts. Thus, among the whole population of
the ten towns of Exeter, Derby, Cheltenham, Leicester,
^Nottingham, Eochdale, jN'orwich, Preston, Halifax, and Eir-
mingham, amounting to 057,000, there were no more than
238 deaths from cholera ; whilst, in an equal jitopulation
inhabiting the towns of iN'ewcastle-nnder-Lyne, Plymouth,
Brighton, Merthyr Tydvil, Portsea, Tynemouth, Wigan, Hull,
Wolverhampton, and Leeds, the number of deaths was no
fewer than 10,4:15, ov fo7'ti/-three times as great. So again, in
twenty-five Metropolitan districts, chiefly on the north side of
the Thames, having a total population of about 310,000, the
number of deaths from cholera was only 389 ; whilst in
twenty- two districts, almost entirely on the south side of the
river, the number of deaths, out of a population of almost
exactly the same amount, was 5,932, or more than twelve
times as great. In no instance is there the least difiieulty in
accounting for these contrasts. They all point to the same
general conclusion; that, namely, of the immense influence
which is exercised over human health by the purity of the air
that is breathed, and of the water that is drunk ; and it is
because these two conditions are in a great degree capable of
public regulation, that legislative interference has so much in
its power, and is so imperatively called for by the interests of
humanity, which S23eak solemnly and distinctly to all who
claim the rights of property in the foul " plague-spots " which
deface our country, of their bounden diet?/ to render them not
unfit for human occupation.

But although the magnitude of the evils resulting from the
neglect of the conditions of Public Health, gives to this sub-
ject the first claim on our consideration, yet it is not the less
important that every individual should acquire as much
knowledge of the constitution of his body, and of the right
means of keeping it in working order, as will save him from
seriously damaging either himself or other peoj)le by his
ignorance of such matters. It is less than ten years since a
fearful sacrifice of life occurred among the deck-j)assengers on
board the Irish steamer " Londonderry," who were ordered

INTRODUCTION. 11

below by tlie Captain on account of the stormy cliaractcr of
the weather, and on whom the hatches Avere closed down,
although the cabin which was crowded l)y tlieni had scarcely
any other means of ventilation. Out of 150 of these unfor-
tunates, no fewer than 70 died of suffocation before the
morning, — a catastrophe only second to that which occurred
in the "Black Hole of Calcutta," in which 123 out of 14:6
died during one night's coniinement in a room eighteen feet
square, provided with only two small windows. Yet the
Captain of the " Londonderry " was acquitted of all blame ;
since he had done what seemed to him best for the welfare of
his passengers, the result being due simply to his astound-
ing ignorance of the fact that men cannot live without having
air to breathe. Not a year passes without the occurrence of
numerous deaths from the like cause ; and yet these are
really insignificant, when compared with the vast amount of
disease which is constantly attributable to inattention, on the
part of individuals, to those simple ineans of securing an
adequate supply of air which are within the reach of every
one. And when we bear in mind that the respiratory func-
tion is only one of the processes whose due performance has
to be provided for, and that the regulation of the food and
drink, of the excretions, of clothing and tempera,ture, of
exercise (bodily and mental) and repose, and of the repro-
ductive functions, all fall within rules which it is the pro-
vince of Physiology to prescribe, we see how vain it is to
expect that the body can be maintained in health, without
some acquaintance with that science, or at least with the
rules which it lays down. For, although it is quite true that
man has within himself certain instincts which afford him a
considerable measure of guidance in all these particulars, —
hunger and thirst, for example, leading him to take the
sustenance v/hich his body requires, weariness tem^^ting hun to
needed repose, and so on, — yet it is no less certain that in a
state of artificial civilisation these instincts are so often over-
borne by acquhed tastes, or by the pressure of other circum-
stances, that they cannot alone be safely relied on. Hence it
is all the more important that the rules for j)reserving health
should be based on an intelligent knowledge of Physiological
principles ; otherAvise, like the natural instmcts, they are likely
to be put aside as occasion prompts ; Avhereas, in proportion as

12 INTRODUCTIOK

the individual is possessed of tlieir rationale, will he be likely
to shape his conduct in accordance with them.

The general principles of Physiological science, again, will
be likely to be thoroughly apprehended, in proportion as they
are based on an extended recognition of the phenomena
which they comprehend. Every physiologist is now satisfied
that the life or vital actions of no one species of animal can
be correctly understood, unless compared with those of other
tribes of diiferent conformation. Hence, for the student of
13hysiology to confine himself to the observation of what
takes place in Man alone, would be as absurd as for the astro-
nomer to restrict liimself to the observation of a single planet,
or for the chemist to endeavour to determine the properties of
a metal by the study of those of that one only. There is not
a single species of animal, that does not present us with a set
of facts which we should never learn but by observing it ;
and many of the facts ascertained by the observation of the
simplest and most common animals, throw great light upon
the great object of all our inquiries, the Physiolog}^ of Man.
For though in liim are combined, in a most wonderful and
"unequalled manner, the various faculties which separately
exhibit themselves in various other animals, he is not the
most favourable subject for observing their action ; for the
obvious reason that his machinery (so to speak) is rendered
too complex, on account of the multitude of operations it has
to perform : so that we often have to look to tjie lowest and
simplest animals for the explanation of what is obscure in
man, their actions being less numerous, and the conditions
which they require being more easily ascertained.

The diffusion of Animal life is only one degree less exten-
sive than that of vegetable existence. As animals cannot,
like plants, obtain their support directly from the elements
around, they cannot maintain hfe, where life of some kind
has not preceded them. But vegetation of the humblest
character is often sufficient to maintain animals of the highest
class. Thus the lichen that grows beneath the snows of
Lapland, is, for many months in the year, the only food of
the rein-deer ; and thus contributes to the support of human
races, which depend almost solely upon this useful animal for
their existence. No extremes of temj^erature in our atmo-
sphere seem inconsistent with animal life. In the little pools

INTRODUCTION. 13

formed by the temporary influence of the sun upon the sur-
face of the arctic snows, animalcules have been found in
a state of activity ; and the ocean of those inhosj^itable regions
is tenanted, not only by the whales and other monsters which
w^e think of as their chief inhabitants, whose massive forms
are only to be encountered " few and far between," but by
the shoals of smaller fishes and inferior anunals of various
kinds upon Avhicli they feed, and through vast fleets of which
the mariner sails for many miles together.

On the other hand, even the hottest and most arid portions
of the sandy deserts of Africa and Asia are inhabited by
animals of various kinds, provided that vegetables can find
sustenance there. The humble and toilsome ants make these
their food, and become in turn the prey of the cunning ant-
lion and of the agile lizard ; and these tyrants are in their
turn kept under by the voracity of the birds which are
adapted to prey upon them. The waters of the tropical ocean
never acquire any high temperature, owing to the constant
interchange which is taking place between them and those of
colder regions ; but in the hot springs of various parts of the
world, we have examjoles of the compatibility of even the
heat of almost boiling water v/ith the preservation of animal
life. Thus in a hot spring at Manilla which raises the ther-
mometer to 187°, and in another in Barbary whose usual tem-
perature is 172°, fishes have been seen to flourish. Fishes
have been thrown up in very hot water from the crater of a
volcano, which, from their lively condition, was apparently
their natural residence. Small caterpillars have been found
in hot springs of the temperature of 205 ° ; and small black
beetles, which died when placed in cold water, in the hot
sulphur baths of Albano. Intestinal worms within the body
of a carp have been seen alive after the boihng of the fish for
eating ; and the inhabitants of some little snail-shells, which
seemed to have been dried up within them, have been caused
to revive by placing the shells in hot water for the purpose of
cleaning them.

The lofty heights of the atmosphere, and the dark and
rayless depths of the ocean, are tenanted by anunals of
beautiful organisation and wonderful powers. Vast flights of
butterflies, the emblems of summer and sunshine, may some-
times be seen above the highest peaks of the Alps, almost

14 INTRODUCTION.

touching witli their fragile wings the hard surface of the
never-melting snow. The gigantic condor or vulture of the
Andes has been seen to soar on its widely-expanded wings
far above the highest peak of Chimborazo, where the baro-
meter would have sunk below ten inches. The existence of
marine fishes has been ascertained at a depth of from 500 to
600 fathoms ; and in the deep recesses of those caverns in
Styi'ia and Carniola, which are inhabited by the curious
Proteus (ZooL. § 532), numerous species of insects are found,
all of which, hov^ever, like the Proteus, are blind.

Having thus glanced at some of those facts which demon-
strate the practical importance of the study of Physiology,
and having indicated ' the mode in which that study should
be pursued, it remains to offer a few observations upon its
value with reference to the culture and discipline of the
mind itself One of its great advantages is, that it not
only calls forth, in a degree second to no otlier, both the
ohserving and the reasoning powers ; but that it oifers so
much that is attractive by its novelty to those who enter
upon it seriously, and make it an object of regular pursuit.
For it affords abundant opportunities, even to the beginner,
of adding to the common stock of information respecting the
structure and habits of the vast number of living beings that
peoj^le our globe. The immense variety of the objects which
come under the investigation of the physiologist, so far from
discouraging the learner, should have the effect of stimulating
his exertions, by opening to him new fields for productive
cultivation. Of by far the larger part of the organised crea-
tion, little is certainly known. Of no single species, — of
none of our commonest native animals, — not even of Man
himself, — can our knowledge be regarded as anything but im-
perfect. Of the meanest and simplest forms of animal life, we
know perhaps even less than we do of the more elevated and
complex ; and it cannot be doubted that phenomena of the
most surprising nature yet remain to be discovered by patient
observation of their actions. It was not until very recently,
that the existence of a most extraordinary series of metamor-
phoses, more wonderful than those of the insect, has been
discovered in the jelly-fish of our seas, in the barnacles that

INTRODUCTION.

15

attacli tliemsolves to floating pieces of timber, and in the crabs,
lobsters, and shrimps of our shores. The very best accounts
we have, of the structure, habits, and economy of the loAver
tribes of animals, have been furnished to us by individuals
who did not think it beneath them to devote many years to
the study of a single species ; and as there are very few
which have been thus fully investigated, there is ample
opportunity for every one to suit his own taste in the choice
of an object.

And none but those who have tried the experiment, can
form an estimate of the pleasure Avliich the study of Nature
is capable of affording to its votaries. There is a simple
pleasure in the acquisition of knowledge, worth to many far
more than the acquisition of wealth. There is a pleasure in
looking in upon its growing stores, and watching the expan-
sion of the mind which embraces it, far above that which the
miser feels in the grovelling contemplation of his hard-sought
pelf. There is a pleasure in making it useful to others, com-
parable at least to that which the man of generous benevo-
lence feels in ministering to their relief with his purse or his
sympathy. There is a pleasure in the contemplation of beauty
and harmony, wherever presented to us. And are not all
these pleasures increased, when we are made aware, — as in the
study of ]^ature Ave soon become, — that the sources of them
are never-ending, and that our enjoyment of them becomes
more intense in proportion to the comprehensiveness of our
knowledge ? And does not the feeling that we are not look-
ing upon the inventions or contriA'ances of a skilful human
artificer, but studying the wonders of a Creative Design
infinitely more skilful, immeasurably heighten all these
sources of gratification? If it is not every one Avho can
feel all these motives, cannot every one feel the force of
some ?

•There is certainly no science Avhich more constantly and
forcibly brings before the mind the poAver, the Avisdom, and
the goodness of the Creator. For whilst the Astronomer has
to seek for the proofs of these attributes in the motions and
adjustments of a universe, Avhose nearest member is at a
distance which imagination can scarcely realize, the Physio-
logist finds them in the meanest Avorm that aa^c tread beneath
our feet, or in the humblest zoophyte dashed by the waves

16 INTRODUCTION.

upon our shores, no less than in the gigantic whale, or massive
elephant. And the wonderful diversity which exists amongst
the several tribes of animals, presents us with a continual
variety in the mode in which these adjustments are made,
that prevents us from ever growing weary in the search.

But it is not only in affording us such interesting objects
of regular study, that the bounty of Nature is exiubited.
Perhaps it is even more keenly felt by the mind which,
harassed by the cares of the world, or vexed by its disap-
pointments, or fatigued by severer studies, seeks refuge in
her calm retirement, and allows her sober gladness to exert
it? cheering and tranquilhzing influence on the spirit.

" With tender ministrations, tliou, Nature,
Healest thy wandering and distracted child;
Thon ]3onrest on him thy soft influences,
Thy sunny hues, fair forms, and breathing sweets,
The melody of woods, and winds, and waters.- —
Till he relent, and can no more endure
To he a jarring and dissonant thing
Amidst the general voice and minstrelsy, —
But bursting into tears wins back his way,
Hia angry spix'it healed and harmonized
By the benignant touch of love and mercy."

COLEBIDGB,

DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. l7

CHAPTER I.

OP THE VITAL OPERATIONS OP ANIMALS, AND THE INSTRUMENTS BY
WHICH THEY ARE PERFORMED.

1. Living beings, wlietlier belonging to tlie Animal or to
the Vegetable kingdom, are distinguished from the masses of
inert matter of which the Mineral kingdom is made np, by
peculiarities of form and size, of structure, of elementary-
composition, and of actions. — Wherever a definite form is
exhibited by Mineral substances, it is bounded by plane
surfaces, straight lines, and angles, and is the effect of the
process of crystallization, in which particles of like nature
arrange themselves on a determinate plan, so as to produce a
regular aggregation; and there is, probably, no Inorganic
element or combination which is not capable of assuming such
a form, if placed in circumstances adapted to the manifestation
of its tendency to do so. The number of different crystalline
forms is by no means large ;' and as many substances crystal-
lize in several dissimilar forms, whilst crystals resembhng one
another in form often have a great diversity of composition,
there is no constant correspondence between the crystalline
forms and the essential nature of the greater number of
mineral substances. If that peculiar arrangement of the
molecules which constitutes crystallization should be wanting,
so that simple cohesive attraction is exercised in bringing
them together, without any general control over their direc-
tion, an indefinite or shapeless figure is the result. With
this indefiniteness of form, there is an absence of any limit
whatever in regard to size : a crystal may go on increasing
continuously, so long as there is new material supplied ; but
this new material is deposited upon its surface merely, and
its addition involves no interstitial change ; the older particles,
which were first deposited, and which continue to form the
nucleus of the crystal, remaining just as they were. In Or-
ganized bodies, on the other hand, we meet with convex
surfaces and rounded outlines, and with a general absence of
angularity ; and the simplest grades, both of Animal and of

c

18 DISTINCTIVE CHARACTEES OF ORGANIZED BODIES.

Vegetable life, present themselves under a shape which ap-
proaches more or less closely to the globular. From the
highest to the lowest, each species has a certain characteristic
form, by which it is distinguished ; this form, however, often
presents marked diversities at different periods of life, and
it is also liable to vary within certain limits among the
individuals of which the species is composed. The size of
Organized structures, like their form, is restramed within
tolerably definite limits, which may nevertheless vary to a
certain extent among the individuals of the same species.
These limits are most obvious in the higher animals, whilst
they seem almost to disappear among certain members
both of the Animal and the Vegetable kingdoms, which tend
to increase themselves almost indefinitely by a process of
gemmation or budding, so as to produce aggregations of
enormous size. Such aggregations, however, being formed
by the repetition of similar parts, wliich can maintain their
existence when detached from one another, may, in some
sense, be regarded as clusters of distinct organisms, rather
than as smgle individuals. Such is the case, for example,
with the wide-spreading forest-tree, and with those enormous
masses of coral of which reefs and islands are composed in
the Polynesian Archipelago. For every separate leaf-bud of
the tree, Uke every single polype of the coral, if detached
from its stock, can, under favourable circumstances, perform
all the functions of life, and can develop itself into a new
fabric resembling that from which it was separated.

2. The differences between Organized and Inorganic bodies,
in regard to their structure, are much more important than
those which relate to their external configuration. . Every
particle of a mineral substance, in which there has not been
a mere mixture of components, exhibits the same properties
as those possessed by the whole ; the minutest atom of car-
bonate of lime, for instance, has all the properties of a crystal
of calc-spar, were it as large as a mountain. Hence it is the
essential nature of an Inorganic body that each of its particles
possesses a separate individuality, and has no relation but
that of juxtaposition to the other particles associated with
itself in one mass. — The Organized structure, on the other
hand, receives its designation from being made up of a greater
or less number of dissimilar parts or or<jans ; each of these

DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 19

beiiig tlie instrument of some special action or f miction, which
it performs under certain conditions ; and the concurrence of
all these actions being necessary to the maintenance of the
structure in its normal or regular state. Hence there is a
relation of mutual dependence among the parts of an Organized
fabric, which is quite distinct from that of mere proximity ;
and this relation is most intnnate, not in the case of those
beings which have the greatest multipUcation of parts, but
among those in which there is the greatest dissimilarity
among the actions of the several organs. Thus it has been
just shown that among Plants and Zoophytes, a small fraction
of an organism may live independently of the rest; the
necessar^r condition being that it shall either itself contain
all the organs essential to life, or shall be capable of pro-
ducmg them, — as when the leaf-bud develops rootlets for
its nutrition. Tliis "vegetative repetition," and consequent
capacity of sustaining the loss of large portions of the fabric,
still shows itself in animals much higher in the scale than
Zoophytes ; thus it is not uncommon to meet with Star-fish
in w^hicli not only one or two, out of the five similar arms,
but even three or four, have been lost, without the destruction
of the animal's life ; and this is the more remarkable, as these
arms are not simply members for locomotion or prehension,
but are really divisions of the body, containing prolongations
of the stomach. In like manner, many of the Worm tribes,
whose bodies show a longitudinal repetition of similar parts,
can lose a large number of their joints without sustainmg any
considerable damage. In the bodies of the higher annuals,
however, where there are few or no such repetitions (save
in the two lateral halves of the body), and where there is,
consequently, a greater diversity in character and function
between the different organs, the mutual dependence of their
actions upon one another is much more intimate, and the loss
of a single part is much more likely to endanger the existence
of the whole. Such structures are said to be more highly
organized than those of the lower classes ; the principle of
" division of labour " being carried much further in them,
a much greater variety of objects being attained, and a much
higher perfection in the accomplishment of them being thus
provided for. Thus the individuality of a plant or a zoo-
phyte may be said to reside in each of its multipHed parts ;

c2

20 DISTINCTIVE CHARACTERS OF ORGANIZED BODIES.

whilst that of one of the higher animals resides in the sum
of all its organs.

3. The very simplest Organized fabric is further dis-
tinguished from Inorganic bodies by marked differences in
regard to intimate structure and consistence. Inorganic sub-
stances can scarcely be regarded as possessing a structure,
since their perfection consists in then' homogeneousness and
their solidity. It is the essential character of Organized
fabrics, on the other hand, that they are formed by a com-
bination of solid and liquid components, so intimately
combined and arranged as to impart a heterogeneous cha-
racter to almost every portion of their substance ; and in aU
the parts which are most actively concerned in the vital
operations, softness of texture seems an essential condition, —
those parts only being so consolidated as to acquire anything
comparable to the density of mineral bodies, which are
destined to possess the simply physical property of resistance,
so as to be subservient either to support, to protection, or
to mechanical movement. A comparison between the pulpy
portion of the leaves of Plants and the heartwood of the stem,
between the membranous tissues of the Coral-polypes and the
stony masses which they form, between the firm shell of the
Crab or the Oyster and the substance of the included body,
or between the solid bones of Man and the flesh which clothes
them, will serve to illustrate this principle. It is in such
sohdified portions of the Organized fabric, that the greatest
resemblance exists to Inorganic bodies ; but even these
portions all pass tlirough the condition of soft tissue, the
consolidation of which is effected by the deposit of some
hardening material (generally carbonate or phosphate of lime),
in its interstices. — It is by the reaction which is contmually
taking place between the solid and the liquid parts of
Organized structures, that their integrity is maintained. For
we shall find it to be a result of their peculiar composition,
that they are prone to continual decay ; and this decay would
speedily destroy them altogether, if it were not compensated
by new formation. The materials for their reproduction
must always be presented to the tissues in a liquid state, and
all the dead and decomposing matter must be reduced to the
same form, in order that it may be carried off; so that the
intermingUng or mutual penetration of sohds and liquids, in

DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 21

the minutest parts of Organized bodies, is a necessary condition
of their existence.

4. Organized structures are further distinguished from In-
organic masses by the pecuharity of their chemical constitution.
This pecuharity does not consist, however, in the presence of
any elementary substances which are not found elsewhere ;
for all the elements of which Organized bodies are composed,
exist abundantly in the world around. This, indeed, is a
necessary consequence of the mode in which they are built
up ; for that which the parent commimicates in giving origin
to a new being, is not the structure itself, but the capacity to
form that structure from materials supplied to it ; and it is
by progressively converting these materials to its own use,
that the germ develops itself into the complete fabric. — ISTow
out of about seventy simple or elementary substances which
are known to occur in the Mineral world, not above twenty
present themselves as constitueoits of Vegetable and Animal
fabrics ; and many of these occur there in extremely minute
proportion. Some of them, indeed, appear to be introduced
merely to answer certain chemical or mechanical purposes ;
and the composition of the parts which j^ossess the highest
vital endoTVTnents is extremely uniform. They are nearly all
formed at the expense of certain " organic compounds," which
are made up of the four elementary substances, oxygen, hy-
drogen, carbon, and nitrogen ; and these elements appear to
be united, — not as in the case of inorganic compounds, two
by two, or after the binary method, — but all four together,
so as to form a compound atom of great complexity. Thus
common nitre is regarded as a binary compound of nitric acid
and potass, since it can be decomposed into those two con-
stituents and can be re-formed by their union; and in the
same manner, its nitric acid is a binary compound of nitrogen
and oxygen, whilst its potass is a binary compound of 2:)otassium
and oxygen. But neither albumen nor gelatine, which are
the principal materials of the animal tissues; can be resolved
into any two other substances, by the union of wliich it can
be re-formed ; and when once it has been decomposed by che-
mical agencies, no means known to the chemist can reproduce
it. Albumen can, in fact, be generated only by the Hving
Plant, at the expense of the carbon, hydrogen, oxygen, and
nitrogen, wliich it draws from the elements around; and

22 DISTINCTIVE CHARACTERS OF LIVING ORGANISMS.

gelatine can only be formed in the animal body by a meta-
morphosis of the albumen which it derives from the Plant.
The peculiar mode in vi^hich the elements of these substances
are held together, renders them very prone to decomposition ;
so that Organized bodies, when no longer alive, rapidly pass
into decay, unless they are secluded from the contact of
oxygen, or are kept at a very low temperature. Such decay,
however, is continually taking place during life, and would
make itself obvious if its joroducts were not carried out of
the system as fast as they are generated within it. It
essentially consists in the resolution of the four principal
components of organic compounds — carbon, hydrogen, oxy-
gen, and nitrogen, in combination with oxygen drawn from
the atmosphere — into the three binary compounds, water,
carbonic acid, and ammonia, which thus restore to the In-
organic world the original materials of Organized fabrics, in
the very forms from wliich those materials were first derived
by the agency of the growing Plant. (See Veget. Physiol.)
5. It is, however, by their pecidiar actions, that living
Organisms are most completely differentiated from the inert
bodies of which the Mineral kingdom is composed. There
can be no doubt that of many of the changes which take
place during the life of an Organized being, a large proportion
(especially in the Animal kingdom) are effected by the direct
agency of physical and chemical forces ; and there is no
reason to believe that these forces have any other operation
in the living body, than they would have out of it under
similar circumstances. Thus the propulsion of the blood by
the heart, through the large vessels, is a purely mechanical
phenomenon ; as is also the movement of the limbs by the
lever-action of the forces brought to bear on their bones.
So, again, the digestive operations which take place in the
stomach are of a purely chemical nature ; and the interchange
of gases between the air and the blood, which takes place in
the act of respiration, must be regarded in the same light. — •
But after every possible allowance has been m.ade for the
operation of physical and chemical forces in the living or-
ganism, there stiU remain a large number of phenomena
which cannot be iu the least explained by them, and which
must be regarded as the result of an agency that differs from
these as they differ from each other ; and this agency, which

DISTINCTIVE CHARACTERS OF LIVING ORGANISMS. 23

is recognised by tlie effects it produces — in tlie same manner
as we recognise heat or electricity by their effects — may be
conveniently designated vital force} Thus, to revert to our
previous illustrations, the mechanical power employed in the
propulsion of the blood, or in the movements of the limbs, is
evolved by muscular contraction, a phenomenon altogether
pecuhar to the living muscle ; and the muscle derives its pro-
perty of contractility from the previous development of its
pecidiar tissue in the act of nutrition. So the solvent
fluids by which the digestion of food is accomplished, arc
separated from the blood by an act of secretion, which can
only be performed by a glandular apparatus in the living
walls of the alimentary canal. And the materials for the
nutrition of the muscular tissue, and for the secretion of the
digestive solvent, as of all the other acts of nutrition and
secretion which are continually going on in the living body, are
derived from the blood, — a lirpiid which possesses properties
very different (as we shall hereafter see) from any mere
mixture of chemical compounds, and which is prepared by
actions totally beyond the power of the chemist to imitate, — •
the laboratory of the living organism being requisite for their
performance.

6. The whole assemblage of vital actions which is per-
formed by the living Animal, may be arranged under two
principal groups ; one of them consisting of those which are
directly concerned in the development and maintenance of
its Organized fabric ; the other including all those by which
it is brought into conscious relation with the world around.
The former group includes the acts of digestion, absorption,
and assimilation, by wliich the nutritive materials are pre-
pared for becoming part of the living fabric ; the circulation
of the assimilated materials through the body ; their conver-
sion, by the act of nutrition, into the solid textures ; the
formation of various secretions, having various purposes to
serve in the economy ; the removal, by the acts of respiration

^ The Author has elsewhere given his reasoDs for the belief, that
Vital force bears the same " correlation " to the Physical and Chemical
forces, as the latter bear to each other ; but the discussion of this sub-
ject is not suited to an elementary treatise ; and the essential peculiarity
of the manifestations of vital force in the phenomena of life, requires
that it should be treated as belonging to a distinct category.

24 DISTINCTIVE CHARACTERS OF ANIMALS.

and excretion, of the effete matters with which the blood he-
comes charged by the decomposition continually going on in
the body ; the maintenance of animal-heat by the same process ;
and the act of reproduction, whereby the race is perpetuated,
in spite of the limited duration of the individual. The fore-
going, which are for the most part common to the Animal and
the Plant, are termed Organic Functions, or Functions of
Vegetative Life. But, in addition to these, it is the character-
istic of Animals generally, that they are sensible to impressions
made by surrounding objects, so that they possess some con-
sciousness of what is going on about them ; and that they also
possess the ^Dower of re-acting on those objects by movements
of their own, so as to change either their own places, or
the places of surrounding objects in relation to them-
selves. These two functions, sensibility and the power of
spontaneous motion, being peculiar to animals, are distin-
guished as Animal Functions, or Functions of Animal Life. In
the higher animals, they are the most important and charac-
teristic phenomena of their existence ; so that it would seem
as if the whole assemblage of organic functions had no other
destination in them, than to build up and keep in order the
apparatus by which the functions of animal life are ]3erformed.
But this state of things is entirely reversed among those
lower tribes of animals which border most closely on the
Vegetable kingdom ; for we find that among such, the mani-
festations of sensibility and power of spontaneous movement
are so feeble, that it may be doubted whether these attributes
are really present in them ; and even in higher orders, there
are many in which the proper animal powers are in such a
low grade of development, that they appear as if they were
destined merely to minister to the organic functions.

7. Thus, although the characteristic difference between the
Animal and the Vegetable kingdom, taking each as a whole,
may be truly said to consist in the possession by the former
of endowments which do not exist in the latter, this does not
express the essential difference between Animals and Plants ;
since, while there are many tribes among the former in which
the proper animal powers are reduced to so low a degree as
to prevent it from being certainly afhrmed that they are
present at all, there are many tribes among the lower plants
which exhibit a power of spontaneous movement fully as

DISTINCTIVE CHARACTERS OF ANIMALS. 25

great as tliat which exists among the lowest animals ; so that
no positive line can be drawn between the two kingdoms on
the basis of this distinction alone. There is another very
important j^hysiological difference, however, between the two
kingdoms, which seems to afibrd an adequate means of
settling the true place of those tribes wdiose position would
otherwise be doubtful. This lies in the nature of their food,
and the source from which it is obtained. Eor although it. is
now known that the primary tissues of plants are originally
formed of the same albuminous material as are those of
animals (the cellulose layers which constitute the great
bulk of the vegetable fabric being a subsequent deposit), yet
this material is generated in the Plant by the combination of
the elements which it obtains from the carbonic acid, water,
and ammonia of the soil or of the atmosphere ; whilst the
Animal is destitute of all jDOwer of thus forming it for itself,
and is hence entirely dependent upon the plant for its sup-
plies of nutriment. Thus, whilst the very humblest forms of
Vegetation, in common with the highest, are found to have
the power of decomposing carbonic acid under the influence
of sunlight, setting free its oxygen and retaining its car-
bon, the humblest forms of Animal life, in common with
the highest, derive their nutriment either directly from
plants, or from the bodies of other animals which have sub-
sisted on vegetable food, whilst they produce a converse
change in the atmosphere by their respiration, absorbing
from it oxygen, and giving forth to it carbonic acid. This
criterion will serve, it is beheved, to distinguish the very
lowest forms of Animal life from those humble forms of Vege-
tation wliich they most closely resemble in the simplicity of
their organization (§ 128); and its application will generally
be found to be very easy. There is now no longer any doubt
that a large proportion of the beings formerly ranked as
Animalcules, are really to be regarded as Plants, notwithstand-
ing that they possess a power of active and apparently
spontaneous movement, far greater than that of many unques-
tionable animals. And generally it may be said that the
presence of a bright-green or bright-red colour in any of these
simple organisms, where it is not derived from coloured sub-
stances taken in as food, affords a strong probability of their
vegetable character; these colours being produced in the

26 DISTINCTIVE CHARACTERS OP ANIMALS.

course of tliat series of cliemical changes, by wliicli, under the
influence of light, the Kving plant can unite, inorganic elements
into organic compounds.

8. jN'ot only clo Animals differ from Plants in the nature
and sources of their aliment, but also in the mode in which
■ it is taken into their bodies ; and this difference is related
alike to the character of the food of anunals, and to the general
conditions of animal existence. For the Plant extends its
roots through the soil in search of liquid, and spreads out its
leaves to the air for the purpose of imbibing some of its
gaseous ingredients. But the Animal could not so exist, and
be at the same time endowed with the power of moving from
place to place ; nor could it appropriate solid nutriment, if it
were not provided with some pecuUar means of receiving and
preparing this. For these purposes, animals (with few
exceptions) are provided with an internal cavity or stomach
into which the food is received from time to time, in v/hich
it can be carried about in the general movements of the body,
and within which it can be ^^repared for being received by
absorption into the current of nutrient liquid which circu-
lates through the body. This stomach is nothing else than a
bag formed by the prolongation of the external covering of
the body into its interior (§ 36); its cavity receives the food
introduced into it by the mouth ; its walls pour out or secrete
a fluid which acts upon the food in such a manner as to dis-»
solve it ; and through its walls are absorbed those portions
of the food which are fit to be employed as nutriment, while
the remainder is cast forth from the cavity, either by the
aperture which first admitted it, or by a distinct orifice. The
exceptional cases, in which no stomach exists, chiefly occur
in one particular tribe of animals, the Entozoa (§ 105), which
live either in the intestinal canal or in the substance of the
tissues of other animals, and which are supported by the
nutrient juices of these ; such an organ obviously not being
requii-ed by creatures which have no j^ower of locomotion,
and which can imbibe liquids already prepared for their use,
through the whole of the soft surface of their bodies. But
there is a large tribe of very simple animals, the lihizopoda
(§ 129), in which, notwithstanding the absence of any regular
stomacli, the food is. received into the very substance of the
jelly-hke particle of wiiich the body consists ; a mouth and

DISTINCTIVE CUARACTERS OP ANIMALS. 27

stomach being extemporized, as it were, on each occasion that
ahment is ingested ; and an anal orifice being extem.porized
in lilce manner, when the indigestible residue has to be cast
forth. All true Animalcules (§ 133) have a proper mouth,
into which food is drawn by the current created by the cilia
(§ 45) wherewith it is fringed ; and this mouth leads to the
general cavity of the body, within which the food is subjected
to the digestive process. hiZoo})hytes (§ 121) which possess
a proper stomach, this organ forms so large a part of the
animal, that its entire body may be almost said to consist of
the stomach and of the prehensile appendages by which it
draws in its food. But in all the higher tribes, the stomach,
with the alimentary canal proceeding from it, arc suspendecl
freely within the general cavity of the body ; and we shall
find that the space that surrounds these viscera is extremely
important in the economy of all but vertebrated animals, as
being a sort of reservoir into wdiich the nutrient materials
prepared by the digestive process first transude, and from
which it is carried into the remoter parts of the system. In
vertebrated animals, this cavity — called in them the peritoneal
cavity, from its being lined with a serous membrane (§ 2^),
termed the peritoneum— ^is not subservient to the same pur-
poses ; the nutrient materials being taken up from the walls
of the digestive cavity, both by the blood-vessels and by
special absorbents, and being by them carried into the current
of the circulation. It is obvious that until they have found
their way, through one or other of these channels, into the
general system, the nutrient materials introduced as food into
the stomach of an animal are not within its body, properly so
called, any more than a fluid is within a plant when it bathes
the exterior of its roots, or within an entozoon when in con-
tact with the soft surface of its integument. In each case,
the ah&oriotion of the fluid is first requisite ; and it is with
this that its application to the reciuirements of the living
body really commences.

9. But further, when we compare together, not the lowest,
but the highest members of the Vegetable and Animal king-
doms respectively — those in which their respective attributes
are most characteristically displayed, — we find that they
present such differences as to render it quite impossible to
confound the one with the other. Although it is easy even

28 DISTINCTIVE CHARACTERS OP ANIMALS.

for tlie scientific naturalist to mistake a Protopliyte (or one
of the simplest forms of vegetation) for an Animalcule, and
although Zoophytes are continually ranked in the popular mind
with the Plants they so much resemble in form, no one is in .
any danger of confounding the Oak and the Elephant, the Palm
and the Whale. Por among the liigher Animals, not only the
principal organs, but the greater part of their elementary
parts or tissues, are formed upon a plan entirely different
from that which prevails in Plants. All the arrangements
of their organism or corporeal edifice are made for the pur-
pose of enabling them to perform, in the most advantageous
manner possible, those peculiar functions with which they have
been endowed, — to receive sensations, — to feel, thuik, and
will, — and to move in accordance with the directions of the
instinct or the judgment. Por these purposes we find a
peculiar apparatus, termed the Nervous system, adapted. .This
apparatus consists of a vast number of fibres, spread out over
the surface of the body, and especially collected in certain
parts, called Organs of Sense (such as the eye, nose, ear,
tongue, lips, and jDoints of the fingers). These have the
peculiar proj)erty of receiving imjDressions which are made
upon their extremities, and of conveying them to the central
masses of nervous matter (known in the higher animals as
the Brain and Spinal Cord\ by the instrumentality of which
they are communicated to the mind.

10. Prom the Nervous centres, other cords proceed to the
various Muscles, by which the body is moved. These muscles,
commonly known as " flesh," are composed of a tissue which
has the power of contracting suddenly and forcibly, when
peculiar stimuli are applied to it. In this respect, it bears a
resemblance to the contractile tissues by which the move-
ments of plants are produced (Veget. Phts. § 390) ; but it
differs from them in being thrown into action, not only by
stimuli that are applied directly to itself, but by an influence
conveyed through the nervous system. Thus, in an annual
recently dead, we may excite any muscles to contraction, by
sending a current of electricity into the nerves supplying
them ; and in a living animal we may do the same by simply
touching those nerves. But the stimulus which these nerves
ordinarily convey, originates in an act of the mi7id, which is
connected in some mysterious and inscrutable manner with

DISTINCTIVE CHARACTERS OP ANIMALS. 29

the central masses of tlie nervous system. Thus, we desire
to perform a certain movement or set of movements; this
desire leads to an act of volition or luill; and the will causes
a certain force or motor impulse to issue from the brain and
travel along the nerves, so as to produce the desired motion,
by exciting contractions in the muscles that perform it. Or
again, a certain sensation calls forth an emotion^ which
prompts a certain muscular movement, and may even ctiuse it
to take place against the will, — as when a strong sense of the
hidicrous produces laughter, in spite of our desire (owing
to the uniitness of the time and place) to restrain it; for
the emotion, lilve the act of vohtion, produces a change in
the nervous centres, which causes a motor impulse to travel
along the nerves, and thus calls the muscles into contraction.
_Vnd it seems to be in the same manner that those instinctive
actions are produced, which, although few in adult JMan when
compared with those resulting from his will, predominate in
his infant state, and tln^ough the whole hfe of the lower
animals (Chap. xiv.). We shall also find that the nervous and
inuscular systems of animals are concerned in a class of actions
with Avhich the mind has no necessary connexion ; these autom-
atic actions, such as those of swallowing (§195) and breathing
(§ 340), having for their object to assist in the performance of
the organic functions, and to protect the body from danger.
^11. In the higher Animals, then, the presence of this
JS^'ervo-Muscular apparatus is an essential and obvious dis-
tinction between their structure and that of Plants ; and we
find that it constitutes a large part of the bulk of the body.
Thus the whole interior of the skull of Man is occupied by his
brain ; his hmbs are composed of the muscles, and of the bones
which support them and which are put in motion by them ;
and it is only in the interior of his trunk, that we find organs
corresponding with those which form the entire fabric of the
Plant. These organs of K'utrition have for their main pur-
pose, to supply the wants of the organs of animal life ; every
exercise of which is accompanied by a certain decay or wear
of their structure, and which consequently require to be con-
tinually nourished and repaired, by the materials provided
l>y what may be termed the vegetative organs. But in
the lower of tribes of Animals, we do not find the animal
lunctions to possess this predominance. In fact, among the

30 DISTINCTIVE CHARACTERS OF ANIMALS.

many wliicli are fixed to one spot during nearly their whole
lives, and which grow and extend themselves like plants, the
movements of the body are but few in number, and trifling
as to their variety; these movements are only destined to
assist in the performance of the organic functions, as by
bringing food to the mouth, and water to the respii'atory
organs ; and the nervo-muscular apparatus by which they
are effected, bears so small a proportion to the organs of
nutrition, as to seem hke a mere appendage to them, and is
sometimes altogether undiscoverable. Tliis is the case, for
example, in the lowest kinds of shell-fish, such as the Oyster,
and in the Coral-polypes.

12. Hence we perceive, as we descend the Animal scale, a
nearer and nearer approach to the character of Plants ; and
this we shall find to be the case, not only in the general
arrangement of the organs, but also in the nature of the
elementary tissues of which these are composed. For in the
higher animals, the whole organism is constructed in such a
manner as to admit a free motion in its individual parts.
The different portions of the skeleton or hard framework are
connected with each other by flexible ligaments, which are
adapted to resist a very powerful strain; the muscles are
attached to these by fibrous cords or tendons, which, also,
can support a vast weight ; and the several muscles and
other ]Darts, which need to be mutually connected, but also
require a certam power of moving independently of one
another, are bound together by a very elastic loosely-arranged
tissue, consisting of fibres crossing and interlacing in every
direction, the interstices between which are filled with fluid.
J^ow to these fibrous tissues, there is nothing analogous in
plants, because no freedom of motion is required, or even
permitted, among their parts ; and we find them bearing a
less and less proportion to the whole, as Ave descend the
animal scale. On the other hand, we find the various forms
of true cellular tissue, such as predominate in plants (Yeget.
Phys. Chap. III.), becoming more and more abundant, as we
pass from the highest to the lowest animals, and having
more and more important duties to fulfil. But even in the
highest Animals, as will hereafter appear, they are the im-
mediate instruments of the most important among the organic
functions, just as they are in Plants.

CIIEMICxVL CONSTITUENTS : ALBUMEN. 31

Chemical Constitution of the Animal Body.

13. By far tlic larger proportion of the Animal fabric is
formed at tlie expense of the substance termed A Ibumen ; the
composition and properties of which, tlierefore, claim our
iirst attention. The fundamental importance of albumen in
the animal economy, is shown by the fact that it constitutes,
with fat, and a small proportion of certain mineral ingredients,
the whole of that mass of nutrient material stored up in the eggs
of oviparous animals, which, being appropriated by the germ
to the building up of its fabric, is converted by it into the
bones, muscles, nerves, tendons, ligaments, glands, mem-
branes, &c. of the embryo. We find it also constituting a
large proportion of the solid matter of the blood and other
nutrient lluids of the adult animal ; and it is the fundamental
form to which the various azotized substances employed as
food (§ 153) — such as animal flesh, or the gluten of bread —
are first reduced by the act of digestion. It is composed
of 49 carbon, 36 hydrogen, 14 oxygen, 6 nitrogen, with a
minute proportion of sulphur ; it is generally blended, also,
with more or less of fatty matter, and with saline and earthy
substances.

14. Albumen may exist in two states, — the soluble and
insoluble. In the animal fluids it exists in its soluble
form; and is united (as an acid to its base) with about 1^
per cent, of soda, forming an albuminate of soda. It is not
altered by being cbied at a low temperature, but still retains
its power of being completely dissolved in water. When a
considerable quantity of it exists in a fluid (as in the white of
the G-gg), it gives to it a glairy tenacious character ; but it is
nearly tasteless. When such a fluid is exposed to a tempe-
rature of about 150°, a coagulation or 'setting' takes place, as
in the famihar process of boiling an egg. But if the albumen
be present in smaller quantity, the fluid does not form a
consistent mass, but only becomes turbid ; and this only after
being boiled. Albumen which has been dried at a low
temperature, however, may be heated to the boiling point of
water, without passing into the insoluble condition ; a fact
which is of peculiar interest in relation to the power which
the Tardigrada (ZoOL. § 841) possess, of sustaining a very
high temperature without the loss of their vitality, when

32 CHEMICAL CONSTITUENTS : ALBUMEN, CASEIN.

their bodies have been completely dried up in , the first
instance. ISTo trace of organization can be detected in
coagulated albumen, which seems to be composed only of a
mass of granules ; and in this respect it differs in an im-
portant degree from fibrin — as we shall presently see.
Albumen may also be made to coagulate readily by the action
of acids, especially the nitric (aqua-fortis) ; so that a very
small quantity of it may be detected in water, by the tur-
bidity produced by adding to it a di'op or two of nitric acid,
and then heating it. Now, when thus coagulated, albumen
cannot be dissolved again by any ordinary process ; but its
solution may be accomplished by rubbing it in a mortar with
a caustic alkali, potass or soda. From this solution it may be
precipitated again on the addition of an acid in sufficient
({uantity to neutralise the alkali. Albumen is distinguished,
then, by its peculiar property of coagulating on the applica-
tion of heat, or on being treated with certain acids.

15. JSTearly allied to albumen is the substance termed
Casein, which replaces it in milk ; and this is specially
worthy of notice here, because it is the sole form in which
the young Mammal receives albuminous nourishment during
the period of suckling, in which it draws its sustenance from
its parent. Like albumen, this substance may exist in two
forms, the soluble, and the insoluble or coagulated ; and the
jDresence of a small quantity of free alkali seems essential to
its continuance in the soluble form. Casein differs from
albumen, however, in this, that it does not coagulate by
heat, and that it is precipitated from its solution by organic
acids, such as the acetic and lactic, which have no coagulating
action on albumen. It is further remarkable for the facihty
with which its coagulation is effected by the contact of
certain animal membranes ; as we see when a small piece of
■rennet (which is the dried stomach of the calf) is put into a
large pan of milk in the process of cheese-making, the ' curd'
which then separates being composed of casein entangling the
oily particles of the milk. In the coagulated state, casein
differs but very little from albumen, and is readily converted
into it by the gastric fluid. It is remarkable for its power of
dissolving the earthy phosphates, as much as 6 per cent, of
phosphate of lime being usually obtainable from it ; and it is
in this combinatioji, that the large quantity of bone-earth

required for the consolidation of the skeleton of the young
animal, is introduced into its system. A substance resembling
casein is obtainable from the serum of the blood, especially in
pregnant females ; and also from the serous fluid which
occupies the interstices of the tissues. It is found, also,
mingled with albumen, in the yolk of the egg, forming a
compound which (before its true character was known) has
been distinguished as vitellin. Now as all the liquids con-
taining casein have it for their special function to supply
formative materials to rapidly-growing tissues, we may with
much probability regard it as still more closely related to
them than is albumen itself. It differs from albumen but little,
if at all, in the ultimate proportions of its elements (§ 1 3).

16. The substance of which muscles are composed, has
been commonly considered to be Fibrin (§ 17) ; but it diflers
essentially from hbrin in its properties, and is now dis-
tinguished as Syntonin. Its chief peculiarity is its solubility
in very dilute muriatic acid (1 part to 100 of water), and its
precipitation in the form of a jelly when the acid is neutra-
lised ; this jelly treated with dilute alkalies forms a solution
which coagulates by heat ; and thus it seems to be reduced
nearly to the condition of albumen. This is, in fact, very
much what takes place in the act of digestion of flesh-meat ;
the muscle-substance being first dissolved by the muriatic or
other acid of the gastric fluid, and the solution being then
rendered alkaline by the mixture of bile and other secretions
in the small intestine.

17. In the blood and other nutrient fluids of the animal
body, there is found a substance which is so closely related to
albumen in its ultimate chemical composition, as not to be dis-
tinguishable from it with any certainty ; but which, though
fluid whilst circulating in the living vessels, coagulates spon-
taneously after having been for a short time withdrawn from
them, the coagulum or clot bemg distinguished from that of
albumen or fifei^H by the fibrillar arrangement of its particles,
which indicates an incipient organization. This substance,
termed Fibr'm, may be obtained in a separate form, by
stirring fresh-drawn blood with a stick, to which it adheres
in threads. In this condition it possesses the softness and
elasticity which characterise the flesh of animals, and con-
tains about three-fourths of its weight- of water. It may be

o

34: CHEMICAL CONSTITUENTS : — FIBRIN.

deprived of this water by drying, and then iDecomes a hard
and brittle substance ; but, like dried flesh, it imbibes water
again when moistened, and recovers its original softness and
elasticity. From the recent experiments of Dr. Eichardson,
it appears that the coagulation of blood-fibrin depends upon
the escape of ammonia, being accelerated by such conditions
as favour the liberation of this gas, and retarded or prevented
by such as cause its retention in the liquid; whilst, even
after the clot has been formed, it may be dissolved by
ammonia, forming again when that gas is set free. Pibrin
differs from syntonin or muscle-substance in not being dis-
solved by very dilute muriatic acid, but being merely caused
to swell up into a gelatinous mass, which contracts again
when more acid is added. It combines with the earthy
phosphates, of which as much as 2^ per cent, is sometimes
found in the ash left by its combustion.

18. There can be no doubt that fibrin is formed in the
blood and in the other fluids in which it presents itself, at
the expense of albumen. What is its precise destination,
cannot as yet be clearly specified ; but there are several
circumstances which point to the conclusion that it is to be
regarded as a transitional stage in the metamorphosis of
albumen into the simple fibrous tissues (§ 23.) Thus, when
the ordinary clot of blood is examined microscopically, it is
found to consist, not, like an albuminous coagulum, of a
homogeneous mass of granules, but of a network of im-
perfectly-formed fibres, enclosing the red corpuscles in its
interstices. A much more distinct network of the same kind
may be seen in the colourless coagulum formed by the liquid
which may be skimmed off the surface of the blood drawn
from persons sufiering under any severe inflammation*; such
blood coagulates slowly, and its red corpuscles and the fluid
in which they float have an unusual tendency to separate
from each other ; and the fibrin previously dissolved in the
latter sets into definite fibres, which continue for some days
to increase in firmness. It is a liquid of the same kind,
charged with fibrin in a peculiarly " plastic " condition, that is
poured forth for the formation of new tissue when the repa-
rative processes are at work for the healing of a wound or the
reunion of divided parts ; and it is by a plug of coagulated
fibrin, which gradually comes to present a more and jnore

CHEMICAL CONSTITUENTS : — FIBRIN, GELATIN. 35

distinctly fibrous structure, tliat the mouths of divided blood-
vessels are closed up, when the flow of blood from them
spontaneously stops. In all such cases, tlie fibrous network,
if formed out of connexion with a living body, passes after a
time into decay ; but if it be formed in a])positioii with living-
parts, blood-vessels gradually extend into it from these, its
nutrition is maintained and improved, and it progressively
comes to present the ordinary characters of the simple fibrous
tissues (§ 22).

19. Although the tissues most actively concerned in
carrying on the vital operations, retain for the most part the
composition of albumen, yet that very large proportion of the
fabric of the higher animals whose offices are essentially
mechanical, has a very different chemical constitution. • If we
boil down either their bones, their skin, or their internal
membranes, we shall get a considerable quantity of the sub-
stance scientifically termed Gelatin, familiarly glue. Though
consisting of the same elements as albimien, its composition is
simpler, because these elements are united in smaller propor-
tions ; the atom or combining equivalent of gelatin being
made up of 13 Carbon, 10 Hydrogen, 5 Oxygen, 2 Nitrogen.
The distinctive character of gelatin consists in its solubility
in warm water, its coagulation on cooling into a uniform jelly
which can be liquefied again by warmth, and its formation of
a peculiar insoluble compound v/ith tannin. Gelatin is very
sparingly soluble in cold water, though made to swell up and
soften by prolonged contact with it. A solution of only one
part of gelatin in 100 of hot water is sufiiciently strong for
the whole to form a consistent jelly on cooling. The re-
action of gelatin with tannin is so decided, that the presence
of only one 2:)art in 5000 of water is at once detected by
infusion of galls ; and it is in this action that the process of
tanning consists, — the gelatinous fibre of the skin, which
would speedily pass into decay, being converted into a com-
paratively unchangeable substance. The different tissues
which have gelatin for their base, yield it to boiling water
with different degrees of facihty ; this diversity ap2:)arently
depending in some degree upon the definiteness of their
organization. Thus the " sound " or air-bladder of the cod,
sturgeon, and other fish, which, when dried and cut into
strips, is knov/n as isinglass, is very readily acted on ; the

d2

36 CHEMICAL CONSTITUENTS : — GELATIN, CHONDRIN.

same is the case with, the animal substance of bones from
which the earthy matter has been removed ; and in each case
the fibrous texture of the hving tissue is but very imperfectly
developed. For the extraction of gelatin from the skin, the
ligaments, the tendons, and various internal membranes,
whose fibrous texture is more pronounced (§ 29), a much
longer action of boiling water is required.

20. A peculiar modification of gelatin, which presents
itself in Cartilage (or gristle), is distinguished as Chondrin.
This requires longer boiling than gelatin for its solution in
water; as is seen when a knuckle of veal or of mutton
is cooked, the tendons and ligaments about the joint
being almost reduced to pulp, whilst the cartilages are scarcely
at all softened. The essential properties of chondrin are
nearly the same as those of gelatin, and its composition
seems nearly identical ; but it is thrown down from its
solution by muriatic and acetic acids and some other reagents,
which do not disturb a solution of gelatin.

21. It is not yet f Lilly known how the material of the
gelatinous tissues is produced in the animal body. There
can be no doubt of its being producible from albumen ; since
we find it in large proportion in the tissues of animals that
have never received gelatin into their bodies in any shape.
And although carnivorous animals will receive it as part of
their ahment, yet there is strong reason to believe that the
gelatin which is thus supplied to them does not really serve
to nourish their bodies, but that it is speedily decomposed
and got rid of (§ 159). It may be considered as quite certain
that the albuminous tissues cannot be formed by the meta-
morphosis of gelatin ; whilst conversely, looking to the fact
that in the egg and in milk no gelatin is provided for the
young animal, although the gelatinous tissues form a yet
larger proportion of its body than they do in the adult, we
seem entitled to question whether it is possible that these
tissues can be formed in any other way than at the expense
of the albuminous constituents of the blood.

Structure of the Primary Tissues.

22. In considering the structure of the " primary tissues,"
of which the various organs of animals are composed, it will
be convenient first to treat of those which are subservient

PRIMARY TISSUES : SIMPLE FIBROUS TISSUES.

37

merely to the physical actions of the framework ; as, for ex-
ample, by holding its parts together, by communicating motion,
or by giving them mechanical support and protection. — Tlie
several parts of the body, even to the very minute divisions
of its organs, are held together by what may be termed, in
contradistinction to Muscular and Nervous fibre, the simple
Jlhrous tissues ; and these are merely endowed, like ordinary
cords, with the power of resisting tension or strain, either
without themselves yielding to it at all, or with a certain
amount of elasticity, which enables them first to yield to
a certain degree, and then to recover their previous state.
These two quahties are characteristic of two distinct forms of
simple fibrous tissue, the v)hite and the yellow.

23. The White fibrous tissue presents itself under various
forms, being sometimes composed of fibres so minute as to be
scarcely distinguishable, but
more commonly presenting
itself under the aspect of
flattened bands, which are
but imperfectly divided into
fibres, and have more or less
of a wavy aspect (fig. 1). This
tissue is resolved, by long
boiling, into gelatine ; and
when treated with acetic acid,
it swells up and becomes
transparent, by w^hich peculiarity it can

White Fibrous Tissue.

dis-
The

be readily
tinguished from the other kind, to be next described.
Yellow fibrous tissue presents
itself in the form of long,
separate, clearly defined fibres,
which sometimes branch, and
which break short off when
overstrained, their extremi-
ties bemg disposed to curl up
(fig. 2). They are, for the most
part, between 1 -5,000th and
l-10,000th of an inch in
diameter ; but they are often
met with both larger and smaller. This kind of tissue un-
dergoes but very little change from long boiling, and it is

Yellow Fibrous Tissue.

38

PRIMARY TISSUES I AREOLAR TISSUE.

not acted on by acetic acid. It is but little prone to decom-
position, and will exliibit its peculiar elasticity long after it
lias been separated from the body, provided it be kept moist.
— These two forms of tissue exist separately in certain parts
of the fabric, but they are much more frec[uently combined ;
and the proportion of the yellow elastic tissue which exists in
any such combination, may be readily determined under the
microscope by the use of acetic acid, which renders all the
white fibrous structure so transparent, that the yellow fibres
are seen completely isolated in the midst of it.

24, One of the tissues which is composed of such an
admixture of white and yellow (or non-elastic and elastic)
fibres, is the one which was formerly called "cellular," but
which is now more correctly designated as Areolar.^ This
is composed of a mesh-work of fibres, and of bands of fibrous
membrane, which are interwoven in such a manner as to leave
very numerous interstices and cavities amongst them, having
a tolerably free communication with each other (fig. 3). These

Fig. 3. — Portion of Areolar Tissue.

cavities are filled during life with a serous fluid ; " and it is a
necessary result of the communication between them, that if
an accumulation of this fluid takes place to an undue extent,

^ From the Latin areola, a small open space.

^ A fluid resembling the serum of the blood, diluted with water
(§ 236).

PRIMARY TISSUES : AREOLAR TISSUE. 39

as in dropsy, it descends by gravity to the lowest situation.
Hence, tlie legs swell more frequently than any other parts.
In its natural state, this tissue possesses considerable elas-
ticity ; hence, when we press upon any soft part, and force out
the fluid beneath into the tissue around, the original state
returns as soon as the pressure is removed. Eut in dropsy,
it appears as if the elasticity of the hbres were impaired or
destroyed by their being over-stretched ; for when we press
with the finger upon a dropsical part, a j^it remains for some
time after the finger has been removed.

25. This Areolar tissue is diffused through almost the
whole fabric of the adult animal, and enters into the compo-
sition of almost every organ. It binds together the minute
parts of which the muscles are composed ; it lies amongst the
muscles themselves, connecting them together, but yet per-
mitting them sufficient freedom of motion ; it exists in large
amount between the muscles and the skin ; it forms sheaths
to the blood-vessels and nerves, and so connects them with
the muscles that they shall not be strained or suddenly bent
by the movements of the latter ; and it enters mto the struc-
ture of almost every one of the organs which are contained
in the cavity of the trunk, uniting its parts to each other, and
keeping the whole in its place. But it is a great mistake to
assert, as it was formerly common to do, that it penetrates the
harder organs, such as bone, teeth, and cartilage. Its ]3urpose
obviously is to allow a certain amount of motion among the
jiarts it unites ; and we find that the more free this motion is
required to be, the larger is the proportion borne by the
yellow or elastic fibres, to the white or non-elastic.

2Q. Although the Areolar tissue contains a very large
number of blood-vessels and nerves, yet it does so merely
because it furnishes the bed or channel in which they are
conducted to the parts where they are really wanted. Its
own vitality is low, and its sensibility very slight. It is
quickly reproduced after injury ; and it is by its means that
losses of substance are repaired in tissues of a more elaborate
kind, which are not so easily regenerated.

27. The continuity or connectedness of this tissue over the
whole siu'face of the body, admits air to pass readily from
one part to another ; and the inflation or blowing-up of its
cavities with air, which has sometimes happened accidentally,

40 PRIMARY TISSUES : SEROUS MEMBRANES.

and lias sometimes been purposely effected, does not produce
any disorder in the general functions of the body. In blow-
ing the nose violently, some j^art of the membrane lining its
cavity has occasionally given way, so as to allow air to pass
into the areolar tissue of the face, and especially into that
contained in the eyehds, which is particularly loose ; an enor-
mous swelling of these parts then takes place, presenting a
very frightful appearance, but not attended mth the least
danger, and subsiding of itself in a few days. This swelling
presents a character to the touch quite different from that
which would be occasioned by a similar distension with liquid;
for it gives somewhat of the crackling feel that is occasioned
by pressing on a blown bladder. A similar inflation of the
areolar tissue of the body has sometimes occurred from the
formation of an aperture, by disease or injury, in the walls of
the lungs or air-passages, and the consequent escape of air
during the act of breathing : in one remarkable case of this
kind, the skin of the whole body was so tightly distended
with air as to resemble a drum. It is intentionally practised
by butchers, who " blow up " the areolar tissue of their veal,
in order to increase its plumpness of aspect ; and the in-
flation of the areolar tissue of the head, in the living state,
has been sometimes practised by impostors, in order to excite
commiseration.

28. Fibres and shreds of fibro-membrane, resembling those
of which areolar tissue is composed, may be so interwoven as
to form a continuous sheet of membrane, having a smooth
and glistening surface j and in this manner are produced the
Serous Membranes that line the different cavities in which the
viscera (or organs contained within the skull, the chest, and
the abdomen) are lodged. The peculiar manner in which
these membranes are arranged, will be explamed hereafter
(§ 43). One of their surfaces is always free or. unattached,
whilst the other is in contact with the outer wall of the
cavity ; and from the free surface, which is covered with a
layer of flattened epithelium-cells (fig. 10), a serous fluid is
exhaled, which adds to its smoothness. It is by an accumula-
tion of this fluid, that di^opsies of the cavities are produced, —
such as water on the brain, or in the chest.

29. By the union of fibres of a stronger kind, those firmer
tissues are produced, which are employed wherever a greater

FIBROUS MEMBRANES AND LIGAMENTS. 41

strain has to be borne. This is the case with the Ligaments,
which bind together the bones at the joints, the Tendons, by
which the muscles are usually attached to the bones, and the
tough Fibrous Membranes that envelope and protect many
of the most important viscera. In these any considerable
amount of elasticity would be misplaced ; and we conse-
c^uently find that they are chielly or entirely composed of the
wJiite fibrous tissue. "Whenever an elastic ligament is re-
quired, however, we find the white replaced by yellow. One
of the best examples of this is seen in the ligament of the
neck of many quadrupeds, commonly known as the paxy-
waxy ; which is given to the large herbivorous quadrupeds,
such as the ox, to assist them in supporting their heavy
heads with as Httle exertion as possible ; whilst carnivorous
quadrupeds, such as the Hon and tiger, are endowed with it
to give them additional power of carrying away heavy bur-
dens in their mouths. In Man we scarcely find a trace of
it. This yellow fibrous tissue is found, moreover, in the walls
of the arteries (§ 248), to which it gives their peculiar elas-
ticity; and it also forms the vocal cords of the larynx (§ 681),
It is by the same kind of elastic ligament that the claws of
the Feline tribe are ch-awn back into their sheaths when not
in use, being projected (when required) by muscular action ;
and that the two j)ieces of the shell of Bivalve MoUusks are
united at the hinge, and are at the same time kept apart for
the admission of water between them, except when the
animal forcibly draws them together by its adductor muscle
(§ 113).

30. All these fibrous tissues, then, are concerned in actions
purely mechanical; and there is nothing in their properties
which is so distinct from those of inorganic substances, as to
require to be considered as vital. We may consider them,
therefore, as among the lowest forms of animal tissue ; and
accordingly we find that, when the higher forms degenerate
or waste away, these appear in their place. Such a degene-
ration may take place simply from want of use. Thus if,
from palsy or want of power of the nerves, the muscles of
the legs are disused for several years, they will lose their
peculiar property of contractility (§ 5) ; and it will be found
that scarcely any true muscular structure remains, but that it
is replaced by some form of fibrous tissue. Or again, if the

42

BASEMENT MEMBRANE : CEIiLS.

front of the eye be so injured by accident or disease, that light
cannot pass through it to make its impression on the nerve,
that nerve, being thrown into disuse, will gradually degenerate
into fibrous tissue. Moreover, this change may take place as
a part of the regular actions of life ; for there are certain
organs in the young animal previous to birth, which are not
required afterwards ; and these degenerate in like manner,
gradually wasting away, and leaving only traces behind them,
— tubes shrivelling into fibrous ligaments, and glandular
structures remaining only as areolar tissue.

31. Along every free surface of the body, both external
and internal, is spread out a delicate structureless layer, which
is termed the Basement or Primary Membrane. This forms
the outer layer of the True Skin, lying between it and the
Epidermis or scarf-skin (§ 37) ; in the same manner it
underhes the Epithelial layer of the Mucous membjanes
which line the open cavities of the body (§ 39), and of the
Serous membranes which line its closed cavities (§ 43) ;
and it occupies the same position in the walls of the blood-
vessels, gland-ducts, and other tubes. It is difficult to sepa-
rate it, in any of these parts, from the tissues with which it
is in contact ; and its characters may be well studied by dis-
solving the calcareous part of an oyster or mussel-shell in
dilute acid, when it mil be found that layers of a thin trans-
parent membrane are left, which have been thrown off at
each act of shell-formation, from the surface of the mantle.
This elementary membrane, like that which forms the walls
of cells (§ 32), is remarkable for the readiness with which

it is permeated by fluid, al-
though no visible pores can
be seen in it.

32. A considerable part of
the fabric of even the highest
Animal is formed, like the
entire organism of the Plant,
of Cells, either unchanged or
in some way metamorphosed.
A cell is a minute bag or

Fig. 4.-NUC.EATED Cells; a a, nuclei. ^^^.^^^^ ^^^^^^^ ^^ ^ structure-
less membrane, and having its cavity filled with fluid of some
kind. In some part of its interior, most commonly adhering

JL // '""^C^ K ' \

\

"•:.^

4

i

li

>

u

cells; their mode of multiplication. 43

to its wall, there is usually to be observed a solid collection
of granular matter, wbich is termed tlie nucleus (fig. 4, a a).
The typical form of tlie cell is globular or oval (fig. 5) ; but
when a number of cells are in contact with each other, and
are pressed together, their sides become flattened; so that
when they are cut across no intervals are seen between them,
but their walls are everywhere in contact (fig. 6), just as in

»:p^^ '1

Fig. 5. Fig. 6.

Rounded Cells in Cartilage Polygonal Cells from Car-

OF Bat's Ear. tilage in Mouse's Ear.

the section of a vegetable pith. The chemical composition of
the nucleus differs from that of the cell- wall ; for whilst tlie
latter is dissolved by acetic acid, the former (like the yellow
elastic tissue, with which its substance appears to have some
relationship) is unchanged by it. When the formation of a
cell is complete, and it is not destined to reproduce its kind,
the nucleus frequently disappears ; tliis is the case, for
example, with the red corpuscles of the blood of Mammaha
(§ 229), and also with Fat-cells (§ 46).

33. ]^ew cells may originate in one of two very distinct
modes ; either from a pre-existing cell, or by an entirely new
production in the midst of an organizable fluid or blastema.
The most remarkable example of the first process is presented
in the early development of the germ, which entirely consists
of an aggregation of cells, every one of which undergoes
successive subdivisions into two, so that the total number
in the germ-mass is repeatedly doubled (Chap. xv.). The
same method of multiplication by hinary suhdivisio7i may be
seen to continue throughout life in Cartilage-cells (§ 47), the
growth of which almost exactly repeats the history of the
growth of the lowest forms of Sea-weeds. The process of sub-
division seems to commence in the nucleus, which begins to
separate itself into two equal parts, and each of these draws

44 MULTIPLICATION AND NEW PRODUCTION OF CELLS.

around it a portion of tlie contents of the cell ; so tliat the
cell-wall, which is at first merely doubled inwards by a sort
of hour-glass contraction, at last forms a complete partition
between the two halves of the original cavity. The process
may be repeated either in the same or in a transverse direc-
tion, so as to produce four cells, which may be either arranged
in a single line O O O O or may form a cluster gg ; and
another subdivision of each cell will, of course, again double
the entire number. In other cases, however, the nucleus
appears to break up at once into several fragments, each of
which may draw around it a portion of the contents of the
parent-cell, which becomes invested by a cell-wall of its own ;
and thus the cavity of the parent-cell may at once become
filled with a whole brood of young cells, without any successive
subdivision. Generally spealdng, the former method seems
to prevail in structures which, lilie Cartilage, have a com-
paratively joermri??e?i^ destination ; whilst the latter is followed
in cases in which the cells thus formed are destined only
for a transitory existence. This is the case especially in Can-
cerous structures, which are particularly distinguished by
their proneness to the rapid production of cells within cells.

34. The production of new cells in the midst of an or-
ganizable blastema or formative fluid, such as is poured out
from the blood for the reparation of an injury, is a very
different process. This blastema, when first effused, is an
apparently homogeneous semi-fluid substance ; as it solidifies,
however, it becomes dimly shaded by minute dots, and as it
is acquiring further consistence, some of these dots seem to
aggregate, so as to form little round or oval clusters, bearing
a strong resemblance to cell-nuclei. These bodies appear to
be the centres of the further changes which take place in the
blastema ; for if it be about to undergo development into a
fibrous tissue (§ 18), they seem to be the centres from which
the fibrillation spreads ; whilst, if a cellular structure is to be
generated, it is from them that the cells take their origin.
The first stage of the latter process appears to consist in the
accumulation of the substance which the cell is to include,
about each nucleus, and around this the cell-membrane is
subsequently developed. It is in this mode that the de-
velopment of new structures, for the filling up of losses of
substance, is provided forj and it appears, from recent

ISOLATED CELLS OF ANIMAL FLUIDS. 45

inquiries, tliat the blastema will resolve itself into fibres or
into cells, according as the wound is completely secluded from
the air, or is exj30sed to it. It is under the former condition
that losses of substance are most rapidly and most completely
repaired ; whilst it is under the latter that inilanmiation is
most Hkely to arise, in consequence of the bad effect pro-
duced by the contact of air with the raw surface ; the
process of healing, when thus interfered with, going on less
favourably as w^eU as more slowly.

35. The very simplest and most independent condition of
the animal Cell, is probably to be found in the nutritive fluids
of the body ; in which we meet with floating cells that are
completely isolated from each other, and which are conse-
quently just as self-sustaining as are the separate vesicles of
the Yeast-plant, of the Eed Snow, or of other simple cellular
Plants. These cells are of two classes. In the blood of
animals generally, and in the chyle and lymph of Yertebrata,
we find a larger or smaller proportion of colourless corpuscles,
which are usually nearly spherical in form, and which exhibit
various stages of development into cells, being sometimes
little else than collections of granules, without any distinct
enveloping membrane, whilst, in other instances, there is a
distinct cell-wall, cell-cavity, and nucleus. These bodies, if
watched under a sufficiently pov.^erful microscope, may often
be seen to undergo very curious changes of form, resembhng
those of the Amoeba (§ 129). Besides the foregoing, however,
the blood of Yertebrated animals contains a far larger pro-
portion of reel corpuscles, which are flattened disks, sometimes
circular but more commonly oval, having pellucid and colour-
less walls, but having their cavities filled with a pecuhar
coloured fluid. As these will be more fully described here-
after (§ 229), it is not requisite to do more than notice them
here as constituting a most important part of the animal
organism, probably not less than a twelfth part of the entire
weight of Man and the higher animals, being thus composed
of nothing else than these isolated cells. ^

36. ISText in independence to the cells or corpuscles float-
ing in the animal fluids, are those which cover the free

^ The entire weight of the blood of Man seems to be about one-sixth
part of that of the body ; and the moist corpuscles constitute about half
the entire weight of the blood.

46 SKIN AND MUCOUS MEMBRANES.

membranous surfaces of tlie body, and which form the
Epidermis, or superficial layer of the skin, and the Epithelium
of the internal membranes. And it will be convenient here
to consider the entire structure of the Skin, the Mucous
Membranes, and the Serous Membranes, which are complex
fabrics, chiefly made up of the elementary tissues already
described. — These membranes may each be considered as
composed of three principal parts, namely, the superficial
layer or layers of cells, the basement-membrane whereon the
cells lie, and the subjacent texture covered by this, which
consists of fibrous tissue compactly interwoven and traversed
by blood-vessels, nerves, absorbents, and also containing
glands of various kinds. The Skin and Mucous Membrane
may, in fact, be regarded as belonging to one and the same
type ; for they are continuous with each other wherever one
of the open cavities of the body conmiunicates with the
surface, as at the mouth, nostrils, and anus ; and in the
Hydra (§ 121) it has been experimentally found that the
membranous layer covering the body may be made to change
places with that which lines the stomach, without any sensible
disturbance in the functions of either. The difl'erence between
the two essentially consists in this ; that the Skin, being
destined especially for the reception of sensations, and for the
protection of the soft parts beneath, is more copiously furnished
with nerves than with blood-vessels, and has its surface
covered by a firm, dry cuticle ; whilst the Mucous Membrane,
ministering especially to the organic functions, is comparatively
little supplied with nerves, but is abundantly furnished with
blood-vessels, and in certain parts with absorbents, whilst its
cellular layer is soft and easily permeable by liquids. Both
in the skin and in mucous membrane we find a multitude of
minute glands, for the separation of particular fluids from the
blood ; the nature of these differs with the locality.

37. The fibrous mesh-work of the Cutis or True-Shin is con-
tinuous with that of the Ai-eolar tissue which lies immediately
beneath it ; so that the two textures are not separated one
from the other by any definite boundary (as the examination
of a vertical section (fig. 7) clearly proves), but are dis-
tinguishable only by the compactness of the one, as contrasted
with the looseness of the other. The outer surface of the
Cutis usually presents numerous minute elevations or papillce

STRUCTURE OF THE SKIN. 47

(fig. 7, i i), wliicli are commonly arranged in rows ; of these,
some are organs of touch, being furnished with sensory nerves
that end upon a peculiar cushion-like organ in their interior
(§ 490) ; but into others no nerves can be traced, so that,
as these are copiously supplied with blood-vessels, it is pro-
bable that they minister to the nutrition of the epidermis.

/

Fig. 7. — Vertical Section of the Skin,

Showing the diflferent structures which it contains. A, Epidermis ; a a, its outer
surface ; a — b, its horny layer ; h — c its inner soft layer, dipping down into the
hollow between the papillae ; B, Cutis ; d, arterial twig supplying its vascular
papillae; e e, perspiratory glandulas ; /, cluster of fat-cells ; g ff, perspiratory duct,
traversing the true skin; h, its continuation through the epidermis; j i, tactile
papillae, with their nerves.

This is the more probable from the fact that we find these
vascular papillse very large and full of blood-vessels in the
interior of corns,' warts, and other such productions, formed
by a "hypertrophy" or over-nutrition of the epidermis in
particular spots ; and also in situations in which the ordinary
epidermis is very thick, as it is on the black pads of the foot
of the dog or cat. And a highly vascular structure of the

48

STKUCTURE OF THE SKIN.

same kind is foinid in tlie matrix or receiDtacle of the growing
roots of nails, hoofs, horns, &c, which are only modified forms
of epidermis. Imbedded in the substance of the cutis we
find, in most situations, the perspiratoyy glands (fig. 7, ee), by
which the watery fluid that is continually being exhaled from
the skin, is separated from the blood (§ 371) ; these send forth
their secretion by canals {g h), which traverse the epidermis
in a corkscrew-like manner, and then open upon its surface
by oblique valvular orifices. In the Cutis, also, are lodged
the hair-follicles (§ 38), which are really pits or depressions
of its surface, with a vascular papilla at the bottom of each,
supplying nutriment for the abundant development of the
cells in which the hair originates, as will be presently
described. "Wherever the hair-fol-
licles occur, there do we also find
sebaceous follicles (fig. 8, a a) ; these
are peculiar glandulas, secreting fatty
matter, which is poured into the hair-
canal, so as to come through it to the
surface of the epidermis ; and the use
of this secretion, which is particularly
abundant in the dark skins of the
natives of warm climates, is to pre-
vent the cuticle and the hair from
being too much dried up by exposure
to air. — The surface of the Cutis is
covered by a layer of basement-mem-
brane (§ 31), which is not traversed
either by blood-vessels, nerves, or
absorbents ; so that none of these
pass into the epidermis which lies on
its outer side.

38. The Epidermis, otherwise
j,.^ termed the cuticle, or " scarf-skin," is

Tkin section'ok the Human composcd of numerous layers of nu-

ScALP;— a a, sebaceous glands; cleatecl Cclls j of wllich We find those

b, a hair, with its follicle c. -^^ immediate contact with the base-
ment-membrane to be nearly spherical ; those a little removed
from it to be rendered polygonal by the mutual pressure of
their sides ; those nearer the outer surface to be flattened,
and this in an increased degree, as we pass from within

STRUCTURE OF THE SKIN : EPIDERMIS. 49

outwards, until we arrive at layers composed entirely of dry
flat scales, which show but little indication of ever having
been cells. There is no doubt, however, that all these forms
are but different stages of the existence of one and the
same set of epidermic cells ; these taking their origin in the
formative fluid exuded on the surface of the basement-mem-
brane, and being progressively carried towards the surface by
the . successive development of new layers beneath them,
whilst the layers above them are thro'wn off", or are worn
away ; and at the same time undergoing a change of form, in
the first instance from mutual pressure, and afterwards from
the loss of their contained fluid. At the same time they are
rendered more firm in texture, by the formation of a horny
secretion in their interior ; so that the outer layers of epi-
dermis form a consistent membrane, which is raised from the
surface of the Cutis when fluid infiltrates between them (as
when the hand has been long soaked in water), or is poured
out by the vessels of the latter (as when a blister is applied) ;
whilst the soft internal layers remain in contact with the
basement-membrane. — The number of layers varies greatly in
different parts, being usually found to be greatest where
there is most pressure or friction, as if the irritation deter-
mined an increased supply of blood to the spot, and thus
favoured an augmented development of epidermic cells.
Thus, on the soles of the feet, particularly at the heel and
the ball of the great toe, the Epidermis is extremely thick ;
and the palms of the hands of the labouring man are
distinguished by the horny hardness of their thick cuticle.
— It was formerly supposed that a special layer of a soft
spongy tissue, termed the rete mucosum, intervenes between
the Cutis and the Epidermis ; and that this was the special
seat of the colour of the skin in the dark races. It is
now well ascertained, however, that this supposed rete con-
sists of notliing else than the newly-forming soft layers
of the true epidermis ; and that the colouring matter is
diffused through the epidermic cells, so as to tinge the
entire thickness of the cuticle, although its presence is
particularly obvious in the deeper layers. — The Nails may
be considered as nothing more than an altered form of
Epidermis ; when examined near their origin, they are found
to consist of cells which gradually dry into scales that remain

E

50 EPIDERMIC APPENDAGES : — NAILS, HAIR, &C.

coherent; and when thin sections are treated by a dilute
solution of soda, these scales swell out again (as do also those
of the cuticle) into globular cells. A new production is
continually taking place in the groove of the skin in which
the root of the nail is imbedded, and also from the whole of
the surface beneath it ; the former adds to the length of the
nail • the latter to its thickness. — The structure of Hairs is
essentially the same. The base of each is formed of a " bulb,"
which consists of a mass of epidermic cells developed from
the vascular papilla at the bottom of the hair follicle (fig.
8, c) ; and as this narrows into the " shaft " of the hair, a
difference shows itself between the cortical or outer layer, and
the medullary or pith-like substance of the interior. The
former, which is continuous with the outer layers of the epi-
dermis, is composed of flattened scales, arranged in an imbri-
cated (tile-like) manner, so that the surface of the hair is
usually marked by transverse jagged lines ; the latter consists
of cells which frequently retain their spheroidal form, hke the
inner layers of the epidermis ; but in the human hair these
cells are elongated into fibres. It is very seldom that there is
any canal in the interior of the Hair, although irregular spaces
are not unfrequently left by the drying-up of the fluid con-
tents of the cells. The structure of Quills is essentially the
same as that of hairs on a large scale ; and we there see the
difference very distinctly marked between the cortical portion
which forms the "barrel" of the quill, and the medullary
portion which forms the white pith-like substance of the
stem of the feather. The Scales, where they are really epi-
dermic appendages, as is the case in serpents and lizards, are
formed upon the same pattern ; and we liave a good example
of tJie detachment of the entire epidermis at once (reminding
us of the casting of the shell of the crab and lobster) in tlie
" sloughing " of the snake.

39. The Mucous Membranes form a sort of internal skin,
lining those cavities of the body which open on its surface ;
mid the elements of which they are composed are essentially
the same, though combined and arranged in a different
manner, in accordance with their difference of function. The
principal part of the thickness of every ordinar}^ mucous
membrane is made up, as in the skin, by the consolidation of
areolar tissue, the fibres of which arc continuous with those

MUCOUS MEMBRANES : EPITHELIUM. 61

of the ordinary areolar tissue on wliicli the membrane rests ;
this layer is copiously furnished with blood-vessels, but it is
seldom supplied with many nerves. Thus the mucous mem-
brane lining the stomach possesses in health so little sensi-
bility, that we are not aware of the contact of the substances
taken in as food, unless they are of an acrid character, or of
a temperature very dilferent from . that of the body ; and
though the mucous membrane linmg the air-passages is very
susceptible of certain kinds of irritation, jet it has but little
ordinary sensibility in the state of health, except near the
entrance to the windpipe. The large supply of blood which
these membranes receive, has reference to their active j)artici-
pation in the functions of secretion and absorption. One
secretion is common to all, that of the mucus by wliicli they
are covered ; tliis serves to j^rotect them from the irritation
that would otherwise be produced by the contact of solid or
liquid substances, or even of aii', with thek free surfaces ;
and we see the results of its deficiency, in the inflammation
which attacks the membrane, sometunes j^roceeding to its
entire destruction, when from any cause the secretion is
checked, as it sometimes is by injuries of the nerves sup-
plying the part.

40. In every mucous membrane, as in the skin, the fibrous
texture is bounded on the free surface by basement-mem-
brane, beyond which no blood-vessels pass. And the surface
of the basement-membrane is covered by cells, arranged either
in a single layer or in multiple layers, conscituting the
Epithelium. This, although answering to the Epidermis in
structure and position, has a very different character ; for its
cells neither dry up nor become horny ; nor do they adliere
in such a manner as to form a contmuous membrane, except
in the interior of the mouth and 02Sophagus (gullet), where
the epithehum is endowed mth somewhat of the firmness
of cuticle, in order to resist the abrading contact of hard
substances. The epithelium cells of mucous membranes are
commonly somewhat flattened ; but in some situations, as on
the villi of the intestinal canal (fig. 9, d), they have more of
a cylindrical, or rather conical shape, their smaller extremities
being in contact with the basement-membrane. The epi-
thelial cells are frequently cast off, like the epidermic, espe-
cially from the parts that are most concerned in secretion ;

E 2

52

MUCOUS MEMBRANES

-EPITnELIUM.

and tlicy arc as continually replaced by newly-formed cells,
■which are produced on the surface of the basement-mem-
brane, at tlie expense of the fluid that transudes through
it from the blood-vessels copiously distributed to its under
surface.

41. Mucous membrane may either exist in the condition of
a simple expanded surface, or may have a much more complex
arrangement, by which its surface is greatly increased. The
simple mucous membrane, such as that which lines the nose
and air-passages, is found, for the most part, where no ab-
sorj)tion has to be performed, and where only a moderate
amount of secretion is necessary. But where it is to absorb
as well as to secrete, it is usually involuted or folded upon
itself, in such a manner as to form a series of little projec-
tions, and also a number of minute pits (fig. 0). These pro-

Fig. 9. — DlAGKAM UEPUF.SKNTING THE MuCOUS MeMBRANE OF THE

Intestinal Canal.

a a, absorbent vessels ; b b, basement membrane ; c c, epithelium-cells of level
surface of membrane ; d d, cylindrical epithelium-cells of villus ; e e, secreting
cells of follicle.

jections sometimes have the form of long folds ; in other
instances they are narrow filaments, crowded together so as
almost to resemble the pile of velvet. In either case, the
absorbent surface is vastly increased ; but chiclly so by these
filaments, which are termed mlli, and act as so many little
rootlets. On the other hand, it is in the pits or follicles, that
the production of the fluid which is to be separated or
secreted from the blood, chiefly takes place. — 'Not only are
the flat expanded surfaces of the mucous membrane covered
with epithelium cells, but the villi also arc sheathed by

them ; and the

secreting follicles

are lined by the same.

STRUCTURE OF GLANDS. SEROUS MEMBRANES. 53

The cells covering the villi (fig. 9, d) perform the important
function of selecting and absorbing certain nutritious ele-
ments of the food, which they communicate to the absorbent
vessels in the interior of the villi. On the otlier hand, the
epithelium-cells of the follicles (e) seem to be the real agents
in the secreting process ; drawing from the blood, as materials
for their own growth, certain elements contained in it ; and
falling oflj, when mature, so as to discharge these substances
as the product of secretion, giving place to a fresh crop or
generation of cells, which go through a series of changes
precisely similar to the preceding.

42, Now these follicles arc the simplest types or examples
of all the Glandular structures, by which certain products arc
separated from the blood, some to be cast forth from the body
as unfit to be retained in it, and some to answer particular
purposes in the system. In all of them the structure ulti-
mately consists of such follicles, sometimes swollen into
rounded vesicles, and sometimes extended into 'long and
narrow tubes. Each follicle, vesicle, or tube, is composed of
a layer of basement-membrane, lined with epithelium-cells,
and surrounded on the outside with miimtely distributed
blood-vessels ; and it seems to be by the peculiar powers of
these cells, that the products of the secreting action, whether
bde, saliva, fatty matter, or gastric fluid, are formed (see
Chap. VII.). — Hence we see that the act of Secretion is, in
animals as in plants, really performed by cells. It is neces-
sary to bear in mind, however, that a simple transudation of
the watery parts of the blood may take place without any
proper secreting action, in the dead as in the living body ; it
is in this manner that the serous fluid of areolar tissue and
serous membrane is poured out, and that the watery portion
of the urine is separated.

43. The Serous Membra7ies which line the closed cavities of
the body, though composed of the same elements as the skin
and mucous membranes, hfive a much simpler structure, and
can scarcely be said to minister directly to any important
vital function. The tissue of which Serous membrane is
principally composed, scarcely differs, except in its greater
density, from the laxer areolar tissue whereby the membrane
is attached to the walls which it covers like i^laster ; it is but
sparingly supplied either with blood-vessels or absorbents; and

54

ARRANGEMENT OF SEROUS MEMBRANES.

it contains very few nerves. The smooth surface of the mem-
brane forms one unbroken plane, being neither raised into
villi, nor depressed into follicles ; and its basement-membrane
is covered with a single layer of flat epithehum-cells, which
are closely applied to it and to each other,
like the pieces of a pavement (fig. 10). It
is with such a membrane that every one of
those great cavities is lined, which contains
important viscera ; and it is also continued
on to the outer surface of these viscera,
so as to afford them an external coating*
over every part save that by which they
are attached. Thus the heart is suspended
Pavemen?' ^^epxthe- freely, by the large vessels proceeding from
LiuM Cells OF Serous its summit, witliin a bag or sac of fibrous
Membrane. membrane peculiar to itself, which is termed

the pericardium. The cavity of this bag is completely Imed
by the serous membrane (fig. 11, p' ), which closely embraces

the vessels, and which then
bends down over the surface
of the heart, so as to enclose
it in the envelope p. Hence
it will be seen that tliis
membrane, whilst including
the heart, and allowing it to

?\

"^m-''

I \ communicate with its vessels,
lf/^,,J"""^P forms a completely shut sac;
and it may be likened to a
/ common double cotton or

woollen night-cap, which has
a similar cavity between its

Fig.ll.-DlAGRAMOFTHEPERICARmUM. ^WO ^CrS, tllO hCad bcing

o a, auricles; t; «, ventricles ;/;, pulmonary really On the OUtside of
artery, c, aorta; py. pericardium. ^j^-^^ ^j^-j^^ SCemiug tO be

within the envelope. The two layers of the pericardium,
though separated in the diagram for the sake of distinctness,
are really in mutual contact, save when separated by the in-
terposition of fluid poured out in disease. Each of the lungs,
in like manner, is suspended in a closed sac of its own, termed
the pleura; and the surface of the lung is covered by a serous
membrane, which is reflected over the wall of the pleural cavity.

SEROUS AND SYNOVIAL MEMBRANES.

55

A similar arrangement exists in the great cavity of the ab-
domen ; hut the number and the complex relations of the
viscera which this contains, give to the disposition of its
serous membrane, termed the peritoneu7n, a peculiar complica-
tion. The cavity of the skull also is lined by a serous mem-
brane, termed the arachnoid, and this is prolonged over the
surface of the brain, and enters its lateral ventricles (§ 458).
The chief purpose of these membranes appears to be to faci-
litate the movements of the included organs, by forming
smooth surfaces which shall freely glide over each other ; this
, is evidently of great importance, where such constantly-
moving organs as the heart and lungs are concerned. Their
surfaces are kept constantly moist with a serous fluid which
exudes from the blood ; but in the state of health this fluid
does not accumulate in their cavities, being al^sorbed as fast
as it is poured out. Various forms of dropsy, however, —
such as "water on the brain," "water on the chest," and
" ascites," or dropsy of the abdomen — are the result of the
increased outpouring of fluid into the serous cavities of the
arachnoid, the pericardium, the pleura, and
the peritonemn respectively.

44. ISTearly allied to the Serous mem-
branes are the Synovial, which form closed
sacs in the interior of joints, covering the \
ends of the cartilages, and then hning the t\
fibrous capsule which passes from one bone '
to the other. The mode of their arrange-
ment will be understood from the accom-
panying diagram ; in which a a represent
the extremities of the two bones which
are jointed together, h b the layers of car-
tilage with which they are severally covered,
and the dotted hne c c the synovial mem-
brane, which is seen to form the sac or
bag cf c', whilst at the points c c c c it is
reflected upon the cartilages of the joints.
In point of fact, however, the Synovial
membrane is not ordinarily traceable as a
distinct layer over the surface of these
cartilages, but seems to have become incorporated with them ;
for though in the embryo its presence may be distinctly proved

Diagram OF THE STRUC-
TURE OF A Joint.

a a, extremities of the
bones, covered with
cartilage ; b b, layer of
cartilage closely co-
vered with synovial
membrane : c c' c, re-
flected layer of syno-
vial membrane form-
ing synovial capsule.

5Q

SYNOVIAL MEMBRANES. — CILIATED EPITHELIUM*.

hj the continuity of its blood-vessels over tlie entire car-
tilage, yet these are found to retreat gradually as the joint
is brought into use, until at last they only form a circle round
the border of the cartilage. Some of the Synovial mem-
branes, as that of the knee-joint, are furnished with little
fringe -like projections, somewhat resembling the villi of
mucous membranes (§ 41) ; these are extremely vascular,
and are furnished with an epithelium which very readily
falls off; and there is a strong probability that they are
concerned in the secretion of the synovial fluid, which is
much denser than the ordinary serous transudation, having
from 6 to 8 per cent, of additional albumen, and presenting a
glairy appearance like that of white of egg. It is interesting
to see that the same purpose may thus be served by the
extension of the membrane in either direction, either out-
wards into a villous filament, or imvards into a follicle ; the
function being determined in each case rather by the
attributes of the cells, and by the
supply of blood, than by the form
which the secreting surface may
happen to present.

45. The cells of Epithelium,
whether flattened or cylindrical,
are observed to be furnished in
particular situations with a fringe
of delicate filaments, which are
termed cilia. These, although of
extreme minuteness, are organs of
great importance in the animal
economy, on account of the extra-
ordinary motor powers Avith which
they are endowed. The form of
the cilia is usually a little flattened,
and tapering gradually from the
base to the point. Their size is
extremely variable ; the largest that

and in transverse section at B; j ^ obscrVcd being about

their cilia are seen at b, their i • i i

nuclei at c; at a is shown one of l-500th of an incli ]n length, and

these cells unusually elongated, the Smallest 1-1 3,000th. "When in

motion, each filament appears to bend from its root to its
point, returning again to its original state, like the stalks of

Fig. 13. — Ciliated Epithelium
Cells; as seen sideways at A,

CILIARY MOVEMENT. 57

corn when depressed by the wind; and if a number be
affected in succession with this motion, the appearance of
progressive waves following one another is produced, as when
a corn-field is agitated by repeated gusts. When the ciliary
motion is taking place in full activity, however, nothing can
be distinguished save the whirl of particles in the surround-
ing liquid; and it is only when the rate of movement
slackens, that the shape and size of the individual filaments,
and the manner in which their stroke is made, can be made
out. The motion of the cilia is not only quite independent
(in all the liigher animals at least) of the will of the animal,
but is also independent even of the life of the rest of the
body ; being seen to continue after the death of the animal,
and even going on with j)erfect regularity in parts separated
from the body. Thus, isolated epithelium-cells have been
seen to swim about actively in water, by the agency of their
cilia, for some hours after their detachment from the mucous
membrane of the nose ; and the regular movement of cilia
has been noticed fifteen days after death, in the body of a
tortoise in which putrefaction was already far advanced. In
the gills of the Eiver Mussel, which are amongst the best
objects for the study of this most curious phenomenon, the
movement endures with similar pertinacity. — The purpose of
this remarkable agency is obviously to propel fluids over the
surfaces which are furnished with cilia. We find it taking
the most important share in the functions of life among the
lowest classes of animals. Thus, in Animalcules of various
kinds, the cilia are the sole instruments, not merely for the
production of those currents in the water which may bring
them the requisite sujDplies of air and food, but also for pro-
pelling their own bodies through the hc|uid. In most
Zoophytes, and in the inferior Mollusks, which pass their
lives with little or no change from one spot to another, the
motion of the ciha lining the alimentary canal and clothing
the gills (where such have a special existence), draws into the
mouth the minute currents wliich serve as food, and also
renews the layer of water in contact with the respiratory
surface. The gills of Fishes are not furnished with ciha,
another provision being .made by muscular action for conti-
nually driving fresh streams of water over them ; but the
motion may be very well seen upon the gills of the young

58 CILIA. — FAT CELLS.

Tadpole or larva of the Water N"ewt, wliicli liang down as fringes
on either side of the neck. In the higher air-breathing
animals, the function of the cilia is much more limited. They
clothe the mucous membrane which hnes the air-passages ;
and their function appears to be, in that and other cases, to
prevent the accumulation of the secretion with which the
membrane is kept moist, by keeping up a continual onward
movement of it towards the outlet of the passage. In some
other cases, however, we find the ducts of secreting organs
furnished with cilia, whose action is obviously to assist in
carrying the products of secretion towards their outlet.

46. Passing on, now, to those tissues of animals of which
cells constitute the permanent components, instead of being
successively thrown .off and replaced as they are in the
Epidermis and Epithelium, we may first notice the Adipose
tissue, or Fat, in which the oily and fatty matters of the body
are for the most part contained. This tissue is composed of
minute cells or vesicles (fig. 14), having no communication
with each other, but lying side by side in the meshes of the

areolar tissue, which serves
to hold them together, and
through which also the blood-
vessels find their way to
them. Erom the fluid in these
vessels, the fatty matter is
separated in the first place by
the secreting action of the
ff? ■ >jf cells j and it is prevented

^ 14.-F.T CELLS, HIGHLY ^om malvlug its way through
MAGNIFIED. tlie vcry thin walls of the

cells, by the simple expedient of keeping these constantly
moist with a watery fluid, the blood. ^ The blood-vessels
have also the power of taking back the fatty matter again
into the circulation, when it is wanted for other purposes in
the economy. These deposits of fatty matter answer several
important objects. They often assist the action of moving
parts, by giving them support without interfering with their
free motions ; thus the eye rests on a sort of cushion of fat,
on which it can freely turn, and through which the muscles

^ Thus oil will nob pass into blotting-paper, if this have been
previously moistened with water.

FAT. CARTILAGE. 59

pass that keep it in play. It also affords, by its power of re-
sisting the passage of heat, a warm covering to animals that
are destined to live in cold climates ; and it is in these that
we find it accumulated' to the largest amount. Further,
being deposited when nourishment is abundant, it serves as a
store of combustive material, which may be taken back into
the system, and made use of in time of need. The causes
which peculiarly contribute to the production of fat, will be
considered hereafter (§ 1 G2).

47. Another tissue of which cells form the principal part,
is that termed Cartilage or gristle. Its simplest state is that
of a mass of firm substance, composed of
chondiin (§20), through which are scat-
tered a number of cells, at a greater or less
distance from one another. In the sunple
cellular cartilages, such as those wliich
cover the ends of' the bones where they
glide over one another so as to form Ni^>^-^

moveable joints, no trace of structure can
be seen in the intervening substance. Fig. i5, — Section op
But in cartilages which have to resist not ^^ . cartilage,

•^ - . . . Showing its cells imbea-

oniy pressure but also extension or strain, ded in intercellular sub-
we find the space between the cells partly stance.
occupied by fibres, Avhich resemble those of ligaments ; and
such are tevmed Jlbro-cartilages. . They are found in Man be-
tween the vertebrae of which the spinal column is made up
(§ 71); and also uniting the bones of the pelvis (§ 645).
Sometimes, where elasticity is required, the fibres are those
of the yellow fibrous tissue (§ 23) ; this is the case with the
cartilage which forms the external ear. Cartilage is not
penetrated by blood-vessels, at least in its natural state. The
blood is brought to its surface by a set of vessels which bulge
out into dilatations or swellings upon it, so that a large quan-
tity of fluid comes into the immediate neighbourhood of the
cartilage, being only separated from it by the thin walls of
the vessels ; and it appears that this fluid, or so much of
it as is required, is absorbed by the nearest cells, and trans-
mitted by them to the cells in the interior, so that the whole
substance is nourished. This is precisely the mode in which
the interior of the large sea-weeds (whose tissue consists of
cells imbedded in a gelatmous substance, and therefore bears

GO CARTILAGE. BONE.

a close resemblance to animal cartilage) obtains its nourish-,
ment from the surrounding fluid.

48. The permanent Cartilages seem to undergo very little
change from time to time. Their wear is slow; and, being
purely mechanical, it is confined to the surface. It is replaced
by the materials absorbed from the blood, which are employed
in the development of new cells, — sometimes within the old
ones, sometunes in the space between them. When a portion
of cartilage has been destroyed, however, by disease or injury,
it is not renewed by true cartilaginous structure, but by what
seems a condensed areolar tissue. Although cartilage does
not usually contain vessels, yet these may be rapidly deve-
loped in its substance, by a process which will be described
hereafter (§ 393), when it becomes inflamed. This may be
often seen to take place. The front of the eye is formed by
a transparent lamina of a substance somewhat resembUng
cartilage, which bulges like a watch-glass : this, which is
termed the cornea (§ 533), is properly nourished only by
vessels that bring blood to its edge, where it is connected
with the tough membrane that forms the white of the eye.
But when the cornea becomes inflamed, minute vessels may
be seen to spread over it, proceeding from its circular edge
towards its centre ; and at last some of these often become of
considerable size. Under proper treatment, however, these
vessels gradually shrink and disappear ; and the cornea
becomes nearly as transparent as before.

49. Many parts exist in the state of Cartilage in the young
animal, which are afterwards to become Bone; and it has
been commonly believed that all bone has its origin in a
cartilaginous structure. This, however, is not the fact, as
will be presently shown. Before attempting to explain the
formation of Bone, it will be desirable to describe its
structure. When we cut through a fully formed bone, such
as that of the thigh, we find that the shaft or elongated
portion is a hollow c^dinder ; of which the walls are formed
by what appears to be solid bone ; whilst the interior is filled,
in the living state, by an oily substance laid up in cells, and
termed marrow. Towards the extremities, however, the struc-
ture of bone is very different. The outside wall becomes
thinner ; and the interior, instead of forming one large cavity,
is divided into a vast number of small chambers, like those

STRUCTURE OP BONE. 61

of areolar tissue, by thin bony partitions, whicli cross each
other in every direction, forming what is called the " cancel-
lated " structure. These chambers or cancelli are filled with
marrow, like the central cavity, with which they communi-
cate. In the flat bones, moreover, — such as those of the
head — we find that the two surfaces are composed of dense
plates of bone, Hke that which forms the shaft of the long
bones ; but that between them there is a layer of cancellated
structure, filled in like manner with marrow. But when we
examine with the microscope a thin section of even the
densest bony matter, we find it traversed by a network of
minute canals, continuous with the central cavity. These
canals usually run, in the shafts

of long bones, in the direction *'''- —^

of their length ; and are con- i f

nected, every here and there, by j i liiiiF r^

cross branches (fig. 16). They i. ,|i|||

are termed the Haversian canals, \ : ' "!

after the name of their disco- i^ : j

verer, Havers. — The hning mem- »'

brane of the large central cavity ■; .

is copiously supplied with blood- i jljljll

vessels; and this sends off pro- 5^/,. . : . , ., .lilllll

longations into the cancelli at ^

the extremities of the bone, and ^^^- ^6.— diagram reprksenting

. , ,, T-T • 1 rm ^"^ STRUCTURE OF A PORTION OF

into the Haversian canals, ihus the shaft of a long bone.
blood is conveyed into the in- %L''3vie''ecUo";%%yc/ruriace

terior of the bone : but no vessels seen m longitudinal section ; i, Ha-

1, 1111.1-i-j. veisian systems cut across, each

can be traced absolutely into its having an Haversian canal in its

texture, so that all the spaces f^ntre; 9,, Haversian systems cut

,.,'.,. ., __ -'• . lon^tudinally ; /, lamellae near the

which lie between the Haversian surface of bone, destitute of Haver-
canals are as destitute of vessels as ''^" systems.
is healthy cartilage. These spaces are provided with nutriment
by the following very remarkable arrangement.

50. When we cut across the shaft of a long bone, and
examine a thin section with a microscope, we of course see
the open extremities of the Haversian canals (fig. 17, a);
just as we 'see the cut ends of the ducts and vessels of
wood, when we make a transverse section of a stem.
Around each of these apertures, the bony matter is arranged
in concentric rings, which are marked out and divided

62 STRUCTURE OF BONE.

by circles of little dark spots; and when these spots are
examined with a higher magnifying power, it is seen that
they ai-e small flattened cavities, from which proceed a number
of extremely minute tubules (A). These tubules pass out

Fig. 17.— Transverse Section of Bone.

Showing the concentric rings round a a, the Haversian canals. At A are seen
some of tlie cavities with^their radiating tubes, more highly magnified.

from the two flat sides of each cavity; one set passes invv'ards,
towards the centre of the rmg, and the other outwards, to-
wards the rmg that next surrounds them. These minute
tubuh, which are far smaller than the smallest blood-vessels,
may thus be traced into every part of the substance of the
bone ; and those proceeding from dilTcrent rings are so con-
nected with each other, that a communication is- established
between the innermost and the outermost circles. The tubuli
which open upon the sides of the Haversian canals, are thus
enabled to take up the nourishment with which they arc

COMPOSITION OF BONE. 63

supplied by the blood-vessels, and to transmit it to tlie
outer circles, or those furthest removed from those vessels ;
and in this manner, a much more active nutrition takes place
in bone than that vi^hich is performed in cartilage. It has
been proved by various experiments, that the substance of
bone is undergoing continual change ; and it is owing to the
comparative activity of its nutritive processes, that bone is so
readily and perfectly repaired, when it has been broken by
violence or has been injured by disease.

51. But the peculiarity of Bone consists, not so much in
this remarkable arrangement of its organic structure, as in its
solidity and firmness. This is given to it by the union of
a large quantity of mineral matter with the organic substance
of its tissue. The mineral matter of bones consists almost
entirely of two compounds of Lime ; the carbonate, with
which we are familiar in the form of limestone and chalk ;
and the phosphate, which is seldom found as an ingredient of
rocks or soils, except where it has been derived from animal
remains. The latter greatly predominates, at least in the
bones of the higher animals. We may easily separate the
animal and the mineral portions of the bony tissue. If we
soak a small bone for some time in muriatic acid much
diluted with water, the compounds of lime are entirel}'-
removed from it, and the organic substance remains ;
the latter is now quite flexible, and almost trans2:)arent, so
that the distribution of its vessels (if they have been pre-
viously injected with colouring matter) may be distinctly
seen. On the other hand, if we subject a bone to strong-
heat, the animal portion will be burnt out, and the earthy
matter will remain. The form of the bone will be still
retained ; but the cohesion between the earthy particles is so
slight, that the least touch will break them asunder. Thus
we see that the hardness of bone, or power of* resisting pres-
sure, is given by the earthy matter; whilst its tenacity, or
power of holding together, depends upon the animal portion.
Although the animal substance which remains after the solu-
tion of the mineral matter, has been commonly described as
Cartilage, yet it is not so in reahty ; for it consists not of
chondrin, but of gelatin ; and instead of being made up of an
aggregation of cells united by an intervening substance, it may
be torn into layers of an indistinctly-fibrous matting. In fact,

64 COMPOSITION AND DEVELOPMENT OP BONE.

it corresponds closely with the wliite fibrous tissue (§ 23), both
in structure and composition ; and so far from this view of its
nature being inconsistent with the history of the formation of
bone, it will be found to be in entire harmony with it. The
proportion which the mineral bears to the annual substance of
bone is very constant, when the 'proper 'osseous tissue alone is
taken into account ; being almost exactly two of the former to
one of the latter, or 66f per cent, to 33^ per cent. But when
the composition of entire hones, including the contents of the
Haversian canals and cancelli, is compared, the proj)ortion of
mineral to animal matter is found to vary greatly in different
classes of anunals, in the same animal at different ages, and
even in different bones of the same individual ; the mineral
matter predominating in bones of a comj^act texture, and the
animal in those Avhose substance is more sjoongy.

52. In the first development of the embryo, a sort of mould
of cartilage is laid down for the greater part of the bones ;
though, in the case of the fiat bones, this mould is generally
limited to the central portion, the place of their marginal part
being occupied by a fibrous membrane only. The process of
ossification, or bone-formation, commences with the deposit
of calcareous matter in the intercellular substance of the
cartilage, so as to form a sort of network, in the interspaces
of which are seen the remains of the cartilage-cells. The
tissue thus formed can scarcely be considered as true bone,
for it contains neither lacunae nor cancdicidi. Before
long, however, it undergoes very important changes ; for
many of the partitions are removed, so that the minute
chambers which they separated coalesce into larger ones ; and
thus are formed the cancelli of the spongy substance, and the
Haversian canals of the more compact. These are at first
much larger than they are subsequently to become ; for they
are gradually narrowed by deposits of true bony tissue,
which successively take place upon their interior walls, at the
expense of the materials supphcd by the blood brought
thither by their contained vessels ; and it is by this forma-
tion of concentric layers around the cavities of the Haversian
canals, that the appearance of concentric rings is produced,
wliich we have just seen to be presented by transverse sec-
tions of long bones. In old bones the Haversian canals are
80 nearly filled by these deposits, that there is barely room

DEVELOPMENT OF BONE : OSSIFICATION. G5

for the blood-vessels to pass along them. And it is through
their complete blocking up, by a continuance of the same
gi'owth, that the supply of blood is cut oif from the interior
of the bone which forms the antlers of the deer, so that they
die and fall off ; their shedding and rcne^ral being an annual
process.^ — Whilst the formation of the Haversian canals and
cancelli is being effected by the partial removal of the first
formed partitions, a complete cavity is formed in the centre of
the shaft of every lo7ig bone (at least in Mammals and Birds),
by the entire removal of the solid tissue. This cavity is at
first not much larger than one of the Haversian canals ; but
as the bone grows in diameter by additions to the exterior of
its shaft, so is the cavity in its interior augmented by the
removal (by absorption) of the first-formed bone ; and this
double process continues until the bone has attained its full
diameter. The formation of new bone on the exterior of
the shaft seems to be the result of the consolidation of the
fibrous tissue of the periostetn^i (or membrane covering the
bone) by calcareous deposit ; the lacunse being probably the
cavities of cells which were entangled in the fibres, and the
canalicuii being outgrowths from these ; and new fibrous
tissue being formed on the outside of the periosteum, to replace
that which has been taken into the bone. Thus it comes to
pass, that after a time none of the bone first formed in its
cartilaginous mould any longer remains, the whole of it
having been removed by absorption ; since the central cavity
of the perfect bone is much larger than the entire carti-
laginous shaft in which it originated. And thus it also
comes to pass, that (as gelatin is the basis of fibrous tissue)
bones yield gelatin, not chondrin, upon being long boiled, —
The increase of the shaft in length, however, is the result of
a different process. In all bones of any considerable dimen-
sions, the process of ossification commences in more than one
l^oint at a time. In the long bones, there are usually three
such points; one for the shaft, and the others for the two

^ It is commonly stated that the death of the antlers is due to the
formation of a bony I'ing at their base, which cuts off the supply ot
blood from the " velvet" vv'hich covers them ; but though this may con-
tribute to produce the effect, it is by no means the sole cause, as the
interior of the antlers is supplied with blood from the vessels of the
bone from which they sprout, and not from those of the "velvet"
only.

QQ DEVELOPMENT OP BONE : OSSIFICATION.

extremities. Long after the ossification of the shaft and of
the extremities has been completed, these parts remain sepa-
rated from each other by tlie interposition of a thin layer of
unconsolidated cartilage ; so that, although the bone appears
firm and complete, its three portions fall apart, if it be
macerated sufficiently long in water for the cartilage to
decay. Now it is by the progressive consolidation of the
cartilage at these two junctions, and by the continual forma-
tion of new cartilage as the old is taken into the bone, that
the length of the shaft continues to increase up to adult
age ; and then, its full size having been attained, the whole
thickness of the intervening layer of cartilage is replaced by
bone, so that the shaft and extremities become firmly con-
solidated. — The general history of the formation of the Jiat
bones is nearly the same. In these, when they are large, or
have projecting out-growths, there are several centres of ossi-
fication ; and although the first ossification takes place in the
substance of cartilage, yet the subsequent growth seems to
be effected mainly by the consolidation of fibrous mem-
brane.

53. The foregoing description applies chiefly to those
higher and more complete forms of Bone, which are found in
Birds and ]\Iammals. In lieptiles and Fishes, the process of
ossification is stopped short, as it were, at an early period ;
and thus the texture of their bones resembles that which we
find the skeleton to present in the earlier life of the higher
animals. — The long bones of Eeptiles (with one remarkable
exce})tion in the Fterodactylus, § 6G9, which is adapted to
the life of a Bird) have no one central cavity, but are pene-
trated by numerous large Haversian canals, like those of very
young bone ; and various pieces remain separate in them
throughout life, which, originating in distinct centres of ossi-
fication, subsequently coalesce in Birds and Mammals. Tliis
permanent separation is still more remarkable in the bones
of Fishes ; and it is consequently in them that we can best
.study the real composition of the skeleton, — every piece
which originates in a distinct centre of ossification, being,
in the eye of the philosophical anatomist, a separate bone.
Further, there is a large group of Fishes in which the
skeleton retains the cartilaginous character through life ; a
certain quantity of mineral matter being deposited in the

BONES OF FISHES : TEETH.

67

cartilage, but its conversion into true bony structure never
taking place. In a few, not even a firm cartilage is produced;
and all the trace of a skeleton is a cylinder formed of hex-
agonal cells, resembling those of the pith of plants, which
takes the place that is generally occupied by the " boclies " of
the vertebrae (§ 71). Such a cylinder, "vvhich is termed the
chorda dorsalis, precedes the formation of the vertebral
column in other vertebratecl animals (§757). In the curious
Arnphioxzis (ZooL. § 642), even this is wanting ; and the
only rudiment of the bony skeleton is to be found in the
fibrous sheath that surrounds the nervous centres, and sends
off prolongations between the successive transverse bands of
muscles, which are attached to these, as they are in other fishes
to the ribs and the spines of the vertebrge.

54. In connexion with the structure of Bone, it will be
convenient to describe that of Teeth, although the general
description of the form and development of these organs will
be more appropriately given in connexion with the account
of their instrumental uses (§§ 181 — 183). The principal part
of the substance of all teetli is made up of a solid tissue,
which has been appropriately called Dentine. Of tliis sub-
stance, one variety, which is peculiarly close in texture, and
susceptible of a high polish, is familiarly known as 'diiori/.
The more perfect forms of dentine, such as present them-
selves in Man and the Mammalia generally, consist of a- hard
transparent substance formed by the union of animal matter
and calcareous salts (chiefly phos-
phate of lime), in the proportion
of about 28 of the former to 72 of
the latter ; the mineral matter thus
bearing a somewhat larger ratio
to the organic, than it' does. in
bone. This dentinal substance is
traversed by minute tubuli of
about 1-1 0,000th of an inch in
diameter, Avliich appear as dark
Lines, generally very close to- Fig. is.

gether ; these pass in a radiating poetiok of df.ntink (highly magni-
manner from the central cavity fied), showing its tubular structure,
of the tooth, diverging from each other as they approach
its exterior; but when seen in only a small part of their

p 2

68 STRUCTURE OF TEETU.

course, they appear to be nearly parallel (fig. 18), tliougli
visually more or less wavy. They occasionally divide into
two branches, which continue to run, at a little distance from
one another, in the same parallel direction ; and they also
frequently give off small lateral branches, which again send
off smaller ones. In some animals the tubuli may be traced
at their extremities into minute cavities analogous to the
lacunas of bone ; and the lateral branchlets also occasionally
terminate in similar cavities. Thus the whole tooth may be
likened, in some degree, to a single Haversian system in
bone ; the central cavity, which is lined by a vascular mem-
brane, representing the Haversian canal, while the radiating
tubuli of the former correspond with the radiating canaliculi
of the latter ; the chief difference lying in the absence of
lacunae along the course of the radiating tubes. In a large
proportion of Fishes, however, there is no single central cavity,
but the whole tooth is traversed by a system of medullary
canals, not only resembling the Haversian, but actually con-
tinuous with those of the bone on which the tooth is im-
planted; and as each of these is the centre of a distinct
system of radiating tubuli, the resemblance of their dentine
to bone is very close. A somewhat similar condition of the
dentine (obviously a lower or less specialized form of this
substance) presents itself in certain Eeptiles and Mammals. —
In the Teeth of Man and most other Mammals, and in those
of many Reptiles and some Fishes, we find two other sub-

__„_ stances, one of them harder and

/~r^p |tr~7 " ''' !'a.^ ' the other softer than dentine.
^^~£^ 'ri ' ^'-^-^^ former, which is called

^P-^^^^ 'W] Ena7nel^ consists of long pris-
C- — --~-w '|il>s j matic cells, which pass from one

v~--~--J^--'^--jv4 surface to the other of the thin
\3J^pifr:£Ez£?|/|i' layer formed by this substance
w-^isf— Hs^/# jj I over the crown, or sometimes in
I C^g^?tS=f^f|| the interior of the tooth (§ 182).

I — ^Si^^&MiEi These prisms are usually hex-
Fig. 19- agonal in form, as is seen in

Portion OF Enamkl (highly niap;ni-|;j,^^j^S^ej,Sg gCCtion (fig. 19); and
ned), showing its component prisms. . . \ r> J } ^

their course is usually more or
less wavy. In teeth which have to sustain an extraordinary
amount of compression (as is especially the case with those of

TEETH. MUSCLE AND NERVE. 69

the Eodentia), the enamel-prisms cross and interlace with one
another, in such a manner as to prevent that separation
which would readily occur if the direction of all of them
were the same. Of all the tissues of the animal body, the
Enamel is the most remarkable for the predominance of
mineral ingredients ; these amount to no fewer tlian 98
parts in 100, leaving wdien removed only 2 per cent, of
organic matter. The softer component of Teeth, known as
the Cemcntum, or Crusta jietrosa, possesses the essential
characters of true bone ; but when only a thin layer of it is
present, we do not find it traversed by medullary canals, its
system of lacunse and canaliculi being then in relation to the
nearest vascular surface, — as is the case also with very thin
laminae of ordinary bone, such as we find in the scapula
(blade-bone) of a Mouse.

55. We come, lastly, to the two tissues whicli are of the
highest importance in the Animal fabric, and to which all the
rest are merely subsidiary ; namely, the Muscular and the
Nervous. It is through tlie instrumentality of these, that
all ^ the actions are performed which essentially constitute
Animal life ; for the nervous apparatus is the medium by
which the consciousness of the individual is affected by what
takes place around him, or within his own body, and by
whicli, in his turn, he originates movements in his body,
and through it in things external to it ; whilst the muscles
are, so to speak, the servants of the nerves, doing, with a
force of their o^\m, the work which the nerves direct. The
relation between the two may be likened to that of the rider
and his horse, or of the engine-driver and his locomotive ; for
the nerves can put forth no motor power by themselves ;
whilst, on the other hand, the muscles (with certain excep-
tions) remain inert except when stimulated to contract by the
agency of the nerves. The muscles use the tendons and the
framework of bones, joints, &c., for the mechanical appli-
cation of their power, as will be shown hereafter (Chap, xii.);
but these parts of the fabric have not the slightest power of
originating motion by themselves. Hence, all Animal Force
takes its rise in one or other of these two tissues ; and we
shall find that the special purpose of the whole apparatus of
Organic life, is, by providing materials for their nutrition and
renovation, to build them up in the first instance, and then

70 STRUCTURE OF MUSCLE.

h-) keep them in working order. For every development of
animal force involves a change of state of the Nervo-mus-
cular substance : a certain amount of it ceasing to exist as
living tissue, and passing into the condition of dead matter ;
and its elements resolving themselves, under the influence of
the free oxygen brought to them by the blood, into new combi-
nations, which are carried forth from the body as quickly as
possible. Consequently, if the ISTervo-muscular tissues be not
renewed as rapidly as they are used up, their powers must
speedily fail from the progressive loss of their substance. In
this particular they are on a difi'erent footing from the other
elementary parts of the organism ; for although each of these
seems to have a certain term of life, the length of which is
in some degree related inversely to its functional activity, —
those wliicli live the fastest having the. shortest individual
duration, and vice versd, — there are none which are called
upon to give forth their whole vital energy in one effort, and
which may thus have their existence as parts of the living
organism terminated at any moment by a demand for their
peculiar power.

56. Muscular Fibre presents itself under two forms, which
are ordinarily very distinct from each other ; although it is
prolmble that they may ultimately prove to be but modifi-
cations of one and the same. The first, which is known as
the striated fibre, is that of which all those muscles are com-
posed, which constitute what is commonly designated as
"flesh" or the "lean" of meat. If any "joint" of meat
be even cursorily examined, it will be seen that its whole
substance is made up of distinct masses, held loosely together
by areolar tissue ; and these masses, Avhicli are known as
" muscles," are easily isolated from each other by dissection.
Every such Muscle is formed by the union of a number of
bundles, having a generally parallel arrangement, which are
closely bound together by areolar tissue, and are themselves
composed of bundles still more minute, united in a similar
manner. These, again, may be separated in the same way ;
and at last Ave come to the primitive fibres of which this
tissue is composed. Each of these primitive fibres termi-
nates at either extremity in tendinous fibre, which unites
with other fibres to form the tendinous cords or bands, that
are attached to the points of the skeleton which the muscle

STRIATED MUSCULAR FIBRE.

has to bring together. The muscular fibre itself consists of
a delicate membranous tube, enclosing a great number of
Jlbrillce, or extremely minute fibrils, -which are not capable
of further division (lig. 20). The peculiar transverse marking

Fi"-. 20.— Striated Musculau Fibri: separ.atii;g into Fibrill;e.

or striation by which this form of muscular hbre is characterised,
is found, when the fibre is separated into its fibrilla), to be due
to the pecuhar markings which every fibril presents. These
markings, consisting of alternate light and dark spaces, give
to the fibril a beaded appearance ; but this is only an optical
deception, since its form is in reality cylindrical, or nearly
so. It is easy to see how the correspondence of the hght
and dark spaces respectively, throughout the whole bundle of
the Jihril, will give rise to the banded appearance which the
entire fibre presents. The form and diameter of the fibres
vary considerably, both in different tribes, and in different
parts of the same animal. In the higher classes, their form
usually approaches a cyhnder ; but the parts which press
against one another are somewhat flattened, so that it is more
or less prismatic. In Insects, on the other hand, the fibrillae
are arranged in flat bands, so that the fibre often consists of
but a single layer of them. The diameter of the fibres in Man
averages about l-400th of an inch, and does not differ very
widely in either direction ; in the cold-blooded Yertebrata,
however, the average size is greater, and the extremes are
also wider ; the diameter of the fibres varying in the Frog
from 1-lOOth to 1-lOOOth of an inch, and in the Skate from
l-65th to l-300th of an inch. The diameter of tho Jibrils is
nearly the same in all classes, seldom departing much from
1-1 0,000th of an inch ; and the average distance of the dark
strise from each other is nearly the same.

72

NON-STRIATED MUSCULAR FIBRE.

57. Ilic other form of Muscular Fibre, whicli, from the
absence of transverse striation, is distinguished as smooth or
non-striated, is found not in large masses, but in thin layers,
forming jiart of the wall of various hollow organs, such as the
stomach and intestinal canal, the bladder, the principal gland-
ducts, and the larger blood-vessels. In all these situations it
is so exclusively concerned in the performance of the vege-
tative or nutritive functions, and it is so entirely mthdrawn
from the influence of the will, that it has been frequently
designated as " the muscular fibre of organic life ;" the striated
fibre, of which the voluntary muscles are composed, being
distinguished as the "muscular fibre of animal life." Eut
these designations are not by any means consistent with the
facts of the case ; for in a large proportion of the ]\Iolluscous
classes, the muscles of animal life are composed of non-
striated fibre, whilst the heart of Man and of other Yerte-
brata, though a muscle of organic life, is made up of striated
fibre. In fact, the employment of the one or of the other
kind of fibre would seem to be chiefly determined by the
kind of contraction which is required from it (§ 59). The
non-striated fibres are arranged, like those of the other
muscles, in a parallel manner into bands or bundles; but
these bundles, instead of being them-
selves grouped into larger ones having
a like parallel arrangement, are gene-
rally interwoven into a kind of network,
having no fixed points of attachment.
The form of the indivicbial fibres is
much more variable than tliat of the
striated kind, being often very much
flattened out ; and hence their general
dimensions cannot well be estimated.
By macerating a portion of this kind
of tissue in dilute nitric acid, each fibre
may be resolve< ' into bundles of long
spindle-shaped bodies, which, contain-
ing elongated staff-shaped nuclei
(fig. 21), may be regarded as cells, al-
though it is diificult to distinguish their
walls from their contents, aiiis form of muscular tissue is
commonly mingled with a large quantity of the ordinary fibrous

Fi-. 21.
A, Portion of a band composed
of non-striated muscular
fibre, showing, a a, the
spindle-shaped cells, and,
b b, the elonfrated nuclei :
B, a single cell isolated, and
more highly magnified ; C. a
similar cdl treated with
acetic acid.

MUSCULAR CONTRACTION. 73

structure ; and we find it dispersed in small quantity through
the latter in the skin, to which (especially in jDarticular
regions) it gives a contractility that is manifested under the
influence of cold or of mental emotions, and thus produces
that general roughness and rigidity of the surface which is
known as cutis anserina, or "goose's skin."

5S. Under the influence of certain exciting causes, or
stimuli (Chap, xii.), striated muscular fibres suddenly and for-
cibly contract. Their two ends approach one another, and their
stria3 become closer ; but they bulge out in the middle to a
corresponding degree. This causes a like change in the bundles
which are made up of these fibres ; and thus the whole muscle,
when shortened by the drawing together of its two ends, is
greatly enlarged in diameter, especially towards its middle.
Of this any one may convince himself, by bending his fore-
arm upon the arm (as when the hand is brought to the
mouth), and feeling the fleshy mass upon the front of the
latter. The muscle, in fact, does not in the least degree
change its ow^n bulk in the act of contraction ; for its enlarge-
ment in diameter is exactly equivalent to the shortening of
the distance between its extremities. The contraction of a
muscular fibre is ordinarily followed, after a short interval,
by its relaxation ; of this we have a remarkable illustration in
the contractions excited by the electric stimulus. But relax-
ation of individual fibres is not incompatible Avitli the con-
tinuance of the state of contraction of the muscle as a wdiole.
For it appears that wdien an ordinary muscle is thrown into
contraction, all its fibres do not usually contract together, but
only a small part of them ; and that, as long as its contraction
is maintained so as to exert a constant force, a continual in-
terchange is taking place in the action of the fibres by which
this is kept up — those which have been shortened becoming
slack, ancl being replaced (as it were) by others, which pass
into the contracted state for a time, and then relax again,
being succeeded by another set. I^ow as the ends of those
fibres which are actually in a relaxed condition, are brought
near together by the contraction of the rest, the fibre is
thrown out of the straight line, and assumes a wavy or zigzag
form, which was formerly supposed to be the state of con-
traction, but is now known to be otherwise. This peculiar
arrangement gives place to the straight form, either when the

74 MUSCULAR COXTRACTIOX. NERVOUS TISSUE.

fibre passes into the state of contraction, or when, by the
relaxation of the whole muscle, its ends are separated again
to their full extent.

59. Kow the alternate contraction and relaxation, which
is thus made to produce a continued contraction in ordinary-
muscles, elsewhere occasions a different effect. Thus in the
heart, all the fibres of the ventricles seem to contract to-
gether and all to relax together, — those of the auricles contract-
ing whilst the others are relaxing, and vice versd; — and in this
way the alternate contractions and dilatations of that most
important organ are continually kept up. Again, in the muscular
coat of the intestinal canal, we observe the contraction of each
part to be almost immediately followed by its relaxation ; but
the peculiarity of its movement is, that the contraction is jDro-
pagated on (as it were) to the succeeding part, which in its turn
contracts and then relaxes, producing the same action in the
part that follows it, — and so on along the whole canal. This
peristaltic motion (§ 215), as it is called, is obviously adapted
to propel the contents of the intestinal tube from one ex-
tremity of it to the other ; just as the peculiar action of the
heart is adapted to receive and propel the blood alternately,
or as the mode of contraction of the ordinary muscles enables
them to keep up a continued strain for a great length of time.
It is much less rapid and energetic than the action of the
heart ; for it is the characteristic of the non-striated fibre, that
its contraction follows much less closely on the application of
the stimulus, and is much less rapidly succeeded by relaxa-
tion, than that of the striated fibre.

60. The Nervous tissue consists of two distinct structures,
of one of which the trunks of the nerves are entirely made
up, whilst the other enters largely into the composition of
the ganglia or centres of action (§ 61). The former, termed
the ivhiie or fibrous tissue, consists of straight fibres, lying
side by side, and bound together by areolar tissue into
bundles (fig. 22); these, again, are united with others into a
larger group ; and by the union of a considerable number of
such groups, the nervous trunks are formed, which are dis-
tributed through the body, especially to the skin and muscles.
Nervous Fibre, like muscular, presents itself in the higher
animals under two forms, of which one may be considered as
more completely developed than the other ; these are known

STRUCTURE OF TUBULAR NERVE-FIBRES.

U)

as the tubular and the geJafinous. The " tubular " fibres are so
named because each poss.' ^os a distinct tubular sheath of a
delicate structureless membrane (fig. 22, a), which encloses the
proper nerve-substance, and isolates it completely from the

Fig. 22.— Structure of Nerve-Tubes.
Tubular Nerve-fibres ; A, from a nerve-trunk; B, from the substance of the brain

blood-vessels and other surrounding structures ; this tube
does not either l^ranch or unite with others, and there is
reason to believe it to be continuous from the origin to the
termination of the nerve-trunk. Within the tube is a hollow
cylinder of a material known (after its discoverer) as the
" white substance of Schwann ;" and tliis encloses a sort of
central pith, which is transparent and semi-fluid in the living
state, but undergoes a kind of coagulation into a granular sub-
stance after death, and under the influence of chemical
re-agents. There is reason to believe that this central pith or
" axis-cylinder " is the essential component of the nervous
fibre, and that the hollow cylinder which surrounds it serves
only to isolate it more completely; for we not nnfrequently
see the former to be alone contmued, both the tubular sheath
and the white substance stopping short ; and this at either
extremity of the fibre, where it separates itself from those
with which it is bound up in the nerve-trunk. The proper
form of the fibre seems always to be truly cylindrical ; though

76 TUBULAR AND GELATINOUS NERVE-FIBRES.

it is very liable to be altered by manipulation, a small excess
of pressure in one part forcing the contents of the tube
towards some other where they are more free to distend it,
and thus producing a swelling. The greater delicacy of
the tubular sheath in the hbrous substance of the brain
and spinal cord, renders its fibres peculiarly susceptible of
this kind of alteration, so that they often present under the
microscope a somewhat beaded appearance (fig. 22, b) ; when
carefully examined, however, without any previous disturb-
ance, these fibres are found to be as cylindrical as those of
the iicrve-trunks. The diameter of the nerve-tubules is
usually between l-2000th and l-4000th of an inch ; but it
may be somewhat greater or considerably less than this
average. They are larger in the. nerve-trunks than they are
near their central termination in the brain ; and it is a remark-
able circumstance that the fibres of the nerves of " special
sense " are considerably smaller than the average in every
part of their course. — The " gelatinous" fibres cannot be shown
to consist of the same variety of parts as the preceding ; for
neither the tubular sheath nor the white substance of
Schwann can be distinguished in them. They are flattened,
soft, and apparently homogeneous, sometimes showing a dis-
position to split into very delicate fibrillae ; being of a
yellowish-grey colour, they are sometimes designated the
grey fibres. Their diameter averages between the 1 -4000th
and l-6000th of an inch. As these "gelatinous" fibres
form a considerable proportion of the trunks of the Sympa-
thetic system of nerves (§ 461), they have been supposed to
belong properly to it, and to minister exclusively to the
organic functions, like the non-striated muscular fibre (§ 57);
but-Uhere is no doubt that this is an incorrect notion, and
that even the fibres of the ordinary nerve-trunks ma}^ present
the " gelatinous " aspect, probably from incompleteness of
development.

Gl. In the central organs of the Nervous system — namely,
the brain and spinal cord of the Vertebrata, the ganglia or
knot-like swellings on the nervous cords which take their
place in the lower animals, and similar ganglia belonging to
the Sympathetic system — we find a form of nervous tissue
altogether distinct from the preceding ; which, from its con-
sisting ol' lavLC cells or vesicles, is generally known as the

VESICULAR OR GANGLIONIC NERVE-SUBSTANCE. 77

vesicular. Tliese nerve-vesicles, sometimes knov/n as gan-
giion-giobiiles, may be regarded as originally spherical, or
nearly so, in form (fig. 23, a); but they often present one or
more prolonged extensions ; and as these when single re-
semble tails, and when multiple are like the rays proceeding
from a star, the cells are said in the first case to be "caudate,"

Fig. 23. — Vesicular Nerve-subst.vnce.

A, combination of Ganglion-cells (of which one is shown separately at a, more highly
magnified), and Nerve-fibres in the grey substance of the brain, which is also
traversed by a capillary vessel, b; B B, Ganglionic cells with caudate pro-
longations.

and in the second to be stellate (b). These prolongations
have been traced into continuity, in some instances, with the
axis-cylinders of nerve-tubes, whilst in other cases they seem
to unite with those proceeding from other vesicles. It is not by
any means certain, however, that the nerve-tubes thus connect
themselves with the nerve-vesicles in all instances ; since it
frequently appears as if the former passed in among the
latter, without coming into direct continuity with them.
Sometimes a ganglion-cell seems to lie in the course of a
tubular fibre, which enlarges to envelope it, and then con-
tracts again to its former dimensions. There can be no
reasonable doubt, however, that in some way or other the
nerve-fibres and the nerve-vesicles come into some kind of
communication in the ganglionic centres. The vesicles arc

STRUCTURt: OF GANGLIA. — NERVOUS ACTION.

filled ^vith a finely-gi^anular substance, wliicli extends into
their prolongations ; and in the warm-blooded Yertebrata they
contain pigment-granules, which give them
a reddish or yellowish-brown colour ; so
that the aggregations of vesicular substance
which we hnd in the larger nervons centres,
are distinguishable by their greyish hue.
This "grey matter,"as it is frequently called,
is disposed on the surface of the brain;
but it occupies the interior of the spinal
cord, and holds the same position in the
smaller ganglionic centres (fig. 24). It
is not only, however, in the central organs
that nerve-vesicles are found ; for they
present themselves also in certain situa-
Fig. 24.— Thin slice of tions at the other extremities of the nerve-
o?^THF symi^\tuftic f^^^'^^' Tlius we find a large proportion
Systkm, siiowiiig the of the retina (§ 535), which is commonly
libSes^'^amonJst^^'Inl described as a mere expansion of the optic
giionic cells. ncrvc, to be composed of nerve-vesicles

that arc scared}^ distinguishable from those of the brain ;
and it is probable that the ultimate branches of other sensory
nerves have some such termination. Wherever we meet mth
vesicular substance, we find it imbedded in a minute net-
work of blood-vessels ; and a copious supply of oxygenated
blood is requisite to the due performance of its actions.

(j2. There can be no doubt that the special office of the
l^ei'VQ-Jlbres is to convey the influence of the changes which
are eflectod in one part of the system, to other and remote
parts ; just as the Avires of a galvanic battery conduct the
electric influence from the instrument which excites it, to
some distant point where it is to be applied to some use.
The effects of such changes in the state of the iS^ervous
System are propagated in two opposite directions ; — the im-
pressions made upon the skin and other parts possessed of
sensibility, being conveyed towards a portion of the nervous
centres called the sensorium, and there giving rise to sensa-
tions; — and the influence of the emotions or volitions to
which these sensations give rise (§ 7), being propagated /rom
the central organs to the muscles, which tliey excite to con-
traction. And by the discoveries of Sir C. Bell, hereafter to

ACTIONS OF NERVOUS SYSTEM. 79

be described it has been fully proved that these opposite
changes are conducted by two different sets of fibres ; — one
conveying to the central organs those Avhich originate in the
cii'cumference ; — and the other conveying to the circum-
ference those which originate in the centre (§ 451), The
transmission of these changes is completely interrupted by
division of the nervous trunk, or by pressure upon it ; and it
sometimes happens that one set of conducting fibres is thus
affected, whilst the functions of the other are not impaired;
so that a limb may retain its sensibility and yet be totally
destitute of the power of motion, or may be completely
obedient to the will though totally destitute of sensibility. In
Yertebrated animals, w^e find some nerves in which there is
only one set of fibres, so that the trunk is only sensory or only
motor (§ 459); but in general, the two sets are bound up
together in the same sheath.

63. The motor fibres may bo considered as originating in
the vesicular substance of the central organs, and as termi-
nating in the muscles ; the power which is generated in the
former being conveyed by their means to the apparatus through
Avhich it operates to produce mechanical motion. When the
nerve-trunks reach the muscles, they divide into branches
which penetrate their substance, and these again subdivide
and ramify minutely, so that at last the fibres may often be
observed running singly, passing amongst the muscular fibres,
but not appearing to penetrate their tubular sheaths. These
terminal fibres seem often to double back upon themselves, so
as to form loops, either re-entering the branch from which
they issued, or connecting themselves with some neighbour-
ing branch ; so that the ultimate distribution of the motor
nerves in the muscular substance, is a sort of plexus or net-
work. The sensory fibres, on the other hand, may be con-
sidered as originating in the sensory surfaces, such as the
skin, the interior of the nose, the lining membrane of the
cavities of the internal ear, the retina of the eye, &c. ; and
as passing towards the central organs, conveying to these the
impressions they have received, which impressions may either
affect the consciousness, or may excite respondent move-
ments, or may act in both modes, through the instrumentality
of the vesicular substance to wliich they are transmitted.
The immediate dependence of the functional activity of this

«0 SIMPLIFICATION OF STRUCTURE IN LOWEST ANIMxVLS.

substance upon the supply of blood which it receives, is
shown by the fact, that if this supply be temporarily cut off,
either by failure of the heart's action (as in fainting), or by
pressure on the blood-vessels which convey it, immediate
insensibility, with loss of all power of motion, is the result.
And the same is the case with regard to the organs of sense ;
for if the circulation through them be interrupted, no sensory
impression can be made upon the nerve-fibres which originate
in them, as we see when the movement of blood in a limb is
suspended by pressure upon its artery.

G4. The foregoing constitute the principal tissues among
the higher animals, in which the principle of division of labour
is most fully carried out, every component part having its
own peculiar structure and its own special action. As we de-
scend in the scale, we find these distinctions less and less
obvious, so that when we come down to Zoophytes (§ 121), we
meet with but little differentiation either in the textures or in
the actions of the several parts of the body ; the whole sub-
stance of these animals being composed of a tissue, which
very closely resembles that which is first formed in higher
animals for the reparation of wounds, having the appearance
of a solidified blastema (§ 34), wdtli nuclear particles, in
various phases of development into cells and fibres, more
or less thickly scattered through it ; and this substance
being everyAvhere contractile, and everywhere (at least in
many instances) equally capable of participating in the func-
tions of nutrition and reproduction. And when we pass still
lower, to that simplest type of animal life, which is pre-
sented to us in the Rliizopods (§ 129), we do not meet with
even this amount of definite structure, but find the entire sub-
stance of their bodies composed of an apparently homogeneous
jelly, which, like the more organized tissue of the Zoophytes,
is everywhere contractile, and which has also the power of
performing every operation required for its growth and main-
tenance as a living being. In such creatures there is not the
sliglitest vestige of a Nervous system ; and it remains a question
whether, in consequence of this deficiency, they are altogether
destitute of consciousness, or whether this endo\vment is dif-
fused, as it were, through the whole substance of their bodies.

G-5. Every component part of the fabric must be regarded

INDEPENDENT VITALITY OF PARTS OF ORGANISM. 81

as having a life of its own, wliicli it maintains by drawing to
itself the nutrient material supplied by the circulating cur-
rent ; but as the continuance of its vital activity is dependent
upon the continuance of its nutrition, the life of no tissue
can be prolonged for any considerable period after the circu-
lation has ceased. But after the movement of the blood has
come to an end, though the body as a whole is dead, its parU
may remain alive for a certain time, and may perform their
functions, so long as they are supplied with the necessary
materials. Thus, various secretions, the growth of hair, and
muscular movements, have been ol3served to take place in
dead bodies. But they cannot continue, because the neces-
sary conditions are withheld by the stoppage of the circu-
lation, — a function which thus binds, as it were, into one
whole the scattered elements, and causes the different opera-
tions to minister one to another. As every component part
has an independent life, so has it a limited duration, quite
irrespective of that of the organism as a whole. Thus the
cells which float separately in the blood, seem to be con-
tinually undergoing change, — dying, and giving place to new
ones. We have seen that the cells of the epidermis and of
some parts of the epithelium are being constantly thrown off
and renewed. The duration of the cells of fat and cartilage
appears to be much greater ; in fact, we have no precise
knowledge of their term of life. That of the bony tissue is
probably greater still ; yet there is adequate evidence that
it is by no means indeterminate. But that of the muscular
and nervous tissues seems to depend almost entirely on the
use that is made of them. Thus we may justly say, — how-
ever startling the assertion may seem, — that death and decay
are continually going on in every living animal body, and are
essential to the activity of its functions.

QQ. Many animals are reduced to a state of apparent dec^th
by dryness, by cold, or by exclusion of" the air. A curious
example of the first kind is furnished by the Tardigrada
(Zoology, § 841) ; some species of which may not only be
completely dried up, but may even be exposed in that state
to a temperature much exceeding that of boiling water,
without losing the power of recovery when moistened. A
similar power of revival after being dried up is possessed by
the common Wheel Animalcule, and probably also by the

G

82 SUSPENDED ANIMATION.

eggs of many minute Entoiuostracous Crustacea (Zoology, §§
883, 931). It is unquestionable that many Fishes, especially
those of fresh- water lakes, will revive on being thawed after
havin<T been completely frozen; and the same has been ascer-
tained'' of certain Caterpillars. The Snail, when retiring for
the whiter, seals the orifice of its shell with an impervious
hd ; and in this cavity it may remain shut up for years, until
re-excited to activity by warmth and moistui'e. Animals in
such states of torpidity strongly resemble seeds that are pre-
vented from germinating, apparently for unlimited periods,
by being kept at a moderate temperature, and excluded
from the influence of air and moisture, which, with adequate
warmth, would call them into active growth, but which, at
a lower temperature, would occasion their decomposition.
There are no positive facts which enable us to say how long
Animals may remain in a parallel condition ; but there seems
no reason why it might not be indefinitely prolonged.

67. The death of the body, then, does not consist in the
mere suspension of its vital activity; for so long as that
activity may be renewed when the requisite conditions are
supplied, so Jong must the organism be considered as alive,
however deatli-like its condition may seem. Among warm-
blooded animals, such a suspension, if complete, cannot be
endured for more than a very brief period, without the
extinction of life ; for the substance of_ their tissues is so
jDrone to decomposition, tliat it speedily passes into decay
unless prevented from doing so either by a reduction of tem-
perature, or by complete drymg-up, or by entire seclusion
from air; and although each of these methods, practised
uj^on animal substances already dead, may prevent the occur-
rence of decomposition for ahnost unlimited periods, yet
neither can be applied to the hving tissues of any of the
higher animals, without occasioning the entire loss of their
vitahty, as we see (in regard to cold) in the loss of members by
" frost-bite." Such parts die, because not only is their vital
activity suspended, but their vital properties are annihilated.
Their death, however, does not necessarily involve that of
the organism as a whole ; since the stoppage of their function
may not disarrange the general train of vital operationc, or
their duty can be discharged by other organs. And among
many of the lower animals, we find that there is a provision

DECAY CONSTANT DURING LIFE. 83

for tlieir replacement by ordinary acts of growth ; and that
even when the body has been so severely injured that the
organic functions are seriously disturbed for a time (as
when a Hydra is divided into two or more pieces, § 122),
the vitality of the individual parts is sufficiently enduring,
and their reparative powers sufficiently energetic, to enable
them to reproduce all that is wanting for the completion of
the organism, and for the renewal of its ordinary actions.
Among the higher animals, the death of the organism at
large may be said to take place when the circulation finally
ceases ; since, as we have just seen, every individual part
must ere long lose its peculiar functional activity, and the
entire body be subject to decay.

^'^. From what has been stated, it will be seen that Life
cannot be regarded as a conchtion in which decay is resisted;
for an incessant decay is taking place in every living organism
as a necessary condition of its vital activity, being only
checked when that activity is itself suspended. But it is a
condition in which, by the wonderful harmony and mutual
adaptation of the operations of the different parts, the repa-
rative action of the Organic Functions is made to countervail
the destructive action involved in the exercise of the Animal
Faculties ; whilst the latter, in their turn, serve to furnish
the conditions requisite for the maintenance of the former.
So long as all these actions go on with regularity and com-
pleteness, so long the whole body lives ; but if any one of the
more important among them be interrupted, the stoppage of the
whole is the result. This relation of mutual dependence is most
intimate in the higher animals ; in which, by the differentia-
tion of the several tissues and organs, and the specialization
of their functions, the division of labour is carried to its
greatest extent, so that no part can entirely fulfil the duty of
any other. On the other hand, it is among those lowest
forms of animal life, in which there is the greatest multipli-
cation of similar parts, and the greatest diffusion of the same
endowments amongst them all, that we find the dependence
of the several parts of the organism upon each other to be
the slightest, and severe injuries to be tolerated with the
least general disturbance.

o '1

84 PKINCIPAL TYPES OF ANIMAL STRUCTURE.

CHAPTEE II.

GENERAL VIEW OF THE ANIMAL KINGDOM.

(j9. When we examine the Aniiual Kingdom as a whole, it
is easy to distinguish in it four general plans or t7/pes of struc-
ture, by which, with ahnost infinite variations in detail, the
formation of the several beings that compose it has been
guided. As specunens of these four plans or types, we may
name four animals which are familiar to almost every one, —
the Dog, the Lobster, the Snail, and the Star-Jish. The dif-
ferences by which these types are distinguished, are mani-
fested in the arrangement of the different organs of the body ;
and particularly in the form of the nervous system and its
instruments. It has been already stated (§ 4 ) that the power
of feeling, and of spontaneous motion, is that which peculiarly
distinguishes the Animal from the Plant ; and as these powers
are possessed in very different degrees, and exercised in very
different modes, by the various tribes of animals, — whilst the
operations' of' nutrition are performed, as in plants, in a much
more uniform manner, — they afford us a satisfactory means of
separating these tribes from one another. For the nervous
system is the organ to which these powers are due ; and we
find it presenting forms so different in the four great divisions
already alluded to, that wo can at once distinguish them by
this alone, even where (as sometimes happens) there may be
such a blending, in a particular animal, of the general characters
of two of them, as to lead us to hesitate in assigning its precise
place in the animal kingdom.

70. The highest of these four divisions is that denominated
Vertebrata, or Vertebrated Animals; it receives its name
from the structure characteristic of it, — the possession of a
jointed back-bone or vertebral column, — which will be pre-
sently described. This is the group to which Man belongs ;
and all the animals it contains bear a greater or less resem
bianco to him in structure. AVe notice in regard to their
external form, that they arc alike on the two sides of their
body; every part having its fellow on the other side. This

VERTEBRATED TYPE OF STRUCTURE.

85

*^ bi-lateral s}Tiimetry '' extends to the arrangement of those
internal parts which are connected with the functions of
animal life ; namely, the nervous system, the organs of sense,
and the muscular apparatus. But it does not always extend
to the organs of nutrition, which are unequally disposed on
the two sides : thus, in Man, the heart and stomach are on the
left side, and the liver on the right, while the lungs are much
larger on the right side than on the left. But in many of the
lower Vertebrata, there is an almost perfect symmetry in the
disposition of these organs, as there is also in the early embryo
of those in which this symmetry is subsequently departed
from ; so that it may be truly said that this symmetry is cha-
racteristic of the Vertebrate type, although for special purposes
it is frequently superseded.

Fig. 25. — Skeleton of the Ostricu.

71. In all Yerteb rated animals, the skeleton is chiefly
internal (fig. 25); and consists of bones, which arc capable of

8G

VERTEBRAL COLUMN.

growing, and of being reiDroduced after injury, like any other
part of the living tissue ; being copiously supplied with blood-
vessels, which penetrate into their interior. These bones give
support, and afibrd points of attachment, to the soft parts, in the
limbs (where they exist) as well as in the trunk ; but the former
arenotunfrequently wanting, as in Serpents : and we must look
in the trunk, therefore, for that peculiar arrange-
ment which is characteristic of this division of
the Animal Kingdom. The back-bone, as it is
commonly termed, is found in all Yertebrated
animals ; though in a few among them (the
lowest Fishes) it is very imperfect (§ 53). It
consists of several pieces jointed together, so as
to possess great flexibility; whilst they are so
firmly connected by ligaments, that they cannot
easily be torn asunder or displaced. The number
of these pieces varies considerably ; in Man there
are only 33 ; in some long-tailed Mammals there
are more than 70 ; but in many Serpents there
are several hundred. Each of them is termed a
vertebra; and the whole structure, composed of the
^i'r\1''c^Iumn^'^^^'^^^^^^ vertebra?, is termed the vertebral column
(fig. 2Q). The ordinary character of the vertebrre
is, that each is perforated by an aperture, which, united to the
corresponding apertures of those above and below^ it, forms
a continuous canal ; and in this canal, one of the most im-
^ portant parts of the nervous system, the spinal

cord (commonly but erroneously termed the spinal
marrow), is contained. The soKd portion of the
vertebra (fig. 27, a) is termed its body; and the
projections, 6 and c, are termed its processes, the
former spinous, the latter transverse. The row
o of spinous processes forms the ridge which we

^' vIrt'^bLI^'''' ^^^^ passing down the back; it is seen on the
right-hand side of fig. 26. To the transverse
processes the ribs are attached. The vertebral column is ex-
panded (as it were) at its upper extremity, to form the skull ;
in the large cavity which it contains, the brain is lodged ; and
its bones are so arranged as to give protection to the organs of
fiense also. At the opposite extremity we see it contracted
into the tail; which is composed of a series of vertebrae

NERVOUS SYSTEM OF VERTEBRATA.

87

resembling those of the back, but simpler in their form, and
not possessing a cavity for the spinal cord. We commonly
find that in those animals in which the skull is very large,
the tail is short ; and that where the tail is very long or
powerful, the head is small. Thus in man and in the apes,
the head is large, and there is no
external appearance of a tail ; but there
are some very imperfect vertebrae at the
lower end of the spinal column, which
constitute the rudiment of it. In the
long-tailed monkeys and in the kan-
.^aroo (whose tail is like a third hind-
leg), the head is comparatively small.
But this rule does not hold good uni-
versally.

72. The Nervous system of Yerte-
brated animals consists of a Brain and
Spmal Cord (fig. 28), which are lodged
mthin the skull ancl vertebral column ;
and of nervous trunks proceeding from
these, which are distributed to all parts
of the body. The Brahi is not (as
commonly reputed) a single organ, but
is composed of a number of ganglionic
masses, differing considerably in their
functions. Thus each of the nerves
of special sense (smell, sight, hearing,
and taste) has its own proper centre ;
and there is another of considerable
size, which seems to perform the same
office in regard to common sensation.
These are found in Vertel^rata generally ;
and their proportionate size corresponds
with the relative development and ac-
tivity of the several organs of sense
with Avhich they are connected. The
bulk of the brain of Man, however, is ^^^•''c^rd^J.^JS^'"'''*'
made up by two large masses of nervous
matter, which are known as the Cerebral Hemispheres; these,
as will be shown hereafter (chap, x,), are so small in the brains
of Fishes as to be scarcely distinguishable ; and their relative size

gS NERVOUS SYSTEM OF VERTEBRATA.

and complexity of structure increase as we ascend the scaie,
in pretty close accordance with the increase of the intelligence
or reasoning faculty. There is also another large nervous
mass, called the Cerebellum ; the function of wliich seems to
consist in the regulation of the more complex movements.
The Spinal Cord is made up of a longitudinal succession of
independent centres, of which one corresponds with each of
the vertebral segments of the body.

73. The distinguishing feature of the Nervous system in
Vertebrata is, that its several centres are thus united into one
large mass, instead of forming a number of separate small
masses or ganglia, as we shall find that they do in the lower
classes of animals : and that it is incloned in the bony casing
which has been described as peculiarly destined for its pro-
tection, instead of being enveloped with all the other organs
in a hard covering, as in the Lobster, or of being entirely
destitute of protection, as in the Slug. That it should receive
this peculiar protection is quite necessary, in consequence of
the much higher development which it attains, and the much
greater importance which it possesses, in this division of the
animal kingdom, than in any other. In all but the very
lowest Vertebrata, all five kinds of sensation exist ; — namely,
sight, hearing, smell, taste, and touch. We find in this group
more intelligence than in any other ; that is to say, the animals
composing it act more with a designed adajjtation of means to
ends ; instead of being impelled by a blind instinct to perform
actions of whose objects they are not aware. And we find, by
observing and comparing the structure and actions of the dif-
ferent groups, that the intelligence gains upon the instinct, as
we ascend from the lowest Fishes towards Man, in whom the
intelligence is at its highest ; whilst we observe a similar
increase in the proportion which the brain bears to the rest
of the nervous system. Hence we conclude, that the brain is
the organ oUntelligence, or of the reasoning faculties.

74. The general arrangement of the other organs in Yerte-
bratcd animals, is shown in fig. 29. At m is seen the mouth,
forming the entrance to the digestive cavity, of which the
termination is at the opposite extremity of the body ; i, i, is
the intestinal canal, and /, the liver : these organs occupy the
part of the body which is called the abdomen or belly. The
moutli also opens, however, into the windpipe, or trachea, t.

GENERAL STRUCTURE OF VERTEBRATA.

89

which conducts air into the lungs, p ; these organs, with the
heart, h, are contained in the portion, of the trunk called the

t p

Fig. 29. -Diagram,

showing the position
Vertebrata

OF THE TRINCIPAL OltGANS IN

thorax, or chest. At 6 is seen the position of the brain ; and
at s that of the spinal cord.

75. The foregoing characters apply, with greater or less
modihcation as to details, to the classes of Mammals (com-
monly termed Quadrupeds), Birds, Reptiles, and Fishes; and
these further agree in the following points, all of which,
therefore, enter into our idea of a Vertebrated animal. The
number of limbs or members never exceeds four ; and of
these, two, or even all four, may be absent. In all the
classes just named, four is the general number ; and the
absence of two or more is the exceptio7i. Thus in Mammals,
we find all four present in every tribe save that of Whales,
which want the hinder pair ; though the upper or anterior
pair may take the form of arms, wings, legs, or fins, accord-
ing to the element which the animal is formed to inhabit.
In Birds we find the posterior pair invariably present in the
form of legs ; whilst the anterior pair, though almost always
developed into wings, is absent in a few instances. In Reptiles
we find considerable variety ; all four members are present in
the Turtle tribe, and in most Lizards, as well as in the Frog
tribe ; but they are entirely absent in the whole tribe of Ser-
pents ; and there are Lizards which have only one pair. And
in Fishes, we usually find two pairs, constituting the pectoral
and ventral fins ; but one or both pairs are sometimes absent,
as in the Eel, Lamj)rey, &c. We have further to remark, in
regard to the general characters of Yertebrated animals, that,

90 CLASSES OF VERTEBRATA : MAMMALS.

with one exception, they have all red blood (§ 22Q) ; and
that the}' possess a complex apparatus for circulating this
through the body.

7G. The four principal modifications under which the Yer-
tebrated type presents itself, constituting the classes of Mam-
mals, Birds, IvEptiles, and Fishes, are respectively character-
ised by the mode in w^hich the principal functions of life are
performed in each.* Thus there are some Vertebrated animals
which produce their young alive, and which nourish them
afterwards by suckhng ; while the greater part rear them
from eggs which contain a store of nutritive matter, and
do not afford them any further nourishment from their own
bodies. Again, some breathe air ; whilst others live con-
stantly in water, and have no direct communication with the
atmosphere. Some, moreover, have the power of keeping
up a high temperature, so that their bodies always feel warm
to the touch ; whilst the temperature of others varies with
that of the atmosphere, so that their bodies give a feeling of
coldness : the former are teimed warm-hlooded — the latter
cold-blooded. There is a like difference in their mode of life ;
some of them being destined to Hve on the surface of the
earth, whilst others are chiefly inliabitants of the air, and
others again are the tenants of the ocean.

77. Mammals are distinguished from all other Yertebrata
by the first of the characters just adverted to ; being the only
animals that produce their young alive, and nourish them
afterwards by suckling. Like Birds and Eeptiles, they
breathe air by means of lungs ; and, in common with Birds,
they are warm-blooded and have a complete double circula-
tion of their blood, carried on by a heart with four cavities.
They are for the most part quadruped (that is, four-footed), and
are destined to live upon the surface of the earth ; but Man,
and the Apes that approach nearest to him, are biped, having
the power of walking on two limbs, and of using the others
for different ])urposes ; whilst the Bat tribe have the two
arms converted into w^ngs, which enable them to fly through
\\\o. air like birds (for which the older naturalists mistook

Many Zoologists range the Frofjs and their allies in a separate class,
under the name of Amphibia; but when looked at from a physiological
point of view, the author does not see that they require to be separated
from the true Reptiles.

GENERAL STRUCTURE OF MAMMALS. 91

tliem); and the Whale tribe are adapted in their general
form to lead the life of fishes (among which they are still
commonly ranked by persons ignorant of natural history).
Notwithstanding these marked differences in external form,
there is a great correspondence as to internal structure ; for
bats and whales, as well as ordinary quadrupeds, produce
their young alive, and suckle them afterwards ; they are also
warm-blooded, breathing air, and having an active circulation.
The bodies of Mammals are, for the most part, more or less
completely covered with hair, which serves to keep in their
warmth ; and this is seldom absent, except in such as inhabit
warm climates and do not require this provision. In the
Wliales, the same end is answered by the thick layer of oil in
the substance of the skin, constituting the blubber ; and Man
is left to form a protective covering for his body by the exer-
cise of his own ingenuity. The general arrangement of the

Sub-maxillary Gland Parotid Gland

Windpipe «., ^-^.

Colon

Ccscum

Small Intestines

Rectum
U-Bladder

Fig. 30.— Interior of a Monkey.

internal organs of Mammals will be seen from the accom-
panying figure of the body of a Monkey, laid open in such

92 GENERAL STRUCTURE OF BIRDS.

a manner as to exhibit the chief of them. The cavity of
the trunk is completely divided, by the muscular partition
termed the diaphragin, into two portions — the tliorax^ con-
taining the heart and lungs ; and the abdomen, containing
the digestive apparatus. It is chiefly by the alternate con-
traction and relaxation of this muscle, that the act of
breathing is performed in Mammals, as will be explained
hereafter (§ 331).

78. In EiRDS there is a much closer conformity to one
general plan than we find among Mammals. The covering of
feathers, by which we ordinarily disting-uish the members of
this class, prevails universally ; and there is no wide depar-
ture from the typical form. This class belongs to the
oviparous division of the Vertebrata ; the young being reared
from eggs. But it is distinguished from Eeptiles, which are
also oviparous and air-breathing, by being warm-blooded;
and by having a very energetic instead of a very slow circu-
lation. The warmth of the maternal body, moreover, is im-
parted to the a^^f^ in the act of incubation ; and without the
heat thus communicated (unless it be supplied from some
other source) the embryo cannot be developed. The covering
of feathers is given, not only to keep in the heat of the body,
which is even greater than that of Mammals, but also
to afford the required surface for the wings, on which the
Bird is supported and propelled through the air. The
feathered portion of the wHngs is stretched out upon the
bones wliich answer to those of our arm, and is moved by its
muscles. The wings are very small, or are entirely absent, in
the Ostrich and a few other birds, which present the nearest
approach to the JMammaha in their internal structure; and
these cannot rise from the ground, but run swiftly along it,
by means of their powerful legs. In the Penguin, also, the
wings are small ; and they are used as fins, by the assistance
of wliich this bird, which can neither walk nor fly with
rapidity, can swim very quickly through the water.

79. Generally speaking, Birds are characterized by the
extraordinary power of motion which they possess, and by
the great acuteness of the sense of sight, by which their
movements are chiefly directed. They are also remarkable for
their instinctive actions, which are chiefly related to their care
of their young, for whom they usually construct a protective

GENERAL STRUCTURE OF BIRDS.

03

nest. The nutritive functions are performed with extra-
ordinary activity in Birds, that the means may be supphed
for the maintenance of their locomotive activity. Their blood
is particularly rich in red particles, and its heat is usually
considerably above that of Mammals. Its circulation is very
energetically carried on ; and although the lungs themselves
are constructed upon a type inferior to that of Mammals,
and the mechanism of respiration is less complete, yet, by an
extension of the respiratory organs through the whole fabric,
the aeration of the blood is carried on with unequalled
energy (§ 326).

80. The arrangement of the organs contained in the cavity
of the trunk of Birds differs from that which has been
described in Mammals, chiefly

in this, — that there is usually
no diaphragm to separate the
chest from the abdomen, and
that although the lungs them-
selves are confined to the upper
part of this cavity, they are con-
nected with a series of air-sacs
which are distributed through
the whole of it. In the accom-
panying figure, which repre-
sents the internal organs of the
Ostrich, the heart is seen at a,
the stomach at h, and the in-
testinal tube at c. The windpipe,
d, opens into the lungs, e, which
are themselves small, and are
attached to the ribs, instead of
lying freely in the cavity of the
chest ; but the space they would
otherwise have occupied is filled
up by the large air-cells, /, /,
which communicate freely with
the lungs and with each other, and which even occui)y a large
part of the cavity of the abdomen, as seen in the figure.

81. In the class of Eeptiles we find a variety of form so
remarkable, that, if we were influenced by this alone, we
should scarcely regard the animals it contains as belonging to

Fig. 31. — Lungs of the Ostrich.

2, the heart; b, the stomach ; c c, the
intestines; d, the trachea; e, the
lungs; ///, air-cells, in which are also
seen the tubes by wliich these air-
cells communicate with the hvngs.

94 GENERAL STRUCTURE OF REPTILES.

the same group ; yet the structure of the internal organs, on
which classification is founded, is essentially alike in all;
and their physiological condition presents no important dis-
similarity. Four obviously different tribes, Turtles, Lizards,
SerpenU, and Frogs, are brought together by the following
characters. They are all oviparous, in this respect agreeing
mth Birds and Fishes ; but they are cold-blooded, and have
not a complete apparatus for the double circulation of the
blood, in which respect they differ from Birds; and they
breathe air by means of lungs, instead of breathing water by
gills, in which respect they difier from Fishes. But by the
lowest group, that of Frogs and their allies, this class is
united to that of Fishes in a most remarkable manner ; for
these animals in then' young state breathe by gills, and
lead the life of a fish ; and some of them retain their gills
during the whole of Kfe, even after the lungs are developed
(§ 87). The first three of the tribes just mentioned un-
dergo no such change : and they further agree in this, that
they breathe air during the whole of their lives, coming forth
from the egg in the same condition as that in which they are
subsequently to live, and also in having their bodies covered
with horny scales or plates, whilst the skin of the Frog tribe
is soft and unprotected.

82. The class of Eeptiles presents a marked contrast to
that of Birds, in the comparative slowness and feebleness of
its movements, the duhiess of its sensibility, and the in-
activity of its organic functions. As there is no fixed tempe-
rature to be maintained, one important source of demand for
food is withdrawn; and when not excited to activity by
external Avarmth, these animals may pass long periods without
fresh supplies of food. Their blood is very poor in red
corpuscles, and its circulation is comparatively languid. A
reduction of the temperature of their bodies to within a few
degrees of freezing point, induces complete torpidity, which
continues until they are roused by a renewal of warmth.

83. The Turtle tribe is peculiarly distinguished by the
inclosui-e of the body in a bony covering; of which the
upper arched portion (termed the carapace) is formed by
the coalescence of the ribs with a set of bony plates deve-
loped in the substance of the skin ; whilst the lower flat
plato (termed the plastron), which is often incomplete, is

STRUCTURE OF TURTLES AND LIZARDS.

95

formed by an expansion of the sternum, or breast-bone, wliicli
is spread out sideways, instead of being raised into a project-
ing keel as in Birds. The carapace and
plastron are covered with large horny
plates, variously arranged in the dif-
ferent species, and constituting what is
commonly called tortoise-shell. These
plates are often very beautifully disposed,
forming a kind of tesselated pavement ;
as in the common Tortoise (fig. 32),
which is often preserved alive in our
gardens.

84. In the tribe of Lkards., the body
has no such covermg ; but these animals,
having more activity than the tortoises
(which are proverbially slow), are enabled
to make their escape from danger, whilst the latter are obliged
to trust to their bony casing for protection from it. In their
general form. Lizards approach Mammals, being four-footed,
and living for the most part on land ; but they difter from
them not only in their essential reptilian characters, but also
in several others of less consequence. Their bodies are
usually covered with scales, which lap over one another like
the tiles of a roof; but in the Crocodile tribe, many parts of

Fig. 33.— Crocodile.

the surface are covered with large knotted horny plates, tiiat
meet at their edges like the scales of tortoise-shell, and aiford
an almost impenetrable covering. Although some of the
Lizard tribe spend a large part of their time in water, they
all breathe air ; but, as their respii'ation is very inactive, they
can remain for long periods beneath the surface, witliout
bemg obliged to come up to breathe.

85. The tribe of Serpents may be regarded as lizards with-
out feet ; their spinal column is immensely prolonged ; and

96

STRUCTURE OF SERPENTS.

their ribs are also very numerous ; and tliey are able to crawl
upon tlio points of these, using them almost as Centipedes do
their le-s (ii<- 421 r>ut in general the movement of their

Fig. 34. — Anatomy of a Coluher

bodies is accomplished by their being drawn-up into folds,
and then straightened so as to project the head. The pro-
longed form of the body in Serpents occasions a curious
variation in the arrangement of the principal organs, which
is shown in the accompanying figure. The oesophagus or

STRUCTURE OF SERPENTS AND BATRACHIA. 97

giiUet, ce, whicli leads from the mouth, to the stomach, is a
long and very wide canal, being even larger than the stomach
at its commencement ; a portion of it is removed at ce', in
order to show the heart, &c., which would otherwise be con-
cealed by it. The stomach, i, is long and narrow ; and the
intestinal tube, i', after making a few turns or convolutions,
passes backwards in a straight line, to terminate in the cloaca,
d, which opens externally by the orifice, an. The liver, /, is
also much lengthened. From the mouth also proceeds the
long ^vindpipo, t t, which conveys air to the lungs, or rather
to the single lung ; for the lung on the left side, j)', is scarcely
at all developed, whilst that on the right, p, extends along a
great part of the body. At o is seen the ovariimi, in which
the eggs, o' o', are produced ; and this also is very much
lengthened, extending from the cloaca a good way up the
body, so as nearly to meet the lung. The other references
are to the parts of the heart, and the principal vessels ; the
structure and arrangement of which will be explained here-
after (§ 284).

86. The Batrachia, or animals of the Frog tribe, are
readily distinguished from all the preceding, by their soft
naked skins ; even when the form of the body, as in the com-
mon Salamander or Water-]N'ewt, resembles that of the lizards.
They are also remarkable for the metamorphosis which they
undergo in the early part of their lives : for they come forth
from the egg in a condition which is, in all essential particu-
lars, that of a fish, and undergo a gradual series of changes,
by which their form and structure become assimilated to those
of the true reptiles. This change is most complete in the
!Frogs and Toads ; the early form of which is known as the
tadpole. The principal stages of this change are represented
in figs. 35 to 39 ; in which, however, the relative sizes are
not preserved, the tadpoles being much larger in proportion
(for the sake of displaying their form and the gradual
development of their legs) than the complete frog. Soon
after the young tadpole has come forth from the egg, it pre-
sents the form which is shown in fig. 35 ; its head and
trunk are large, and the latter is prolonged into a flattened
tail, by which the little animal swims freely through the
water. There is not the least appearance of limbs or mem-
bers. It breathes by gills, which are long fringes, hanging

H

98

3IETAM0RPH0SIS OF BATRACHIA.

loosely in the water on either side of the head. At a later
period, however, these gills, which are merely temporary,
disappear ; and the breathing is carried on by another set,
which are situated behind the head, and are covered in by a
fold of skin; the water gains access to these by passing
through the mouth, exactly as in Fishes. The form is then
that which is represented in fig. 36. In a short time after-
wards, the animal still breathing by its gills, the hind-legs
begin to sprout forth, as it were, at the base of the tail ; this

Fig. 36.

Fig. 37.

Fig. 35.

Fig. 39.

stage is shown in fig. 37. At a still later period, the fore-
legs begin to be developed, as seen in fig. 38 ; and from that
time they are nourished at the expense of the tail, which
gradually disappears, as seen in fig. 39, a, h. During this
period, other important changes are taking place in the inte-
rior of the body ; the chief of which are the development of
the lungs and the gradual disuse of the gills, so that the
animal becomes fitted to live on land and breathe air, and is
no longer capable of remaining long under water without
coming to the surface to respire.

87. The metamorphosis in other members of the group is

PERENNIBRANCHIATE BATRACHIA.

99

less complete than in the Frog, being checked at a less
advanced stage. Thus in the common Water-Newt, the tail
is retained during the whole of life, and the animal continues
to be an inhabitant of the water, though breathing air alone.
There are some very curious animals, however, in which the
change is stopped, as it were, at a much earlier period, so that
the gills also are retained ; and in these, the lungs are suffi-
ciently developed to enable the animals to breathe air, so that
they can live either on land or in water. Such Batrachia are
scientifically known as perennihranchiate, this term express-
ing the persistency of their gills. In tig. 40 is represented

Fig. 40.— AxoLOTL.

an animal of this kind, the Axolotl, which inhabits some of the
lakes of Mexico. And in fig. 41 is shown the form of a still
more remarkable animal, the Lepidosiren, or mud-fish, recently

41. — Lepidosiren.

brought from th ; rivers of Africa, the metamorphosis of
wliich appears to be checked at a still earlier period, so that
it is very difficult to decide whether it should be regarded as

H 2

100 STRUCTURE OF FISHES.

a Fisli or as a Eeptile, so complete is the mixture of charac-
ters which it presents.

88. The class of Fishes is distinguished from all other
Vertebrata, by the adaptation of the animals composing it to
breathe by means of water in their adult state, so as to be
capable of living in that element only. Like Eeptiles, they
are oviparous and cold-blooded ; and in these characters they
differ completely from the Whales and other Mammals,
which are, like them, inhabitants of the great deep, but which
are warm-blooded, viviparous, and air-breathing animals.
There is a simple external character, by which the members
of the two classes may be at once distinguished. The animals
of the Whale tribe are, like fishes, chiefly propelled through
the water by means of a flattened tail ; but in the former the
tail is flattened horizontally, so that its downward stroke may
serve to bring the animal to the surface to breathe; whilst in
Fishes it is flattened vertically, that its strokes from side to
side may simply propel the fish through the water. A
liattenmg or compression of the body is seen more or less
in almost all fishes, and is intimately connected with the
nature of their motion through the element they inhabit ; as
it serves the double purpose of diminishing the resistance
which is offered to their progress, and of increasing the extent
of the oar-like surface, by the lateral stroke of wliich the
body is propelled forwards (Chap. xii.). This stroke is given
by a series of muscles of great power, Avhich pass from the
prolonged extensions of one vertebra to those of another, and
altogether make up the principal part of the bulk of the
animal. The fins which represent the limbs are not so much
used in propelHng the Fish, as in changing its direction
either laterally or vertically. Thus in the lowest group of
the Vertebrated series, the act of motion is chiefly performed
by the vertebral colimin itself, instead of being committed to
the limbs, as in Mammals, Eirds, and most Eeptiles. The
larger number of Fishes swim with great activity ; and their
lives may be said to be passed in seeking their subsistence
and in flying from their enemies.

89. Fishes are for the most part very voracious, and their
food consists in great part of the members of their own class.
In seeking it, they appear to be chiefly guided by the sight ;
fur their eyes are usually large and highly developed, while

STRUCTURE OF FISHES. 101

the other organs of sense are formed upon a very inferior
t}^e. They swallow it without much division in the mouth ;
but it seems to undergo rapid digestion. The blood of some
rish, whose muscular activity is pecuHarly great, is rich in
red corpuscles, and of a temperature not much lower than
that of Mammals ; but, generally speaking, it contains much
less solid matter than that of the w^arm-blooded Vertebrata,
and its temperature follows that of the surrounding medium.

90. Although Fishes breathe by gills instead of by lungs,
these gills are connected with the mouth, so that the water
which passes over them is received into it, in the same man-
ner as the air is in the higher Vertebrata. This is a character
which distinguishes the position of the gills of fishes from
that of the corresponding organs of any of the inferior tribes.
They are lodged in a cavity on each side of the throat ; and this
cavity opens outwardly, either by one large valve-hke aperture
on either side, or by several ; through these apertures the
streams of water which have been taken in by the mouth,
and forced over the gills by the action of its muscles, make
their exit.

9 1 . All Fishes are oviparous ; and the number of eggs which
they produce is generally prodigious. It is very seldom that
after the eggs have been deposited and fertilized, the parents
take any further concern in regard to them ; though there
are a few instances in which a kind of nest is made, and
others in which the egg is retained and hatched within the
body, so that the young comes forth alive. This last is the
case with the Sharks and Eays, which, notwithstanding that
their skeleton is cartilaginous, are higher than Fishes generally
in several other parts of their organization.

92. All the animals which are destitute of a vertebral
column are called Invertehrata ; and this division into the
Vertebrated and Invertebrated groups was formerly regarded
as the first step in the classification of the animal kingdom.
But it was pointed out by Cuvier, that in the Invertebrated
division are comprehended three groups, of which the mem-
bers differ as much from one another as they do from Verte-
brated animals ; and that each of these ought, therefore, to
rank with the first, as a primary division. This is evident
to those who are but slightly acquainted with the structure
of the animals already named (§ 69) as characteristic speci-

102

GENERAL STRUCTURE OF ARTICULATA.

mens of these divisions ; and it will become more apparent
as we proceed.

93. In the second division, that of Articulata, or Articu-
lated (jointed) animals, we find a conformation very different

from that which has been just described. The
exterior of the body is still perfectly symme-
trical, as in the Yertebrata ; and the interior is
even more symmetrical ; for the organs that
represent the heart and lungs are equally dis-
posed on the two sides of the central line of the
body. But the skeleton, instead of being internal,
is external; and is composed of a series of pieces
jointed together, which form a casing that in-
cludes the whole body. In general, these pieces
are very similar to each other ; so that the whole
body appears like the repetition of a number of
similar parts, as we see in the Centipede (fig. 42).
The limbs are usually very numerous, where
they exist at all ; and they have a jointed cover-
ing, like that of the body. But in the lower
tribes of this group, such as Leeches and Worms,
the limbs or members are but sHghtly developed,
or are altogether absent; and in the highest,
wliicli approach most nearly to the Yertebrata
in their general organization, the number of
members is much reduced, — although it is never
less than six. The hard matter of which the
cen'ti ^^* external skeleton is composed, undergoes little
or no change when it is once fuUy formed ; and,
in order to accommodate it to the increasing size of the
animal, this covering is thrown off and renewed at intervals
during the period of growth.

94. The nervous system consists of a series of separate
ganglia, which are arranged in a cord or chain along the
central line of the body. There is iisuaUy a pair of large
gangha in the head, bearing a reseml) lance (in their peculiar
connexion with the eyes) to the ganglionic centres of the
optic nerves in Yertebrata; and there" is commonly one for
each segment or division of the body, from which the nerves
pass to supply its muscles, as they do from the spinal cord of
Yertebrata. The cord which connects these ganglia is double.

GENERAL STRUCTURE OP ARTICULATA.

103

and the ganglia tliemselves are composed of two halves,
which have little connexion with each other. The chain
thus formed (fig. 43) passes along the
nnder-side of the trunk of the animal
(as seen at g, fig. 44), not on what
seems its back ; and by the presence
of tliis double chain of ganglia an
Articulated animal may be distin-
guished, even when, in its general
structure, it should seem to belong to
the group of Mollusca (§ 102).

95. The general arrangement of the
organs in the Articulata is shown in
the accompanying figure of a Cray-
fish. The mouth, situated on a pro-
jecting head, opens into s, the stomach,
from which passes backwards the in-
testinal tube, i, i, to terminate at the
opposite extremity of the body. Tlie

upper part of the tube is surrounded by the liver, /, which is
here very large. In the head are seen the ganglia, c; and
along the under-side of the body is seen the chain of ganglia,

Fig. 43.

-Nervous System
AN Insect.

Fig. 44.— Diagram showing the position of the piuNcirAt, Organs ik
THE Articulata.

g. The blood is nearly colourless, and is usually impelled
through the body not by a single organ or heart, but by a
succession of contractile cavities, one for each segment, which
open into one other longitudinally, forming what is known as
the dorsal vessel; in the Cray-fish and its aUies, however, one
part of this, h, is specially enlarged, so as in great degree
to serve as a heart for the system generally. The respiratory
organs are not connected with the mouth ; and are not usually

104 STRUCTURE OP INSECTS.

restricted to one part of the body, but are diffused either on
its outside or through its substance.

96. The organs of sense, in this group, are less numerous
than in Yertebrata, and are inferior in perfection ; those of
sight are the most developed, and are formed upon a very
peculiar plan (§ 573); but all organs of special sense appear
wanting in the lowest tribes. Yet we find that the muscular
power is very great ; for the animals of this group, taken as a
whole, can move faster in proportion to their size, and possess
greater strength, than those of any other. We observe, too,
that -with little or no intelligence, they are prompted to the
most remarkable actions by instinct alone. They seem to act
like machines, doing as they are prompted, without choice, or
knowledge of the end to be gained ; and consequently the dif-
ferent individuals of the same species have not that difference
of capacity and of disposition, which we see in animals whose
endowments are higher.

97. In the highest division of the Articulated series, we
easily recognise, as forms quite distinct from each other, the
Insects, the Spiders, the Crustaceous animals (crabs, lobsters,
&c.), and the Centipedes. The class of Insects is distinguished,
for the most part, by the presence of wings ; but to this there
are exceptions. It includes those of the higher Articulata,
which breathe air by means of air- tubes distributed through
the body (§ 320), which have no more than six legs, and
whose body, in its perfect form at least, manifests a division
into tliree distinct parts — the head, thorax, and abdomen
(fig. 45). To the thorax alone are attached the six legs, as
well as the wings ; and its cavity is principally occupied by
the muscles that move them : the abdomen contains the
organs of digestion and reproduction, as in vertebrated animals.
In the greater part of this class, the young animal comes forth
from the egg in a condition very different from that which it
is ultimately to possess ; and it undergoes a complete meta-
morphosis, the larva wliich the egg produces bearing a close
resemblance in form to the lower Articulata, and only attain-
ing the condition of the imago or perfect insect by passing
again into a state of inactivity, during which the store of
nutriment which it has acquu-ed is appUed to the development
of new organs. This p)upa or chrysaUs condition may be
considered as a sort of postponed completion of the embryonic

STEUCTUKE OF INSECTS AND ARACHNIDxV.

10^

life, which was interrupted at a very early period. In some
tribes, however, the genenil form is the same from the first,
and the wings are the only parts deficient ; these gradually

Antennae
Eyes

1st pair of Legs -

1st pair of Wings
2nd pair of Legs

2nd pair of Wings
3rd pair of Legs

Tibia
Tarsus

^-^^^S^.

Head

Abdomen

Fig. 45.— Skeleton of an Insect.

make their ajDpearance, and the insect is then complete. Such
is the case with the Grasshopper and Cricket; and a change
of this kind is termed an incomjylete metamorphosis.

98. The animals of the class Arachnid a, which includes
the spiders, scorpions, and mites, are, hke Insects, articulated,
breathing aii', and possessing legs, but the number of these
legs is never less than eight; there is an entire absence of
wings, and the head is united with the thorax, so that the
body seems to bo formed of two principal divisions, — the
cephalo-thovax (as it is termed), and the ahdomcn. In fig. 46
we have a representation of the arrangement of the parts con-

106 STRUCTURE OF ARACHNIDA AND CRUSTACEA.

tained in these cavities. At c ^ is seen the cephalo-thorax
opened from below, and giving attachment to the legs ; at m
is shown the place of the mandibles or jaws ; at ^ is seen one

ct pa af> pa *

Fig. 4G. — Anatomy of

of the palpi, which arc appendages to the mouth ; pa is the
foremost leg ; t, the large nervous mass, from which the legs
arc supplied; a, the collection of ganglia supplying the
abdomen ; a b, the abdomen ; p a, the respii-atory chambers ;
s s the stigmata or openings into these ; I, the leaf-like folds
within them (§ 323); w a, the muscles of the abdomen ; a n,
the termination of the intestine ; /, the spinnerets ; o, the
ovaries ; and o r, the opening of the oviduct.

99. The class of Crustacea, of which the Crab, Lobster,
and Craij-Jish are the best-known forms, differs from both
the preceding, in being adapted to breathe by means of gills,
and thus to reside in or near water, instead of inhabiting
the air. Moreover, the body is inclosed in a hard covering,
which generally contains a good deal of carbonate of lime, and
which is thrown off at regular intervals. This covering also
incloses the members, wliich are never less than ten in
number, and are frequently more numerous. There is great
variety of form among the animals of this group, which is
altogether one of great interest. — In the Crab tribe, the head,
thorax, and abdomen are all drawn together, as it were, into
one mass ; and the general arrangement of the organs it con-
tains is exhibited in the succeeding figure, which shows them
nearly as they are found to lie, when the upper part of the
shell, or carapace, is removed. At t there is left a portion of

STRUCTURE OF CRUSTACEA.

107

the membrane whicli lines the carapace and covers in the
■vascera. On the central line, at c, is seen the heart, which in
the Crustacea is large and powerful in its action ; from it
there passes forwards the artery a o, which supplies the eyes

Fig. 47. — Anatomy of a Cviab.

and the front of the body ; whilst the artery a a passes to the
lower and hinder parts ; at b are seen the gills of the left side
in their natural position ; whilst at b' are seen those of the
right side, turned back to show their under-surface, and to
disclose the lower portion of the shell, fl. At e is seen the
stomach, situated close behind the mouth ; and at ttl are
pointed out its powerful muscles, by the action of which the
food is ground down. The bulky ovary is seen on either side
of the stomach ; and the space between this and the edge of
the shell is occupied by the very large liver, / o.

100. In most of the Crustacea, however, the body is more
prolonged. In some, as the Lobster, there is an indication of
a division of the body into three parts, representing the head,
thorax, and abdomen of insects ; whilst in others, as the Sand-

108 STRUCTURE AND METAMORPHOSIS OF CRUSTACEA.

Iiopper, the rings or segments are almost as similar to each
other as they are in the centipede tribe. There is no class in
which we find the same parts exhibiting so great a variety of
forms, and rendered subservient to so many uses. _ Thus in
the Crab and Lobster the members of the first pair are not
used for walking, but form the claws or arms by which the
food is seized ; in the Cray-fish, these members may be used
either as legs or claws; whilst in the Sand-hopper, they
closely resemble the other legs. And the jaws of the higher
Crustacea, of which there are several pairs, are really meta-
morphosed legs ; as may be seen by comparing them with the
corresponding appendages of the Limulus or king-crab, the
first joints of which act as jaws, whilst the remaining portions
of these members serve either as legs for locomotion, or as
claws for prehension.

101. Most of the Crustacea, like insects, come forth fi'om

the egg in a state very different from
their adult form ; and afterwards undergo
a series of changes, which are in some
instances so remarkable as to approach
the complete metamorphosis of insects,
and which end in the production of the
complete form. An early form of the
common crab, at a time when it is of
the minute size indicated on the scroll,
is shown in fig. 48. The immature
Crustacea of different tribes bear much
more resemblance to each other, than do
the forms into which they are ulti-
mately to be developed ; and the dit-
ferences they afterwards present are
chiefly due to a variety in the amount
of growth which the different parts
undergo.

102. It is one of the most remarkable results of modern
zoological research, that in immediate connexion with the
class of Crustacea, if not as actual members of it, we have
to place a group of animals which were for some time asso-
ciated with the Mollusca; their bodies being inclosed .in
shells, which do not fit closely around them, nor give more
than a general protection to theii' members. This group is

Fig. 48. — ZOEA, OR LARVAL

FORM OP THE Crab.

STEUCTUEE OF CIEEHIPEDA.

109

the Barnacle tribe, forming the class Cieehipeda, or tendril-
footed animals. They agree with the lower Mollusca, in
being fixed to one spot diu-ing all but the earhest period of
theii' lives ; the shell being sometimes attached by a long
membranous or leathery tube, as that of the Barnacle
(fig. 49) j and sometimes being itself fixed on the surface of

Fig. 49.— Shell of
THE Barnacle.

Fig. 50. -Body of
THE Barnacle.

a rock, or on another shell, as is that of the Balanus or
acorn-shell. In both cases, the form and structure of the
animal are essentially the same. When taken from the shell
(in which it lies doubled up, as it were) and spread out, its
articulated nature is evidenced by its division into segments,
and by the regularity of the arrangement of their tendril-like
appendages. These are not formed like legs, since they could
be made no use of, the animal being incapable of moving
from place to place ; but they serve to produce currents in
the surrounding water, by which food is brought to the
mouth, and the blood is submitted to the influence of a
fresh supply of air. The nervous system of this group
is formed precisely upon the plan of that of the Articulata
generally (§ 94) : and if any doubt could have remained as to
its true place in the series, it is removed by the knowledge
of the fact, that the animals composing it bear a strong
resemblance in their early condition to some of the lower
Crustacea, possessing eyes and legs, and swimming freely
about; and that they attain their adult form by passing

110 STRUCTURE OF MYRIAPODA AND ANNELIDA.

through a series of metamorphoses, in which they lose their
eyes and legs, and become fixed for the remainder of their
lives.

103. 'Wg now pass back to another class of the higher
group of Articulata, adapted to breathe air and to inhabit
the land, — the Myriapoda or Centipede tribe (fig. 42). Both
these names are derived from the great number of legs
possessed by these animals, which often amount to 60 pairs
or even more. In this class we see a more perfect equahty
of the segments or divisions of the body than in any others
among the higher Articulata; and the similarity is scarcely
less complete in the internal arrangement, than it is in the
external form. In its lower tribes (fig. 51), the legs are so

Fig. 51. — luLu

weak as scarcely to be able to sustain the body, which moves,
therefore, partly in the manner of that of a worm. The
animals of this class undergo no proper metamorphosis ; but
there is a considerable adcUtion to the number of their seg-
ments and legs after they have come forth from the egg.

104. AVe now pass to the lower division of Articulata, in
which the body possesses no jointed members ; and the animals
belonging to this group are for the most part included in the
class of Annelida, the Leech and Worm tribe. We here find
the body enveloped, — not in a hard casing, formed of distinct
pieces united by a flexible membrane, — but in a skin which
is altogether flexible, and which gives little indication of a
division into segments. This class includes several distinct
tribes, which aU agree, however, in the long worm-like form
of the body, and in the similarity of the different ganglia oi
their nervous system. The Earth-worm and its allies are
adapted to live on land and to breathe air ; but the greater
number of Annelids arc purely . aquatic ; and these breathe
by gills, which form tufts that are disposed on various parts
of the body. In the Nereis^ or Sea-centipede (fig. 52), these

STRUCTURE OF ANNELIDA AND ENTOZOA.

Ill

tufts are arranged regularly on tlie several segments, and the
animal can swim by the motion that it gives them ; besides
these, it has a kind of bristle-shaped aj^pendage, that seems

^^^^Ws^^^

Fig. 52. — Nereis.

like a rudimentary leg, which assists it in crawhng.

But

there are others of these marine-worms, that form a tubular
shell, in which they reside during the greatest part of their
Lives ; and in these the gills, if disposed along the body,
would have been removed from the access of water ; they are
therefore arranged round the head, often forming (as in the
Serpuloe, fig. 145) tufts of great brilhancy and elegance.

lOo. Below the Annelida are other worm-like tribes of yet
greater simplicity of conformation, but still presenting the
same general plan of structure. Of one of these the common
Leech may be taken as an example ; of another, the Tape-

Fig. 53.— Tape-worm.

worm (fig. 53). This last belongs to a group termed Entozoa,
from the circumstance that they inhabit the bodies of other
animals. They are remarkable for the very low development
of their digestive apparatus, — their nourishment being appa-

112 GENERAL STRUCTURE OF MOLLUSCA.

rently imbibed through, the whole surface of their bodies
from the juices in the midst of which they live ; whilst, on
the other hand, their reproductive apparatus is enormously
developed, the multii^Hed segments of the Tape-worm (for
example) containing this alone, and the head (as it is com-
monly termed, though really the body) being able to repro-
duce these to an indefinite extent after they have been
throA\Ti off. The group of Eotifera, or Wheel- Animalcules,
which is one of great interest to the Microscopist, also belongs
to this lower section of the Articulated sub-kingdom.

lOG. The general character of the animals composing the
group or division Mollusc a, is, in many respects, the very
opposite of that which prevails in the Articulated animals.
The body is soft (whence the name of the group is derived),
neither possessing an internal skeleton, nor any proper ex-
ternal skeleton. In some of the most characteristic specimens
of the group, such as the Slug, there is no hard frame- work
or skeleton whatever, the body being alike destitute of
support and protection. In most MoUusks, however, the
body has the power of forming a shelly covering, which serves
for its protection ; but this does not give any assistance in its
movements by affording fixed points for the attachment of the
muscles ; in fact, when the animal puts itself in motion, it is
obhged to make its locomotive organs project beyond the
shell. AVe must not regard the shell as an essential part of
the Molluscous animal ; because there are many tribes entirely
destitute of it ; and also because some of the Articulata have the
power of forming a shell (§ 102), which bears a close resem-
blance to that produced by the animals of this group. N"ot un-

Fig. 54.— Testacei.la.

frequently we see that, of two animals whose general structure
ifl almost exactly the same,— as that of the Simil and Slug,—

STRUCTURE OF MOLLUSCA. 113

one possesses a shell into whicli it can withdraw its whole
body for the sake of protection, whilst the other has none;
and several intermediate forms exist, in which the shell
bears a larger or smaller proportion to the body, sometimes
being able to contain nearly the whole of it, and sometimes
being a mere rudiment, as in the Testacella (hg. 54).

107. The external form of the body of Mollusks is subject
to great variation ; and generally has a good deal to do with
the degree in wliich the organs of sense and the instruments
of motion are developed in the particular animal. For these
are almost always symmetrical, being arranged ^vith equality
on the two sides of a middle line ; whilst the rest of the
body, containing the organs of nutrition, is often unequal on
the two sides. But in the lower Mollusca, which have little
or no power of moving from place to place, even this degree
of symmetry is altogether lost. Few of the Mollusca have
any powers of active movement ; in fact, the term sluggish-
ness, derived from a characteristic member of the group, very
well expresses their general habit. The Gasteropods, which
may be regarded as the types of the whole series, crawl upon
a flesh}^ disk, by the successive contractions and relaxations
of which they advance slowly along the surface over which
they move ; this kind of action is easily studied, by causing
a Snail or Slug to crawl- upon a piece of glass, and by looking
through this at the under side of its foot. Hence, there is a
great contrast between the inertness of the Mollusca, and the
liigh activity of the Articulata, This contrast shows itself in
the structure of their bodies ; for whilst the chief part of the
interior of an Insect is made up of the muscles which move
its legs and wings, — the apparatus of nutrition being small, —
the chief part of the hulk of a Slug or Snail is given by its
very complex apparatus for nutrition — there being no other
muscles (except some small ones connected with the mouth
and head) than the fleshy disk already mentioned. The blood
of the Mollusca is nearly colourless, as it is in the Articulata ;
but the organ by which it is circulated through the body is
much more powerful and complete, bearing more resemblance
to the heart of Vertebrated animals. The skin is usually
thick and spongy in its texture ; having muscular fibres inter-
woven in its substance, so that it can contract or extend itself
in any part ; and having the power of exuding shelly matter

I

lU

STRUCTURE OF MOLLUSCA.

from its surface, in those si3ecies wliicli form such, a pro-
tection. This envelope, which is called the mantle^ is very
loosely applied round the parts which it contains; and it
frequently extends itself into folds or duplicatures, which
wrap round the gills, and sometimes meet and adhere so as to
inclose them within a cavity of their own. In the Cuttle-
fish, the water within this cavity is renewed from time to time
by the muscular movements of its walls ; but usually a
current of fluid is kept up over the surface of the gills, by the
action of the c'dla (§ 45) with which they are covered.

m vb ah b ov

Fig. 55.— Anatomy of Turbo Pica.

108. Tlie accompanying figure of the interior of a Turho
\s\S\ show the very large size of the digestive apparatus, and
of the other organs of nutrition. The muscular disk or foot
is seen at/); and this carries the operculum o, which serves
to close the mouth of the shell when the body of the animal
is dra^vn within it. At t is sho^vn the proboscis, on either
side of which are the tentacula or feelers, ta, bearing the eyes
at y. Just behind the tentacula is seen the large cephalic
ganglion, sending nerves to the eyes ; and behind this again
are the saUvary glands. The mantle, m, is opened and folded
hack to show the respiratory cavity, in which lie the gills

STRUCTURE OF MOLLUSCA.

115

hh; to this cavity, water has access by means of a wide slit,
of which the edge, t\ of the mantle forms one part of the
border, whilst at d is seen a fringed membrane that forms
another part. At c is seen the heart, which receives the
l)lood from the gills by v b, the branchial vein, and then
transmits it to the body generally ; at e, far np in the spire,
are the stomach and liver ; at a, the anal orifice of the intes-
tine within the branchial cavity, and at ov the oviduct, which
opens in the same situation.

109. Thus it is seen that, — whilst the body of an Articu-
lated animal may be compared to that of a man in whom the
apparatus of nutrition (contained in the chest and abdomen)
is of the smallest possible size, but whose limbs are strong,
and his movements agile, — the body of a Mollusk resembles
that of a man " whose god is his belly," his digestive appa-
ratus becoming enormously developed, whilst his limbs ar©
feeble, and his movements heavy. Such varieties, in a greater
or less degree, are continually presenting themselves to our
notice.

110. The nervous system of the MoUusca generally consists
of a single ganglion or pair of ganglia, which are placed in the
head, or (when that is deficient) in the neighbourhood of the
mouth ; and of two or more separate ganglia, which are found
in different parts of the body, and are connected with the
preceding by nervous cords. The
former correspond to those con-
tained in the head of Insects ; but
of the latter, one only is connected
with the foot or organ of motion,
the remainder having for their
function to regulate the action of
the gills, and to perform other
movements connected with the
operations of nutrition. In fig. 56
is represented one of the simpler
forms of this nervous system, —
that of the Pecten or Scallop-shell ;
A A are the ganglia near the mouth, Fig- sc-
from which the organs of sense
are supplied ; b is the ganglion connected with the gills ;
and c is that from wliich power is given to the foot. The

" I 2

-Nervous System of
Pecten.

116 • GENERAL STRUCTURE OF MOLLUSCA. — CEPHALOPODS.

two first lie wide apart, but are connected by an arched
band that passes over the gullet, e. The organs of sense in
the higher forms of Mollusca are more developed than those
of motion. They serve to direct the animal to its food, and
to warn it of danger ; but there seems an absence, in all save
the highest species, of that ready and acute sensibility which
is so remarkable in the preceding groups ; and the variety of
impressions which they can receive appears to be but small. In
no instance has a special organ of smell been certainly dis-
covered ; the organ of hearing is always imperfect, and fre-
quently absent altogether ; and the eyes are very often wanting.
In the lower Mollusca there are no certain indications of the
existence of any organs of special sense ; and there is probably
but a limited amount of general sensibility.

111. As the Articulata are divided into two subordinate
groups, according to the presence or absence of articulated
limbs or meml^ers, so may we arrange the Mollusca in two
subdivisions, according to the presence or absence of a dis-
tinct head^ that is, a projecting part of the body, containing
the mouth or entrance to the digestive cavity, and also bearing
the organs of sense which guide the animal in the discovery
and selection of its food. In the higher Mollusca, there is a
distinct head, furnished with eyes, and sometimes with im-
perfect ears ; but in the lower, the entrance to the digestive
cavity or stomach is buried deep among other parts, and is
guarded ])y no other organs of sense than the tentacula or
sensitive lips. These are termed acephalous, or headless
Mollusca: and among the lowest of them (§ 114), we meet
with composite fabrics, formed by the process of multiplica-
tion by budding, which was formerly regarded as peculiar to
Zoophytes. — The highest group of Mollusca, in regard to the
approach of several parts of its structure to that of Yerte-
brated animals, is the class of Cephalopoda, or Cuttle-fish
Ifibe : which receives its name from the peculiar arrangement
of the arms or feet around the mouth, which is the cha-
racteristic of its members (fig. 57). The common Cuttle-fish
and its allies are destitute of any external protection ; but
they usually have a flat sheh, commonly known as the cuttle-
fish bono, inclosed in a fold of the mantle, and lying along
the back. In the Calamary, this is horny in its texture, and
is sufficiently flexible to ofler no resistance to the action of

STRUCTURE OF CEPHALOrODS A^'D I'TEROPODS.

117

the fin-like tail, By which the animal is propelled through the
water very much in the manner of a fish. The Pearly
Nautilus is the only type now existing of an inferior order of

Fig. 58. — Ammonite.

Fig. 57. — Calamary.

Cephalopods, which approaches the Gasteropoda in many parts
of its organization. The body is inclosed in the last chamber
of a shell (usually spiral in form),
the cavity of which is divided by
numerous transverse partitions ; and
such shells, the fossihzed remains of
very numerous forms of this group
that existed in the ancient seas, con-
stitute the nautilites, ammonites,
helemnites, Szo,., wliicli abound in
many rocks (fig. 58). Tlie Cuttle-
fish are animals of considerable
activity; their mouth is furnished
with a horny beak, strongly reseni-
bling that of the parrot ; and their arms are provided with
a series of very curiously constructed suckers, by the action
of which they can take a very firm
hold of anything which they desire
to grasp.

112. The class of Pteropoda, or
wing-footed MoUusks, consists of
but few species, and the animals
which it contains arc all of them of
small size ; but the individuals are
often very numerous, whole fleets
of them being sometimes seen
covering the ocean, especially in
the Arctic and Antarctic regions,
where they constitute one of the ^'^- 59--Htal;ea.

principal articles of food to tlie Whale. The general form of the
body usually difi"ers but little from that represented in fig. 59.

118 STRUCTURE OF GASTEROPODS AND BIVALVES.

On cither side, a little behind the head, the mantle is extended
into a fin-like expansion, by the aid of which the animal can
swim through the water. The hinder part of the body is
usually inclosed, more or less completely, in a shell, which
is conmionly of extreme thinness and delicacy. The head is
not furnished with long arms, to grasp the food ; but it has
a number of minute sucking disks, by which it can lay firm
hold of whatever it attacks : whilst its powerful rasp-like
tongue is set to work upon it. — The class Gasteropoda con-
tains those animals which, hke the Snail and Slug, crawl
upon a fleshy disk on the under side of their bodies ; and
the number of distmct forms which it includes is very large.
The greater part of them are inhabitants of the sea-shore^
rivers, lakes, &c. ; some have the power of swimming freely
through the open sea; and the proportion of those that
breathe air and live on land, is comparatively small. The
general structure of the animals of this group has been
already described (§ 108). Some of them form shells, wMst
others are destitute of them. The shells are composed of a
smglo piece, or are univalve, except in one tribe ;
and they have usually more or less of a spiral
formation (fig. GO). The animals of this class all
])ossess a distinct head ; and this is generally
I'urnished with eyes, as well as with tentacula.
They have often a powerful masticating ap-
paratus, and are voracious in their habits ;
Fir. oo.-siiell some of them feed upon vegetable matter, others
OF PALumsA. ^^^^^^^ animals.

1 1 .'5. The Acephalous MoUusca are divided into two groups,
— those which form shells, and those which do not. The
former are termed Conchifera, or shell-hearing animals ; and
tliis class includes all the MoUusca that form a shell composed
of two parts or valves fitted together (which shell is termed
hivalve), as well as some others whose general structure is the
same, but whose shell is formed in several pieces, or multivalve.
The two valves of a bivalve shell (fig. 61) are connected by a
hinge, where they arc united by a "ligament, which, by its
elasticity, keeps them apart wliile it holds them together. Tliis
is their usual condition when the annual is alive ; and in this
manner the water which is required for their respiration, and
also to convey their supply of food, has free access to the internal

STRUCTURE OF CONCHIFERA, OR BIVALVES.

119

parts. But when any alarm or Kritation causes tlie animal to
so by means of a muscle (sometimes

close its sliell, it does

Fig. 61.— Shell of Tridacne.

single, sometimes double), wliicli stretches across from one
valve to the other, and which, by contracting, draws them
together. Each valve is hned by an extended fold or lobe
of the mantle. In the higher tribes of the class, these lobes
are united along their edges, leaving apertures for the ingress
and egress of water (which are sometimes prolonged into
tubes, fig. 150), and another for the foot. But in the Oy&ter
and its allies, which have no foot, or a very small one, the
mantle-lobes are quite disunited. The accompanying diagram
(fig. 62) gives a general idea of the arrangement of the
organs in one of the higher acephalous Mollusca, the Mactra,
which is among those having two muscles for the drawing
together of the valves. The upper end, as represented in
this figure, is that wliich is considered as the anterior end or
front of the animal, being that nearest which the mouth lies ;
and the posterior extremity (the loAvest in the figure) is that
at which the intestinal canal terminates, and at which the
respiratory tubes are formed. Kear the anterior muscle, we
find the mouth, or entrance to the stomach ; it is furnished
with four riband-shaped tentacula, of which one is seen in
the figure ; and these seem to possess peculiar sensitiveness.
Near the mouth lie the anterior ganglia of the nervous
system, which represent the brain of higher animals ; and
these are connected by long cords with the posterior ganglion,
which Hes near the posterior muscle. The stomach, intes-
tines, and liver occupy the central portion of the cavity of
the shell ; and the intestinal tube is seen to pass backwards,

120

STRUCTURE OF COXCHIFERA OR BIVALVEri.

terminating near one of the canals or siphons, wliich also
carries out the water that has been taken in through the other
for the purposes of respiration. The figure also shows the
large fleshy foot, by which this animal can move itself along

Intestine

Mantle

(Anterior
\Ganglia

Respiratory Tubes,
Fig. 62.— Anatomy of Mactra.

the ground, or bore into sand or mud. The heart and circu-
iutrng system are less complete than in the Gasteropoda ; but
arc far higher in character (as arc most of the other parts

STRUCTURE OP TUNICATA.

121

of tlie nutritive apparatus) than the corresponding parts in
Articulated animals, in which the apparatus for locomotion so
much predominates.

114. The group"'of Acephalous Mollusks wliicli are desti-
tute of the powerJof forming a shell, includes two classes, of
Avhich one does not depart widely from the general Molluscan
iype, whilst the other presents 'so strong a general resem-
blance to Zoophytes, that until recently it has been universally
ranked with it. The first of these classes receives its name
TuNiCATA from the circumstance that the mantle, instead of
secreting a shell, is very commonly condensed into a tough
leathery or cartilaginous tunic. Many of these animals live
separately, and have the power of freely moving through the
water. Others are associated in comj^ound masses, of which,
however, the individuals are not connected by any internal
union. But others form really composite structures, like
those of Zoophytes (§ 124); each individual being able to
live by itself alone, but being connected by a stem and vessels
with the rest. The general structure of the individuals is
the same, however, in the single and in the composite
animals of this class, and may be understood from the accom-

b a

Fig. 63. — Social Ascidiax

panying lig-ure (fig. 63). The cavity of the mantle possesses,
as in the former instance, two orifices ; by one of which, 6, a
current of water is contmually entering, whilst by the other,
a, it is as continually flowing out. These orifices lead into a
large chamber, the lining of which, folded in various ways,
constitutes the gills ; and at the bottom of this chamber lie
the stomach, e, and the intestinal canal, i, which terminates
near the aperture for the exit of the Avater. All these parts

122 STRUCTURE OF TUNICATA AND POLYZOA.

are covered with cilia, by the action of wliicli a continual
stream is made to flow over the gills and to enter the stomach ;
and the minute particles which the water brings with it, and
which are adapted to serve as food, are retained and digested
in the stomach. Even these animals, fixed to one sjDot during
all but the early part of their hves, and presenting but very
slight indications of sensibility, possess a regular heart and
system of vessels ; and these vessels form part of the stem, t,
by which the compound species are connected. A single
nervous ganglion is found between the two orifices ; this
seems to receive sensory fibres from tentacula situated around
the oral orifice, and to transmit motor filaments to the mus-
cular coat wliicli underhes the outer tunic, so that any irrita-
tion applied to the former occasions a contraction of the
latter, which tends to expel the ofiending particle. — This
class is one of particular interest to the naturahst, since we
see in it the tendency to the formation of compound struc-
tures, by a process resembling that of the hudding of plants,
which is essentially characteristic of Zoophytes; this ten-
dency, however, is more fully manifested in the succeeding
class.

115. The animals forming the class Polyzoa (more com-
monly known as Bryozoa) are seldom or never found solitary ;
since, in consequence of their universal tendency to multiply
by gemmation, they form clusters or colonies of various kinds.
The body of each individual is inclosed in a sheath or " cell,"
which is somcthncs horny, sometimes calcareous ; and the
composite skeleton formed by the aggregation of these, which
has sometimes a branching or leaf-like form, but sometimes
possess the compactness of a stony coral, is known as the
" polyzoary." In their general structure the animals of this
class possess considerable analogy to the Tunicata ; but the
Molluscan type presents itself under a more degraded aspect,
no vestige of a heart or of blood-vessels being here dis-
cernible, and the general structure being so simplified as to
manifest no great degree of elevation above that of Polypes.
The typical structure of these animals may be understood
from that of the Bowerhankia (fig. 64), which is one of those
whose cells are not in contact with each other, but grow forth
at intervals from a creeping stem. The mouth, a, is situated
in tlio midst of a circle of arms fringed with cilia : these

STRUCTURE OF POLYZOA AND RADIATA.

123

arms do
the food

not serve, however, like those of polypes, to grasp
but the vibration of their cilia produces a powerful
current which brings both food and oxygen.
The mouth leads by a large funnel-shaped
oesophagus or gullet, to a gizzard, h ; in which
the particles of food that enter it are ground
down, by the action of its muscular walls and
of the tooth-like processes that line it. Below
this gizzard is the true digestive stomach, c,
around which the rudiment of a liver may
be traced ; and from tliis stomach there passes
upwards an intestinal tube, which terminates
by a distinct orifice at d, on the outside of the
circle of arms. The digestive apparatus is evi-
dently formed, therefore, upon a much higher
plan in these animals than it is in the true
polypes, which have no true anal orifice. The
Molluscan character of these animals is further
shown by the presence of a single nervous
ganglion, situated between the two orifices,
as in the Tunicata ; this acts uj^on a complex
apparatus of muscles, by which the animal
?'^orifice^of"Sti- can be either drawn into its cell or projected
tine. forth from it, with great rapidity.

116. The fourth subdivision, that of Eadiata, includes
those animals which have the parts of the body arranged in
a circular manner around a common centre, so as to present a
radiated or rayed asj)ect. This arrangement is well seen in
the common Star-fisic (fig. Q5), which has five such rays, all
having a precisely similar structure, and thus repeating each
other in every respect. The mouth of this animal is in the
centre ; and it opens into a stomach, which occupies the cen-
tral disk, and sends prolongations into the rays. The nervous
system is, in like manner, composed of a repetition of similar
parts. A plan of it is seen in fig. 6Q ; where a shows the
position of the mouth, which is surrounded by a ring or
nervous cord, having five ganglia, corresponding to the five
arms. From each of these ganglia proceeds a branch along
its arm, terminating in a little organ at its extremity, which
is believed to be an imperfectly-developed eye. 'No other
organs of special sense can be cletected in any of these ani-

Fig. 64.

BOWERBANKIA

a, cesop]iag;us ; b, s

12-i

STRUCTURE OP RADIATA.

Dials ; and it is only in a few that even these imperfect
can ho discovered. ~ ^ ^^^^ -~^ ^

In the inferior Eadiata, not the sUghtest

Fig. 65.— SuKLL OF Star Fish.

traces of a nervous system liave yet been discovered; and
it is very doubtful whether any such structure exists in
them. It is only among the higher
liadiata that any locomotive power
exists ; and this is usually so feeble
that the animals remain in the same
locality during the greater portion
of their lives. Generally speaking,
there is a period in the history of
each species, in which there is a
more active movement, that serves
to prevent the accumulation of indi-
viduals in one spot ; but this move-
ment is of a purely automatic
••haracter, rather resembling that of the " zoospores " of plants,
tlian the intentional change of place of the higher animals.

Fig. 6G. — Nervoxts System
OF Star Fish.

STRUCTURE OF RADIATA. ECHINODERMATA. 125

117. The circular arrangement of the organs of Eadiatcd
animals is a striking point of resemblance to the Vegetable
kingdom ; and it has frequently caused mistakes to be made
in regard to the Sea- Anemones and other large polypes,
which, when their mouths are open and their arms spread
out, look so much hkc the blossoms of some of the Com-
posite tribe of plants, as to have received the name of animal
fiowers. But there is yet a stronger analogy between the
lower members of the Eadiated group and the Vegetable
kingdom ; for among the former, as in the latter, we find
a union of many individuals, which are capable of existing
separately, into one compound structure, having a plant-like
form. This is the nature of the stem of Coral (fig. 76);
which is, in fact, the skeleton of one of these compound
systems, consisting of a number of polypes united by a jelly-
like flesh ; just as the woody stem of a tree is the skeleton
that supports a vast number of buds, each of which is capable
of living by itself. This aggi-egation results from the in-
definite multiplication of parts by the process of gemmation
or budding, and from the persistence of the connexion
between these parts, notwithstanding that, if separated,
they can maintain an independent existence. To the tree-
like fabrics thus produced, the name Zoophytes (animal
plants) is commonly given; and ordinary observers often
find it difficult to get rid of the idea of their vegetable
origin. The animals that formed them are, of course, fixed
to one spot during all but the earliest periods of life ; and
the amount of movement which they perform, for the pur-
pose of obtaining and secui'ing their food, is very little
greater than that which is witnessed in the Sensitive plant
and Venus's fly-trap.

118. The class of Echinodermata receives its name from
the prickly character of its covering, which is evident enough
in the Echinus or Sea- Urchin, and in the Star-fish; but there
are other animals, sufficiently resembling these in general
structure to be united in the same class, which have a body
entirely soft, — namely, the Holothurioe (fig. 67), commonly
termed Sea-Cucumbers. This class ranlis as the highest
among the Eadiata, in regard to general complexity of struc-
ture. The skeleton of the Sea-lJrchin, Star-fish, and other
animals resembling them, is a box-Uke shell or " test," formed

126

STEUCTURE OF ECHINODERMATA.

of a great nimiber of pieces, very regularly arranged and
united together (fig. 69, e); but these pieces are for the most

Fig. 67. — HoLOTiiURiA.

part only repetitions of one another ; and the different
portions have not that variety of uses which we see in higher
animals. With the exception of
the tribe of Encrinites or lily-like
animals (fig. 68), of which there are
very few now existing, but which
were very abundant in former ages,
all the animals belonging to this
class are unattached, and are capa-
ble of moving freely from place to
place. Their motions are very
sluggish, however, and are princi-
pally effected by means of an im-
mense number of minute tubular feet
(fig. 6S, c), furnished with suckers
at their extremities, which can be
projected from ahnost any part of
the body. These are seen in rows
on the under side of each arm of
the Star-fish ; they are put forth
through rows of very minute aper-
tures in the shell of the Sea-
TJrohin (commonly termed the Sea-

STRUCTURE OF ECHINODERMATA.

127

Egg) ; and they are also arranged in rows on the surface of the
body of the Holothuria, as seen in fig. 67.

119. The radiated arrangement is very evident in the
whole bodies of the Star-Fish (fig. 65), and Echinus or Sea-
Urchin (fig. 69); but in the Holothuria (fig. 67) it is nearly
confined to the parts about the mouth; which, however,
exhibit it so completely, that such an animal cannot be mis-
taken for one of the Articulated series, even though, as some-
times happens, the body is prolonged into a worm-like
form. The digestive apparatus in this class has usually a
high degree of complexity, as will be seen by the accompany-

Fig. 69.— Interior op Echinus

ing figure (fig. 69), which shows the interior of an Echinus^
whose globular shell has been sawn across its equator, so as

128 STRUCTURE OF ECHIXODERMATA AND ACALEPH^.

to alloAV of the separation of its two halves. The mouth, A-,
situated at one of the poles of the shell, is surrounded by a
very curious apparatus of jaivs and teeth (fig. 69), which
forms what is termed the "lantern;" from the mouth com-
mences the long narrow oesophagus, m, that leads to the
stomach, n, which is merely a dilated portion of the alimen-
tary tube ; the continuation of this, o, q, r, forms the intestinal
canal, which winds once round the shell, and then doubles
back and winds in the opposite direction, terminating at the
anal orifice, s, which is situated at the opposite pole. The
intestine is held in its place by a double fold of" the mem-
brane lining the shell, resembling the mesentery of higher
animals ; the blood is distributed over this membrane, to be
exposed to the aerating influence of the water admitted into
the canity of the shell ; and the water is kept in movement
by the cilia with which the membrane is clothed. Eound the
anus, s, are seen the five branching ovaries, each of wliich
discharges its contents by a distinct orifice. The circulating
apparatus is imperfect, the blood not being impelled by a
distinct heart ; still, however, it moves in great part of its
course through proper vessels, and not through mere chan-
nels in the tissue. — In the Star-fish, however, the body is
very much flattened ; and the stomach, instead of having a
separate intestinal tube with a distinct orifice, is a mere bag
with a single aperture, which serves both to take in food and
to cast forth the indigestible remains. This character will be
Ibund to prevail among all the inferior Eadiata.

120. The radiated structure is also w^ell seen in the greater
number of animals forming the group of Acaleph^, or Sea^
Nettles. Their bodies are entirely soft and jelly-hke ; contain-
ing so small a quantity of solid matter, that, when upon being
taken out of the water their fluid drains away, there is
scarcely anything left; hence they are commonly termed
Jelhj-Fish. They derive their other name of Sea-Nettles from
the stinging power wliich most of them possess. They are
formed to float freely in the water; but they do not in
general possess any means of actively propelling themselves
through it. The radiated arrangement is very regularly pre-
served in some of this group, whilst it is less evident in
others. The accompanying figure (fig. 70) represents one of
the Medusa tribe, as seen floating in water. The umbrella-

STRUCTURE OF ACALEPHiE.

129

shaped disc above contains the stomacli, which is placed in
the centre, and which opens by a single oritice or mouth,
directed downwards. Around the stomach are four chambers,
in wliich the eggs are prepared. The mouth is surrounded by
four large tentacula, which bring to it the necessary supply of

70. — Pelagia.

food; and other tentacula are seen, in this species, to be
hanging from the edge of the disc. In the edge of this disc,
the nutritious fluid, which flows in channels prolonged from
the stomach and excavated out of the soft tissues, seems to be
exposed to the influence of the surrounding water ; but
nothing like a heart or a regular circulation exists. — Eecent
discoveries in regard to the developmental history of the
Medusce and their allies, have rendered it very doubtful
whether the Acalephce should continue to take rank as a dis-
tinct class ; since many of them constitute only a jiarticular
phase in the life of the Hydroid Zoophytes (§ 124).

121. The class of Polypifera, or coral-forming animals,
commonly known as Zoophytes, includes two principal tribes,
which differ from one another in structure to such a degree as to

130

STRUCTUKE OF HYDRA.

Fig. 71.

Hydra, or Fresh-water
Polype.

require separate notice. The group of Hydrozoa, or Hydroid
Zoopliytes, so named from the little Hydra, or fresh- water

polype, which may be regarded
as its type, will be first described
on account of its near con-
nexion with the preceding. The
Hydra (fig. 71) is a solitary
polype, not at all uncommon
in ponds or other collections of
fresh water, where it is found
attached to aquatic plants, or
to floating sticks, straws, &c.,
by means of a kind of sucker
at its lower extremity, stretching
out its tentacles in search of its
food, which consists of minute
aquatic worms and insects. These
are securely laid hold of by one
or more of the tentacles, and are
drawn into the mouth, a, which
leads to the stomach or general
cavity of the body, in which they are digested, and from
the walls of which the nutritious portions are absorbed, the
portions of the food which are not capable of being digested
being cast out through the mouth.

122. The Hydra multiplies in two ways ; namely, by gem-
mation or budding, and by a proper generative process. Little
bud -like processes are developed from various parts of the
walls of the stomach, which gradually assume the form of the
parent, possessing a mouth surrounded by tentacles, and a
digestive cavity which is at first in connexion with that
of tlie parent ; the communication is gradually cut off, how-
ever, by the closure of the canal of the footstalk of the young
polype ; and ere long the footstalk itself separates, and the
young polype henceforth leads an entirely independent Hfe.
Not uniVequently, however, the young polype itself puts forth
buds before its separation ; and as many as nineteen young
IIydra>, in difrercnt stages of development, have been seen to
be thus connected with one and the same stock. Another
very curious endowment of the Hydra depends upon the same
facihty of developing the whole structure from any part of it ;

HYDRA, AND HYDROID ZOOPHYTES.

131

for into whatever number of parts its body may be cut up,
each, under favourable circumstances, can give origin to
a new and entire polype, so that tliirty or forty individuals
may thus be produced by the division of oue.

123. The proper generative process, here reduced to its
utmost simplicity, consists in the development of a germ-cell
and of sperm-cells in the substance of the w^all of the stomach,
the former being produced near the footstalk, the latter just
beneath the arms. The egg which is evolved from the former,
being fertilized by the products set free from the latter, gives
origin to a young Hydra, which resembles its parent. The
two reproductive processes, however, are perlbrmed under
very ditferent conditions ;
for whilst multiplication Ijy
gemmation is favoured by
warmth and a copious sup-
ply of food, the true gene-
rative process seems to be
brought about by a lower-
ing of the temperature, and
to have for its object the
perpetuation of the race
through the winter, the egg
being capable of enduring a
degree of cold which would
be fatal to the polype itself.

124. The group of Hy-
drozoa is for the most part
made up of composite fabrics
more or less resembling the
Gampanularia (fig. 72),
which may be likened to
a Hydra whose buds do not
detach themselves, but re-
main in connexion with the
stock that produced them ;
the whole plant-like struc-

Fig. 72.— Camj

ture, moreover, being strengthened by the consolidation of its
external layer into a horny sheath, which retains its form
after the destruction of the soft parts. Thus each comes to
consist of a stem and branches, on the sides or ends of which

k2

132 REPRODUCTION OF HYDROID ZOOPHYTES.

are a number of little cells or bell-shaped chambers, with
their mouths upwards, every one of them containing a polype
that bears a strong resemblance to the Hydra. Each of these
polypes is capable' of living independently of the rest, obtains
its nourishment by means of its own arms, and digests it in
its own stomach ; but all are connected by a canal that passes
along the stem and branches, in which a kind of cii'culation
takes place, that strongly reminds us of that of the compound
Tunicata (§ 114). This plant-like structure extends itself by
budding; new branches are formed from those previously
existing ; and these are enlarged at a certain point into cells,
in which after a time polypes make their appearance.

125. Besides the cells containing the polypes, however, we
find capsules in which are evolved buds of a different nature,
that form within themselves the generative products. These
buds in some instances assume the form of Medusae, and,
becoming detached from the stalk that put them forth, swim
about freely, living upon food obtained by themselves, and
setting free either sperm-cells or germ-cells, by the concur-
rence of whose contents eggs are formed, from which new
polype-growths arise. In otlier instances the Medusoid bodies
give forth their generative products, without ever leaving the
capsules in which they were themselves developed. And in
other cases, again, it does not seem that any Medusoid form
intervenes at all, the germ-cells and sperm-cells being evolved
from the Zoophytic structure itself But since it is also
known that even the most characteristic Medusan forms are
evolved as buds from a Zoophytic stock (Chap, xv.), and since
those comj)osite forms of Acalephse whose structure has until
lately been most obscure, turn out to be, as regards their
essential characters, Ilydrozoa organized for floating, there
seems to be no longer any sufiicient ground for ranking the
Acalephse as a separate class.

12G. It is not, however, by animals of this very simple
structure, that the massive stony fabrics are built up, which
constitute the coral islands of the Pacific Ocean, and of which
a large portion of our limestone rocks seems to be composed.
These are constructed by animals belonging to the group of
Anthozoa, and formed upon the same general plan with the
Sea- Anemone,— di plan which is higlier than that of the Hydra,
inasmuch as we find the interior uf the body containing other

STRUCTUBE OP ANTHOZOA : SEA- ANEMONE.

133

cavities around the stomach, which are destined to pre-
pare the generative products. In lig. 73, we have a repre-
sentation of the Sea- Anemone, as seen from above ; showing
its mouth in the centre, surrounded by its numerous radi-
ating tentacula ; these are often brightly coloured, and give to
the animal the appearance of a beautiful flower. In fig. 74, a
siniihir animal is represented, cut open to sho\v its interior.

Fig. 73.— Sea-Anemone, seen
from above.

Fig. 74. — Section of Sea-Anemone.
a, cavity of stomach; b, surrouuding
chambers.

The mouth is seen to open into a rounded stomach, a, which
has no other orifice outwards ; and round this stomach there is
a series of radiating membranous partitions, which divide the
space intervening between it and the outer covering of the
body into numerous chambers, b. Witliin tliese chambers, and
attached to their partition- walls, are found the bodies which
are commonly designated ovaries, but which contain sperm-
ceUs or germ-cells according to the sex. It is doubtful
whether these two products are ever formed by the same
individual, as they are in the Hydra. The Sea- Anemone does
not usually multiply itself by budding, though some species
do so ; but large numbers of young are produced from the
eggs, which are fertilized and partly developed whilst still
within the ovarian chambers, and these make their Avay into
the stomach through an aperture at its deepest point, and
finally escape by the mouth.

127. The Sea- Anemone itself, like the Hydra, is a solitary
animal, capable of shifting its place at will ; and it forms no
stony skeleton or support. But there are other animals of the
same general structure, which have the power of depositing
stony matter in the membrane of their base or foot, and in
the membranous partitions between the chambers ; and this
stony deposit forms a Coral or Madrepore, such as is shown

134

ANTHOZOA : — STONY CORALS.

in the accompanying figure (lig. 75). The particular arrange-
ment of the radiating plates of the Madrepore (shown at the
top of each stem) is the result of the
form of the soft structures by which
it was deposited; aiid wherever we
see a structure of this kind in coral,
whether upon a large or a small
scale, we may infer that it was formed
by an animal nearly allied in structure
to the Sea- Anemone. Of the stone
depositing coral- animals, a large
number are often associated in a com-
pound structure, as in fig. 76 ; this
consists of a stony tree-like stem and
branches ; but instead of the soft ani-
Fig. 75.-CAKYOPHYLLIA. ^^^^ mnUev bciug contained in its
interior, as in tlie Hydrozoa, it usually forms a kind of flesh

Fipf. 76.— Stem of Cural.

tliat clothes the surface, and connects together the diilereut

STRUCTURE OF PROTOZOA.

135

polypes ; and new branches, are formed either by the sub-
division of the polypes, or by gemmation from the connecting
substance.

128. When we pass from Zoophytes to animals of still
simpler organization, we lose all trace of definite symmetry, and
find ourselves amid forms which cannot be referred to any
particular plan of growth. These, moreover, are for the most
part distinguished by an extreme simplicity of structure ; no
such differentiation of parts exhibiting itself among them, as
is shown in the " organs " of even the simplest Zooj^hyte or
Worm. Hence they are appropriately designated Protozoa.
They may, in fact, be considered as essentially consisting of
homogeneous particles of a jelly-like substance, to which the
name of Sarcode has been given ; and the chief modification
this undergoes, consists in the consolidation of certain parts
of it by the deposit of horny, calcareous, or siliceous matter,
so as to form a skeleton. This may take place on the outer
surface only, so as to form shells very like those of MoUusks
in miniature, as we see amoiig For am in if era (fig. 78); or it
may occur in the midst of the fleshy substance, so as to
form an internal network, such as presents itself in the
Sponge. The endowments of the " sarcode " are very extra-
ordinary ; and will be best understood by observation of the
life-history of one of those simplest Protozoa, in which the
whole body consists of but a minute particle of it.

Fig. 77.— RiiizopoDA :— A, Amceba ; B, Actiiiopliiys.

129. Such an example is afforded by the Amoiha (fig. 77 a),
—a creature frequently to be met with in great abundance
in fresh and stagnant waters, vegetable infusions, &c. Its

136 RHizoroDA : — amoeba; actinophrts.

organization is so low, that there is not even that distinct
differentiation into containing and contained parts which is
necessary to constitute a cell (§ 32) ; for although the super-
ficial layer of the sarcode possesses more consistence than the
interior, it is nevertheless obvious that it has not the tenacity
of a membrane, since (as will be presently seen) it does not
oppose the passage of solid particles into the interior. How-
ever inert this creature may seem when first glanced at, its
possession of vital activity is soon made apparent by the
movements which it executes and the changes of form it
imdergoes ; these being, in fact, parts of one and the same
set of actions. For the shapeless mass puts forth one or
more finger-like prolongations, which are simply extensions
of its gelatinous substance in those particular directions ;
and a continuation of the same action, first distending the
prolongation, and then, as it were, carrying the whole body
into it, causes the entire mass to change its place. After
a short time another prolongation is put forth, either in the
same or in some different direction ; and the body is again
absorbed into it, so as to shift its place still more. It is by
means of this movement that the creature obtains its supplies
of food ; for when, in the course of its progress, it meets with
a particle appropriate for its nutriment, its gelatinous body
spreads itself over this, so as to envelope it completely ; and
the substance (sometimes animal, sometimes vegetable), thus
taken into this extemporized stomach, undergoes a sort oi
digestion therein, the nutrient material passing into the sub-
stance of the sarcode, and any indigestible portion making its
way to the surface, from some part of which it is (as it were)
finally squeezed out.

130. Many other forms of this group, which has received
the designation of Bhizopoda, ha^-e less power of movmg from
place to place, but obtain their food by a modification of the
same arrangement : of this we have an example in Actinophrys
(lig. 77 b). The body being stationary, i! ; gelatinous substance
extends itself into long filaments, ten;.-, I pseudopodia : these
ollcn divide themselves again like the roots of a tree (whence
the designation of the group), so as to form threads of ex-
treme tenuity; and sometimes these threads meet again and
coalesce, so as to form a sort of irregular network. When any
niinutc animal or vegetable organism happens to come in contact

RHIZOrODA : FORAMINIFERA. 137

with one of these thrccads, it is usually held by adhesion to
it, and the filament forthwith begins to retract itself; as it
shortens, the surrounding filaments also apply themselves to
the captive particle, bending their points together, so as gra-
dually to inclose it, and themselves retracting until the prey
is brought to the surftice of the body ; and the substance of
the threads being itself drawn into that of the body, the
entrapped particle is embedded along with this, and under-
goes digestion in the surrounding sarcode, any indigestible
particle being subsequently extruded from the surface of the
body, just as in the Amoeba. The reproduction of these
creatures, so far as is yet known, is effected by self-division,
like that of the Infusoria (§ 135); but there is reason to
believe that a "conjugation," or reunion of two individuals,
sometimes occurs, and that this is to be looked on as repre-
senting the sexual propagation of higher animals.

Fig. 78. — FORAMINIFERA.

A, Oolina; B, C, Nodosaria; D, rristellaria ; E, Polystomella ; F, Dendritina,
G, Globigerina; l\, Textularia; I, Quinqueloculina.

131. This Ehizopod type of animal hfe is manifested in
two ga^oups of great interest, which are characterised by
the possession of hard shells, formed by the consolidation
of the external layer of sarcode. The Foraminifera have
calcareous shells, which often bear a strong resemblance to
those of Nautili, &c. in miniature (fig. 78), but which really
have an entirely different relation to the animals that form
them. For whilst the Nautilus occupies only the last or
outer chamber of its shell, the chambers previously formed

138

FORAMINIFERA AND TOLYCYSTINA.

beiii'-' empty and deserted, each cliainber of tlie B,otalia, or
any other i^oraminiferous sliell, is occupied by a segment of
sarcode, which is to a great degree independent of the rest,
and is only connected with those on either side of it by
dehcate threads of the same substance ; and the extension of
the shell is due to the formation of an additional segment of
sarcode on the outside of the last-formed chamber. Each
segment has usually the power of gutting forth its own
"pseudopodia" through minute apertures in the shell, and
thus of drawing in its own nourishment through these ; but
even when (as sometimes happens) these food-collecting
threads are put forth from the last chamber alone, the nutri-
ment there obtained is transmitted to the segments within by
percolation through the substance of the sarcode, and not
through any tubular canal. — The accumulation of the shells
of Foraminifcra in some parts of the existing sea-bottom is
very remarkable ; and similar accumulations in past ages
have formed no unimportant part of the crust of the earth —
a large part of the Chalk-formation having had its origin in
theni, as well as nearly the whole of the Nummulitic limestone
by which it was succeeded.

132. But animals whose essen- a b

tial structure seems to be nearly
the same, may form siliceous in-
stead of calcareous shells ; and
thus are produced those beautiful
organisms, known under the
name of Polycystina (fig. 79),
which arc occasionally found in
the existing seas, but whose re-
mains are met with under a far
greater variety of forms in certain
of the newer marine deposits.
Tliere is not in these the same
tendency to form composite
sti-uctures by the multiplication
of segments, as in the Foraminifera ; but the complication of
the individual form is often much greater. Yet, however
complex the form, the essential composition of these crea-
tures seems to retain the same attrilmte of simplicity, which
cannot bu conceived capable of further reduction.

Fig. Id. — Polycystina.
A, Podocyrtis ; B, Rhopalocanium.

INFUSORY ANIMALCULES.

139

133. The Animalcules to which the name of Infusoria
may be properly restrictcil (the Eotifera, or Wlieel- Animal-
cules, § 105, whose organization is much higher, together with
many organisms whose true nature is vegetable, being ex-
cluded), present an advance upon the simplicity of the Rliizo-
poda in this, — that whilst their bodies consist for the most
part of sarcode, and present scarcely anything that can be
termed a distinction of organs, their external surface is con-
densed into a membrane too firm to admit either of indehnite
extension into pseudopodia, or of the passage of alunentary
particles through it ; and consequently the form of the body,
although not insusceptible of being temporarily changed by
pressure, possesses a considerable degree of constancy for each
species (fig. 80). A mouth, or definite aperture for the in-
gestion of food, is provided; with an additional orifice in
some instances, through which indigestible or effete matters
may be discharged from the interior. Into this moutli, ali-

Infusory Animalcules.

I. Monads ; ii. Trachelis anas; iii. Enchelis, discharging fecal matter, iv. Para-
ma'cium ; v. Kolpoda ; vi, Trachelis fasciolarls.

mentary particles are drawn by the agency of the cilia with
which some part of the surface of the body is provided ;
these cilia being always so disposed as to serve at the same
time for the general locomotion of the animalcule, and for the
production of currents that shall bring food to its interior.

134. Although most Infusoria move freely through the
water in which they live, yet certain kinds of them attach
themselves by footstalks to marine plants or other floating
bodies, during at least a part of their lives ; and in this con-
dition bear no slight resemblance to Zoophytes, tho^igh of far
simpler organization. It is in these sessile forms that the
agency of the cilia in creating currents which l)ring food to

140 INFUSORIA. — PORIFERA OR SPONGES.

the mouth, becomes most conspicuous. The alimentary par-
ticles introduced into the mouth commonly have to pass
down a short canal before they enter the general cavity of
the body ; and within this cavity a number of minute par-
ticles are commonly aggregated into a sort of little pellet, as
may be Avell seen when Infusoria are fed with carmine or
indigo. One after another of these pellets being thus intro-
duced into the interior, which is occupied by a soft sarcode,
each seems to push onwards its predecessors ; and a kind of
circulation is thus occasioned in the contents of the cavity.
The pellets that first entered make their way out after a time
(their nutritive materials having been yielded up), generally
by a distinct anal orifice, sometimes, however, by any jiart of
the surface indifferently, and sometimes by the mouth.

1 35. The multiplication of Infusoria ordinarily takes place
by spontaneous fission, precisely after the manner of the
multiplication of ordinary cells (§ 33). This process, under
favourable circumstances, may be performed with such
rapidity, that, according to the computation of Ehrenberg, no
fewer than 268 millions might be produced in a month by
the repeated subdivision of a single Paramecium. Sometimes,
instead of undergoing subdivision into two equal parts, the
Animalcule puts forth a bud, which gradually increases, and
then detaches itself from the parent stock. Whether any-
thing equivalent to the sexual generation of higher animals
occurs among Infusoria, is yet uncertain ; but recent re-
searches aftbrd a probability in the affirmative.

13G. In the tribe of Porifera, or SjmiigeSj we seem to
have the connecting link between Protozoa and Zoophytes.
For their animality does not lie so much in the individual
particles, as in those aggregations which begin to shadow
forth that distinction into organs which is carried out more
completely among Zoophytes : and there is a large section of
the last-named group, in which the polypes are connected
together, not by a regular stony or horny stem, but by a
sponge-like mass ; while the extension of the flibric is provided
for -by the budding out of this spongy portion of it, the
orifices of whose canals after a time become fu;'nished mth
polype-mouths. The true Sponge (fig. 81) consists of a fleshy
sub.stiince, composed of an aggregation of particles of sarcode,
supjM.rtcd upon a skeleton which usually consists of a net-

PORIFERA OR SPONGES. 141

work of horny fibres, strengthened by spicules of mineral
matter, sometimes calcareous, but more commonly siliceous.
The entire mass is traversed by a great number of canals,
which may be said to commence in the small pores upon its
surface, and which discharge themselves into the wide canals
that terminate in the large orifices, or vents, that usually pro-
ject more or less from the surface
of the Sponge. Through this sys-
tem of canals, there is continually
taking place, during the living state
of the animal, a circulation of water,
which is drawn in from without
through the minute pores, then
passes into the large canals, and is
ejected in a constant stream from
the vents. The immediate cause of this movement seems
to lie in the vibration of cilia so extremely minute that their
existence can only be detected by the most careful micro-
scopic examination. Its purpose is evidently to convey to
the animal the nutriment which it requires, and to carry off
the matter which it has to reject. No distinct indications of
sensation, or of power of locomotion, have been seen in the
Sponge : but changes in the form of its projecting vents
may be seen to take place from time to time, if it be watched
sufficiently long.

137. The reproduction of the Sponge is commonly effected
by the budding forth of little particles of sarcode, from the
layer which lines the larger canals ; these become furnished
with cilia, and, when detached and carried out by the current
that issues from the vents, swim freely about for some time ;
so as, before fixing themselves and beginning to develope
into Sponges, to spread the race through distant localities.
But it appears that Sponges are also reproduced by a true
sexual process ; " sperm-cells " and " germ-cells " being pro-
duced (as in the Hydra, § 123) in different parts of the
organism, and a true embryo taking its origin in the action
of the contents of the former upon those of the latter.

142 NATURE AND SOURCES OF ANIMAL FOOD.

138. We thus conclude our general survey of the Animal
Kingdom ; which, it is hoped, will be found to answer the
puj'pose for which it was designed, — that of giving such an
amount of preparatory knowledge respecting the principal
types of animal structure, as may enable even the beginner to
comprehend what will hereafter be stated of their physiological
actions. It has not been attempted to observe any proportion
in the notice of these several types ; the higher forms having
been slightly passed over, because the details of their vital
phenomena wdll constitute the principal subject of the follow-
ing pages ; whilst some among the lower have been more
fully treated, because the ordinary reader cannot be expected
to liave even that outline-acquaintance with their nature and
actions, which he can scarcely help possessing in the case of
animals that are familiar to him.

CHAPTEE III.

NATURE AND SOURCES OF ANIMAL TODD.

139. Before we examine the nature of the process by
which the food of animals is prepared for absorption into
their bodies, it will be desirable to consider the characters of
the aliment itself, and the purposes to which it is to be appro-
priated. The term food or aliment may be applied to all
those substances which, when introduced into the living
body, serve as materials for its growth, or for the repair of
the losses which it is continually sustaining (§ 55). When
animals are deprived of these materials, we see their bodies
]irogressively diminishing in bulk, their strength decreases,
and death at last takes place, after sufferings more or less
prolonged. In warm-blooded animals, however, a yet more
urgent demand for food is created by the requirements of the
licat-producing process ; and many substances are fitted to
supply this, which cannot serve for the nourishment of the
tissues.

UO. The demand of the body for food is made known by
a peculiar sensation, which has its seat in the stomach, namely,
hunger. It is increased by mental and bodily exercise, and

NATURE AND SOURCES OF ANIMAL FOOD, 143

by everything which augments the general energy of the
system ; whilst, on the contrary, everytliing which tends to
retard the operations of life, such as bodily and mental inac-
tivity, sleep, or depression of spirits, tends also to render the
demand for food less imperious. Thus, cold-blooded animals,
particularly Eeptiles, can sustain a very prolonged abstinence,
when the general activity of their functions is kept down by
a low temperature; and hybernating Mammals, which pass
the winter in a state of torpidity, require no food during the
continuance of their lethargy. But with this exception,
warm-blooded animals require a constant supply of nutriment,
not merely for the maintenance of their proper heat, but also
for the repair of the waste resulting from that continuous
activity which the uniform temperature of their own bodies
enables them to keep up. This is the case with Man and
the MammaUa generally, and still more with Birds, whose
temperature is higher, and whose movements are more active
and energetic. It is also more the case with young animals
than with adults ; since in the former the changes in the
tissues, in consequence of the increase they are undergoing,
take place with much more rapidity than in the latter, the
bulk of whose bodies remains stationary. Hence, if children,
young persons, and adults be shut up together, and deprived
of food, the younger will usually perish first, and the adults
will survive the longest. The Itahan poet Dante has given
a terrible picture of such an occurrence, in his history of the
imprisonment of Count Ugolino and his children.

141. The difference in the demand for food between the
young growing animal and that which has arrived at maturity,
is very remarkable in the case of Insects. There are no
animals more voracious than the larva or caterpillar; and
there are none that can sustain abstinence, with little dimi-
nution of their activity, better than the imago or perfect
insect. The larva3 of the Flesh-fly, produced from the eggs
laid in carrion, are said to increase in weight 200 times in
the course of 24 hours ; and theu^ voracity is so great as to
have caused Linnceus to assert, that three individuals and
their immediate progeny (each female giving birth to at least
20,000 young, and a few days sufficing for the production of
a third generation) would devour the carcase of a horse with
greater celerity than a lion. The larva of the Silk-worm

144 NATURE AND SOURCES OF ANIMAL FOOD.

weighs, ^vllCIl hatched, about 1-lOOth of a grain ; previously
to ils first metamorphosis it increases to 95 grains, or 9,500
times its original weight. The comparative weight of the
full-grown caterpillar of the Goat-moth to that of the young
one just crept out of the egg, is said to be as 72,000 to 1.
For this enormous increase a very constant supply of material
is necessary, and many larva3 perish if left unsupplied with
food for a single day. On the other hand, a black beetle
(Melasoma) has been known to live seven montlis, pinned
down to a board ; and another beetle (Scarabseus) has been
kept three years without food,— and this ^vithout manifesting
any inconvenience or loss of activit}^ There are many perfect
insects which never eat after their last change, but die as soon
as they have performed their part in the propagation of the race.

142. The nature of the food of animals is as various as the
conformation of their different tribes. It always consists,
however, of substances that have previously undergone organ-
ization. There are some apparent exceptions to this, in the
case of animals which seem to derive their support, in part at
least, from mineral matter. Thus, the Spatangus (an animal
allied to the Echinus, § 119) hlls its stomach with sand ; but
it really derives its nourishment from the minute animals
which this contains. The Earthworm and some kinds of
Beetles are known to swallow earth ; but only to obtain from
it the remains of vegetable matter that are mixed with it.
By some races of Man, too, what seems to be mineral matter
is mixed with other articles of food, and is said to be nutri-
tious ; this may be beneficial, in part, by giving bulk to the
aliment, and thus exciting the action of the stomach (§ 205);
but it has been found, in one case at least, that the supposed
earth consists of the remains of animalcules, and contains no
inconsiderable portion of organic matter.

143. There arc many instances in which, no obvious sup-
plies of food bemg afforded, the mode of sustenance is obscure ;
and it has been frequently supposed that, in such cases, the
animals are sustained by air and water alone. But it will
always be foTind that, wliere food is taken in no other way,
a supply of tlic microscopic forms of animal or vegetable life
is introduced by ciliary action (§ 45); and it is on these,
indeed, that a large proportion of the lower forms of aquatic
annuals depend entirely for their sui)port.

NATURE OF THE FOOD OF ANIMALS. 145

144. The first division of aliments is naturally into those
which are derived from the Animal and Vegetable kingdoms
respectively. Wherever plants exist, we find animals adapted
to make use of the nutritious products they furnish, and to
restrain their luxuriance within due limits. Thus among
Mammals, the Dugong (an animal having the general form
and structure of the whale, but adapted to a vegetable diet)
browses upon the sea-weeds that grow beneath the surface
of the tropical ocean ; the Hippopotamus roots up with his
tusks the plants growing in the beds of the African rivers,
and fills his huge paunch, not only with these, but with the
decaying vegetable matter which he finds in the same situa-
tion ; the Antelopes, Deer, Oxen, and other Euminants, crop
the herbage of the plains and meadows ; the Giraffe is enabled
by his enormous height to feed upon the tender shoots which
are above the reach of ordinary quadrupeds ; the Sloths, living
entirely in trees, and hanging from their branches, strip them
completely of their leaves ; the Squirrels extract the kernels
of the hard nuts and seeds ; the IMonkeys devour the soft
pulpy fruits ; the Boar grubs up the roots and seeds buried
under the soil ; the Eeindeer subsists during a large part of
the year upon a lichen that grows beneath the snow ; and
the Chamois finds a sufficient supply in the scanty vegetation
of Alpine heights, Not less is this the case among Birds ;
but in the classes of Reptiles and Fishes, the number of
vegetable-feeders, and consequently the variety of their food,
is much less.

145. Among Insects, a very large proportion derive their
food entirely from Plants, and many from particular tribes of
plants only; so that, if from any cause these should fail, the
race may for a time disappear. There is probably not a
species of plant which does not furnish nutriment for one or
more tribes of insects, either in their larva state or their per-
fect condition ; and in this manner it is prevented from mul-
tiplying to the exclusion of others. Thus, on the Oak no less
than two hundred kinds of caterpillars have been estimated
to feed ; and the Nettle, which scarcely any beast will touch,
supports fifty different species of insects, — but for which
check it would speedily annihilate all the plants in its neigh-
bourhood. The habits and economy of the different races
existing on the same plant, are as various as their structure.

L

146 VORACITY OF INSECTS.

Some fcGcl only upon the outside of the leaves ; some upon
the internal tissue ; others upon the flowers or on the fruit ;
a few wiil cat nothing but the bark ; while many derive their
nourishment only from the woody substance of the trunk.

146. The excessive multiplication of certain tribes of
Insects has sometimes had the effect of devastating an entire
coiuitry. Tims the " plague of locusts " is not unfrequently
repeated in tropical countries, and is dreaded by the inhabi-
tants even more than an earthquake. These insects are of
such extreme voracity that no gi-een thing escapes them;
and when their nimibers are so increased that they fly in
masses which look like dark clouds, and cover the ground
where they aUght for miles together, it may be easily con-
ceived that the devastation they create must produce incal-
culable injury. The north of Africa and the west of Asia are
the countries most infested by these pests. It is related by
Augiistin, that a plague, induced partly by the famine they
had created, and partly by the stench occasioned by theii'
dead bodies, carried off 800,000 inhabitants from the kingdom
of i!^umidia and the adjacent parts. They occasionally attack
the south of Europe. It is recorded that Italy was devastated
by them in the year 591 ; and that a prodigious number both
of men and beasts perished from similar causes, — no less
than 30,000 persons in the kingdom of Venice alone. These
tremendous swarms usually advance towards the sea ; and
being there checked, and having completely exhausted the
country behind them, they themselves die of famine, or are
blo^vn into the sea by a gale. In 1784 and 1797, they de-
vastated Southern Africa ; and it is stated by Mr. Barrow
(in his Travels in that country) that they covered a surface
of 2,000 square miles ; that, when cast into the sea by a
strong wind from the north-east, and washed upon the beach,
they formed a line fifty miles long, and produced a barrier
along the coast three or four feet high ; and that, when the
wmd again changed, the stench created by the putrefaction
of their bodies was perceived at a distance of 150 miles
inland. A similar event occurred in the Earbary States in
1790, and was followed, as in the other cases, by a plague.

147. We have occasionally an example of similar devasta-
tion in our own country, thougli on a smaller scale. Thus,
a few years ago, the turnip-crops of some parts of England

VORACITY OF INSECTS. 147

were almost entirely destroyed by the larva' of an insect
called the " turnip-fly." The parent insects were seen buzzing
over the fields, and depositing their eggs in the plants, which
they do not themselves employ as food ; and in a few days all
the soft portions of the leaves were destroyed, and nothing
but the skeletons and stalks were left. — Some kinds of timber
occasionally suffer to no less an extent from the devastations
of insects, whose operations are confined to the wood, and do
not manifest themselves externally, until the tree is seen to
languish and at last to die. The pine-forests of the Hartz
mountains in Germany have been several times almost de-
stroyed by the ravages of a single species of beetle, less than a
quarter of an inch in length. The eggs are deposited beneath
the bark ; and the larvae, when hatched, devour the sap-
wood and inner bark (the parts most concerned in the func-
tions of vegetation) in their neighbourhood. It was estimated
that, in the year 1783, a million and a half of pines were
destroyed by tliis insect in the Hartz alone ; and other forests
in Germany were suffering at the same time. The wonder is
increased, when it is stated that as many as 80,000 larvse are
sometimes found on a single tree.

148. But every class in the Animal Kingdom has its car-
nivorous tribes, which are adapted to restrain the too rapid
increase of the vegetable-feeders (by which a scarcity of their
food would soon be created), or to remove from the earth the
decomposing bodies that might otherwise be a source of dis-
ease or annoyance. The herbivorous races, being for the most
part very prolific, would very rapidly increase to such an
extent as to produce an absolute famine, if not kept in check
by the races appointed to limit their multiphcation. Thus,
the myriads of Insects which find their subsistence in our
forest-trees, if allowed to increase without restraint, w^ould
soon destroy the hfe that supports them, and must then all
perish together ; but another tribe (that of the insectivorous
Birds, as the woodpecker) is adapted to derive its .subsistence
from them, and thus to keep their numbers wdtliin salutary
bounds. Their occasional multiplication to the enoimous
extent mentioned in the preceding paragraphs, is probably
due in general to the absence of the races that should keep
them in check. This may occur from accidental causes, or
may be produced by the interference of Man. Thus, a set of

l2

148 BALANCE AMONG DIFFERENT RACES.

ignorant farmers have imagined that a neighbouring rookery
was injurious to them, because they saw the rooks hovering
over the newly-sown corn-fields, and seeming to pick the
grains out of the ground ; and ha^dng extirpated the rookery,
they have found in the course of a year or two that they
have done themselves an immense injury, — the roots of their
corn and grasses being devoured by the grubs of cockchafers
and other insects, the multiplication of which was before
prevented by the rooks, whose natural food they are.

149. On the other hand, by an intelligent application of
this principle, the excessive multiplication of insects has been
prevented where it had already commenced. Thus, no means
of extirpating the larvse of the turnip-fiy was found so suc-
cessful, as turning into the fields a number of ducks, which
quickly removed them from the plants. And in the island of
Mauritius, the increase of locusts, which had been accidentally
introduced there, and which were becoming quite a pest, was
checked by the introduction from India of a species of bird,
the grakle, which feeds upon them.

150. Of the carnivorous tribes themselves, however, the
increase might be so great as to destroy all the sources of their
food, were it not that they are kept in check by others, larger
and more powerful than themselves, which, not being prolific,
are not likely ever to gain too great a power. Thus, among
birds, the eagles, falcons, and hawks rear only two or three
young every year, whilst many of the smaller birds produce
and bring up four or five times that number. — The following
is a curious instance of the system of checks and counter-
checks, by which the "balance of power" is maintained
amongst the different races. A particular species of moth
having the fir-cone assigned to it for the deposition of its eggs,
the young caterpillars, coming out of the shell, consume the
cone and superfluous seed ; but, lest the destruction should
be too great, another insect of the ichneumon kind lays its eggs
in the caterpillar, inserting its long tail in the openings of
the cone until it touches the included insect, its own body
being too large to enter. Thus it fixes upon the caterpillar
its minute egg, which, when hatched, destroys it.

151. The peculiarity of the agency of Insects, in the
economy of nature, has been justly remarked to consist in their
power of very rapid multiplication, in order to accomplish a

VARIATIONS IN POWER OP ABSTINENCE. 149

certain object, and then in their as rapidly dying off. In tliis re-
spect they resemble the Fungi among plants. ( Botany, § 789.)

152. There are great variations in the degree of power
possessed by animals of different species to sustain abstinence
from food, which appear to be related to their respective
habits of life ; such as most easily obtain a constant supply
of food being immediately dependent upon it, and vice versd.
Thus, among the larv?e of Insects, those that feed upon vege-
tables or dead animal matter (in the neighbourhood of which
theii' eggs are usually deposited by the parent) speedily die if
placed out of reach of their aliment ; whilst those that lie in
wait for living prey, the supply of which is uncertain, are able
to endure a protracted abstinence, even to the extent of ten
weeks, without injury. Again, carnivorous Birds and Mam-
mals are generally able to exist for some time without food ;
their natural habits leading them to glut themselves upon the
carcase of the animal they have destroyed, in such a manner
as to prevent them from requiring any new supply for some
time : thus the wild cat has been kept twenty days without
food, the dog has lived for thirty-six days in the same circum-
stances, and the eagle for a similar period. But some herbi-
vorous animals, such as the camel and the antelope, whose
habits are such as to keep them out of the reach of food for
several days together, are able to endure a similar abstinence ;
whilst among the insectivorous Mammals, which naturally
take food often, and but little at a time, the powxr of absti-
nence is much less, — the mole, for instance, perishing in
confinement, if not fed once a day, or even more frequently.

153. We have next to consider the different substances
used as food, in regard to their chemical composition ; and to
inquire for what purposes in the nutrition of the body they are
respectively destinecl. The Vegetable tissues are chiefly made
up of the three components, oxygen, hydrogen, and carbon ;
the oxygen and hydrogen having the same proportions as in
water. Their composition being thus nearly the same as that
of starch, gum, and sugar (into which, indeed, they may for
the most part be converted by a simple chemical process),
alimentary substances of this kind form a natural group to
which we may give the name of Saccharine (sugary). — But in
many vegetable substances used as food, there is a considerable
quantity of oily matter, stored up in cells ; and the same kind

loO ORGANIC CONSTITUENTS OP ANIMAL FOOD.

of matter constitutes the principal part of the fat of animals.
Of these oily and fatty matters, also, the chemical elements,
oxygen, hydrogen, and carbon, are the only ingredients ; but
they are combined in i)roportions different from the last, the
two latter predominating considerably. Hence they consti-
tute another group of alimentary materials, to which the
term Oleaginous maybe given. — Lastly, most Vegetables con-
tain, in greater or less amount, certain compounds M-hich
consist of the four elements, oxygen, hydrogen, carbon, and
nitrogen, of which the animal tissues are composed. These
compounds exist most largely in the corn-grains, and also in
the seeds of the pea and bean tribe ; but there are few vege-
table substances used as food by animals, that do not contain
them in some small amount. The gluten of wheat, the legu-
min of peas, and other vegetable substances of this kind,
together v/ith the flesh of animals, the composition of which
(§ 13) is identical with theirs, are united into a third group,
10 which the name Albuminous is given. — "We cannot pro-
perly include in this group, however, the gelatinous portions
of the animal tissues, which exist largely in gristle, bone, the
skin, and other ])arts ; because gelatin (the substance that
forms glue), though it agrees with albumen in being made up
of WiQj'our ingredients just named, differs from it extremely
in the proportions of tliose elements (§ 19) ; so that, although
there is good reason to believe that gelatin may be formed out
of albumen, it does not seem that any albuminous compound
can be formed out of gelatin. Hence we must consider the
gelatinous compounds separately.

154. Of these four groups, the last two are distinguished as
azotized compounds, or substances that contain azote or nitro-
gen ; whilst the first two are spoken of as non-asotizecl, being
destitute of this element. The distinction is a very important
one ; and must be kept steadily in view in considering the ulti-
mate destination of each kind of food. It is obvious from what
has'been already stated as to the composition of the animal tis-
sues (§§13—21), that azotized compounds must supply the chief
materials for their nutrition and re-formation. The non-azotized
substances must be for the most part destined, unless converted
mto azotized compounds within the living body, either to be
sniiply deposited in its interstices, or to be thrown ofl' from it
again witliout ever actually forming part of its organised
structure.

DESTINATION OF NON-AZOTIZED ALIMENTS. 151

155. Now, in regard to the non-azotized, or the saccharhie
and oleaginous groups of aUmentary substances, it aj^pears to
be an established fact, that none of the higher animals can be
permanently supported upon them alone. Thus, dogs that
have been fed on sugar and starch only, do not survive long ;
and it is evident, before their death, that their tissues are
gradually undergoing decay. It has been thought that such
results might be partly explained upon the fact, that animals
fed upon one simple substance soon become disgusted with it,
and will even refuse it altogether ; but the experiments have
been repeated with a combination of various non-azotized sub-
stances, and the same result has occurred. Still it is too much
to afiirm, as some have done, that these substances do not con-
tribute in any degree to the nutrition of the animal tissues;
since there is ample evidence that the presence of fatty matter in
the blood is a condition essential to the production of newly
forming tissue ; and we find that either oleaginous substances,
or substances belonging to the saccharine group which can be
readily converted into fat within the bod}^, constitute an im-
portant part of the food of Man, and of animals generally.^

156. That such a conversion can take place, has been de-
monstrated by experiments carefully conducted upon bees,
which have been found to generate wax when fed upon sugar
only ; and also upon cows, which give off in their milk so
much larger a quantity of butter than can be produced at the
expense of the fat contained in their food, that there is no
other mode of accounting for its 2)resence, than by regarding
it as generated from the starchy portion of their diet. And
the fattening power of starchy and saccharine articles of diet
is well known to breeders of cattle ; though the articles which
contain oily matter in addition seem to possess a higher value
in this respect.

157. But if these non-azotized compounds, which exist so
largely in the food of herbivorous animals, are not destined
to form any other permanent part of the anunal organism
than the oleaginous contents of the fat-cells (§ 4G), the ques-
tion again arises, — what becomes of them 1 It is not enough

^ The value of cod-liver oil, which is now so extensively used in the
treatment of diseases of imperfect nutrition, seems to depend upon the
readiness with which it can be digested and assimilated, so as to furnish
the supply of fat required by the formative processes.

152 DESTINATION OF NON-AZOTIZED ALIMENTS.

to say that tlicy are deposited as fat ; since it is only when
a largo quantity of them is taken in, that there is any in-
crease in the ijuantity of fat already in the body. We shall
hereafter see that they are used up in the process of respira-
tion, one great object of which is, to produce a certain amount
of heat, sufficient to keep up the temperature of the body, in
^^•arm-blooded animals, to a high standard. We might almost
say mth truth, that a great part of the oleaginous and sac-
charine principles is burned within the body, for this pur-
|)Ose. The process will be hereafter considered more in
detail (§§ 412, 413) ; and at j)resent we need only stop to
remark upon the adaptation between the food provided for
animals in different climates, and the amount of heat which
it is necessary for them to produce. Thus the bears, and
seals, and whales, from which the Esquimaux and the Green-
lander derive their support, liaA^e an enormous quantity of
fat in their massive bodies : this fat is as much esteemed as
an article of food amongst these people, as it would be thought
repulsive by the inhabitants of southern climates ; and by the
large quantity of it they consume, they are able to support
the bitterness of an Arctic winter, without appearing to suffer
more from the extreme cold than do the residents in more
temperate climes during their winter. On the other hand,
the antelopes, deer, and wild cattle, which form a large pro-
l)ortion of the animal food of savage or half-cultivated nations
inhabiting tropical regions, possess very little fat; and the
comparatively small supply of carbon and hydrogen, of which
the combustion is required to keep up the bodily temperature
of the inhabitants of those regions, is derived from i\ie flesh of
these animals, in the manner that will be presently explained.
158. The application of the substances forming the albu-
minous group, to the support of the animal body, by affording
the materials for the nutrition and re-formation of its tissues,
needs little explanation. The proportions of the four ingre-
dients of which they are all composed, are so nearly the same,
that no essential difference appears to exist among them ; and
it is a matter of little consequence, except as far as the gra-
tification of the palate is concerned, whether we feed upon
the llcsh of animals (syntonin, § IG), upon the white of agg
(albumen, § 1 ;^), the curd of milk (casein, § 15), the grain of
wheat (gluten), or the seed of the pea (legumin). All these

DESTINATION OF NON-AZOTIZED ALIMENTS. 153

substances are reduced in the stomach to the form of albumen ;
which is the raw material out of which the various fabrics of
the body are constructed. But the rule holds good with re-
gard to these also, that by being made to feed constantly on
the same substance, — boiled white of q,^^^ for instance, or meat
deprived of the principle that gives it flavour, — an animal may
be effectually starved ; its disgust at the food being such, that
even if it be swallowed it is not digested. It is very interest-
ing to remark that, in the only instance in which Nature has
provided a single article of food for the support of the animal
body, she has mingled articles from all the three preceding
groups. This is the case in Milk ; which contains a consider-
able quantity of the albuminous substance, casein., that forms
its curd ; a good deal of oily matter, the butter ; and no in-
considerable amount of sugar, which is dissolved in the whey.
The proportions of these vary in different Mammalia, being-
related as it would seem to the habits of the young animal
thus sustained, while they depend in part upon the nature of
the food supplied to the animal that forms the milk ; but the
three substances are thus combined in every instance.

159. But although the greater part of the organised tis-
sues of animals have a composition nearly allied to that of
albumen, many of them also contain a large quantity of
gelatin (§ 19). It seems certain that this gelatin may be pro-
duced out of albuminous substances ; since in animals that
are supported on these alone, the nutrition of the gelatinous
tissues does net seem to be impaired. But it appears equally
certain, that gelatin cannot be applied to the nutrition of the
albuminous tissues. Many series of experunents have been
made on this subject, with a view of determining how far
gelatin-soup made from crushed bones (such as that which
long constituted a principal article of diet in the hospitals of
Paris) is adequate for the support of the body in health.
The result of these has been uniformly the same, — namely,
that although gelatin may be advantageously mixed with
albumen, fibrin, gluten, &c., and those other ingredients which
exist in meat-soup and bread, yet that, when taken alone, it
has little (if any) more power of sustaining life, than sugar
or starch possesses. Although it might have been thought
hkely that gelatin employed as food might be applied witliin
the body to the nutrition of its gelatinous tissues, yet there

1,')4 SOURCES OF DEMAND FOR ALIMENT.

is strong reason to believe that these, like the albuminous,
are formed at the expense of the albuminous matter of the
blood, and that gelatin thus introduced undergoes a rapid
decomposition, yielding up a considerable part of its carbon
and hydrogen to the combustive process, which is the only
function to wliich it affords any substantial imhidum. Con-
sequently the current idea regarding the nutritive value of
jdliea of various kinds, has little or no real foundation.

IGO. It has been already stated (§ ^^) that all the living
tissues of the body are continually undergoing a sort of death
and decay ; and that they do this the more rapidly, in pro-
])ortion as they are called upon for the discharge of their
functions. The need of material capable of rejDlacing that
Avhich has been lost, is consequently the chief source of the
constant demand for aliment. Even in young, actively
growing animals, the quantity required for the increase of
their bodies constitutes but a very small proportion of that
which is taken in ; of the remainder, a part is at once re-
jected as indigestible ; and the rest is appro j)riated to the
ix'pair of the loade which is continually going on. This wast^e
is much greater in young animals than in adults ; for all their
vital i)rocesses are more actively and energetically performed :
their movements are quicker in proportion to their size ; and
injuries are more speedily repaired. To remove the products
of this decomposition is the special object of the various pro-
cesses of excretion ; and among these, the resjnration, by
which a large quantity of carbon and hydi-ogen is carried
off in the form of carbonic acid and water, is of the most
constant importance, on account of the heat which it thus
enables the animal body to maintain. This temperature, in
Carnivorous animals, appears to be sufficiently kept up by
the combustion of the carbon and hydrogen set free by the
decay (or metamorphosis, as it may be termed) of their tis-
sues ; but this combustion goes on with much more rapidity,
in consequence of their almost unceasing acti^dty, than it does
in the Herbivorous animals, which lead comparatively inac-
tive lives. Every one who has visited a menagerie must have
noticed the continual restlessness of the Tigers, Leopards,
Hyenas, &c., which keep pacing from one end of their narrow
cages to the other ; and it would seem as if this restlessness
were a natural instinct, impelling them to use muscular exer-

NUTRITION OF CARNIVOROUS ANIMALS. 155

tion sufficient for the metamorphosis of an adequate amount
of tissue, that enough carbon and hydrogen may be set free
for the support of the respiratory process. And we see a cor-
responding activity in the Human hunters of the swift-footed
antelope and agile deer, which answers a similar purpose ; and
which is remarkably contrasted with the stupid inertness of
the inhabitants of the frigid zone, that is only occasionally
interrupted by the necessity of securing the supplies of food
afforded by the massive tenants of their seas.

161. The nutrition of the Carnivorous races may, then, be
thus described. The bodies of the animals upon which they
feed, contain flesh, fat, &c., in nearly the same proportion as
their own ; and all, or nearly all, the aliment they consume,
goes to supply the waste in the fabric of their own bodies,
being converted into its various forms of tissue. After having
remained in this condition for a certain tune, varying ac-
cording to the use that is made of them, these tissues un-
dergo another metamorjDhosis, which ends in restoring them
to the condition of inorganic matter ; and thus give back to
the mineral world the materials wliich were drawn from it by
plants. Of these materials, part are burned off, as it were,
within the body, by union with the oxygen of the air taken
in through the lungs, from which organs they are discharged
in the form of carbonic acid and water : the remainder are
carried off in the liquid form by other channels. Hence
we may briefly express the destination of their food in the
following manner : —

l^ Carbonic acid and
^ , . ^. . ,. \ water, tlirown ofT

^0"^,^""^}^*"^^ ) Livin- ) and this (by respiration,

of albuminous I converted \ ^r^anised metamorphosed / Uren and biliary
ami other com- r into ( -^^^^^^ | i„^|, matter, &c.,

P°""^s J ' ^1 thrown off by

' other excretions.

162. But ill regard to the Herbivorous animals, the case
is different. They perspire much more abundantly, and their
temperature is thus continually kept down (§ 372). They
consequently require a more active combustion, to de-
velop sufficient bodily heat ; and the materials for this are
supplied, as we have seen, by the non-azotized constituents of
their food, rather than by the metamorphosis of their own
tissues, which takes place with much less rapidity than in
the carnivorous tribes. Hence we may thus express the

156 NUTRITION OF IIERBIVORA AND OF MAN.

destination of this part of tlieir food ; that of the albuminous
matters, here mueh smaller in amount, being the same as in
the preceding case : —

Starch, oil, and 'i partly ( Fatty and -j but chiefly j' Carbonic acid and water,
other lion -azo- > converted J other animal [ thrown otf < disengaged by the respi-
tized compounds j into ( tissues, ; directly as Uatory process.

The proportion of the food deposited as fat, will depend in
part upon the surplus which remains, after the necessary sup-
ply of materials has been afforded to the respiratory process.
Hence, the same quantity of food being taken, the quantity
of fat will be increased by causes that check the perspiration,
and otherwise prevent the temperature of the body from being
lowered, so that there is need of less combustion within the
body to keep up its heat. This is consistent with the teach-
ings of experience respecting the fattening of cattle ; for it is
well known that this may be accomplished much sooner, if
the animals are shut up in a warm dwelling and are covered
with cloths, than if they are freely exposed in the open air.

163. Now the condition of Man may be regarded as inter-
mediate between these two extremes. The construction of
his digestive apparatus, as well as his own instinctive pro-
pensities, point to a mixed diet as that which is best suited
to his wants. It does not appear that a diet composed of
ordinari/ vegetables only, is favourable to the full develop-
ment of either his bodily or his mental powers ; but this
cannot be said in regard to a diet of which the corn-grains
furnish the chief ingi-edient, since the gluten they contain
appears to be as well- adapted for the nutrition of the animal
tissues, as is the flesh of animals. On the other hand, a diet
composed of animal flesh alone is the least economical that
can be conceived ; for, since the greatest demand for food is
created in him (taking a man of average habits in regard to
activity and to the climate under which he lives) by the ne-
cessity for a supply of carbon and hydrogeii to support his
respiration, this want may bo most advantageously fulfilled
by the employment of a certain quantity of non-azotized food,
in which these ingredients i)redominate. Thus it has been
calculated that, since lifteen pounds of flesh contain no more
cai-]jon than four pounds of starch, a savage with one animal
and an equal weight of starch, could support life for the same
length of time during which another restricted to annual

COMPOSITION OF ARTICLES OF HUMAN FOOD.

157

food would require five such animals, in order to procure the
carbon necessary for respiration. Hence we see the immense
advantage as to economy of food, which a fixed agricultural
population possess over the w\andering tribes of hunters which
still people a large part both of the Old and Xew Continents.
164. The following Table exhibits the proportions of albu-
minous, starchy or saccharine, fatty, and saline substances,
contained in various articles ordinarily used as food by Man ;
together with the proportion which ivater bears in each case
to the solid constituents of the food, which becomes a most
important element of consideration when the nutritive value
of diiferent kinds of food is compared : —

Substances, 100 parts.

Human Milk

Cow's Milk

Skimmed Milk ...

Butter Milk

Beef and Mutton

Veal

Poultry

Bacon

Cheese (Cheddar)
,, (Skimmed)

Butter

Eggs

White of Egg

Yolk of Egg

White Fish

Salmon

Eel

Wheat Flour

Barley-meal

Oat-meal

Rye-meal

Indian-meal

Rice

Haricots

Peas

Beans

Lentils ,

Wheat-bread

Rye-bread ,

Potatoes

Green Vegetables .
Arrow-root

1 ^

III

73 X ^

i

89

3.5

4.2

3.0

8(5

4.5

5.0

4.1

87

4.5

5.0

2.7

87

4.5

5.0

0.5

73

19.0

5.0

77

19

1.0

74

21.0

3.0

20

0.8

70.0

.•!6

29.0

30.0

44

45.0

6.0

15

830

74

14.0

10.5

78

20.0

52

Ib.O

30.0

79

19.0

...

1.0

78

17.0

4.0

80

10.0

..,

8.0

15

11.0

70.0

2.0

15

10.0

70.0

2.4

15

12.0

620

6.0

15

9.0

66.0

2.0

14

9.0

65.0

8.0

14

7.0

76.0

0.3

19

23.0

45.0

3.0

13

22.0

58.0

2.0

14

24.0

44.0

1.4

14

29.0

44.0

1.5

44

9.0

49.0

1.0

48

5.3

4'i.O

1.0

74

2

23.0

0.2

8G

2.0

4.0

0.5

18

...

82.0

...

0.2
0.7
0.7
0.7
2.0
0.6
1.2
1.3
4.5
5.0
2.0
1.5
1.6
1.3
1.2
1.4
1.3
1.7
2.0
3.0
1.8
1.7
0.3
3.G
3.0
3.6
23
2.3
1.4
0.7
0.7

i3 3

11.4

14.8

11.5

6.0

12.0

2.4

7.2

168.0

72.0

14.4

199.0

25.0

72.0
2.4
9.6
19.2
74.8
75.8
76.4
70.8
84.2
76.7
52.2
C2.8
47.4
47.6
51.4
48.4
23.5
5.0
82.0

3.5

4.5

4.5

4.5

19.0

19.0

21.0

0.8

29.0

45.0

14.0
20.0
16.0
19.0
17.0
10.0
11.0
10.0
12.0
9.0
9.0
7.0
23.0
22.0
24.0
29.0
9.0
5.3
2.0
2.0

14.9
19.3
16.0
10.5
31.0
21.4
28 2
168.8
101.0
59.4
199.0
39.0
20.0
88.0
21.4
26.6
29.2
85 8
85.8
88.4
79.8
93.2
83.7
75.2
84.8
71.4
76.6
60.4
53.7
25.5
7.0
82.0

* The value of the Fat is stated in this cohimn according to its
heating equivalent of starch, which is larger in the ratio of 2,4 to 1.
Hence, in the last column, the proportion of nutriment in aliments
containing fat, comes to be greater than the weight of their solids
would indicate.

158 ECONOMY OF HUMAN DIET.

Those articles of food in which tlie nitrogenous compounds
predominate, are especially fitted for the maintenance of the
soHd fabric of the body ; whilst those in which the carbon-
aceous compounds are in largest excess, are those which are
most efi'ective as supplying materials for the combustive pro-
cess. Conspicuous among the former are the various kinds
of animal flesh, as also the white of eggs ; whilst among the
latter the most noticeable are bacon and butter, rice and
potatoes, the former consisting almost wholly of fat, the latter
being chiefly composed of starch. Of all single articles of
food, good wheaten bread, in which the proportion of nitro-
genous to carbonaceous components is about as 5.7 to 1,
seems to be the one best suited to the ordinary wants of
Man ; but this acquires much additional value from the con-
current use of a moderate amount of fatty matter in the form
of butter,

165. If the more highly azotized forms of food be em-
ployed exclusively, a great excess of them must be consumed
to supply the carbon needed for respiration ; whilst if the
more carbonaceous kinds of food be used as the sole susten-
ance, unless the quantity ingested be large enough to afford
the requisite supply of azotized material for the maintenance
of the tissues, their nutrition must be imperfectly effected,
and the strength must fail. I^ot only in the instance just
cited, but in a variety of others, the instincts of mankind
have led to such a combination of different articles of diet,
as includes in their appropriate proportions the albuminous,
the saccharine, and the oleaginous principles. Thus with
meat we eat potatoes ; and with the white meats which are
deficient in fat, we eat bacon. We use melted butter with
most kinds of fish, or fry them in oil ; whilst the herring, the
salmon, and the eel, are usually fat enough in themselves, and
are dressed and eaten alone. A similar adjustment is made
when we mix eggs and butter with sago, tapioca, and rice ;
when we add oil and the yolk of an egg to salad ; when we
boil rice with milk, and combine cheese with maccaroni.
Bacon and greens, and pork and pease-pudding, again, are
combinations founded in taste, which approve themselves to
the judgment; as is also the Irish dish termed kolcannon, con-
sisting of potatoes and cabbage, with a little bacon or fat pork.
So arc the mixture so common in Ireland and Alsace, of butter-

ECONOMY OF HUMAN DIET. 159

milk or curdled-milk with potatoes ; and the combination of
rice and fat, which is the staple of the diet of many Eastern
nations. Even the morsel of butter or the bit of cheese
which the English labourer eats with his hard-earned bread,
are not matters of luxury, but have a positive importance ;
and the existence of these tastes and habits shows how by
long experience Man has at last learned to adjust the com-
position of his food, so as best to maintain the health and
vigour of his body. With a difference of requirement comes
a difference of tastes. Thus men who are going through a
very laborious course of exertion, prefer meat to bread or
vegetables, feeling it to be more sustaining to their strength.
On the other hand, those who are continuously exposed to
the severity of an Arctic winter, eat with relish large masses
of fat, on which they would look with disgaist under other
circumstances. The quantity of work which a man can do,
and his power of sustaining extreme cold, both depend in
gi-eat part, as has now been abundantly proved, upon the
adequacy of the sustenance he takes : the demand, in the first
case, being for albuminous material to supply the waste of his
tissues ; whilst in the second it is for combustive material
suitable to generate heat iii large measure, — a purpose which
is far more efficiently answered by oleaginous substances, than
by those of a starchy or saccharine nature. Experience fur-
ther shows that the healthy condition of the blood of Man
can only be maintained by the use of fresh vegetables as part
of his ordinary diet. When these are withdrawn for any
length of time, the disease known as Scurvy is certain to
appear, unless lemon -juice or some other efficacious anti-
scorbutic be employed as a substitute. This is a fact of the
utmost importance in provisioning ships for long voyages ;
the tendency to scurvy being increased by confinement
and insufficient ventilation, and by the exclusive use of salt
provisions.

166. Besides these organic substances, there are certain
Mineral ingredients, which may be said to constitute a part
of the food of Animals ; being necessary to their support, in
the same manner as other mineral substances are necessary to
the support of Plants. Of this kind are common salt, and
also phosphorus, sulphur, lime, and iron, either in combina-
tion or separate. — The uses of Salt are very numerous and

I GO MINERAL INGREDIENTS OF FOOD.

iiiiportaiit. It exists largely in the blood, and in the various
aninial fluids which are secreted from it ; and it is also an
essential ingi-edient of most of the solid tissues. Its presence
obviously tends to prevent that spontaneous decomposition
to whicli organic substances are liable. Fhosphoi^us is chiefly
required to be united with fatty matter, to serve as the
material of the nervous tissue ; and to be combined with
oxygen and lime, to form the bone-earth by which the bones
are consolidated. Sulphur exists in small quantities in several
animal tissues ; but its part seems by no means so important
as that performed by phosphorus. Lime is required for the
consolidation of the bones, and for the production of the
shells and other hard parts that form the skeletons of the
Invertebrata. Of the limestone rocks of which a great part
of the crust of our globe is composed, a very large proportion
is made up of the remains of animals that formerly existed in
the ocean. Thus some almost entirely consist of masses of
Coral, others of beds of Shells, and others of the coverings
of minute Foraminifera (§131). To these mineral ingredients
we may also add Iron, which is a very important elem.ent in
the red blood of Vertebrated animals.

167. These substances are contained, more or less abun-
dantly, in most articles generally used as food ; and where
they are deficient, the animal suffers in consequence, if they
be not supplied in any other way. Common Salt exists, in
no inconsiderable quantity, in the flesh and fluids of animals,
in milk, and in the egg : it is not so abundant, however,
in plants ; and the deficiency is usually supplied to herbi-
vorous animals by some other means. Thus salt is purposely
mingled with the food of domesticated animals ; and in most
parts of the world inhabited by wild cattle, there are spots
Avherc it exists in the soil, and to which they resort to obtain
it ; such are the " buffalo-licks" of North America. Fhos-
phorus exists also in the yolk and white of the egg, and in
jnilk, — the substances on which the young animal subsists
< hiring the period of its most rapid gi-owth ; it abounds not
only in many animal substances used as food, but also (in the
state of phosphate of lime or bone-earth) in the seeds of many
plants, especially the grasses ; and in smaller quantities it
is found in the ashes of almost every plant. When flesh,
bread, fruit, and husks of grain, arc used as the chief articles

MINERAL INGREDIENTS OF ANIMAL FOOD. 161

of food, more phosphorus is taken into the body than it
requires ; and the excess has to be carried out in the excre-
tions. 6'ulphur is derived aUke from, vegetable and animal
substances. It exists in flesh, eggs, and milk ; also in the
azotized compounds of plants ; and (in the form of sulphate
of lime) in most of the river and spring water that we drink.
Iron is found in the yollc of egg, and in milk, as well as in
animal flesh ; it also exists, in small quantities, in most
vegetable substances used as food by man, — such as potatoes,
cabbage, peas, cucumbers, mustard, &c. ; and probably in
most articles from which other animals derive their support.

168. Lirne is one of the most universally diffused of all
mineral bodies ; there being very few animal or vegetable
substances in which it does not exist. It is most commonly
taken in, among the higher animals, combined with phos-
phoric acid, so as to form bone-earth, in which state it exists
largely in the seeds of most grasses. A considerable quantity
of lime exists, moreover, in the state of carbonate and sul-
phate, in all hard water.

169. When an unusual demand exists for lime, however,
for a particular purpose, an increased supply must be aflbrdecl.
Thus a hen preparing to lay, is impelled by her instinct to
eat chalk, mortar, or some other substance containing the car-
bonate of lune which is required for the consolidation of the
shell ; and if this be withheld, the egg is soft, its covering
being composed of animal matter alone, not consolidated by
the deposit of earthy i)articles. The thickness of the shells
of aquatic Mollusks depends greatly upon the quantity of
hme in the surroundmg water. Those which inhabit the sea,
find in its waters as much as they require ; but those that
dwell in fresh-water lakes, which contain but a small quan-
tity of lime, form very thin shells ; whilst, on the other hand,
those that inhabit lakes in which, from peculiar local causes,
the water is loaded with calcareous matter, form shells of
remarkable thickness.

170. The mode in which the Crustacea, whose calcareous
shell is periodically thrown ofi' (§ 99), are able to renew it
with rapidity, is very curious. There is laid up in the walls
of their stomachs a considerable supply of calcareous matter,
in little concretions, which are commonly known as " crabs'
eyes." When the shell is cast, this matter is taken up by

M

1C2 DIGESTION AND ABSORPTION.

the Llood, and is thrown out from the surface, mingled with
animal matter. This hardens in a day or two, and the new
covering is complete. The concretions in the stomach are
then found to have disappeared ; but they are gradually
replaced, before the supply of lime they contain is again
required.

CHAPTER IV

LIGESTION AND ABSORPTION.

171. Having now considered the nature of the food of
Animals, and the sources from Avliich it is obtained, we have
next to consider the process by which the aliment is received
into their bodies, and prepared to form a part of their own
fabric. This process, termed Digestion, is naturally divided,
among the higher animals at least, into various stages. In
the first place, there is the prehension or laying hold of the
aliment, and its introduction into the mouth or entrance to
the digestive cavity. In the mouth it usually undergoes a
preparation ; which consists partly in its being cut, ground,
or crushed, by mechanical action, into minute pieces ; and
partly in the working-up of these pieces with a fluid that is
poured into the mouth, — the saliva. These two processes are
termed mastication and insaUvation ; similar processes are
performed, in some animals, in a part of the digestive tube
intermediate between the mouth and the stomach, and even
in the latter itself. The stomach is usually situated at some
distance from the mouth, and is connected with a tube called
the oesophagus or gullet ; and the passage of the food into
this, constituting the act of swallowing, is termed deglutition.
T]ie food, having arrived in the stomach, is acted-upon by a
peculiar fluid which it contains, and much of its alimentary
portion is dissolved, so that a pulpy mass is formed which is
termed chyme ; hence this process, which is the first stage of
digestion properly so called, is termed clujmification or the
manufacture of chyme. The chyme, which passes into the
intestines, is further acted-on by secretions that are poured
into thorn ; and a certain nutritive combination of albuminous

PREHENSION OF FOOD. 163

and fatty matters, termed chyle, is sejoarated from the matters
that are to be thrown off : this process, which is the second
stage of true digestion, is termed chylification. The rejected
portions of the food, with secretions poured into the alimen-
tary canal, find their way out through the intestinal tube ;
and are voided at its terminal orifice by the act of defecation.
And lastly, the nutritive materials are taken up by absorption
into vessels that are distributed upon the walls of the diges-
tive cavity, and undergo a gradual change, by which they are
converted into blood. These two processes are called absorp-
tion and sanguification (or manufacture of blood). Each of
the foregoing stages will now be separately considered.

Prehension of Food.
172. The introduction of aliment within the entrance to the
digestive cavity is accomphshed in various methods in dif-
ferent animals. In the Mammalia in general, the aperture of
whose mouth is guarded by fleshy lips, these, with the jaws
and teeth, are the chief instruments of this operation. But
ill Man and the Monkey tribe the division of labour is
carried further ; the food being laid hold of by the anterior
members, or hands, and by them carried to the mouth.
Where the hand has the power of grasping, and especially
where the thumb can be opposed to the fingers, the action of a
single member is sufficient ; but there are several animals
which, lilvC the Squirrel, use both limbs conjointly to hold
their food, the extremity not having itself the power of grasp-
ing. The Ant-eaters, Woodpeckers, Chameleons, and other
insect-eating annuals, obtain their food
by means of a long extensible tongaie ;
this either serving to transfix the insect,
or being covered with a viscid saliva
which glues it to the surface. The Giraffe
uses its long tongue to lay hold of the
young shoots on which it browses ; and
the Elephant employs its trunk, which is
nothing else than a prolonged nose, for
every kind of prehension (fig. 82). Many
of the Invertebrata are furnished with ^ ''^'^^

Tiji 1 ^ A^ • ill Head of Elephant.

little appendages round their mouths by

which the food is conveyed into th(^m ; such are the palpi

m2

164

BECEPTION OF SOLID AND LIQUID ALIMENT.

of Insects, of which a pair is attached to each jaw (fig. 84);
the tentacula of Mollusks, which are sornetiiues extremely
prolonged, as in the Cuttle-fish tribe (fig. Sb) ; and the
similar organs of the polypes (fig. 71).

Carabos.

Fig. 84.— Jaws of the
SAME Insect.

173. The reception of liquids is accomplished in two ways.
Sometimes the liquid is made to fall into the mouth, simply
by its own weight (fig. 8G) ; in other instances it is drawn or
pumped up into this cavity, — cither by the expansion of the
chest, which causes a rush of air towards the lungs, — or by
the movement of the tongue, which, being drawn back like a
piston, produces the action of sucking. Some of the lower
animals are destined to be entirely supported by liquids which
they find in plants, or which they draw from the bodies of
other animals whereon they live as parasites. This is the
case with many Insects ; and their mouth, instead of present-
ing the ordinary structure, is formed into a sort of tube or
trunk, very much extended, through which the juices arc
drawn up according to the wants of the animal. Such a
conformation exists in the butterfly and moth tribe, whose

SUCTION OF LIQUIDS.

165

trunk, wlien not in use, is coiled up in a spiral boneath the
head ; as is shown in fig. 87, representing the head of a

Fig. 87.— Trunk of a
Butterfly.

Fig. 86. — Chimpanzee dkinking.

Butterfly, or, of which the eye is seen at c, the base of the
antennae at 6, the palpi at e, and the trunk at d. In some of
the Fly tribe, the trunk attains a length several times greater
than that of the body, as shown in fig. 88, representing a

Fig. 88. — Nemestrina longirostris.

dipterous (two-winged) insect from the Cape of Good Hope,
which sucks the juices of a single kind of flower, the length
of whose tube just equals that of its long proboscis.

16G

DEVELOPMENT OF TEETH.

Mastication.

174. The act oi Mastication, or the mechanical division of
the ahmcntary matter, is effected in most of the higher animals,
by the Teeth ; which are implanted in the jaws, and are so fixed
as to act ao-ainst one another, with a cntting, crushing, _ or

power, according

Grinding

Fig. 89. — Devklopment
the gum ; h, the lower jaw ;

jaw ; d, dental capsules

01' Teeti

to the nature of the food on
which they have to oj^erate.
The manner in which they
are form.ed is worthy of
note. In Man, who may be
taken as a fair example,
each tooth is developed in
the interior of a little mem-
angieofthe brauous sac, wliich is lodged
in the thickness of the jaw-
bone ; as seen in the accompanying figure, wliich represents
half the lower jaw of a very young infant, from which the
outside has been removed. This sac, which is named the
dental capsule («, tig. 90), is composed of two membranes,
abundantly furnished with blood-vessels ; and it encloses in
its interior a little bud-like protuberance, h, in which ramify
a great number of nervous filaments and minute vessels, c.
The matter composing this little body, which is termed the
j)idp, is gradually converted into the dentine (§ 54) of the tooth,
d d which in Man constitutes nearly its whole

structure ; this conversion takes place first
at its highest points, d, d. The crown or
ui)pcr portion of the tooth receives a
(•overing of enamel (§ 54). Gradually the
process of conversion extends more and
more to the interior of the pulp ; and at
last the whole is changed into dentine,
with the exception of a small portion
occupying what is termed the cavity of
is frequently laid open by decay of its

Fig. 90.— L)KK'r.vr,
Capsi LE.

that still
the tooth

reuuiin>
, which

external wall. The fang of the tooth, which is the part
last formed, receives an envelope of cementum (§ 54), wliich
invests it up to the part at which the enamel begins. As the

DEVELOPMENT OF TEETH. 167

root of the tooth is developed, the crown is gradually pushed
upwards, so as to 23ress against the upj)er portion of the
capsule and the gum by which this is covered. These
parts yield slowly to the pressure ; and the tooth makes its
way to the surface ; or, in common language, is cut.

175. The process of "cutting teeth" is usually not a severe
one in the healthy and well-managed infant ; but it occasions
the death of vast numbers of children who are injudiciously
treated ; and it is especially fatal to those who have a ten-
dency to disease of the nervous system. The irritation
caused by the pressure of the tooth against the gum, is
liable to excite, in such cases, convulsive actions of various
kinds, on the principles hereafter to be explamed (§ 473) ;
and, as the removal of the source of irritation is of the
most urgent importance, the lancing of the gums, — doing
that in an instant which the pressure of the tooth might not
accomplish for days, — is a measure of most obvious utility ;
however unnecessary it may seem, in ordinary cases, to in-
terfere with the course of nature. But it is of the utmost
importance at the same time to bring the nervous system into
a less excitable condition ; and no measure is commonly more
efficacious in this respect, than removal into a fresh and pure
atmosphere.

176. At the same time that the development of the tooth
is thus taking place, the bone of the jaw is becoming liardened,
and closes round its root, formuig a complete socket. This
partly interrupts the passage of vessels and nerves to the
tooth, which, when once fully formed, seems to acquire no
further growth, and to possess but little power of repairing
injuries occasioned by disease or accident. Hence a tooth
which is broken or decayed, is not restored as a bone would
be. Still, however, its root or fang is penetrated by a small
nerve and artery, which are distributed to the membrane
that lines the cavity ; and it is to the action of air upon the
former, when the cavity is laid open by decay, that the pain
of tooth-ache is chiefly due. The remedies which are most
effectual in removing this pain, such as kreosote, nitric acid,
or a heated wire, are those which destroy the vital jwwer of
the nerve.

177. But there are teeth, in many animals, which never
cease to grow, and in which the central cavity is always filled

168 TEETH OF RODENTS. MOTION OF JAWS.

with pulp. Such have no proper root ; for additional matter
is being continually formed at their base, and thus the whole
tooth i^ pushed upwards. This is the case with the Elephant's
tusks ; and also with the large teeth that occupy the front of
the iaw in Eabbits, 8(iuirrels, Rats, and other gnawing ani-
mals (fig. 91). The
upper edges of these
teeth are being con-
stantly worn away by
use : and they are
kept up to their
proper level by the
growth of the tooth

Fig. 91. -Jans AND Teeth OF RAniur. from beloW. But it

sometimes happens that one of these teeth is broken off ; and
the one opposite to it in the other jaw is then thrown into dis-
use. It continues, however, to grow up from below ; but,
not being worn down at the top, its length increases greatly,
so that it may l)ccome a source of great inconvenience to the
animal.

178. The teeth are but passive instruments in the act of
mastication. They are put in movement by the jaws in which
they are fixed ; and these are made to act against each other
by various muscles. The upper jaw is usually fixed to the
head; and has not, therefore, any power of moving inde-
pendently of it. But the lower jaw is connected with the
skull by a regular joint on either side ; and is so moved by
the muscles attached to it, as to cut, crush, or grind the food,
according to the nature of the teeth.

179. There is considerable variety, in different animals, as
to the extent of motion which the lower jaw possesses. In
the purely Carnivorous quadrupeds, it has merely a hinge-like
action, that of opening and shutting ; and by the sharpness of
the edges of the molar teeth, it is thus rendered a powerful
cutting instrument. But in the Herb-rorous animals, which
have to grind or triturate their food 1 . Iween the roughened
surfaces of their molars, such a limited motion would be of
no avail ; and we accordingly notice, if we watch an ox or a
horse whilst masticating its food, that the lower jaw has con-
siderable power of motion from side to side. On the other
hand, in the Ilodents, or gnawing animals furnished with

MOVEMENTS OF LOWER JAW.

169

two large front teeth, tlie lower jaw has no power of moving
from side to side, but is rapidly drawn backwards and for-
wards ; and, as the ridges of the molar teeth are arranged in
the opposite direction, they become very powerful filing in-
struments, by which the toughest vegetable sulxstances are
quickly reduced.

180. In the Human jaw, there is a moderate power of
motion in all these different directions ; and it is furnished
with all the muscles by which they are effected in the
different animals that perform them ; but these are not so
large or strong. The most powerful of the muscles of the
lower jaw, in all animals, is that by wdiich it is drawn up
against the upper, so as to close the mouth. This arises from
the side of the skull in the region of the temple, and is hence
called the temporal muscle. It covers at its origin a large
surface of bone ; but its fibres approach one another as they
descend, and pass under a bony arch (which may be felt
between the cheek and the ear), to attach themselves to
a process or projection of the
lower jaw^ (cr, fig. 92), about
an inch in front of the joint.
As the distance from the ful-
crum of the point a, at which the
powder is applied, is thus much
less than that of the front of
the jaw h, where chiefly the
resistance is encountered, the
powder of the muscle is applied
at a mechanical disadvantage ;
and, to overcome a given resist-
ance, the muscle must itself be
several times more powerful.
Thus the Tiger and Lion, which
can lift and carry away the bodies of animals weighing several
hundred pounds, must possess temporal muscles that shall
contract wdth a force of two thousand, or even more.

181. In Man, as in most of the other Mammalia, there are
three kinds of teeth, adapted for different purposes. The
first terminate in a thin cutting edge, and are intended simply
to divide the food introduced into the mouth ; these are termed
inciso?' teeth (fig. 93). Others have more of a conical form.

Fig. 92.— Human Skull.

170

DIFFERENT KINDS OP TEETH.

and ill many animals (especially those of carnivoi'ous liabits)
project far beyond the former ; they are adapted not to cut
the food, but, by being deeply fixed in it, to enable the
animal to tear it asunder : these are termed ca^iine teeth.
The teeth of the third kind have large irregular flattened
surfaces, and are adapted to bruise and grind the food ; these
are called mola?- (or mill-like) teeth. The manner in which
these different teeth are implanted in the jaw, varies with the
ibrm of their crowns, and is in accordance with their several
uses. The incisors, whose action tends as much to bury
them in their sockets as to cb^aw them forth, have but a single
root or fang of no great length. The canine teeth, on which
there is often considerable strain, penetrate the jaw more
deeply than the incisors ; esj)ecially when they are large and

Molars. Bicuspid. Canine.

Fig. 93.— Human Teeth.

long, as in the Cat tribe (fig. 94). And the molars, whose
action requires great firmness, have two, three, or even four
roots or fangs, which spread out from each other ; and these
at the same time increase the solidity of their attachment to
the jaw, and prevent the teeth from being forced into their
sockets by any amount of pressure.

182. The arrangement of the dental apparatus varies, in
different Mammalia, accordmg to the nature of the aliment
on which they are destined to feed ; and this correspondence
is so exact, that the anatomist can generally determine by the
simple inspection of the teeth of an animal, not only the
nature of its food, but the general structure of the body, and
even its ordinary habits. Thus, in those that feed exclusively
on annual lU'sh, the molar teeth are so compressed as to form

DIFFERENT KINDS OF TEETH.

171

cutting edges, which work against each other Hke the blades
of a pair of scissors (fig. 94) ; whilst in animals that live on
insects, these teeth are raised into conical points, which lock

Fig. 94. — Teeth of Carnivorous Animai

Fig. 05.
Teeth of Insectivorous Animal.

into corresponding depressions in the teeth of the opposite
jaw (fig. 95), When the nourishment of the animal con-
sists principally of soft fruits, these teeth are simply raised
into rounded elevations (figs. 97, 98) ; and when they are

Fig. 06.
Teeth of Herbivorous Animal. Fig. 97.— Teeth of Frugivorous Animal.

destined to grind harder vegetable substances, they are termi-
nated by a large flat and roughened surface (figs. 96, 99).
The roughness of this surface is maintained by the peculiar
arrangement of the three substances of which the tooth is
composed. The enamel, instead of covering its crown, is
arranged in upright plates, which are dispersed through the
tooth ; and the space between them is filled up by plates of
ivory and of cementum (§ 54). These last, being softer than
the enamel, are worn down the soonest ; and thus the plates
of enamel are left constantly projecting, so as to form a rough
surface admirably adapted to the giinding action which the
tooth is destined to perform. The mode in which these
plates are disposed, afibrds a most characteristic distinction
between the two species of Ele2:)hant at present existing,

DIFFERENT KINDS OF TEETH.

namely, the African and tlie Indian ; as also between each
of these and the great extinct species known as the Mam-
moth (tig, 99). In the great gnawing teeth of the Eabbit,

Fig. 98. — Molar Tooth of Mastodon.

Fig. 99.— Molar Teeth of Elephants.

1 , African Elephant ; 2, 1 ndian Elephant ;

3, Mammoth.

&c., the front surfoce only is covered with enamel ; and as
this is worn away more slowly than the ivory, it stands uj) as
a sharp edge (fig. 91), which is always retained, however
much the tooth may be worn away.

183. Of all the teeth, the molars may be regarded as the
most useful. They are seldom absent in the Mammalia ; and
their office is usually essential to the proper digestion of the
food. Animal flesh (the most easily digested of all substances)
needs but to be cut in small pieces ; but the hard envelopes

of beetles and other insects
must be broken up ; and the
tough woody structure of the
grasses, and the dense coverings
^ /Ji^^^^^-xjiXMSb^r^^" J *^'^ ^^^^ seeds and fruits on which
V^V^iJ^-^^-^^'-^^^^^^'/^ ^^^^ herbivorous animals are

supported, must be ground
down. The incisors and canines
are chiefly employed among
purpose of seizing their living
prey, and are never delicient in them ; but they are less re-
quired in Herbivorous animals ; and either or both kinds are
not uufro(piently dcificient. Sometimes, however, they are not

Fig. 100. —Skull of Boar.

Carnivorous animals for the

SUCCESSION OF TEETH. WHALEBONE. 173

only present in the latter, but are largely developed, serving as
weapons of attack and defence ; as in the Boar (fig. 100).

184. In the Mammalia in general, as in Man, the teeth are
not much developed at the time of birth, that they may not
interfere with the act of sucking; and they do not make
theii- appearance above the gum, until the time approaches
when the young animal has to prepare its own food, instead
of smiply receiving that wdiich has been prepared by its
parent. The teeth which are first formed are destined to be
shed after a certain period, and to be replaced by others.
They are called milk-teeth ; and in Man they are twenty in
number, — namely, four incisors in the front of each jaw, and
two canines and four molars on each side. These begin to fall
out at about the age of seven years ; previously to which,
however, the first of the permanent molars appears above the
gum, behind those of the first set. The incisors and canines
of the first set are replaced by incisors and canines respec-
tively ; but the molars of the first set are replaced by teeth
like small molars, having only two fangs ; these are called
false molars, or, more properly, hicuspid teeth (fig. 93). The
second of the true molars does not make its appearance until
all the milk-teeth have been shed ; since it is only then that
the jaw becomes long enough to hold any additional teeth.
The third does not usually come up until the growth of the
jaw is completed ; and as this time corresponds w-ith that at
wdiich the mind as well as the body is matured, they are
commonly known as wise or wisdom teeth. There are then
thirty- two teeth in all, or sixteen in each jaw ; — namely, four
incisors, two canines, four bicuspid, and six true molars. — In
extreme old age, these teeth fall out like those of the first
set ; but they are not replaced by others, and their sockets
are gradually obliterated.

185. There are a few Mammalia which do not possess teeth.
This is the case with the common A¥hale, in which they are
replaced by an entirely different structure. From the upper
jaw (fig. 102) there hang down into the mouth a number of
plates of a fibrous substance (fig. 101), to which we give the
name of whaleho7ie, though it is really analogous to the gum
of other animals. The fibres of these plates are separate at
their free extremities, and are matted (as it were) together, so
as to form a kind of sieve. Through this sieve the "Whale

174 ABSENCE OF TEETH IN WHALE, ANT-EATER, ETC.

draws water in enormous quantities, whenever it is in want
of food ; and in this manner it strains out, as it w^ere, the
minute gelatinous animals upon wdiich it lives, from the w^ater
of the seas it inhabits. The water thus taken in is expelled
from the nostrils or blow-holes, which are situated at the top

Fig. 102.— Skull of Whale,

Fig. 101. — WnALKlioNE.

of the head. ^Most of the Whale tribe have short fringes of
this kind in the roof of the mouth ; but in none, except the
Balosna, or Greenland Whale, is it long enough to make it
w^orth separating ; all the other sjDecies havmg teeth, either in
one or both jaws. — It is a curious fact, that the rudiments of
teeth may be discovered in both jaws of the young Greenland
whale, although they are never to be developed. And the
rudiments of incisor teeth in the upper jaw, and of canine
teeth in both jaws, may also be discovered in the young of the
Euminant quadrupeds (oxen, sheep, &c.), though they never
show themselves above the gum.

18G. The Ant-eaters, also, are destitute of teeth, and usually
'»btain their food by means of their long extensible tongues,

which are covered with a viscid
saliva ; this being pushed into
the midst of an ant-hill, and
then drawn into the mouth,
brings into it a large number
of

Fig. 103.— Skill oi- the Ant-eater,

these insects, which are
sufficiently bruised between the toothless jaws (fig, 103).
Lastly, may be mentioned as a curious exception to the general
rules respecting the teeth of Mammalia, the remarkable Orni-
thorhyncus of New Holland (Zoology, § 317), which feeds,

TEETH OF RErTILES AND FISHES.

17^

like the duck, upon the water-insects, shell-fish, and aquatic
plants, that it obtains from the mud, into which it is continu-
ally plunging its singular bill ; and its jaws, entirely destitute
of teeth, are furnished with horny ridges, by which it can in
some degree masticate its food.

187. Among Birds, there is an entire absence of teeth;
and the mechanical division and the reduction of food is per-
formed in the stomach, in the manner hereafter to be men-
tioned (§ 200). The mouths of almost all Reptiles, excepting
the Turtle tribe, are furnished with numerous teeth (fig.
104) ; but these are not
adapted for much variety of
purposes, being prmcipally
destined to prevent the
escape of the prey which
the animals have secured ;
and their shape is conse-
quently nearly uniform, being for the most part simply
conical. There are some Lizards, however, which are herbivo-
rous ; and these have large rough teeth, somewhat resembling
the molars of Mammalia. The Igiianodon, an animal of this
tribe, attained a gagantic size in past ages of the world.

188. In Fishes, the teeth are commonly very numerous (fig.
105), but they have for their object only to separate and retain

■Head of Gavi
of the Ganges.)

(Crocodile

Fig. 105.— Head of Shark.

their food ; and there is little variety in their form. Fre-
quently they have no bony attachment, being only held by
the gum, as in the Shark ; and they are consequently often
torn away, but they are as readily replaced. Sometimes, how-

17G

MASTICATING INSTRUMENTS OF INVERTEBRATA.

ever, the tooth seems like a coutiiiiiation of the ])one of the
jaw, not being in any way separated from it, and the tubular
structure of the latter being continued into it without any
interruption. The teeth of fishes are often set, not only upon
the proper jaw-bones, but upon the surface of the palate, and
even in the 'pharynx or swallow.

189. In the Invertebrata there are generally no proper
teeth ; in the Articulated and sometimes in the Molluscous
series, however, we meet with firm horny jaws, which are
often furnished with projections that answer the same pur-
pose ; and in most Gasteropods we find a very curious organ,
commonly designated as the tongue^ more correctly the
imlate, the surface of which is beset with innumerable tootli-
like points (fig. 106), by whose rasping action the food is
reduced. These teeth present great varieties of form and

arrangement in the different
genera andsjDecies of this group ;
and these varieties appear to
bear some relation to the nature
of the food on which the animals
respectively live. It is remark-
able that in an animal so low
in the scale as the Echinus or
Sea- Urchin (§ 1 19), a very com-
plex dental apparatus should
exist. This consists of five long
hard teeth, which surround the
mouth ; and these are fixed in
p. ,„, ,. ^ ,, a framework which is worked

Fig.ioe.—DKNTAL Organ OF Nerixa. ■, ^ ^ , i^ ^

by a power! ul set oi muscles,
and thus serve effectually to grind down the food.

Insalivation.

190. The act of mastication is connected with another;
which is also of great importance in preparing for the sub-
sequent process of digestion. This is the blending of the
sahva with the food, during its reduction between the teeth,
—an act which is termed inmUmtion. The saliva is separated
Irom the blood, by glands which arc situated in the neigh-

SECRETION OP SALIVA, AND ITS USES. 177

l30iirliood of the mouth ; of these there are three pair in Man,
two beneath the tongue (fig. 107), and one in the cheek, each
pouring-in its secretion by a separate canal. The salivary fluid
is prmcipally composed of water, in which a small quantity
of animal matter and some saline substances (chiefly common
salt) are dissolved ; the whole amount of these, however, is
not more than 1 part in 100. The secretion of saliva is not
constantly going on ; but the fluid is formed as it is wanted.
The stimvlus by which the gland is set in action may be simply
the motion of the jaws ; thus, on first waking in the morning,
the mouth is usually dry, but it is soon rendered moist by the
movements which take place in speaking. The contact of
solid substances with the membrane lining the mouth appears
also to excite the How ; hence dryness of the mouth may
often be remedied for a time, when no water is at hand,
by taking a pebble into its interior, and moving this from
side to side. There are certain substances, however, whose
presence in the mouth has a special influence in provoking
an increased secretion of saliva ; and every one knows, too,
that the simple idea of savoury food will excite an increased
flow, making the "mouth water" as it is popularly termed.
These are instances of the power of the nervous system,
through which such impressions are conveyed, over the act of
secretion.

191. In the case of farinaceous or starchy food, the admix-
ture of saliva occasions the commencement of that chemical
change in which its digestion consists, namely, its conversion
into sugar ; but in general, the benefit derived from this pro-
cess of insalivation is just that which is obtained by the
chemist, when he bruises in a mortar, with a small quantity
of fluid, the substances he is about to dissolve in a larger
amount of the same. If the preliminary operations of masti-
cation and insalivation be neglected, the stomach has to do the
whole of the work of preparation, as well as to accomplish
the digestion ; thus more is thrown upon it than it is adapted
to bear ; it becomes over- worked, and manifests its fatigue by
not being able to discharge even its own proper duty. Thus
the digestive function is seriously impaired, and the general
health becomes deranged in consequence. A malady of this
kind is very prevalent in the United States ; and is almost
universally attributed by medical B'^en, in part at least, to the

N

DEGLUTITION OR SWALLOWING.

general habit of very rapidly eating or rather " bolting " the
meals. There is another evil attendant on this practice, — that
much more food is swallowed than is necessary to supply the
wants of the system ; for the sense of hunger is not so readily
abated by food which has not been prepared for digestion ;
and thus the feehng of satiety is not produced, until the
stomach has already received a larger supply than it is well
able to dispose of. Imperfect mastication of the food is very
apt to occur, in persons who are losing their teeth by old age
or decay; and where these are not replaced by artificial means,
the next best remedy is to cut the food into very small por-
tions, before it is taken into the mouth, and to masticate it
there as thoroughly as possible.

Deglutition.
192. In the Mammalia, the cavity of the mouth is guarded
behind by a sort of moveable curtain, which is known as the
veil of the palate (fig. 107) ; and this hangs down during

Veil of the palate

Nose

Pharynx

(Esophagus

Tongue

Salivary glands
Os hyoides

Larynx
Thyroid gland

Trachea

Fig. 107.-PKnPKNDICULAR SeCTIOK of TlIK MoUTH AND THROAT.

mastication, in such a manner as to prevent any of the food
trom pas.-iiig backwards. Tliis partition, which does not exist

DEGLUTITION OR SWALLOWING. 179

in Birds and other animals that do not masticate their food,
hangs from the arch and sides of the paUite, so as to touch
the tongue by its lower border ; but it can be lifted in such a
manner as to give the food free passage beneath it, into the
top of the gullet. When mastication is completed, the food
is collected on the back of the tongue into a kind of ball ;
and this, bemg carried backwards by the action of its muscles,
presses against the partition just mentioned, and causes it to
o^eii. The food thus passes into a sort of funnel, formed by
the expansion of the top of the oesophagus or gullet ; this
cavity, termed the phari/nx, communicates above with the
nostrils, and in front with the larynx, which is at the top of
the trachea or windpipe. The oesophagus is a long and narrow
tube, which descends from the pharynx to the stomach, lying
just in front of the vertebral column, and behind the heart
and lungs. It is surrounded by muscular fibres, disposed in
various ways ; by the action of which the food that has once
passed into the pharynx is propelled downwards to the
stomach.

193. But in order to reach this tube, the alimentary ball
must pass over the glottis or aperture of the larynx. With
a view to prevent its falling-in, the larynx is drawn, in the
very act of swallowing, beneath the base of the tongue ; and
this action presses down a little valve-like flap, the epiglottis,
upon the aperture, so as in general effectually to prevent any
solid or fluid particles from entermg it. But it sometimes
happens that, if the breath be drawn-in at the moment ot
swallomng, a small particle of the food, or a drojD of fluid, is
drawn into the glottis ; and this action (commonly termed
" passing the wrong way,") excites a violent coughing, the
object of which is to drive up the particle, and to prevent it
from finding its way into the lower part of the windpipe. It
may also happen that a larger substance may slip backwards,
by its own weight, into the glottis, when there was no
intention of swallowing, and when the larynx was conse-
quently not drawn forwards beneath the tongue. The presence
of such a substance in the windpipe excites a violent and fre-
quently ahnost suffocating cough (§ 342) ; the effect of which
is sometimes to drive it up through the glottis, and thus to
get rid of the source of irritation.

194. The act of swallowing is itself involuntary, and may

n2

180 MOVEMENTS OF DEGLUTITION.

be even made to take place against the will. ^ This may seem
contrary to every one's daily experience ; but it is nevertheless
true. The movement by which the food is carried back,
beneath the arch of the palate, into the pharynx, is effected by
the will ; but when the food has arrived there, it is laid hold of,
as it were, by the muscles of the pharynx, and is then carried
down involuntarily. It has several times happened, that a
feather, with which the back of the mouth was being tickled
to excite vomiting, having been introduced rather too far, has
been thus grasped, by the pharynx, and has been swallowed.
Moreover, we cannot perform the act of SAvallowing, without
carrying something backwards upon the tongue ; and it is the
contact of this something, even if it be only a little saliva,
A\'ith the membrane lining the pharynx, that produces the
muscular movement in question.

195. This action is one of the kind now denominated rejlex
(§ 430). It is produced through the nervous system ; for if
tlie nerves supplying the part be divided, it will not take
place. But it does not depend upon the Brain ; for it may
be performed after the brain has been removed, or when its
power has been destroyed by a blow. It is caused by the
conveyance to the top of the Spinal Cord, of the impression
made on the lining of the pharynx ; this impression, brought
thither through one set of nerves, excites in the spinal cord a
motor impulse ; which, being transmitted thence through
another set of nerves, calls the muscles into action.

196. This action is, therefore, necessarily connected with
tlie impression, so long as this portion of the spinal cord, and
the nerves proceeding from it, are capable of performing their
functions : and it is one of those to which we may give the
name of instinctive, to distinguish it from those which are
eifected by an effort of the Will, intentionally directed to
accomplish a certain purpose. It may even take place without
the animal being aware of the contact of any substance to be
swallowed with the lining of the pharynx ; for there is good
reason to believe that when the brain has been destroyed, or
])aralyzed by a blow, all sensibility is destroyed ; and we have
also sufficient reason to consider it as suspended in profound
sleep or apoplexy, in which states swallowing is still per-
formed. In the severest cases of apoplexy, however, the
power of swallowing is lost ; and this is a symptom of gi-eat

DEGLUTITION DIGESTIVE APPAllATUS.

181

danger, since it shows that not the brain alone, but the ujiper
part of the spinal cord, is suffering from the pressure ; and
that the movements of respiration, which depend upon a
sunilar action of the nervous system (§ 340), will probably-
soon cease, so that death must ensue.

Digestive AppcnxUiis.

197. Tlie food, thus propelled downwards by the action of
the muscles of the pharynx and of the esophagus (gullet),

Gall-Bladder ■

Large Intestine

Spleen

Colon

- Small Intestine
• Colon

Small Intestine Rectum

Fig. 108.— Digestive AppAnAxus of Man.

arrives, in Man and the Mammalia, at the stomach ; which is
a large membranous bag, placed across the upper part of the

232 FORM OF THE STOMACH.

abdomen (fig. 108). The form of this stomach varies much,
according to the nature of the ahmcnt to be digested. Wliere
tlie food is animal flesh, which is easily dissolved, the stomach
is small, and appears like a mere enlargement of the alimentary
tube ; this is the case in the Cat tribe, for example. In Her-
bivorous animals, on the contrary, the stomach is very large,
the food being delayed there a long time on account of the
difficulty Avith which it is digested ; and the principal part of
its cavity is not a simple enlargement of the alimentary tube,
but a bag or sac that bulges out, as it were, on the left side of
that canal. By the degree of this bulging, we can judge of
the nature of the food on which the animal is destined to
live. Thus in Man (fig. 108), the large end of the stomach,
situated on tlie left side (the y^iglit side of the figure as w^e
look at it), is moderately developed ; showing, as we might
expect from the form of his teeth, as w^ell as from his natural
tastes, that he is adapted for a diet in which animal and
vegetable food are mixed. In the purely carnivorous tribes,
this large end of the stomach is almost deficient ; whilst in
the herbivorous races, it is enormously developed, and some-
times forms a distinct pouch.

(Esophagus

Intestine

Pylorus 4tliStom. 2d Stoni. 1st Stem.
Fig. 109.— Stomachs of the Siif.ep.

108. The most eom[)lcx form of the stomach among Mam-
mals, is that which we find in the animals that ruminate or
chew the cud. It possesses, in fact, no less tlian four distinct
cavities, through all of Avhich the food has to pass during the

STOMACH OF RUMINANTS.

183

process of digestion. The external appearance of the stomach
of the Sheep is seen in fig. 109 ; and its interior is displayed
in fig. 110. The food of the Euminant animals is not
chewed by them before it is first swallowed. In their wild
state, they are pecuKarly exposed to the attacks of their car-
nivorous enemies, w^hen they come down from their rocky
heights to browse upon the rich pastures of the valleys. If
they were then obliged to masticate every mouthful, they
would be subjected to long-continued danger at every meal ;
but, by the curious construction of the digestive apparatus,
this is spared to them ; for they are enabled to swallow their
food as fast as they can crop it, and afterwards to return it to
their mouths, so as to masticate it at their leisure, when they
have retreated to a place of safety. The crude unmasticated
food, wliich is brought-down by the oesophagus, first enters the
large cavity on the left side, which is commonly termed the
paunch. It is there soaked, as it were, in the fluid secreted

Intestine Honeycomb Paunch

Fig. 110. — Section of the Stomachs of the Shf.ef.

by its walls ; and is then transmitted to the second cavity,
which, from the sort of network produced by the irregular
folding of its lining membrane, is called the reticulum or
honey-comb stomach. This stomach also has a direct commu-
nication with the a^sophagus, and appears destined especially
to receive the fluid that is swallowed ; for this passes im-
mediately into it, without going into the first stomach at all.
The folds of its lining membrane present a large surface,
through which fluid may be absorbed into the system. It is

134 ACT OF RUMINATION.

here that we find the curious arrangement of water-cells in the
stomach of the Camel, by which that animal is enabled to
retain a supi^ly of water for several days. These cells corre-
spond with the little pits which are seen in the honey-comb
stomach of the Sheep, but are much deeper, and their orifices
may be closed by the action of a set of muscular fibres wdiicli
pass in every direction round each, so as to form a net- work
including these orifices in its meshes.

199. After the food has been macerated in the fluids of the
lirst and second stomachs, it is returned to the mouth by
a reversed peristaltic action of the oesophagus, which brings
it up as a succession of globular pellets, that are formed by
compression in a sort of mould at the lower end of the oeso-
l)hagus. These pellets are subjected within the mouth to
mastication and insalivation ; and the food is then ready for
the real process of digestion. It is this mastication which is
commonly known as the " chewing of the cud ; " and the
animal, whilst performing it, seems the very picture of placid
enjoyment. When again swallowed, the food is directed, by
a pecuhar valvular groove at the bottom of the oesophagus,
into the third stomach, commonly termed the manyplies,
from the pecuhar manner in which its lining membrane is
arranged. This presents a number of folds, lying nearly close
to one another, like the leaves of a book, but all directed, by
their free edges, towards the centre of the tube, — a narrow
fold intervening between each pair of broad ones. The food
has, therefore, to pass over a large surface, before it can reach
the outlet of the cavity ; and this leads to the fourth stomach,
commonly termed the reed. This is the seat of the true digestive
process,^ the gastric juice (§ 204) being formed here only ;
and it is from this that the rennet is taken, which is used
in making clieese to cause the milk to coagulate or curdle.
In tlie sucking animal, the milk passes directly into this
fourth stomach, without entering either the first or second
stomachs, and without being delayed in the third, the folds
of which adhere together so as to form a narrow undi^•ided
tube. The paunch is at that time comparatively small, being
of less size than the reed ; and its dimensions increase, as
soon as the young animal begins to distend it by swallowing
solid vegetable matter.

200 lu the digestive apparatus of P.irds, we find a con-

DIGESTIVE APPARATUS OF BIRDS.

185

siderable modification of form, resulting from the fact that, as
these animals do not masticate their food, they require somQ

(Esophagus

Ventriculus\
Succenturiatusi

Gizzard

Pancreas

Duodenum

Liver

Gall-bladder
13ile ducts

Cceca

Large Intestine

Ureter

Oviduct

Cloaca

Fig. 111. — Digestive Apparatus of Fowl.

other means of reducing it. lliis means is provided for them
in their stomach. In the tribes whose food is of such a
nature as to require being moistened before it is rubbed down,
and especially in those which feed upon grains, the oesopha-
gus has a pouch-like dilatation, termed the crop or craw
(iig. HI) ; in iliis it is retained, and exposed to the action

286 TRITURATING ACTION OF GIZZARD.

of fluid secreted by its walls, just as it is in the pauncli of
ruminant quadrupeds. This crop is of enormous size in some
of the granivorous (grain-eating) birds, such^ as the Turkey.
The second stomach (or ventriculus succenturiatus) is the one
in which the gastric juice is secreted ; but this is seldom
large enough to retain the food, which passes-on through it
to the gizzard, a hollow muscle, furnished with a hard tendi-
nous lining. In the granivorous birds this is extremely
strong and thick ; and pieces of gravel are swallowed by
them, which, being worked-up with the food by the action of
the gizzard, assist in its reduction. In the rapacious flesh- or
flsh-eating birds, however, no such assistance is required, the
food being easy of solution ; the walls of their gizzard are
thin, possessing but few tendinous fibres ; and the three
cavities of the stomach are almost united into one.

201. Various experiments have been made to test the
mechanical powers of the gizzard of Birds. Balls of glass
which they were made to swallow with their food, were soon
ground to powder ; and the points of needles and of lancets,
fixed in a ball of lead, were blunted and broken-off by the
power of the gizzard, whilst its own internal coat did not
appear to be in the least injured. On the other hand it has
been ascertained, that grain enclosed in metal balls which
protected it from the mechanical action of the gizzard, but
which were perforated so as to afford the gastric fluid free
access to their contents, was not in the least digested ; so that
the utility, and even the necessity of this operation, become
evident.

202. As there are few animals, save the Mammalia, that
perform any proper masticaton in their mouths, the grinding
down of their food (where it is of such a nature as to require
it) must bo performed in the stomach ; and accordingly we
find many tribes, belonging to different divisions of the animal
kingdom, in which a gizzard, or something analogous to it,
exists. It is possessed by almost all Cephalopods, and by
many of the Gasteropoda. In the walls of the stomach of
some of these last, there is a considerable amount of mineral
matter deposited, intermixed with the hard tendinous fibres
of which they chiefly consist. A powerful gizzard is also
found in many Insects, but here it is placed above the diges-
tive stomach (fig. 112, c). The accompanying figure exhibits

187

the alimentary canal of a Beetle, from its commencement to
its termination. At a is seen the head, bearing the jaws, &c. ;
from this the gullet passes straight backwards, and is dilated

into a crop at 6, below
which is the gizzard, c. This
opens at its lower end into
the true digestive stomach,
d; which is surrounded by
an immense number of little
follicles or bags, by which
the secretion of the gasUic
juice is effected (§ 204).
Into the lower end of this,
the long vessels, e, open,
which constitute in Insects
ths only rudiment of a liver
(§ 358). In many of the
Crustacea, the walls of the
stomach are beset with re-
gular rows of teeth, which
are moved by the action of
powerful muscles. These
teeth are cast or shed at the
same time with the shell.
In the Wheel-Aniinalcules,
the place of the gizzard is
occupied by a curious pair of
jaws, armed with teeth ; by
the working of which, the
food is effectually crushed.
In the Bryozoa, a gizzard
exists between the oesopha-
gus and the true digestive
stomach ; and the stomach
itself is surrounded by the
little follicles which secrete
the bile, and pour it into
that cavity (§ 115).
203. In animals which subsist exclusively on flesh, how-
ever, no such complicated apparatus exists. Thus in Serpents
(fig. 34), the stomach is but a slight dilatation of the alimen-

112.— Digestive Apparatus of
Beetle.

188

DIGESTIVE APPAEATUS — GASTRIC DIGESTION.

tary tube ; and it is not easy to say where it commences and
terminates. In Spiders and Scorpions, too, which live upon
the juices they suck from other animals, the alimentary tube
is very simple ; and it is scarcely dilated into a proper sto-
mach. And in most of the Eadiated classes, we find the
stomach to possess only one orifice, through which the undi-
gested residue of the food is cast out, as well as fresh sup-
plies taken in. But this stomach is not always a simple
bag ; thus in the Star-fish it sends prolongations into the
rays, the use of which is at present un-
determined. There are certain animals in
which no digestive cavity exists : their
sustenance being derived either from the
juices jDrepared by other animals, in
whose tissues or cavities they are im-
bedded, and being introduced by absorp-
tion through the whole surface, as is the
case in the lower Entozoa (fig. 53) ; or
from particles which are drawn into the
midst of the soft gelatinous substance of
their bodies, and undergo a sort of diges
tion there, as is the case with the lihko-
poda (§ 129).

Gastric Blgestion : — Chymification.

204. The food which has been re-
chtred. in the mouth by the action of the
teeth, or in the stomach itself by the
movement of its own tendinous walls, is
prepared for the real process of digestion;
by which it is converted into a fluid,
and thus made fit to be truly received
into the system, by being absorbed into
its vessels. The chief agent in the
digestive process is a fluid termed the
gastric juice, which is secreted or sepa-
38 seen in a vL-rticai section rated from the blood bv a vast number

of thcStoniacli, maRnified , !• ^•ll^ \ o ^^• i /n t i .->\ •

three diameters at a, and ^^ ^itflc bagS Or lolllcleS (fig. 113), IIU-

tventy diameters at 1). bcddcd in thc walls of the stomacli.
When the cavity is empty, this fluid is secreted in very small
quantities ; but, like the sahvary secretion, it is poured out

Fig. 113.
Gastkic Follicles

SENSE OF HUNGER — SECRETION OF GASTRIC JUICE. 189

in abundance when the lining membrane is stimulated by
the contact of food, especially solid food. Only a limited
(quantity is secreted at any one time ; and this quantity is
just that which is sufficient to dissolve food enough for the
supply of the natural wants of the system. The contact of
any solid substances with the interior of the stomach, is suffi-
cient to produce a flow of this fluid into its cavity ; but the
secretion soon ceases if the substance be not of an alimentary
nature.

205. The sense of hunger appears due to the distension of
the blood-vessels of the stomach, which takes place in pre-
paration for the secretion of the gastric fluid. This deter-
mination of blood towards the stomach seems to occur when-
ever the body needs a fresh supply of nourishment ; and it
ceases as soon as a sufficient amount of gastric fluid has been
drawn off. Hence it is, that hunger is relieved by eating ;
and hence it is, also, that hunger is for a time relieved by
taking solid substances into the stomach, even though they
contain no nourishing matter. It is from having experienced
this, that savage nations are in the habit of mixing indiges-
tible solid matter with the fluids that sometimes constitute
their principal articles of food. Thus the Kamschatdales mix
earth or saw-dust with the train-oil on which alone they are
frequently reduced to live ; and the Yeddahs, or wild hunters
of Ceylon, mix the pounded fibres of soft or decayed wood
with the honey on which they feed when meat is not to be
had. One of them being asked the reason of the practice,
replied, " I cannot tell you, but I know that the belly must
be filled." It has been found by experunent, that soups and
other forms of liquid aliment are not alone fit for the support
of the system, even though they may contain a large amount
of nutritious matter ; and the medical man well knows, that
many persons have stomachs too weak and irritable to retain
"slops" (as they are commonly termed), who can yet digest
solid food of a simple kind. All these instances show, that
the contact of a solid substance with the walls of the stomach,
is the proper stimulus or excitement to the secretion of the
gastric fluid.

206. This fluid, when poured upon the food, is thoroughly
mixed-up with it by a peculiar movement of the walls of the
stomach, which is continually bringing fresh portions of the

190 TROPERTIES OF GASTRIC JUICE.

alimentary mass into contact with its sides, so that the whole
is after a time equally exi^osed to the influence of the gastric
secretion. If this movement Avere not to take ^^lace, only the
outside of the mass would be digested, and the central portion
would remain but httle affected.

207. The nature of the gastric fluid, and the mode of its
operation upon the food, have been studied by withdrawing
a portion of it from the stomach, and by observing its pro-
perties and actions out of the body. A sufficient quantity
for this purpose cannot be easily procured. Spallanzani, an
Italian physiologist of the last century, contrived to obtain
it, by causing birds and other animals to swallow sponges to
which pieces of thread were attached j these, when they had
remained long enough in the stomach to cause a secretion of
the gastric juice, were drawn up again ; and the fluid they
had absorbed was pressed out into vessels, in which its pro-
perties could be examined. More recently, however, an
advantageous opportunity has presented itself for obtaining
supplies of gastric fluid in a less objectionable manner. A
young man, named Alexis St. Martin, received a very severe
wound in his left side, by the bursting of a gun ; and al-
though this wound laid open the cavity of his stomach, he
recovered his health completely, and subsequently married
and had a family. There remained, however, an aperture in
his stomach, which would not close up ; and through this
orifice, which was usually covered by a bandage, the contents
of the stomach could be drawn out. The gastric juice was
obtained by introducing an India-rubber tube into the sto-
mach when it was empty, and by moving it about within the
(cavity ; the contact of the tube then excited the folHcles to
.secretion (on the principle already mentioned, § 204) ; and
the fluid thus poured into the stomach was drawn off through
the tube.

208. The Gastric Juice is very like saliva in its appearance,
Imt it is distinctly acid to the taste ; and it is found, by
chemical examination, to contain a considerable quantity of
muriatic acid '^ in an uncombined state. Besides this, it con-
tains a considerable quantity of a peculiar animal substance
which seems like altered albimien, and which has been desig-
nated pepsin; as well as other ingredients of less importance.

* Muriatic acid is commonly known as spirit of salt.

ACTION OF GASTRIC JUICE. 191

This fluid possesses the power of dissolving albuminous sub-
stances of various kinds, when these are submitted to its
action at the constant temperature of 100° (which is about
that of the stomach), and are frequently shaken-up with it.
The solution appears to be in all resjoects as jDcrfect as that
which naturally takes place in the stomach, but requires a
longer time. It does not seem, however, that the gastric juice
has a special solvent power for any other than albummous
substances. Gelatinous and saccharine matters are taken-up
by it, as by other watery fluids ; but neither starchy nor
oleaginous substances undergo any other change by its action,
than consists in the separation of their particles by the solu-
tion of the membranes and fibres which held them together.
There is every reason to believe that what is true of artificial
is true of natural digestion ; and that so far from the whole
operation being performed in the stomach, as was formerly
supposed, gastric digestion is limited to the solution of the
albuminous, gelatinous, and saccharme constituents of the
food.

209. With regard to the precise mode in which the gastric
fluid acts in dissolving albuminous substances, there is yet
some uncertainty ; although there can be no longer any rea-
sonable doubt, that the operation is of a purely chemical
nature. An artificial gastric fluid, capable of eflecting all
that can be done by that which is secreted in the liviaig
stomach, may be made, by macerating (or soaking) a portion
of the membrane lining the stomach of a pig, or of the fourth
stomach of a calf (even after it has been washed and dried)
in water, which dissolves a portion of the pepsin ; and by
then acidulating this solution with muriatic or acetic acid.
It has been proved that both the acid and the pepsin are
essential to the process of solution ; for the acidulated fluid
without the animal matter acts extremely slowly upon pieces
of meat, hard-boiled qq^, &c., submitted to it ; and water in
which the stomach has been macerated, but which contains
no acid, will not act at all. But the acidulated water alone
will readily dissolve the substances just mentioned, at a higher
temperature ; and thus it appears that the acid is the real sol-
vent ; and that the pepsin has for its oflice to produce some
change in the albuminous substances, by whicli they are more
readily dissolved, The recent inquiries of Liebig and other

192 GASTRIC DIGESTION : CHTMIFICATION.

Chemists, render it probable that this change is of the nature
of fermentation.

210. It is a fact of gi-eat practical importance, that a cer-
tain quantity of the gastric fluid can act only upon a limited
amount of alimentary matter ; so that, if more food be taken
into the stomach than the gastric fluid can dissolve, it remains
there undigested. Isow it has been already mentioned, that
the quantity of the gastric fluid secreted at any one time, is
proportional, not to the amount of food in the stomach, but
to the wants of the system ; so that, if more food be swal-
lowed than is required to repair the waste of the body, it
Ues for some time unchanged in the stomach, and becomes a
source of irritation which prevents the due discharge of its
functions ; and the evil goes on increasing with every addi-
tion to the contents of the cavity. This may not be felt by
the individual at the time ; but it leaves permanent effects,
which manifest themselves sooner or later in derangement of
the general health. The habit of taking more food than is
really necessary, and of irritating the stomach by stimulating
substances or fluids (such as pepper, mustard, spirits, &c.), is
a fertile source of disease. The injurious effects of these are
manifested by the thirst which is the consequence of their
use, and which is a call (as it were) on the part of the stomach,
to prevent their irritating action by diluting them with water.

211. By the solution of its albuminous portion, and the
separation of its other component particles, the food is re-
duced in the stomach to a kind of pulp, which is termed
chyme. The consistence of this will of course vary accord-
ing to the nature of the food, and the quantity of fluid in the
stomach ; but in general it is grayish, semi-fluid, and uniform
tliroughout. When the food has been of a rich character, the
aspect of the chyme resembles that of cream ; but when the
food has consisted of farinaceous substances (rice, potatoes,
&c.), the chyme is more like gruel. At the point where the
stomach opens into the intestinal canal, which is called the
'pylorm, there is a kind of valve, which permits the chyme
to pass as fost as it is formed, but closes against the portions
of the food which are yet solid and undigested ; and thus the
chyme escapes from the stomach in successive waves, slowly
at iirst, but afterwards more rapidly, as the digestive process
approaches its completion.

INTESTINAL DIGESTION. 193

Intestinal Digestion; Chylification.

212. The process of digestion is by no means completed in
the stomach ; for much of the matter which escapes from it
in the chyme, is destined to undergo a further cliange whilst
"passing through the intestinal canal ; especially in the her-
bivorous tribes, whose food, being less digestible than that of
the carnivorous races, requires to be longer delayed in the
intestinal canal, in order that it may yield up its nutritious
portion. Hence we find this canal of enormous extent in
most animals whose food is vegetable, being in the Sheep
about twenty-eight times the length of the body ; in the
purely carnivorous animals, on the other hand, it is compara-
tively short, being in the Lion only about three tunes the
length of the body, while in the Serpent it runs almost
straight from one extremity to the other ; and in animals
which hve on a mixed diet, it is of medium length, being
in Man about six times as long as his body. The intes-
tinal tube is usually distinguished into the small and the
large intestine ; of which the small is the first portion, and
the large the second. The former, as shown in fig. 108, is
disposed in a convoluted or twisted manner, so that a great
extent of it may be packed within a small compass ; it
usually forms about three-fourths of the whole length of the
canal. It is held in its place by a serous membrane termed
the peritoneum, which forms an immense number of folds
that suspend it (as it were) from the vertebral column ; but
these stiU allow it a considerable power of movement.

213. Soon after passing from the stomach into .the intes-
tinal canal, the food is mingled with three secretions, which
have an important influence on the changes it is further to
undergo ; these are the Bile, the Pancreatic fluid, and the In-
testinal juice. The two former are prepared by two large glan-
dular masses, the Liver and the Pancreas (or sweetbread),
which, in all the higher animals, are completely detached
from the alimentary canal, and send their secretions into it
through special ducts ; the latter, like the gastric juice, is
formed in little follicles lodged in the wall of the canal itself.
The peculiar matter which forms the chief solid constituent of
hile, is essentially a soap formed by the union of two resinoid
acids, with soda as a base (§ 364). The composition of the

o

194 BILIARY, PANCREATIC, AND INTESTINAL SECRETIONS.

pancreatic fluid closely corresponds with that of saliva, which
it much resembles in appearance. The intestinal juice, like
the gastric, is a nearly colourless, somewhat viscid fluid, con-
tainilig an organic compound not far removed from albumen ;
but it°differs from the gastric juice in being alkaline instead
of acid. The relative offices of these three fluids have not
yet been determined with certainty ; but there appears good
reason to believe : (1) that the bile, by its alkalinity, neutralizes
the acidity which the chyme derives from the gastric juice,
and that this neutralization favours the metamorphosis of
starch into sugar, which has been almost suspended in the
stomach ; (2) that the bile aids the pancreatic fluid in re-
ducing the oleaginous particles to the condition of an emul-
sion, that is, in bringing them into a state of very minute
division, in wliich tliey remain suspended in the albuminous
solution ; (3) that the pancreatic fluid aids the salivary mat-
ter which was swallowed with the food, in the transforma-
tion of starch into sugar ; (4) that the intestinal juice has a
solvent power for albuminous substances which is scarcely
inferior to that of the gastric juice, with a power of converting
starch into sugar which is scarcely inferior to that of saliva
or pancreatic fluid. The fluid of the Small Intestine, com-
pounded of the salivary, gastric, intestinal, biliary, and pan-
creatic secretions, appears to possess a far greater digestive
power than that of the stomach, being capable of dissolving,
or at any rate of reducing to an absorbable condition, nutri-
tious substances of every class. This process goes on during
the passage of the alimentary mass along the small intes-
tine ; and the nutritious materials are progressively with-
drawn by absorption, partly into the blood-vessels, which
appear to receive whatever are in a state of perfect solution
(§ 218), and i)artly into the lacteal absorbents, which take up
nothing but that peculiar emulsion of albumen and fatty matter
which is termed chyle (§ 222).

214. At the extremity of the Small Intestine, there is a
kind of pouch, called the coecum ; which in some animals
seems almost like a second stomach, and which is furnished
with one or more little appendages, termed coeca."^ This is very
small in Man, and does not seem to perform any important

* The word coecum is used in Anatomy to denote a tube closed at one
extremity.

PERISTALTIC MOVEMENT DEFECATION. 195

fimction ; but in most herbivorous animals it is larger (as
in the Monkey, fig. 30) ; and it is found to secrete an acid
fluid, which resembles the gastric juice, and which may have
for its office to perform a second digestion upon the sub-
stances which have escaped the first. These ca^ca are some-
times very large in the intestinal canal of Birds (fig. 111). —
From the coecum, the Large Intestine ascends as high as the
liver, crosses the upper part of the abdomen, and then
descends again, as shown in fig. 108 ; this portion is termed
the colon ; and it terminates in the rectum^ which forms the
extremity of the intestinal tube.

215. The alimentary mass is propelled along the first part
of the intestinal canal, — and the residue left after the absorp-
tion of the nutritive materials is carried along the continua-
tion of it, — by the contraction of its muscular coat, producing
what is termed the perutaltic motion of the bowels. The fibres
of this muscular coat are chiefly arranged in a ring-like
manner around the tube ; so that, when they contract, they
narrow the diameter of the tube. They are stimulated to
contract by the contact of the solid or liquid matter passing
through it (Chap, xii.) ; and their contraction forces this matter
onwards, into the succeeding portion of the tube. This con-
tracts in its turn, so as to propel its contents further ; and thus
the mass is gradually diiven from one extremity of the canal
to the other. The peristaltic movement does not seem to
depend (as do the contractions of the muscles concerned in
swallowing, § 195) upon the nervous system ; for it will take
place after the intestinal tube has been completely separated
from the principal nervous centres ; and also after the death
of the animal, if this have been produced by a sudden cause.
Thus, if a Kabbit be killed by a smart blow at the top of the
neck, and the abdomen be immediately opened, the peristaltic
movement will be seen in vigorous action, especially if the
animal have eaten a full meal an hour or two previously.

Defecation.

216. In passing through the large intestine, the undigested
residue is still more completely deprived of the nutritive
matter it may contain ; and its fluid portion is absorbed, so
that it becomes more solid. It is allowed to accumulate in the
rectum, until its bulk occasions inconvenient pressure upon

o2

196

DEFECATION LACTEAL ABSORPTION.

the surrounding parts ; and it is kept-in by a circular muscle
or sphincter, which surrounds the outlet of the alimentary
canal. Eut when the accumulation has taken place beyond
this amount, it excites a reflex action (§ 195) in the muscles
that surround the abdomen ; and these make pressure suf-
ficient to overcome the resistance of the sphincter, and to
force out the contents of the rectum.

Absorption of Nutritive Material.

217. We have only now to inquire into the mode, by which
the nutritive matter extracted from the food is taken-up from
the alimentary canal and applied to the nutrition of the body.
In all Vertebrated animals, there exists a special set of vessels
termed Ahsorhents: of which those formino; one division.

Thoracic Mesenteric

Aorta Duct

Glands

Origins of
--'' Lacteal Vessels

• — Intestine

Lymphatic Mesentery

Vessels

Fig. 114,— Chyle-vessels,

known as Lacteals, from the milk-like character of their con-
tents, originate in tlie numberless villi or minute projections
Avith which tlie mucous membrane that lines the small intes-
tmo IS covered (§ 41). During the act of digestion, the

ABSORPTION BY LACTEALS AND BLOOD-VESSELS. 197

epithelmm-cells, which clothe the extremity of each vilhis
(iig. 9), become distended with an opalescent fluid, the chyle
(§ 222), which they select from the contents of the small
intestine ; and this is subsequently given up by them to a
lacteal tube, which, without any open mouth, commences in
the midst of each villus. The vessels which thus originate,
unite into minute trunks, and these again into larger ones ; and
these pass between the two layers of the mesentery (or fold of
peritoneum by which the intestines are suspended, § 212)
towards the lower part of the spinal column : where they
deliver their contents into a sort of reservoir, which thus
becomes the receptacle for all the chyle that has been collected
from the alimentary canal (fig. 114). In traversing the me-
sentery, the lacteals of the higher animals pass through little
knot-like bodies of a peculiar nature, which are called mesen-
teric glands. These appear to afford the means for the per-
formance, within a more concentrated space, of the assimi-
lating action which is carried on during the passage of the
chyle through the lacteal system ; for in Reptiles, in which
these glands do not exist, the absorbent vessels are much
more extended and spread out than they are in Birds and
Mammals.

218. ]!^ear the surface of each of these villi, moreover, lies
a minute network of Blood-vessels ; and there is now no
longer any doubt that these receive, by simple imbibition,*
any substances, whether alimentary or otherwise, which exist
in a state of perfect solution in the contents of the intestinal
canal. For a great variety of such substances have been
detected, by chemical analysis, in the blood which is returned
from the walls of the intestines by the mesenteric veins ;
whilst it is seldom that anything is found in the lacteals,
save the proper constituents of chyle. It is through this
channel that poisonous substances are taken into the circula-
tion ; and these may be absorbed from the walls of the
stomach (on which there are no villi or lacteals), without ever
passing from it into the intestinal tube. Hence it is a great

* That tendency — called Endosmose—v^hich. thinner liquids have to
pass-towards and mix-with such as are more viscid, even through an
intervening membrane, seems to be the physical cause (as experi-
ment indicates) of this imbibition ; which is greatly promoted by the
movement of blood in the vessels.

198 ABSORPTION BY LYMPHATICS.

mistake to characterise the lacteals (with the lymphatics) as
Absorbents in any exclusive sense ; the fact being that their
function is limited to a special selective absorption, whilst
the more general action is performed by the blood-vessels.

219. But the reservoir above-mentioned receives, not only
the lacteal vessels that bring nutritious matter from the intes-
tinal tube, but also lymphatics, which are absorbent vessels
of similar character, that originate in every part of the body.
These, also, pass through a set of (so-called) glands, in their
way towards this receptacle ; and the structure of these glands,
of wliicli many are seated in the neck, some in the arm-pit,
others in the groin, &c., is exactly the same as that of the
mesenteric glands. The fluid they convey, which resembles
very dilute liquor sanguinis (§ 229), seems evidently destined
to be again a2)plied to the purposes of nutrition. There is
some obscurity as to its source ; but it seems probable that
it may partly consist of the residual fluid, which, having
escaped from the blood-vessels into the tissues, and having
furnished the latter with the materials of their nutrition, is
now to be returned to the former ; and partly of those par-
ticles of the body, which, though they have lost their vitality
in the course of the change it is continually undergoing,
have not undergone a degree of decay that unfits them for
serving, like the dead bodies of other animals, as a material
for reconstruction by the organizing process. The lymphatics,
being copiously distributed in the true Skin, absorb substances
which are introduced into its tissue ; and if these substances
be of an irritating nature, they may occasion an inflammatory
action in the absorbents and their glands. Thus when poisoned
wounds in the hand have been received, as in opening the
bodies of men or animals that have died of particular diseases,
the cfl'ect is usually manifested at first by heat and pain in the
arm, along which the inflamed absorbents can be traced as
hard cords ; and the glands in the arm-pit swell and become
tender.

220. The lymphatics do not appear destined, however, to
absorb from the surface of the skin ; this function being per-
formed by the blood-vessels which are distributed abundantly
in its substance. It is a fact now well established, that when
the quantity of fluid in the body has been greatly reduced,
absorption of water through the skin may take place to a

ABSORPTION THROUGH SKIN THORACIC DUCT. 199

considerable amount. Thus there is a case recorded by Dr.
Currie, of a patient who suffered under obstruction of the
gullet, of such a kind that no nutriment, either solid or fluid,
could be received into the stomach ; and who was supported
for some weeks by immersion of his body in milk and water,
and by the introduction of nutritive liquids into the lower
end of the intestine. During this time, his weight did not
diminish ; and it was calculated by Dr. Currie, that from one
to two pints of fluid must have been daily absorbed through,
the skin. The patient's thirst, which had been very trouble-
some previously to the adoption of this plan, was removed by
the bath, in which he experienced the most refreshing sensa-
tions. — It is well known that shipwrecked sailors and others,
who are suffering from thirst owing to the want of fresh
water, find it greatly alleviated, or altogether relieved, by
dipping their clothes into the sea, and putting them on whilst
still wet.

221. From the receptacle into which the chyle, and a con-
siderable proportion of the contents of the Ijonphatics, are
delivered, a tube passes upwards in front of the spine (fig.
114) ; and this tube, called the Thoracic Duc% conveys these
nutritious fluids to the point where they are to be delivered
into the current of blood. This dehvery takes place at *he
angle where two great veins unite, — a point at which there is
less resistance than in any other part of their walls. These
veins are the Jugular, which brings the blood from the neck,
and the Subclavian, which conveys it from the arm, of the
right side (fig. 122) ; on the left side there is a smaller duct,
which receives some of the lymphatics of the left side, and
opens into the blood-vessels at a corresponding point between
the left jugular and subclavian veins.

Sanguification.

222. The Chyle of Vertebrated animals, as taken-up by the
lacteals, may be regarded as blood in an early stage of its
formation, with a large excess of fatty matter. It contains
about 90 parts of water in 100 ; about 3| j)arts of albumen,
and the same of fat ; and about 3 parts of other animal and
saline matter. Its appearance and characters differ, according
to the part of the lacteal system from which it is drawn. If
obtained near the surface of the intestines, before it has passed

200 PROPERTIES OF CHYLE SANGUIFICATION.

through the glands, it is entirely destitute of that power of
spontaneously coagulating, or clotting, which is so remarkable
in blood : and when examined with a microscope, it is seen
to present a number of oily globules of various sizes ; together
with an immense number of very minute particles or mole-
cules, which also seem of a fatty nature ; and to these last,
whose diameter is between l-24,000th and l-36,000th of an
inch, the milky whiteness which characterises chyle appears
principally due. But the chyle drawn from the lacteals, after
they have passed through the mesenteric glands, possesses the
power of coagulating slightly ; hence it would seem that some
of its albumen has undergone a transformation into fibrin
(§ 17). At the same time, a great increase is observed in
the number of certain floating corpuscles, which are occa-
sionally to be noticed in the first chyle, but which are very
abundant in the fluid drawn from the glands and from the
lacteals that have passed through them ; of these, which bear
a strong resemblance to the colourless corpuscles of the blood
(§ 234), the average diameter is about 1-4, 600th of an inch. —
By the time that the chyle reaches the central receptacle, its
power of coagulating has still further increased ; so that its
resemblance to blood, except in regard to colour, is much
stronger. The proportion of hbrin and albumen which it
contains, is much greater than that which existed in the first
chyle, whilst the amount of oily matter is less.

223. There can be little doubt that the change which the
chyle undergoes in its passage through the lacteals, is partly
due to the influence of the living walls of these vessels upon
the fluid in contact with them, and partly to that of the
colourless corpuscles which float in the fluid, and which form
the principal constituents of the absorbent glands. The whole
apparatus, indeed, may be looked upon as one great Assimi-
lating Gland, having for its function to make blood out of
crude nutriment ; provided-for in the higher Vertebrata by the
convolution of the lacteals in the mesenteric glands, and in the
lower, by the simple extension of the vessels themselves. It is
probable that, by being brought into very close neighbourhood
with the blood in these glands, the chyle may be made to
undergo some further change ; although, as each fluid is con-
tained in its own tubes, which do not communicate, there can
be no proper intermixture.

ASSIMILATING GLANDS ABSORPTION IN INVERTEBRATA. 201

224. There are certain glandular bodies, disposed in various
parts of the system, which seem to discharge a similar office ;
withdrawing the raw material (so to speak) from the general
current of the circulation, and returning it again in a state of
higher elaboration. Such are the Spleen, the Thyroid and
Thymus glands, and the Supra-Eenal capsules. Besides these,
the Liver probably exerts an assimilating action upon the crude
materials which are made to pass through its substance, almost
immediately after having been received into the blood-current,
and before they are allowed to pass into the general circula-
tion ; the whole of the blood returned by the gastric and
mesenteric veins from the walls of the alimentary canal, being
conveyed through the liver by the portal system, in its way to
the heart (§ 267).

2'25. In the Invertebrated animals, neither lacteals nor
lymphatics exist; and the blood-vessels, whose absorbent
powers are to a certain extent restricted in the higher animals,
have to perform the functions of these. There are animals,
however, which are destitute not only of lacteal and lymphatic
vessels, but even of blood-vessels ; and in these, as in the
CeUular Plants, there is but little transmission of fluid from one
part of the body to the other ; for every portion, both of the
internal surface (or lining of the stomach), and of the external
surface which is bathed in the surrounding fluid (for most of
these animals are aquatic), seems equally to possess the power
of absorption ; and the parts to w^hose nourishment the fluid
thus received into the body is to be appropriated, are in the
immediate neighbourhood of those which have absorbed it.
This is the case, for example, in the Hydra and Sea-Anemone,
and, more or less, in all the Polypes ; as well as in the lower
Worms. Between these, therefore, and the Cellular Plants, a
remarkable analogy exists in regard to the mode in which the
nutriment is absorbed and applied ; the diff"erence being, that
the Animal possesses a digestive cavity, lined by an inward
extension of the external surface, which does not exist in
Plants (§ 8). And it is upon the walls of this cavity, that
the absorbent vessels of the higher Animals (whether lacteals
or blood-vessels) are distributed, collecting the nourishment
in contact with them ; just as the roots of a Plant, spread
through the soil, draw up that which it contains. But among
those lowest animals in which the digestive cavity altogether

202 OF THE BLOOD, AND ITS CIRCULATION.

disappears (§ 203), the function of absorption is not in any
wa}' limited ; since every part seems to have the power of re- »
ceiving from without, and of assimilating to its own substance, I

the nutrient materials which it needs.

CHAPTER V.

OF THE BLOOD, AND ITS CIRCULATION.

226. The processes that have been already explained, have
for their object to prepare the nutritious fluid, which suppUes
the materials for the growth of the several parts of the body,
and which is conveyed through them by the apparatus to be
presently described. In Man and the higher animals, this
fluid, which is known as tlie Blood, has a red colour, and con-
tains a large quantity of solid matter. The redness of the blood
has been mentioned as a distinctive character of the Verte-
brated classes (§ 75) ; it exists in Mammalia, Birds, Eeptiles,
and Fishes, and in these alone. In the Molluscous classes, as
also in most of the Articulated, the nutritious fluid is nearly
colourless ; and it will hereafter appear that this fluid bears,
in some respects, a stronger resemblance to the chyle and
lymph of the Vertebrata, than to their blood (§ 234). There is
an apparent exception in the case of certain marine Worms,
the fluid circulating in whose vessels has a reddish hue ; this
does not depend, however, upon the presence of any red par-
ticles, such as are characteristic of the blood of Vertebrata
(§ 229), but upon a reddish tinge in the fluid itself, which
does not seem altogether to answer to the character of
blood (§ 294).

227. The blood of all the higher animals exists in two
difl'crent states. Wlien it is drawn from a slight scratch or
other wound of the skin, it is of a bright red hue ; whilst that
which is drawn in bleeding from the arm, is of a dark purple.
The former is termed arterial blood, because it is contained, for
the most part, in the tubes which are called Arteries, and
which are conveying it from the heart to the tissues it has to
nourish. The latter is called venous blood, because it is drawn
from the Veins, by which it is returned from the tissues to
the heart, after having performed its part in them. Hence it

VENOUS AND ARTERIAL BLOOD. 203

is evident that this change of character has been produced
during the passage of the blood through the tissues ; and so
important is the alteration, that the blood which has been
subjected to it is not fit to pass again into the arteries of the
body, until it has been renewed by exposiu-e to air in the
Lungs, In their vessels, the contrary change — of which the
nature will be presently explained (§ 253) — is effected, the
dark hue of venous blood giving place to the bright red of the
arterial fluid ; this is again changed during the passage of the
blood through the body, to be again restored in the lungs.
The same is the case in regard to Fishes, whose gills perform
the same function as the lungs of air-breathing Vertebrata.
And among the Invertebrated classes, although the deteriora-
tion of the blood in its passage through the body is not made
manifest by any change of colour, yet its renewal by exposure
to air in the respiratory organs is not less requisite.

228. Hence the continual movement of the blood is neces-
sary for two purposes in particular ; — first, to convey the
nutritive materials from the place where they are received and
prepared, to that in which they are appropriated, and thus
to afford to every organ a constant supply of the materials
which it requires ; — and, second, to carry this fluid, at regular
intervals, to certain organs by whose instrumentality it may
be exposed to the influence of the air, so as to regain the
qualities it has lost, and part with what it has taken-up to its
prejudice. But there are many other objects fulfilled by it,
which will unfold themselves as we proceed.

Properties of the Blood.

229. Wlien the circulating blood of a red-blooded animal
is examined with a microscope, it is seen to consist of two
distinct parts ; — a clear and nearly colourless fluid, to whicli
the name of liquor sanguinis (or liquor of the blood) is given ;
and of an immense number of rounded particles floating in
this fluid, which are often termed the glohides of the blood. The
shape and size of these particles are, for the most part, very
uniform in animals of the same species ; but in no instance
are they globular ; and it is better, therefore, to term them
corpuscles. In Man and most other Mammals, they are
nearly flat discs, resembling pieces of money, but usually
exliibiting a slight depression towards the centre (fig. 115).

204 BLOOD-DISCS OF MAN AND MAMMALS.

No nucleus can be distinguislied in tliem, but they present a
dark central spot, which is an optical effect of their bi-concave
form ; and this spot may be made to disappear by the addition

Fig. 115. — Red Cokpuscles of Human Blood.
Seen separately at a, a a showing the front view, b the profile or edge view, and * a
three-qi:arter view; at b united with each other so as to form columns likepiljs
of money; at c in a state of alteration such as exposure to air will produce;
D shows a colourless corpuscle, or lymph-globule.

of water to the liquid in which they are suspended, the discs
first becoming flat, then bulging- out on either side, and at
last swelling so as to burst. The reason of this will be pre-
sently explained (§ 231). In Man and Mammals generally,
the diameter of these blood-discs varies from about l-2800th to
1 -4000th of an inch ; but in the small Musk-deer, it is less
than 1-1 2,000th. In the Camel tribe, the discs are oval, as
in the lower Vertebrata.

230. In Birds, Eeptiles, and Fishes, the blood-particles
present some curious differences from those of Mammalia.
In the first place, they are much larger ; their form, also, is
oval instead of being round ; and instead of being depressed
in the centre, they bulge-out on each side. Tliis bulging is

^

Fig. 116. — Blood Corpuscles of Pigeok.
At A are seen the red corpuscles a, 6, and the colourless, or lymph globules c, c; at
D, a red corpuscle treated with acetic acid ; and at c, the same treated with water,
80 as to render the nucleus more distinct.

ovidently occasioned by the presence of a nucleus which is
more sohd than the rest ; the nucleus, however, is not so well

BLOOD-DISCS OF BIRDS AND REPTILES.

205

seen in the corpuscles of circulating or of freshly-drawn blood,
as it is in that of blood which has been drawn for some little
time ; and it is best brought into view by treating the blood
either with water or with acetic acid. The long diameter of
the oval discs of Birds (fig. 116) varies from about l-1700th
to 1 -2400th of an inch ; and the short diameter from about

Fig. 117.— Blood Corpuscles of Frog.

At A are seen the red corpuscles a, b, and the colourless corpuscle c ; at b, a red

corpuscle treated with acetic acid.

1 -300th to l-4800th. Thus the discs, though much longer than
those of Man, are not in general much broader. In Eeptiles,

Fig. 118. — Blood Corpuscles of Proteus.

a, b, red corpuscles ; a*, corpuscle showing the nucleus ; c, colourless corpuscle ■

d, red corpuscle treated with water.

206 BLOOD-DISCS OF REPTILES AND FISHES.

there is considerable diversity as to the size of the discs ; but
the largest particles are found in the group of Amphibia, and
especially in those species which retain their gills through
life. The oval discs of Frogs (fig. 117) have a long diameter
of about 1-1 000th of an inch, and a transverse diameter of about
1-1 800th. Those of the perennibranchiate Amphibia (§ 87)
may even be distinguished by the naked eye ; those of the
Siren having a long diameter of about 1 -435th of an inch, whilst
in the Proteus (fig. 118) the long diameter is stated occasionally
to reach 1 -337th of an inch. In Fishes, also, the size of the
blood-discs is variable ; they are
sometimes smaller (fig. 119), though
generally larger, than those of the
Frog ; but they never approach those
of the last-named remarkable ani-
mals. Hence the great size of the

Fig. 119.-BL00D Corpuscles OP blood-discs of the CUrioUS Lcpido-

RoACH. si^^n (fig. 41) is strongly indicative

a, a, b, red corpuscles ; c, colour- £ ^j Ecptilian affinities of that

less corpuscle; d, red corpuscle '^^ , ^ J^

treated with water. SpecicS.

231. It is by observing the large blood-discs of the Frog,
and still better those of the Proteus and Siren, that we can
obtain the best information as to their structure. They are
exidenilj fattened cells, having an envelope or cell-wall, which
consists of an extremely delicate membrane, and which con-
tains a fluid. The nucleus consists of an assemblage of minute
granules, which seem adherent to each other and to the wall
of the cell ; and it corresponds, in all essential particulars, to
the nuclei of the cells of other Animal tissues (§ 32). The
fluid contained in the cells has a red colour.; and it is to this
that the peculiar hue of the blood of Yertebrata is owing.
When we are looking at a single layer of blood-discs, how-
ever, their red colour is not apparent, but they have rather
a yellowish tint; and it is only when we look through a
number at once, that the characteristic hue is seen. The
fluid is of about the same density as that in which the par-
ticles float ; and thus neither will have a tendency to pass
towards the other. T3ut, if Ave dilute the liquor sanguinis
with water, the fluid outside the cells will have a tendency
to pass towards their interior, according to the law of Endos-
mose. The cells Avill in consequence be first distended, and

STRUCTURE AND COMPOSITION OF RED CORPUSCLES. 207

will then burst ; and their contents will be diffused through
the surrounding fluid, whilst their membranous walls will
subside to the bottom. On the other hand, if the liquor
sanguinis be rendered denser than the fluid in the blood-
discs, as by the admixture of gum or syrup, the latter will
pass towards it, and the cells will become still more flattened,
and more or less completely emptied. The flexibility and
elasticity of the blood-discs are well seen, in watching (with
a microscope) its flow tlirough the minute vessels ; for if one
of them meets with an accidental obstruction to its progress,
its form becomes accommodated to that of the space left for
it to pass, and it makes its way tlirough a very small ajDcrture,
recovering its usual form immediately afterwards.

232. The Red Corpuscles differ considerably in chemical
composition from the liquid in wliich they float. Of the
solid residue obtained by drying, about one-eighth is formed
by their cell-walls, the remainder being yielded by the cell-
contents. The latter portion seems to consist chiefly of a
mixture of two components, which have been named glohulin
and h(Ematin. The former is a colourless substance, nearly
allied to albimien in composition, but differing from it in
some of its reactions ; its most characteristic peculiarity, how-
ever, being its power of crystallizing. Its crystals, the form
of which varies in different animals, are usually tinged deeply
with haematin, from which they cannot easily be freed. The
composition of hsematin, to which alone the colour of the red
corpuscles (and consequently of the whole mass of the blood)
is due, is notably diflerent from that of the albuminoid
compounds ; the proportion of carbon to the other components
being much greater, and a definite quantity of iron being an
essential part of it. This iron, in a certain state of oxidation,
has been supposed to be the source of the red colour; but
such is certainly not the case ; and this hue must be, like the
colours of Plants, a peculiar attribute of the organic compound
which presents it. — Besides their globulin and haematin, the
red corpuscles contain a certain proportion of fatty and
mineral matters. The former, which are united with phos-
phorus, are of a kind which are scarcely traceable in the
liquor sanguinis ; and the latter are remarkable as having
potass for their principal base, whilst the base of the salts of
the liquor sanguinis is chiefly soda. Hence it appears that

208 PROPORTION OF RED CORPUSCLES.

the Eed Corpuscles draw into themselves nearly the whole
of the iron, phosphorus, and potass, which the chyle pours
into the circulating current; and that they modify a large pro-
portion of the solid matter of the blood, that which they con-
tain being notably different in composition from that of the
liquor sanguinis, which does not differ, save in the proportion
of its components, from the liquid portion of Chyle or Lymph.

233. The proportion of Eed Corpuscles to the whole mass
of the blood varies greatly in different animals, and even in
different states of the same animal. It is greatest in those
which have the highest muscular vigour and activity, and
which consume the largest quantity of oxygen by respiration ;
hence these particles are rather more numerous in the blood
of Birds than in that oi Mammals, and far more abundant
in these last than in Eeptiles or Fishes. Again, they are
more numerous in Men of ruddy complexion, strong pulse,
and active habits, than in those of pale skins, languid circu-
lation, and comparatively feeble powers. In a healthy Man
they seem to constitute about half the mass of the circulating
blood ; but they contain as much as three-fourths of its solid
matter, the proportion of dri/ corpuscles being about 150 in
1000 parts of blood, whilst that of the other solid matters
is about 50. A very marked decrease occasionally presents
itself in disease ; the proportion of dry corpuscles being some-
times reduced as low as 27. When too abundant, they pro-
duce what is known as the plethoric condition of the body,
in which haemorrhage from the bursting of a blood-vessel is
liable to occur. Their number is effectually reduced by bleed-
ing ; and the aspect of those who have suffered from extreme
loss of blood, gives sufficient evidence that the deficiency is
not made-up for a long period. The most effectual means of
restoration, in cases where the proportion of blood-corpuscles
is too low, is a highly nutritious diet, with the administration
of iron as a medicine ; for this substance seems to have the
power of hastening the reproduction of the corpuscles, being
itself an essential ingredient in their contents ; and there
arc facts which show its remarkable power of increasing their
amount in proportion to the mass of the blood.

234. It appears that the red corpuscles, like other cells,
have a certain allotted term of life ; and as they are con-
tii.ually dying, they must be as continually reproduced. The

COLOURLESS CORPUSCLES — USES OF RED CORPUSCLES. 209

mode in which this reproduction is effected has not yet been
clearly made out ; hut there is strong reason to believe that the
red corpuscles are developed from the corpuscles of the chyle
and lymph (§ 222) which are continually being poured into
the circulating current, and of which isolated examples, known
as the white or coloitrless corpuscles, are met with in every
drop of blood that is examined under the microscope. The
size of these is pretty much the same in all Vertebrata, their
diameter being usually about l-3000tli of an inch. In the
blood of Maji and the Mammalia in general (fig. 115, d) they
are not easily distinguished from the red particles ; their
diameter being nearly the same, while the colour of single
discs of the two kinds is not very dissimilar. But in the lower
Vertebrata, whose blood has large oval red particles, the differ-
ence between the two kinds is very obvious ; and the resem-
blance which the colourless globule!^ (c, figs. 116-119) bear to
those of the chyle and lymph, is very striking. Similar colour-
less particles exist, to a variaoie amount, in the nutritive fluid
of Invertebrated animais ; so that in this, as in some other
respects, that fiaid bears a stronger resemblance to the chyle
and lymph of the Vertebrata, than it does to their blood,
which is characterised by the presence of the red particles.

235. Physiologists are now generally agreed, that one of
the functions of the Eed Corpuscles is to convey oxygen from
the lungs to the tissues and organs through which the blood
circulates, and to bring back the carbonic acid which is set
free in these, so as to deliver it at the lungs. For although
it is certain that the liquor sanguinis can also convey these
gases, yet experiment shows that the red corpuscles can take
up, bulk for bulk, a much larger proportion of them ; and
that the blood w^hich is richest in these particles is, therefore,
most fit to serve as the medium for the transmission between
the respiratory organs and the body at large. I^^ow it is in
the nervo-muscular apparatus that there is the greatest demand
for oxygen ; for this apparatus is not capable of vigorous
action, unless oxygen be freely supplied to it. The quantity
of this it requires, however, depends upon the exercise of its
powers ; for when at rest, it needs little or no more than is
made use of by the other tissues ; but whilst in activity, it
needs a greatly-increased supply. The quantity of oxygen
which the animal takes-in by its lungs, and the amount of

p

210 USES OF RED CORPUSCLES LIQUOR SANGUINIS.

carbonic acid whicli it gives-off by the same channel, vary,
therefore, with the muscular exertion it makes. This variation
is most easily observed and measured in Insects ; and it is found
in them to be enormous (§ 308). As, however, the blood of
the Invertebrata does not contain these red particles, to which
so important a function has been assigned, it may be asked,
how the conveyance of oxygen to their tissues is provided
for. The reply is very simple. In Insects, and other Arti-
CULATA which have active powers of motion, the air is con-
veyed to the tissues, not through the medium of the blood,
but directly through air-tubes which convey it to every part
of the body (§ 321). And in the Molluscous classes, as
among the Crustacea also, the nervo-muscular system forms
so subordinate a part of the general mass of the body, and its
movements are so sluggish, that the quantity of oxygen which
the fluid part of the blood conveys to them, is sufficient for their
need.

236. Of the properties of the Liquor Sanguinis, whilst it
is circulating in the vessels, the microscope tells us nothing ;
since it constantly remains in the state of a transparent fluid.
Eut if the blood be withdrawn from the living body, it soon
imdergoes a very curious and important change. A large
portion of it passes into the solid state, forming the crassa-
mentum or clot ; whilst there remains a transparent liquid of
a yellowish hue, which is termed the serum. It is evident
that the clot contains all the red particles ; but it is easily
proved that its coagulation is not due to them. For the blood
of a Frog, or of any other animal having blood-discs suffi-
ciently large, may be caused to pass through filtering-paper,
which will retain and collect its blood-discs, allowing the
liquor sanguinis to flow through it ; and this fluid will coagu-
late just as completely as if these particles were retained in
it. Again, in certain conditions of the blood (generally result-
ing from disease), even when the coagulation is allowed to
take place in the ordinary manner, the fibrin and the red
])articles separate from one another, — the latter gradually
su})siding, whilst the former are left at the surface ; and the
upper part of the clot is then nearly colourless, exhibiting
what is commonly known as the huffy coat or crust ; whilst
the lower part of it includes the red particles, and has a very
deep colour. The bufly coat, being composed almost exclu-

LIQUOR SANGUINIS COAGULATION. 211

sively of the fibrous network, is very firm in its texture,
being sometinjes almost leathery in its character ; whilst the
lower part of the clot, which is chiefly composed of the red
particles, loosely bound together by scattered fibres, is very
soft, and easily broken asunder. This effect may be also
produced, by acting on healthy blood with certain substances
which retard its coagulation, such as a strong solution of
Glauber's salt ; for if sufficient time is allowed, the red par-
ticles will subside in consequence of their greater specific
gravity, leaving a colourless layer of fibrin above them. — It
is of the liquor sanguinis, in a concentrated form, that those
exudations consist, which are poured out from the blood for
the repair of injuries, and which pass spontaneously into the
condition of a simple form of tissue (§ 393).

237. When a very thin slice of the clot is examined with
a microscope, it is found to be made up of a net-work of an
imperfectly fibrous character, interlacing in every direction,
and including the blood-discs in its meshes. These fibres are
produced by the spontaneous change in the fibrin of the blood,
from the fluid to the solid form. So long as the blood is
circulating in the vessels of the living body, so long does its
fibrin remain dissolved in the watery part of it ; but so soon
as it is withdrawn from these, and is allowed to remain at
rest, it undergoes this remarkable change. If fresh-drawn
blood be continually stirred with a stick or beaten w4th twigs,
the fibrin coagulates in irregular strings, which adhere to the
stick or twigs ; and it does not then include the red particles,
which are left behind in the fluid. In this maimer it may be
completely separated from the other elements of the blood,
which have not in themselves the least tendency to coagulate
spontaneously. Although forming a large proportion of the
substance of the clot, the fibrin, when dried, does not consti
tute more than from 2 to 3 parts by weight in 1000 of blood.
This proportion is augmented to 6, 8, or even 10 parts, in
severe inflammatory diseases.

238. When the fibrin and the red particles have both been
separated from the blood, there remains a fluid, the serum,
in which a good deal of albumen is dissolved, together wlvii
fatty matter, and other organic substances ; with the addition
of saline matter, of which a considerable pr( portion is chloride
of sodium, or common salt. The proportion which the solid

p2

212 SERUM USES OF BLOOD.

matter of the serum bears to the whole mass of blood, in
health, is about 53 parts in 1000 ; and of these about 40
parts are albumen, 8 parts saline matter, and 5 parts fat, with
certain ill-defined substances, of which some appear to be
organic compounds that are undergoing metamorphosis into
sohd tissues, whilst others are the products of the decay of
the tissues, which are being progressively withdrawn and
ehminated by the excretory organs.

239. The influence of the Blood as a whole upon the
anunal as well as on the nutritive functions, is easily proved.
When an animal is bled largely, it is gradually weakened as
the flow proceeds, and at last it seems to lose all consciousness
and power of movement. If allowed to remain in this con-
dition, it seldom or never recovers of itself. But if we inject
into its veins, by small quantities at a time, blood similar to
that which it has lost, the apparent corpse becomes as it were
reanimated, and all its functions are completely re-established.
The importance of the red particles is manifestly seen in the
(tfifect of this remarkable operation, which is called the tra7is-
/usion of blood ; for if, instead of blood freshly obtained from
another living animal, we inject serum without these particles,
the effect is but httle greater than if so much water were
introduced, and the animal dies of the hasmorrhage. By this
operation, practised on the Human subject, many valuable
hves have been saved, that would otherwise have been de-
stroyed by loss of blood. Again, if, by mechanical means, as
by tying the principal blood-vessel going to any organ, we
cause a permanent diminution to any considerable extent, in
the quantity of blood with which it is supplied, a decrease in
its size is soon apparent, and it may even shrink almost to
nothing. On the other hand, we observe that, the more active
the function of a part, the larger is the quantity of blood with
which it is supplied. Thus, when the antlers of the Stag,
which fall off every year, are being renewed, the arteries that
supply the parts of the skull from which they spring, are
greatly increased in size ; but they shrink again, as soon as
the growth of the horns is completed for that year. A similar
increase takes place among animals tliat suckle their young,
in the size of the arteries that supply the mammary glands,
by which the milk is formed ; and these also shrink, when
this liquid is no longer required.

USES OP SEPARATE CONSTITUENTS OF BLOOD. 5l3

240. The following appear to be tlie chief uses of the
principal constituents of the Blood, considered separately, in
the general economy : — The fibrin is the material which is
most assimilated to the condition of the solid tissues, having
the power of passing from the Hquid state into a low and
simple form of organization. It was formerly supposed to be
the nutritive material at the expense of which the solid
tissues generally are immediately produced ; the muscular
substance, in particular, being regarded as chemically identical
with it. But there is now good reason to think that the
greater part of the tissues form themselves at the expense of
the albumen of the serum and perhaps of the globulin of the
red corpuscles ; and that the purpose of the fibrin is chiefly
to give origin to those simple forms of fibrous or connective
substance, the production of which is the first step in the
reparation of injuries. Were it not for its power of coagula-
tion, the slightest cut or scratch might become fatal, from the
gradual draining-away of the blood ; and such, in fact, has
actually happened, in cases of disease in which the fibrin is
deficient. The presence of fibrin also gives a degree of vis-
cidity to the blood, which, as experiment proves, favours
(instead of resisting, as might have been expected) its passage
through capillary tubes ; and thus, when there is a deficiency
in this ingredient, local stagnations and obstructions in the
circulation of the blood are very liable to occur. The albumen
of the blood may be considered, like that of the egg, as the
raw material, at the expense of which (in combination with
fat) every other organic compound in the body is generated.
It is, as we have seen, the substance to which all the tissue-
forming elements of the food are reduced in the process of
digestion ; and in this condition it seems to be continually
appropriated by the acts of self-formation that are taking
place, with varying rapidity, throughout the body, just as the
albumen of the egg is appropriated by the self-formative
operations of the embryo. There is strong reason to believe that
a large proportion of the solid tissues regenerate themselves
by the direct appropriation of this material ; and if (as has
been already stated to be probable) the simple fibrous tissues
find their material in the fibrin, and the muscular substance
in the globulin of the red corpuscles, it is from the albumen
that these substances are themselves elaborated, both of thein

214 USES OF SEPARATE CONSTITUENTS OF BLOOD.

being, as it were, in process of organization. The albumen
of the blood further serves to supply the albuminoid matters
which are required as constituents of various secretions, espe-
cially those which are concerned in the digestive process, as
the saliva, the gastric juice, and the pancreatic fluid. A
large amoimt is daily drawn-ofF for the production of the
peculiar ferments contained in these secretions, whose action
upon the food is necessary for its reduction to the form in
which alone it can be received into the circulating current.
Hence the making of new blood involves a considerable ex-
penditure of the old.

241. The liquid in which the fibrin and albumen are dis-
solved, has a considerable power of absorbing gases ; and this
is greatly increased by the presence of the saline matters
which it holds in solution. Hence the liquor sanguinis not
only sustains the nutrition of the body, but can also serve, to
a considerable extent, as a medium of communication between
the lungs and the tissues. In this kind of activity, however,
it is completely surpassed by the red corpuscles (§ 235).
Independently of their use in ministering to the function of
Respiration, there seems reason to believe that the red cor-
puscles are also subservient to that of Nutrition ; for a certain
conformity which exists between the organic and mineral sub-
stances they contain (§ 232), and the composition of Muscle
and Nerve, taken in connexion with the manifest relation
between their number and the activity of the Nervo-muscular
apparatus, makes it probable that they have it for their especial
office to prepare the materials which are to be used in its pro-
duction and renewal of those tissues. The saline matter of the
blood has many important offices : thus it furnishes the mineral
ingredients which are requisite for the production of the tissues
and secretions ; it helps to preserve the organic substances from
decomposition ; and, in conjunction with the albumen, it keeps
up the density of the scrum to the point at which it is equi-
valent to that of the contents of the red corpuscles, without
which balance the condition of the latter would be seriously
impaired (§ 231). Finally, the fattij matters of the blood are
subservient to two very important functions — the maintenance
of heat, and the formation of tissue. They maintain the
combustive process, whenever there is a deficiency of more
readily combustible material 3 and they also take part with

ASSIMILATING AND SELF-PURIFYING POWER OF BLOOD. 215

albumen in the formation of all new tissue, its nuclear par-
ticles being always found to include fat-granules.

242. The presence of a due proportion of the foregoing
substances in the blood is an essential condition of health ;
and we find it provided-for in the marvellous power w^hich
the blood, like any solid tissue, seems to have of making itself
from the materials supplied to it, and of getting rid of what
is superfluous or unsuitable. Thus an excess of albuminous
matter in the food does not seem to produce more than a very
limited increase in the quantity of albumen in the blood, the
surplus being made to undergo changes within the body,
which issue in its being removed by the excretory organs.
An excess of any of the saline compounds is very speedily
strained off (as it were) into the urine. And an excess of fatty
matters is draT\Ti oif either by the formation of fat as a tissue,
or by the augmented activity of the liver in producing bile.
This conservative power is still more remarkably shown in
the completeness with which the poisons that are generated
in the body by the decay of its tissues, and which are received
into the current of the circulation for the purpose of being
conveyed to the several excreting organs, are drawn off from
it, so as to leave the blood pure. Thus, carbonic acid is being
continually produced in such large quantities, that its accu-
mulation in the blood, even for five minutes, would be fatal ;
yet by the aerating process to which the limgs are subservient,
it is got rid of as fast as formed, so that the blood is restored
to its previous purity. In like manner, the urea, which is
one of the products of the wear and tear of the muscles
consequent upon their use, is so perfectly and constantly
eliminated by the kidneys, that its detection in the circulating
current is a matter of difficulty, although we know that it
must always be passing through this.

243. Thus the circulating current may be likened to a
tidal river running through the midst of a large town, and
supplying it with the water needed for the drink of its
human and other inhabitants, as well as with that w^hich is
required for the various manufacturing and cleansing opera-
tions carried on within its precincts ; the same stream also
receives the drainage of the town, and consequently becomes
charged with the products of animal and vegetable decompo-
sition, and the foul refuse of manufactories j and as the flow

216 CIRCULATION OF THE BLOOD.

of the tide brings back a large proportion of what is carried
down at ebb, the waters speedily become so contaminated with
hurtful and offensive matters as to be unfit for use, unless
means be provided for getting rid of these as fast as they are
poured in. The perfection with which tliis requirement is
fullilled in the Animal body, while it excites our admiration,
should also incite us to imitation, so far as the art of Man
can hope to imitate the works of the Divine Artificer.

Circulation of the Blood.

244. In some of the lower tribes of Animals, the blood
appears to circulate in channels which are merely excavated
in the substance of their tissues and organs. But among all
the Vertebrata, and even in most of the Invertebrated classes,
the movement of the blood takes place in a very complicated
apparatus, which is composed, 1st, of a system of tubes or
canals wliich serve to convey it through every part of the
structure, and 2d, of a special organ for the purpose of
giving motion to that liquid. These canals are known as the
blood-vessels ; and this special organ is the heart.

245. The Zleart is the centre of the circulating apparatus.
It is a kind of fleshy bag, communicating with the blood-
vessels : and it alternately dilates to receive the blood, which
is conveyed to it by one set of these ; and then contracts so as
to force it out into another set of tubes. In this manner a
continual current is kept up. All but tlie lowest animals
have a heart, or something which represents it. Such an
organ exists, not merely among all the Vertebrated classes,
but in all the Mollusca, and in the higher Articulata. But,
as will presently appear, there is a great diversity in its form,
and in the complexity of its construction ; for whilst, in its
simplest condition, it possesses but one cavity, communicating
with both sets of vessels, — it contains, in its highest forms, four
different chambers, each of which has its own peculiar function.

246. The two sets of blood-vessels just adverted-to are, 1st,
the Arteries, which convey the blood from the heart into the
several parts and organs of the body ; and 2d, the Veins,
which collect the blood that has been distributed through
these, and return it to the heart. The Ai'terial system, as it
issues from i\w heart, consists of one or more large trunks,
which divide into branches, very much in the manner of the.

DISTRIBUTION OF ARTERIES AND VEINS.

217

stem of a tree ; these branches again subdivide into othei-s
more numerous but smaller, and these again into twigs still
more nmnerous and more minute ; until almost every portion
of the body is so penetrated with them, that not even a
trifling scratch, cut, or prick, can be made, without wounding
some one of these small divisions (fig. 120). — The Venous
system presents a corresponding distribution, but it is destined
for an opposite purpose ; and we must regard it as commencing
in the tissues by the minuter canals, which run together like the

Fig. 120.— Distribution of thk smaller Blood-Vessels in the Membrane

BETWEEN two OF THE TOES OF THE HINI) FOOT OF THE CoMMON FrOG ; « U,

veins; b b, arteries.

little rivulets that form the origin of a mighty river, or like the
smallest fibres of which the roots of a tree are made up. The
larger canals thus formed gradually unite with each other as
they approach the heart, towards which they all tend, just as
the various tributary streams pour their contents into one prin-
cipal channel : and at last all the veins empty into the heart,
by one or two large trunlvs, the blood which they have conveyed
from the several parts of the body ; just as all the tributaries
which have arisen over a wide extent of country, pour into
the ocean the water they have collected, by one mouth which
is thus common to all of them.

247. Although the number of the Arterial branches increases

218 AREAS OF ARTERIAL TRUNKS AND BRANCHES.

SO vastly, as we proceed from their origin towards their termi-
nation, yet their capacity does not, at least in any considerable
(jegree ; — that is, the first or main trunk will allow as much
fluid to pass through it in a certain time, as will the whole of
the first set of branches into which it divides, or the still more
numerous subordinate branches into which these diverge. Or,
to put this fact in another form, if we cut across the main
trunk, and compare the area, or space included within its
circular walls, with the sum of the areas of all the branches it
supplies at a certain distance — say a foot — from the heart, we
shall find them precisely equal ; and the same will hold good,
if the comparison be made with the sum of the areas of the
more nimierous but smaller branches at a greater distance from
the main trunk. It is quite true that, when an artery divides
into branches, the combined size of these seems to be greater
than that of the trunk ; but this is only because the compa-
rison is made, not between the areas of their circles, but their
diameters. Thus, an artery of 10*1 lines in diameter, may
divide into three branches, two of them having a diameter of
7 lines, and the third a diameter of 2 lines ; — and yet these
will convey no more blood than the single trunk. For,
according to a simple rule in geometry, the areas of circles are
to each other as the squares of their diameters. The area of
the trunk is expressed, therefore, by the square of lO'l,
which is almost exactly 102. The area of each of the two
large branches, in like manner, is expressed by the number
49, which is the square of 7 ; and that of the smaller one by
4, the square of 2 ; and the sum of these (49+49+4) is ex-
actly 102, making the combined areas of the branches the same
as that of the trunk. In like manner, one of the branches of 7
lines diameter might subdivide into two branches of a little
less than 5 lines each ; for, as the square of 5 is 25, and twice
that number is equal to 50, the combined areas of the two
branches of 5 lines each, exceed by very little the area of the
trunk of 7 lines. — Hence it results, that the pressure of the
blood upon the walls of the arteries will be everywhere
almost exactly the same ; — a conclusion which is confirmed
by experiment.

248. Tliere are certain differences in the structure and dis-
tribution of the Arteries and Veins, which it is desirable to
mention. The Arteries receive the blood pressed out from

STRUCTURE AND DISTRIBUTION OP ARTERIES AND VEINS. 219

the heart, and must be strong enougli to resist the force of its
contraction ; otherwise, as there is a considerable impediment
to its onward flow, produced by the minuteness of the tubes
through which it has to pass, and the friction to which it is
subjected against their sides, their walls would give way, and
they would burst. They have, accordingly, a tough elastic
fibrous coat, which contains also more or less of non-striated
muscular fibre. On the other hand, the Veins receive the
blood after the heart's power over it has been almost ex-
pended in forcing it through the capillary system, and when
it is consequently moving much more slowly. They are very
large in proportion to the arteries ; so that, if we were to cut
across a limb at any place, and to estimate the respective areas
of all the veins and arteries, we should find that of the veins
two or three times as great as that of the arteries. Hence the
pressure on their walls is much less ; and their strength does
not require to be so great. Accordingly we find their walls
much thinner, and the tough elastic fibrous coat almost entirely
wanting.

249. The difference in the force with which the blood
presses on the walls of the arteries and veins, is seen when
these vessels are wounded. If a small incision be made into
an artery, the blood spouts from it to a great distance ; but if
a similar incision be made in a vein, the blood merely flows
out, unless we stop its passage to the heart, by making pres-
sure on the vein above the orifice, as in ordinary blood-letting
(§ 277). Hence much greater pressure is requisite to check
bleeding from an artery, than to stop bleeding from a vein ;
and it frequently happens that no amount of pressure can
prevent the continued drain of blood from the former, so that
it becomes necessary to stop the flow of blood through the
artery altogether, by tying a ligature tightly round it.

250. The Arteries are for the most part so distributed, that
their trimks lie at a considerable distance from the surface of
the body, so as to be secluded from injury ; and they are often
specially protected by particular arrangements of the bony
parts. Of the Veins, on the other hand, a large proportion he
near the surface, and they are consequently more liable to be
injured ; but, for the reason just stated, wounds in them are
of comparatively little consequence.

251. The ultimate ramifications of the Arteries are conti-

220

CAPILLARY BLOOD-VESSELS.

nuous with the commencing twigs of the Venous system. The
communication is established by means of a set of extremely
minute vessels, which are termed Capillaries.'^ These capil-
laries form a network, Avhich is to be found in almost every

part of the body (fig. 121).
It is in them alone that the
blood ministers to the opera-
tions of nutrition and secre-
tion. Even the walls of the
larger blood-vessels are inca-
pable of directly imbibing
nourishment from the blood
which passes through them ;
but are supplied with minute
branches, which proceed from
neighbouring trunks, and form
a capillary network in their
substance. The diameter of
the capillaries must of course
bear a certain proportion to
that of the blood-discs which
have to pass through them :
in Man they are commonly
from about 1 - 2500th to
1-1 600th of an inch in dia-
meter. In the true capilla-
ries, it would seem that only one row or file of these particles
can pass at a time ; but we frequently see vessels passing
across from the arteries to the veins, which will admit
several rows. There seems, however, to be a considerable
difference in the diameter of the same capillary at different
tim.es ; a change sometimes taking place from causes which
are not yet understood. f The rate at which the blood moves
* From the Latin capilla, hair ; so named on account of their being,
like hairs, of very minute size. Their diameter is really, however, far
less than that of ordinary hairs.

t The circulation of the blood in the Frog's foot, the tail of the
Tadpole, the gilis of the larva of the Water-Newt, the yolk-bag of
embryo Fish, and other appropriate subjects for the observation, is one
of the most beautiful and interesting spectacles that the Microscope can
open to us. Details of the various modes of exhibiting it will be found
in the Author's treatise on " The Microscope and its Eevelations,"
Chap, xviii.

Fig. 121.— Portion of thic Membrane

BETWEEN THE TUES OF THli HIND

FOOT <iF THE Frog, more highly
magnified than in i^ig. 120, showing the
network of Capillaries that traverses it ;
a, small venous trunk; b b, branches
communicating with the capillaries ;
f, intervening tissue covered with epi-
thelium cells.

RESPIRATORY CIRCULATION. 221

through the capillaries of a warm-blooded animal, has been
determined by microscopic examination to be about 3-lOOths
of an inch per second. From the comparison of this rate with
that of the How of blood through the larger arteries, wliich has
been found by experiment to be nearly 12 inches per second,
it appears that the area of the capillary system must be
nearly four hundred times as great as that of the vessels
which supply it with blood.

252. Thus the Arterial and Venous systems conununicate
with each other at their opposite extremities ; their large
trunks through the medium of the heart ; and their ultimate
subdivisions through the capillaries. Hence we may consider
tliis double apparatus of vessels as forming a complete circle,
through which the blood flows in an uninterrupted stream,
returning continually to its point of departure ; and the term
circulation is therefore strictly applicable.

253. But the conveyance of the nutritive fluid to the several
organs of the body, for their support and maintenance, is not
the only object to which its circulation has to minister. It is
rec[uisite that the blood should be continually exposed to the
influence of the air, by which it may get rid of the carbonic
acid with which it has become charged during its circulation
in the system, and may take-in a fresh supply of oxygen to
replace that which has been withdrawn from it. In order to
efiect this exposure, the blood is conveyed to a particular organ,
in which it is made to pass through a special set of capillary
vessels, that bring it into almost immediate contact with
air. In the lower tribes, in which this aeration is (from
various causes hereafter to be explained) much less constantly
necessary than in the higher, we find the respiratory organ
supplied by a branch from the general circulation ; and the
blood which has passed through it, and which has been sub-
jected to the invigorating influence of the air, is mingled in
the heart with that which has been deteriorated by circulating
through the system, which is again supplied with this mixed,
half-aerated blood. But in the highest classes, there is a dis-
tinct circle of vessels subservient to the respiratory function :
namely, an arterial trunk issuing directly from the heart, and
subdividing into branches which terminate in the caj^illary
system of the respiratory organ ; a set of capillaries, in which
the aeration of the blood takes place ; and a system of veins

222 OTHER USES OP THE CIRCULATION.

which collects the blood from these, and returns it to the
heart. This circuit of the blood is sometimes called the lesser
circulation; to distinguish it from that which it makes
through the general system, which is called the greater
circulation.

254. Although carbonic acid is one of the chief impurities
with which the blood becomes charged during its circulation,
in consequence of the changes of composition which are con-
tinually taking place in the living body, it is by no means the
only one ; and other organs are provided, besides the lungs,
for ^emo^dng the noxious matters from the current of the cir-
culation as fast as they are introduced into it. Thus, in the
course of its movement through the general system, the blood
is made to pass through the liver, the kidneys, and the skin,
each of which has its special purifying office ; these organs,
however, have no such special circulation of their ow^n as the
respiratory apparatus of higher animals possesses, though the
liver, as we shall hereafter see (§ 267), is peculiarly supplied
by a sort of offset from the general circulation, so that the
blood from which its secretion is formed is venous instead of
arterial, like that transmitted to the limgs.

255. The course which the blood takes, and the structure
of the apparatus which is subservient to its movement, differ
very greatly in the several classes of animals. The chief of
these differences will be pointed out hereafter ; and it will be
preferable to commence with the highest and most complex
form of the circulating system, such as we find in Man, that
it may serve as a standard of comparison with which the
rest may be contrasted.

Circulating Apparatus of the Higher Animals.

2o6. In Man, and those animals which approach him most
nearly in structure, the heart is situated between the lungs in
the cavity of the chest, wiiich is termed by anatomists the
thorax. Its form is somewhat conical ; the lower extremity
tapering almost to a point, and the upper part being much
larger. The lower end is quite unattached, and points rather
fbi-wards and to the left ; during the contraction of the heart,
it is tilted forwards, and strikes' against the w^alls of the chest,
between (in Man) the fifth and sixth ribs. It is from the
large or upper extremity that the great vessels arise; and

CIRCULATING APPARATUS IN MAN. 223

these, being attached to the neighbouring parts, serve to
suspend the heart, as it were, in a cavity in which its
movements may take place freely. This cavity is lined by
a smooth serous membrane (§ 43), which, near its top, is

ar vc vr a vl

Fig. 122. — Lungs, Heart, and principal vessels of Man.

ai\ right auricle; vr, right ventricle; vl, left ventricle; a, aorta; vc, vena cava;
ac, carotid arteries ; vj, jugular veins ; as, subclavian artery ; vs, subclavian veins ;
t, trachea.

reflected downwards over the vessels, and covers the whole
oiitei^ surface of the heart. Hence as the surface of the heart,
and the lining of the cavity in which it works, are alike
smooth, and are kept moist (in health) with a fluid secreted
for the purpose, there is as little interruption as possible from
friction in the working of this important machine.

257. The heart may be described as a hollow muscle,
wliich, in Birds and Mammalia, as in Man, is divided into
four distinct chambers. This division is eflected by a strong
vertical partition, that divides the entire heart into two halves,
which are almost exactly similar to each other, excepting in
the greater tliickness of the walls on the left side ; and each
of these halves (which do not communicate with one another)
is again subdivided by a transverse partition, into two cavities,

224

STRUCTURE OF THE HEART.

of which the upper one is termed the auricle, and the lower
the ventricle. Thus we have the right and left auricles, and
the right and left ventricles. Each auricle communicates
witli its corresponding ventricle, by an aperture in the

Superior Pulm. Pulmonary

vena cava art. Aorta artery

Pulmonary veins »<!_

Right auricle

Tricuspid valves

Inferior vena cava "'

Right ventricle

Pulmonary veins

Left auricle
Mitral valve

Left ventricle

Partition Aorta
Fig. 123.— Ideal Section of the Human Heart.

transverse partition, which is guarded by a valve. The walls
of the ventricles are much tliicker than those of the auricles ;
and for this evident reason, — that the ventricles have to
propel the blood, by their contraction, through a system of
remote vessels ; whilst the auricles have only to transmit the
fluid tliat has been poured into them by the veins, into the
ventricles, which dilate themselves to receive it. The difference
in the thickness of the walls of the left and the right ventiicles
is explainable on the same principle ; for the left ventricle
has to send the blood, by its contractile power, through the
remotest parts of the body; whilst the right has only to
transmit it through the lungs, which, being much nearer,
require a far less amoimt of force for the circulation of the
blood through them.

258. The arterial system of the greater circulation entirely
springs from one large trunk, which is called the aorta (see
figs. 122-124); this originates in the left ventricle, and is
the only vessel which passes forth from that cavity. It first
ascends towards the bottom of the nock ; then forms what is
termed the arch, a sudden curve, which gives it a downward

ARTEBIAL SYSTEM OF MAN.

225

direction ; and then descends along the front of the spinal
column^ behind the heart, as far as the lower part of the

Antenor
tibial artery

Art. of foot,..,

Peroneal
artery

Fig. 124.— Arterial Sysiem of Max
Q

226 ARTERIAL SYSTEM OF MAN.

tmnk, where it divides into two great branches, which
proceed to the lower extremities. From the arch of the
aorta are given off the arteries which sui3ply the head and
upper extremities. These are, the two carotids, which ascend
on either side of the neck; and the two subclavian, w^hich
pass outwards beneath the clavicles, so as to arrive at the
arms, — becoming successively in their course the axillary and
brachial arteries, as they pass through the axilla or arm-pit,
and along the arm. The subclavian and carotid arteries of
the right side arise together from the aorta, in Man, by a
conmion trunk ; but this arrangement varies much in different
Mammals. Thus in the Elephant, the two carotids arise by
a common trunk, — the two subclavians separately. In some
of the AVhale tribe, all four are separate. In the Bat, the
subclavian and carotid of the left side arise from a common
trunk, like those of the right. And in those Emninating
animals which possess a long neck, all four arteries come off
from the aorta together, by a large trunk, which first gives off
the subclavians on either side, and then divides into the
carotids. All these varieties occasionally present the^nselves in
Man ; — a fact of no small interest.

259. The descending aorta, in its progress along the trunk,
gives-off several important branches ; — as the coeliac, from
which the stomach, liver, and spleen are supplied ; the 7r7ial,
to the kidneys ; and the mesenteric, to the intestines. It
divides at last into the two iliac arteries ; which, after giving
off branches for the supply of the lower bowels, pass iuto the
thighs, where they become the femoral arteries ; and these
again subdivide into branches for the supply of the leg.

2 GO. For the sake of comparison, a figure of the arterial
system of a Bird is introduced ; from which it will be seen
that by far the larger proportion of its blood is distributed
to its upper extremities. In Man, the descending aorta is
evidently the continuation of the aortic arch ; and the parts
which it supplies receive far more blood than the head and
upper extremities, — the locomotion of biped man being per-
formed almost entirely by his lower limbs. In Quadrupeds,
which require nearly as much strength in their fore feet as in
their hind, the subclavian arteries bear a larger proportion to
the iUac. But in Birds, the function of locomotion is abnost
entirely performed by the wings ; and their powerfid nniscles,

ARTERIAL STSTPLM OF BIRD.

227

■whicli constitute the mass of flesh lying on the breast, are
supplied Trith blood by the arteries of the upper extremities,
which here possess a manifest predominance. The aorta, soon
after its origin, subdivides into three large branches ; of which
the first two (one on either side giving-off the subclavian and
carotid arteries) convey the blood to the head, the wings, and

Lingual artery

Femoral
artery

Sacral artery Cloaca

Fig. 125. — Arterial System of Bird.

the muscles lying on the thorax ; whilst the middle one curves
backwards and downwards, and becomes the descending aorta.
ISTow that which is here the continuation of the great side

q2

228 DISTRIBUTION OF ARTERIES.

branch, is ncitlier the carotid nor the subclavian, both of
which are subordinate branches given-off from it; but it is
the trunk which distributes the blood to the muscles of the
breast, and which in Man is a subordinate branch of the sub-
clavian artery (the mammary). The descending aorta is seen
to lose itself almost entirely in supplying the viscera of the
trunk ; so that the branches into which it divides at last for
the supply of the legs, are very small. These limbs, in birds,
are usually required only for the support of the body at times
of rest, and are seldom much concerned in locomotion ; so that
they possess little muscular power, and require but a small
supply of blood.

261. It is very interesting to trace such differences in the
arrangement of the vascular system, corresponding with vari-
ations in the general plan of structure, yet not exhibiting
any actual departure from the general type. Thus, there is
probably not a single large artery in Man, to which a corre-
sponding branch might not be found in the Eird ; on the
other hand, there is perhaps not a single large artery in the
Bird, to which there is not an analogous branch in Man.
The chief dilference consists in the relative sizes of the seve-
ral trunks ; and these correspond closely with the amount of
tissue they have respectively to supply. Here, then, we have
one example, out of many that might be adverted-to, of that
Unity of Design which we see every^vhere prevalent through-
out nature ; manifesting itself in the close conformity of a
great number of apparently-diiferent structures to one general
plan, whilst there is, at the same time, an almost infinite
variety in the details.

^ 262. There is a very interesting peculiarity in the distribu-
tion of the arteries, by which the due circulation of blood in
their branches is provided for, even though there should be
an obstruction in the main trunk. The branches Tvhich are
given-otf from it at different points, have frequent communica-
tions or anastomoses with each other ; so that blood may pass
from an upper part of a main artery into the lower, by means
of these lateral communications, even though its flow through
the trunk itself should be completely stopped.

263, These anastomoses are very numerous in the arteries
of the limbs, and particularly about the joints ; and it is well
that they are so ; for, by relying on the maintenance of the

ANASTOMOSES OF ARTERIES ANEURISM. 229

circulation througli them, the Surgeon is often able to save a
limb, or even a life, wliicli would otherwise be sacrificed.
Arteries are liable to a peculiar disease, termed aneurisvt,
wliich consists in a thinning-away, or mpture, of the tough
fibrous coat, and a great dilatation of the other coats, so that
a pulsating tumour is formed. This change takes plac