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THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

> >

ANIMAL PHYSIOLOGY

BY

WILLIAM B. CARPENTER, M.D,

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

REGISTRAR OF THE UNIVERSITY OF LONDON.

THOROUGHLY REVISED, AND PARTLY RE-WRITTEN.

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

1859.

LONDON:

R. CLAY, PRINTER, BREAD STREET HILL.

TO

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

PHYSICIAN IN OKDINABY TO THE QUEEN AND TO PRINCE ALBERT,
ETC. ETC.

MY DEAR SIR JAMES,

I cannot more appropriately inscribe this Treatise,
having for its object the general diffusion of sound Physio-
logical knowledge, than to one whose Professional eminence
is founded 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 B. CAKPENTEK.

UNIVERSITY HALL, LONDON,
January} 1859.

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 country, — that, namely,
of an Educational Treatise on Animal Physiology, which
should at the same time communicate to its readers the facts
of greatest importance as regards their practical bearing,
and present these in such a form as to place the learner
in possession of the essential principles 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 " Cours Elementaire
d'Histoire Naturelle" 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

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 country, — that, namely,
of an Educational Treatise on Animal Physiology, which
should at the same time communicate to its readers the facts
of greatest importance as regards their practical bearing,
and present these in such a form as to place the learner
in possession of the essential principles 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 " Cours Elementaire
d'Histoire Naturelle " 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

6

PREFACE.

of his own materials than of those supplied 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 phenomena
of Eeproduction 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 being
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 who 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 which this country is now so abundantly furnished, it
forms a part of the regular course of instruction.

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

PREFACE. Vli

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 different
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 Reform, 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.

INTRODUCTION 1

CHAPTER I.

ON THE VITAL OPERATIONS or 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 92

REPTILES 93

BATRACHIA 97

100

ARTICULATA 102

INSECTS 104

ARACHNIDA 105

CRUSTACEA 106

CIRRHIPEDA 109

MYRIAPODA 110

ANNELIDA ib.

ENTOZOA Ill

MOLLUSCA 112

CEPHALOPODA 116

PTEROPODA 117

GASTEROPODA 118

CONCHIFERA ib.

TUNICATA ]21

POLYZOA . 122

CONTENTS.

CHAPTER II.— Continued.

PAGE

RADIATA 123

ECHINODERMATA 125

ACALEPHJS . • 128

POLYPIFERA 129

PROTOZOA 135

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

INSALIVATION 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 AKIMALS . . 222

FORCES THAT MOVE THE BLOOD 232

COURSE OF THE BLOOD IN THE DIFFERENT CLASSES OF

ANIMALS . . . , , 240

CONTENTS. XI

CHAPTER VI

PAGE

OF RESPIRATION 258

NATURE OP THE CHANGES ESSENTIALLY CONSTITUTING

RESPIRATION 259

STRUCTURE AND ACTIONS OF THE RESPIRATORY APPARATUS 265

CHAPTER VII.

OF EXCRETION AND SECRETION 292

GENERAL PURPOSES OF 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 . . ib.

FORMATION OF THE TISSUES 317

REPAIR OF INJURIES 323

CHAPTER IX.

ON THE EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY BY

ANIMALS 327

ANIMAL LUMINOUSNESS ib.

ANIMAL HEAT 332

ANIMAL ELECTRICITY 340

CHAPTER X.

FUNCTIONS OF THE NERVOUS SYSTEM 345

STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM IN THE

PRINCIPAL CLASSES OF ANIMALS 350

FUNCTIONS OP THE SPINAL CORD. — REFLEX ACTION . . 374
FUNCTIONS OF THE GANGLIA OF SPECIAL SENSE. — CON-
SENSUAL ACTIONS 380

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

FUNCTION OF THE CEREBRUM.- INTELLIGENCE AND WILL . 385

Xii CONTENTS.

CHAPTER XI.

PAGE

ON SENSATION, AND THE ORGANS OF THE SENSES 387

SENSE OF TOUCH 390

SENSE OF TASTE 395

SENSE OF SMELL 398

SENSE OF HEARING 401

SENSE OF SIGHT 413

CHAPTER XII.

OF ANIMAL MOTION, AND ITS INSTRUMENTS 443

CONTRACTILE TISSUES. — MUSCULAR CONTRACTILITY . . 444

APPLICATIONS OF MUSCtfLAR POWER. — BONES AND JOINTS . 453

MOTOR APPARATUS OF 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.

ENTBODUCTIOK

THE importance of the 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 Dot hitherto been
so, may be attributed to several causes. 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. We
almost daily see vast numbers of animals of different tribes,
in active hfe 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 familiar, 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 1

2 INTRODUCTION.

In reply to this it is sufficient to state, that millions annually
perish from a neglect of the conditions which Divine wisdom
has appointed as requisite for the preservation of the body 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 the 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, upon the care
with which 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
during the few first days of infancy, 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
suffer from this cause, the seeds of future disease are sown,
by inattention to the simple physiological principles, which
should regulate its clothing in accordance with the cold or
lieat of the atmosphere around. Nor 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 up
in a sound and healthy state of body and mind, in the midst
of a close, ill-ventilated atmosphere. Those that are least
able to resist its baneful influence, are carried off by the dis-

INTRODUCTION. 3

eases of infancy and childhood ; 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 life, or deprive them of a large part of that
enjoyment which health alone can bring.

!Nor 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
destroyed by excesses, whether in sensual indulgence, in
bodily 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 which 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 guiltless of having impaired its vigour by his
own transgressions.

These statements are not theoretical merely : they are based
upon facts drawn from observations carried on 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, as 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 2,600 to 450 in a year. Here, then,

B 2

4 INTRODUCTION.

was a total of 2,150 instances of loss of life, occurring yearly
in a single institution, chargeable, not against any unalterable
decrees of Providence, as some are disposed to contend as an
excuse for their own negligence ; but against the ignorance,
indifference, 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 ! " l

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 Lying-in Hospital.
At the conclusion of 1782, it was found that out of 17,650
infants born alive, no fewer than 2,944, or one in every six,
had died within the first fortnight. By the more efficient
ventilation of the wards, the proportion of deaths during the
first 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 diminished.

In the island of St. Kilda, the most northern of the Heb-
rides, according to the statement of a gentleman who visited
it in 1838, as many as eight out of every 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 " air is good, and the water excellent : " but
to the " filth in which the inhabitants live, 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 clergyman, who lives
exactly as those around him do, in every respect, except as
regards the condition of his house, has reared a family of four
children, all of whom are well and healthy ; whereas, accord-
ing to the average mortality around him, at least three out of
the four would have been dead within the first fortnight.

It is not a little remarkable that a recent sanitary inquiry
carried out by order of the Danish government, into the con-

1 Dr. A. Combe on the Physiological and Moral Management of
Infancy.

INTRODUCTION. 5

dition of the Icelandic population, should have disclosed the
existence of almost precisely similar habits 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 off at night. Although
the Icelanders enjoy an almost 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. Kilda, to the very high rate of infantile
mortality; a large proportion of, all the 'infants born being
carried off before they are a fortnight old. It is in the little
island of Westmannoe, and the opposite parts of the coast of
Iceland, where 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 localities.

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 was almost entirely referrible
to one single disease, " Trismus nascentium," or, " Lock-jaw
of the New-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
BO 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 preventable than this ;
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 which it falls
as low as 1 1 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 ordinary 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 mortality is
the lowest, we may take 13 per thousand as the average to

INTRODUCTION. 7

which, it may be 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,
showing 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 mortality 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, like 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, which 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 intelligent public opinion shall have been created, by
the general diffusion 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
property 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 immediate neighbourhood; but the condition of
these dwellings is most filthy. A few years ago, as many as
3,000 pigs were kept in this locality (the number has since
been somewhat diminished) ; and the boiling of fat and other
offal, which is carried on by some of the pig-feeders, some-

8 INTRODUCTION.

times taints tlie air for a mile round. Very few of the 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 dwellings 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. Now, 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 than four-fifths of these deaths occurred at, or beneath,
jive 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. During the prevalence of the cholera-epidemic,
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 thoroughly 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 itself in identical localities, provided those

INTRODUCTION. 9

remained in the same foul state as at first ; whilst new loca-
lities have been affected, just 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 Newcastle- 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, — vrhere, 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 Nottingham, which suffered
severely in the first epidemic, escaped comparatively un-
harmed in the subsequent visitations ; and this result is
plainly due 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 localities, which, in spite
had been done, retained much of their previous filth.

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

10 INTRODUCTION.

may be well to subjoin a few additional facts, derived from
the cholera-experience 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, Norwich, Preston, Halifax, and Bir-
mingham, amounting to 657,000, there were no more than
238 deaths from cholera ; whilst, in an equal population
inhabiting the towns of Newcastle-under-Lyne, Plymouth,
Brighton, Merthyr Tydvil, Portsea, Tynemouth, Wigan, Hull,
Wolverhampton, and Leeds, the number of deaths was no
fewer than 10,415, or forty-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 difficulty 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 speak solemnly and distinctly to all who
claim the rights of property in the foul " plague-spots " which
deface our country, of their bounden duty 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 people by his
ignorance of such matters. It is less than ten years since a
fearful sacrifice of life occurred among the deck-passengers on
board the Irish steamer " Londonderry," who were ordered

INTRODUCTION. 1 1

below "by the Captain on account of the stormy character of
the weather, and on whom the hatches were closed down,
although the cabin which was crowded by them 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 146
died during one night's confinement 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 means 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 temperature, 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 which his body requires, weariness tempting him 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 acquired 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 preserving health
should be based on an intelligent knowledge of Physiological
principles ; otherwise, }ike the natural instincts, they are likely
to be put aside as occasion prompts ; whereas, in proportion as

12 INTRODUCTION.

the individual is possessed of their 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 different conformation. Hence, for the student of
physiology to confine himself to the observation of what
takes place in Man alone, would be as absurd as for the astro-
nomer to restrict himself 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 Physiology of Man.
For though in him 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 the 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 life, 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. JSTo extremes of temperature 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 inhospitable regions
is tenanted, not only by the whales and other monsters which
we 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 animals of various
kinds upon which 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 j 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 examples of the compatibility of even the
heat of almost boiling water with 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 boiling 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 animals 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 IXTKODTJCTIOX.

touching with 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
Styria and Carniola, which are inhabited by the curious
Proteus (ZooL. § 532), numerous species of insects are found,
all of which, however, 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 other, both the
observing and the reasoning powers ; but that it offers 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
people 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

attach, themselves 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 lower
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 which 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 Nature we 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 contrivances 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 who can
feel all these motives, cannot every one feel the force of
some ?

There is certainly no science which more constantly and
forcibly brings before the mind the power, the wisdom, 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, whose nearest member is at a
distance which imagination can scarcely realize, the Physio-
logist finds them in the meanest worm that we 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 exhibited.
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
its cheering and tranquillizing influence on the spirit.

" With tender ministrations, thou, 0 Nature,
Healest thy wandering and distracted child ;
Thou pourest 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 be a jarring and dissonant thing
Amidst the general voice and minstrelsy, —
But bursting into tears wins back his way,
His angry spirit healed and harmonized
By the benignant touch of love and mercy."

COLERIDGE.

DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 17

CHAPTER I.

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

1. LIVING beings, whether belonging to the Animal or to
the Vegetable kingdom, are distinguished from the masses of
inert matter of which the Mineral kingdom is made up, 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 resembling 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

18 DISTINCTIVE CHARACTERS OP ORGANIZED BODIES.

Vegetable life, present themselves tinder 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 restrained 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, which can maintain their
existence when detached from one another, may, in some
sense, be regarded as clusters of distinct organisms, rather
than as single 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, like 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 organs ; each of these

DISTINCTIVE CHARACTEES OF ORGANIZED BODIES. 19

being the instrument of some special action or function, 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 intimate, not in the case of those
beings which have the greatest multiplication 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
necessary condition being that it shall either itself contain
all the organs essential to life, or shall be capable of pro-
ducing them, — as when the leaf-bud develops rootlets for
its nutrition. This "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 which 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 sustaining any
considerable damage. In the bodies of the higher animals,
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 multiplied 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 their 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 all
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
solidified portions of the Organized fabric, that the greatest
resemblance exists to Inorganic bodies; but even these
portions all pass through 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 continually
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
intermingling or mutual penetration of solids 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 peculiarity of their chemical constitution.
This peculiarity 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 communicates 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. — JS~ow
out of about seventy simple or elementary substances which
are known to occur in the Mineral world, not above twenty
present themselves as constituents 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 possess the highest
vital endowments 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 potassium
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 which 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 living
Plant, at the expense of the carbon, hydrogen, oxygen, and
nitrogen, which it draws from the elements around; and

22 DISTINCTIVE CHARACTERS OP 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 which 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 products 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 which those materials were first derived
by the agency of the growing Plant. (See VEGET. PHYSIOL.)
5. It is, however, by their peculiar 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 made for the
operation of physical and chemical forces in the living or-
ganism, there still remain a large number of phenomena
which cannot be in 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 the effects it produces — in the same manner
as we recognise heat or electricity by their effects — may be
conveniently designated vital force.1 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
peculiar to the living muscle ; and the muscle derives its pro-
perty of contractility from the previous development of its
peculiar tissue in the act of nutrition. So the solvent
fluids by which the digestion of food is accomplished, are
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 liquid 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 whiqh 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 which 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

1 The Author has elsewhere given his reasons 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 CHAEACTERS OF ANIMALS.

and excretion, of the effete matters with, which the blood be-
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 power 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 performed.
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 affirmed that they are
present at all, there are many tribes among the lower plants
which exhibit a power of spontaneous movement fully as

DISTINCTIVE CHAEACTERS OF ANIMALS. 25

great as that 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 physiological difference, however, between the two
kingdoms, which seems to afford an adequate means of
settling the true place of those tribes whose position would
otherwise be doubtful. This lies in the nature of their food,
and the source from which it is obtained. If or 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 power 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 believed, to distinguish the very
lowest forms of Animal life from those humble forms of Vege-
tation which 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 that series of chemical changes, by which, under the
influence of light, the living plant can unite, inorganic elements
into organic compounds.

8. Not only do 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 animals, 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 peculiar 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 which
it can be carried about in the general movements of the body,
and within which it can be prepared 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
required by creatures which have no power 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 Rhizopoda
(§ 129), in which, notwithstanding the absence of any regular
stomach, the food is.. received into the very substance of the
jelly-like particle of which the body consists ; a mouth and

DISTINCTIVE CHARACTERS OP ANIMALS. 27

stomach, being extemporized, as it were, on each occasion that
aliment is ingested ; and an anal orifice being extemporized
in like 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. In Zoophytes (§ 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, are suspended
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 which 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 (§ 28),
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 absorption of the fluid is first requisite ; and it is with
this that its application to the requirements 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 OF ANIMALS.

for the scientific naturalist to mistake a Protophyte (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. For among the higher 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, think, and
will, — and to move in accordance with the directions of the
instinct or the judgment. For 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 points of the fingers). These have the
peculiar property of receiving impressions 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. From the Nervous centres, other cords proceed to the
various Muscles, by which the body is moved. These muscles,
commonly known as " fiesh," 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 (YEGET. PHYS. § 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 animal
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 mind, which is
connected in some mysterious and inscrutable manner with

DISTINCTIVE CHAKACTEES OP ANIMALS. 29

the central masses of the nervous system. Thus, we desire
to perform a certain movement or set of movements; this
desire leads to an act of volition or will; 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 cause it
to take place against the will, — as when a strong sense of the
ludicrous produces laughter, in spite of our desire (owing
to the unfitness of the time and place) to restrain it; for
the emotion, like the act of volition, produces a change in
the nervous centres, which causes a motor impulse to travel
along the nerves, and thus calls the muscles into contraction.
And it seems to be in the same manner that those instinctive
actions are produced, which, although few in adult Man when
compared with those resulting from his will, predominate in
his infant state, and through the whole life of the lower
animals (Chap. xiv.). "We shall also find that the nervous and
muscular systems of animals are concerned in a class of actions
with which 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
Nervo-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 limbs 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 Nutrition 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
by what may be termed the vegetative organs. But in
the lower of tribes of Animals, we do not find the animal
functions to possess this predominance. In fact, among the

30 DISTINCTIVE CHARACTERS OF ANIMALS.

many which, 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 respiratory
organs ; and the nervo-muscular apparatus by which they
are effected, bears so small a proportion to the organs of
nutrition, as to seem like a mere appendage to them, and is
sometimes altogether undiscoverable. This 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 parts, which need to be mutually connected, but also
require a certain 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.
Now 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 we descend the
animal scale. On the other hand, we find the various forms
of true cellular tissue, such as predominate in plants (VEGET.
PHYS. Chap, in.), becoming mere 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.

CHEMICAL CONSTITUENTS : ALBUMEN. 31

Chemical Constitution of the Animal Body.

13. By far the larger proportion of the Animal fabric is
formed at the expense of the substance termed Albumen; the
composition and properties of which, therefore, claim our
first 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 fluids 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 dried 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 egg), 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 familiar 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. No 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 drop 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
quantity 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. Nearly 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
presence 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 facility
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 combination, that the large quantity of bone-earth

CHEMICAL CONSTITUENTS I — CASEIN, STNTONIN, FIBRIN. 33

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 (§ 13).

16. The substance of which muscles are composed, has
been commonly considered to be Fibrin (§ 17) ; but it differs
essentially from fibrin 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 being distinguished from that of
albumen or fibrin by the fibrillar arrangement of its particles,
which indicates an incipient organization. This substance,
termed Fibrin, 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

D

34: CHEMICAL CONSTITUENTS : — FIBRIN.

deprived of this water by drying, and then becomes 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. Richardson,
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. Fibrin
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 suffering 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, that the mouths of divided blood-
vessels are closed up, when the flow of blood from them
spontaneously stops. In all such cases, the fibrous network,
if formed out of connexion with a living body, passes after a
time into decay j but if it be formed in apposition 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 albumen, 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 with 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 sufficiently 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 part 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 facility ; this diversity apparently
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 known 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 living 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 fully 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 aliment, 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 maybe 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. — The
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
fibrous 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 qualities are characteristic of two distinct forms of
simple fibrous tissue, the white 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 which peculiarity it can be readily dis-
tinguished from the other kind, to be next described. The
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 being disposed to curl up
(fig. 2). They are, for the most
part, between 1-5, 000th and
1-1 0,000th of an inch in
diameter ; but they are often Fig- 2--Y*MO* FIBROUS TISSUE.
met with both larger and smaller. This kind of tissue un-
dergoes but very little change from long boiling, and it is

Fig. 1. — WHITE FIBROUS TISSUE.

38 PKIMARY TISSUES : AEEOLAR TISSUE.

not acted on by acetic acid. It is but little prone to decom-
position, and will exhibit its peculiar elasticity long after it
has 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 frequently 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.1 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 OP AREOLAR TISSUE.

cavities are filled during life with a serous fluid ; 2 and it is a
necessary result of the communication between them, that if
an accumulation of this fluid takes place to an undue extent,

1 From the Latin areola, a small open space.

2 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, the 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. But in dropsy,
it appears as if the elasticity of the fibres were impaired or
destroyed by their being over-stretched ; for when we press
with the finger upon a dropsical part, a pit 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 j 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 into 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 purpose
obviously is to allow a certain amount of motion among the
parts 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.

26. 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 surface 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 I — SEROUS MEMBRANES.

and has sometimes been purposely effected, does not produce
any disorder in the general functions of the body. In blow-
ing the nose violently, some part 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 eyelids, which is particularly loose ; an enor-
mous swelling of these parts then takes place, presenting a
very frightful appearance, but not attended with 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 nbro-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 explained 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 dropsies 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-
quently find that they are chiefly or entirely composed of the
white 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 quadrupecls,
such as the ox, to assist them in supporting their heavy
heads with as little exertion as possible ; whilst carnivorous
quadrupeds, such as the lion 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 drawn back into their sheaths when not
in use, being projected (when required) 'by muscular action ;
and that the two pieces of the shell of Bivalve Mollusks 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 I CELLS.

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
underlies the Epithelial layer of the Mucous membranes
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 j and its characters may be well studied by dis-
solving the calcareous part of an oyster or mussel-shell in
dilute acid, when it will 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._NUCLEATED CELLS; a a, nuclei. ^.^ formed Q£ a stmcture.

less membrane, and having its cavity filled with fluid of some
kind. In some part of its interior, most commonly adhering

CELLS; THEIR MODE OF MULTIPLICATION. 43

to its wall, there is usually to be observed a solid collection
of granular matter, which is termed the nucleus (fig. 4, a a).
The typical form of the 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

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 the
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 ; this is the case, for
example, with the red corpuscles of the blood of Mammalia
(§ 229), and also with Fat-cells (§ 46).

33. New 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 binary subdivision 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 the contents of the cell ; so that 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 speaking, the former method seems
to prevail in structures which, like Cartilage, have a com-
paratively permanent 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 for; and it appears, from recent

ISOLATED CELLS OF ANIMAL FLUIDS. 45

inquiries, that the blastema will resolve itself into fibres or
into cells, according as the wound is completely secluded from
the air, or is exposed 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 inflammation is
most likely 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
iavourably as well 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 powerful microscope, may often
be seen to undergo very curious changes of form, resembling
those of the Amoeba (§ 129). Besides the foregoing, however,
the blood of Vertebrated animals contains a far larger pro-
portion of red corpuscles, which are flattened disks, sometimes
circular but more commonly oval, having pellucid and colour-
less walls, but having their cavities filled with a peculiar
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.1

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

1 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 the 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 communicates 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 difference 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- Skin is con-
tinuous with that of the Areolar 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), which 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 OP THE SKIN,

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

This is the more probable from the fact that we find these
vascular papilla 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 STRUCTURE OF THE SKIN.

same kind is found in the matrix or receptacle of the growing
roots of nails, hoofs, horns, &c. which are only modified forms
of epidermis. Imbedded in the substance of the cutis wo
find, in most situations, the perspiratory glands (fig. 7, e e.), 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 7i), 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 glandule, 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
termed the cuticle, or " scarf-skin," is
OF THE HTTMAN composed of numerous layers of nu-

SCALP;— a a, sebaceous glands; cleated Cells : of which WC find those
6, a hair, with its follicle c. ^ immediate contact with the bage,

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 OP THE SKIN I 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 thrown 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 nothing 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, like the
inner layers of the epidermis j 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 have a good example
of the detachment of the entire epidermis at once (reminding
us of the casting of the shell of the crab and lobster) in the
" 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 ;
and 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 ordinary mucous
membrane is made up, as in the skin, by the consolidation of
areolar tissue, the fibres of which are continuous with those

MUCOUS MEMBRANES : EPITHELIUM. 61

of the ordinary areolar tissue on which 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 diiferent from that of the body; and
though the mucous membrane lining the air-passages is very
susceptible of certain kinds of irritation, yet 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 partici-
pation in the functions of secretion and absorption. One
secretion is common to all, that of the mucus by which they
are covered ; this serves to protect them from the irritation
that would otherwise be produced by the contact of solid or
liquid substances, or even of air, with their free surfaces;
and we see the results of its deficiency, in the inflammation
which attacks the membrane, sometimes proceeding 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, constituting 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 adhere
in such a manner as to form a continuous membrane, except
in the interior of the mouth and oesophagus (gullet), where
the epithelium is endowed with 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 ;

E2

52 MUCOUS MEMBRANES : EPITHELIUM.

and they are as continually replaced by newly-formed cells,
which are produced on the surface of the basement-mem-
brane, at the 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-
sorption 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. 9). These pro-

Fig. 9. — DIAGRAM KEPRESENTING THE Mucous MEMBRANE OP THE
INTESTINAL CANAL.

a a, absorbent vessels; ft 6, 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 chiefly 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 are 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 other 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 off, 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 are the simplest types or examples
of all the Glandular structures, by which certain products aro
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 minutely distributed
blood-vessels ; and it seems to be by the peculiar powers of
these cells, that the products of the secreting action, whether
bile, saliva, fatty matter, or gastric fluid, are formed (see
Chap. VIL). — 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 Membranes which line the closed cavities of
the body, though composed of the same elements as the skin
and mucous membranes, have- 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 diifers, except in its greater
density, from the laxer areolar tissue whereby the membrane
is attached to the walls which it covers like plaster ; it is but
sparingly supplied either with blood-vessels or absorbents ; and

Fig. 10.

PAVEMENT EPITHE

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 epithelium-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
freely, by the large vessels proceeding from
CELLS OF^SEROUS its summit, within a bag or sac of fibrous
MEMBRANE. membrane peculiar to itself, which is termed

the pericardium. The cavity of this bag is completely lined
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 this
membrane, whilst including
the heart, and allowing it to
communicate with its vessels,
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
two layers, the head being

j-'Ag. **• J-'i A. VJJVAUO. v»x i nr* JL X.AJ.VAAI.'IIJ J»« -- _ ... ^^

a a, auricles ; v v, ventricles ; b, pulmonary really On the GUtSlue 01
artery ; c, aorta; pp>, pericardium. ^ whi]jgt geeming 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.

Fig. 11. — DIAGRAM OF THE PERICARDIUM.

SEROUS AND SYNOVIAL MEMBRANES. 55

A similar arrangement exists in the great cavity of the ab-
domen ; but the number and the complex relations of the
viscera which this contains, give to the disposition of ita
serous membrane, termed the peritoneum, 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 absorbed 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 peritoneum respectively.

44. Nearly allied to the Serous mem- T /
branes are the Synovial, which form closed
sacs in the interior of joints, covering the
ends of the cartilages, and then lining the
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, b b the layers of car- Fis- 12«

tilage with which they are severally covered, DIAGRAM OF THE STRUC-

1,, , , . , ,. * ,, •'. , ' TURE OF A JOINT.

and the dotted line c c the synovial mem-
brane, which is seen to form the sac or
bag cf cf, whilst at the points cccc 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

cartilage ; b b, layer of
cartilage closely co-
vered with synovial
membrane ; c c1 c, re-
flected layer of syno-
vial membrane form-
ing synovial capsule.

56 SYNOVIAl MEMBRANES. — CILIATED EPITHELIUM:

by the continuity of its blood-vessels over the 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 mlli 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-
ivards into a villous filament, or inwards 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 with 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

their cina are seen at o, tneir nave been observed being about
nuclei at c -, at a is shown'or.e of l-500th of an inch in length, and

these cells unusually elongated. ^ gmaUest M^OOOth. When in

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

B

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 higher 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 perfect 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 supplies of air and food, but also for pro-
pelling their own bodies through the liquid. 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 cilia lining the alimentary canal and clothing
the gills (where such have a special existence), draws into the
mouth the minute currents which serve as food, and also
renews the layer of water in contact with the respiratory
surface. The gills of Fishes are not furnished with cilia,
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 Newt, which hang 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 lines 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. From the fluid in these
vessels, the fatty matter is
separated in the first place by
the secreting action of the
cells ; and it is prevented
from making its way through
the very thin walls of the
cells, by the simple expedient of keeping these constantly
moist with a watery fluid, the blood.1 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

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

Fig. 14.— FAT CELLS, HIGHLY

MAGNIFIED.

FAT. CARTILAGE. 59

pass that keep it in play. It also affords, by its power of re-
sisting the passage of neat, 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 (§ 162).

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
chondrin (§20), through which are scat-
tered a number of cells, at a greater or less
distance from one another. In the simple
cellular cartilages, such as those which
cover the ends of the bones where they
glide over one another so as to form
moveable joints, no trace of structure can
be seen in the intervening substance. Fig. is. — SECTIOK OF
But in cartilages which have to resist not CARTILAGE,

, . . Showing its cells imbed-

only pressure but also extension or strain, ded in intercellular sub-
we find the space between the cells partly stance-
occupied by fibres, which resemble those of ligaments ; and
such are termed fibro-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 gelatinous substance, and therefore bears

60 CAETILAGE. 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, sometimes 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 resembling
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 cylinder ; 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 OF BONE.

61

of areolar tissue, by thin bony partitions, which 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, like 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-
nected, every here and there, by
cross branches (fig. 16). They
are termed the Haversian canals,
after the name of their disco-
verer, Havers. — The lining mem-
brane of the large central cavity
is copiously supplied with blood-
vessels; and this sends off pro-
longations into the cancelli at

the extremities of the bone, and Fif?- ".-DIAGRAM REPRESENTING

' THE STRUCTURE OP A PORTION OS

into the Haversian Canals. Thus THE SHAFT OF A LONG BONE.

hlonrl is rrmvpvprl into fhp» in a b ° d> the surface as seen in
Lb COnvtyeu IE in- transverse section; b e f c, surface

terior of the bone ; but no vessels

can be traced absolutely into its

texture, so that all the spaces

which lie between the Haversian

canals are as destitute of vessels as

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

seen in longitudinal section ; i, Ha-
versian systems cut across, each
having an Haversian canal in its
centre ; g v, Haversian systems cut
longitudinally { /, lamellae near the
surface of bone, destitute of Haver-
sian systems.

62

STRUCTUEE OP BONE.

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

a —

Fig. 17.— TRANSVERSE SECTION op BONE.

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

from the two flat sides of each cavity; one set passes inwards,
towards the centre of the ring, and the other outwards, to-
wards the ring that next surrounds them. These minute
tubuli, 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 different 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 are

COMPOSITION OF BONE. 63

supplied by the blood-vessels, and to transmit it to the
outer circles, or those furthest removed from those vessels ;
and in this manner, a much more active nutrition takes place
in bone than that which 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 entirely
removed from it, and -the organic substance remains;
the latter is now quite flexible, and almost transparent, 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 reality ; 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 OF BONE.

it corresponds closely with the "white 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 animal 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 bones, including the contents of the
Haversian canals and cancelli, is compared, the proportion of
mineral to animal matter is found to vary greatly in different
classes of animals, in the same animal at different ages, and
even in different bones of the same individual ; the mineral
matter predominating in bones of a compact texture, and the
animal in those whose substance is more spongy.

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 lacunce nor canaliculi. 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 supplied 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,
which we have just seen to be presented by transverse sec-
tions of long bones. In old bones the Haversian canals are
so nearly filled by these deposits, that there is barely room

DEVELOPMENT OF BONE : OSSIFICATION. 65

for the blood-vessels to pass along them. And it is through
their complete blocking up, by a continuance of the same
growth, that the supply of blood is cut off from the interior
of the bone which forms the antlers of the deer, so that they
die and fall off ; their shedding and renewal being an annual
process.1 — 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 long 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 periosteum (or membrane covering the
bone) by calcareous deposit ; the lacunae being probably the
cavities of cells which were entangled in the fibres, and the
canaliculi 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
point at a time. In the long bohes, there are usually three
such points; one for the shaft, and the others for the two

1 It is commonly stated that the death of the anclers is due to the
formation of a bony ring at their base, which cuts off the supply of
blood from the " velvet" which 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.

F

66 DEVELOPMENT OF 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 the 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 fiat
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 Mammals. In Reptiles 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
exception in the Pterodactylus, § 669, 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. This
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 " bodies " of
the vertebrae (§ 71). Such a cylinder, which is termed the
chorda dorsalis, precedes the formation of the vertebral
column in other vertebrated animals (§ 757). In the curious
Amphioxus (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 vertebrae.

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 teeth is made up of a solid tissue,
which has been appropriately called Dentine. Of this sub-
stance, one variety, which is peculiarly close in texture, and
susceptible of a high polish, is familiarly known as "ivory*
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- _T^_mr__TTTTT^_^n_T^.r^rimr_TTr.i
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 an
bone. This dentinal substance is
traversed by minute tubuli of
about l-10,000th of an inch in
diameter, which appear as dark
lines, generally very close to- Fig. is.

gether ; these pass in a radiating PORTION OF DENTINE (highly magni-

manner from the central cavity fied), showing Us tubular structure.
of the tooth, diverging from each other as they approach
its exterior; but when seen in only a small part of their

F 2

68 STRUCTURE OF TEETH.

course, they appear to be nearly parallel (fig. 18), though
usually 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
lacunse 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 Reptiles 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
the other softer than dentine.
The former, which is called
Enamel, consists of long pris-
matic cells, which pass from one
surface to the other of the thin
layer formed by this substance
over the crown, or sometimes in
the interior of the tooth (§ 182).
These prisms are usually hex-
Fig. 19. agonal in form, as is seen in

PORTION OF ENAMEL (highly magni- transverse section (fig. 19): and
fied), showing its component prisms.

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, iii 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 than 98
parts in 100, leaving when removed only 2 per cent, of
organic matter. The softer component of Teeth, known as
the Cementum, or Crusta petrosa, 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 lacunae 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 which 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 the 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
which, 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 own, 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, xn.);
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.

to 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 different 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 which 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
probable 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, which 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 we 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

STHIATED MUSCULAR FIBRE. 71

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

Fig. 20.— STRIATED MUSCULAR FIBRE SEPARATING INTO FIBRILLJE.

or striation by which this form of muscular fibre is characterised,
is found, when the fibre is separated into its fibrillee, to be due
to the peculiar 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 light
and dark spaces respectively, throughout the whole bundle of
the fibril, 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 cylinder; 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 fibrillse
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 1 -400th of an inch, and does not differ very
widely in either direction ; in the cold-blooded Vertebrata,
however, the average size is greater, and the extremes are
also wider ; the diameter of the fibres varying in the Frog
from l-100th to l-1000th of -an inch, and in the Skate from
l-65th to l-300th of an inch. The diameter of the fibrils is
nearly the same in all classes, seldom departing much from
1-1 0,000th of an inch ; and the average olistance of the dark
striee from each other is nearly the same.

72 NON-STRIATED MUSCULAR FIBRE.

57. The other form of Muscular Fibre, which, from the
absence of transverse striation, is distinguished as smooth or
non-striated, is found not in large masses, but in thin layers,
forming part 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 withdrawn
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." But
these designations are not by any means consistent with the
facts of the case ; for in a large proportion of the Molluscous
classes, the muscles of animal life are composed of non-
striated fibre, whilst the heart of Man and of other Verte-
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
c 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 individual fibres is
much more variable than that 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
fibre, showing, a a, the may ^Q resolve<! into bundles of long

spindle-shaped cells, and, . ,, -, -, , -,. n • i . • -

b 6, the elongated nuclei ; spindle-shaped bodies, which, contain-
So?et!MvCe^'iS?na ™S elongated staff-shaped nuclei
similar ceil treated with (fig. 21), may be regarded as cells, al-
though it is difficult to distinguish their

walls from their contents. This form of muscular tissue is
commonly mingled with a large quantity of the ordinary fibrous

MUSCULAR CONTRACTION. 73

structure ; and we find it dispersed in small quantity through
the latter in the skin, to which (especially in particular
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."

58. Under the influence of certain exciting causes, or
stimuli (Chap, xu.), striated muscular fibres suddenly and for-
cibly contract. Their two ends approach one another, and their
striae 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 own 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 with the con-
tinuance of the state of contraction of the muscle as a whole.
For it appears that when 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, and 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. Now 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 CONTRACTION. 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. Now 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 pro-
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 white 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. 75

as the tubular and the gelatinous. The " tubular " fibres are so
named because each poss.' ;*es 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 branch 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 this 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 unfrequently
see the former to be alone continued, 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 fibrous 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 nerve-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 fibrillse ; 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^there is no doubt that this is an incorrect notion, and
that even the fibres of the ordinary nerve-trunks may present
the " gelatinous " aspect, probably from incompleteness of
development.

61. 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 of large cells or vesicles, is generally known as the

VESICULAR OR GANGLIONIC NERVE-SUBSTANCE. 77

vesicular. These nerve-vesicles, sometimes known as gan-
glion-globules, 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-SUBSTANCE.

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 are

78 STRUCTURE OF GANGLIA. — NERVOUS ACTION.

filled with a finely-granular substance, which extends into
their prolongations ; and in the warm-blooded Vertebrata they
contain pigment-granules, which give them
a reddish or yellowish-brown colour; so
that the aggregations of vesicular substance
which we find in the larger nervous 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-

™ETHE ™M,^HE"IC fibreS' ThuS We filld a }™SB Pr°POrtion

SYSTEM, showing the of the retina (§ 535), which is commonlv
flK*ea£oiV»tne££ described as a mere expansion of the optic
giionic ceils. nerve, to be composed of nerve-vesicles

that are scarcely 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 with
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.

62. There can be no doubt that the special office of the
'Neive-Jibres is to convey the influence of the changes which
are effected in one part of the system, to other and remote
parts ; just as the wires 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 Nervous
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 from
the central organs to the muscles, which they 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 which originate in the
circumference; — 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, we 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 be 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
which 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 plexu,s 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 which they are transmitted.
The immediate dependence of the functional activity of this

80 SIMPLIFICATION OF STRUCTUBE IN LOWEST ANIMALS.

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.

64. 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), with nuclear particles, in
various phases of development into cells and fibres, more
or less thickly scattered through it ; and this substance
being everywhere 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 Rhizopods (§ 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
slightest 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 endowment is dif-
fused, as it were, through the whole substance of their bodies.

65. Every component part of the fabric must be regarded

INDEPENDENT VITALITY OF PARTS OF ORGANISM. 81

as having a life of its own, which 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 part&
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 observed 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.

66. Many animals are reduced to a state of apparent death
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 Entomostracous Crustacea (ZOOLOGY, §§
883, 931). It is unquestionable that many Fishes, especially
those of fresh- water lakes, will revive on being thawed after
having been completely frozen ; and the same has been ascer-
tained of certain Caterpillars. • The Snail, when retiring for
the winter, seals the orifice of its shell with an impervious
lid ; and in this cavity it may remain shut up for years, until
re-excited to activity by warmth and moisture. 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 long must the organism be considered as alive,
however death-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
prone to decomposition, that it speedily passes into decay
unless prevented from doing so either by a reduction of tem-
perature, or by complete drying -up, or by entire seclusion
from air; and although each of these methods, practised
upon animal substances already dead, may prevent the occur-
rence of decomposition for almost unlimited periods, yet
neither can be applied to the living tissues of any of the
higher animals, without occasioning the entire loss of their
vitality, 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 operations, 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 their 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.

68. Prom what has been stated, it will be seen that Life
cannot be regarded as a condition 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.

84 PRINCIPAL TYPES OP ANIMAL STRUCTURE.

CHAPTEE II.

GENERAL VIEW OF THE ANIMAL KINGDOM.

69. WHEN we examine the Animal Kingdom as a whole, it .
is easy to distinguish in it four general plans or types of struc-
ture, by which, with almost infinite variations in detail, the
formation of the several beings that compose it has been
guided. As specimens 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-fish. 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 we 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
blance to him in structure. We notice in regard to their
external form, that they are 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 symmetry " 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 op THE OSTRICH.

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

86 VERTEBRAL COLUMN.

growing, and of being reproduced 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 afford points of attachment, to the soft parts, in the
limbs (where they exist) as well as in the trunk ; but the former
are not unfrequently 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 Vertebrated
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
^JRAif coIuMTE~lin^'e(^ Vertehra3, is termed the vertebral column
(fig. 26). The ordinary character of the vertebras
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 solid portion of the
vertebra (fig. 27, a) is termed its body; and the
projections, b and c, are termed its processes, the
former spinous, the latter transverse. The row
of spinous processes forms the ridge which we
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
sense also. At the opposite extremity we see it contracted
into the tail; which is composed of a series of vertebrae

NERVOUS SYSTEM OF VEBTEBRATA.

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-
garoo (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 Verte-
brated animals consists of a Brain and
Spinal Cord (fig. 28), which are lodged
within the skull and vertebral column ;
and of nervous trunks proceeding from
these, which are distributed to all parts
of the body. The Brain 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 Vertebrata generally ;
and their proportionate size corresponds
with the relative development and ac-
tivity of the several organs of sense
with which they are connected. The
bulk of the brain of Man, however, is *
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

88 NERVOUS SYSTEM OF VERTEBRATA.

and complexity of structure increase as we ascend the scale,
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 which 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 inclosed 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 adaptation 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 of intelligence, or of the reasoning faculties.

74. The general arrangement of the other organs in Verte-
brated 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 I, the liver : these organs occupy the
part of the body which is called the abdomen or belly. The
mouth also opens, however, into the windpipe, or trachea, t,

GENERAL STRUCTURE OP VERTEBRATA. 89

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

s I

Fig. 29.— DIAGRAM, SHOWING THK POSITION op THE PRINCIPAL ORGANS IN
VERTEBRATA.

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
modification 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 exception. 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 Eeptiles
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, Lamprey, &c. We have further to remark, in
regard to the general characters of Vertebrated animals, that,

90 CLASSES OF VERTEBEATA : MAMMALS.

with one exception, they have all red blood (§ 226) ; and
that they possess a complex apparatus for circulating this
through the body.

76. The four principal modifications under which the Ver-
tebrated type presents itself, constituting the classes of MAM-
MALS, BIRDS, REPTILES, and FISHES, are respectively character-
ised by the mode in which 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 suckling; 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 termed warm-blooded — the latter
cold-blooded. There is a like difference in their mode of life ;
some of them being destined to live on the surface of the
earth, whilst others are chiefly inhabitants of the air, and
others again are the tenants of the ocean.

77. MAMMALS are distinguished from all other Vertebrata
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 Reptiles, 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 purposes ; whilst the Bat tribe have the two
arms converted into wings, which enable them to fly through
the air like birds (for which the older naturalists mistook

* Many Zoologists range the Frogs 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

them); 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
Whales, 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 «..„ ~"*v..

'

Pharynx

-Oesophagus

Gall-Bladder

Colon .^, _
Caecum....
Small Intestines

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 diaphragm, into two portions — the thorax, 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 BIRDS there is a much closer conformity to one
general plan than we find among Mammals. The covering of
feathers, by which we ordinarily distinguish 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 Reptiles, 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 egg 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 wings is stretched out upon the
bones which 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 Mammalia 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 which 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.

93

nest. The nutritive functions are performed with extra-
ordinary activity in Birds, that the means may be supplied
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 beeD
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 6, 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

Fig. 31. — LUNGS OF THE OSTRICH.

,

; ///, air-cells,in which are also
seen tne tubes b^ which these air'

cellg communicate Wuh the lu-ngs.

«. the lieart; b> the stomach ; c c, the

.. . , • j • fm j intestines; d, the trachea; e, the

Otherwise have OCCUpied IS filled lungs;
n-n Vnr fhp IflTfTP flir rpll<3 f f

up by tne large air- eiis, /,/,

which communicate freely with

the lungs and with each other, and which even occupy 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

94 GENERAL STRUCTURE OP REPTILES.

the same group j 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,
Serpents, and Frogs, are brought together by the following
characters. They are all oviparous, in this respect agreeing
with 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 differ 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 their young state breathe by gills, and
lead the life of a fish ; and some of them retain their gills
during the whole of life, 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 dulness 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 warmth, 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
inclosure 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 fiat
plate (termed the plastron), which is often incomplete, is

STRUCTURE OP TURTLES AND LIZARDS. 95

formed by an expansion of the sternum or breast-bone, which
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 Lizards, the body
has no such covering ; but these animals,
having more activity than the tortoises

/ !_• i. i • n i \ -ui j Fig. 32. — TORTOISE.

(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 differ 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, that
meet at their edges like the scales of tortoise-shell, and afford
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 respiration is very inactive, they
can remain for long periods beneath the surface, without
being 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 they are able to crawl
upon the points of these, using them almost as Centipedes do
their legs (fig. 42). But in general the movement of their

Fig. 34.— ANATOMY OF A COLUBER

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

gullet, oe, which 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 oe', 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,
cl, which opens externally by the orifice, an. The liver, /, is
also much lengthened. From the mouth also proceeds the
long windpipe, t t, which conveys air to the lungs, or rather
to the single lung ; for the lung on the left side, p'} is scarcely
at all developed, whilst that on the right, p, extends along a
great part of the body. At o is seen the ovarium, 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-Newt, 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

METAMORPHOSIS 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, b. 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 perennibranchiate, this term express-
ing the persistency of their gills. In fig. 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

Fig. 41. — LEPIDOSIREN.

brought from th.3 rivers of Africa, the metamorphosis of
which 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 STBUCTUEE OP FISHES.

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

88. The class of PISHES 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 Reptiles, 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
flattening 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 which the
body is propelled forwards (Chap. xn.). This stroke is given
by- a series of muscles of great power, which 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 propelling 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 column itself, instead of being committed to
the limbs, as in Mammals, Birds, and most Reptiles. 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 ;
for their eyes are usually large and highly developed, while

STBUCTUEE OF FISHES. 101

the other organs of sense are formed upon a very inferior
type. They swallow it without much division in the mouth ;
but it seems to undergo rapid digestion. The blood of some
Fish, whose muscular activity is peculiarly 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 warm-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-like 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 Invertebrata ; 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 Vertebrata ; 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 slightly developed,
or are altogether absent; and in the highest,
which 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
external skeleton is composed, undergoes little
or no change when it is once fully 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 usually a pair of large
ganglia in the head, bearing a resemblance (in their peculiar
connexion with the eyes) to the ganglionic centres of the
optic nerves in Vertebrata ; 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 OF ARTICULATA. 103

and the ganglia themselves are composed of two halves,
which have little connexion with each other. The chain
thus formed (fig. 43) passes along the
under-side of the trunk of the animal
(as seen at g, fig. 44), not on what
seems its back ; and by the presence
of this 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-

, . , , , r . . , , . ., Fig. 43.— NERVOUS SYSTEM OF

testmal tube, i, ?., to terminate at the AN INSECT.

opposite extremity of the body. The

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. 44. — DIAGRAM SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN
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 allies, however, one
part of this, k, 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 OF 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 Crustaceans 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 three 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 which 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 acquired is applied to the development
of new organs. This pupa or chrysalis condition may be
considered as a sort of postponed completion of the embryonic

STRUCTURE OF INSECTS AND ARACHNIDA.

105

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

Antennae _

Eyes —

Head

1st pair of Legs

1st pair of Wings -—
2nd pair of Legs -

2nd pair of Wings •"
3rd pair of Legs

Tibia

Tarsus

Abdomen

Fig. 45.— SKELETON OF AN INSECT.

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

98. The animals of the class ARACHNIDA, which includes
the spiders, scorpions, and mites, are, like Insects, articulated,
breathing air, 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 be formed of two principal divisions, — the
cephalo-thorax (as it is termed), and the abdomen. 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 t 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 p is seen one

pa ab pa s

p pa t a I s ma o

Fig. 46. — ANATOMY OF SPIDER.

of the palpi, which are appendages to the mouth ; pa is the
foremost leg ; t, the large nervous mass, from which the legs
are supplied; a, the collection of ganglia supplying the
abdomen ; a 6, the abdomen ; p a, the respiratory chambers ;
s s, the stigmata or openings into these ; Z, the leaf-like folds
within them (§ 323) ; m 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 Cray-fifth 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, which 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 which, lines the carapace and covers in the
viscera. 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 CRAB.

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.

hopper, 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 from

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 dif-
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 their members. This group is

STKUCTUKE OP CIRBHIPEDA. 109

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

Fig. 49.— SHELL or Fig. 50. -BODY OP

THE BARNACLE. 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. We 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 equality
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.— IULUS.

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 110 proper metamorphosis ; but
there is a considerable adoption to the number of their seg-
ments and legs after they have come forth from the egg.

104. We 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 all agree, however, in the long worm-like form
of the body, and in the similarity of the different ganglia 01
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 are 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 axe arranged regularly on the several segments, and the
animal can swim by the motion that it gives them ; besides
these, it has a kind of bristle-shaped appendage, that seems

Fig. 52. — NEREIS.

like a rudimentary leg, which assists it in crawling. Eut
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
fSerpulce, fig. 145) tufts of great brilliancy and elegance.

105. Eelow 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 multiplied 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
thrown off. The group of ROTIFERA, or Wheel-Animalcules,
which is one of great interest to the Microscopist, also belongs
to this lower section of the Articulated sub-kingdom.

106. The general character of the animals composing the
group or division MOLLUSCA, 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 Mollusks, 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
obliged to make its locomotive organs project beyond the
shell. We 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. Not un-

Fig. 54.— TESTACEILA.

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

STRUCTURE OF MOLLUSCA. 113

one possesses a shell into which 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 (fig. 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 which the organs of sense and the instruments
of motion are developed in the particular animal. For these
are almost always symmetrical, being arranged with 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 fleshy 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
high 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 bulk 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

114 STEUCTURE OF MOLLUSCA.

from its surface, in those species which 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 cilia (§ 45) with which they are covered.

vb ab b ov

Fig. 55.— ANATOMY OF TURBO PICA.

108. The accompanying figure of the interior of a Turbo
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 cloge the mouth of the shell when the body of the animal
is drawn within it. At t is shown 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 salivary glands. The mantle, m, is opened and folded
back to show the respiratory cavity, in which lie the gills

STEUCTURE OP MOLLUSCA.

115

b b; to this cavity, water has access by means of a wide slit,
of which the edge, /, 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
blood from the gills by v b, the branchial vein, and then
transmits it to the body generally ; at e, far up in the spire,
are the stomach and liver ; at cc, 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 are
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 Mollusea 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,
from which the organs of sense
are supplied; B is the ganglion connected with the gills;
and c is that from which power is given to the foot. The

i2

Fig. 56. — 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 members, 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 by 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 Verte-
brated animals, is the class of CEPHALOPODA, or Cuttle-fish
1f ibe : 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 shell, commonly known as the cuttle-
fish bone, 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 offer no resistance to the action of

STRUCTURE OF CEPHALOPODS AND PTEROPODS. 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. 57. — CALAMARY.

Cephalopods, which approaches the Gasteropods 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 fossilized remains of
very numerous forms of this group
that existed in the ancient seas, con-
stitute the nautilites, ammonites,
belemnites, &c., which abound in
many rocks (fig. 58). The Cuttle-
fish are animals of considerable
activity; their mouth is furnished Fig SS—
with a horny beak, strongly resem-
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 Mollusks, consists of
but few species, and the animals
which it contains are 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 Fig> 59--HTAI-*A-
principal articles of food to the Whale. The general form of the
body usually differs but little from that represented in fig. 59.

118 STRUCTURE OP GASTEROPODS AND BIVALVES.

On either 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 commonly 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, like the Snail and Slug, crawl
upon a fleshy disk on the under side of their bodies ; and
the number of distinct 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, whilst
others are destitute of them. The shells are composed of a
single piece, or are univalve, except in one tribe ;
and they have usually more or less of a spiral
formation (fig. 60). The animals of this class all
possess a distinct head ; and this is generally
furnished with eyes, as well as with tentacula.
They have often a powerful masticating ap-

paratus, and are voracious in their habits ;

Fig. GO.— SHELL some of them feed upon vegetable matter, others

OF PALUDINA. ^^ animalg>

113. The Acephalous Mollusca are divided into two groups,
— those which form shells, and those which do not. The
former are termed CONCHIFERA, or shell-bearing animals ; and
this class includes all the Mollusca that form a shell composed
of two parts or valves fitted together (which shell is termed
bivalve), 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 are united by a ligament, which, by its
elasticity, keeps them apart while it holds them together. This
is their usual condition when the animal 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 CONftHIFERA, OR BIVALVES. 119

parts. But when any alarm or irritation causes the animal to
close its shell, it does so by means of a muscle (sometimes

Fig. 61. — SHELL op TRIDACNE.

single, sometimes double), which stretches across from one
valve to the other, and which, by contracting, draws them
together. Each valve is lined 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 Oyster
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 which is considered as the anterior end or
front of the animal, being that nearest which the mouth lies ;
and the posterior extremity (the lowest in the figure) is that
at which the intestinal canal terminates, and at which the
respiratory tubes are formed. JSear 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 lies 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 OP CONCHIFERA OR BIVALVES.

terminating near one of the canals or siphons, which 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

Foot -

Intestine T.

Stomach -

Gills.—

Mantle

Anas

Respiratory Tubes.
Fig. 62.— ANATOMY OF MACTRA.

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

STRUCTURE OP TUNICATA. 121

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

114. The group" of Acephalous Mollusks which are desti-
tute of the power*of forming a shell, includes two classes, of
Avhich one does not depart widely from the general Molluscan
type, 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 compound 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-

t

Fig. 63.--SOCIAL ASCIDIANS.

panying figure (fig. 63). The cavity of the mantle possesses,
as in the former instance, two orifices ; by one of which, 5, a
current of water is continually 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 water. All these parts

122 STRUCTURE OF TUNICATA AND POLYZOA.

are covered with cilia, by the action of which 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 spot during
all but the early part of their lives, 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 which underlies the outer tunic, so that any irrita-
tion applied to the former occasions a contraction of the
latter, which tends to expel the offending particle. — This
class is one of particular interest to the naturalist, since we
see in it the tendency to the formation of compound struc-
tures, by a process resembling that of the budding 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 sometimes 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 Eowerbankia (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 the midst of a circle of arms fringed with cilia; these

STRUCTURE OF POLYZOA AND RADIATA.

123

arms do not serve, however, like those of polypes, to grasp
the food ; 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, b ; 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 this 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 upon a complex
a, oesophagus ; b, giz- apparatus of muscles, by which the animal
rorificesof™nfehsi can be either drawn into its cell or projected
tiue. 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 aspect. This arrangement is well seen in
the common Star-fisti (fig. 65), 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. 66 ; 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 detected in any of these ani-

Fig. 64.

BOWERBANKIA.

124

STRUCTURE OF RADIATA.

mals ; and it is onjy in a few that even these imperfect eyes
can be discovered. In the inferior Eadiata, not the slightest

Fig. 65.— SHELL OF STAR FISH.

traces of a nervous system have yet been discovered; and
it is very doubtful whether any such structure exists in
them. It is only among the higher
Radiata 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
character, rather resembling that of the " zoospores " of plants,
than the intentional change of place of the higher animals.

Fig. 66. — NERVOUS SYSTEM
OF STAR FISH.

STRUCTURE OF RADIATA. ECHINODERMATA. 125

117. The circular arrangement of the organs of Eadiated
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 like the blossoms of some of the Com-
posite tribe of plants, as to have received the name of animal
flowers. 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 aggregation 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 securing 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 Holothurice (fig. 67), commonly
termed Sea-Cucumbers. This class ranks as the highest
among the Eadiata, in regard to general complexity of struc-
ture. The skeleton of the Sea-Urchin, Star-fish, and other
animals resembling them, is a box-like shell or " test," formed

126

STRUCTUKE OF ECHINODERMATA.

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

Fig. 67. — HOLOTHURIA.

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. 68, c), furnished with suckers
at their extremities, which can be
projected from almost 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-
Urchin (commonly termed the Sea-

Fig. 68. — ENCRINITE.

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-

. — 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 OP ECHINODERMATA AND ACALEPH^.

to allow of the separation of its two halves. The mouth, &,
situated at one of the poles of the shell, is surrounded by a
very curious apparatus of jaws 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 fl.1imp.n-
tary tube ; the continuation of this, o} <?, 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 cavity of the shell ; and the water is kept in movement
by the cilia with which the membrane is clothed. Round the
anus, s, are seen the five branching ovaries, each of which
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
found to prevail among all the inferior Radiata.

120. The radiated structure is also well seen in the greater
number of animals forming the group of ACALEPH.E, or Sec,*
Nettles. Their bodies are entirely soft and jelly-like ; 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
Jelly-Fish. They derive their other name of Sea-Nettles from
the stinging power which 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 OP ACALEPH^. 129

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

Fig. 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
Medusae 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 particular
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

STRUCTURE OF HYDRA.

Fig. 71. — HYDRA, OR FRESH-WATER
POLYPE.

require separate notice. The group of Hydrozoa, or Hydroid
Zoophytes, 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 the 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 life.
Not unfrequently, however, the young polype itself puts forth
buds before its separation ; and as many as nineteen young
Hydrse, in different 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
facility 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 thirty or forty individuals
may thus be produced by the division of one.

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 wall 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 performed under
very different conditions;

for whilst multiplication by
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
Campanularia (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 StniC- Fig. 72.— CAMPANULARIA.

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 OP 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 circulation
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 Medusce, 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 other 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 composite forms of Acalephse whose structure has until
lately been most obscure, turn out to be, as regards their
essential characters, Hydrozoa organized for floating, there
seems to be no longer any sufficient ground for ranking the
Acalephse as a separate class.

126. 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, — a plan which is higher than that of the Hydra,
inasmuch as we find the interior of the body containing other

STRUCTURE OF ANTHOZOA : - SEA- ANEMONE.

133

cavities around the stomach, which are destined to pre-
pare the generative products. In fig. 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
similar animal is represented, cut open to show its interior.

Fig. 73. — SEA-ANEMONE, seen Fig. 74. — SECTION OF SEA-ANEMONE.

from above. a, cavity of stomach; b, surrounding

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. Within these chambers, and
attached to their partition- walls, are found the bodies which
are commonly designated ovaries, but which contain sperm-
cells 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 way 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 I — STONY CORALS.

in the accompanying figure (fig. 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; and 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-
^ matter keing contained in its
interior, as in the Hydrozoa, it usually forms a kind of flesh

Fig. 76. — STEM OF CORAL.

that clothes the surface, and connects together the different

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 Zoophyte 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 Mollusks
in miniature, as we see among Foraminifera (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.

•>. 77.— RHIZOPODA :— A, Amoeba; B, Actinophrys.

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

136 RHIZOPODA : — AIKEBA; 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
undergoes; 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 01
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 Rhizopoda, have less power of moving from
place to place, but obtain their food by a modification of the
same arrangement : of this we have an example in ActinopJirys
(fig. 77 B). The body being stationary, its gelatinous substance
extends itself into long filaments, termed pseudopodia : these
often 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
minute animal or vegetable organism happens to come in contact

RHIZOPODA I FORAMINIFERA. 137

with one of these threads, 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 surface 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 4in the Amoeba. The reproduction of these
creatures, so far as is yet known, is effected by sell-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. — FORAMINIPERA.

A, Oolina ; B, C, Nodosaria ; D, Cristellaria ; E, Polystomella ; F, Dendritina ,
G, Glolngerina ; H, Textularia; I, Quinqueloculina.

131. This Bhizopod type of animal life is manifested in
two groups 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 POLYCYSTINA.

being empty and deserted, each chamber of the Rotalia, or
any other Foraminiferous shell, 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
delicate 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 putting 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 Foraniinifera 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
them, as well as nearly the whole of the Nummulitic limestone
by which it was succeeded.

132. But animals whose essen-
tial structure seems to be nearly
the same, may form siliceous in-
stead of calcareous shells; and
thus are produced those beautiful
organisms, known tinder the
name of Polycystina (fig. 79),
which are 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.
There is not in these the same

t -i i J* '±. Fig. 79. — POIYCYSTIKA.

tendency to lorrn composite A „ _,

i ji TI- v ,. A, Podocyrtis ; B, Rhopalocanium.

structures 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 attribute of simplicity, which
cannot be conceived capable of further reduction.

INFUSORY ANIMALCULES. 139

133. The Animalcules to which the name of INFUSORIA
may be properly restricted (the jRotifera, or Wheel- 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 Khizo-
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 indefinite
extension into pseudopodia, or of the passage of alimentary
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 mouth, ali-

Fig. 80. — INFUSORY ANIMALCULES.

i. Monads ; n. Trachelis anas ; in. Enchelis, discharging faecal matter; iv. Para-
nuEcium; v. Kolpoda; vi. Trachelis fasciolaris.

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, though of far
simpler organization. It is in these sessile forms that the
agency of the cilia in creating currents which bring 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 well 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 part 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 afford a probability in the affirmative.

136. In the tribe of PORIFERA, or Sponges, 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 whioh 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 fabric is provided
for by the budding out of this spongy portion of it, the
orifices of whose canals after a time become furnished with
polype-mouths. The true Sponge (fig. 81) consists of a fleshy
substance, composed of an aggregation of particles of sarcode,
supported upon a skeleton which usually consists of a net-

PORIFEBA 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 pig gl _SPONQE

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 oif
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.

1 37. 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 OP ANIMAL FOOD.

138. We thus conclude our general survey of the Animal
Kingdom ; which, it is hoped, will be found to answer the
purpose 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 will 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 have 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 FOOD.

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
progressively 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
heat-producing process ; and many substances are fitted to
supply this, which cannot serve for the nourishment of the
tissues.

140. 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, everything 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 Reptiles, 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 Mammalia 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 Italian 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 larvaa 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 their voracity is so great as to
have caused Linnaeus 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 Hon. The larva of the Silk-worm

144 NATURE AND SOURCES OF ANIMAL FOOD.

weighs, when hatched, about 1-1 00th of a grain ; previously
to its 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 larvae perish if left unsupplied with
food for a single day. On the other hand, a black beetle
(Melasoma) has been known to live seven months, pinned
down to a board ; and another beetle (Scarabseus) has been
kept three years without food, — and this without manifesting
any inconvenience or loss of activity. 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) fills 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 are many instances in which, no obvious sup-
plies of food being 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 found that, where food is taken in no other way,
a supply of the 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
animals depend entirely for their support.

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 Ruminants, 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 Monkeys devour the soft
pulpy fruits ; the Boar grubs up the roots and seeds buried
under the soil ; the Reindeer 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 feed only upon the outside of the leaves ; sortie upon
the internal tissue ; others upon the flowers or on the fruit ;
a few will eat 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
country. Thus 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 green thing escapes them;
and when their numbers are so increased that they fly in
masses which look like dark clouds, and cover the ground
where they alight 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
Augustin, that a plague, induced partly by the famine they
had created, and partly by the stench occasioned by their
dead bodies, carried off 800,000 inhabitants from the kingdom
of Numidia 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
blown 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
wind 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
1799, 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, though 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 larvse of an insect
called the " turnip-fly." The parent insects were seen buzzing
over the fields, and depositing their eggs in the plants, \vhicli
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 this 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 larvae 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 multiplication. Thus,
the myriads of Insects which find their subsistence in our
forest-trees, if allowed to increase without restraint, would
soon destroy the life 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 within salutary
bounds. Their occasional multiplication to the enormous
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 having 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 larvae of the turnip-fly 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 eggsr
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 this 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 Iarva3 of Insects, those that feed upon vege-
tables or dead animal matter (in the neighbourhood of which
their 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 power 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 destined. 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

150 ORGANIC CONSTITUENTS OF 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 proportions different from the last, the
two latter predominating considerably. Hence they consti-
tute another group of alimentary materials, to which the
term Oleaginous may be given. — Lastly, most Vegetables con-
tain, in greater or less amount, certain compounds which
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 with the flesh of animals, the composition of which
(§ 13) is identical with theirs, are united into a third group,
to 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 parts ; because gelatin (the substance that
forms glue), though it agrees with albumen in being made up
of the/<mr ingredients just named, differs from it extremely
in the proportions of those 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.

1 54. 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-azotized, being
destitute of this element. The distinction is a very important
one j 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 (§§ 1 3 — 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
into azotized compounds within the living body, either to be
simply deposited in its interstices, or to be thrown off from it
again without ever actually forming part of its organised
structure.

DESTINATION OP NON-AZOTIZED ALIMENTS. 151

155. Now, in regard to the non-azotized, or the saccharine
and oleaginous groups of alimentary substances, it appears 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 affirm, 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 body, constitute an im-
portant part of the food of Man, and of animals generally.1

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 presence, 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 animal organism
than the oleaginous contents of the fat-cells (§ 46), the ques-
tion again arises, — what becomes of them ? It is not enough

1 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 they are deposited as fat ; since it is only when
a large quantity of them is taken in, that there is any in-
crease in the quantity 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
warm-blooded animals, to a high standard. "We might almost
say with truth, that a great part of the oleaginous and sac-
charine principles is burned within the body, for this pur-
pose. The process will be hereafter considered more in
detail (§§ 412, 413) ; and at present 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, have 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-
portion 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 the 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 al1 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 flesh of animals (syntonin, § 16), upon the white of egg
(albumen, § 13), the curd of milk (casein, § 15), the grain of
wheat (gluten), or the seed of the pea (legumin). All these

DESTINATION OF NOX-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 egg 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 chat
are supported on these alone, the nutrition of the gelatinous
tissues does not seem to be impaired. But it appears equally
certain, that gelatin cannot be applied to the nutrition of the
albuminous tissues. Many series of experiments 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
likely that gelatin employed as food might be applied within
the body to the nutrition of its gelatinous tissues, yet there

154 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 which it affords any substantial pabulum. Con-
sequently the current idea regarding the nutritive value of
jellies of various kinds, has little or no real foundation.

160. It has been already stated (§ 68) 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-
portion as they are called upon for the discharge of their
functions. The need of material capable of replacing that
which 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 appropriated to the
repair of the waste which is continually going on. This waste
is much greater in young animals than in adults ; for all their
vital processes 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 respiration, by
which a large quantity of carbon and hydrogen 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 activity, 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 tjie 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 time, varying ac-
cording to the use that is made of them, these tissues un-
dergo another metamorphosis, which ends in restoring them
to the condition of inorganic matter ; and thus give back to
the mineral world the materials which 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 : —

( Carbonic acid and

\ water» thrown Off

—

other excretions.

162. But in 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 HERBIVORA AND OF MAN.

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

Starch, oil, and1} partly ( Fatty and \ but chiefly f Carbonic acid and water,
other non-azo- > converted < other animal [thrown off< disengaged hy the respi-
tized compounds J into ( tissues, ) directly as (. ratory 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
ordinary 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 ingredient, 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 hydrogea to support his
respiration, this want may be most advantageously fulfilled
by the employment of a certain quantity of non-azotized food,
in which these ingredients predominate. Thus it has been
calculated that, since fifteen pounds of flesh contain no more
carbon 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 animal

COMPOSITION OP 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 wandering tribes of hunters which
still people a large part both of the Old and New 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 water bears in each case
to the solid constituents of the food, which becomes a most
important element of consideration when the nutritive value
of different kinds of food is compared : —

Substances, 100 parts.

jj

"5

F

Albumi-
nous sub-
stances.

& £

S Id

£o><a

i

K

£

1

. J. *
J %
uǤ

A

o •

£ S

£§

«'£c'

0 3 S
H e S

Human Milk

89

3 5

4 2

3 0

0 2

11 4

3 5

14 9

Cow's Milk

8fi

4.5

5 0

4.1

0.7

14.8

4 5

19 3

Skimmed Milk
Butter Milk

87

87

4.5
4.5

5.0
5.0

2.7
0.5

0.7
0.7

11.5
6.0

4.5
4 5

16.0
10 5

Beef and Mutton

7?

19.0

5 0

2.0

12 0

19 0

31 0

Veal

77

19 0

1 0

0.6

2 4

19 0

21 4

Poultry

74

21.0

3.0

1.2

7 2

21.0

28 2

20

0.8

70.0

1.3

168 0

0 8

168 8

Cheese (Cheddar)
„ (Skimmed)
Butter

.36
44
IS

29.0
45.0

.'..'

30.0
6.0
830

4.5
5.0
2.0

72.0
14.4
99 0

29.0
45.0

101.0
59.4
199 0

Eggs
White of Egg

74

78

14.0
20.0

...

10.5

.5

.6

25.0

14.0
20.0

39.0
gO o

Yolk of Egg
White Fish

52

79

lti.0
19 0

...

30.0
1 0

.3
2

72.0
2 4

16.0
19 0

88.0
21 4

78

17.0

4.0

.4

9 6

17 0

26 6

Eel

80

10 0

8.0

3

19 2

10 0

29 2

Wheat Flour

IS

11.0

70.0

2.0

.7

74.8

11.0

85 8

15

10.0

70.0

2.4

2.0

75 8

10 0

85 8

Oat-meal

15

12 0

62 0

6 0

3 0

76 4

12 0

88 4

Rye-meal

15

9.0

66.0

2.0

1.8

70 8

9 0

79 8

14

9 0

65 0

80

1 7

84 2

9 0

93 2

Rice

14

7 0

76 0

0 3

0 3

76 7

7 0

83 7

Haricots

10

23.0

45 0

3.0

3.6

52 2

23 0

75 2

Peas

13

22 0

58 0

2.0

3 0

62 8

22 0

84 8

Beans

14

24 0

44 0

1 4

3 6

47 4

24 0

71 4

Lentils

14

29.0

44 0

1.5

2 3

47.6

29 0

76 6

Wheat-bread

44
48

9.0
5 3

49.0

4/i 0

1.0
1 0

2.3
1 4

51.4
48 4

9.0
5 3

60.4
53 7

Potatoes

74

20

23.0

0.2

0.7

23.5

2 0

25 5

Green Vegetables

86
18

2.0

4.0
82 0

0.5

0.7

5.0
82 0

2.0

7.0
82 0

* The value of the Fat is stated in this column according to its
Jieating equivalent of starch, which is larger in the ratio of 24 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 the nitrogenous compounds
predominate, are especially fitted for the maintenance of the
solid fabric of the body ; whilst those in which the carbon-
aceous compounds are in largest excess, are those which are
most effective 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 Not 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 are the mixture so common in Ireland and Alsace, of butter-

ECONOMY OF HUMAN DIET. 159

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 disgust under other
circumstances. The quantity of work which a man can do,
and his power of sustaining extreme cold, both depend in
great 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 in 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

160 MINERAL INGREDIENTS OF FOOD.

important. It exists largely in the blood, and in the various
animal fluids which are secreted from it ; and it is also an
essential ingredient of most of the solid tissues. Its presence
obviously tends to prevent that spontaneous decomposition
lo which organic substances are liable. Phosphorus 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 element 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
where it exists in the soil, and to which they resort to obtain
it ; such are the " buffalo-licks" of North America. Phos-
phorus exists also in the yolk and white of the egg, and in
milk, — the substances on which the young animal subsists
during the period of its most rapid growth ; 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, are 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. Sulphur is derived alike 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 limp.) in most of the river and spring water that we drink.
Iron is found in the yolk 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. Lime 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 afforded.
Thus a hen preparing to lay, is impelled by her instinct to
eat chalk, mortar, or some other substance containing the car-
bonate of lime 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 particles. The thickness of the shells
of aquatic Mollusks depends greatly upon the quantity of
lime in the surrounding 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 off (§ 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

162 DIGESTION AND ABSORPTION.

the blood, 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

DIGESTION AND ABSORPTION.

171. HAVING now considered the nature of the food of
Animals, and the sources from which 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 insalivation ; 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.
The 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 chymijlcation or the
manufacture of chyme. The chyme, which passes into the
intestines, is further acted-on by secretions that are poured
into them ; and a certain nutritive combination of albuminous

PEEHENSION OF FOOD. 163

and fatty ^ matters, termed chyle, is separated 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 accomplished 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
in 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 j but there are several animals
which, like 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 animals, obtain their food
by means of a long extensible tongue ;
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

T, ji T -, , -, . ,1 -i HEAD OF ELEPHANT.

little appendages round their mouths by

which the food is conveyed into them ; such are the palpi

M2

164

.RECEPTION OF SOLID AND LIQUID ALIMENT.

of Insects, of which a pair is attached to each jaw (fig. 84);
the tentacula of Mollusks, which are sometimes extremely
prolonged, as in the Cuttle-fish tribe (fig. 85); and the
similar organs of the polypes (fig. 71).

Fig. 84. — JAWS OF THE
SAME INSECT.

Fig. 85.— LOLIGOPSIS.

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. 86) ; in other instances it is drawn or
pumped up into this cavity, — either 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 are
drawn up according to the wants of the animal. Such a
conformation exists in the butterfly and moth tribe, whose

SUCTION OP LIQUIDS.

165

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

Fig. 87.— TRUNK OF A
BUTTERFLY.

Fig. 86. — CHIMPANZEE DRINKING.

Butterfly, a, of which the eye is seen at c, the base of the
antennae at b, the palpi at <?, 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.

166

DEVELOPMENT OF TEETH.

Mastication.

174. The act of Mastication, or the mechanical division of
the alimentary 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 against one another, with a cutting, crushing, or
a grinding power, according

to the nature of the food on
which they have to operate.
The manner in which they
are formed is worthy of
note. In Man, who may be
taken as a fair example,
c each tooth is developed in

6 _. the interior of a little mem-

Fig. 89.— DEVELOPMENT OF TEETH. , , . , . , n

a, the gum ; b, the lower jaw ; c, angle of the DrailOUS SRC, which IS lodged
jaw ; d, dental capsules. in the tl^knegg Of the jaw.

bone ; as seen in the accompanying figure, which 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 (a, fig. 90), is composed of two membranes,
abundantly furnished with blood-vessels ; and it encloses in
its interior a little bud-like protuberance, b, in which ramify
a great number of nervous filaments and minute vessels, c.
The matter composing this little body, which is termed the
pulp, is gradually converted into the dentine (§ 54) of the tooth,
d a which in Man constitutes nearly its whole

structure ; this conversion takes place first
at its highest points, d, d. The crown or
upper portion of the tooth receives a
covering 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
that still remains, occupying what is termed the cavity of
the tooth, which is frequently laid open by decay of its
external wall. The fang of the tooth, which is the part
last formed, receives an envelope of cementum (§ 54), which
invests it up to the part at which the enamel begins. As the

Fig. 90.— DENTAL
CAPSULE.

DEVELOPMENT OF TEETH. 167

root of the tooth is developed, the crown is gradually pushed
upwards, so as to press against the upper 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 explained (§ 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 hardened,
and closes round its root, forming 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 power 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 is pushed upwards. This is the case with the Elephant's
tusks ; and also with the large teeth that occupy the front of
the jaw in Rabbits, Squirrels, 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.— JAW AND TEETH OF RABBIT. 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 become 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 Herbivorous animals, which
have to grind or triturate their food 1 ., 'tween 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 Rodents, or gnawing animals furnished with

MOVEMENTS OF LOWER JAW.

169

two large front teeth, the 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 substances 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 which 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 (a, fig. 92), about
an inch in front of the joint.
As the distance from the ful-
crum of the point a, at which the
power is applied, is thus much
less than that of the front oi
the jaw b, where chiefly the
resistance is encountered, the
power 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 with 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
incisor teeth (fig. 93). Others have more of a conical form,.

Fig. 92.- HUMAN SKULL.

170

DIFFERENT KINDS OF TEETH.

and in many animals (especially those of carnivorous habits)
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 canine teeth.
The teeth of the third kind have large irregular flattened
surfaces, and are adapted to bruise and grind the food ; these
are called molar (or mill-like) teeth. The manner in which
these different teeth are implanted in the jaw, varies with the
form 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 draw 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 ; especially 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, according 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 animal flesh, the molar teeth are so compressed as to form

DIFFERENT KINDS OF TEETH.

171

cutting edges, which work against each other like 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. 05.
Fig. 94. — TEETH OF CARNIVOROUS ANIMAL 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. 96.
TEETH OF HERBIVOROUS AMMAL. 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 tipright 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 grinding action which the
tooth is destined to perform. The mode in which these
plates are disposed, affords a most characteristic distinction
between the two species of Elephant at present existing,

172

DIFFERENT KINDS OF TEETH.

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

Fig. 98.— MOLAR TOOTH OF MASTODON.

1. 2. 3.

Fig. 99. — MOLAR TEETH OF ELEPHANTS.

1, African Elephant; 2, Indian Elephant ;

3, Mammoth.

&c., the front surface only is covered with enamel ; and as
this is worn away more slowly than the ivory, it stands up 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
of the seeds and fruits on which
the herbivorous animals are
supported, must be ground
down. The incisors and canines
Fig. IOO.-SKULL OF BOAR. are ^fly employed among

Carnivorous animals for the purpose of seizing their living
prey, and are never deficient in them ; but they are less re-
quired in Herbivorous animals ; and either or both kinds are
not unfrequently deficient. Sometimes, however, they are not

SUCCESSION OP TEETH. — WHALEBONE. 173

only present in the latter, but are largely developed, serving as
weapons of attack and defence ; as in the Boar (rig. 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
their appearance above the gum, until the time approaches
when the young animal has to prepare its own food, instead
of simply receiving that which 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, bicuspid 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 with that at
which 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 Whale, 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 whalebone, 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-EATEB, ETC.

draws water in enormous quantities, whenever it is in want
of food ; and in this manner it strains out, as it were, the
minute gelatinous animals upon which it lives, from the water
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.— WHALEBONE.

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
Balcena, or Greenland Whale, is it long enough to make it
worth separating ; all the other species having 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
Ruminant quadrupeds (oxen, sheep, &c.), though they never
show themselves above the gum.

186. The Ant-eaters, also, are destitute of teeth, and usually
obtain 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

Fig. 103. — SKULL OF THE ANT-EATER. „ 5? . , °, . ,

of 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 KEPTILES AND FISHES.

175

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 famished 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 principally
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 Iguanodon, an animal of this
tribe, attained a gigantic 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

Fig. 104.— HEAD OF GAVIAL. (Crocodile
of the Ganges.)

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, bow-

176 MASTICATING INSTRUMENTS OF INVERTEBRATA.

ever, the tooth seems like a continuation of the bone 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
palate, the surface of which is beset with innumerable tooth-
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 and species 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
a framework which is worked

-

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
saliva with the food, during its reduction between the teeth,
— an act which is termed insalivation. The saliva is separated
from the blood, by glands which are situated in the neigh-

SECRETION OF SALIVA, AND ITS USES. 177

bourhood 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 principally 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 stimulus 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 flow; 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 men, in part at least, to the

N

178

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 feeling 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

Salivary glands
Os hyoides

Larynx
Thyroid gland

(Esophagus

mm ma

Trachea
Fig. 107. — PERPENDICULAR SECTION OF THE MOUTH AXD THROAT.

mastication, in such a manner as to prevent any of the food
from passing backwards. This partition, which does not exist

DEGLUTITION OB SWALLOWING. 179

in Birds and other animals that do not masticate their food,
hangs from the arch and sides of the palate, 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, being carried backwards by the action of its muscles,
presses against the partition just mentioned, and causes it to
open. 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 pharynx, 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 entering it. But it sometimes
happens that, if the breath be drawn-in at the moment 01
swallowing, a small particle of the food, or a drop 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 almost 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 swallowing, without
carrying something backwards upon the tongue ; and it is the
contact of this something, even if it be only a little saliva,
with the membrane lining the pharynx, that produces the
muscular movement in question.

195. This action is one of the kind now denominated reflex
(§ 430). It is produced through the nervous system ; for if
the 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
the 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
effected 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
paralyzed 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 great

DEGLUTITION DIGESTIVE APPARATUS.

181

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

Digestive Apparatus.

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

Large Intestine

Spleen

.----Colon

- Small Intestine
- - Colon

Small Intestine Rectum

Fig. 108.— DIGESTIVE APPARATUS op MAH.

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

182 FORM OF THE STOMACH.

abdomen (fig. 108). The form of this stomach varies much,
according to the nature of the aliment to be digested. Where
the 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 with 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 the left side (the right side of the figure as we
look at it), is moderately developed; showing, as we might
expect from the form of his teeth, as well 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 ^,^_^^^^mi

Pylorus 4thStom. 2d Stom. 1st Stom.
Fig. 109. — STOMACHS OF THE SHEEP.

198. The most complex 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 than four distinct
cavities, through all of which 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 Kuminant animals is not
chewed by them before it is first swallowed. In their wild
state, they are peculiarly exposed to the attacks of their car-
nivorous enemies, when 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, which 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

Heed

Intestine Honeycomb Paunch

Fig. 110. — SECTION OF THE STOMACHS OF THE SHEEP.

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 oesophagus, 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

184 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 supply 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 which
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
first 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-
phagus. 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 peculiar valvular groove at the bottom of the cesophagus,
into the third stomach, commonly termed the manyplies,
from the peculiar 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 cheese to cause the milk to coagulate or curdle.
In the 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 undivided
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 In the digestive apparatus of Birds, 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 some

CEsophagus

Ventriculus\
Succenturiatusj

Gizzard

Pancreas

Duodenum —

Coeca

Large Intestine

Ureter

Oviduct

Cloaca
Anus

Fig. 111. — DIGESTIVE APPARATUS OF FOWL.

other means of reducing it. This 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
\&g. Ill); in this it is retained, and exposed to the action

186 TRITURATING ACTION OF GIZZARD.

of fluid secreted by its walls, just as it is in the paunch 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
fish-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 be 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 Gasteropods. 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

REDUCING APPARATUS OF INSECTS, ETC.

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 tme digestive stomach,
d; which is surrounded by
an immense number of little
follicles or bags, by which
the secretion of the gastric
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 W keel- Animalcules,
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 alirnen-

Fig. 112.— DIGESTIVE APPARATUS OF
BEETLE.

188

DIGESTIVE APPAKATUS — 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 prepared 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 Rhizo-
poda (§ 129).

Gastric Digestion : — Chymification.

204. The food which has been re-
duced 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-
as seen in a vertical section rated from the blood by a vast number
*£Cd£Se^maJT£d ^ little bags or follicles (fig. 113), im-
twenty diameters at B. bedded in the walls of the stomach.
When the cavity is empty, this fluid is secreted in very small
quantities ; but, like the salivary secretion, it is poured out

Fig. 113.
GASTRIC FOLLICLES,

SENSE OF HUNGER — SECRETION OP 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 Veddahs, 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 experiment, 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 PROPERTIES OF GASTRIC JUICE.

alimentary mass into contact with its sides, so that the whole
is after a time equally 'exposed to the influence of the gastric
secretion. If this movement were not to take place, only the
outside of the mass would be digested, and the central portion
would remain but little 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 ; 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 follicles 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,
but 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 albumen, 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 respects as perfect 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 albuminous
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 saccharine 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 effecting all
that can be done by that which is secreted in the living
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 egg, &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 office to produce some
change in the albuminous substances, by which they are more
readily dissolved. The recent inquiries of Liebig and other

192 GASTRIC DIGESTION : CHYMIFICATION.

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

210. It is a fact of great 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. Now 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
lies 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
throughout. 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
pylorus, there is a kind of valve, which permits the chyme
to pass as fast 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 first, 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 j for much of the matter which escapes from it
in the chyme, is destined to undergo a further change 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 times the
length of the body, while in the Serpent it runs almost
straight from one extremity to the other ; and in animals
which live 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 still 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 itsel£
The peculiar matter which forms the chief solid constituent of
bile, is essentially a soap formed by the union of two resinoid
acids, with soda as a base •(§ 364). The composition of ttie

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-
taining 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 which they 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 partly 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 ccecum is used in Anatomy to denote a tube closed at one
extremity.

PERISTALTIC MOVEMENT DEFECATION. 195

function ; 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 coeca 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 peristaltic motion of the bowels. The fibres
of this muscular coat are chiefly arranged in a ring-like
manner around the tube j 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, xn.) j 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 driven 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 Rabbit 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. But 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 Absorbents; of which those forming one division,

Thoracic Mesenteric
Aorta Duct Glands

Origins of
Lacteal Vessels

— Intestine

i .' /

Lymphatic Mesentery
Vessels

Fig. 114.— CHYLE-VESSELS.

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

ABSORPTION BY LACTEALS AND BLOOD-VESSELS. 197

epithelium-cells, which clothe the extremity of each villus
(fig. 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. Near 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— which 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 which 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 applied 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 effect 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 lymphatics, are
delivered, a tube passes upwards in front of the spine (fig.
114) ; and this tube, called the Thoracic Duct, conveys these
nutritious fluids to the point where they are to be delivered
into the current of blood. This delivery takes place at the
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 3J parts 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 dotting, 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 fibrin 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).

225. 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
Cellular 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 whose 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 difference 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
way limited ; since every part seems to have the power of re-
ceiving from without, and of assimilating to its own substance,
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 supplies
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 the 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 Yerte-
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 Yertebrata, 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 Yertebrata
(§ 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
different states. When 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 Yeins, 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 exposure 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. When 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 which
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 globules 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
exhibiting a slight depression towards the centre (fig. 115).

204 BLOOD-DISCS OF MAN AND MAMMALS.

No nucleus can be distinguished in them, 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

T>

Fig. 115. — RED CORPUSCLES OF HUMAN BLOOD.

Seen separately at A, a a showing the front view, b the profile or edge view, and * a
three-quarter view; at B united with each other so as to form columns likepilis
of money; at c in a state of alteration such as exposure to air will produca;
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, Reptiles, 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. This bulging is

Fig. 116.— BLOOD CORPUSCLES OF PIGEON.

At A are seen the red corpuscles a, b, and the colourless, or lymph globules c, c; at
B, a red corpuscle treated with acetic acid ; and at c, the same treated with water,
so as to render the nucleus more distinct.

evidently occasioned by the presence of a nucleus which is
more solid 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 1-1 7 00th
to 1 -2400th of an inch ; and the short diameter from about

Fig. 117. — BLOOD CORPUSCLES OF FROO.

At A are seen the red corpuscles a, b, and the colourless corpuscle c ; at B, a red
corpuscle treated with acetic acid.

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

Fig. 118. — BLOOD CORPUSCLES OF PROTEUS.

«, bt 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. 1 1 9), 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. ii9.— BLOOD CORPUSCLES OP blood-discs of the curious Lcpido-
RoACH- siren (fig. 41) is strongly indicative

a, a, b, red corpuscles ; c, colour- nf xi^ PprvHlinTi flffim'tiPQ nf fhnt
less corpuscle ; d, red corpuscle Ol tjie -ttepttUan affinities 01 tnat
treated with water. Species.

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
evidently 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 Vertebrata 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. But, if we 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 will 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 through 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 through a very small aperture,
recovering its usual form immediately afterwards.

232. The Eed Corpuscles differ considerably in chemical
composition from the liquid in which 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 globulin
and hcematin. The former is a colourless substance, nearly
allied to albumen 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 hsematin, 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 different 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 OP RED CORPUSCLES.

the Red 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 Red 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 of Mammals, and far more abundant
in these last than in Reptiles 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 ajbout half the mass of the circulating
blood ; but they contain as much as three-fourths of its solid
matter, the proportion of dry 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
are 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-
tinually 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 ; but 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 colourless 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 1 -3000th of an inch. In the
blood of Man 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 globuler (c, figs. 116-119) bear to
those of the chyle and lymph, i? very striking. Similar colour-
less particles exist, to a variaole amount, in the nutritive fluid
of Invertebrated animals ; so that in this, as in some other
respects, that fluid 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 Red 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 which 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. Now 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

210 USES OF EED CORPUSCLES LIQUOR SANGUINIS.

carbonic acid which, 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
$he 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.
But if the blood be withdrawn from the living body, it soon
undergoes 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
particles separate from one another, — the latter gradually
subsiding, 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 buffy coat, being composed almost exclu-

LIQUOR SANGUINIS — COAGULATION. 211

sively of the fibrous network, is very firm in its texture,
being sometimes 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 with 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 manner 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 wii-n
fatty matter, and other organic substances ; with the addition
of saline matter, of which a considerable proportion 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
solid tissues, whilst others are the products of the decay of
the tissues, which are being progressively withdrawn and
eliminated by the excretory organs.

239. The influence of the Blood as a whole upon the
animal 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
effect of this remarkable operation, which is called the trans-
fusion of blood ; for if, instead of blood freshly obtained from
another living animal, we inject serum without these particles,
the effect is but little greater than if so much water were
introduced, and the animal dies of the haemorrhage. By this
operation, practised on the Human subject, many valuable
lives 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 that 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. 213

240. The following appear to be the 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 liquid 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 j 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 them

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 amount 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
Inspiration, 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 serum 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, \hsfatty 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; 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 th.e blood is an essential condition of health ;
and we find it provided-for in the marvellous power which
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 drawn off 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 lungs 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 which 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 ; 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 this requirement is
fulfilled 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 which 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 Heart 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 the 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 Arterial system, as it
issues from the 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 others
more numerous but smaller, and these again into twigs still
more numerous 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
I

Fig. 120.— DISTRIBUTION OF THE SMALLER BLOOD-VESSELS IN THE MEMBRANE

BETWEEN TWO OF THE TOES OF THE HIND FOOT OF THE COMMON FROG; a O,

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 trunks, 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
degree ; — 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 numerous 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 1O1 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. Fof,
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 10-1,
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. There 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 OF ARTERIES AND VEINS. 219

the heart, and must be strong enough 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 trunks 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 lie
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.

rnious 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, which 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

J . A, . . *, ,f , . ,

that 01 tllC DlOOd-dlSCS WIUCI!

hovp fn r»qqq tVirmio-Ti fhpTn •
naVG t0 PaSS mrougn

in Man they are Commonly
« i_ j_ i O^AA.L'U t.

irom about 1 - 2oOOth 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
times ; 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.

f The circulation of 'the blood in the Frog's foot, the tail of the
Tadpole, the gills 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 Revelations,"
Chap, xviii.

Fig. 121.— PORTION OF THK MEMBRANE

BETWEEN THE TOES Of THE HIND

FOOT OF THE FROG, more highly
magnified than in fig. 120, showing the
network of Capillaries that traverses it ;
a, small venous trunk; b b, branches
communicating with the capillaries ;
c intervening tissue covered with epi-
thehum cells.

RESPIRATORY CIRCULATION. 221

through the capillaries of a warm-blooded animal, has been
determined by microscopic examination to be about 3-100ths
of an inch per second. From the comparison of this rate with
that of the flow of blood through the larger arteries, which 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 communicate
with each other at their opposite extremities ; their large
trunks through the medium of the heart j and their ultimate
subdivisions through the capillaries. Hence we may consider
this 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
requisite 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
effect 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 capillary
system of the respiratory organ ; a set of capillaries, in which
the aeration of the blood takes place ; and a system of veins

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 removing 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 own 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 lungs.

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.

256. 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, which 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
forwards and to the left ; during the contraction of the heart,
it is tilted forwards, and strikes against the walls 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

ac vj

vl

Fig. 122. — LUNGS, HEART, AND PRINCIPAL VESSELS op MAN.

a r, right auricle; IT, 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
outer 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,
which, in Birds and Mammalia, as in Man, is divided into
four distinct chambers. This division is effected 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 thickness 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 STRUCTUEE OP 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
with its corresponding ventricle, by an aperture in the

Superior Pulm. Pulmonary

vena cava art. Aorta artery

—7 Pulmonary veins
Pulmonary veins

Right auricle (^^BT /HHW- Left auricle

Tricuspid valves - ~ SftlS ' Mitral valve

Inferior vena cava — ; ,^^

/ x'HHffiSlw / "^*-«.«.

^ ^^ Left ventricle

Right ventricle

Partition Aorta
Fig. 123. — IDEAL SECTION op THE HUMAN HEART.

transverse partition, which is guarded by a valve. The walls
of the ventricles are much thicker 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 that 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 ventricles
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 amount 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 neck ; then forms what is
termed the arch, a sudden curve, which gives it a downward

ARTERIAL SYSTEM OF MAX.

225

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

Anterior
tibial artery

Art. of foot

Peroneal

artery

Fig. 124.— ARTERIAL SYSTEM OF MAX

Q

22 G AETERIAL SYSTEM OF MAN.

trunk, 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 supply the head and
upper extremities. These are, the two carotids, which ascend
on either side of the neck; and the two stibclavian, which
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
common 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 Whale 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 Euminating
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 themselves 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 cceliac, from
which the stomach, liver, and spleen are supplied ; the renal,
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 into the
thighs, where they become the femoral arteries ; and these
again subdivide into branches for the supply of the leg.

260. 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 iliac. But in Birds, the function of locomotion is almost
entirely performed by the wings ; and their powerful muscles,

ARTERIAL SYSTEM OP BIRD.

227

which, constitute the mass of flesh lying on the breast, are
supplied with 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

Aorta

Sacral artery Cloaca

Fig. 125. — AKTERIAI, 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

228 DISTRIBUTION OF AKTERIES.

branch, is neither 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 Bird ; 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 difference 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 everywhere prevalent through-
out nature ; manifesting itself in the close conformity of a
great number of apparently-different 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 which are
given-off 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 OP ARTERIES ANEURISM. 229

circulation through them, the Surgeon is often able to save a
limb, or even a life, which would otherwise be sacrificed.
Arteries are liable to a peculiar disease, termed aneurism,
which consists in a thinning-away, or rupture, of the tough
fibrous coat, and a great dilatation of the other coats, so that
a pulsating tumour is formed. This change takes place most
frequently at the bend of the thigh, the ham, the shoulder,
and the elbow ; where the artery, in the working of these
joints, often has to undergo sudden twists. The result of the
disease Avould be generally fatal, in consequence of the gradual
thinning-away of the walls of the tumour, which at last
bursts, allowing the blood to escape from the arterial trunk
with such rapidity as, if unchecked, to cause almost instanta-
neous death. In order to prevent this, the surgeon ties the
artery at some little distance above the aneurism, — that is, he
puts a thread round it, which is drawn so tight as to prevent
the passage of any blood to the aneurism. The circulation in
the lower part of the limb is at first retarded ; its temperature
falls ; and it becomes more or less insensible. But after the
lapse of a few hours, the circulation becomes quite vigorous,
the pulsations strong, the temperature rises, and the numb-
ness passes off; and as the main trunk still continues com-
pletely obstructed, this can only have been brought about by
the flow of blood through the anastomoses, which must in
that short period have undergone considerable enlargement.
Examination of the vessels after death shows that this has
been actually the case. Even the aorta has thus been tied in
dogs, without causing death ; the anastomoses of the branches
given-off from its upper part, with those proceeding from the
lower, being sufficient to maintain the circulation in the latter,
when the current through the main trunk is obstructed.

264. A very complex series of anastomoses, forming a com-
plete network of large tubes, is found in several situations,
where it seems desirable that the flow of blood to a particular
organ should be retarded, whilst a large amount is to be
allowed to pass through. Thus in animals which keep their
heads near the ground for some time together, as in grazing,
the arteries which supply the brain suddenly divide, on their
entrance within the skull, into a great number of branches,
by the anastomoses of which a complex network is formed ;
and from this network, by the reunion of its small vessels,

230 PECULIARITIES OF DISTRIBUTION OF ARTERIES.

originate the trunks which supply the brain in the usual
manner. The object of this apparatus appears to be, to pre-
vent the influence of gravitation from causing a too great rush
of blood towards the brain, when the head is in a depending
position ; for the rapidity of its flow will be checked, as soon
as it enters the network, and is distributed through its
numerous canals. A similar conformation is found in the
blood-vessels of the limbs of the Sloth, and of some other
animals which resemble that animal in the sluggishness of
their movements ; and its object is probably to prevent the
muscles from receiving too rapid a supply of blood, which
would give them what (for these animals) would be an undue
energy of action ; whilst, by the very same delay, their power
of acting is greatly prolonged, — as we find it to be in Eeptiles,
whose circulation is languid (§ 284).

265. In the Whale tribe, and some other diving animals
that breathe air, we find a curious distribution of the blood-
vessels, which has reference to their peculiar habits. The
intercostal arteries (which are sent-off from the aorta to the
spaces between the ribs on each side) are enormously dilated,
and are twisted into thousands of convolutions, which are
bound together into a mass by elastic tissue. This mass,
which is of considerable bulk, lies at the back of the chest,
along both sides of the vertebral column ; and it serves as a
reservoir, in which a great quantity of arterial blood may be
retained. The veins also have very large dilatations, which
are capable of being distended, so as to hold a considerable
amount of venous blood ; and thus, while the animal is pre-
vented from breathing by its submersion in the water, the
circulation through the capillaries of the system is sustained,
by the passage of the blood stored up (as it were) in the
arterial system, into the venous reservoirs. If this provision
did not exist, the whole circulation would come to a stand,
in consequence of the obstruction it meets with in the lungs,
when the breathing is stopped.

266. With regard to the Venous system, there is little to be
added to what has been already stated (§§ 248-250) as to its
general character and distribution. The large proportion which
its capacity bears to that of the arterial system, is shown by
the fact, that every main artery is accompanied by a vein (fre-
quently by two) considerably larger than itself ; and that the

DISTRIBUTION OF VEINS PORTAL SYSTEM. 231

superficial veins, which lie just beneath, the skin, are capable
of conveying at least as much more. The veins of the body
in general unite in two large trunks, the superior and inferior
vena cava ; which meet as they enter the right auricle of the
heart (fig. 123). The superior vena cava is formed by the
union of the veins which return the blood from the neck (the
jugulars) with those which convey it from the arms (the
subclavians), as shown in fig. 122; and the inferior cava
(v c, fig. 122) receives the blood from the trunk, the organs
contained in the abdomen, and the lower extremities.

267. There is, however, an important peculiarity in the
distribution of the veins of the Intestines, which should not
pass unnoticed. Instead of delivering their blood at once into
the inferior vena cava, these veins unite into a trunk, called
the Vena Portce (fig. 134), which enters the liver and subdivides
into branches, whence a capillary network proceeds that per-
meates the whole of its mass. It is from the venous blood,
as it traverses this network, that the secretion of bile is
formed ; and the blood which is brought by the hepatic artery
serves chiefly to nourish the liver, — no bile being formed from
it, until it has become venous. The blood is carried-off from
this double set of capillaries by the hepatic vein, which conveys
it into the inferior vena cava. In Fishes, not only the blood
of the intestines, but that of the tail and posterior part of the
body, enters this "portal" system, which is distributed to
their kidneys as well as to their liver. Thus all the blood
which flows through the portal system, has to go through two
sets of capillaries, between each period of its leaving the heart
by the aorta, and its return to it by the vena cava.

268. We have yet to notice the lesser circulation, which
is confined to the Lungs only. The venous blood which is
returned to the heart by the vense cavae, enters the right
auricle, and thence passes into the right ventricle. By the
contraction of this last cavity, it is expelled through the. pul-
monary artery (fig. 123), which soon divides into two main
trunks that proceed to the right and left lungs respectively.
The right trunk again subdivides into three principal branches,
which are distributed to the three lobes or divisions of the
right lung ; whilst the left divides into two branches, which
are in like manner distributed to the two lobes of the left
lung. The capillaries, into which these branches ultimately

232 LESSER CIRCULATION — FORCES THAT MOVE THE BLOOD.

subdivide, are distributed upon the walls of the air-cells (fig.
162), and the character of the blood is in them converted, by
exposure to the air, from the dark venous to the bright arte-
rial. From this capillary network the pulmonary veins arise ;
and the branches of these unite into trunks, of which two
proceed from each lung, to empty themselves into the left
auricle (fig. 123). This auricle delivers the blood, now arte-
rialized or aerated (§ 253), into the left ventricle, whence the
aorta arises ; and by the contraction of this cavity, it is
delivered through that vessel to the system at large. — It
will be observed that the vessel which proceeds from the
heart to the lungs is called the pulmonary artery, although it
carries dark or venous blood. This is because it conveys the
blood from the heart towards the capillaries. And, for a
similar reason, the vessels which return the blood from the
capillaries to the heart are termed pulmonary veins, although
they carry red or arterial blood.

Forces that move the Blood.

269. The mechanical action, by which the blood is caused
to circulate in the vessels, is easily comprehended. The cavi-
ties of the heart, as already explained (§ 245), contract and
dilate alternately, by the alternate shortening and relaxation
of the muscular fibres that form their walls (Chap, xn.) ; and
the force of their contraction is sufficient to propel the blood
through the vessels which proceed from them. The two
ventricles contract at the same moment ; the auricles contract
during the relaxation of the ventricles, and relax whilst the
ventricles are contracting. The series of movements is there-
fore as follows : — The auricles being full of the blood which
they have received from the venae cavse and pulmonary veins,
discharge it by their contraction into the ventricles, which
have just before emptied themselves into the aorta and pul-
monary artery, and which now dilate to receive it. When
filled by the contraction of the auricles, the ventricles contract
in their turn, so as to propel their blood into the great vessels
proceeding from them ; and whilst they are doing this, the
auricles again dilate to receive the blood from the venous
system, after which the whole process goes-on as before. It
is when the ventricles contract, that we feel the beat of the
heart, which is caused by the striking of its lower extremity

MECHANISM OF THE HEART.

233

Fig. 126.

against the walls of the chest ; and it is by the same action
that the pulse in the arteries is produced (§ 276V

270. The combined actions
of each auricle and its ventri-
cle, may be illustrated by an
apparatus like that repre-
sented in fig. 126. It con-
sists of two pumps, a and
b, of which the pistons move
np and down alternately ;
and these are connected with
a pipe c /, in which there are
two valves d and e, opening
in the direction of the arrow.
The portion c of the pipe
represents the venous trunk
by which the blood enters

the heart ; the pump a represents the auricle, and the raising
of its piston enables the fluid to enter and fill it. When its
piston is lowered, its fluid is forced through the valve d into
the pump b (which represents the ventricle), whose piston
rises at the same time to receive it ; and when this piston is
lowered in its turn, the fluid (being prevented from returning
into a by the closure of the valve d) is propelled through the
valve e into the pipe f, which may represent an arterial tube ;
whilst at the same time a fresh supply of blood is received
into the pump a by the raising of its piston.

271. The number of contractions of the heart ordinarily
taking place in an adult man, is from 60 to 70 per minute.
It is usually rather greater in women ; and in children it is
far higher, being from 130 to 140 in the new-born infant, and
gradually diminishing during the period of infancy and child-
hood. It is rather greater in the standing than in the sitting
posture, and in sitting than in lying down : it is increased by
exercise, especially by ascending a steep hill or going upstairs,
and also by any mental emotion. It is important to remember
these facts, in reference to the management of those who are
suffering under diseases of the heart or of the lungs, which
prevent the ready passage of the blood through these organs ;
for if more blood be brought to the heart by the great veins,
than it can propel through the pulmonary arteries, a feeling of

234 VALVES OF THE HEAET.

very great distress is experienced; and there may be even
danger of rupture of the heart or large vessels, or of sudden
cessation of the heart's action, causing instant death. Such
persons ought, therefore, carefully to refrain from any violent
muscular movement, and also to avoid giving way to strong
mental emotions. — In syncope or fainting, the heart's action is
so weakened as to be scarcely perceptible, though it does not
entirely cease ; and this state may be brought on by several
causes which make a strong impression on the nervous system,
such as violent mental emotion (whether joy, or grief, or terror),
sudden loss of blood, and the like.

272. The blood which has been received by each ventricle
from its auricle, is prevented from being driven back into the
latter, on the contraction of the former, by a valve that guards
the aperture through which it entered. This valve consists of
a membranous fold, surrounding the borders of the aperture,
and so connected with the neighbouring parts, as to yield when
the blood passes from the auricle into the ventricle, but to
be tightened so as completely to close the aperture when
the blood presses in the contrary direction. The manner
in which these valves act will be seen from fig. 127, which
is a section of the right auricle with its ventricle. The
auricle, a, 'receives its blood from the two venae cavse, e, e'}
and transmits it into the ventricle, b, by the orifice, c. On
either side of this orifice are seen the membranous folds,
which are kept in their places by the tendinous cords, d.
a Now when the blood is passing from

a to 5, these folds yield to the current ;
L. / hut when the cavity b is filled and begins

to contract, the blood presses against
ff their under sides, so as to make them

close against each other, as far as they
b are permitted to do by the tendinous

cords. In this manner the aperture is

completely shut, and no blood can flow
Fig. 127.— SECTION OP ONE back. A valve of this kind exists on
HEART> each side of the heart; but there is
a slight difference between the forms of the two, whence
they have received different names. That on the right side
has three pointed divisions, to which the tendinous cords
are attached, and it is hence called the tricuspid valve;

SEMILUNAB VALVES. 235

whilst that on the left side has only two, so as to bear some
resemblance to a bishop's mitre, whence it is called the mitral
valve.

273. The aorta and pulmonary artery are in like manner
furnished with valves, which prevent the blood that has been
forced into them by the contraction of the ventricles, from
returning into those cavities when they begin to dilate again.
These valves, however, are formed upon a different plan, and
more resemble those of the veins, which will be presently
described. They consist of three little pocket-shaped folds of
the lining membrane of these arteries (similar to those at b b,
fig. 128), which are pressed flat against the walls of those
tubes when the blood is forced into them ; but as soon as they
are filled, and the ventricles begin to dilate, so that the blood
has a tendency to return, it presses upon the upper side of
these pockets, and fills them out against one another, in such
a manner as completely to close the entrance into the ven-
tricle. The three little pocket-shaped folds, however, would
not close the centre of the aperture, were it not that each of
them has a little projection from its most prominent part,
which meets with those of the others, and effects the requi-
site end. The situation of these valves (which are termed
semilunar from their half-moon shape) is seen at g, fig. 127,
/ being the pulmonary artery.

274. The amount of blood sent-out from either ventricle
at each contraction, in a middle-sized man, seldom exceeds
3 ounces ; but the whole quantity of blood contained in the
body is not less than 181bs. : hence, it would require 96
contractions of the heart to propel the whole of this blood
through the body, and these (at the ordinary rapidity) would
occupy about Ij- minute. It has been calculated, from recent
experiments, that the usual force of the heart in man would
sustain a column of blood about 7 feet 2 inches high, the
weight of which would be about 4 Ibs. 3 oz. on every square
inch. The backward pressure of this column upon the walls
of the heart, or in other words, the force which they have to
overcome in propelling the blood into the aorta, is estimated
at about 13 Ibs.

275. From the mode in which the blood is forced into the
arterial system by a series of interrupted impulses, it might
be supposed that its course would be a succession of distinct

236 EQUALIZING ACTION OP ARTERIES I PULSE.

jets ; but this is prevented, so that the current is reduced to
an equable stream by the time it reaches the capillaries,
through the elasticity of the walls of the arteries. In order
to comprehend how this acts, we may suppose a forcing-pump
(§ 270) to propel its fluid, not into a hard unyielding tube of
iron or lead, but into an elastic tube of india-rubber. The
effect of each stroke of the pump will be partly expended in
distending the tube, so as to make it contain an additional
quantity of water ; and the suddenness of the jet at its oppo-
site extremity will be diminished. In the interval of the
stroke, the elasticity of the wall of the tube will cause it to con-
tract again, and to force-out the added portion of its contents ;
this it will not have completed by the time that the action of
the pump is renewed ; and in this manner, instead of an inter-
rupted jet at the mouth of the tube, we shall have a continuous
flow, which, if the tube be long enough, will become quite
equable.* It is precisely in this manner that the elasticity of
the arteries influences the flow of blood through them, by
converting the interrupted impulses which the heart com-
municates to it, into a continued force of movement. In the
large arteries, these impulses are very evident ; in the smaller
branches they are less so, but they still manifest themselves
by the jerking in the stream of blood proceeding from a
wound in one of these vessels ; whilst in the capillaries, the
influence of the heart's interrupted impulses cannot usually
be seen at all, the streams that pass through them being
perfectly equable.

276. The phenomenon which we call the pulse, is nothing
else than the change in the condition of the artery occasioned
by the increased pressure of the fluid upon its walls, at the
moment when the heart's contraction forces an additional
quantity of blood into the arterial system. By the frequency
and force of this change, we can judge of the power with
which the blood is being propelled. But the pulse can only
be well distinguished, when we can compress the artery
against some resisting body, so that there is a partial obstruc-
tion to the flow of blood through it, which causes the disten-
sion to be more powerful ; the most convenient artery for this

* The same effect is obtained in an ordinary fire- or garden-engine,
by the interposition of an air-vessel, in which the elasticity of com-
pressed air is substituted for that of the wall of the pipe.

PULSE : — WOUNDS OF ARTERIES. 237

purpose is the radial artery (fig. 124) at the wrist ; but the
carotid artery in the neck, and the temporal artery in the
temple, may be felt, when it is desired to know the force of
the circulation in the head; as may the arteries supplying
other parts, when we wish to gain information respecting
the organs they supply. An increased action in the organ,
whether this be due to inflammation, or to a state of unusual
activity of its function, causes an increase of size in the artery
which supplies it ; and thus the pulsation may be unusually
strong in a particular trunk, when the heart's action and the
general circulation are not in a state of excitement. For
instance, a whitlow on the thumb will occasion its artery to
beat almost as powerfully as the radial artery usually does ;
and excessive activity of the mind, prolonged for some hours,
greatly increases the force of the pulsations in the carotid
arteries, from which the brain is chiefly supplied.

277. When an artery is wounded, there is often great
difficulty in controlling the flow of blood ; for pressure can
seldom be effectually applied in the situation of the wound ;
and the surgeon is generally obliged to tie the vessel above
the orifice. As a temporary expedient, the loss of blood may
be prevented by making firm pressure upon the artery above
the wounded part, that is, nearer the heart ; and many valu-
able lives have been saved by the exercise of presence of
mind, guided by a little knowledge. The best means of
keeping-up the requisite pressure, until the proper instrument
(the tourniquet) can be applied, is to lay over the artery (the
place of which may be found by its pulsation) a hard pad,
made by tightly rolling or folding a piece of cloth ; this pad
and the limb are then to be encircled by a bandage, by which
the pressure is maintained ; and this bandage may be tightened
to any required degree, by twisting it with a ruler or a piece
of stick. Thus a constant pressure may be exercised upon
the artery, which will be generally sufficient to control the
bleeding from it. But there are, unfortunately, many cases
in which pressure of this kind cannot be applied ; as for
instance when the femoral artery is wounded high up in the
thigh, or the carotid artery in the neck. And nothing else
can then be done, but to compress the artery with the thumb,
or with some round hard substance (such as the handle of an
awl), until proper assistance can be obtained.

238 FLOW OF BLOOD THROUGH THE VEINS.

278. The impulse of the heart, and the elasticity of the
arteries, which together propel the blood through the capillary
system, continue to act upon it after it is received into the
veins ; and are in fact the chief causes of its movement in
them. If we interrupt the current of blood through an artery
by making pressure upon it, and open the corresponding vein,
the fluid will continue to flow from the latter, so long as the •
artery contains blood enough to be forced into the vein by its
own contraction ; but as soon as it is emptied, the flow from
the orifice in the vein will cease, even though the vein itself
remains nearly full. If the pressure be then taken off the
artery, there is an immediate renewal of the stream from the
vein, which may be again checked by pressure on the artery.
In the ordinary operation of bleeding, we cause the superficial
veins of the arm to be distended, by tying a bandage round
them above the point at which we would make the incision ;
and when an aperture is made, the blood spouts forth freely,
being prevented by the bandage from returning to the heart.
But if the bandage be too tight, so that the artery also is
compressed, the blood will not flow freely from the vein ; and
the loosening of the bandage will then produce the desired
effect. When a sufficient quantity of blood has been with-
drawn, the bandage is removed ; and the return-flow through
the veins being now unobstructed, the stream from the orifice
immediately diminishes so as to be very easily checked by
pressure upon it, or may even cease altogether.

a 279. The veins contain a great

number of valves, which are
formed, like the semilunar valves
of the aorta (§ 273), by 'a
doubling of their lining mem-
brane. Their situation may be
known by the little dilatations
which the veins exhibit at the
points where they occur ; and
which are yery obvious in the
arm of a person not too fat,

•„ -__•£__ ;.j»w'j|^ M ' when it is encircled by a bandage
that causes distension of the

Fig. 128,-VEiK LAID OPEK, TO superficial veins. _ The structure
SHOW ITS VALVES. oi these valves is seen at o 0,

FLOW OP BLOOD THROUGH THE VEINS. 239

fig. 128; they consist of pocket-like folds of the lining mem-
brane, which allow the blood free passage as it flows towards
the heart, but check its reflux into the arteries. Hence it
follows, that every time pressure is made upon the veins, it
will force towards the heart a portion of the blood they con-
tain, since this cannot be driven in a contrary direction. IsTow,
from the manner in which the veins are distributed, some of
them must be compressed by almost every muscular move-
ment; these will become refilled as soon as the muscles relax;
and they will be again pressed-on, when the movement is
repeated. Hence a succession of muscular movements will act
the part of a diffused heart, over the whole of the venous
system, and will very much aid the flow of blood through its
tubes. It is partly in this manner, that exercise increases the
rapidity of the circulation. If the blood is brought to the heart
by the great veins more rapidly than usual, the heart -must go
through its operations more rapidly, in order to dispose of the
fluid ; and if these actions be impeded, great danger of their
entire cessation may exist. Hence the importance of bodily
tranquillity to those affected with diseases of the heart or
lungs (§ 271).

280. Besides the aid thus afforded to the venous circulation,
it is probable that there is another cause of the motion of the
blood in them, which is independent of the action of the
heart and of the arteries. Many facts lead to the belief that
a new force is produced, while the blood is flowing through
the capillary vessels, — a force which may, in some instances,
maintain the circulation by itself alone. Thus in many of
the lower animals, it seems as if the power of the heart were
so unequal to the maintenance of the circulation, that this
must partly depend upon some other influence ; and even in
the highest, there is evidence that the movement of blood in
the capillaries may continue for a time, after the action of the
heart and of the arteries has ceased, to affect it.* This
movement seems intimately connected with the changes
to which the blood, is subservient in the capillaries; for,
if these be checked, not even the heart's action can
propel the blood through them, although no mechanical

* For a full consideration of this question, see the Author's Principles
of Comparative Physiology (4th edition), §§ 247-251 ; and Principles
of Human Physiology (5th edition), §§ 267-275.

240 CIRCULATION IN MAMMALS AND BIRDS.

obstruction exists. Thus, when the admission of air to the
lungs is prevented, the blood will not pass through the
pulmonary capillaries, since it cannot undergo the change
which ought to be performed there ; and it therefore accumu-
lates in the pulmonary artery, the right side of the heart, and
venous system ; and if no relief be afforded by the admission
of air into the lungs, the whole circulation is thus brought to
a stand. This condition, which is termed Asphyxia, occurs
in drowning, hanging, and other forms of suffocation (§ 338).

Course of the Blood in the different Classes of Animals.

281. The Circulation of the Blood takes place on the
same general plan in all other MAMMALS, and in BIRDS, as
in MAN. In all the animals included in these groups, the
heart is composed of two halves quite distinct from each
other ; each possessing an auricle or receiving cavity, and a
ventricle or propelling cavity. The course of their blood,
which goes through a complete double circulation, is shown by
the diagram (fig. 129). The vessels and cavities of the heart
which contain venous blood are shaded ; whilst those which
convey arterial blood are left white : and this distinction is
kept-up in the other figures. The direction of the blood is
indicated by the arrows. Every drop of blood which has
passed through the capillaries of the system, is transmitted
to the lungs before it is allowed again to enter the aorta ;
and the whole mass of "the blood passes twice through the
heart, before any part of it is transmitted a second time to the
vessels from which it was before returned.

282. The two sides of the heart do not possess, when that
organ is perfectly formed, any communication with each other,
except through the pulmonary vessels ; and thus they might
be regarded as two distinct organs, united for the sake of
convenience. The right side of the heart, being placed at
the origin of the pulmonary artery, and having for its office
to propel the blood through the lungs so as to receive the
influence of the air, may be called the respiratory heart :
whilst the left side, which is placed at the origin of the
aorta, and has to propel the blood to the body in general, may
be called the systemic heart. The circulation would be per-
formed precisely in the same manner, if these two organs

DIFFERENT FORMS OF CIRCULATING APPARATUS. 241

Lesser circulation.

Pulmonary veins

Pulmonary artery —

Left auricle

Right ventricle ,'

Aorta

Left ventricle

Greater circulation.
Fig. 129.— DIAGRAM OF THE CIRCULATION IN MAMMALS AND BIRDS.

were quite distinct from each other ; and in fact they are
almost so in the Dugong, one of the herbivorous Whales-
Lesser circulation.

Heart

Vena cava

Aorta

Single ventricle
Greater circulation.

Fig. 130.— DIAGRAM OF THE CIRCULATION IN REPTILES.
R

242 DIFFERENT FORMS OF CIRCULATING APPARATUS.

Lesser circulation.

;— Heart

Dorsal artery

Veins

Greater circulation.
Fig. 131. — DIAGRAM OF THE CIRCULATION IN FISHES.

(ZOOLOGY § 305). In the lower tribes of animals we shall

Lesser Circulation.

Bran chio- cardiac canals

:..._ Heart

Veins

Arteries

Greater circulation.
Fig. 132. — DIAGRAM OF THE CIRCULATION IN CRUSTACEA.

CIKCULATION IN FCETUS AND IN REPTILES. 243

presently find that there is but a single, instead of a double,
heart ; and that the organ which is absent is sometimes the
systemic, and sometimes the respiratory heart.

283. Previously to birth, when the lungs are not yet dis-
tended with air, and the aeration of the blood is provided-for
in other ways, the circulation takes place on a different plan
from that on which it is afterwards performed. There exists
at that period an opening in the partition between the two
auricles, by which they have a free communication; and
there is also a large trunk which passes from the right
ventricle into the aorta. By these channels, the blood which
is received from the systemic veins can pass at once into the
aorta, without going through the pulmonary vessels. But
when the young animal begins to breathe, these communi-
cations are speedily obliterated; the blood is transmitted
through the pulmonary vessels to the lungs ; and the whole
circulation takes place upon the plan just described. There
are occasional instances, however, in which the communica-
tion between the auricles remains open, so that the double
circulation is never perfectly established ; for a portion of the
blood is allowed to pass from the right to the left side of the
heart, without being aerated in the lungs, so that the blood
which is sent to the system contains a mixture of venous with
the proper arterial fluid, — a state which will be presently seen
to be natural in the Reptile. Such cases are recognised by
the blueness of the skin, the lividity of the lips, and the
indisposition to bodily or mental exertion. Persons affected
with this malformation seldom reach adult age.

284. In the class of REPTILES, there is not a complete
double circulation ; for a mixture of arterial and venous blood
is sent alike to the lungs and to the general system ; and no
part is supplied with the pure arterialized fluid. In general
the heart contains only three cavities, — two auricles and one
ventricle (fig. 133). One of the auricles receives the venous
blood from the system ; whilst the other receives the arterial-
ized blood from the lungs. Both these pour their contents
into the same ventricle, where they are mingled together;
and this mingled blood is transmitted, by the contraction of
the ventricle, partly into the lungs, and partly into the aorta
(fig. 130). In some Reptiles there is a partial division of
the ventricle, so that the mixture of the arterial and venous

R2

244 CIRCULATION IN REPTILES.

blood is not complete ; and whilst the Wood transmitted to
the lungs is chiefly that which has returned from the systemic
veins, the blood which enters the aorta for the supply of the

Pulmonary artery ^^jtJ^f^^WjS^^S*^** Pulmonary artery

fll /^Jsi»40fe. fflxmPulmonary vein
Pulmonary vein ]

Right auricle-

Left auricle

Single ventricle

Ventral aorta
Fig. 133.— HEART OF TORTOISE.

system is chiefly that which has returned from the lungs in an
arterialized state. Hence such animals have a circulation
which approaches very closely to that of Mammals and Birds ;
and it is among them that we find the greatest vigour and
activity in this generally inert and sluggish class.

285. The general arrangement of the blood-vessels in
Eeptiles is shown in fig. 134. It is seen that the aorta,
soon after its origin, divides into three arches on either side ;
and that these, after sending off branches to the head and to
the lungs, reunite into a single trunk, which corresponds
exactly with the aorta of the higher animals. These arches
are in fact the remains of a set of vessels, which will be
found to be of the highest importance in Fishes, being there
subservient to the aeration of the blood : in the true Reptiles,
however, they are never concerned in this function, but they
still remain, as if to show the unity of the plan on which
this apparatus is formed. Precisely the same arrangement of
the vessels may be seen in Birds and Mammalia, at an early
stage of their development; but it afterwards undergoes
considerable changes, by the obliteration of several of the
arches ; for of the four pairs which may be seen at one period,
a single branch only remains on either side ; and one of these
becomes the permanent arch of the aorta, whilst the other
becomes the permanent pulmonary artery.

CIRCULATION IN REPTILES.

245

Arches of aorta

Left auricle

\

\

Super, vena cava _

Ventral aorta

Pulmonary artery

Inferior vena cava

Liver and hepatic
vein

Kidneys —
Ventral Aorta

Carotid artery

Arches of Aorta
Right Auricle

Ventricle
Pulmonary vein
Brachial artery

Pulmonary artery

Lungs
Stomach

Gastric vein

Vena portas
Intestines

Fig. 134. — CIRCULATING APPARATUS OB LIZARD.

286. In the class of FISHES, the circulating apparatus is
still more simple. The heart only possesses two cavities, an

246

CIECULATION IN FISHES.

auricle and a ventricle. It is placed at the origin of the
vessels which are concerned in the aeration of the blood ; and

Branchial artery

Arterial bulb ,

Ventricle -^

Auricle —

Venous sinus

Vena portae, liver, &c. —

Intestine —

Venacava —

Vessels of the gills

, Dorsal artery

.„. Kidneys

. Dorsal artery or aorta

Fig. 135. — CIRCULATING APPARATUS OF FISH.

it receives and transmits venous blood only; hence it is
analogous to the right side, or respiratory heart, of Birds and
Mammalia. The venous blood, which is brought to it by the

CIKCULATION IN BATEACHIA. 247

systemic veins, is transmitted by its ventricle into a trunk,
which subdivides into four or five pairs of branches or arches
(fig. 135). These branches run along the fringes which form
the gills of the fish, and send a minute vessel into every one
of their filaments (§ 312). Whilst passing through this
vessel, the blood is submitted to the influence of the air
diffused through the water, to which the gills are freely
exposed, and is thus aerated; and it is then collected from
the several filaments and fringes, into a single large trunk,
analogous to the aorta of the higher animals, by which the
whole body is supplied "with arterialized blood. After circu-
lating through the system, the blood returns to the heart in a
venous condition, and again goes through the same course.
This course is represented in a simple form in the diagram,
fig. 131 ; and it will be seen, on a little consideration, that
it does not differ from that which exists in Animals with a
complete double circulation, in any other essential particular
than this, — that there is no systemic heart to receive the blood
from the gills or aerating organs, and to convey it to the body
at large. But, though all the blood must necessarily pass
through the gills before it can again proceed to the body, it
does not follow that the blood should be as completely aerated
as in Reptiles, in whose circulation there is a mixture of
venous and arterial blood; for the exposure of the blood to
the small quantity of air which is diffused through water is
not nearly so effectual as its direct exposure to air.

287. There is a group of animals which forms the transition
between Fishes and Eeptiles ; some of them being Fishes at
one part of their lives, and Reptiles at another ; whilst others
remain, during their whole lives, in a condition intermediate
between the two groups. Of this group (§ 86), the common
Frog is a familiar example. In the Tadpole state, it is essen-
tially a Fish, breathing by means of gills, and having its cir-
culation upon a corresponding plan; but after it has gone
through its metamorphoses, it breathes by lungs, its heart
acquires an additional auricle, and the whole plan of the circu-
lation is changed, so as to become comformable to that of the
true Reptile. This process takes place, not suddenly, but by
progressive stages ; and as these are extremely interesting,
they will now be briefly described. In fig. 136 we have a
representation of the circulating apparatus of the Tadpole in

248 CHANGE OP CIRCULATION IN TADPOLE.

its fish-like condition. At a is seen the large trunk which
issues from the ventricle, forming a bulbous enlargement like
that which is seen in the corresponding part of the Fish.
From this enlargement proceed three trunks on each side,
called the branchial arteries (br\ br2, br*), which convey the
blood to the gills or branchice; and after being aerated bypassing
through their filaments, the blood is collected by the bran-

ab

a ap av c ab 2 vb

Fig. 136.— BLOOD-VESSELS OF THE TADPOLE, IN FIRST STATE.

chial veins (vb, vb). Of these, the first pair transmits its blood
by the vessels o, o, t, (which are also formed in part by the
"•econd pair) to the head and upper extremities ; whilst the
greater part of the blood of the second pair, with the whole
of that of the third, is discharged into the trunk c on either
side. Ey the union of that vessel with its fellow, the trunk
a v is formed ; which conveys the blood that has been aerated
in the gills, to the general system, and is thus to be evidently
regarded as the aorta. But we find here three small vessels
(1, 2, and 3), which do not exist in the Fish; and which
establish a communication between the branchial arteries and
the branchial veins, in such a manner, that the blood may
pass from the former into the latter, without going through
the filaments of the gills. These communicating vessels are
very small in the Tadpole, and scarcely any blood passes
through them ; but it is chiefly by their enlargement, that the
course of the blood is subsequently altered. There is also a
fourth branch, ap, which proceeds to the lungs on either
side ; and as these organs are not yet developed, this pulmo-
nary artery also is at first of very small size.

288. As the metamorphosis of the other parts proceeds,

CIRCULATION IN TADPOLE. 249

however, and the animal is being prepared for its new mode
of life, the lungs are gradually developed, and the pulmonary
arteries greatly increase in size ; whilst the gills, on the other
hand, do not continue to grow with the animal, but rather
shrink, from the diminished supply of blood wThich they
receive. For, during this period, the communicating branches

ap av ap

Fig. 137. — THE SAME, IN A MORE ADVANCED STATE.

increase in size ; so that a considerable part of the blood which
has been transmitted into the branchial arteries passes at once
into the veins, and thence into the aorta, without being made
to traverse the gills ; its aeration being partly accomplished
by the lungs. This state of things is seen in fig. 137 ; where
ap, ap, are the enlarged pulmonary arteries ; and where the-
communicating branches are seen almost to form the natural
continuations of the branchial arteries. A condition of this
kind exists permanently in those Batrachia which retain their
gills during their whole lives, and have the lungs imperfectly
developed (§ 87). When the metamorphosis is complete, the
branchial vessels altogether disappear, but the arches still
remain, as shown in fig. 138. The first of these arches sup-
plies the vessels of the head, 1 1 ; which also, however, receive
a branch o from the second arch. The second arch, after
giving off that branch, unites with its fellow to form the
aortic trunk av. The third arch has completely shrivelled
up. And the fourth arch or pulmonary artery has now
attained its full size, and is become the sole channel through
which the aeration of the blood is effected.

250 CIRCULATION IN INVERTEBRATED CLASSES.

289. Among the Invertebrated classes generally, the condi-
tion of the circulating apparatus differs from that which prevails

br 1

ap av ap

Fig. 138. — THE SAME, IN THE PERFECT ANIMAL.

throughout the Vertebrata, in one remarkable feature; —
namely, that whereas in the latter the blood moves in every
part of its course through a set of closed vessels, it meanders
in the former through a set of channels or sinuses excavated
in the substance of the tissues, and communicating with the
" general cavity of the body " in the midst of which the viscera
lie. Generally speaking, it is in the venous system that the
greatest deficiency exists ; for the heart usually sends forth
the blood by definite arterial trunks, which distribute it by
its ramifications through the substance of the various parts of
the body ; and it is in its course from these to the respiratory
organs that it is least restrained within definite boundaries.
The degree of this imperfection differs considerably in the
several groups of Invertebrata ; for whilst, in the highest Mol-
lusca and Articulata, the vascular system is almost as complete
as in Vertebrated animals, we find it gradually becoming less
and less distinct as we descend, so that in the lower forms of
both series it presents itself merely as an extension of the
general cavity of the body, and is not furnished with any
special organ of impulsion.

290. In the greater part of the MOLLUSC A, the circulation

CIRCULATION IN MOLLUSCA.

251

takes place nearly on the same general plan as in Fishes ; the
heart having.two cavities, and the whole of the blood travers-
ing "both the respiratory and the systemic vessels, between
each time of its leaving the heart and returning to it again.
But this heart is systemic, and not pulmonary ; for it receives
the arterial blood from the gills, and transmits it to the great
systemic artery ; and after the blood has been rendered venous
by its passage through the tissues of the body, it enters the
channels which distribute it to the gills, before being again
subjected to the action of the heart. The accompanying figure
(fig. 139) of the circulation in the Doris (a kind of sea slug)

Fig. 139.— CIRCULATING APPARATUS OF DORIS.

will serve to show the general distribution of the vessels in
this group. The heart consists of the ventricle a, whence
issues the main artery I ; and of a single or double auricle c,
in which terminate the veins, d, of the branchial apparatus e.
The aerated blood which these convey to the heart, is trans-
mitted by it, through the artery 6, to the system at large ; and
from this it is collected, in the* state of venous blood, by the
sinuses which terminate in the large trunk //. By this trunk it

252 CIRCULATION IN GASTEROPODA AND CEPHALOPODA.

is distributed to the gills e; and thence it returns to the heart,
after having undergone aeration. Now if a second heart had
been placed on the trunk //, just as it is about to subdivide
for the distribution of the blood to the gills, the circulation
would have been analogous to that of Birds and Mammals.
There is a great variety in the position of the gills in Mollus-
cous animals, and a corresponding variety in the situation of
the heart, which is usually placed near them. In the Doris
the gills are arranged in a circular manner, round the termina
tion of the intestinal canal ; but in many Mollusca they form
straight rows of fringes on the two sides of the body. In
these last, the heart not unfrequently has two auricles ; but
these are not analogous to the two auricles of Eeptiles ; for
each has the same function with the other — the reception of
the blood from the gills of its own side.

291. There is a very interesting variety in the conformation
of the heart in the CEPHALOPODA, or Cuttle-fish tribe; which

vb

.... br

ab

\

vv av a cs vv

Fig. 140. — CIRCULATING APPARATUS OF CUTTLE-FISH.

seems to form a connecting link between the plan of the cir-
culation that prevails among the Mollusca in general, and that

CIRCULATION IN CEPHALOPODA AND CRUSTACEA. 253

•which we have seen in the class of FISHES. The auricle and
ventricle of the heart are separated from each other; and
whilst the latter remains in the position just described, the
auricle occupies the place which the whole heart possesses in
the class above. The course of the blood in these animals is
shown in fig. 140; where c represents the ventricle or sys-
temic heart, from which arises the aorta a, a, as, av, that
supplies the body with arterial blood. The venous blood is
returned through the great vein vc, covered with a curious
spongy mass cs, the use of which is not known; this also
receives the blood from the intestinal veins vv ; and it divides
into two trunks which convey the blood to the gills or branchiae
(br and br), where it undergoes aeration. On each of these
trunks is an enlargement, cb, which has the power of con-
tracting and dilating, and thus of assisting the transmission
of the blood through the arteries of the gills, a b. The blood
is returned to the ventricle by the branchial veins, vb, on
each of which there is another dilatation, bu, which might be
regarded as analogous to the auricle of the other Mollusca,
but that it is not muscular. Thus in the Cuttle-fish, the blood
receives an impulse from the systemic heart, by which it is
transmitted into the main artery ; and when it returns by the
systemic veins, it receives another impulse from the branchial
hearts, before it passes through the gills ; — an arrangement
obviously analogous to that which we meet with in the highest
Vertebrata.

292. In the Crab and Lobster, and other animals of the
class CRUSTACEA, the blood for the most part follows the same

e f i a d b

Fig. 141. — CIRCULATING APPARATUS OF LOBSTER.

course as in the Mollusca, excepting that the heart contains
but a single cavity. The arrangement of the circulating appa-

254 CIRCULATION IN CRUSTACEA.

ratus of a Lobster is seen in fig. 141, in which, a is the heart ;
6 and c, the arteries to the eyes and antennae ; d, the hepatic
artery ; and e and f, the arteries which supply the abdomen
and thorax. The blood that has been propelled through these
by the action of the heart, finds its way into the great venous
sinus g g} which receives the fluid collected from all parts of
the body ; from this it passes to the gills, h ; and thence it
is returned to the heart by the branchial veins, i. Another
view of a portion of the circulating apparatus is given in fig.
14:2, which represents a transverse section of it in the region

b ve c f vb

st ce
Fig. 142. — BRANCHIAL CIRCULATION OF LOBSTER.

of the heart, with one pair of gills. The heart is seen at c ;
and from its under side proceeds one of the arterial trunks
which convey the blood to the system. Returning thence,
the blood enters the venous sinus s, which has an enlarge-
ment at the base of each gill ; and this seems to act the part
of a branchial heart, like the corresponding enlargement on
the branchial vessels of the Cuttle-fish. From this cavity, it
is carried by the vessel va into the branchiae 5 ; and after it
has passed through the capillaries of the gill-filaments, it is
collected by the vessels ye, which carry it to the branchial
veins, v!>, and thence to the heart. The general plan of the
circulation in this class is shown in fig. 132.

293. In the class of INSECTS we find a still greater incom-
pleteness in the system of vessels for the conveyance of blood.
Arterial trunks can only be traced to a short distance from
the dorsal vessel, which answers the purpose of a heart ; and
the nutritive fluid which they convey is delivered into the
channels or sinuses that exist among the different organs.

CIRCULATION

INSECTS.

255

Nevertheless, it has a tolerably regular circulation ; and the
organ by which this movement is chiefly effected is .a long
tube, termed the dorsal vessel, which seems to propel it for-
wards, whilst two principal sinuses, one on either side, convey
it backwards. The dorsal vessel, seen at a, is a membranous

Fig. 143.--CiiicuLATioN IN INSECTS.

tube lying along the back of the insect, and partly divided
into several compartments by incomplete valvular partitions,
which bear no inconsiderable resemblance to the valves of veins
(§ 279). By the successive contraction of these different por-
tions, the blood which entered at the posterior extremity of
the dorsal vessel is gradually propelled forwards ; and when
it arrives at the front of the body, it passes out by a series of
canals, some of which convey it to the head, whilst others
pass sideways and backwards for the supply of the body, with
its appendages, the legs and wings. On returning from these
parts, it re-enters the posterior end of the dorsal vessel. But,
Asides ministering to this general circulation, the several
compartments of the dorsal vessel seem to act as independent
hearts, each for its own segment ; into which they send forth
blood ^by rainute arterial trunks, and from. which they receive
it again by minute apertures furnished with valves. It is to
be remarked that in Insects no special arrangement of vessels
for the aeration of blood is required ; since this aeration is

256

CIRCULATION IN ARACHNIDA AND ANNELIDA.

Fig. 144.— DORSAL VESSEL
OP SPIDER.

accomplished by the conveyance of air, by means of minute
air- tubes, into every part of the body, however small (§ 321) ;
a mode of respiration different from
any that we notice elsewhere. — A very
similar arrangement of the circulating
apparatus is met with in the Spider
tribe ; but as the body is not so long,
the dorsal vessel is less extended in
length, and is of larger diameter. This
is seen in fig. 144 ; where a represents
the abdomen of the animal; ar, the
large dorsal vessel or heart ; c} a trunk
passing forwards to the head; and v,
vessels communicating with the re-
spiratory organs.

294. In the animals of the Worm tribe,
belonging to the class Annelida, there
is a general similarity in the course of
the blood to that which prevails in Insects ; but as the respi-
ration is accomplished by means of special organs, which are
sometimes diffused along the entire body, and sometimes
restricted to one part of it (§ 314), there is considerable variety
in the provisions for submitting the blood to the influence of
the air. In those which possess red blood (§ 226), this fluid
can be seen to move in a closed system of vessels ; whilst a
colourless fluid containing numerous corpuscles flows through
a set of canals prolonged from the general cavity of the body.
It may be surmised that the two principal offices to which the
circulation of the blood is subservient, are here separately
performed; the red non-corpusculated fluid having for its
office to aerate the tissues, whilst the colourless but corpus-
culated fluid serves for their nutrition.

295. A very curious departure from the normal type of the
circulation presents itself in the class of TUNICATA, the lowest
of the Molluscous series (§ 114). The heart in these animals
is much less perfectly formed than in the higher tribes ;
though it still contains two cavities, one for receiving and the
other for impelling the blood. The blood may be sometimes
seen to flow in the direction customary among Mollusks;
coming to the heart from the respiratory surface, and then
going forth through an arterial trunk that conveys it into a

CIRCULATION IN LOWER INVERTEBRATA. 257

system of channels excavated through the tissues, after passing
through which it finds its way again to the respiratory sur-
face, and thence to the heart. But after a certain duration of
its flow in this direction, the current stops, and then re-com-
mences in the contrary direction, proceeding first to the
respiratory organs, and then to the system in general. It
would seem as if in this, one of the lowest forms of animals
possessing a distinct circulation, the central power were not
yet sufficiently strong to determine the course which the fluid
is to take. In the group of JBryozoa, which forms a connecting
link between the Tunicata and Zoophytes, we lose all trace
of a distinct circulation, which is only represented by the
movement of fluid in the general cavity of the body, and in
the prolongations of this cavity in the arms that surround
the mouth (fig. 64). — In the Star-fish, Sea- Urchin, and other
animals belonging to the class ECHINODERMATA, there seems
to be a regular circulation of nutritious fluid, carried on through
distinct vessels, but without any definite heart The only
trace, indeed, of anything like a propelling organ, is an en-
largement of one of the trunks, which pulsates with tolerable
regularity ; but this would not seem to have force enough to
propel the fluid through a complex system of vessels ; and
the circulation seems to be carried on chiefly by some force
produced in the capillaries (§ 280).

296. The circulating apparatus of higher animals is only
represented in Zoophytes, Medusae, and the lower Worms, by
ramifying prolongations of the digestive cavity, which extend
throughout the body, and are specially distributed to the
respiratory surface, so as to subject the products of digestion
to the aerating process. Thus, in the stony corals which are
formed by animals constructed upon the general plan of the
Sea Anemone (§ 127), the gelatinous flesh that connects the
polypes is traversed by a network of canals that open freely
into the sides of their visceral cavities, of which they may be
regarded as extensions ; whilst in the Campanularia (fig. 72)
and other composite Hydraform Zoophytes, a like communi-
cation is established by a system of canals passing along the
stem and branches, and becoming continuous with the base of
each polype. In this system of canals, viewed under a sufficient
magnifying power, a granular fluid may be seen to move, the
direction of the flow being sometimes from the stem towards

258 CIRCULATION IN ZOOPHYTES AND SPONGES.

the polypes, and sometimes from the polypes towards the
stem ; the rapidity and constancy of these currents depending
apparently on the activity of the growth of the parts towards
which they are directed. In the Medusae we find the central
stomach sending out prolongations towards the margin of the
disk, where they frequently inosculate so as to form a net-
work, which seems to have for its purpose to expose the
product of digestion to the aerating action of the surrounding
water ; and in this system of canals, also, a movement of fluid
may be observed, which appears to depend upon the action of
cilia in their interior. In all these cases, it is to be observed
that the circulation of nutritive fluid is really effected by a
modification of the digestive apparatus, instead of by an appa-
ratus set apart for this sole purpose ; and the blending of the
two functions is still more remarkably exhibited in the Sponge,
the inosculating canals of which (§ 136) may be regarded
alike as constituting a ramifying digestive cavity, or as a simple
form of circulating apparatus. The most correct method is
perhaps to consider it as representing both these systems,
which are here blended (as it were) into one ; the simplicity
of structure characteristic of this type not admitting of the
division of labour which we meet with in higher organisms.

CHAPTER VI.

OP RESPIRATION.

297. WE have seen that arterial blood, by its action on the
living tissues, loses those qualities which rendered it fit for
the maintenance of life ; and that after having undergone
this change, it recovers its original properties by exposure to
air. This exposure is necessary, therefore, to the continued
existence of Animals in general. If we place an animal
under the receiver of an air-pump, and exhaust the air either
partially or completely, a great disturbance soon shows itself
in its various functions ; shortly afterwards, the several
actions of life cease to take place; and a state of apparent
death comes on, which speedily becomes real, if air be not
re-admitted. The influence of air is not less necessary to

NATURE OP RESPIRATION. 259

Plants than to Animals; for they also die when excluded
from it : and thus its presence may be stated to be a general
condition, necessary for the continuance of the life of all
organised beings. — There is, however, a marked difference in
the rapidity with which the deprivation of air occasions
death in different animals (§ 310).

298. At first sight it might be thought that Animals which
always live beneath the surface of the water, such as Fishes,
Zoophytes, and many Mollusca, are removed from the influence
of the air ; and that they hence constitute an exception to this
general law. But such is not the case ; for the liquid which
they inhabit has the power of absorbing, and of holding dis-
solved in it, a certain quantity of air, which they can easily
separate from it, and which is sufficient for the maintenance
of their life. They cannot exist in water which has been
deprived of air (as by boiling, or by being placed under the
exhausted receiver of an air-pump) ; for they then become
insensible and die, just as do Mammalia and Birds when
prevented from inhaling air in the ordinary manner.

299. The changes which result from the exposure of the
blood or nutritious fluid of Animals to the air, either in the
atmosphere, or through the medium of water, form a very
important part of their vital actions ; and the changes them-
selves, together with the various mechanical operations by
which they are effected, constitute the function of Respiration.
The nature of these changes will be first explained ; and the
structure and operations of the organs by which they are
performed will be afterwards described.

Nature of the Changes essentially constituting Respiration.

300. Atmospheric air, it has been stated, is necessary to
the continued life of all animals ; but this fluid is not com-
posed of one element alone. By the science of Chemistry,
it is shown to be a mixture of three gases possessing very
different properties. Besides the watery vapour with which
the atmosphere is always more or less charged, the air con-
tains 21 parts in 100 of oxygen, and 79 parts of nitrogen or
azote ; with about l-5000th part of carbonic acid gas. The
question immediately presents itself, therefore, whether these
gases have the same action on animals ; or, if their actions

s2

260 CHANGES IN AIR BY RESPIRATION.

be different, to which, of them specially belongs the property
of thus contributing to the maintenance of life. This question
may be decided by a few simple experiments. If we place
a Bird or small Mammal in a jar filled with air, and cut off
all communication with the atmosphere, it perishes by suffo-
cation in a longer or shorter time ; and the air in the vessel,
which has thus lost the power of maintaining life, is found
by chemical analysis to have lost the greater part of its
oxygen. If we then place another animal in a jar filled with
nitrogen gas, it perishes almost immediately; whilst if we
place a third in pure oxygen, it breathes with greater activity
than in air, and shows no sign of suffocation. It is then
evident, that it is to the presence of oxygen that atmospheric
air owes its vivifying properties.

301. But the change produced in the atmosphere by animal
respiration is not limited to this. The oxygen which disap-
pears is replaced by carbonic acid ; which, instead of being
favourable to the maintenance of life, causes the death of
animals which inhale it, even in small quantities. The
exhalation of this substance is an action not less general in
the Animal kingdom than the absorption of oxygen ; and it
is in these two changes that the act of respiration essentially
consists.

302. The quantity of nitrogen or azote in the air that has
been respired, varies but very little. There appears, however,
to be a continual absorption of nitrogen by the blood, and
as continual an exhalation of it. When the quantity exhaled
and the amount absorbed are equal, or nearly so, no change
manifests itself in the air which has been breathed ; when the
quantity absorbed is the greater, there is a diminution in that
which the respired air contains; and when the quantity
exhaled is the greater, there is a corresponding increase. An
exhalation of nitrogen seems to be ordinarily taking place in
warm-blooded animals, to an extent varying between l-50th
and 1-1 00th of the oxygen consumed; but when the same
animals are partially or wholly deprived of food, an absorption
of nitrogen usually occurs.

303. The differences in the character of the blood which
are produced by its exposure to the air, have already been
noticed (§ 227); and we now see that they are essentially due
to the absorption of oxygen, and the setting free of carbonic

CHANGES IN BLOOD BY RESPIRATION. 261

acid. These changes will take place out of the living body as
well as in it ; provided that the blood can be exposed as com-
pletely to the influence of the atmosphere. When blood is
drawn from a vein into a basin • or cup, the dark hue of the
surface is usually seen to undergo a rapid alteration, so as to pre-
sent the arterial tint ; but this is confined to the upper surface,
because it alone is exposed to the influence of the atmosphere.
The alteration takes place still more rapidly and completely
if the blood be exposed to pure oxygen gas ; but even then it
is almost confined to the surface. It is not prevented, even
though the direct communication between the blood and the
gas be cut off by a membranous partition, as it is in the living
animal ; for if the "blood be tied up in a bladder, the gas has
still the power of penetrating to it, and of effecting the change
in it ; and the change is manifested, not only by the alteration
in the aspect of the blood, but by the disappearance of a
certain quantity of oxygen, and its replacement by carbonic
acid. Now if, by spreading out the blood in a very thin layer,
we expose a much larger surface to the air, or if, by frequently
shaking it, we continually change its surface, we render the
change more complete. But still it is accomplished far less
effectually than it is in the lungs or gills of a living animal ;
for when it is passing through their capillaries, it is divided
into an immense number of very minute streams, each of
which is completely exposed to the influence of the air, and
the combined surface of which is very great.

304. The question next arises, what becomes of the Oxygen
which disappears, and what is the origin of the Carbonic acid
which is thus produced by respiration ? This question will
now be considered.

305. When charcoal is burned in a vessel filled with air,
the oxygen disappears, and is replaced by an equal bulk of
carbonic acid : at the same time there occurs a consider-
able disengagement of heat. During respiration, the same
phenomena occur : there is always an evident relation between
the quantity of oxygen employed by an animal, and the
amount of carbonic acid it produces (the latter being usually
somewhat less than the former) ; and, as we shall see hereafter
(Chap, ix.), there is always a greater or less amount of heat
produced. There exists, then, a great analogy between the
principal phenomena of respiration, and those of the combus-

262 SOURCE OF CARBONIC ACID EXHALED.

tion of carbon ; and this agreement in tlie results naturally
leads to the belief that the causes of both are the same. — It
is to be borne in mind, however, that the substitution of
carbonic acid for oxygen is not the only change produced by
respiration in the air ; for there is nearly always a disap-
pearance of oxygen (to an amount sometimes equal to one-
third of that exhaled in combination with carbon), which
is taken into the system to be applied to other uses
(§ 343).

306. It was at one time supposed that the oxygen of the
inspired air combines, in the lungs themselves, with the carbon
brought there in the blood ; and thus produces the carbonic
acid which is expired, occasioning at the same time the
development of heat. But this theory is inconsistent with
experiment ; for it has been proved that the carbonic acid, is
not formed in the lungs, but that it is brought to them in
the venous blood of the pulmonary artery ; and that their
office is to disengage or get rid of it, whilst they at the same
time introduce oxygen into the arterial blood. For in the
first place, it can be shown that a considerable quantity of
carbonic acid exists in venous blood, from which it may be
removed by drawing it into a vessel filled with hydrogen or
nitrogen, or by placing it under the vacuum of an air-pump ; it
can also be shown that arterial blood contains a consider-
able quantity of oxygen. Again, if Frogs, Snails, or other
cold-blooded animals, be confined for some time in an atmo-
sphere of nitrogen or hydrogen (neither of which gases in itself
exerts any injurious effect upon them), they will continue for
some tune to give off nearly as much carbonic acid as they
would have done in common air; thus proving that the
carbonic acid is not formed in the lungs by the union of carbon
brought in the venous blood with the oxygen of the air, since
here no oxygen was supplied ; and showing that the carbonic
acid must have been brought ready-formed. This process, ,
however, could not be continued for any great length of time,
even in cold-blooded animals ; since a supply of oxygen is
necessary to the performance of their various functions. And
in warm-blooded animals, a constant supply of this element
is so much more important, that they will die if cut off from
it, even for a short time.

307. The quantity of oxygen thus taken in, and of carbonic

SOURCE OF CARBONIC ACID EXHALED. 263

acid thus disengaged, bears a very regular proportion to the
amount of exertion which is made during the same time.
Hence it is much greater in tribes whose habits are active,
than in those which are inert ; and it is also greater in any
individual, during a day spent in active exercise, than it is in
the same person during a day passed in repose. This obviously
results from the fact, now established beyond all doubt, that
every muscular contraction or production of muscular-force,
and every production of nerve-force by which muscular contrac-
tion is usually called forth, involve, as their essential condition,
the death and disintegration of proportionate amounts of
muscular and nervous substances, which pass from the state of
living tissues to that of dead matter ; and for this operation,
the presence of the oxygen in arterial blood is required. This
oxygen combines with part of the materials thus set free as
waste (§ 55), and converts them into the products that are
thrown off by the various excretions. One of the chief of
these products is carbonic acid, which is carried off by the
lungs in the manner already described. Thus the presence
of oxygen in the blood is essential to the development of
nervo-muscular force ; while the elements of the blood
itself are required to re-form the tissues which have been thus
destroyed.

308. It is among Birds and Insects that we find the greatest
quantity of carbonic acid produced, in proportion to the size
of the animals ; and in both these classes we find extraordi-
nary provisions for the energetic performance of this function
(§§ 321 and 326). The greater energy of the respiration of
Birds than that of Mammals, when compared with the greater
number of the red corpuscles in their blood, gives an increased
probability to the idea, that the red corpuscles are the chief
carriers of oxygen from the lungs to the capillaries of the
system, and of carbonic acid from the capillaries of the system
to those of the lungs (§ 235). The energetic respiration of
Insects, though these corpuscles are absent, is fully accounted
for by the peculiar manner in which the air enters every part
of their bodies (§321). In no case do we see the influence
of muscular activity, on the amount of carbonic acid thrown
off, more strongly manifested than in Insects. A humble-bee,
while in a state of great excitement after its capture, made
from 110 to 120 respiratory movements in a minute; after

264 DIMINUTION OF RESPIRATION IN TORPID STATE.

the lapse of an hour, they had sunk to 58 ; and they sub-
sequently fell to 46. In the first hour of its confinement it
produced about l-3rd of an inch of carbonic acid (a quantity
many times greater, in proportion to its size, than that which
Man would have set free in the same time) ; and during the
whole twenty-four hours of the subsequent day, the insect
produced a less amount than that which it then evolved in a
single hour. In the Larva state, which is usually one of com-
parative inactivity, the respiration is not much above that of
the Worm tribes ; and in the Chrysalis state of those which
become completely inactive, the amount of respiration is still
lower.

309. This chrysalis state, indeed, bears a strong resemblance
to the condition of. torpor in which many animals pass the winter.
Reptiles, and most Invertebrata that inhabit the land, become
(to all appearance) completely inanimate when the temperature
is lowered below a certain point ; yet retain the power of
exhibiting all their usual actions when the temperature rises
again. In this state, their circulation and respiration appear
to cease entirely ; or, if these functions are carried on at all,
they are performed with extreme feebleness ; and the animals
may be prevented from reviving for a long time, without their
vitality being permanently destroyed, if they be surrounded by
an atmosphere sufficiently cold. Thus Serpents and Frogs have
been kept for three years in an ice-house, and have completely
revived at the end of that period. Among Mammals there are
several species which pass the winter in a state of torpidity ;
but this is less profound than the torpidity of cold-blooded
animals, for the circulation and respiration never entirely
cease, though they become very slow. There are various
gradations between this condition and ordinary deep sleep.
Thus some of the animals which hybernate or retire to winter
quarters, lay up a supply of food in the autumn, and pass the
cold season in a state differing but little from ordinary sleep,
from which they occasionally awake, and satisfy their hunger.
But others, like the Marmot, are inactive during the whole
period, taking no food, and exhibiting scarcely any evidence
of life, unless they are aroused. The consumption of oxygen
and the production of carbonic acid, under such circumstances,
are extremely slight, as might be anticipated from the languor
of the circulation and the inactivity of the nervo-muscular

TOLERANCE OP DEPRIVATION OF AIR. 265

system. But a very slight irritation is sufficient to produce
respiratory movements ; the heart's action is quickened ;
and the animal for a time shows an increase of its general
activity.

310. Animals will in general bear deprivation of air well
or badly, according as the respiration is more or less active.
Thus a warm-blooded animal usually dies, if kept beneath
water for more than a few minutes ; though there are some
which are enabled, by peculiar means, to sustain life much
longer (§ 265). In cold-blooded animals, however, whose
demand for oxygen is much less energetic, this treatment may
be continued for a much longer time without the loss of life.
Thus the common Water-Newt naturally passes a quarter of
an hour or more beneath the surface, without coming up to
breathe j and it may be kept down for many times that
period without serious injury. And, as we might expect from
their peculiar condition, warm-blooded animals, when hyber-
nating, may be kept beneath water for an hour or more,
without apparent suffering; although the same animals, in
their active state, would not survive above three minutes.
There is reason to believe that a similar condition may be
produced in Man, by the influence of mental emotion, or of a
blow on the head, at the moment of falling into the water ;
so that recovery is by no means hopeless, even though the
individual may have been more than half an hour beneath the
surface,

Structure and Actions of the Respiratory Apparatus.

311. In animals whose organization is most simple, the act
of respiration is not performed by any organ expressly set
apart for it ; but it is effected by all the parts of the body that
are in contact with the element in which the animal lives,
and from which they derive their necessary supply of oxygen.
This is the case, for example, in the lower class of Animal-
cules, in the Polypes, Jelly-fish, Entozoa, and many other
animals. Even in the higher classes, a considerable amount
of respiratory action takes place through the skin, especially
when it is soft and but little covered with hair, scales, &c., as
in Man, and in the Frog tribe ; but we almost invariably find
in them a prolongation of this membrane, specially designed to
enable the blood and the air to act upon each other, and having

266 STRUCTURE OF RESPIRATORY ORGANS.

its structure modified for the advantageous performance
of this function. This modification consists in the peculiar
vascularity of this membrane, that is, in the large number of
vessels that traverse its surface ; and also in the thinness of
the membrane itself, by which gases are enabled to permeate
it the more readily. Moreover, we always find this membrane
so arranged, that it exposes a very large surface to the air or
water which comes into contact with it ; and this surface may
be immensely extended, without any increase in the size of
the organ. Thus the small lungs of a Eabbit really expose
a much larger respiratory surface to the air, than is afforded
by the large air-sacs of a Turtle which are ten times their size.
This is effected by the minuteness of the subdivision of the
former into small cavities or air-cells, whilst the latter remain
as almost undivided bags.

312. It is desirable to possess a distinct idea of the mode
in which the surface is thus extended by subdivision. We
may, for the purpose of explanation, compare the lung to a
chamber, on the walls of which the blood is distributed, and
to the interior of which the air is admitted. This chamber,
for the sake of convenience of description, we shall suppose to

have two long and two short sides, as at A. JSTow if a parti-
tion be built-up in the direction of its length, as at B, a new
surface will be added, equal to that which the two sides
previously exposed ; since both the surfaces of this partition
are supplied with blood, and are exposed to the air. Again,
if another partition be built-up across the chamber, as at c, a
new surface will be added, equal to that which the ends of the
chamber previously exposed. And thus, by the subdivision
of the first chamber into four smaller ones, the extent of sur-
face has been doubled. Now if each of these small ones were
divided in the same manner, the surface would again be
doubled ; and thus, by a continual process of subdivision, the

STRUCTURE OF RESPIRATORY ORGANS. 267

surface may be increased to almost any extent compatible with
the free access of air to the cavities, and of blood to the walls.
In the same manner, where the respiratory membrane is pro-
longed outwardly, so as to form gills, which hang from the
exterior of the body (as is the case in most aquatic animals),
its surface is very much extended by disposing it in folds, and
by dividing these folds into fringes of separate filaments. It
has been calculated that, by this kind of arrangement, the
gills of the Skate present a surface four times as great as that
of the Human body.

313. The structure and arrangement of the Eespiratory
organs differ, according as they are destined to come in con-
tact with air in the state of gas, or to act upon water in which
a certain amount of air is dissolved. In the former case, we
usually find the respiratory membrane (which is but a pro-
longation of the skin or general envelope) forming the wall of
an internal cavity, — just in the same manner as the membrane,
through which the act of absorption takes place in animals,
is prolonged from the skin so as to form the wall of the
digestive cavity (§ 14). Such a cavity for the reception of
air into the interior of the body, exists in all air-breathing
animals ; and in the Vertebrata it receives the name of lung.
On the other hand, in animals that breathe by means of water,
the respiratory surface is prolonged externally, so as to be
evidently but an extension of the general surface, — -just in the
same manner as the roots of plants are prolonged into the soil
around them. These prolongations, termed branchice or gills,
which may have various forms, carry the blood to meet the
surrounding water, and to be acted-on by the air it contains ;
and then return it to the body in a purified condition.

314. The form and arrangement of the gills vary greatly
in the different classes of aquatic animals. Sometimes they
simply consist of little leaf-like appendages, which have a
texture rather more delicate than that of the rest of the skin,
and which receive a larger quantity of blood. In other in-
stances, they are composed of a number of branching tufts,
which are more copiously supplied with vessels. Among
the ANNELIDA we observe a great variety in the mode in
which these tufts are disposed; and this is connected with
the general habits of the animal. Thus in the Serpula (fig.
145), whose body is inclosed in a tube, the tufts are disposed

268

RESPIRATORY ORGANS OP AQUATIC ANIMALS.

around the head alone, and spread out widely into the sem-
blance of a flower. In the Nerds (fig. 52) and its allies, they
are set upon nearly every
division of the body, and are
much smaller. Their usual
arrangement in these marine
worms may be seen in fig.
146, which represents one
of the appendages of Eunice.
The tuft of gills is shown
at b ; at c is seen a bristle-
shaped filament, which may
perhaps be regarded as the
rudiment of a leg ; and the
projections to which the
letters t and ci point, also
seem connected with the
movements of the animal.
In the Arenicola (the lob-
worm of fishermen) we find
the respiratory tufts dis-
posed on certain segments

only, and possessing more of an arborescent (tree-
like) form (fig. 147).

315. A somewhat similar
arrangement is seen in the
larvae of many aquatic IN-
SECTS, which breathe by
means of gills ; although all
perfect Insects breathe air
in the manner to be pre-
sently described. In fig.
148 is represented the larva
of the Ephemera (Day-fly),
which breathes by means

of a series of gill-tufts disposed along the abdomen,
and also prolonged as a tail. In the CRUSTACEA, we usually
find the gills presenting the form of flattened leaves or plates.
In the lower tribes of the class, they project from the surface
of the body; but in the higher, they are inclosed within
a cavicy, through which a stream of water is made con-

. 145.
GILL-TUFTS OF SERPULA.

Fig. 146.
GILL-TUFT OF EUNICE.

Fig. 147.
ARENICOLA.

RESPIRATORY ORGANS OF AQUATIC ANIMALS.

269

stantly to flow, by mechanism adapted for the purpose. Their

form and position in the Crab are shown at b, b', fig. 47

Although these animals usually reside in the water,

or only quit it occasionally, there are some species,

known under the name of land-crabs, which have

the power of living for some time at a distance

from water. In order to prevent their gills from

drying up, which would destroy their power of

acting on the air, there is a kind of spongy

structure in the gill-chamber, by which a fluid is

secreted that keeps them constantly moist.

316. In theMoLLuscA we find the gills arranged
in a great variety of modes. In the lowest class,
the TUNICATA, the respiratory membrane is merely
the lining of the large chamber formed by the
mantle (fig. 63), through which a stream of water
is continually made to flow by ciliary action
(§ 319); and this surface is sometimes extended by the
folding or plaiting of the membrane. In most of the CON-
CHIFERA, however, we find four lamellae or folds of membrane

Fig. 148.

LARVA OF

EPHEMERA.

Fig. 149.— RESPIRATORY APPARATUS OF THE OYSTER.

», one of the valves of the shell ; v1, its hinge ; m, one of the lobes of the mantle;
m', a portion of the other lobe folded back; c, muscles of the shell; br, gills;
b, mouth ; t, tentacula, or prolonged lips; /, liver; i, intestine; a, anus- co
heart.

270

RESPIRATORY ORGANS OF AQUATIC ANIMALS.

(br, fig. 149), lying near the edge of the shell, and copiously
supplied with blood-vessels. In the Oyster, these are freely
exposed to the surrounding element, the lobes of the mantle
being separated along their entire length; but where the

mantle-lobes are united along
their margin, so as to shut-in
the gills, there are two ori-
fices, often prolonged into
tubes (as in the Tellina, fig.
150), through one of which
the water is drawn-in for the purpose of respiration, whilst
through the other it passes out, as in the Tunicata. In the
aquatic GASTEROPODA there is scarcely any part of the body to
which we do not find the gills attached in some species or other.
In the naked marine species, which may be called Sea- slugs,
they form fringes which are sometimes disposed along the sides
of the body, as in the Tritonia and Glaucus (figs. 151, 152),

50. — TELLINA.

Fig. 151.— TRITONIA.

Fig. 152.— GLAUCUS.

sometimes arranged in a circle around the end of the intestine,
as in the Doris (fig. 153, — see also fig. 139); and are some-
times covered-in, more or less
completely, by a fold of the mantle.
In most of the species that pos-
sess shells, the gills form comb-
like fringes, which are lodged in
a cavity inclosed in the last turn
Fig. 15S.-DOHU. of the ,3^ shell. and to this

cavity the water is admitted, sometimes by a large opening,
sometimes by a prolonged tube. In the CEPHALOPODA, we find

RESPIRATORY ORGANS OF AQUATIC ANIMALS. 271

the gills composed of a collection of little leaf-like folds, placed
on a stalk (6, fig. 154); they are inclosed in a cavity which is
covered-in by the mantle ; and the walls of this cavity have
the power of alternately dilating and contracting, so as to
draw-in and expel water. It communicates with the exterior
by two orifices, one of which, o, a wide slit, is for the entrance

Fig. 154.— GILLS OF POULP.

of water ; whilst the other, t, is tube-like, and serves not only
to carry-off the water that has passed over the gills, but also
to convey away the excrements, and the fluid ejected by the
ink-bag. This is called the funnel.

317. In FISHES, the gills are composed of fringes, which
are disposed in rows on each side of the throat, and are
covered by the skin. The cavity in which they lie has two
sets of apertures ; • one communicating with the throat, and
the other opening on the outside. In the Fishes with a car-
tilaginous skeleton, we usually find as many of these external
orifices as there are rows of gills ; thus in the Lamprey there
are seven, as shown in the succeeding figure (a). But in
Fishes with a bony skeleton, there is usually but a single
large orifice on either side ; and this is covered with a large
valve-like flap, which is termed the operculum or gill-cover.
A continual stream of water is made to pass over the gills by
the action of the mouth, which takes-in a large quantity of

272

RESPIRATORY ORGANS OF FISHES.

fluid, and then forces it through the apertures on each side of
the throat, into the gill-cavities ; and from these it passes out
by the other orifices just described. Fishes, in common with
other animals that breathe by gills, can only respire properly

Fig. 155.— LAMPREY.

when these are kept moist, and are so spread out as to expose
their surface to the surrounding element. The act of respira-
tion can take place when they are exposed to air, provided
these conditions be fulfilled ; but in general it happens that,
when a Fish is taken out of water, its gills clog together and
dry up, so that the air cannot exert any action upon them ;
and the Fish actually dies of suffocation, under the very cir-
cumstances which are necessary to the life of an air-breathing
animal.

318. There are certain Fishes, however, which are provided
with an apparatus for keeping the gills moist, somewhat re-
sembling that which has been already noticed in the land-
crab. The bones of the pharynx are extended and twisted

in such a remarkable
manner (fig. 156), as
to form a number of
small cavities ; these
cavities the Fish can
fill with water; and
they form a reservoir
of fluid, from which
the gills may be sup-
plied with a sufficient
amount to keep them
moist during some
time. The gill-fila-
ments themselves are

so arranged that they do not clog together ; and by this combi-
nation of contrivances, the species of Fish that are furnished
with it can live for a long time out of water, so as to be able to
journey for a considerable distance on land. Such a provision

Fig. 156. — RESPIRATORY APPARATUS OF ANABAS.

AQUATIC RESPIRATION : USE OF CILIA. 273

is especially desirable in tropical climates, where shallow
lakes are often dried-up by continued drought, so that their
inhabitants must perish, if they were not thus enabled to
migrate. One of the most curious of these Fishes (most of
which are inhabitants of India and China) is the Andbas or
climbing-perch of Tranquebar ; which climbs bushes and trees
in search of its prey, a species of land-crab, by means of the
spines on its back and gill-covers. — The gills of the Amphi-
bious Reptiles, in their Tadpole state, resemble those of Pishes,
and are connected with the mouth in the same manner.

319. In the respiratory actions of nearly all these animals,
a very important part is performed by the cilia (§ 45) which
cover the surfaces of the gills. Even in such as do not
possess any special respiratory organs (§ 311), the action of
the cilia is very important, in causing a constant change -in
the water that is in contact with their surface. Thus in
Zoophytes, which are for the most part fixed to one spot, the
action of the cilia produces currents in the surrounding water.
On the other hand, in the actively-moving Animalcules, the
same action propels their bodies rapidly through the water ;
though in some of them, which occasionally fix themselves
like Polypes, the action of the cilia resembles theirs. In
either case there is a continual change in the layer of water
which is in immediate contact with the surface ; and thus a
constant supply of the air contained in the water is secured.
A similar action goes-on, still more energetically, over the gill-
tufts of the Annelida; and this action continues after the
death of the animal, or after the tuft has been separated from
it, producing evident currents in the water in which it is
placed. It is by the action of the cilia alone, that the con-
tinual current of water is kept-up through the respiratory
chamber of the lower Mollusca; but this is superseded in
Cephalopods and Fishes by the other means for sustaining
this current which have been already noticed. Ciliary action
may be well observed in the young Tadpole of the common
Water-Newt, whose gills hang freely from the neck on either
side; the cilia are themselves so minute that they cannot be
readily distinguished ; but the motion of the water produced
by them may be at once perceived in a tolerable microscope,
especially when small light particles, such as those of
powdered charcoal, are diffused through it.

274

ATMOSPHERIC RESPIRATION.

320. In animals whose blood is made to act directly upon
the air, we usually find a provision of some kind for intro-
ducing the air into the interior of the body. The simplest
arrangement is that which we meet-with in the Snail and other
terrestrial GASTEROPODS ; and it consists merely of a cavity
(p, fig. 157), resembling that in which the gills are disposed in
the aquatic Mollusca, but having a free communication with

* d /

Fig. 157. — ANATOMY OF SNAIL.

/", muscular disc or foot; t, tentacula; d, diaphragm separating the respiratory
cavity p from other organs, but here turned back; s, stomach; o, ovary; or,
arterial trunk supplying the system; i, r, intestine; I, liver; h, heart ; ap, vascular
trunk spreading over the pulmonary cavity p ; cv, canal for excreting the viscid
mucus secreted by the gland v.

the external air, and having the blood minutely distributed by
vessels upon its walls. — In the MYRIAPODA or Centipede tribe,
in conformity with the general plan of Articulated structure
(§ 93), we find a repetition of similar cavities along the body,
one pair usually existing in each segment;
and these open externally by small apertures,
which are termed spiracles.

321. In INSECTS, the same general arrange-
ment is modified in the most remarkable manner.
The spiracles do not open into distinct air-sacs,
but into canals, which lead to two large tracheae
which run along the sides of the body, and are
connected by several tubes that pass across it —
one usually for each segment. From these
tracheae others branch off, which again subdivide into more

Fig. 158.

AIB.-TUBE OP IN-
SECT.

RESPIRATION OP INSECTS.

275

minute tubes ; and, by the ramifications of these, even the
minutest parts of the body are penetrated (fig. 159). These
tubes are formed upon a similar plan with the air-vessels
of Plants, having a spiral fibre winding inside their outer
membranous coat (fig. 158) ; by the elasticity of which
fibre, the tube is kept from being closed by pressure. In
this manner the air is brought into contact with almost

Head.

1st Pair of legs

1st Segment of\
thorax)

Origin of wing —
2d Pair of legs
3d Pair of legs

Tracheae

Stigmata

Air- sacs.

Fig. 159. — RESPIRATORY APPARATUS OP INSECT (NEPA).

every portion of the tissue, and is enabled to act most ener-
getically upon it ; and thus the feeble circulation of these
animals (§ 293) is in a great degree counterbalanced by

T2

276 RESPIRATION OF INSECTS AND SPIDERS.

the extraordinary activity of their respiration. There are
no animals which consume so much oxygen, in proportion
to their size, as Insects do when they are in motion (§ 308) ;
but when they are at rest, their respiration falls to the
low standard of the tribes to which they bear
the greatest general resemblance. Although, as
we have seen, the respiration of aquatic larvae is
sometimes accomplished by means of gills, yet
many aquatic Iarva3 breathe air by means of
tracheae ; and such are consequently obliged, like
Whales and other aquatic Mammals, to come
occasionally to the surface for the purpose of
gaining a fresh supply of air. The larva of the
Gnat, which breathes in this manner, has one of
the spiracles of its tail-segment prolonged into a
tube ; and it may often be seen suspended, as it
were, in the water, with its head downwards, the
end of this tube (t, fig. 160) being at the surface.

322. In the greater number of perfect Insects, we find the
trachese dilated at certain parts into large air-sacs (fig. 159) ;
these are usually largest in Insects that sustain the longest
and most powerful flight ; in some of which, as in the
common Bee, they occupy a greater portion of the trunk than
they do in the insect whose system of air-tubes has been just
represented, — this insect, the Nepa or water-scorpion, being
of aquatic habits, and seldom using its wings for flight.
There can be little doubt that one use of these cavities is to
diminish the specific gravity of the Insect, and thus to render
it more buoyant in the atmosphere ; but it would not seem
improbable that they are intended to contain a store of air
for its use while on the wing, as at that time a part of
the spiracles are closed. We shall find in Birds, the Insects
of the Vertebrated division, a structure bearing remarkable
analogy to this (§ 326).

323. In some of the ARACHNIDA, such as the Cheese-mite,
the respiration is accomplished by tracheae, as in Insects ; but
in the Spiders it is performed by a different kind of apparatus.
Instead of opening into a system of prolonged tubes, each
spiracle leads to a little chamber, the lining membrane of
which is arranged in a number of folds that lie together like
the leaves of a book; and thus a large surface is exposed

KESPIEATION OF AIR-BREATHING VERTEBRATES. 277

to the air which is admitted through the spiracles. This
arrangement is shown in fig. 46, I.

324. Hitherto it has been seen that the respiratory appa-
ratus is not connected with the mouth, which in the Inverte-
brated classes has the reception of food as its sole function.
On this account, we cannot regard the air-sacs of Insects as
bearing any real analogy to the lungs of Vertebrata. The
simplest condition of the true lung is that which constitutes the
air-bladder (or " sound ") of Fishes. This we sometimes find in
the condition of a closed bag, lying along the spine ; and its use
cannot be that of assisting respiration, since air is not ad-
mitted to it from without. But in other cases we find it
connected with the intestinal tube, by means of a short wide
duct ; and since many Fishes, as the Loach, are known to
swallow air, which is highly charged with carbonic acid when
it is again expelled, it seems probable that their air-bladder
effects this change in precisely the same manner as a lung
would do — the air being transmitted to it from the intestine.
There are some Fishes in which the resemblance of the air-
bladder to a lung is more decided, and its connexion with the
function of Eespiration is evidently more important. The
canal by which it communicates with the alimentary canal
opens into the latter above the stomach, and even, in some
instances, at the back of the mouth; so that a gradual
approach is seen to the arrangement which exists in air-
breathing animals. In these Fishes, as in the Amphibia that
retain their gills (§ 87), it would appear that the respiration
is accomplished partly by the lungs, and partly by the gills ;
this is the case in the curious Lepidosiren (fig. 41), which, as
formerly mentioned, is regarded by some naturalists as a Fish,
and by others as a Eeptile.

325. The lungs of EEPTILES are for the most part but little
divided ; so that, although they hold a very large quantity of
air, this does not act advantageously upon the blood, in con-
sequence of the small surface over which the two are brought
together (§ 312). In Serpents we find but a single lung, that
of the other side not being developed (fig. 34) ; and this lung
extends through nearly half the length of the body. Eeptiles
have no power of filling their lungs by a process resembling
our inspiration, or drawing-in of air ; but they are obliged to
swallow it, as it were, by the action of the mouth. The skin

278 RESPIRATION OP REPTILES AND BIRDS.

of Frogs is of great importance in their respiration — in fact,
of almost as much consequence as their lungs. The necessity
for more energetic respiration increases in these animals with
the temperature, every rise in which excites them to greater
activity. During the winter, which they pass beneath the
water in a state of torpidity, the action of the water upon
their skin is sufficient to aerate their blood. When the re-
turning warmth of spring arouses them from their inaction,
they need a larger amount of respiration, and come occa-
sionally to the surface to take-in air by their lungs. And
when summer comes on, the greater heat increases their need
of respiration ; and they quit their ditches and ponds, so as
to allow the atmosphere to act upon their skin as well as
upon their lungs. If they are prevented from doing so, they
will die ; and the same result follows if the skin be smeared
with grease, so that the air cannot permeate it. Moreover, if
the lungs be removed, and the animal be made to breathe by
its skin alone, it may live for some time, if the temperature
be not high. These facts show the great importance of the
skin as a respiratory organ in Frogs.

326. The respiration of BIRDS is more active than that of

Trachea

Pulmonary vessels

___jBronchial tube

Orifice of bronchial)
tube)

("Bronchial tube
(.opened.

Fig. 161. — AIR-TUBES AND LUNGS OF BIRDS.

any other Yertebrata ; that is, they consume more oxygen,
and form more carbonic acid, in proportion to their size.

RESPIRATION OF BIRDS AND MAMMALS. 279

Their lungs, however, are not so minutely subdivided as are
those of Mammals ; but the surface over which the air can
act upon the blood is immensely extended, by a provision
which is peculiar to this class. The air introduced by the
windpipe passes not only into the lungs properly so called,
but into a series of large air-cells, which are disposed in
various parts of the body, and which even send prolongations
into the bones, especially in Birds of rapid and powerful
flight, whose whole skeleton is thus traversed by air. The
mode in which some of the bronchial tubes, or subdivisions of
the windpipe, pass from the lungs to these air-cells, is shown
in fig. 161. Now, by this arrangement, a much largtr quan-
tity of air is taken-in at once, and a much more extensive sur-
face is exposed to its action, than could otherwise be provided
for ; and as the air which is received into the air-cells has to
pass through the lungs, not only when it is taken-in, but when
it is expelled again, its full influence upon the blood is secured.

327. Birds, like Eeptiles, are destitute of the peculiar
apparatus by which Mammals are enabled to fill their lungs
with air ; but it is introduced without any effort on their
parts. For the cavity of their trunk is almost surrounded by
the ribs and breast-bone ; and the elasticity of the former
keeps it generally in a state corresponding to that of our own
lungs when we have taken-in a full breath. Thus the state
of fullness is natural to the lungs and air-cells of Birds.
When the animal wishes to renew their contents, however, it
compresses the walls of the trunk, so as to diminish its cavity
and to force out some of the air contained in the lungs, &c. ;
and when the pressure is removed, the cavity returns to its
previous size by the elasticity of its walls, and a fresh supply
of air is drawn into the lungs. The air-sacs answer the same
purpose in Birds as in Insects, diminishing the specific gravity
of the body, by increasing its size without adding to its weight,
and thus rendering it more buoyant.

328. In Man and other MAMMALS, the lungs are confined
to the upper portion of the cavity of the trunk, termed
the thorax; which is separated from the abdomen by the
diaphragm, a muscular partition, whose action in respiration
is very important. (An imperfect diaphragm is found in
some Birds, which approach most nearly to Mammals in their
general structure.) The lungs are suspended, as it were, in

280

RESPIRATORY APPARATUS OF MAMMALS.

this cavity, by their summit or apex ; and are covered by a
serous membrane termed the pleura, which also lines the
thorax, being reflected from one surface to the other precisely
in the manner of the pericardium (§ 43). Thus the pleura of the
outer surface of the lung is continually in contact with that
which forms the inner wall of the thorax ; they are both kept
moist by fluid secreted from them ; and they are so smooth,
as to glide over one another with the least possible friction.

The lungs themselves are
very minutely subdivided ;
and thus expose a vast ex-
tent of surface in proportion
to their size. The air-cells of
the human lung, into which
the air is conveyed by the
branches of the wind-pipe, and
on the walls of which the
blood is distributed, do not
average above the 1-1 00th of
an inch in diameter. In the
accompanying figure is repre-
sented, on one side, the lung,
d, presenting its natural ap-
pearance ; and on the other,
the ramifications of the air-
passages or bronchial tubes,
c, e, by which air is con-
veyed into every part of the

Fig. ^.-AIR-TUBES AND LUNG OF MAN. lungs. The trachea^ or wind-
pipe, 6, opens into the

pharynx or back of the mouth, by the larynx, a. The con-
struction of this is especially destined to produce the voice,
and will be explained under that head (Chap, xin.); but
it may be here mentioned that the entrance from the
pharynx into the larynx consists of a narrow slit, capable of
being enlarged or closed by the separation or approximation
of its lips, which form what is called the glottis. The aper-
ture of the glottis is regulated by the muscular apparatus of
the larynx ; the actions of which are not under the direct
control of the will, but are automatic, like those concerned in
swallowing (§ 194) ; and the purpose of this provision is to

RESPIRATORY APPARATUS OP MAMMALS. 281

prevent the entrance of anything injurious into the windpipe.
Thus if we attempt to breathe carbonic acid gas, which would
produce an immediately fatal result if introduced into the
lungs, the lips of this chink immediately close together, and
so prevent its entrance. The contact of liquids or of solid
substances, too, usually causes the closure of the aperture, so
that they are prevented from finding their way into the wind-
pipe ; but this does not always happen, especially when the
glottis is widely opened to allow the breath to be drawn- in
(§ 193).

329. The larynx, trachea, and bronchial tubes, to their
minutest ramifications, in all air-breathing Vertebrata, are
lined by a mucous membrane continued from the back of the
throat ; and this membrane, like the gills of aquatic animals,
is covered with cilia, which are in continual vibration. It is
obvious, however, that the purpose of this ciliary movement
must be here different from that which is fulfilled by the same
action on the surface of the gills (§ 319); and it probably
serves to get rid of the secretion which is being continually
poured out from the surface of the mucous membrane, and
which, if allowed to accumulate there, would clog up the air-
cells, and in time produce suffocation. The vibration of the
cilia is observed to be always in one direction, — towards the
outlet ; and it is in this direction, therefore, that the fluid is
gradually but regularly conveyed. The ciliary movement may
be seen to take place on the surface of the mucous membrane
of the nose ; but not on that of the pharynx, where it would
be continually interrupted by the passage of food.

330. The constant renewal of the air in the lungs is pro-
vided for, in Mammals, by a peculiar mechanism, which accom-
plishes this purpose most effectually, though itself of the
most simple character. We must recollect that the thorax in
this class is an entirely closed cavity. It is bounded above
and at the sides by the ribs (the space between which is filled
up by muscles, &c.), and below by the diaphragm, which here
forms a complete partition between the thorax and abdomen.
The whole of this cavity, with the exception of the space
occupied by the heart and its large vessels (and also by the
gullet, which runs down in front of the spine), is constantly
filled-up by the lungs. Now the size of this cavity may
be made to vary considerably; — in the first place, by the

282 RESPIRATORY MOVEMENTS OF MAMMALS.

movement of the diaphragm ; and in the second, by that of
the ribs.

331. i. The diaphragm, in a state of rest or relaxation,
forms a high arch, which rises into the interior of the chest,
as at <7, fig. 163 ; but when it contracts, it becomes much
natter (though always retaining some degree of convexity
upwards), and thus adds considerably to the capacity of the
lower part of the chest. The under side of the diaphragm is
in contact with the liver and stomach, which, to a certain

c a h

Fig. 163. — THORAX OP MAN.

degree, rise and fall with it. It is obvious that, when the
diaphragm descends, these organs, with the abdominal viscera
in general, must be pushed downwards ; and as there can be
no yielding in that direction, the abdomen is made to bulge
forwards when the breath is drawn-in. On the other hand,
when the contraction of the diaphragm ceases, the abdominal
muscles press back the contents of the abdomen, force up the

RESPIRATORY MOVEMENTS OF MAMMALS. 283

liver and stomach against the nnder side of the diaphragm,
and cause it to rise to its former height.

332. ii. The play of the ribs is rather more complicated.
These bones, c c, and c c' (to the number of twelve on each
side in Man), are attached at one end by a moveable joint to
the spinal column, a \ whilst at the other they are connected
with the sternum (breast-bone) by an elastic cartilage. Now
each rib, in the empty state of the chest, curves downwards
in a considerable degree ; and it may be elevated by a set of
muscles, of which the highest, i, are attached to the vertebrae
of the neck and to the first rib, whilst others, e, e, e (termed
intercostals), pass between the ribs. The cartilages also curve
downwards in the opposite direction, from their points of
attachment to the sternum. When the breath is drawn-in,
the first rib is raised by the contraction of the muscles, i ; and
all the other ribs, which hang, as it were, from it, would of
course be raised by this action to the same degree. But each
of them is raised a little more than the one above it, by the
contraction of its own intercostal muscle ; and thus the lower
ribs are raised very much more than the upper ones. Now
by the raising of the ribs, they are brought more nearly into
a horizontal line, as are also their cartilages ; and since the
combined length of the two is greater, the nearer they approach
to a straight line, it follows that the raising of the ribs must
throw them further out at the sides, and increase the pro-
jection of the sternum in front, thus considerably enlarging
the capacity of the chest in these directions. When the
movement of inspiration is finished, the ribs fall again, partly
by their own weight, partly by the elasticity of their carti-
lages, and partly by the contraction of the abdominal muscles
which are attached to their lower border. — For the full under-
standing of this description, it is desirable that the reader
should examine the movements of his own or another person's
chest, by placing his fingers upon different points of the ribs,
and watching their changes of position during the drawing-in
and the expulsion of the breath.

333. Now the cavity of the thorax is itself perfectly
closed; so that, if it were not for the expansion of the
lungs, a void or vacuum would be left when the diaphragm
is drawn down and the ribs elevated. The atmosphere
around presses to fill that vacuum; but this it can only

284 RESPIRATION IN MAN.

do by entering the lungs through, the windpipe, and inflating
them (or blowing them out), so as to increase their size in
proportion to the increase of the space they have to fill. In this
manner the lungs are made constantly to fill the cavity of the
chest, however great may be the increase in the latter. But
if we were to make an aperture through the walls of the chest,
the air would rush directly into its cavity, when the move-
ments of inspiration are performed, and the lung of that side 1
would not be dilated. The same thing would happen if there
were an aperture in the lung itself, allowing free communica-
tion between one of the larger bronchial tubes and the cavity
of the chest ; for the air, although still drawn-in by the wind-
pipe, would pass directly into the cavity of the chest, rather
than dilate the lung, which would thus become entirely useless.
Such an aperture is sometimes formed as the result of disease ;
and if the action of both lungs were thus prevented, death
must immediately take place from suffocation.

334. The extent of the respiratory movements varies con-
siderably ; but in general it is only such as to change about
the seventh part of the air contained in the lungs. (It may
be generally noticed, that every fifth or sixth inspiration in
Man is longer and fuller than the rest.) Their rate varies
according to age, and to the state of the nervous system ; being
faster in infants and in young persons than in adults; and
more rapid in states of mental excitement, or irritation of the
bodily system, than in a tranquil condition. In a state of rest,
from 14 to 18 inspirations take place every minute in an
adult, and at each about 20 cubic inches of air are drawn-in ;
but both the depth and frequency of the inspirations are con-
siderably increased by exercise. Taking an average alterna-
tion of activity and repose, it appears that about 360 cubic
feet of air pass through the lungs every twenty-four hours,
or 15 cubic feet every hour; and as the air which has once
passed through the lungs contains about 1-2 4th part of
carbonic acid, about 15 cubic feet of that gas, containing
nearly 8 ounces of solid carbon, are thrown-off in the course
of twenty-four hours.

335. Now carbonic acid, when diffused through the atmo-
sphere to any considerable amount, is extremely injurious to

1 Each lung has its own cavity ; the two being completely separated
from each other by the pericardium (§43).

IMPOETANCE OF FREE VENTILATION. 285

animal life ; for it prevents the due excretion by the lungs of
that which has been formed within the body ; and the latter
consequently accumulates in the blood, and exercises a very
depressing influence on the action of the various organs of the
body, but particularly on that of the nervous system. The
usual proportion is not above 1 part in 5000 ; and when this
is increased to 1 part in 100, its injurious effects begin to be
felt by Man, in headache, languor, and general oppression.
ISTow it is evident, from the statements in the last paragraph,
that, as a man produces in twenty-four hours about 15 cubic
feet of carbonic acid, if he were inclosed in a space containing
1500 cubic feet of air (such as would exist in a room 15 feet
by 10, and 10 feet high), he would in twenty-four hours
communicate to its atmosphere from his lungs as much as
1 part in 100 of carbonic acid, provided that no interchange
takes place between the air within and the air outside the
chamber. The amount would be further increased by the car-
bonic acid thrown off by the skin, the quantity of which has
not yet been determined.

336. In practice, such an occurrence is seldom likely to
take place; since in no chamber that is ever constructed,
except for the sake of experiment, are the fittings so close as
to prevent a certain interchange of the contained air with
that on the outside. But the same injurious effect is often
produced by the collection of a large number of persons for a
shorter time, in a room insufficiently provided with the means
of ventilation. It is evident that if twelve persons were to
occupy such a chamber for two hours, they would produce the
same effect with that occasioned by one person in twenty-four
hours. Now we will suppose 1200 persons to remain in a
church or assembly-room for two hours ; they will jointly
produce 1500 cubic feet of carbonic acid in that time. Let
the dimensions of such a building be taken at 100 feet long,
50 broad, and 30 high ; then its cubical content will be
(100 x 50 x 30) 150,000 cubic feet. And thus an amount
of carbonic acid, equal to 1-1 00th part of the whole, will be
communicated to the air of such a building, in the short space
of two hours, by the presence of 1200 people, if no pro-
vision be made for ventilating it. And the quantity will
be greatly increased, and the injurious effects will be pro-
portionably greater, if there be an additional consumption of

286 IMPORTANCE OF FREE VENTILATION.

oxygen, produced by the burning of gas-lights, lamps, or
candles.

337. Hence we see the great importance of providing for
free ventilation, wherever large assemblages of persons are
collected together, even in buildings that seem quite adequate
in point of size to receive them ; and much of the weariness
which is experienced after attendance on crowded assemblies
of any kind, may be traced to this cause. In schools, facto-
ries, and other places where a large number of persons remain
during a considerable proportion of the twenty-four hours, it
is impossible to give too much attention to the subject of
ventilation ; and as, the smaller the room, the larger will be
the proportion of carbonic acid its atmosphere will contain,
after a certain number of persons have been breathing in it
for a given time, it is necessary to take the greatest precaution
when several persons are collected in those narrow dwellings,
in which, unfortunately, the poorer classes are compelled to
reside. Even the want of food, of clothing, and of fuel, are
less fertile sources of disease than insufficient ventilation;
which particularly favours the spread of contagious diseases,
on the one hand by keeping-in the poison, and thus concen-
trating it upon those who expose themselves to its influence ;
and, on the other, by obstructing the elimination of the waste
matter from the system, the presence of which in the blood
renders it peculiarly liable to be acted-on by all poisons
having the nature of " ferments."

338. When the quantity of carbonic acid in the air accu-
mulates beyond a certain point, it speedily produces suffocation
and death. This is occasioned by the obstruction to the flow
of blood through the capillaries of the lungs, which takes
place when it is no longer able to get rid of the carbonic acid
with which it is charged, and to absorb oxygen in its stead.
The general principle to which this stagnation may be referred
has already been noticed (§ 280). Now, as all the blood of
the system, in warm-blooded animals, is sent through the
lungs before it is again transmitted to the body, it follows
that any such obstruction in the lungs must bring the whole
circulation to a stand. The functions of the nervous system
are directly dependent upon a constant supply of arterial
blood (Chap, x.) ; and, accordingly, as this supply becomes
progressively diminished in quantity and deteriorated in qua-

SUFFOCATION FROM WANT OP RESPIRATION. 287

lity, its actions first become irregular, producing violent con-
vulsive movements, and at last cease altogether, the animal
becoming completely insensible. In this condition, which is
termed A. sphyxia, the pulmonary arteries, the right side of the
heart, and the large veins which empty themselves into it, are
gorged with dark blood ; whilst the pulmonary veins, the left
side of the heart, and the arteries of the system, are compara-
tively empty. Hence the action of the right side of the heart
comes to an end, through a loss of power in its walls, occa-
sioned by their being over-distended ; whilst that of the left
side ceases for want of the stimulus of the contact of blood,
by which the muscular fibre is excited. If this state be
allowed to continue, death is the consequence ; but if the
carbonic acid in the lungs be replaced by pure air, the flow of
blood through their capillaries recommences, — the right side
of the heart is unloaded and begins to act again, — arterial
blood is sent to the left side, and excites it to renewed motion,
— and the same being propelled by it to all parts of the body,
their several functions are restored, the nervous system re-
covers its power of acting, and all goes on as before. These
changes occur in exactly the same manner when a warm-
blooded animal is made to breathe nitrogen or hydrogen ;
since these gases do not perform that which it is the office of
oxygen to effect, — the removal of carbon from the system, in
the form of carbonic acid. And they also take place in a
perfectly pure atmosphere, when the individual is prevented
from receiving it into its lungs by an obstruction to its passage
through the windpipe, such as that produced by hanging,
strangulation, drowning, &c. For the air in the lungs, not
being renewed, speedily becomes charged with carbonic
acid, to an extent that checks the circulation through their
capillaries; and all the consequences of this follow as
before.

339. The most efficient remedy in all such cases is evidently
that suggested by the facts stated in the last paragraph, —
the renewal of the air in the lungs. But with this, other
means should be combined; and the general directions1
of Dr. Marshall Hall, with the method of producing artificial

1 The instructions, though specially intended for the resuscitation of
persons apparently drowned, are applicable with slight modification to
other forms of Asphyxia.

288 TREATMENT OF THE APPARENTLY-DROWNED.

respiration suggested by Dr. H. E. Silvester, seem most likely
to answer in practice : —

Treat the patient instantly, on the spot, in the open air,
exposing the face and chest to the breeze, except in severe
weather.

i. To clear the Throat, place the patient gently on the
face, with one wrist under the forehead — (all fluids and the
tongue itself then fall forwards, leaving the entrance into the
windpipe free). If there be breathing, wait and watch ; if
not, or if it fail, —

ii. To excite Respiration, turn the patient well and in-
stantly on his side, and excite the nostrils with snuff, or the
throat with a feather, &c., and dash cold water on the face
previously rubbed warm. If there be no success, lose not a
moment, but instantly, —

in. To imitate Respiration, lay the patient on his back,
with the head and shoulders slightly elevated ; then let the
arms be raised and steadily extended upwards, by the sides of
the head, so as to draw-up the shoulders. In this way, the
ribs are drawn-up by the muscles passing to them from the
shoulders, and the cavity of the chest is enlarged. If the arms
be then carried-down to the sides of the body, the shoulders
fall, the ribs are lowered, and the sides of the thorax approach
one another, as in natural expiration, — an effect which
may be increased by moderate pressure on the front and
sides of the chest. By an alternation of these movements,
an artificial Inspiration and Expiration will be effected,
which, though imperfect, may restore life.

iv. To induce Circulation and Warmth, meantime rub
the limbs upwards, with firm grasping pressure and with
energy, using handkerchiefs, &c. (by this measure the blood
is propelled along the veins towards the heart) ; let the limbs
be thus warmed and dried, and then clothed, the bystanders
supplying the requisite garments j avoiding the continuous
warm bath, and the position on, or inclined-to, the back.

340. The ordinary movements of respiration belong, like
those of swallowing, to the class of reflex actions (§ 430). We
have seen that, in every such movement, a great number of
muscles are called into play simultaneously; and this is
effected by means of the stimulus which is sent to them from
the spinal cord. But this stimulus does not originate there ;

CAUSE OF RESPIRATORY MOVEMENTS. 289

for it is the consequence of impressions conveyed to the spinal
cord, and especially to its upper end, by several nerves, — some
originating in the lungs, and others in the general surface.
The nerves originating in the lungs convey to the spinal cord
the impression produced by the presence of venous blood in
their capillaries : of this impression we are not ordinarily
conscious ; but if we hold our breath for a few moments, we
become aware of it ; and it speedily becomes so distressing as
to force us to breathe, even though we may try to resist it by
an effort of the will. The impression conveyed by the nerves
of the general surface is chiefly that produced by the applica-
tion of cold to the skin. It is this which is the cause of the
first inspiration in the new-born infant ; which is not unfre-
quently prevented by the seclusion of its face (the part most
capable of receiving this impression) from the influence of the
air. Every one knows that, when the face is dipped into
water, an inspiratory movement is strongly excited ; and the
same happens when a glass of water is dashed over the face.
This simple remedy will often put a stop to hysterical laughter,
by producing a long sighing inspiration. A still- stronger
tendency to draw-in the breath is experienced in the first
dash of water over the body in the shower-bath. The respi-
ratory movements, in the higher Animals, are placed under
the control of the will, to a certain extent, because on them
depend the production of sounds, and in Man the actions of
speech ; but that they are quite independent of the will, and
even of sensation, is shown by the fact that they will continue
after the brain has- been completely removed, provided the
spinal cord and its- nerves are left without injury. In most
of the Invertebrata they are connected with distinct ganglia,
which minister to them alone. (See Chap, x.)

341. The actions of sighing, yawning, sobbing, laughing,
coughing, and sneezing, are nothing else than simple modifica-
tions of the ordinary movements of respiration, excited either
by mental emotions, or by some stimulus originating in the
respiratory organs themselves. Sighing is nothing more than
a very long-drawn inspiration, in which a larger quantity of
air than usual is made to enter the lungs. This is continually
taking place in a moderate degree, as already noticed (§ 334) ;
and we notice it particularly, when the attention is released
after having been fixed upon an object which has excited it

u

290 VARIOUS MOVEMENTS CONNECTED WITH RESPIRATION.

strongly, and which, has prevented our feeling the insufficiency
of the ordinary respiratory movements. Hence this action is
only occasionally connected with mental emotion. Yawning
is a still deeper inspiration, which is accompanied by a kind
of spasmodic contraction of the muscles of the jaw, and also
by a very great elevation of the ribs, in which the shoulders
and arms partake. The purely involuntary character of this
movement is sometimes seen in a remarkable manner in cases
of palsy, in which the patient cannot raise his shoulder by an
effort of the will, but does so in the act of yawning. Never-
theless the action may be performed by the will, though not
completely ; and it is one that is particularly excited by an
involuntary tendency to imitation, as every one must have
experienced who has ever been in company with a set of
yawners. Sobbing is the consequence of a series of short
convulsive contractions of the diaphragm ; and it is usually
accompanied by a closure of the glottis, so that no air really
enters. In Hiccup, the same convulsive inspiratory movement
occurs, the glottis closing suddenly in the midst of it ; and
the sound is occasioned by the impulse of the column ol
air in motion against the glottis. In Laughing, a precisely
reverse action takes place ; the muscles of expiration are in
convulsive movement, more or less violent, and send out the
breath in a series of jerks, the glottis being open. This some-
times goes on until the diaphragm is more arched, and the
chest more completely emptied of air, than it could be by an
ordinary movement of expiration. The act of Crying, though
occasioned by a contrary emotion, is, so far as the respiration
is concerned, very nearly the same. We all know the effect
of mixed emotions in producing something " between a laugh
and a cry."

342. The purposes of the acts of coughing and sneezing are,
in both instances, to expel substances from the air-passages,
which are sources of irritation there ; and this is accomplished
in both by a violent expiratory effort, which sends forth a
blast of air from the lungs. — Coughing occurs when the source
of irritation is situated at the back of the mouth, in the
trachea, or bronchial tubes. The irritation may be produced
by acrid vapours, or by liquids or solids that have found
their way into these passages, or by secretions which have
been poured into them in unusual quantity as the result of

COUGHING AND SNEEZING — AQUEOUS EXHALATION. 291

disease ; and the latter will be the more likely to produce the
effect, from the irritable state in which the lining membrane
of the air-passages already is. The impression made upon
this membrane is conveyed by the nerves spread out beneath
its surface to the spinal cord; and the motor impulses are
sent to the different muscles, which they combine in the act
of coughing. This act consists, 1st, in a long inspiration,
which fills the lungs ; 2d, in the closure of the glottis at the
moment when expiration commences ; and 3d, in the burst-
ing-open, as it were, of the glottis, by the violence of the
expiratory movement, so that a sudden blast of air is forced
up the air-passages, carrying before it anything that may offer
an obstruction. — Sneezing differs from coughing in this, that
the communication between the larynx and the mouth is
partly or entirely closed, by the drawing-together of the sides
of the veil of the palate over the back of the tongue ; so that
the blast of air is directed more or less completely through
the nose, in such a way as to carry-off any source of irritation
that may be present there.

343. Every one is aware that the air he breathes-forth con-
tains a large quantity of vapour : this is not perceptible in a
warm atmosphere, because the watery particles remain dis-
solved in it and do not affect its transparency ; but in a cold
atmosphere they are no longer held in solution, and conse-
quently present the appearance of fog or steam. The quantity
of fluid which thus passes off is by no means trifling, —
probably not less than from 16 to 20 ounces in the twenty-
four hours; a portion of it undoubtedly proceeds from the
moist lining of the mouth, throat, &c., but the greater part
is thrown-off by the lungs themselves. This fluid, when col-
lected, is found to contain a good deal of decomposing organic
matter, especially in cases in which the respiratory process
has not been carried on with perfect freedom; such matter
being oxydized and thrown-off under other forms, when the
blood is duly aerated. Various substances of an odoriferous
character, which have been taken into the blood, manifest
their presence in this exhalation : thus turpentine, camphor,
and alcohol, communicate their odour to the breath; and
when the digestive system is out of order, the breath fre-
quently acquires a disagreeable taint, from the reception of
putrescent matters into the blood, and their exhalation through

u2

292 ABSORPTION OF VAPOUR — POISONOUS GASES.

this channel. — Of the water of the blood, from which this
exhalation is given-off, a small part is most probably formed
by the direct union of the hydrogen contained in the food
(especially when this is one of its predominating components,
§ 153) with the oxygen absorbed. For it has been found by
careful experiment, that the proportion of inspired oxygen
which disappears (not being contained in the carbonic acid
expired, § 305), is much greater in animals that are fed on a
flesh diet, than in those living on farinaceous food. Another
portion of such oxygen probably unites with the sulphur and
phosphorus of the food and tissues, to form sulphuric and
phosphoric acids, which are excreted through the kidneys in
combination with alkaline bases (§ 367).

344. Certain gases act as violent poisons, even when respired
in very small proportion. Thus, a Bird is speedily killed by
breathing air which contains no more than 1-1 500th part of
sulphuretted hydrogen ; and a Dog will not live long in an
atmosphere containing 1 -800th part of this gas. The effects
of carburetted hydrogen are similar ; but a larger proportion
is required to destroy life. Both these gases are given-off by
decomposing animal and vegetable matter ; the neighbourhood
of which is consequently very injurious to health. Several
cases of arsenical poisoning have occurred, from the accidental
inhalation of a small quantity of arseniuretted hydrogen, the
amount of arsenic contained in which must have been so
minute as to be scarcely appreciable.

CHAPTER YIL

OF EXCRETION AND SECRETION.

General Purposes of the Excreting Processes.

345. WE have seen that the Blood, in the course of its
circulation, not only deposits the materials that are converted
into the several fabrics of which the body is composed, but
also takes-up into itself the products of the decomposition
which is continually going-on in its various parts ; and it is
to replace this, that the constant Nutrition of the tissues is
required. In order that the blood may retain its fitness foi

OF EXCRETION AND SECRETION. 293

ae purposes to which it is destined, it is requisite that these
>roducts should be drawn-off from the current of the circula-
don, as constantly as they are received into it ; and this is
accomplished by the various processes of Excretion, which are
continually taking place in different parts of the body. The
uninterrupted performance of these is even more essential to
the maintenance of life, than is an uninterrupted supply of
nutritive materials ; for an animal may continue to exist for
some time without the latter, but it speedily dies if either
of the more important excretions be checked. We have a
striking instance of this in the case of the Eespiration, which
may be regarded as a true function of Excretion, having for
its object to set free Carbonic acid from the blood in a gaseous
form, — thereby contributing to the introduction of Oxygen
into the blood, for the various important actions to which
that element is subservient, especially the maintenance of
Animal Heat. (Chap, ix.) The effects of the suspension of
vhe respiratory process, even for a few minutes, in a warm-
)looded animal, have been shown (§ 338) to be certainly and
speedily fatal ; and they are as certainly fatal in the end in
cold-blooded animals, though a longer time is required to
produce them.

346. The products of excretion are the same, as to their
essential characters at least, through the whole Animal king-
dom ; and for this it is not difficult to find a reason. It will
be remembered that the ultimate elements of the Animal
tissues are four in number : oxygen, hydrogen, carbon, and
nitrogen ; and that the materials which make up the chief
part of the fabric of different classes of animals — albumen,
gelatin, fatty matter, &c. — contain these elements united in
constant proportions, from whatever source we obtain them.
Hence we should expect to find the products of their decom-
position also the same ; and this is, for the most part, the
case. Of these four ingredients, oxygen can never be said
(in the healthy state at least) to be superfluous in the body ;
for a large and constant supply of it is required, to unite with
the others and carry them off in their altered conditions.
Thus, unless oxygen were continually introduced into the
system, for the sake of uniting with the carbon that is to be
thrown off by Respiration, that excretion must be checked ;
and it is required, in like manner, for uniting with hydrogen

294 NATURE AND OBJECTS OF EXCRETORY ACTIONS.

to form water, and with compounds of nitrogen to form urea.
Hence there is no need of an organ to carry off the super-
fluous oxygen ; but an organ to introduce it is rather required ;
and this purpose, as we have seen, is answered by the Respi-
ratory apparatus. But we find organs of excretion specially
destined to carry off the carbon, hydrogen, and nitrogen, which
are set free, under various forms, by the decomposition of the
tissues. Thus the Respiratory organs, as we have seen, throw
off carbon in the form of carbonic acid, and hydrogen which
has been in like manner united with oxygen so as to form
water. The Liver has for its office partly to separate these
same elements from the blood in a different form, throwing
them off in the condition of a peculiar fatty matter, which
consists almost entirely of carbon and hydrogen. But it has
another function of no less importance in animals whose
respiration is active ; for by its agency the hydro-carbonaceous
matter circulating in the blood is brought into a state in
which it readily combines with oxygen to form carbonic acid
and water ; and thus the liver may be said to prepare the
pabulum for the combustive process. Lastly, the Kidneys
have for their chief object to throw off the azotized compounds
which result from the decomposition of the tissues ; these
contain a very large proportion of azote or nitrogen, which is
united with the other elements into the crystalline compounds,
urea, and uric or lithic acid, the latter of which is usually
thrown off in combination with soda or ammonia And the
kidneys further serve as the channel through which soluble
matters of various kinds, which have found their way into
the current of the circulation, and are foreign to the composi-
tion of the blood, are eliminated from it.

347. It is obvious that, when an animal has retained its
usual weight for any length of time without change, the total
weight of its excretions must be equivalent to the total weight
of the solids and fluids it has taken-in. If these last have been
no more in amount than was absolutely necessary for the main-
tenance of the body during that period, all the azotized portion
of the food was first appropriated to the formation of the
azotized tissues ; whilst the non-azotized portion was used-up
i»n maintaining the respiration (§ 157), Consequently, no
part of the food would pass at once into the biliary and urinary
excretions ; and these would have no other function than to

EXCRETION OF SUPERFLUOUS AZOTIZED NUTRIMENT. 295

separate or strain-off, as it were, the products of the decompo-
sition of the tissues formed from it, when their term of life
had expired (§ 161). But it is certain that Man (as well as
other animals which have in some degree learned his habits)
frequently consumes much more food than is necessary for
the supply of his wants ; and a little consideration will show,
that the surplus must pass-off by these excretions, without
ever forming part of the living fabric. For new muscular
tissue is not formed in proportion to the quantity of aliment
supplied, but in proportion to the demand created by the
exercise of it (§ 587); consequently, if more food be taken-in
than is necessary to supply that demand, no use can be
made of it. We never find that a Man becomes more fleshy
by eating a great deal and taking little exercise ; indeed, the
very contrary result happens, his flesh giving place to fat.
But let him put his muscles to regular and vigorous exercise,
and eat as much as his appetite demands, and they will then
increase both in strength and bulk.

348. Hence, if more azotized food be taken-in, than is
required to supply the waste of the muscular and other azotized
tissues, the surplus must be carried-off by the organs of
excretion — chiefly, indeed almost entirely, by the Kidneys.
By throwing upon them more than their proper duty, they
become disordered and unable to perform their functions ;
hence the materials which they ought to separate from the
blood accumulate in it, and give rise to various diseases of a
more or less serious character, which might have been almost
certainly prevented by due regulation of the diet. The most
common of these diseases among the higher classes are Gout
and Gravel ; both of these may be often traced to the same
cause, — the accumulation in the blood of lithic acid, which
results from the decomposition of the superfluous azotized
food, and which the kidneys are not able to throw-off in the
proper state, that is, dissolved in water. That these diseases
are, comparatively speaking, rare among the lower classes, is
at once accounted-for by the fact, that they do not take-in
any superfluous azotized food; — all that they consume being
appropriated to the maintenance of their tissues, and the
kidneys having only to discharge their proper function of
removing from the blood the products of the decomposition
of these.

296 EXCRETION OF SUPERFLUOUS NON-AZOTIZED NUTRIMENT.

349. Hence the radical cure of these diseases, in most
persons who have a sufficiently vigorous constitution and firm
resolution to adopt it, is abstinence from all azotized nutri-
ment— whether contained in animal flesh, bread, or other
articles of vegetable diet, — save such as is required to supply
the wants of the system. If such abstinence be carried too
far, however, it will produce injurious instead of beneficial
results, weakening the fabric, and impairing the digestive
powers ; and if food be employed of a kind which is liable to
produce lactic acid (the acid that appears in milk, when it turns
sour), much disorder may still remain, which must be avoided by
using the kind of diet that is least liable to undergo this change.

350. Again, if more non-azotized food is taken into the
system than can be got rid of by Eespiration, it must either
be deposited as fat, or it must be separated from the blood,
and carried-off by the excretion of the Liver. But if too
much work be thrown upon this organ, its function becomes
disordered, from its inability to separate from the blood all
that it should draw-off ; the injurious substances accumulate
in the blood, therefore, producing various symptoms that are
known under the general term of bilious ; and to get rid of
these, it becomes necessary to administer medicines (especially
those of a mercurial character) which shall excite the liver to
increased secretion. The constant use of these medicines has
a very pernicious effect upon the constitution; and careful
attention to the regulation of the diet, and especially the
avoidance of a superfluity of oily or farinaceous matter, will
generally answer the same end in a much better manner.

351. That the materials of the Biliary and Urinary excre-
tions pre-exist (like the carbonic acid thrown-off by respiration)
in the blood, in forms which, if not identical, are at any rate
closely allied to those under which they present themselves
in the bile and urine, has now been fully proved. The
quantity of them present in the circulating fluid, however, is
usually very small ; for the simple and obvious reason that,
if the excreting organs are in a state of healthy activity, these
substances are drawn-off by them from the blood, as fast as
they are introduced into it. But if the excretions be checked,
they speedily accumulate in the blood, to such a degree as to
be easily detected by the Chemist, and also to make their
presence evident by their effects upon the animal functions,

NATURE AND PURPOSES OF ANIMAL SECRETIONS. 297

especially those of the nervous system. This sometimes
happens in consequence of disease, and it may be imitated by
experiment ; for when the trunk of the blood-vessel convey-
ing the blood to the liver or kidney is tied, the excretion is
necessarily checked, and the same results take place as when
the stoppage has depended on want of secreting power. The
biliary and urinary matters have the effect of narcotic poisons
upon the brain ; when they have accumulated in the blood,
their symptoms begin to manifest themselves ; and these
symptoms increase in intensity, as the amount of the sub-
stances becomes augmented, until death takes place.

352. Besides the Excretions, we find various Secretions
elaborated in different parts of the bodies of animals, with a
view not so much to the purification of their blood, as to the
fulfilment of special purposes in their economy. These vary
considerably in the different classes of animals ; though some
of them, being concerned in functions almost universally per-
formed, are equally general in their range. Thus we find the
Salivary and Gastric fluids poured into the mouth and stomach,
for the reduction and solution of the food (§§ 190 and 204);
and the Lachrymal secretion poured out upon the surface of
the eye, for the purpose of washing it from impurities (§ 541) :
while the secretion of Milk for the nourishment of the
young is limited to Mammals ; and poisonous secretions are
formed in Serpents ami Insects, for the destruction of their
prey or for means of defence. Any one of these may be
checked, without rendering the blood impure by the accumu-
lation of any substances that should be drawn-off from it ;
but its cessation may produce effects fully as injurious, by
disordering the function to which it is subservient. Thus, if
the salivary and gastric secretions were to cease, the reduction
of the food could not be effected, and the animal must starve,
though its stomach were filled with wholesome aliment. — It
is to be observed, in regard to nearly all these secreted fluids,
that they contain but a small quantity of solid matter, and
that this matter seems to be formed from the albumen of the
blood by a process of incipient decomposition, which gives it
the character of a " ferment."

353. The various acts of Secretion and Excretion which are
continually taking place in the living body, are, like those of
Nutrition, completely removed from the influence of the will;

298 INFLUENCE OF EMOTIONS UPON SECRETIONS.

but they are strongly affected by emotions of the mind. This
has been already pointed out in regard to the Saliva (§ 190);
and it is equally evident in the case of the Lachrymal secre-
tion (§ 541). In these instances, however, the effect of the
emotion is manifested upon the quantity only of the secretion ;
in the case of the secretion of Milk, not only the quantity "but
quality is greatly influenced by the mental state of the nurse.
The more even her temper, and the more free from anxiety
her mind, the better adapted will be her milk for the nourish-
ment of her offspring. There are several instances now on
record, in which it has been clearly shown, that the influence
of violent passions in the mother has been so strongly exerted
upon the secretion of milk, as almost instantaneously to com-
municate to it an absolutely poisonous character, which has
occasioned the immediate death of the child. 1 The influence of
emotional states upon the Secretions is probably communicated
by the Sympathetic system of Nerves (§ 461), which is very
minutely distributed, with the blood-vessels, to the several
glands which form them.

Nature of the Secreting Process. — Structure of the Secreting
Organs.

354. Notwithstanding the different characters of the pro-
ducts of Secretion and Excretion, and the variety of the pur-
poses to which they are destined to be applied, the mode in
which they are elaborated or separated from the blood is
essentially the same in all. The process is performed, in the
Animal, as in the Plant, by the agency of cells; and the
variety of the structure of the different Glands, or secreting
organs, has reference merely to the manner in which these,
their essential parts, are arranged. The simplest condition
of a secreting cell, in the animal body, is that in which it
exists in Adipose or fatty tissue ; which is composed, as
formerly explained (§ 46), of a mass of cells, bound together
by areolar tissue that allows the blood-vessels to gain access
to them. Every one of these cells has the power of secreting
or separating fatty matter from the blood ; and the secreted
product remains stored-up in its cavity, as in Plants (VEGET.

1 See the Author's Principles of Human Physiology, chap. xv. ; and
Dr. A. Combe on the Management of Infancy, chap. x.

ESSENTIAL STRUCTURE OF SECRETING ORGANS. 2? 9

PHYS. § 324) — not being poured forth, as it is in most other
cases, by the subsequent bursting of the cell.

355. But when the secreting cells are disposed on the
surface of a membrane, instead of being aggregated in a mass,
it is obvious that, if they burst or dissolve-away, their contents
will be poured into the cavity bounded by that membrane;
and this is the mode in which secretion ordinarily takes
place. Thus, the Mucous Membranes (§ 39) are covered with
epithelium-cells, which are continually being cast-off, and
which are replaced as constantly by a fresh crop ; and they
form by their dissolution the glairy viscid substance termed
mucus, which covers the whole surface of the membrane,
and serves for its protection. In parts of the membrane
where it is necessary that the secretion should be peculiarly
abundant, we find its secreting surface greatly increased,
by being prolonged into vast numbers of little pits or bags,
termed follicles, which are lined with epithelium-cells, that
resemble those of its general surface (see fig. 9). Such
follicles are very abundant along the whole alimentary canal
of Man ; and the glandulae in which the Gastric and Intes-
tinal fluids are elaborated, are almost equally simple in their
structure (§ 204).

356. Now although the most
important Secretions and Ex-
cretions are separated, in Man
and the higher animals, by
organs of a much more com-
plex nature, yet in the lower
we find them generated after
the same simple fashion. Thus
in the little Bowerbankia (§
115), the bile is secreted by
minute follicles which are
lodged in the walls of the
stomach (fig. 64, c) and pour
their secretion separately into
its cavity, having no communi-
cation with one another. In
more complex forms of glan-
dular structure, however, several follicles open together
into a tube, which discharges the product of their secretion

Fig. 164.— GLAND THAT SECRETES THE
ACRID FLUID DISCHARGED BY THE
BOMBARDIER BEETLE.

300

ESSENTIAL STRUCTURE OF SECRETING ORGANS.

(fig. 164) j and thus the entire mass may be composed of
numerous lobules, each having its own duct. Passing to still
higher forms, we find all the ducts coalescing into a common
trunk, so that the gland bears a strong resemblance to a bunch
of grapes ; as is seen in fig. 1 65, which represents the structure

Fig. 165. — INTIMATE STRUCTURE OF A COMPOSITE GLAND (THE PAROTID).

of the Parotid (one of the salivary glands) of Man. The
main stalk is the duct into which all the others enter ; from
this pass off several branches, and these again give off smaller

twigs, the extremities of which
enter the minute follicles in
which the secretion is formed.
These follicles are lined, as in
their simple condition, with cells,
which are the essential instru-
ments in the production of the
secretion; the fluid which they
separate is poured, by the
giving-way of their walls, into
the small canals proceeding
from the follicles, thence into
the larger branches, and finally
into the main trunk, by
into the
is to be

employed or from which it is to pass out. The Liver will
be seen to possess a structure exactly resembling this, in the

Fig. 166. — PORTION OF ONE OF THE
TUBI;LI URINIFERI OF THE
HUMAN KIDNEY;

Showing its lining of flattened epithe- which it is Carried

Uum ceils. situation where it

ESSENTIAL STRUCTURE OF SECRETING GLANDS.

301

Crustacea, by referring to fig. 47, fo ; and in the Mollusca it is
nearly the same (figs. 157, /, and 149,/).

357. The required extent of secreting surface is not unfre-
quently given, however, by the prolongation of the follicles
into tubes, rather than by a great multiplication in their
number. Of this we have a remarkable example in the
Kidney of the higher animals (§ 368), which is entirely com-
posed of such tubes, together with areolar tissue which binds
them together, and the blood-vessels distributed amongst them.
These tubes, like the follicles, are lined with epithelium-cells
(fig. 166), which are the real instruments in the separation of
their secreted product.

358. That there is nothing in the form of any secreting
apparatus, however, which determines the peculiar nature of
its secretion, is evident from this

fact, — that, in glancing through the
Animal series, we find the same secre-
tion elaborated by glandular struc-
tures of every variety of form. Thus,
we have seen that the bile is secreted,
in the lowest animals in which we
can distinguish it, by a number of
distinct follicles, as simple in their
structure as are those by which the
mucous secretions are formed in the
highest. Again, the bile is secreted
in Insects, by a small number of long
tubes, which open separately into the
intestinal canal j ust below the stomach
(fig. 112); and these tubes appa-
rently differ in no respect from those
that form the urinary secretion in the
same animals, which open nearer the
outlet of the intestinal canal. In
fact, the distinct function of the
latter was not known, until it was
ascertained that uric acid is to be
found in them. In fig. 167, which
represents the digestive apparatus of the Cockchafer, it is
seen that the biliary vessels are only four in number, but
are very long ; and that, for a good part of their length,

Fig. 167.

ALIMENTARY CANAL AND
HEPATIC TUBULES OF COCK-

CHAFER.

302 ESSENTIAL STRUCTURE OF SECRETING GLANDS:

they are beset with a series of short tubes opening from
them, by which the extent of secreting surface is much in-
creased.— On the other hand, although the urinary secretion
is generally formed by long tubes, yet in the Mollusca it is
secreted by follicles, according to the general plan of their
glandular structures.

359. The secreting cells not unfreqnently possess the power
of elaborating a peculiar colouring matter, either separately,
or along with the substances which seem more characteristic
of the secretion. Thus the ink of the Cuttle-fish is in reality
its urine, charged with a quantity of black matter formed in
the pigment-cells (resembling those of the interior of the eye,
§ 533) that line its ink-bag ; and the corresponding secretion
in other Mollusca is rendered purple by the same cause.
The bile seems to be universally tinged with a yellow or
greenish colouring matter, which may be regarded, therefore,
as an essential part of the secretion ; and the urine of Mam-
mals is also tinged by a yellow pigment, which seems related
in its nature to that of the bile. In all these pigments, carbon
is the predominating ingredient ; and their amount is increased
when the respiratory process is insufficiently performed.

360. It appears, then, that the different secreting cells have
the power of elaborating a great variety of products ; and that
no essential differences can be discovered in the structure of
the glands into whose composition they enter, which can
account for that variety. We are entirely ignorant, therefore,
of the reason why one set of cells should secrete biliary matter,
another urea, another a colouring substance, and so on ; but
we are as ignorant of the reason why, in the parti-coloured
petal of a flower, the cells of one portion should secrete a red
substance, whilst those in immediate contact with it form a
yellow or blue colouring matter ; and we know as little of the
cause, which occasions one set of the cells of which the embryo
is composed to be converted into muscular tissue, another
into cartilage, and so on,

361. One of the most curious points in the Physiology of
Secretion, is the interchange which sometimes occurs in the
functions of particular glands. "When the operation of some
one gland is checked or impaired by disease, it not unfrequently
happens that another gland, or perhaps only a secreting sur-
face, will perform its functions more or less perfectly; this

RECEPTACLES FOR SECRETED PRODUCTS. 303

happens most frequently in regard to the important Excretions,
as if Nature had especially provided for their continued sepa-
ration from the blood, that its purity may be unceasingly
maintained. Thus the urinary secretion has been passed off
from the surfaces of the skin, stomach, intestines, and nasal
cavity, and also from the mammary gland; the colouring
matter of the bile, when it accumulates in the blood (as in
jaundice), is separated from it in the skin and conjunctival
membrane of the eye (§ 537) ; and milk has been poured
forth from pustules on the skin, and from the salivary glands,
kidneys, &c. Such cases have been regarded as fabulous ;
but they rest upon good authority, and they are quite consistent
with physiological principles.

362. Some of the main ducts or channels, through which
the glands pour forth their secretions, are provided with
enlargements or receptacles, which serve to retain and store
up the fluid for a time, until it may be desirable or convenient
that it should be discharged. Thus, in most of the higher
animals, the duct which conveys into the intestinal tube the
bile secreted by the liver, is also connected with a receptacle
termed the gall-bladder ; the bile, as it is secreted, passes into
this, and is retained there until it is
wanted for assisting in the digestive
process (§ 213); when it is pressed out
into the intestinal canal. It is a curious
fact, that in most persons who die of
starvation, the gall-bladder is found dis-
tended with bile; showing that the
secretion has continued, although it has
not been poured into the intestine for
want of the stimulus occasioned by the
presence of food in the latter. In many
quadrupeds, especially those of the
Euminant tribe, the milk-ducts are in
a, kidneys; 6, ureters; e, ^ke manner dilated into a large re-
hiadder ; d, its canal, the ceptacle, the udder, which retains the

urethra. , . •.!_•/» i . -i .-i

secretion as it is formed, until the
period when it is needed. In all Mammals, and in some
Eeptiles, Mollusks, and Insects, but not in Birds or Fishes,
we find the ureters, which convey away the urinary excretion
from the kidneys, dilated at their lower extremity into a

*. /*

Fig. 168.— URINARY AP-
PARATUS.

304

STRUCTURE OF THE LIVER.

bladder (fig. 168), which serves to retain all the fluid that is
poured forth by the gland during a considerable length of
time, and thus prevents the necessity for its being continually
passed out of the body.

Characters of Particular Secretions.

363. In nearly all animals, the Liver holds the first rank
among Glands or secreting organs, in regard both to its size
and to the obvious importance of its function. The principal
varieties of its plan of structure in the Invertebrated classes
having been already noticed (§ 356), we shall here limit
ourselves to a sketch of that peculiar arrangement of its
elementary parts, which presents itself in Man and other
Yertebrata. The position of this organ in the abdominal
cavity is shown in fig. 30. It is chiefly composed of a mass
of cells of a flattened spheroidal form (fig. 169, B), the dia-
meter of which is usually from l-800th to 1-1 600th of an inch ;
each cell presents a distinct nucleus, which is surrounded by
yellow biliary matter in a finely granular condition ; and in
the midst of this there are usually one or two large fatty
globules, or five or six small ones. The quantity of fat in
the liver is very liable to increase, however, when there is a
large amount of oily or fatty matter in the food, or when the
respiratory function is not performed with sufficient activity.

The hepatic cells are
clustered together into
lobules of irregular form,
but about the average
size of a millet-seed;
these lobules are disposed
upon the ramifications of
the hepatic vein (fig.
169, A), like leaves upon
the branches of a tree j
and they are separated
from one another by the
peculiar distribution of
the " portal " vessels and
of the hepatic ducts.
The Vena Portce, it will be remembered, collects the blood
that has been distributed to the alimentary canal, and conveys

Fig. 169.— PORTION OF THE HUMAN LIVER.

A, Showing the manner in which the substance
of its lobules is disposed around the branches
of the hepatic vein a ; B, cells of which the
lobules are composed, more highly magnified.

STRUCTURE OF THE LIVER.

305

it to the liver, through, which it is distributed by the sub-
divisions of this vessel, which acts the part of an artery
(§ 267). Its branches proceed to the surfaces of the lobules,
amidst which they form by mutual inosculation a tolerably
regular network (fig. 170, 6, 6, 6); and from these branches a

Fig. 170. — TRANSVERSE SECTION OF THREE LOBULES OF THE LIVER;

Showing the passage of the ramifications of the portal vessels from the network
b b bb, which surrounds the lobules, towards the centre of each lobule, near
which they become continuous with the rootlets a a a of the hepatic veins.

set of capillary twigs proceeds inwards towards the centre of
each lobule, traversing in their course its aggregation of
secreting cells. These capillaries finally terminate in the
rootlets of the hepatic veins, which diverge from the centre of
each lobule (fig. 170, a, a, a), and which collect the blood
that has traversed its capillary system, to transmit it through
larger trunks into the Vena Cava (§ 266), and thence to the
heart. The liver is also supplied with arterial blood by the
Hepatic artery ; but this seems to have for its function rather
to nourish the solid tissues of the organ, than to supply the
materials for secretion. The bile-ducts, which convey away
the fluid that is elaborated by the hepatic cells, appear to form
a network which surrounds the lobules, connecting them
together and sending branches towards the interior of each
(fig. 171). It is still doubtful, however, whether they extend
through the entire substance of the lobules, and whether the

306 STBUCTUBE OF THE LIVER ! — BILE.

hepatic cells are really included within their extensions (as
they are within the tubes or follicles of the liver of Inverte-
brata); or whether the cells lie outside the bile-ducts, in
immediate contact with the capillary blood-vessels that tra-
verse the lobule, filling up the entire space not occupied by
them, and transmitting the products of their secretion from
one to another, until these reach the exterior of the lobule,
where they find their way into the bile-ducts and are carried

*•&
-Jfisini

Fig. 171. — TRANSVERSE SECTION op TWO LOBULES OP THE LIVER;

Showing the bile-ducts distended by injection ; a a, ramifications of the hepatic
vein, occupying the centres of the lobules ; b b b, branches of the hepatic
ducts, which are largest in the space c, between the lobules, and which pass
towards the centre through d d, the substance of the lobules.

off by them. — The bile may flow directly, as it is secreted,
into the intestinal tube (§ 213); but if digestion be not going
on, so that its presence there is not required, it regurgitates
into the gall-bladder (fig. 30), which stores it up until it is
needed. In this reservoir it undergoes a certain degree of
concentration by the removal of its watery part.

364. Bile is a yellowish (sometimes a greenish-yellow)
fluid, somewhat viscid and oily-looking, and having a very
bitter taste, followed by a sweetish after-taste. It is readily
miscible with water, its solution frothing like one of soap ;
and it has the power, in common with soap, of dissolving oily
matters ; so that ox-gall is not unfrequently used to remove
grease-spots from woollen stuffs. The basis of the principal
ingredient of biliary matter, which constitutes about 5 parts
in 100 of the secretion, is a fatty or resinoid acid which is
termed the Cholic; this consists of 49 Carbon, 39 Hydrogen,
and 9 Oxygen ; and it forms, by " conjugation " with glycine (a

SECRETION OF BILE. 307

sugary substance that is derivable from the decomposition of
gelatin and albumen) and withtaurine (a substance distinguished
for the large proportion of sulphur it contains, — no less than
25 per cent), two other acids, the Glycocholic and the Tauro-
cholicj which are mingled in different proportions in the bile
of different animals, both being combined with soda as a base.
Bile also contains a white crystallizable fatty substance
resembling spermaceti, which is termed Cholesterin ; this
consists of 36 Carbon, 32 Hydrogen, and 1 Oxygen; and
though its quantity in healthy bile appears to be very small,
yet it occasionally increases to such an extent as to form the
concretions known as "gall-stones," which, getting into the
bile-duct, are transmitted along it with great pain and diffi-
culty, or block it up altogether. The peculiar colouring
matter of bile is quite distinct from the preceding substances ;
but like them it is extremely rich in carbon and hydrogen.

365. The bulk of the Liver, and the activity of the Respira-
tory apparatus, seem generally to bear an inverse ratio one to
the other. Thus we find in Insects, a respiratory system
possessing enormous extension and activity of function, and a
liver so slightly developed, that for a long time it was not
recognised as such. On the other hand, in the Mollusca, we
find the respiration carried-on upon a lower plan, and with
far less activity; but the liver is of enormous size, often
making up a large part of the bulk of the body. Moreover,
in the Crustacea, which are formed upon the same general
plan with Insects, but which have an aquatic and therefore
less energetic respiration, we find the liver very large, as in
the Mollusca. In Reptiles and Fishes, again, whose respira-
tion and temperature are low, the liver is comparatively larger
than in Birds and Mammals, in which classes the respiration
is more energetic, and the blood warm. In all these in-
stances, however, the bulk of the liver depends in great part
upon the accumulation of fat in its cells ; and the secreting
activity may be positively less in them, than it is in animals
which have a comparatively small biliary apparatus.

366. The materials of the secretion of Bile are probably
derived in part from the disintegration of the tissues, and in
part more directly from the food. It is an interesting fact
that the composition of bile and urine, taken together, corre-
sponds closely with the composition of the blood ; so that it

x2

308 ASSIMILATING ACTION OF LIVER.

would appear as if the nutritive materials, in their ultimate
metamorphosis, resolved themselves chiefly into these two
excretory products. The greater part of the biliary matter
poured into the intestinal canal seems to be ordinarily re-
absorbed with the fatty matter of the food, and to be, like it,
carried out of the system through the lungs in the form of
carbonic acid and water; it being only when the bile has
either been formed in excessive amount, or has been pro-
pelled along the intestinal tube with undue activity, that it is
discharged in any quantity from the rectum, as in bilious
diarrhoea. — The secreting action of the Liver, however, is by
no means its sole mode of influencing the composition of the
blood ; for it has been shown by the recent researches of
M. Bernard, that the blood which leaves the liver by the
hepatic vein contains a peculiar substance of a saccharine
nature,1 which does not exist in the blood brought to the
organ by the portal vein. This substance appears to be
elaborated by the converting power- of the liver, either from
materials supplied by the food, or from the products of the
waste of the system ; and it seems to be specially destined as
a pabulum or fuel for the combustive process, being usually
eliminated from the blood in the form of carbonic acid and
water during its passage through the lungs, so as not to pass
into the systemic circulation unless either its quantity be un-
usually great, or its elimination be interfered with by imperfect
respiration. The liver seems also to form a peculiar fat, which
is usually consumed in the same manner ; but if the respiratory
process be feeble, this fat accumulates in the cells of the liver
itself.

367. The Urinary excretion has for its chief purpose to
throw off those products, formed in a similar .manner, which
are highly charged with azote. The most important of its
ingredients, in Man and the Mammalia, is the substance termed
Urea, which has a crystalline form, and is very soluble in
water. It contains 2 equivalents of Carbon, 4 of Hydrogen,
2 of Azote, and 2 of Oxygen ; and it will be seen, by referring
to the statement formerly given of the composition of albumen

1 This substance is spoken of by M. Bernard as sugar : it has been
demonstrated, however, by the recent researches of Dr. Pavy, that the
liver does not form sugar, but a substance that becomes sugar almost
immediately upon contact with albuminous matters.

SECRETION OF URINE. 309

(§ 13) and gelatin (§ 19), that the amount of azote in propor-
tion to that of the other elements is much greater in urea
than it is in these substances, which form the materials of
the animal tissues. The quantity of Urea which is daily
excreted is very considerable, the average in an adult being
about an ounce, and in a child of eight years old about half
as much. — There is another compound which does not usually
exist in large amount in the urine of the Mammalia, but
which makes up a considerable part of the solid matter of
this secretion in Birds and the lower Vertebrata ; this is uric
or lithic acid, which consists of 10 equivalents of Carbon, 4 of
Hydrogen, 4 of Azote, and 6 of Oxygen. It is almost entirely
insoluble in water, unless it be combined with soda or am-
monia ; and in this state it ordinarily exists. When formed
in too large quantity, however, it may be deposited in an
insoluble form, constituting gravel (§ 348) ; and the same
effect may result from the presence of some other acid, which,
combining with the ammonia, precipitates or sets free the
lithic acid. In the urine of herbivorous animals, uric acid is
replaced by Hippuric acid, which contains a much larger
proportion of carbon, its composition being 18 Carbon,. 8
Hydrogen, 1 Nitrogen, and 5 Oxygen. Urine also contains
a considerable quantity of Saline matter; part of which
consists of what has been introduced into the system in the
same form, and has to be got rid of as superfluous; whilst
another part results from the conversion of the sulphur and
phosphorus of the food into sulphuric and phosphoric acids by
union with atmospheric oxygen (§ 343), and from the com-
bination of these acids with alkaline bases furnished by the
food. The amount of alkaline phosphates contained in the
urine may be considered as in some degree a measure of the
expenditure of nervous tissue ; whilst that of alkaline sulphates
has some relation to the expenditure of muscular substance.

368. The Kidney, by which the secretion of Urine is eli-
minated from the blood, is an organ whose structure in the
higher animals is very peculiar, although in the lower it is a
mere aggregation of tubes or of follicles. If we make a ver-
tical section of the kidney of Man or any of the higher Mam-
malia (fig. 172, A), we find that it seems composed of two
different substances, one surrounding the other ; to the outer;
a, the name of conical (bark-like) substance has been given ;

310

STRUCTURE OF THE KIDNEY.

whilst the inner, 6, is termed medullary (or pith-like). In
the cortical substance, no definite arrangement can be de-
tected by the naked eye ; it chiefly
consists of a very intricate network
of blood-vessels, surrounding the
extremities of the tubes. But in
the medullary substance we can
trace a regular passage of minute
tubes, from the circumference to-
wards the centre. They commence
in the midst of the network of
blood-vessels (B, a), and then pass
down in clusters, nearly in a
straight direction, and slightly con-
verging towards each other, until
each cluster terminates in a little
body, called the calyx or cup, which
discharges the fluid it receives into
the large cavity of the kidney,
termed the pelvis or basin (A, c).
From this it is conveyed away by
the ureter d, which terminates in
the bladder.

369. One of the most interesting
circumstances in reference to the
Urinary secretion, is the very large
quantity of water which, in the
higher animals, is got rid of through
this channel, and the means by
which it is drawn off. The kidneys
seem to form a kind of regulating valve, by which the quan-
tity of water in the system is kept to its proper amount. The
exhalation from the Skin is liable to sustain great variations
in its amount from the temperature of the air around ; for
when this is low, the exhalation is very much diminished ;
and when it is high, the quantity of fluid that passes off in
this manner is increased (§ .371). Hence, if there were not
some other means of adjusting the quantity of fluid in the
blood-vessels, it would be liable to continual and very inju-
rious variation. This important function is performed by the
kidneys, which allow such a quantity of water to pass into

Fig.

172.— STRUCTURE OP
KIDNEY OF MAN.

A, vertical section of the kidney ;
a, cortical substance ; b, tubular
substance ; c, calyx and pelvis ;
d, ureter.

B, portion of the gland enlarged;
a, extremity of the uriniferous
tubes; b, straight portion; c,
their termination in the calyx.

MALPIGHIAN BODIES OF THE KIDNEY;

311

Fig. 173.

-MALPIGHIAK BODIES
THE KIDNEY.

the urinary tubes, as may keep the pressure within the vessels
very nearly at a uniform standard ; an«l a distinct and very
curious provision exists for its separation. The extremity of
many of the uriniferous tubes is made to include little knots
or bunches of capillary vessels,
which have extremely thin
walls (fig. 173) ; and a vast
number of such knots, which are
termed " Malpighian bodies,"
after the name of their dis-
coverer, are scattered through
the cortical portion (§ 368) of
the kidney. To these the blood
brought to the organ by the
renal artery is first conveyed;
and the membranes that sepa-
rate the interior of the capil-
lary vessels from the cavity of
the uriniferous tube, being of
extreme thinness, water is
readily able to traverse them; and will do so in larger or
smaller quantity, according as the pressure upon the walls
of the capillaries is greater or less. The blood which has
passed through these is next conducted to another set of
capillaries, which form a network upon the part of the tube
that is lined by the secreting cells ; and it is there subservient
to the elaboration of the solid part of the secretion. Hence
the quantity of water in the urinary secretion depends in part
upon the amount exhaled from the skin, — being greatest
when this is least, and vice versa, — and in part upon th&
quantity which has been absorbed by the vessels. The quan-
tity of solid matter in the secretion has but little to do with
this ; for it depends upon the amount of waste of the muscular
and other tissues that has been occasioned by their activity
(§367); and also upon the quantity of surplus aliment which
has to be discharged through this channel, there being no
other vent for it (§ 348).

370. Next to the excretions formed by the liver and the
kidneys, that of the Skin probably ranks in importance. A
large quantity of watery vapour is constantly passing-off from
the whole surface of Man and other sofVskin&ed animals;

312 EXHALATION FKOM THE SKIN.

and this amount is greatly increased under particular circum-
stances. A continual evaporation takes place from the surface
of the skin, wherever it is not protected by hard scales or
plates ; and the amount of it will depend upon the warmth,
dryness, and motion of the surrounding air, exactly as if the
fluid were being evaporated from a damp cloth. Every one
knows that the drying of a cloth is much more rapidly effected
in a warm dry atmosphere, than in a cold moist one ; more
quickly, too, in a draught of air, than in a situation where
there is no current, and where the air is consequently soon
charged with moisture. That all these influences affect the
evaporation from the bodies of Animals, there is ample evi-
dence derived from experiment.

371. But besides this continual evaporation, a special
exhalation of fluid takes place from the vast number of
minute perspiratory glands imbedded in the fatty layer just
beneath the Skin (§ 37). Every one of these glandule, when
straightened out, forms a tubule about a quarter of an inch in
length ; and as it has been estimated that in a square inch of
surface on the palm of the hand there are no fewer than 3528
of these glandule, the length of their tubing must be 882
inches or 73 J feet. The average number in other parts of the
body may be estimated at about 2800 per square inch ; and
as the number of square inches of surface on a man of ordinary
stature is about 2500, the total number of perspiratory glan-
dulse must be not less than seven millions, and the length of
their tubing nearly twenty-eight miles. The fluid which these
perspiratory glands ordinarily exhale, is dissolved by the atmo-
sphere, and carried off in the state of vapour, so as to pass
away insensibly ; but they are stimulated to increased action
by the exposure of the body to heat, which causes them to
pour forth their secretion in greater abundance than the air
can carry off, and this consequently accumulates in drops upon
the surface of the skin. The amount of perspiration may be
considerably increased, without its becoming sensible, if the air
be warm and dry, so as to carry off, in the form of vapour,
the fluid which is poured out on the skin ; but, on the other
hand, a very slight increase in the ordinary amount immedi-
ately becomes sensible on a damp day, the air being already
too much loaded with moisture to carry off this additional
quantity. The distinction between insensible and sensible

COOLING EFFECT OF CUTANEOUS EXHALATION. 313

perspiration, is not the same, therefore, with the difference
between simple evaporation and exhalation from the skin ; for
a part of the latter is commonly insensible ; and the degree
in which it is so depends upon the amount of fluid exhaled,
and the state of the surrounding atmosphere. If the fluid
thus poured forth be allowed to remain upon the surface of
the skin, it produces a very oppressing effect ; most persons
have experienced this, when walking in a mackintosh cloak or
coat, on a damp day. The waterproof garment keeps in the
perspiration, almost as effectually as it keeps out the rain;
and consequently the air within it becomes loaded with fluid,
and the skin remains in a most uncomfortable as well as pre-
judicial state of dampness.

372. The purpose of this watery exhalation, and of its
increase under a high temperature, is evidently to keep the
heat of the body as near as possible to a uniform standard.
By the evaporation of fluid from the surface of the skin, a
considerable quantity of heat is withdrawn from it, becoming
latent (according to ordinary phraseology) in the change from
fluid to vapour : of this we make use in applying cooling
lotions to inflamed parts. The more rapid the evaporation,
the greater is the amount of heat withdrawn in a given time ;
hence, if we pour, on separate parts of the back of the hand,
small quantities of ether, alcohol, and water, we shall find
that the spot from which the ether is evaporating feels the
coldest, that which was covered by the alcohol less so, whilst
the part moistened with water is comparatively but little
chilled. The greater the amount of heat applied to the body,
then, the more fluid is poured out by the perspiratory glands ;
and as the air can carry it off more readily in proportion to
its own heat, the evaporation becomes more rapid, and its
cooling effect more powerful. It is in this manner that the
body is rendered capable of sustaining very high degrees of
external heat, without suffering injury. Many instances are
on record, of a heat of from 250° to 280° being endured in
dry air for a considerable length of time, even by persons
unaccustomed to a peculiarly high temperature ; and indi-
viduals whose occupations are such as to require it, can sustain
a much higher degree of heat, though perhaps not for any
great length of time. Thus, the workmen of the late Sir F.
Chantrey were accustomed to enter a furnace in which his

314 IMPOETANCE OF CUTANEOUS EXHALATION.

moulds were dried, while the floor was red-hot, and a ther-
mometer in the air stood at 350° ; and Chabert, the " Fire-
king," was in the habit of entering an oven whose temperature
was from 400° to 600°. It is possible that these feats might
be easily matched by many workmen, who are habitually
exposed to high temperatures; such as those employed in
iron-foundries, glass-houses, and gas-works.

373. That the power of sustaining a high temperature
mainly depends upon the dryness of the atmosphere, is evident
from what has just been stated j since, if the perspiration that
is poured-forth upon the skin is not carried-off with sufficient
rapidity, on account of the previous humidity of the air, the
temperature of the body will not be sufficiently kept down.
It has been found, from a considerable number of experiments,
that when warm-blooded animals are placed in a hot atmos-
phere saturated with moisture, the temperature of their bodies
is gradually raised 12° or 13° above the natural standard ; and
that the consequence is then inevitably fatal.

374. The amount of fluid exhaled from the skin and lungs
(§ 343) in twenty-four hours, probably averages about three
or four pounds. The largest quantity ever noticed, except
under extraordinary circumstances, was 5 Ibs. ; and the smallest,
If Ibs. It contains a small quantity of solid animal matter,
besides that of the other secretions of the skin which are
mingled with it ; and there is good reason to think that this
excretion is of much importance, in carrying off certain sub-
stances which would be injurious if allowed to remain in the
blood. That which is called the Hydrophatic system, proceeds
upon the plan of increasing the cutaneous exhalation to a
very large amount ; and there seems much evidence, that
certain deleterious matters, the presence of which in the blood
gives rise to Gout, Rheumatism, &c., are drawn off from it
more speedily and certainly in this way, than in any other.

375. Besides the perspiratory glands, the skin contains
Others, which have special functions to perform. Thus in
most parts which are liable to rub against each other, we find
ft considerable number of sebaceous follicles (fig. 8, a a), which
secrete a fatty substance that keeps the skin soft and smooth.
Besides these, the skin contains other follicles in particular
parts, for secreting peculiar substances ; as, for instance, those
•which form the cerumen, a bitter waxy substance that is

MAMMAKY GLAND : SECRETION OF MILK. 315

poured into the canal leading to the internal ear, for the pur-
pose (it would seem) of preventing the entrance of insects.

376. The secretion of Milk is important, not so much to
the parent who forms it, as to the offspring for whose nourish-
ment it is destined. It does not seem to carry off from the
system any injurious product of its decomposition ; for it bears
a remarkable analogy to blood in the combination of substances
which it contains ; nevertheless it is found that, when this
secretion is once fully established, it cannot be suddenly
checked, without producing considerable disturbance of the
general system. The structure of the Mammary gland closely
resembles that of the parotid already described (fig. 165). It
consists of a number of lobules, or small divisions, closely
bound together by fibrous and areolar tissue ; to each of these
proceeds a branch of the milk-ducts, together with numerous
blood-vessels ; and the ultimate ramifications of these ducts
terminate in a multitude of little follicles, about the size (when
distended with milk) of a hole pricked in paper by the point
of a very fine pin.

377. The nature of the composition of Milk is made evident
by the processes to which we commonly subject it. When it is
allowed to stand for some time, its oleaginous part, forming the
cream, rises to the top. This is still combined, however, with
a certain quantity of albuminous matter, which forms a kind
of envelope round each of the oil-globules ; but in the process
of churning, these envelopes are broken, and the oil-globules
run together into a mass, forming butter. In ordinary butter
a certain quantity of albuminous matter remains, which, from
its tendency to decomposition, is liable to render the butter
rancid ; this may be got rid of by melting the butter at the
temperature of 180°, when the albumen will fall to the bottom,
leaving the butter pure and much less liable to change. In
making cheese, we separate the albuminous portion, or casein,
by adding an acid which coagulates it. The buttermilk and
whey left behind after the separation of the other ingredients,
contain a considerable quantity of sugar, and some saline
matter. The proportion of these ingredients varies in different
animals ; and also in the same animal, according to the sub-
stances upon which it is fed, and the quantity of exercise it
takes. The amount of casein seems to be greatest in the milk
of the Cow, Goat, and Sheep ; that of oleaginous matter in the

316 GENERAL REVIEW OP NUTRITIVE OPERATIONS.

milk of the Human female ; and that of sugar in the milk of
the Mare. The milk of the Cow, if a portion of its casein
were removed, would resemble Human milk more nearly than
any other ; and it is therefore hest for the nourishment of
Infants, when the latter cannot be obtained. The important
influence of Mental emotion on this secretion has already been
noticed (§ 353) ; and many more instances might be related,
were not the ordinary facts in regard to it generally known.

CHAPTEE VIII.

GENERAL REVIEW OF THE NUTRITIVE OPERATIONS — FORMATION
OF THE TISSUES.

General Review of the Nutritive Operations.

378. IN the preceding Chapters (HI. to v.) those processes
have been described, by which the alimentary materials that
constitute the raw material of the tissues, are converted into
a fluid adapted for the Nutrition of the body ; and we then
(CHAPS, vi. and VH.) considered those functions, by which this
fluid is kept free from the impurities it acquires during its
circulation through the body, and is maintained in the state
which alone can adapt it to the purposes it is destined to fulfil.
These purposes may be regarded as fourfold. In the first
place, the Blood is destined to supply the materials of the
fabric of the body ; which, as it is continually undergoing
decay (§ 68), requires the means of as constant a renovation.
Secondly, the Blood (in most animal& at least) serves to convey
to the tissues the supply of oxygen which is required by
them, — especially by the muscular and nervous tissues, — as a
necessary stimulus to the performance of their functions.
Thirdly, the Blood furnishes to the secreting organs the
materials for the elaboration of the various fluids, which have
special purposes to serve in the Animal economy, — such, for
instance, as the Saliva, Gastric juice, Milk, &c. And lastly,
the Blood takes up, in the course of its circulation, the pro-
ducts of the waste or decomposition of the various tissues,
which it conveys to the several organs, — the Lungs, Liver,

FORMATION OF THE TISSUES. 317

Kidneys, &c., — destined to throw them off by Excretion.
The greater number of these processes have already been
treated of in more or less detail. Those included under the
first head were considered, in a general form, in Chap. i. of
this Treatise. Those which are comprehended under the
second head have been dwelt-on in Chaps, v. and vi. ; and
will be again noticed, when the actions of the Nervous and
Muscular tissues are described. And the varied actions which
are included under the third and fourth classes, have been
discussed in the two Chapters which precede the present one.
We have now to enter, in more detail, into the mode in which
the circulating fluid is applied to the Nutrition and Formation
of the Tissues.

Formation of the Tissues.

379. There is sufficient reason to believe that every living
being is developed from a germ; no organized structure being
able to take its origin (as some have supposed) in a chance
combination of inorganic elements. All the facts relating to
the production of Fungi and Animalcules, which have been
imagined to favour this doctrine, may be satisfactorily ex-
plained in other ways (VEGET. PHYS. § 779 ; ZOOL. § 1213).
Now the first structure developed from this germ, in the
Animal as in the Plant, is a simple cell; and the entire fabric
subsequently formed, however complex and various in struc-
ture, may be considered as having had its origin in this cell.
The cells of Animals, like those of Plants, multiply by the
development of new cells within them; each of these be-
comes in its turn the parent of others ; and thus, by a con-
tinuance of the same process, a mass consisting of any number
may be produced from a single one. It is in this manner that
the first development of the Animal embryo takes place, as
will be shown hereafter (Chap. xv.). A globular mass, con-
taining a large number of cells, is formed before any diversity
of parts shows itself; and it is by the subsequent development,
from this mass, of different sets of cells, — of which some are
changed into cartilage, others into nerve, others into muscle,
others into vessels, and so on, — that the several parts of the
body are ultimately formed.

380. This process of differentiation is carried to very
different degrees in the development of the several classes of

318 DIFFERENTIATION OF STBUCTUEE.

animals ; for in some it is checked so early, that scarcely any
distinction either of organs or of tissues ever shows itself;
whilst in others it continues during a large proportion of the
earlier period of life. It has no relation to growth, or simple
increase of size ; for this may take place by the multiplication
of similar parts, giving rise to that "vegetative repetition"
which is so characteristic of the lower tribes of Animals, and
which gives to many of them so strong a resemblance in
general aspect to Plants ; whilst, on the other hand, the de-
velopmental process by which higher forms of structure are
evolved, sometimes takes place without any increase at all.
It is in its degree of such differentiation, that what is called the
lower or the higher organization of any living being essentially
consists ; for whilst in the simplest forms of Animal struc-
ture every part is similar to every other, so that all the
functions of life are performed in common by each, we find
in Man (whose body may be regarded as presenting the
highest type or example of this differentiating process) that
no two parts are precisely similar, except those on the
opposite sides of the body. This fact is occasionally mani-
fested in a very singular manner, in the symmetry of disease ;
certain morbid poisons (as those of gout, and of several affec-
tions of the skin), which have a tendency to single out par-
ticular spots of the tissues whose nutrition they disturb,
exhibiting their action in those parts of the two lateral halves
of the body which precisely correspond with each other.

381. Now in the lowest grades of Animal structure, we find
that the several tissues of the body can themselves appropriate
from the products of digestion the nutrient materials they
respectively require ; so that, for their growth and mainte-
nance, it is sufficient that these products should be carried
into their neighbourhood by extensions of the digestive cavity
(§ 296). But in all the -more highly-organized animals, it
appears requisite that the nutrient material should pass
through an intermediate stage of preparation, which is termed
assimilation (or making-like); and this is effected by their
introduction into the current of the circulation, and their
mixture with the pre-existing blood, which, in virtue of its
own vital powers, exerts upon them a converting action, which
prepares them for being appropriated by the solid tissues.

382. When once the several forms of tissue have been

MUTUAL DEPENDENCE OF PARTS OP ORGANISM. 319

developed, their nutrition is kept up by the supply of their
respective materials which they derive from the blood. Each
tissue draws from the circulating current that which it re-
quires ] and it is one of the most wonderful proofs of the skill
with which the entire fabric has been planned and constructed,
that the composition of the blood should be maintained at a
nearly uniform standard, in spite of the continual change which
is thus taking place in its actual components. It has been
justly remarked, that each part of the body, by taking from
the blood the peculiar substances which it needs for its own
nutrition, does thereby act as an excretory organ, inasmuch
as it removes from the blood that which, if retained in it,
would be injurious to the nutrition of the body generally.

383. Hence it seems that such a mutual dependence must
exist among the several parts and organs of the body, as
causes the evolution of one to supply the conditions requisite
for the production of another ; and this view is borne out by
a great number of phenomena of very familiar occurrence,
which show that a periodical change in one set of organs
governs changes in others which at first sight might seem to
have no relation to them. Thus the plumage of Birds, at the
commencement of the breeding season, becomes (especially in
the male) more highly coloured, besides being augmented by
the growth of new feathers ; but when the generative organs
pass into their condition of periodical inactivity, the plumage
begins at once to assume a paler and more sombre hue, and
many of the feathers are usually cast, their nutrition being no
longer kept up. So, again, it is no uncommon occurrence
among Birds, for the female, after ceasing to lay, to assume
the plumage of the male, and even to acquire other character-
istic parts, as " spurs " in the fowl tribe. That, in these and
similar instances, the development of organs is immediately
determined by the presence or absence in the blood of the
appropriate pabulum for the parts in question, and that its
existence depends upon changes taking place in other parts,
has been rendered still more probable by the results of expe-
riments, which show that if the ordinary changes in one set
of organs be prevented by their removal, those usually taking
place in the others do not occur.

384. Though all the tissues derive the materials of their
development from the blood which circulates in the vessels,

320 NUTRITION OF NON-VASCULAR TISSUES.

yet there is considerable variety in the mode in which the
supply is afforded ; some tissues being furnished with blood
much more copiously and directly than others, in consequence
of the greater minuteness with which the capillaries are dis-
tributed through their substance. There are several, indeed,
into which no blood-vessels enter, in their natural state ; but
which derive their nutriment by absorbing the liquor san-
guinis that is brought into their neighbourhood. This is
the case, for instance, with the Epidermis and Epithelium
(§§ 38, 40) ; the cells of which are developed at the expense
of the fluid which they absorb, through the basement mem-
brane on which they lie, from the vessels of the skin or
mucous membrane beneath it. In like manner, even the
thick layer of Cartilage which covers the ends of most of the
long bones, is destitute of blood-vessels ; and the small amount
of nourishment it requires, is obtained by absorption from the
vessels which surround it (§ 47). This tissue undergoes very
little change from time to time ; and its growth takes place
chiefly by addition of new matter to its surface ; consequently
there is no necessity for any active circulation through its
interior; and the transmission of nutritive fluid from one
cell to another (as in the cellular tissue of Plants) is sufficient
for its wants. Even in Bone, the blood-vessels are not very
minutely distributed ; for although there is a close network
of capillary vessels on the membrane lining the Haversian
canals and the cells of the cancellated structure (§ 49), yet
none of these pass into the actual substance of the bone.
The simple Eibrous tissues are, for the most part, sparingly
supplied with blood-vessels, as they are but little liable to
decay or injury ; though the Areolar tissue serves as the bed
for the reception of the vessels which are on their passage to
other tissues. Thus it is by its means that blood-vessels are
conveyed into the Adipose tissue ; for the ultimate elements
of that tissue, namely, the fat-cells, are surrounded by capil-
lary vessels, not entered by them. The same important pur-
pose is answered by the areolar tissue that lies amongst the
tubes which form the essential parts of the Nervous and
Muscular tissues ; for these tubes are not perforated by ves-
sels, so that their contents must be nourished by fluid absorbed
through their walls.

385. In no instance that we are acquainted with, in the

IMPERFECT NUTRITION : — CONSUMPTION. 321

higher animals at least, do the vessels directly pour the blood
into any tissue for the purpose of nourishing it. Unless there
have been an actual wound which has artificially opened the
blood-vessels, no fluid can escape from them into the substance
traversed by the capillaries, except by transuding the walls of
the latter ; and hence it would seem impossible that any of
the floating cells contained in the blood can be deposited in
the tissues and contribute to their development. The Liquor
Sanguinis seems, therefore, to furnish all that is wanting for
this purpose ; and it readily permeates the walls of the capil-
laries, the basement-membrane, and any other of the softer
tissues, so as to arrive at the parts where it is to be applied.
As it is withdrawn from the blood, it is continually being
re-formed from the food ; but if it be not supplied in sufficient
quantity by the latter, the tissues are imperfectly nourished,
and the strength of the body and the vigour of the mind are
consequently alike impaired.

386. This imperfect nutrition seems to be the essential
conditjpn of one of the most destructive diseases to which the
human frame is liable, — that commonly known as Consump-
tion ; which is, however, but one out of several diseases that
may result from the same general defect of nutrition. If the
liquor sanguinis be imperfectly elaborated, it is less fit to
undergo organization ; and, consequently, instead of being
converted into living tissue, part of it is deposited, as an
imperfectly organized mass, in the state known to the Medical
man as Tubercle. Such deposits take place more frequently
in the lungs than in any other part ; and besides impeding
the circulation and respiration, they produce irritation and
inflammation, in the same manner as other substances im-
bedded in the tissues would do ; and so far from having, like
many other diseases, a natural tendency to cure, this malady,
if unchecked, almost certainly leads to a fatal termination.
Microscopic examination of tubercular matter shows that it
consists of half-formed cells, fibres, &c., together with a granu-
lar substance which seems to be little else than coagulated
albumen. The only manner in which any curative means can
be brought to bear upon this terrible scourge, is by attention
to the constitutional state from which it results. This is
sometimes hereditary ; and is sometimes induced by insuffi-
cient nutrition, obstructed respiration, habitual exposure to

Y

TUBERCULAR DIATHESIS : ITS TREATMENT.

cold and damp, long-continued mental depression, &c. The
treatment of the Tubercular diathesis (as this state of consti-
tution is termed) must be directed to the invigoration of the
system by good food, active exercise, pure air, warm clothing,
and cheerful occupation; and by the due employment of
these means, at a sufficiently early period, many valuable lives
may be saved which would have otherwise fallen a sacrifice.
The value of cod-liver oil in the treatment of this disease,
which is now a well-established fact, seems to depend upon
the facility with which it is assimilated as a nutritive material.
It is a remarkable fact that the inhabitants of Iceland, the
greater part of whom live under conditions that might be
expected to favour the development of tubercular disease, are
singularly free from it; and the source of this exemption
seems to consist in the very oleaginous nature of their diet.
Consumption presents itself among the inhabitants of all
climates ; and the value of change to a patient who is
affected with this malady, chiefly depends upon the oppor-
tunity which it affords him for abundant exercise in the open
air, without injurious exposure to cold or damp.

387. From the foregoing facts it is evident, that the opera-
tions of Nutrition are due, on the one hand, to the indepen-
dent properties of the several Tissues, which draw from the
blood the materials of their continued growth and renewal ;
and, on the other, to the properties of the Blood, which
supplies them with these materials. The blood, left to itself,
could form no tissue more complex than a mere fibrous net-
work : whilst, conversely, the various tissues of the body
could not draw their nourishment directly from the products
of digestion, and are consequently dependent upon the blood
for their supply. We may illustrate the relation between the
three states, — that of aliment, blood, and organized tissue, —
by comparing them with the three principal states which
Cotton passes through in the progress of its manufacture, —
namely, the raw cotton, spun-yarn, and woven fabric. The
spun-yarn could not of itself assume that particular arrange-
ment which is given to it by the loom ; and the loom could
make nothing of the raw cotton, until it has been spun into
yarn.

388. It is also evident, that the blood-vessels have no other
purpose in the act of Nutrition, than to convey the circulating

ACTION OF BLOOD-VESSELS IN NUTEITION. 323

fluid into the neighbourhood of the part where it is to "be
employed ; and the blood, or at least its organizable portion —
the liquor sanguinis — must quit the vessels before it can be
employed in the development of new tissue. We might illus-
strate this by the distribution of water-pipes through a city ;
they might pass into every house, nay, into every room, and
yet the water must be drawn from the pipes before it can be
applied to any required purpose. The spaces untraversed by
vessels have been shown to be larger in some tissues, and
smaller in others ; the distribution of the capillaries being
more minute, in proportion as the nutritive actions of the
part go on more energetically. Now in the embryo, even of
the most complex and perfect animals, there is a period when
no blood-vessels exist, the whole mass being made-up of cells,
every one of which lives for itself and by itself, absorbing
nutriment from a common source, and not at all dependent
upon its brethren. It is only when a diversity of structure
begins to show itself, — one part undergoing transformation
into bone, another into muscle, and so on, — and when some
portions of the fabric are cut-off from the direct supply of
nourishment, — that vessels begin to show themselves. These
are formed, like the ducts of Plants, by the breaking-down of
the partitions between contiguous cells ; they at first seem
rather like passages or channels, than tubes with walls of their
own ; and this condition they retain in certain cases through
life (§ 289).

Repair of Injuries.

389. Every animal possesses, in a greater or less degree,
the power of not merely maintaining its organized fabric in
its integrity, by the renewal of the parts which are from time
to time passing into decay, but also of reproducing parts of
that fabric which have been lost by disease or accident. This
power seems greatest among the lowest tribes of Animals ; in
many of which the entire organism can be reproduced from a
small portion of it, as is the case with the Hydra (§ 122), and
with some species of Sea-Anemone (§ 126). In the Star-fish,
a far more highly-organized animal, the regenerative power is
more limited, though it is still very remarkably manifested ;
for if one, two, or more of the rays be broken or cut off, they
are gradually restored, provided the central disc be uninjured.

Y2

324 REPARATIVE POWERS OP LOWER ANIMALS.

Of certain kinds of Holothuria (fig. 67), which eject the entire
mass of viscera under the influence of alarm, it has been ob-
served that they not only continue to move about as if nothing
had happened, but that, under favourable circumstances, they
regenerate the whole of the digestive and reproductive appa-
ratus thus parted-with. — Next to Zoophytes, there are no
animals in which the regenerative power is known to manifest
itself so strongly as the lower members of the Articulated
series, such as the inferior Entozoa and the Turbellaria (Zoo-
LOGY, § 924), among which last we find the Planaria almost
rivalling the Hydra, although it is an animal of much more
complex structure. The common Earthworm can reproduce
either the head or any portion of the body of which it may
have been deprived; but it cannot be multiplied by the division
of its body into two or more parts (as asserted by some), since
these parts, although they continue to move for a time, soon
perish. There are Worms allied to it, however, in which the
regenerative power is sufficient to produce the whole body
from a separated fragment; and no fewer than twenty-six
have thus been made to originate by the subdivision of a
single individual. In the higher Articulata, such as Crus-
tacea, Insects, and Spiders, the reparative capacity is limited
to the restoration of limbs ; and even this would seem to be
seldom preserved in perfect Insects, being restricted to the
larval period of their lives. Little is known of the regene-
rative power possessed by the higher Mollusks ; but it has
been affirmed that the head of the Snail may be reproduced
after being cut-off, provided the cephalic ganglion be not
injured, and an adequate amount of heat be supplied.

390. It is only among the cold-blooded members of the
Yertebrated series, that the reparative power extends to the
renewal of entire organs ; and this seems limited in Fishes to
the reproduction of portions of the fins which have been lost
by disease or accident. In JBatrachia, however, it has been
found that entire new legs, with perfect bones, nerves,
muscles, &c., may be reproduced after severe loss or injury
of the original members ; and even a perfect eye has been
formed in place of one that had been removed. It is inte-
resting to observe that the exercise of this reparative power
essentially depends upon the temperature in which the animal
is living; the reproduction of entire members apparently

EEPARATIVE POWERS OF HIGHER ANIMALS. 325

requiring a higher degree of the stimulus of Heat, than does
their ordinary nutrition. In Lizards, an imperfect reproduc-
tion of the tail takes place when part of it has been broken
off; but the newly-developed portion contains no perfect ver-
tebrae, its centre being occupied by a cartilaginous column
like that of the lowest fishes. — In the warm-blooded Verte-
brata generally, the power of reproduction after loss or in-
jury seems much more limited. We do not find that entire
parts or members once destroyed, are completely renewed;
though very extensive breaches of substance are often filled
up. The tissues most readily reproduced are Bone, the Simple
Fibres (§ 22), and the Membranes (such as the Skin, the
Mucous and Serous membranes), of which these tissues form
the basis. As a general rule, losses of substance in Glandular
tissue, Muscle, and other parts of comparatively high organi-
zation, do not seem to be reproduced ; but there is a curious
exception to this in the case of Nervous tissue, which, with
Blood-vessels, is very readily re-formed in the new growths by
which losses of substance are repaired, as we often see in tha
rapid skinning-over of a large superficial wound. One of the
most remarkable manifestations of reparative power in the
Human body, is the re-formation of an entire bone, when the
original one has been destroyed by disease. The new bony
matter is thrown-out, sometimes within and sometimes around
the dead shaft ; and when the latter has been removed, the
new structure gradually assumes the regular form, and all the
attachments of muscles, ligaments, &c., become as complete
as before. A much greater variety and complexity of actions-
are involved in this process, than in the reproduction of whole
parts in the simpler animals ; though its effects do not appear
so striking. It appears that, in some individuals, this regene-
rating power is retained to a much greater degree than it is by
the species at large ; thus, there is a well-authenticated instance,
in which a supernumerary thumb on a boy's hand was twice re-
produced, after having been removed from the joint. In many
cases in which the crystalline lens of the eye has been re-
moved, in the operation for cataract, it has been afterwards
regenerated ; and there is evidence that, during embryonic
life, the regeneration of lost parts may take place in a degree
to which we have scarcely any parallel after birth ; attempts
being sometimes made at the re-formation of entire limbs, in

326 REPAIR OF LOSSES OF SUBSTANCE.

place of such as are lost during the earlier periods of develop-
ment.

391. When an entirely new structure is to be formed, — as
for the closure of a wound, the union of a broken bone, or the
repair of any other injury, — the process is of a kind very much
resembling the first development of the entire fabric. The
neighbouring vessels pour out their liquor sanguinis, which is
known to the Surgeon under the name of coagulable lymph ;
this fills up the open space, and forms a connecting medium
between the separated parts. If this intervening layer be
thin, the two sides of the wound may adhere so closely as to
grow together without any perceptible interposition of new
substance ; this is what is called " healing by the first inten-
tion." But if the loss of substance has been too great to
allow of such adhesion, the vacant space is filled by the
gradual organization of the coagulable lymph ; and this may
take place in one of two very different modes, the determina-
tion being chiefly dependent 011 the condition of the wound
as to seclusion from air or exposure to it.

392. The former of these conditions is by far the more
favourable of the two ; for the reparative material is usually
developed gradually but surely into fibrous tissue, without
any loss, and with very little irritation either in the part
itself or in the system at large. This process seems to take
place naturally in cold-blooded animals, even in open wounds ;
the contact of air not having that disturbing influence in
them, which it exerts in warm-blooded animals. And Nature
frequently endeavours to bring it about in the superficial
wounds of warm-blooded animals, by the formation of a large
scab, which protects the exposed surface ; but this happens
much less frequently in the Human subject than it does
among the lower animals, the unnatural conditions in which
a large proportion of the so-called civilised races habitually
live (especially deficient purity of the air, continual excess in
diet, and the frequent abuse of stimulants) being unfavourable
to it. The performance of many operations which formerly
left open wounds, in such a manner that the air may be
effectually excluded by a valvular fold of skin, is one of the
greatest improvements in modern Surgery.

393. In an open wound, on the other hand, which is
healing by the process termed " granulation," the reparative

HEALING OF OPEN WOUNDS. 327

material is rapidly developed into cells, amongst which, blood-
vessels speedily extend themselves. The formation of new
blood-vessels, in this and other cases, seems to commence in
the giving- way of the walls of some of the previously-existing
capillary loops, at particular spots, and in the escape of blood
corpuscles in rows or files into the soft substance that sur-
rounds them ; thus channels or passages are excavated, which
come into connexion with each other; and these channels,
after a time, acquire proper walls, and become continuous
with the vessels from which they originated, — to be in their
turn the originators of a new series. The vitality of this new
" granulation- tissue," however, is very low; and the part ex-
posed to the air passes into the condition of pus (the yellow
creamy fluid discharged from an open wound), which contains
the same materials in a decomposing state. Thus there is a
constant waste of organizable substance, the amount of which,
in the case of an extensive wound, becomes a serious drain
upon the system ; at the same time, there is a much greater
irritative disturbance both in the part itself and in the system
generally ; and the new tissue that is formed is of such low
vitality that it subsequently wastes away, so as by its disap-
pearance to leave a contracted cicatrix or scar. — The difference
between the two modes of reparation now described is often
one of life and death, especially in the case of large burns of
the body in children.

CHAPTEE IX.

ON THE EVOLUTION OP LIGHT, HEAT, AND ELECTRICITY BY ANIMALS.

Animal Luminousness

394 A large proportion of the lower classes of aquatic
Animals possess, in a greater or less degree, the power of
emitting light. The phosphorescence of the sea, which has
been observed in every zone, but more remarkably between
the tropics, is due to this cause. "When a vessel ploughs the
ocean during the night, the waves — especially those in her
wake, or those which have beaten against her sides — exhibit
a diffused lustre, interspersed here and there by stars or
ribands of more intense brilliancy. The uniform diffused

328 LUMINOSITY OF MAKINE ANIMALS.

light is chiefly emitted by innumerable minute animals, which
abound in the waters of the surface; whilst the stars and
ribands are due to larger animals, whose forms are thus bril-
liantly displayed. This interesting phenomenon, when it occurs
on our own coasts, is chiefly produced by incalculable multi-
tudes of a small creature, termed the Noctiluca, having a nearly
globular form, and a size about equal to that of the head of
a minute pin. When these cover the water, and a boat is
rowed among them, every stroke of the oars produces a flash
of light ; and the ripple of the water upon the shore is marked
by a brilliant line. If a person walk over sands that the tide
has left, his footsteps will seem as if they had been impressed
by some glowing body. And if a small quantity of the water
be taken up and rubbed between the hands, they will remain
luminous for some time. The transparency of the little ani-
mals to which these beautiful appearances are due, might cause
them to be overlooked if they are not attentively sought ; they
somewhat resemble grains of boiled sago in their aspect, but
are much softer. In the general simplicity of their structure,
the Noctilucos appear to correspond rather with the Rhizopoda
(§ 130) than with any other group; but they are distinguished
by some remarkable peculiarities.

395. Of the larger luminous forms which are seen to float
in the ocean-waters, a great proportion belong to the class
Acalephce. The light emitted by these appears to be due to
the peculiar chemical nature of the mucus secreted from their
bodies ; for this, when removed from them, retains its pro-
perties for some tune, and may communicate them to water
or milk, rendering those fluids luminous for some hours, parti-
cularly when they are warmed and agitated. It is probably from
this source, that the diffused luminosity of the sea is partly
derived. The secretion appears to be increased in amount, by
anything that irritates or alarms the animals ; and it is from
this cause that the dashing of the waves against each other,
the side of a ship, or the shore, — or the tread of the foot
upon the sand, — or the compression of the animals between
the fingers, — occasions a greater emission of light. But some
of these causes may act, by bringing a fresh quantity of the
phosphorescent secretion into contact with air, which seems
necessary to maintain the kind of slow combustion on which
the light depends.

LUMINOSITY OP MARINE ANIMALS AND INSECTS. 329

396. But the Noctilucae and Acalephae are by no means the
only luminous animals which tenant the deep. Many Zoo-
phytes appear to have this property in an inferior degree, and
also some of the Echinodermata. Of the lowest class of
Mollusks, the Tunicata, a very large proportion are luminous,
especially those which float freely through the ocean, and
which abound in the Mediterranean and tropical seas ; the
brilliancy of some of these can scarcely be surpassed. Among
some of the Conchifera, also, the phenomenon has been ob-
served ; as well as in certain marine Annelida. Other marine
animals of higher classes are possessed of similar properties ;
thus, many Crustacea, especially the minuter species, are
known to emit light in brilliant jets ; and the same may be
said of a few Fishes : but it is probable that the luminosity
attributed to many of the latter is due to the disturbance they
make in the surrounding water, which excites its phospho-
rescence in the manner just explained. In all these, the
general phenomena are analogous, — the luminous matter ap-
pearing to be a secretion from the surface of the animals, and
to undergo a sort of slow combustion by combination with
oxygen. Wherever it is presented by these animals, it is
always most brilliant upon the surfaces concerned in respira-
tion. The light continues for some days after death ; but
ceases at the commencement of putrefaction.1

397. In the class of INSECTS, there are several species which
have considerable luminous power ; and in these the emission
of light is for the most part confined to a small part of the
surface of the body, from which it issues with great brilliancy.
The luminous Insects are most numerous among the Beetle
tribe, and are nearly restricted to two families, the Elateridas,
and the Lampyridce. The former contains about 30 luminous
species, which are known as fire-flies; these are all natives of
the warmer parts of the New World. Their light proceeds
from two minute but brilliant points, which are situated one
on each side of the front of the thorax ; and from another

1 There are certain cases, however, in which the production of Light,
like that of Electricity (§ 423), appears to be a peculiar manifestation
of Nervous power. There is strong reason to believe that Nerve-force
may be directly metamorphosed (as it were) into these or other forms of
physical and vital force, according to the principle of " Correlation *r
now generally admitted as regards the Physical Forces.

330 LUMINOSITY OF INSECTS.

beneath the hinder part of the thorax, which is only seen
during flight. The light proceeding from these points is
sufficiently intense to allow small print to be read in the pro-
foundest darkness, if the insect be held in the fingers and
be moved along the lines ; and the natives of the coun-
tries where they are found (particularly in St. Domingo,
where they are abundant) use them instead of candles in
their houses, and tie them to their feet and heads, when
travelling at night, to give light to their path through the forest.
In all the luminous species of this family, the two sexes are
equally phosphorescent.

398. The family Lampyridce contains about 200 species
known to be luminous ; the greater part of these are natives

of America, whilst others are
widely diffused through the
Old World. In most of these,
the light is most strongly dis-
played by the female, which
is usually destitute of wings,
so that it might be mistaken
for a larva. The species of

Fig. 174.— MALE AND FEMALE GLOW- OUT own country is known as

the Glow-worm (fig. 174).

399. The light of the Glow-worm issues from the under
surface of the last three abdominal rings. The lumiaous
matter, which consists of little granules, is contained in
minute sacs covered with a transparent horny lid ; and this
exhibits a number of flattened surfaces, so contrived as to
diffuse the light in the most advantageous manner. The sacs
are mostly composed of a close network of finely-divided air-
tubes (§ 321), which ramify through every part of the granular
substance ; and it appears that the access of air through these
is a necessary condition of the phosphorescence. For if the
aperture of the large trachea which supplies the luminous sac
be closed, the light ceases ; but if the sac be lifted from its
place, without injuring the trachea, the light is not inter-
rupted. All the luminous insects appear to have the power
of extinguishing their light ; and this they probably do when
alarmed by approaching danger. The sudden extinction of
the light is probably due to the animal's power of closing the
aperture of the trachea.

LUMINOSITY OF INSECTS. 331

400. There are a few other Insects not included in these
families, which are reputed to possess luminous powers ; and
of these the most remarkable are the Fulgorce, or Lantern-
flies (fig. 175); of which one species inhabits Guiana, whilst
another is a native of

China. These are in-
sects of very remark-
able form, having an
extraordinary proj ection
upon the head ; and
this is the part said to
be luminous. The au-
thority for the assertion,

however, is doubtful; Fig" 175-F»—

and many Entomologists who have captured the insect, have
denied the phosphorescent power imputed to it. But it is not
impossible that the female only may possess it, and that it may
only be manifested at one part of the year. One of the common
English species of Centipede, which is found in dark, damp
places, beneath stones, &c., is slightly luminous ; and the
common Earthworm is also said to be so at the breeding
season.

401. Of the particular objects of this provision in the
Animal economy, little is known, and much has been con-
jectured. It is not requisite to suppose that its purposes are
always the same ; the circumstances of the diiferent tribes
which possess it being so different. The usual idea of its use
in Insects, — that it enables the sexes of the nocturnal species
to seek each other for the perpetuation of the race, — is pro-
bably the correct one. The light is more brilliant at the
season of the exercise of the reproductive functions, than at
any other ; and is then exhibited by animals which do not
manifest it at any other period. Moreover, it is well known
that the male Glow-worm, — which ranges the air, whilst the
female, being destitute of wings, is confined to the earth, — is
attracted by any luminous object; as are also the Fire-flies,
which may be most easily captured by carrying a torch or
lantern into the open air : so that the poetical language
in which this phosphorescence is described as " the lamp of
love — the pharos — the telegraph of the night, which marks
by its scintillations, in the silence of the night, the spot

332 PHOSPHORESCENCE OF DECAYING ANIMAL MATTER.

appointed for the lovers' rendezvous," would not seem so incor-
rect as the ideas of poets on subjects of Natural History too
frequently are. — Regarding the uses of the luminosity of the
lower marine tribes, it is more difficult to form a definite
idea ; since many of them are fixed to one spot during the
whole of life, and in many others the sexes do not require to
seek each other.

402. It not unfrequently happens, that an evolution of
light takes place from the bodies of animals soon after their
death, but before their decomposition has advanced far.
This has been most frequently observed to proceed from the
bodies of Fishes, Mollusks, and other marine tribes ; but it
has been seen also to be evolved from the surface of land
animals, and even from the Human body. Indeed, some
well-authenticated cases have been put on record, in which a
considerable amount of light was given off from the faces of
living individuals, who were near their end. All animal
bodies contain a considerable quantity of phosphorus (§ 166) ;
and it is by no means impossible that some peculiar compound
of this substance may be formed, during the early stages of
decomposition, or even before death, which may, by its slow
combustion, give rise to the luminous appearance. It appears
that the whole substance of the body of the Fire-flies is phos-
phorescent ; for, according to an early historian of the West
Indies, "many wanton wilde fellowes" rub their faces with
the flesh of a killed fire-fly, "with purpose to meet their
neighbours with a flaminge countenance."

Animal Heat

403. One of the conditions necessary for the performance
of Vital action, is a certain amount of warmth ; and we have
seen that the animals which alone are capable of retaining
their activity in the coldest extremes of temperature, are those
which have the power of generating heat within themselves,
and thus of keeping-up the temperature of their bodies to a
high standard. Those which do not possess a power of this
kind, are either rendered completely inactive, even by a com-
paratively moderate cold, or are altogether destroyed by it.
Those which ordinarily do possess this power are destroyed
even more rapidly by cold, if from any cause the production
of heat within their bodies be interrupted ; for they are the

TEMPERATURE OF COLD-BLOODED ANIMALS. 333

animals whose vital actions are the most varied and energetic,
and in which an interruption to any one of them most
speedily brings the rest to a stand. The inquiry into the
amount of heat generated by different animals, and into the
sources of its production, is one, therefore, of great practical
importance.

404. Our knowledge of the heat evolved by the lower In-
vertebrated animals is very limited; but it is probable that
in most of them the temperature of the body follows that
of the element they inhabit, keeping a little above it for a
time, if it be much lowered. Thus, when water containing
Animalcules is frozen, they are not at once destroyed; but
each lives for a time in a small uncongealed space, where the
fluid seems to be kept from freezing by the heat liberated
from its body. The temperature of Earthworms, Leeches,
Snails, and Slugs, ascertained by introducing a thermometer
into the midst of a heap of them, is usually about a degree or
two above that of the atmosphere ; and they also have the
power of resisting for a time the influence of a degree of cold,
which would otherwise iTnTnp.rHa.te1y freeze their bodies.

405. In the cold-blooded Vertebrata, also, the heat of the
body is almost entirely dependent upon that of the surround-
ing element. Thus most FISHES are incapable of maintaining
a temperature more than two or three degrees higher than
that of the water in which they live ; and the warmth of
their bodies consequently rises and falls with that of the sea,
river, or lake they may inhabit. There are, however, a few
marine Fishes which have the power of maintaining a tem-
perature 10 or 12 degrees higher than that of the sea ; and
these are peculiar for the activity of their circulation, and for
the deep colour of their blood, which possesses red particles
(§ 229) enough to give to the muscles a dark red colour, like
that of meat. The Thunny, a fish which abounds in the
Mediterranean, where there are extensive fisheries for it, is
one of this group. — It is to be remembered that the animals
of this class are less liable to suffer from seasonal alternations of
temperature, than are those which inhabit the air. In climates
subject to the greatest atmospheric changes, the heat of the
sea is comparatively uniform throughout the year, and that of
deep lakes and rivers is but little altered. Many have the
power of migrating from situations where they might other-

334 TEMPERATURE OF FISHES AND REPTILES.

wise have suffered from cold, into deep waters ; and those
species which are confined to shallow lakes and ponds, and
which are thus liable to be frozen during the winter, are fre-
quently endowed with sufficient tenacity of life, to enable
them to recover after a process which is fatal to animals much
lower in the scale. Fishes are occasionally found imbedded
in the ice of the Arctic sea.s ; and some of these have been
known to revive when thawed.

406. In REPTILES, the power of maintaining an uniform tem-
perature is somewhat greater; being especially shown when
the external temperature is reduced very low. Thus when
the air is between 60° and 70°, the body of a Reptile will
be nearly of the same heat; but when the air is between
40° and 50°, it may be several degrees higher. Frogs and
other aquatic Reptiles have a remarkable power of sustaining
a temperature above that of freezing, when the water around
is not only congealed, but is cooled down far below the
freezing-point. Thus in ice of 21°, the body of an edible
frog has been found to be 37 1° ; and even in ice of 9°, the
animal has maintained a temperature of 33°. In these cases,
as in Animalcules, the water in immediate contact with the
body remains fluid, so long as the animal can generate heat ;
but at last it is congealed, and the body also is completely
frozen. But it is certain that Frogs, like Fishes, may be
brought to life again, after the fluids of their bodies have
been so completely congealed that their limbs become quite
brittle ; it is not known, however, whether this may happen
with other Reptiles. It would appear that among Reptiles,
as among Fishes, some of the more active species have the
power of maintaining their bodies at a temperature consider-
ably higher than that of the atmosphere ; thus in some of the
more agile of the Lizard tribes, the high temperature of 86°
has been noticed, when the external air was but 71°.

407. The only classes of animals in which a constantly
elevated temperature is kept up, are BIRDS and MAMMALS.
The bodily heat of the former varies from 100° to 112°; the
first being that of the Gull, the last that of the Swallow. In
general we find that the temperature is the highest in species
of rapid and powerful flight ; and least in those which inhabit
the earth. Birds that are much in the water have a special
provision for retaining within their bodies the heat which

TEMPERATURE OF WARM-BLOODED ANIMALS. 335

would otherwise be too rapidly conducted away ; their bodies
being clothed with a thick and soft down, which is rendered
impenetrable to fluid by an oily secretion applied with the
bill. The temperature of MAMMALS generally seems to range
from 96° to 104°; but that of the Bat, and probably of
other hybernating species, appears to be frequently much
below the lower of these limits, even when the animals are in
their ordinary activity. The mean or average heat of the
body of Man is about 100°; but it has been observed as
low as 96^° when the temperature of the air was 60°, and as
high as 102° when the air was at 82°. As a variation of 5J°
may occur when the range of the external temperature of the
air is only from 60° to 82°, it is probable that observations
made in cold climates will show that the temperature of the
body may be still further lowered, when that of the air around
is much depressed. But it seems that, in Man, as in other
animals, the lower the temperature of the air around, the
greater is his power of generating heat within his body, to
keep up the necessary standard; and no observations yet
made indicate that the temperature of the body ever falls
below 95° in health.

408. The young of warm-blooded animals have usually less
power of maintaining an independent heat than adults. The
embryo, whether in the egg, or within the body of the parent,
is dependent, upon external sources for the heat necessary to
its full development. The contents of the egg, when lying
under the body of its parent, are so situated, that the germ-
spot (§ 756) is brought into the nearest neighbourhood of
the source of warmth. It is not usually until some weeks
after the hatching of Birds, or the birth of Mammals, that
the young animals have the power of maintaining an inde-
pendent temperature. Thus young Sparrows, taken from the
nest a week after they were hatched, were found to have a
temperature of from 95° to 97° ; but this fell in one hour to
66J°, the temperature of the atmosphere being at the same
time 621° ; and the rapid cooling was proved not to be due
to the want of feathers alone. There are some birds, how-
ever, which can run about and pick up their food the moment
they are hatched : these come into the world in a more
advanced condition than the rest, and can maintain their
temperature with little or no assistance. We find the same

336 TEMPERATURE OP WARM-BLOODED ANIMALS.

to be the case among Mammals. There are some species
(such as the Guinea-pig) whose young are able from birth to
walk and run, and to take the same food with the mother ;
and these have from the first the power of maintaining a
steady temperature when the air around is not very cold.
But, in general, the young of Mammals are much less advanced
at the time of birth, being not unfrequently born blind as
well as helpless j and they require considerable assistance, in
keeping up their heat, from the parent or nurse. Thus the
temperature of new-born puppies, removed from the mother,
will rapidly sink to between 2° and 3° above that of the air.

409. These facts are of extreme practical importance, in
regard to the treatment of the Human infant. Though not
destitute of sight, at its entrance into the world, like the
young of the Cat, Dog, or Rabbit, it is equally helpless, and
dependent upon its parent not only for support but for
warmth. And as the Human body is longer in arriving at
its full development than is any other, so is it necessary that
this assistance should be longer afforded. This assistance is
the more necessary in the case of infants born prematurely ;
and it should be kept up during the years of childhood,
gradually diminishing with age. It is too frequently neglected,
by those who are well able to afford it, under the erroneous
idea of hardening the constitution ; and the want of it, con-
sequent upon poverty, is one of the most fertile sources of the
great mortality among children of the poorer classes. This
is easily proved by the proportional number of deaths which
take place in different parts of the year, at different ages.
During the first month of infant life, the winter mortality is
nearly double that of the summer ; though there is very little
difference between the two seasons in the mortality of adults.
But in old age the difference again manifests itself to the
same amount as in infants ; for old persons are almost equally
deficient in the power of maintaining heat; they complain
that their " blood is chill," and suffer greatly from exposure
to cold.

410. The class of INSECTS presents us with some very
interesting phenomena. In the larva and pupa states, the
temperature of the body is never more than from J° to 4°
above that of the surrounding medium ; but, in many tribes,
the temperature of the perfect' Insect rises so high, when it is

PRODUCTION OP HEAT BY INSECTS. 337

in a state of activity, that it might be at such times called a
warm-blooded animal ; though in the states of abstinence,
sleep, and inactivity, its temperature falls again nearly to that
of the atmosphere. A single Humble-bee, inclosed in a phial
of the capacity of 3 cubic inches, had its temperature speedily
raised by violent excitement, from that of rest (2° or 3° above
that of the atmosphere) to 9° above that of the external air ;
and communicated to the air in the phial as much as 4° of
heat within five minutes. In another similar experiment, the
temperature of the air in the phial was raised nearly 6° in
eight minutes. It is among the active Butterflies, and the
Hymenopterous insects (Bee and Wasp tribe), which pass
nearly the whole of their active condition on the wing, that
we find the highest temperature ; and next to them are the
more active of the Beetle tribe. Those of the latter which
seldom leave the ground, have little power of producing heat.
411. The greatest manifestation of this power is shown
among Insects which live in societies ; most of which belong
to the order Hymenoptera. It has been seen that the body
of a Humble-bee, in a state of activity, has a temperature of
about 9° above that of the atmosphere ; but its nest has been
found to have an ordinary temperature of from 14° to 16° abova
the air, and from 17° to 19° above that of the chalk bank in.
which it was formed. The production of heat is increased
to a most extraordinary degree, when the pupae are about to
come-forth from their cells as perfect Bees, requiring a higher
temperature for their complete development. This is fur-
nished by a set of Bees termed Nurse-bees, which are seen
crowding upon the cells and clinging to them, for the purpose
of communicating to them their warmth; being themselves
evidently very much excited, and respiring rapidly, even at
the rate of 130 or 140 inspirations per minute. In one
instance, the thermometer introduced amongst seven nursing-
bees stood at 92|°, whilst the temperature of the external air
was but 70°. In Hive-bees, whose societies are large, this
process occasions a still more remarkable elevation of tempe-
rature ; for a thermometer introduced into a hive during May
has been seen to rise to 9 6° or 98°, when the range of atmospheric
temperature was between 56° and 58°. In September, when
the bees are becoming stationary, the temperature of the hive
is but a few degrees above that of the air. It was formerly

338 SOURCES OP ANIMAL HEAT.

supposed that Bees do not become torpid during the winter ;
but this is now known to be a mistake. Bees, like other
Insects, pass the winter in a state of hybernation ; but their
torpidity is never so profound as to prevent their being aroused
by moderate excitement. The temperature of a hive is usually
from 5° to 20° above that of the atmosphere ; being kept at or
above the freezing-point, when the air is far below it. Under
such circumstances, their power of generating heat is most
remarkable. In one instance, the temperature of a hive, of
which the inmates were aroused by tapping on its outside,
was raised to 102°; whilst a thermometer in a similar hive
that had not been disturbed, was only 48^°; and the tempe-
rature of the air was 34|°.

412. The evolution of Heat in the Animal body may now
be stated with tolerable certainty to depend for the most part
on the union, by a process resembling ordinary combustion,
of the carbon and hydrogen which it contains, with oxygen
taken-in from the air in the process of Eespiration. It has
been elsewhere shown that, even in Plants, this union, when
it takes place with sufficient rapidity, is accompanied by the
disengagement of a considerable amount of heat (VEGET. PHYS.,
§ 381); and in all those Animals which can maintain an
elevated temperature, we find a provision for this union, both
in regard to the constant supply of carbon and hydrogen from
the body, and to the introduction of oxygen from the air.
The supply of carbon and hydrogen may be derived (as already
shown, § 157), either directly from the food, a large proportion
of which is thus consumed in many animals without ever
forming part of the tissues of the body ; or it may be the
result of the waste of the tissues, especially of the muscular,
consequent upon their active employment (§ 160), and con-
verted into a substance peculiarly adapted for combustion by
the agency of the liver (§ 366). Or, again, it may be derived
from the store laid-up in the system in the form of fat ; which
seems destined to afford the requisite supply, when other
sources fail. Thus, when food is withheld, or when dis-
ease prevents its reception, the fat in the body rapidly
diminishes ; being burnt off, as it were, to keep up the
temperature of the system. This is the case, too, during
hibernation; the animals which undergo this change usually
accumulating a considerable amount of fat in the autumn, and

SOURCES OF ANIMAL HEAT. 339

being observed to come forth from their winter quarters, with
the return of spring, in a very lean condition.

413. The consumption of oxygen and the production of
carbonic acid are found to bear, in every animal, a very close
relation to the amount of heat liberated at the time. Thus in
warm-blooded animals, the respiratory function is much more
active than in the cold-blooded ; but when the former are
reduced to the state of cold-blooded animals, as occurs in
hybernation (§ 309), their respiration is proportionately low.
On the other hand, whenever the temperature of an animal
is quickly raised by any extraordinary stimulus, above that
which it was previously maintaining, it is always by means of
increased activity of the respiratory movements, and augmented
consumption of oxygen. Thus during the incubation of Bees
(§ 411), the insect, by accelerating its respiration, causes the
evolution of heat and the consumption of oxygen to take place
at least twenty times as rapidly as when in a state of repose.
The same takes place when a hybernating animal is roused ;
and it is remarkable that even extreme cold will effect this
for a time ; but the animal, if exposed for too long a period
to a very low temperature, will not be able to resist its
influence, and will perish.

414. Although the combustion of carbon and hydrogen
within the Animal body is undoubtedly the chief source of
the production of heat, yet it must not be left out of view that
there are other chemical changes in the system, which also con-
tribute to it, though in a minor degree (§ 343). Of this kind
are the oxidation of the sulphur and phosphorus which enter
the body in the organic compounds used as food, and which,
being united by a combustive process with oxygen, pass out
of the system in the urine, in the form of sulphuric and phos-
phoric acids, combined with alkaline bases (§ 367).

415. Besides all these sources, it seems probable from
various considerations, that Heat may occasionally be generated,
like light and electricity, by the direct agency of the Nervous
system; as one of the modes of force into which nervous
power may be metamorphosed. Of course, in any such gene-
ration of heat, there must be the same consumption of nervous
tissue, as would occur if its equivalent of nerve-force had been
manifested.

340 PRODUCTION OF ELECTRICITY BY ANIMA-LS.

Animal Electricity.

416. Almost all chemical changes are attended with some
alteration in the electric state of the bodies concerned ; and
when we consider the number and variety of these changes in
the living animal body, it is not surprising that disturbances
of its electric equilibrium should be continually occurring.
But these, when slight, can only be detected by very refined
means of observation ; and it is only when they become con-
siderable, that they attract notice. Some individuals exhibit
electric phenomena much more frequently and powerfully
than others ; and cases are occasionally recorded in the
Human subject, in which there has been a most decided pro-
duction of electricity, which manifested itself in sparks when-
ever the individual was insulated.

417. The sparks and crackling noise, however, which are
occasionally observed on pulling off articles of dress that
have been worn next the skin, especially in dry weather, are
partly due to the friction of these materials with the surface and
with each other ; the production of electricity being greatly influ-
enced by their nature. Thus, if a black and a white silk stocking
be worn, one over the other, on the same leg, the manifesta-
tion of electricity when they are drawn off, especially after a
dry frosty day, is most decided ; but this would also be the
case if they were simply rubbed together, without any con-
nexion with the body.

418. In most animals with a soft fur, sparks may be pro-
duced by rubbing it, especially in dry weather ; this is familiar
to most persons in the case of the domestic Cat. But the
electricity thus produced seems occasionally to accumulate in
the animal, as in the Ley den jar, so as to produce a shock.
If a cat be taken into the lap, in dry weather, and the left
hand be applied to the breast, whilst with the right the back
be stroked, at first only a few sparks are obtained from the
hair; but after continuing to stroke for some time, a smart
shock is received, which is often felt above the wrists of both
the arms. The animal evidently itself experiences the shock,
for it runs off with terror, and will seldom submit itself to
another experiment.

419. But there are certain animals which are capable of
producing and accumulating electricity in large quantities, by

ELECTRIC FISHES I GYMNOTUS.

341

means of organs specially adapted for the purpose; and of
discharging it at will, with considerable violence. It is re-
markable that all these belong to the class of Fishes ; l and that
they should differ alike in their general conformation, and in
their geographical distribution. Thus, the two species of
Torpedo, belonging to the Eay tribe, are found on most of
the coasts of the Atlantic and Mediterranean ; sometimes so
abundantly, as to be a staple article of food. The Gfymnotus,
or Electric Eel, is confined to the rivers of South America.
The Malapterurus (commonly known as the Silurus) which
approaches more nearly to the Salmon tribe, occurs in the
Niger, the Senegal, and the Nile; and there are two other less
known Fishes, said to possess electric properties, which inhabit
the Indian seas.

420. Of all these, the Gymnotus (fig. 176) is the one which
possesses the electric power in the most extraordinary degree.
It is an eel-like fish, having nothing remarkable in its external
appearance ; its usual length is from six to eight feet, but it
is said occasionally to attain
the length of twenty feet. This
fish will attack and paralyse
horses, as well as kill small
animals ; and the discharges of
the larger individuals sometimes
prove sufficient to deprive even
Men of sense and motion. This
power is employed by the fish to
defend itself against its enemies;
and even, it is said, to destroy
its prey (which consists of other
fishes) at some distance; the
shock being conveyed by water,
as a lightning-conductor conveys to the earth the effects of
the electric discharge of the clouds. The first shocks are
usually feeble ; but as the animal becomes more irritated,
their power increases. After a considerable number of power-
ful discharges, the energy is exhausted, and is not recovered
for some time ; and this circumstance is taken advantage 01
in South America, both to obtain the fishes (which afford

1 Certain Insects and Mollusks have been said to possess electrical
properties ; but no special electric organ has been discovered in them.

Fig. 176.— GYMNOTUS.

342 ELECTRIC FISHES I TORPEDO.

excellent food), and to make the rivers they infest passable
to travellers. A number of wild horses are collected in the
neighbourhood, and are driven into the water ; the Gymnoti
attack these, and speedily stun them, or even destroy their
lives by repeated shocks ; but their own powers of defence
and injury are exhausted in the same degree, and they then
become an easy prey to their captors.

421. The shock of the Torpedo (fig. 177) is less powerful;
but it is sufficient to benumb the hand that touches it. From
its proximity to European shores, this fish
has been made the subject of observation
and experiment more completely than the
other ; and some curious results have been
attained. It seems essential to the proper
reception of the shock, that two parts of the
body should be touched at the same time ;
and that these two should be in different
electrical states. The most energetic dis-
charge is procured from the Torpedo, by
touching its back and belly simultaneously ;
the electricity of the back being posi-
:DO> tive, and that of the belly negative. When
two parts of the same surface, at an equal distance from the
electric organ, are touched, no effect is produced ; but if one
be further from it than the other, a discharge occurs. It has
been found that, however much a Torpedo is irritated through
a single point, no discharge takes place ; but the fish makes
an effort to bring the border of the other surface in contact
with the offending body, through which a shock is then sent.
This, indeed, is probably the usual manner in which its dis-
charge is effected. If the fish be placed between two plates
of metal, the edges of which are in contact, no shock is per-
ceived by the hands placed upon them, since the metal is a
better conductor than the human body ; but if the plates be
separated, and, while they are still in contact with the opposite
sides of the body, the hands be applied to them, the discharge
is at once rendered perceptible, and may be passed through a
line formed by the moistened hands of two or more persons.
In the same manner, also, a visible spark may be produced ;
but this is less easily obtained from the Torpedo than from
the Gymnotus. — As to the uses of the electrical organs to the

STRUCTURE OF ELECTRIC APPARATUS.

343

Fishes which possess them, no definite information can be
given. It is doubtful to what extent they are employed in
obtaining food ; since it is known that the Gymnotus eats
very few of the fishes which it kills by its discharge ; and
that Torpedos kept
in captivity do not
seem disposed to ex-
ercise their powers
on small fishes placed
in the water with
them. The chief use
of their electrical
power appears to be,
to serve as a means
of defence against
their enemies.

422. The electric
organs of the Torpedo
(fig. 178) are of flat-
tened shape, and
occupy the front and
sides of the body,
forming two large
masses, which ex-
tend backwards and
outwards from each
side of the head.
They are composed
of two layers of mem-
brane, the space be-
tween which is di-
vided by Vertical "&*£• l^S. — ELECTRIC APPARATUS OF TORPEDO.

partitions into hex- c, brain; me, spinal cord; o, eye and optic nerve; e,
afOTial fplls ^ lilrp electric organs ; tip, pneumogastric nerve, supplying

us, e, like electric or*gans'. ™ fateral n« - Jn»d ££**

those of a honey-
comb, the ends of which are directed towards the two sur-
faces of the body. These cells — which are filled with a whitish
soft pulp, somewhat resembling the substance of the brain,
but containing more water — are again subdivided horizontally
by little membranous partitions ; and all these partitions are
profusely supplied with vessels and nerves. — The electrical

344 STRUCTURE AND ACTIONS OF ELECTRIC ORGANS.

organs of the Gymnotus are essentially the same in structure,
but differ in shape in accordance with the conformation of the
animal; they occupy one-third of its whole bulk, and run
nearly along its entire length, being arranged in two distinct
pairs, one much larger than the other. In the Malapterurus

Fig. 179.- ELECTRIC MALAPTKRURUS.

(fig. 179), there is not any electrical organ so definite as those
just described ; but the thick layer of dense areolar tissue
which completely surrounds the body, appears to be sub-
servient to this function ; being composed of tendinous fibres
interwoven together, and containing a gelatinous substance in
its interstices, so as to bear a close analogy with the special
organs of the Torpedo and Gymnotus.

423. In all these instances, the electrical organs are sup-
plied with nerves of very great size, larger than any others in
the same animals, and larger than any nerves in other animals
of like bulk. These nerves arise from the top of the spinal
cord, and seem analogous to the pneumogastric nerve (§ 458)
of other animals. The influence of these nerves is essential to
the action of the electric organs. If all the trunks on one
side be cut, the power of the corresponding organ will be
destroyed, but that of the other may remain uninjured. If
the nerves be partially destroyed on either or both sides, the
power is retained by the portions of the organs which are still
connected with the brain by the trunks that remain. Even
slices of the organ entirely separated from the body, except by
a nervous fibre, may exhibit electrical properties. Discharges
may be produced by irritating the part of the nervous centres
from which the trunks proceed (so long, at least, as they are
entire), or by irritating the trunks themselves. In all these
respects, there is a strong analogy between the action of the
nerves on the electric organs and on the muscles (Chap, xn.) ;
and it may be safely affirmed that the Nervous force develops

ELECTRIC MANIFESTATIONS OF MUSCLE AND NERVE. 345

Electricity by its action on the electrical organs, just as it pro-
duces Motion by its action on the muscles.

424. It is another interesting point of analogy between the
action of Muscles and that of Electrical organs, that the former,
like the latter, is attended with a change of electric state. In
any fresh vigorous muscle, there is a continual current from the
interior to exterior, which appears to depend upon the fact
that the actions connected with the nutrition and disintegra-
tion of its tissue go on more energetically in the interior of
the muscle, than they do near its surface, where the proper
muscular fibres are mingled with a large proportion of areolar
and tendinous substance. During the contraction of a muscle,
this current is diminished in intensity, or is even entirely
suspended; but it is renewed again, so soon as the muscle
relaxes. — An electric current has been found to exist also in
Nerves ; and its conditions are in most respects similar to
those of the muscular current.

CHAPTER X.

FUNCTIONS OF THE NERVOUS SYSTEM.

425. WE have now completed our consideration of the
Functions of Organic or Vegetative Life; those changes,
namely, in the Animal body, which are concerned in the
maintenance of its own fabric ; and which, although per-
formed in a different mode, and having different objects to
fulfil, are essentially the same in character with those which
take place in Plants. The first and most striking difference of
mode results, as we have seen, from the nature of the food of
Animals, which requires that they should possess a cavity for
its reception, and a chemical and mechanical apparatus for its
digestion (or reduction to the fluid form), in order that it may
be prepared for absorption into the vessels. In regard to the
absorption of the aliment, and its circulation through the
system, there is but little essential difference between Plants
and the lower Animals ; but in the higher tribes of the latter,
we find that a muscular organ having the action of a forcing-
pump is appended to the system of tubes in which the fluid

346 FUNCTIONS OP ORGANIC AND ANIMAL LIFE.

circulates, in order to drive it through them with the requisite
certainty and energy. The respiration of Animals, again, is
essentially the same with that of Plants j the chief difference
being that, in order to secure the active performance of this
important function, the higher Animals are provided with
a complex apparatus of nerves and muscles, by which the air
or water in contact with the aerating surface is continually
renewed. And in regard to the functions of secretion and
excretion, we have seen that, though there is a wide difference
in the form of the organs by which they are executed, they
are the same in essential structure ; and that the difference in
their mode of operation consists chiefly in this, that their
products in the Animal are destined to be carried out of the
body, instead of being retained within it, as in Plants.

426. In regard to the immediate objects of these functions,
also, there is but little essential difference ; for in both in-
stances it is the conversion of alimentary materials into living
organized tissue. But the ultimate purpose of this tissue is
far from being the same in the two kingdoms. Nearly all the
nourishment taken-in by Plants is applied to the extension of
their own fabric ; and hence there is scarcely any limit to the
size they may attain. There is very little waste or decay of
structure in them, the parts once formed (with the exception
of the leaves and flowers) continuing to exist for an indefinite
time ; this is a consequence of the simply physical nature of
the functions of the woody structure, which has for its chief
object to give support to the softer parts, and to serve as the
channel for the movement of the fluid that passes towards and
from them. — The case is very different in regard to Animals.
With the exception of those inert tribes which may be com-
pared with Plants in their mode of life, we find that the
whole structure is formed for motion ; and that every act of
motion involves a waste or decay of the fabric which executes
it. An energetic performance of the nutritive actions is re-
quired, therefore, in the more active Animals, simply to make
good the loss which thus takes place ; we find, too, that their
size is restrained within certain limits ; so that, instead of the
nourishment taken into the body being applied, as in Plants,
to the formation of new parts, it is employed for the most part
in the simple repair of the old. Thus we may say that, whilst
the ultimate object of Vegetable Life is to build up a vast

GENERAL PURPOSES OP NERVOUS SYSTEM. 347

fabric of organized structure, the highest purpose of the
Organic Life of Animals is to construct, and to maintain in a
state fit for use, the mechanism which is to serve as the
instrument of their Functions of Animal Life, — enabling them
to receive sensations, and to execute spontaneous movements,
in accordance with their instincts, emotions, or will.

427. This mechanism consists of two kinds of structure, —
the Nervous and Muscular, — which have entirely different
offices to perform. The Nervous system is that which is the
actual instrument of the Mind. Through its means, the indi-
vidual becomes conscious of what is passing around him ; its
operations are connected, in a manner we are totally unable to
explain, with all his thoughts, feelings, desires, reasonings,
and determinations ; and it communicates the influence of
these to his muscles, exciting them to the operations which
he determines to execute. But of itself it cannot produce
any movement, or give rise to any action ; any more than
the expansive force of steam could set a mill in motion,
without the machinery of the Steam-engine for it to act upon.
The Muscular System is the apparatus by which the move-
ments of the body are immediately accomplished ; and these
it effects by the peculiar power it possesses of contracting
upon the application of certain stimuli, of which Nervous
agency is the most powerful.

428. Although the presence of a Nervous System is the
most distinguishing attribute of Animals, yet we do not en-
counter it by any means universally. For among certain of
those classes which possess on other grounds a title to be
ranked in the Animal kingdom, it seems beyond a doubt that
no nervous system exists; and there are many others in
which, if it be present at all, its condition is so rudimentary,
that it can take little share in directing the general operations
of the organism. The life of such beings, in fact, is chiefly
vegetative in its nature ; their movements are not dissimilar in
kind to those that we witness in Plants ; and their title to a
place in the Animal kingdom chiefly rests upon the nature of
their food, and the mode in which they appropriated (§§ 7, 8).
This is the case with the Protozoa generally (§§ 128—137),
and in a less degree with Zoophytes (§§ 121 — 127).

429. In proportion as we ascend the Animal series, how-
ever, we find the Nervous System presenting a greater and

348 GENERAL PURPOSES OF NERVOUS SYSTEM.

greater complexity of structure, and obviously acquiring
higher and yet higher ' functions ; so that in Vertebrated
animals, and more especially in Man, it is evidently that
portion of the organism to whose welfare everything else is
brought*into subordination (§ 73). And we observe this to be
the case, not merely in virtue of its direct instrumentality as
the organ of Mind, but also in the intimacy of its relation to
the Organic functions, which are placed in great degree under
its control. Thus we find that the inlets and outlets to the
Digestive apparatus, the mechanism by which food is brought
to the mouth and conveyed into the stomach, and that by
which indigestible matters are voided from the large in-
testine, are subjected to its influence ; although the act of
digestion itself, and the passage of the aliment from one end
of the canal to the other, are performed independently of it.
So, again, the movements of Eespiration, by which the air
within the lungs is renewed as fast as it becomes vitiated, are
not only effected through its instrumentality, but are placed,
for the purposes of Vocalization, as far under the control of
the Will as would be consistent with a due regard to the
safety of life. Yet among many of the lower tribes of Animals,
the ingestion of food and the aeration of the circulating fluid
are provided-for by ciliary action alone (§ 45), in which we
have every reason to believe that nervous agency has no par-
ticipation whatever.

430. If, taking the Nervous System of Man as the highest
type of this apparatus, we analyse in a general' way the actions
to which it is subservient, we find that they may be arranged
under several distinct groups, which it is very important to
consider apart, whether we are studying his psychical J functions
or those of the lower animals. — 1. The simplest mode of its
action is that in which an impression made upon an afferent
nerve excites, through the ganglionic centre in which it ter-
minates, an impulse in the motor nerve issuing from it, which,
being transmitted by it to the muscular apparatus, calls forth
a respondent movement. Of this action, which is called reflex,
or " excito-motor," and which may be performed without any
consciousness either of the impression or of the motion, we
have already seen examples in the movements of Deglutition

1 This term is used to designate the sensorial and mental endowmenti
of Animals, in the most comprehensive sense.

MODES OF ACTION OF NERVOUS SYSTEM. 349

(§ 194) and Eespiration (§ 340). — 2. If the ganglionic centre
to which the impression is conveyed, should be one through
which the consciousness is necessarily affected, sensation be-
comes a necessary link in the circular chain ; and the action
is distinguished as consensual, or " sensori-motor." The closing
and opening of the pupil of the eye, in accordance with the
amount of light that falls upon the retina, together with other
remarkable adjustments which are involuntarily made in the
working of that wonderful organ, are characteristic examples
of this class. In the foregoing operations no mental change
higher than simple consciousness of impressions — that is to
say, Sensation, with which may be blended the simple feelings
of pleasure and pain — is involved. Such would appear to be
the condition of the Human infant on its first entrance into
the world, before the self-education of its higher faculties has
commenced ; and such is probably the state of Invertebrated
animals generally, whose instinctive actions seem to be refer-
able to one or other of the foregoing classes.

431. But Sensation is the very lowest form of purely
Mental action. When the outness or externality of the
objects which give rise to our sensations has been recognised
by perception, we begin to form ideas respecting their nature,
qualities, &c. ; and it is in the various processes of association,
comparison, &c., to which these ideas are subjected, that our
Reasoning faculty consists. Now these processes may go on
in great degree automatically, that is, without any control or
guidance on our own part, as happens in the states of Dream-
ing, Reverie, and Abstraction ; and they may express them-
selves in action, as we see in the movements of a Somnambulist,
who may be said to be acting his dreams. This form (3) of
Nervous activity, which may be termed ideo-motor, seems to be
the ordinary mode in such of the lower animals as are governed
by Intelligence rather than by instinct (Chap, xiv.) ; but it is
abnormal and exceptional in Man. — With ideas are associated
feelings of various kinds, which constitute Passions and Emo-
tions; and these (4), when strongly excited, may become direct
springs of action, so powerful as even to master the control of
the Will, producing emotional movements.

432. In the well-regulated mind of Man, however, the Will
(5) possesses supreme direction over the whole current of
thought, feeling, and action : regulating the succession of the

350 NERVOUS SYSTEM OP BADIATA.

ideas ; keeping in check the passions and emotions, or, on the
other hand, promoting their healthful activity by directing the
attention to the objects of them ; and determining the move-
ments which the reason prompts : — and the acquirement and
right direction of such regulating power is the highest object
of all Education.

433. It will be recollected that every form of Nervous
System essentially consists of two kinds of nervous tissue, —
the tubular or fibrous, whose functions seem to be purely
conductive (§§ 60, 62), — and the vesicular or ganglionic, which
seems to be the seat of all the changes to which this apparatus
ministers, and the source of all its peculiar powers (§§ 61, 63).
— The principal forms under which this apparatus presents
itself in the several divisions of the Animal Kingdom, and
the general nature of the functions to which it is subservient
in each, will now be successively described in the ascending
series, from Zoophytes up to Man.

Structure and Actions of the Nervous System in the principal
Classes of Animals.

434. In most of the EADIATED classes, it is difficult to dis-
cover any distinct traces of a Nervous System ; the general
softness of their tissues being such, that it cannot be certainly
distinguished amongst them. It clearly exists, however, in
the highest group, the ECHINODERMATA ; and it presents an

extremely simple form, which par-
takes of the general arrangement of
parts in these animals. In the
Star-fish, for instance, it forms a
ring which surrounds the opening
into the stomach (fig. 180) ; this
ring consists of a nervous cord that
forms communications between five
ganglia, one of which is placed at
the base of each ray. These ganglia
appear to be all similar to each

Fig. ISO.-

position of the mouth. them sends a large trunk along its
own ray ; and two small branches
to the organs in the central disk. The rays being all similar
to each other in structure, it would appear that no one of these

NERVOUS SYSTEM OF BADIATA AND TUNICATA. 351 .

ganglia can have any controlling power over the rest. All the
rays have at their extremities what seem to be very imperfect
eyes ; and so far as these can aid in directing the movements
of the animal, it is obvious that they will do so towards all
sides alike. Hence there is no one part which corresponds
to the head of higher animals ; and the ganglia of the nervous
system, like the parts they supply, are but repetitions of one
another, and act independently of one another. Each would
perform its own individual functions if separated from the
rest ; but, in the entire animal, they are brought into mutual
relation by the circular cord, which passes from every one of
the five ganglia to those on either side of it. — In Man, as well
as in all the Vertebrated and Articulated animals, and in some
of the Mollusca, there is a like repetition of the parts of the
Nervous System on the two sides of the central line of the
body ; but the organs are only double, instead of being
repeated five times. Still the two hemispheres of the brain,
and the two halves of the spinal cord, in the Vertebrated
animal, — and the two halves of the chain of ganglia, in the
Articulated animal, — are as independent of one another as are
the five separate ganglia of the Star-fish ; and they are made
to act in mutual harmony by similar uniting bands of nervous
fibres, which are termed commissures.

435. In the nervous system of MOLLUSCA, we do not meet
with any such repetition of parts ; the body itself not pre-
senting this character. In the lowest and
simplest animals of this group, there exists
only a single ganglion, which may be
regarded as analogous to any one of the
ganglia of the Star-fish ; but in the higher,
we find the number of ganglia increased,
in accordance with the increase of the
functions which they have to perform.
The simplest form of the nervous system
in this class is seen in the accompanying
figure (fig. 181), which represents one of the
solitary TUNICATA, the A scidia. At a is seen
the orifice by which the water enters for sup- Fig. isi.— NERVOUS SYS-
plying the stomach with food, and for aerat- TEM OF ASCIDIA-
ing the blood (§ 114); and at b is the orifice by which the current
of water passes out again, after it has served these purposes.

352 NERVOUS SYSTEM OF TUNICATA AND CONCHIFEEA.

Between these orifices is the single ganglion c, which sends
filaments to both of them, and other branches which spread
over the surface of the mantle d. These animals are for the
most part fixed to one spot during nearly the whole of their
existence ; and they show but little sign of life, beyond the
continual entrance and exit of the currents already adverted
to. When any substance is drawn-in by the current, however,
the entrance of which would be injurious, it excites a general
contraction of the mantle ; and this causes a jet of water to
issue from both orifices, which carries the offending body to
a distance. And in the same manner, if the exterior of the
body be touched, the mantle suddenly and violently contracts.

436. These are the only actions, which, so far as we know,
the nervous system of these animals is destined to perform.
They scarcely exhibit any traces of eyes or other organs
of special sense; and the only parts that appear peculiarly
sensitive, are the small tentacula which guard the orifice a.
It would seem as if the irritation caused by the contact of any
hard substance with these, or with the general surface of the
animal, caused a reflex contraction of the mantle, having for
its result the getting-rid of the source of the irritation. Such
a movement could only be performed by the aid of a Nervous
system, which has the power of receiving impressions, and of
immediately exciting even the most distant parts of the body
to act in accordance with them. In the Venus' s Fly-trap and
Sensitive Plant (YEGET. PHYS., §§ 214, 391), an irritation
applied to one part is the occasion of a movement in another ;
but this takes place slowly, and in a manner very different
from the energetic and immediate contraction of the mantle
of the Tunicata.

437. In the CONCHIFERA, or animals inhabiting bivalve
shells, there are invariably at least two ganglia, having differ-
ent functions. The larger of these, corresponding to the single
ganglion of the Tunicata, is situated towards the posterior end
of the body (B, fig. 182), in the neighbourhood of the posterior
muscle ; and its branches are distributed to that muscle, the
mantle, the gills, and the siphons. But we find another gan-
glion, or rather pair of ganglia (A A), situated near the front
of the body, either upon or at the sides of the oesophagus, and
connected by a commissural band that arches over it ; these
ganglia receive nerves from very sensitive tentacula which

NERVOUS SYSTEM OF MOLLUSKS. 353

guard the mouth; and they evidently correspond, both in
position and functions, to the sensory ganglia of higher ani-
mals, whilst the posterior gan-
glion has for its office to regulate
the respiratory movements. In
the Pecten, however, as in other
Conchifera which possess a
foot (fig. 62), we find an addi-
tional ganglion (c), the pedal,
connected with the cephalic
ganglia, and sending nerve-
trunks to that organ. There
is good reason to believe that,
whilst the cephalic ganglia alone
are the instruments of sensation,
so that they exert a general

Control and direction over the Fig. m-NBKVous SYSTEM OF PECTEN.
movements Of the animal, the A A, cephalic ganglia ; B, branchial

pedal and branchial ganglia f£g£?; c' pedal ganglion; e'

minister to the reflex actions

(§ 433) of the organs which they supply.

438. A similar arrangement is
found in the higher Mollusks,
among which the ganglia are more
numerous, in accordance with the
greater variety of functions to be
performed. Of this we have an
example in the Aplysia, a sort of
sea-slug somewhat resembling those
formerly alluded to (§ 316). In
proportion as we ascend the scale,
we find the cephalic ganglia rising
higher and higher on the sides of
the oesophagus ; and in the Aplysia
they meet on the central line above
it, forming the single mass (A, fig.
183), which receives the nerves of
the eyes, tentacula, &c., and sends
branches of communication to the
other ganglia. The branches which
it sends backwards are three on each

A A

Fig. 183.— NERVOUS SYSTEM OB
APLYSIA.

354 NERVOUS SYSTEM OF MOLLUSKS.

side. Of these, one passes through the ganglionic masses cc, to
communicate with the ganglion B, which is the one connected
with the respiratory movements. The others are distributed
with the branches of the ganglia cc, the function of which is
double ; for one set of branches from each is distributed to
the mantle in general, every part of which (in these shell-less
Mollusks) is capable of contracting and giving motion to the
body; whilst another set is distributed to that thick and
fleshy part of it which is called its foot, and on which the
animal crawls (§ 107). There is another ganglion, D, lying
in front of the cephalic ganglion, and also receiving branches
of communication from it ; this ganglion is specially connected
with the actions of mastication and swallowing, and is called
the pharyngeal ganglion.

439. Thus we see that the cephalic ganglion sends branches
to all the other ganglia, though these having different functions,
do not communicate with each other; and thus every part
has two sets of nervous connexions, one with the cephalic
ganglia, and the other with its own ganglion. By the former,
the animal becomes conscious of impressions made upon it,
these impressions being converted in the cephalic ganglia into
sensations ; and the influence of its conscious power is exerted
through them upon the several parts of its body, causing
spontaneous motion. By the latter are produced those reflex
actions of the several organs, which do not require sensation,
but which depend upon the simple conveyance of an im-
pression to the ganglion, and the transmission of the resultant
motor impulse from it to the muscles supplied by its nerves.
A small part only of the Nervous System of Mollusks
ministers to the general movements of the body; and this
corresponds with what has been elsewhere stated (§ 107) of
the inertness which is their general characteristic, and of the
small amount of muscular structure which they possess.

440. On the other hand, in the ARTICULATED classes, in
which the apparatus of movement is so highly developed, and
whose actions are so energetic, we find the Nervous System
almost entirely subservient to this function. Its usual form
has been already described (§ 94) as a chain of ganglia con-
nected by a double cord, which commences in the head, and
passes backwards through the body. In general, we find a
ganglion (or rather a pair of ganglia united on the middle

NERVOUS SYSTEM OF ARTICULATED ANIMALS.

355

Fig. 184. — NERVOUS SYSTEM OF
AN INSECT.

line) in each segment; hence in the ANNELIDA and MYRIA-
PODA, the ganglia are very numerous ; but they are pro-
portionably small. In INSECTS (fig. 184), the number of
segments, and consequently of ganglia, never exceeds thirteen ;
and the ganglia are larger. — What-
ever be the number of the ganglia,
they are usually but repetitions of
one another, the functions of each
segment being the same with those
of the rest. The nerves proceeding
from them are chiefly distributed to
the muscles of the legs ; or, where
legs do not exist (as in the Leech),
to the muscles that give motion to
the body. This is the case in the
larva of the Insect, as in the Cen-
tipede or Nereis ; but in the per-
fect Insect the case is different ; for
the apparatus of locomotion is con-
fined to the thorax (§ 97), and
the segments of the abdomen have

no members. We accordingly find that the ganglia of the
thorax, from which the legs and wings are supplied, are very
much increased in size, and are sometimes concentrated into
one mass ; whilst those of the abdomen are very small, one or
two of them occasionally disappearing altogether.

441. A good example of this curious change in the nervous
system of Insects, is seen in the Sphinx ligustri, or Privet
Hawk-Moth, as shown in the succeeding diagrams. In
fig. 185 the nervous system of the Caterpillar is represented;
this consists of a pair of cephalic ganglia (1); from which
proceeds, on each side, a cord of communication to the first
ganglion of the trunk (2), and thence to the other ganglia (3
— 13). No difference is seen in these ganglia, except that
the last two are more closely connected than the rest. The
cephalic ganglia, with their cords of communication, form a
ring, through which the oesophagus passes ; they are situated
above it; but the whole chain of ganglia of the trunk is
situated beneath the alimentary canal. — In fig. 186 is shown
the Nervous System of the perfect Insect; in which it is
seen that the whole is considerably abbreviated (the body of

A A2

356

NERVOUS SYSTEM OF ARTICULATED ANIMALS.

the Moth being much shorter than that of the Caterpillar),
and that great changes have taken place in the relative sizes

of the ganglia. The cephalic
ganglia, being now connected
with much more perfect eyes and
other organs of sense, are greatly
enlarged; the thoracic ganglia,
from which the legs and wings
are supplied, are enlarged and
concentrated ; whilst the abdomi-
nal ganglia are relatively dimi-
nished in size, the 7th and 8th
being entirely wanting.

442. When the structure of
the chain of ganglia is more par-
ticularly inquired into, it is found
to consist of two distinct tracts;
one of which is composed of
nervous fibres only, and passes
backwards from the cephalic
ganglia, over the surface of all
the ganglia of the trunk, giving
off branches to the nerves that
proceed from them; whilst the
other connects the ganglia them-
selves. Hence, as in Mollusca,
every part of the body has two

vus TSTEOP sets of nervous connexions ; one

Fig. 185.— NER-
VOUS SYSTEM OF

LARLViGusFTRiHINX zfioSSw ™*^ the cePnalic g^g1^, tne

other with the ganglion of its

own segment. Impressions made upon it, being conveyed by
the fibrous tract to the cephalic ganglia, become sensations; and
by the influence of the conscious power, operating through
these same ganglia, the general movements of the body are
harmonised and directed. It is obvious that, as the motions
of an animal are chiefly guided by its sight, the cephalic
ganglia would have a governing influence over the rest, if
only from their peculiar connexion with the eyes ; but there
is good reason to believe that their functions are still more
different from those of the ganglia of the trunk, and that
sensation resides in them alone. The motions produced by

EEFLEX MOVEMENTS OF ARTICULATA. 357

the ganglia of the trunk, when separated from the head, are
often very remarkable, and seem at first sight to indicate
sensation and a guiding will; but, when they are carefully
studied, it is found that striking differences are to be detected,
by which their nature is found to be simply reflex, — a certain
stimulus or irritation producing a certain movement, without
any choice or guidance on the part of the animal, and pro-
bably without its consciousness. As there are no animals in
which these reflex movements are more remarkable than they
are in Centipedes and Insects, we shall pause to dwell upon
them here in more detail.

443. If the head of a Centipede be cut off whilst the animal
is in motion, the body will continue to move onwards by the
action of the legs ; and the same will take place if the body
be divided into several distinct portions. After these actions
have come to an end, they may be excited again by irritating
any part of the nervous centres, or the cut extremity of the
nervous cord. The body is moved forwards by the regular
and successive action of the legs, as in the natural state ; but
its movements are always forwards, never backwards ; and are
only directed to one side when the direct movement is
checked by an interposed obstacle. There is not the slightest
indication of consciousness, either in direction of object, or in
avoidance of danger. If the body be opposed in its progress,
by an obstacle not more than one half its own height^ it
mounts over it and moves directly onwards, as in a natural
state ; but if the height of the obstacle be equal to its own, its
progress is arrested, and the cut extremity of the body
remains opposed to it, with the legs still moving. If, again,
the nervous cord of a Centipede be divided in the middle of
the trunk, so that the hinder legs are cut off from connexion
with the cephalic ganglia, they will continue to move, but not
in harmony with those in the fore part of the body, — being
completely withdrawn from any .control on the part of the
animal, though still capable of performing reflex movements
by the influence of their own ganglia. Or, again, if the head
of a Centipede be cut off, and, while it remains at rest, some
irritating vapour (such as that of ammonia or muriatic acid)
be caused to enter the air-tubes on one side of the trunk, the
body will be immediately bent in the opposite direction, so as
to withdraw itself as much as possible from the influence of

358 REFLEX ACTIONS OF ARTICULATA.

the vapour : if the same irritation be applied on the other
side, the reverse movement will take place; and the body
may be caused to bend in two or three curves, by bringing
the irritating vapour into the neighbourhood of different parts
of either side. This movement is evidently a reflex one, and
serves to withdraw the entrances of the air-tubes from the
source of irritation ; just as the act of sneezing in higher
animals causes the expulsion from the air-passages of any
irritating matter, whether solid, liquid, or gaseous, which may
have found its way into them; and we have no reason to
regard the former as more voluntary than the latter, which we
know to be purely reflex (§ 342).

444. Among Insects, we meet with reflex actions yet more
curious. The Mantis religiosa (fig. 187) is remarkable for the

Fig. 187.— MANTIS RELIGIOSA.

peculiar conformation of its first pair of legs, which serve as
claws for seizing its prey ; and also for the peculiar attitude
which it assumes, especially when threatened or attacked.
Supporting itself upon its two hinder pairs of legs, it rears up
its head upon the long first segment of the thorax, elevating
at the same time its large and powerful arms ; and the resem-
blance fancied to exist between this attitude and that of prayer,
is the cause of the epithet religiosa having been given to it.
Now if the first segment 'of the thorax, with its attached
members, be removed, the posterior part of the body will
still remain balanced upon the four legs which belong to it ;
resisting any attempts to overthrow it, recovering its position
when disturbed, and performing the same agitated movements
of the wings as when the unmutilated animal is excited. But
it will remain quite at rest, so long as it is not irritated. On
the other hand, the detached portion of the thorax which

REFLEX ACTIONS OF ARTICULATA. 359

contains a ganglion, will, when separated from the head, set in
motion its long arms, and impress their hooks on the fingers
which hold it. — Again, a specimen of Dytiscus (a water-
beetle), from which the cephalic ganglia have been removed,
executes the usual swimming motions when cast into water,
with great energy and rapidity, striking all its comrades to
one side by its violence ; in these it will persist for half an
hour, though so long as it lies on a dry surface it remains
quiescent.

445. From these and similar facts, it appears that the ordi-
nary movements of the legs and wings of Articulated animals
are of a reflex nature, and are dependent upon the ganglia
with which these organs are severally connected ; whilst
in the perfect animal they are harmonised, controlled, and
directed by its conscious power, which acts through the
cephalic ganglia and the trunks proceeding from it. When
we come to compare the reflex movements of Insects with
those of the higher animals, we shall perceive that there is no
ground for supposing the ganglia of the trunk to be in them-
selves endowed with sensibility ; so that, when the head is cut
off, or the cephalic ganglia are removed, or their connexion
with any part of the body is interrupted by division of their
nervous cord, no sensation is felt, however much the move-
ments it performs may seem at first to indicate this. (See
§ 467.)

446. From this account of the structure and uses of the
chain of ganglia in the Articulata, it is obvious that these
ganglia are so many repetitions of the pedal ganglia (or gang-
lion of the foot) of the Mollusca ; and we have not yet had
to notice any ganglia appropriated to other functions. In fig.
186, however, is seen a small ganglion in front of the cephalic
mass, which corresponds to the pharyngeal ganglion of the
Aplysia (fig. 183, D) ; and we have now to describe an entirely
distinct system of nerves, appropriated to the function of respi-
ration. As the respiratory apparatus of Articulata, instead of
being confined to one spot, like that of the Mollusca, is dis-
persed througk the body (§§ 315 and 320), the ganglia which
minister to its actions are repeated in the several segments.
There is, in fact, a chain of minute ganglia lying upon the
larger cord, and sending off its nerves between those proceed-
ing from the latter, as seen in fig. 185. These respiratory

360

RESPIRATORY NERVES OF ARTICULATA.

ganglia and their nerves are best seen in the front of the body,
where the cords that pass between the ganglia diverge or
separate from each other. This is shown
on a larger scale in fig. 188; where AB,
A B, are two pairs of ganglia in the thoracic
region, connected by two cords which di-
verge from one another; and between these
are seen the small respiratory ganglion a, and
its branches b b. These branches are distri-
buted to the air-tubes and other parts of the
respiratory apparatus, and communicate
with those of the other system. "We shall
find that, even in the highest Vertebra ta,
there is a portion of the nervous centres
which is set apart for the maintenance of the
respiratory actions, and which may be regarded as the respi-
ratory ganglion ; though it is so closely connected with other
parts of the mass as to seem but a part of it (§ 450).

447. In the higher Invertebrata, among both the Articulated
and the Molluscous classes, we find a tendency to the concen-
tration of the ganglia into one or two masses, — carrying to a

Fig.lSS.— PORTION OF
THE NERVOUS SYS-
TEM OF INSECT;

Showing the respira-
ratory ganglia and
nerves.

Fig. 189. — NERVOUS SYSTEM OF CRAB (Maia).

ca, upper part of the shell laid open ; a, antennae ; y, eyes ; e, stomach ; c, cephalic
ganglion; no, optic nerves; co, cesophageal collar; ns, stomato-gastric nerves;
t, thoracic ganglionic mass ; np, nerves of the legs , na, abdominal nerve.

CONCENTRATION OF GANGLIA IN HIGHEST INVERTEBEATA. 361

greater extent that which has been already noticed in the per-
fect Insect (§ 441). Thus in the Spider, the cephalo-thorax
contains a single large ganglion (t, fig. 46), from which all the
legs are supplied. The same is the case in the Crab, whose
nervous system is represented in fig. 189. Besides this mass,
t, however, which is situated beneath the alimentary canal,
there is a single or double cephalic ganglion, c, which receives
the nerves from the organs of sense, and sends backwards, to
communicate with the mass t, a pair of cords that separate to
give passage to the oesophagus, round which they form a sort
of collar co. And there are other small ganglia and nerves,
connected with the operations of mastication and digestion,
which are called stomato-gastric (from two Greek words,
meaning the mouth and the stomach).

448. A similar concentration, though with a different
arrangement of parts, is seen in the nervous system of the
Poulp, one of the Cephalopoda (§ 111). There is still a
nervous collar through which the oesophagus passes (a, fig.
190) ; but the organs of locomotion being the enlarged tenta-
cula that surround the mouth, the nerves given off to them
arise from ganglia that form part of the cephalic mass, b, b,
instead of being located at a distance from it. At o are seen
the optic nerves, proceeding from distinct ganglia ; and at c
is a heart-shaped ganglionic mass, which seems to bear more
resemblance to the proper brain of higher animals, than does
any that we elsewhere find in the Invertebrata. In front of
this are two ganglia on the middle line, both of which belong
to the stomato-gastric system, one supplying the lips and the
other the pharynx. From the mass g, situated beneath the
oesophagus, there pass backwards two cords m m, each of
which has a ganglion e upon its course, and from this are
given oif nerves to the general surface of the mantle ; and also
other two cords, which run backwards to supply the viscera,
and especially the gills, — each passing through a long narrow
ganglion r, before entering them. It would seem as if the
ganglia e and r corresponded with the ganglia c and B in the
Aplysia ; but as if, in consequence of the great enlargement
of the cephalic mass, they were proportionally reduced in
size.

449. In the nervous system of Vertebrated animals, the
ganglia are no longer scattered through the body, but are

362

NERVOUS SYSTEM OF VERTEBRATA.

united into one continuous mass ; and this mass, constituting
the Brain and Spinal Cord, is inclosed within the bony

Fig. 190.— NERVOUS SYSTEM OF OCTOPUS (PouLP).

casing formed by the skull and vertebral column, in such a
manner as to be protected by it from injuries to which it
would otherwise be continually liable (§§ 72, 73). We have
seen that among the Invertebrated classes the nervous system
has no such peculiar defence, but lies among the other organs,
sharing with them the protection afforded by the general hard
envelope of the body. But in the Vertebrata, its development

NERVOUS SYSTEM OP VERTEBRATA. 363

is so much higher, and its importance so much greater, that
special care is taken to guard it from injury. — The term brain
is commonly applied to the whole mass of nervous matter
contained within the cavity of the skull ; but this consists of
several distinct parts, which have obviously different charac-
ters. The principal mass in Man and the higher Vertebrata
is that which is termed the Cerebrum (fig. 195, a) ; this occu-
pies all the front and upper part of the cavity of the skull, and
is divided into two halves or hemispheres by a membranous
partition which passes from back to front along the middle
line. Beneath this, at the back part of the skull, is another
mass, b, much smaller, but still of considerable size, termed
the Cerebellum; and this also is divided into two hemi-
spheres. At the base or under side of the cerebrum, and
completely covered-in by it, are two pairs of ganglia (1 and g,
fig. 196), which belong to the nerves of smell and sight. We
shall presently find that these are, relatively speaking, much
larger in the lower Vertebrata than in the higher.

450. The several masses of nervous matter contained in the
skull, are connected with each other and with the spinal cord
by bands of nerve-fibres and tracts of vesicular substance,
which serve to bring the brain into connexion with the nerve-
trunks issuing from the spinal cord. But the Spinal Cord
has also distinct properties of its own, analogous to those
which have been shown to exist in the chain of ganglia in
Insects. The upper part of it, which passes-up into the
cavity of the skull, is termed the Medulla Oblongata (/', fig.
197). This is connected with the nerves of respiration, masti-
cation, and deglutition; and may be regarded as combining
together the respiratory and the stomato-gastric systems of
Invertebrata. The remainder of the spinal cord, which de-
scends through the vertebral column, sends its nerves to the
limbs and trunk ; and may be regarded as analogous to the
chain of ganglia by which the corresponding parts are sup-
plied in Insects.

451. The nerves which issue from the Spinal Cord, all
possess two sets of roots ; one from the anterior portion of
the cord, the other from its posterior portion (fig. 191). The
fibres which come-off by these two sets of roots, soon unite
into the trunk of the nerve, which thus possesses the proper-
ties common to both. It was the great discovery of Sir

364 NERVOUS SYSTEM OF VERTEBRATA.

Charles Bell, that the posterior set of roots consists of those
fibres that bring impressions from the body in general to the
Spinal Cord ; which impressions, if carried-on to the Brain,
become sensations. On the other hand,
the anterior roots consist of fibres which
convey motor influence from the Spinal
Cord and Brain, to the muscles of the body.
Thus if the spinal cord of an animal be
laid bare, and the posterior set of roots be
touched, acute pain is obviously produced ;
whilst, if the anterior roots be irritated,
violent motions of the muscles supplied by
that nerve are occasioned. Both these
Fig. i9i.— PORTION op roots contain fibres that connect them with

THE SPIKAL CORD, the brain ag well ag with the g -^ CQrd
Showing the origin of ,-, , ,, , ,, , ± .., „

the nerves : a, spinal so that, through the same trunk, either of

c^ngn00fupon0Us theS6 CentreS maJ act UP°n tlie Par> We

course; d, anterior shall presently find that there is good
b^ne'unTon^Sht reason to believe the Brain to be the seat
/, branch. of sensibility and of voluntary power ; whilst

the Spinal Cord is the instrument of those reflex actions which
take place automatically, as it were, without direction on the
part of the animal, and which are concerned in the mainte-
nance of the organic functions of the body, and in its preser-
vation from injury.

452. The relative proportions which these different parts
present, are very different in the several classes of Vertebrata.
We find that among the lower, the Sensory Ganglia, or gan-
glionic centres immediately connected with the organs of sense
(which are analogous to the cephalic ganglia of thelnvertebrata),
are very large, and occupy a considerable part of the cavity of the
skull ; whilst the Cerebrum and Cerebellum are comparatively
small. — The Cerebrum increases, as we ascend the scale, in
proportion to the development of the intelligence, and the
predominance which it gradually acquires over blind unde-
signing instinct (Chap. xiv.). Its greatest development is
seen in Man. — The Cerebellum seems to be connected with
muscular motion, and to bear a proportion in size with the
variety and complexity of the movements which the animal
performs, serving to harmonise these and blend them together
(§ 480). On the other hand, the Spinal Cord, and the nerves

NERVOUS CENTRES OF FISHES.

365

proceeding from it, are largest in those animals in which the
brain is smallest.

453. It is in FISHES that we find the brain least developed,
and the cerebral hemispheres bearing the smallest proportion
to the other parts. On opening the skull, we usually observe
four nervous masses (three of them in pairs) lying, one in
front of the other, nearly in the same line with the spinal
cord. Those of the first pair are olfactory ganglia, or the
ganglia of the nerves of smell (fig. 192 A, ol). In the Shark,
and some other Fishes, these are separated from the rest by
peduncles or foot-stalks (B, ol) ; a fact of much interest, as
explaining the arrangement

which we find in Man(§ 458).
Behind these is a pair of gan-
glionic masses (c h), of which
the relative size varies con-
siderably in different fishes e£
(thus in the Cod they are
much smaller than those of.
which succeed them, whilst ce
in the Shark they are much
larger) ; these are the cerebral sp
hemispheres. Behind these,
again, are two large masses
(op), the optic ganglia, in
which the optic nerves termi-
nate. And at the back of these, overlying the top of the
spinal cord, is a single mass, the cerebellum (ce) ; this is seen
to be much larger in the active rapacious Shark, the variety
of whose movements is very great, than in the less energetic
Cod. The spinal cord (sp) is seen to be divided at the top by
a fissure, which is most wide and deep beneath the cerebellum,
where there is a complete opening between its two halves.
This opening corresponds to that through which the oesophagus
passes in the Invertebrata ; but, as the whole nervous mass of
Vertebrated animals is above the alimentary canal (§ 74), it
does not serve the same purpose in them ; and in the higher
classes the fissure is almost entirely closed by the union of the
two halves of the cord on the central line.

454. In REPTILES we do not observe any considerable
advance in the character of the brain, beyond that of Fishes ;

Fig. 192. — BRAINS OF FISHES.
A, Cod ; B, Shark.

366 NERVOUS CENTRES OF REPTILES, BIRDS, AND MAMMALS.

save that the Cerebral hemispheres are usually larger, extend-
ing forwards so as to cover-in the Olfactive ganglia (fig. 193).
The Cerebellum is generally smaller, as we should expect from
the inertness of these animals, and the want of
variety in their movements (§ 480). The
Spinal Cord is still very large, in proportion
to the nervous masses contained in the skull ;
and, as we shall hereafter see, its power of
keeping-up the movements of the body, after
it has been cut-off from connexion with the
krain, is very considerable.

455. In BIRDS, however, we find a consi-
derable advance in the character of the brain,
towards that which it presents in Mammalia.
The Cerebral hemispheres (a, fig. 194) are
greatly increased in size, and cover-in, not only the olfactory
ganglia, but also in great part the optic ganglia, 6. The Cere-
bellum, c, also, is much more developed,
as we should expect from the number and
complexity of the movements performed
by the animals of this class ; but it is still
undivided into hemispheres. The Spinal
Cord, d, is still of considerable size, and is
much enlarged at the points from which
the nerves of the wings and legs originate ;
in the species whose flight is most ener-
getic, the enlargement is the greatest in
^e neignDournoo(i °f the wings ; but in
those which, like the Ostrich, move
principally by running on the ground, the posterior en-
largement, from which the legs are supplied with nerves, is
the more considerable.

456. In MAMMALS, we find the size of the Cerebral
hemispheres very greatly increased, especially as we rise
towards Man ; whilst the olfactive and optic ganglia are pro-
portionally diminished, and are completely covered-in by
them. The surface of the cerebral hemispheres is no longer
smooth, as in most of the lower classes, but is divided by
furrows into a series of convolutions (fig. 196) ; by these, the
surface over which the blood-vessels come into relation with
the nervous matter is very greatly increased ; and we find the

NERVOUS SYSTEM OF MAX.

367

Fig. 195.- NERVOUS SYSTEM OF MAN.

368 NERVOUS CENTRES OP MAMMALS.

convolutions more marked as we rise from the lowest Mam-
malia, in which they scarcely exist, towards Man, in whom
the furrows are deepest. The two hemispheres are much
more closely connected with each other, by means of fibres
running across from either side, than they are in the lower
tribes ; and in fact, a considerable part of their mass is made
up of fibres that pass among their different portions, uniting
them with each other. The Cerebellum, also, is divided into
two hemispheres (b, fig. 195) ; and the grey matter in its
interior has a very complex and beautiful arrangement, which
causes it to present a tree-like aspect when it is cut across (d,
fig. 196). The Spinal Cord is much reduced in size, when
compared with the other parts of the nervous centres ; the
motions of the animal now depending more upon its will and
being more guided by its sensations, and the simply reflex
actions bearing a much smaller proportion to the rest.

457. The general arrangement of the nervous centres, and
distribution of the nervous trunks, of Man, are shown in fig.
195. At a are seen the hemispheres of the Cerebrum ; at b
those of the Cerebellum ; and at c, the Spinal Cord. The
principal motor nerve of the face (the facial) is seen at d; and
and at e is seen the brachial plexus, a sort of net- work of
nerves, originating by several roots from the spinal cord, and
going to supply the arm. From this plexus there proceed the
median nerve, // the ulnar nerve, g ; the internal cutaneous
nerve, h; and the radial and musculo-cutaneous nerves, t.
From the Spinal Cord are given off the intercostal nerves, j,
passing between the ribs ; the nerves forming the lumbar
plexus, &, from which the front of the leg is supplied ; and
those forming the sacral plexus, I, from which the back of the
leg is supplied. The latter gives origin to the great sciatic
nerve ; which afterwards divides into the tibial nerve, m ;
the peroneal oifibular nerve, n ; the external saphenous nerve,
o ; and other branches.

458. We shall now examine the structure of the Brain
itself, and the arrangement of the nerves which proceed from
it; confining ourselves to the points of most physiological
importance, and neglecting those which are interesting only
to the professed anatomist. In fig. 196 is represented a per-
pendicular section of the Human Brain down its middle ; the
two hemispheres forming the Cerebrum having been separated

BRAIN OF MAN. 369

from each, other by the division of the broad fibrous band^J
termed the corpus callosum, which unites them. Each, hemi-
sphere is considered as made up of three lobes or divisions,

7 11 9 10 6 e

Fig. 106. — SECTION OF THE BRAIN op MAN.

the anterior a, the middle 6, and the posterior c; but these
are not by any means distinctly marked-out, either on the
external surface or in the internal structure of the organ. The
vesicular or ganglionic nerve-substance is disposed for the
most part upon the exterior, forming a continuous layer, whose
extent is greatly increased by the convoluted folds in which it
lies ; and it is very copiously supplied with blood from the
pia mater, a membrane which consists almost entirely of blood-
vessels and of the areolar tissue that holds them together, and
which so closely enfolds the hemispheres as to dip down into all
the furrows of their surface. The principal part of the internal
substance of each hemisphere is composed of nerve-fibres, of
which some pass between its convolutions and the chain of
ganglionic masses on which the cerebrum is superposed, others

B B

370 BRAIN OF MAN.

pass from each hemisphere to its fellow through the corpus
callosum, whilst others again bring the different convolutions
of the same hemisphere into mutual connexion. The hemi-
spheres are (so to speak) wrapped round the collection of
Sensory Ganglia in which the spinal cord may be said to ter-
minate at its upper end, in such a manner as to leave two
cavities, one on either side, which are called the lateral ven-
tricles.1 The Sensory Ganglia are so small relatively to the
Cerebrum, that they would scarcely attract notice as inde-
pendent centres, if they were not carefully compared with the
ganglionic centres corresponding to them among the lower
animals. The olfactory ganglia are mere bulbous enlarge-
ments upon the cords (1), which, though commonly termed the
olfactory nerves, are really (as in the Shark, § 453) footstalks
connecting these ganglia with the rest of the series ; it being
from these ganglia that the true olfactive nerves are given off
(§ 506). The optic ganglia, g, only in part represent the
optic lobes of Fishes ; the function of the latter being shared
by two large masses termed the thalami optici, which form
the hinder part of the floor of the lateral ventricles, and
which also seem to participate in the sense of touch, as the
sensory columns of the spinal cord may be traced up to them.
This close connexion of the sensorial centres of Sight and
Touch is just what we might anticipate from the continual
co-operation of these two senses (§§ 556, 557). In front of
the optic thalami is another pair of large ganglionic masses,
termed the corpora striata, which is in the like close relation
with the motor columns of the spinal cord ; and it is chiefly
from them and from the thalami optici, that the fibres pro-
ceeding to the surface of the Cerebral hemispheres radiate.
The Cerebellum, which has no direct communication with the
Cerebrum, but possesses independent connexions of its own
with the upper part of the spinal cord, has its grey or vesicular
and its white or fibrous substance so peculiarly disposed, as
to present in section the appearance delineated at d, which
is termed the arbor vita?, or tree of life.

459. Of the nerves given off within the skull (figs. 1 9 6, 1 97),

1 There are other ventricles, which are merely spaces left on the
middle plane by the imperfect coalescence of the two lateral columns
of the nervous axis, like the openings formed by the divergence of the
two halves of the nervous cord in Insects (fig. 188).

CEREBRO-SPINAL NERVES. 371

the first pair are the olfactive, which proceed from the bulbs
(1) of the olfactive peduncles, into the cavity of the nose.
Next to these are the optic nerves (2), which may be partly
traced to the optic ganglia, and
partly to the thalami optici. The
third (3), fourth (4), and sixth pairs
(6), are nerves of motion only, and
are distributed to the muscles of
the eye. The^A pair is for the
most part a nerve of sensation
only. Before leaving the skull, it
divides into three great branches ;
of which the first (5) passes into
the orbit (or cavity in which the
eye is lodged), endows the parts
contained in it with sensibility,
and then comes out beneath the
eyebrow, to be distributed to the
forehead and temples ; the second
(5') passes just beneath the orbit,
and makes its way out upon the
face, supplying the cheeks, nose,
upper lip, &c., which it endows
with sensibility; whilst the third
(5"), which (like the spinal nerves)
possesses a motor root also, supplies
the muscles of mastication with
the power of moving, and the
parts about the mouth with sensi-
bility. The seventh pair (7), or
facial, is the general motor nerve
of the face ; and this does not
endow the parts which it supplies
with the least sensibility. Beneath
the origin of this nerve is seen the
cut extremity of another trunk,
that of the auditory nerve (8), or
nerve of hearing. At 9 is seen the glosso-pharyngeal nerve,
which supplies the back of the mouth and pharynx, and is
concerned in the act of swallowing. Originating from the
upper part of the spinal cord (or medulla bloongata) very near
B B 2

Fig. 197.— BRAIN AND SPINAI
CORD OF MAN.

372 CEREBRO-SPINAL NERVES.

this, is the pneumogastric nerve, or par vagum (10), which
supplies the lungs and air-passages, and also the heart and
stomach. Below this, again, is the hypoglossal nerve (11),
which gives motion to the tongue j at 12 is a nerve termed
the spinal accessory, which is concerned in the acts of respira-
tion; and at 13 and 14 are two of the regular spinal nerves.
The termination of all these nerves is either in that prolonga-
tion of the Spinal Cord into the cavity of the skull, which is
termed the Medulla Oblongata (fig. 197, /'), or in the Sensory
Ganglia which are closely connected with the upper part of
this prolongation. Although some of them seem to pass
directly into the Cerebrum, it is very doubtful if such is really
the case.

460. A general connected view of the Brain and Spinal
Cord is given in fig. 197 ; which represents the front of the
latter, with the Brain a turned back, so as to expose its
under side. At 5 is seen its anterior lobe ; at c its middle
lobe ; and its posterior lobe d is almost entirely concealed by
the Cerebellum e. At /' is shown the Medulla Oblongata,
or upper end of the Spinal Cord ff. The brachial plexus
is seen at g, formed by the nerves that originate in the cervical
region of the cord ; at h is the lumbar plexus formed by the
nerves of the lumbar portion ; and at k is the sacral plexus
formed by the sacral nerves. The spinal cord terminates at
its lower extremity in a bundle of nerves /, to which the name
cauda equina is given, from its resemblance to a horse's tail.
The various pairs of nerves from 1 to 14 are the same as in
the preceding description; 15 and 16 are nerves from the
upper part of the cervical region ; 25, a pair from the dorsal
region ; and 33, a pair from the lumbar region. All these
spinal nerves find their way out through apertures in the
vertebral column, which are formed by a union of two notches,
one in each of the adjoining vertebrae.

461. The system of nerves which has been now described
is termed the Cerebro-Spinal ; but it is not the only set of
nerves and ganglia contained within the bodies of Vertebrated
animals. In front of the vertebral column there is a chain of
oblong ganglia, which communicate with two large ganglia
that lie among the intestines, and with several small ganglia
in the head and other parts. They communicate also with the
posterior roots of the spinal nerves, on which are another set

SYMPATHETIC SYSTEM OF NERVES. 373

of ganglia (c, fig. 191), that seem to belong to the same system.
The nerves proceeding from this system, which is called the
Sympathetic, are distributed, not like those of the cerebro-
spinal, to the skin and muscles, but to the organs of digestion
and secretion, and to the heart and blood-vessels. Hence the
former system of nerves, being that by which sensations are
received and spontaneous motions executed, is called the
nervous system of animal life ; whilst the latter, being con-
nected with the nutritive processes alone, is termed the nervous
system of organic life.

462. What is the nature of the influence which the Sympa-
thetic system exerts over the functions of the parts to which
it is distributed, is not yet clearly made out. The sympathetic
nerves distributed to the alimentary canal have been ascer-
tained to have the power of exciting its peristaltic actions ;
and those which are distributed with the blood-vessels (on the
coats of which they form a minute net- work) have a direct influ-
ence over their calibre, producing changes in the local circulation
in obedience to passions and emotions of the mind, as well as
to states of other bodily organs. Of this influence we have a
familiar example in the acts of blushing and turning pale from
agitation of the feelings, and a more decided but less frequent
one in the fainting which sometimes occurs from a sudden
shock. It is doubtful, however, whether the Sympathetic
system really possesses motor filaments of its own ; its motor
actions being certainly in part dependent upon filaments de-
rived from the cerebro-spinal system. The action of its motor
fibres upon the muscular coats of the blood-vessels supplying
the glands, serves to regulate the quantity of the fluids secreted
by these organs, especially in cases in which the demand for
the secretion is intermittent; but as there is evidence that
the quality of many secretions may be affected by mental states
(§ 353), it seems likely that the fibres peculiar to the Sympa-
thetic system (§ 60) may be the channel of this influence. —
Although it is still impossible to define precisely the functions
of the Sympathetic system, yet it may be stated generally,
that in virtue of the two modes of action just explained, it
seems to harmonise and blend together the various actions of
Nutrition, Secretion, &c., in such a manner as to bring them
into conformity with each other, and with the condition of
the organs of Animal life.

374 FUNCTIONS OF SPINAL COED : EEFLEX ACTION.

463. We shall now consider, in more detail, the functions
of the different parts of the Cerebro-Spinal System in Man
and the higher animals ; referring occasionally to the Inver-
tebrated classes for illustrations which they can best afford.
We shall commence by examining the functions of the Spinal
Cord and Medulla Oblongata, which are the parts concerned
in reflex action.

Functions of the Sjiinal Cord. — Reflex Action.

464. The Spinal Cord of Vertebrated Animals may be con-
sidered as a collection of ganglia, analogous to those of which
the ganglionic cord of Articulata is composed ; these ganglia
being united, however, in an unbroken line, instead of being
distinct from one another and brought into communication by
coiinecting cords. There is great difficulty in tracing-out the
precise course of the nerve-fibres which form the white strands
of the Spinal Cord ; and it is doubtful how far any of them
form a continuous connexion between the roots of the Spinal
Serves and the Brain. But there can be no doubt that such
a connexion is established, either by the fibrous tracts or by
the grey matter of the Spinal Cord; experiment having
unequivocally shown that the latter participates with the
former in this conducting power.

465. When the Cerebro-Spinal system is in full activity,
its nerves convey impressions from every part of the body to
the Brain, where they are communicated to the mind, — that is,
the individual becomes conscious of them, or feels them as
sensations. And by the fibres of the same system which pass
in the contrary direction, the will acts upon the muscles so as
to produce voluntary motion. Now the brain is not in con-
stant action, even in a healthy person ; it requires rest ; and
during profound sleep it is in a state of complete torpor. Yet
we still see those movements continuing, which are essential
to the maintenance of life ; the breathing goes on uninter-
ruptedly, liquid poured into the mouth is swallowed, and the
position is changed when the body would be injured by
remaining in it. The same is the case in apoplexy, in which
the actions of the brain are suspended by pressure upon it.
And the same will take place, also, in an animal from which
the cerebrum has been removed ; or in which its functions
are completely suspended by a severe blow on the head. If

FUNCTIONS OF SPINAL COBD I REFLEX ACTION. 375

the edge of the eyelid be touched with a straw, the lid imme-
diately closes ; if a candle be brought near the eye, the pupil
contracts (§ 534); if liquid be poured into the mouth, it is
swallowed ; if the foot be pinched or be burnt with a lighted
taper, it is withdrawn ; and, if the experiment be made upon
a Frog, the animal will leap away as if to escape from the
source of irritation. The respiratory movements, too, are kept
up with regularity; so that there is no impp.diTnp.Tit to the
continuance of the circulation, and the organic life of the
animal may thus endure for some time. In one of the experi-
ments made with the view of ascertaining the degree in which
the activity of the Cerebrum is essential to the maintenance
of life, a pigeon was kept alive (if alive it could be called) for
some months after the removal of its cerebrum, — running
when it was pushed, flying when it was thrown into the air,
drinking when its beak was plunged in water, swallowing
food which was put in its mouth, — though at all other times,
when left to itself, appearing like an animal in profound
sleep.

466. It is evident, therefore, that we are not to regard the
Brain (according to the former opinion of Physiologists, and
the belief which is still commonly entertained) as the only
centre of nervous power, and as essential to the maintenance
of the life of the body; and that we must attribute to the
Spinal Cord no small amount of independent power. We
might be disposed to infer, from the statements in the last
paragraph, that an animal whose brain has been removed can
still feel and judge and perform voluntary motions by means
of the Spinal Cord; but this, again, would be putting a wrong
interpretation upon the phenomena in question. It is ob-
served that the motions performed by an animal in such
circumstances are never spontaneous ; they are always excited
by a stimulus of some kind. Thus a decapitated Frog, after
the first violent convulsive movements occasioned by the ope-
ration have passed away, remains at rest until it is touched ;
and then its leg, or even its whole body, will be thrown into
sudden action, which immediately subsides again. In the
same manner, the action of swallowing is not performed,
except when it is excited by the contact of food or liquid
(§ 195) ; and even the respiratory movements, spontaneous as
they seem to be, would not continue long, unless they were

376 FUNCTIONS OF SPINAL CORD I REFLEX ACTION.

excited by the presence of venous blood in the vessels — espe-
cially in those of the lungs. These movements are all necessarily
linked with the stimulus that excites them; — that is, the
same stimulus will always produce the same movement, when
the condition of the body is the same. Hence it is evident
that the judgment and will are not concerned in producing
them ; but that they may be rather compared to the move-
ments of an automaton, which are calied-forth by touching
certain springs.

467. The next question is, whether these movements can
be performed without any feeling or sensation, on the part of
the animal, of the cause that produces them. It is difficult
to imagine that an animal, executing such regular and
various actions, which so strongly resemble those it would
execute in its complete state, and which are so perfectly
adapted to their obvious purposes, can do so without con-
sciousness ; and accordingly some Physiologists have regarded
them as furnishing proof that the Spinal Cord possesses the
property of sensibility, or, in other words, that an animal
whose Brain has been removed can still feel. Eut this in-
ference will not bear a close examination. Such movements
take place, not only when the Brain has been removed and
the Spinal Cord remains entire, but even when the Spinal
Cord has been itself cut across into two or more portions.
Thus if the head of a Frog be cut off, and its Spinal Cord be
divided in the middle of the back, so that its fore-legs remain
connected with the upper part, and the hind-legs with the
lower, each pair of members may be excited to movement by
a stimulus applied to itself; but the two pairs will not
execute any consentaneous motions, as they will do when the
Spinal Cord is undivided. Or, when the Spinal Cord is cut
across without removal of the Brain, the lower limbs may be
excited to movement, though completely paralysed to the will;
whilst the upper remain under the control of the animal's
sensation and conscious power.

468. Now although the Frog cannot tell us that it has no
sensation in its lower limbs, we have very strong evidence to
that effect; for cases are of no infrequent occurrence in
Man, in which, the Spinal Cord having been injured in
the middle of the back by disease or accident, there is not-
only loss of voluntary control over the motions of the legs,

REFLEX ACTION WITHOUT SENSATION. 377

but loss of sensation also. Further, in several cases of this
kind, in which the injury was confined to a small portion of
the cord, and the part below was not seriously disturbed, it
has been noticed that motions may be excited in the limbs by
stimuli applied directly to them, — as, for instance, by tickling
the sole of the foot, pinching the skin, or applying a hot plate
to its surface ; and this without the least sensation, on the
part of the patient, either of the cause of the movement, or
of the movement itself; the nervous communication, which
would otherwise have conveyed the impression to the brain
and there given rise to sensation, being interrupted in the
spinal cord.

469. By such cases, then, it appears to be clearly proved,
that the actions performed by the Spinal Cord, when the
Brain has been removed, or its power destroyed, or its com-
munication with the part cut-off, do not depend upon Sensa-
tion; but upon a property peculiar to the Spinal Cord, by
which impressions, made upon certain parts, necessarily excite
motions of an automatic character. By other experiments it
has been shown to be necessary for the exercise of this Reflex
function (as it has been termed), that an impression should be
conveyed by one set of nervous fibres, from the point where
the stimulus is applied, to the Spinal Cord ; and that a motor
impulse, conveyed by another set of filaments, should issue
•from the Cord to the muscles. The excitor and motor fila-
ments distributed to any part are commonly bound up in
the same trunk, and are connected with the same part of the
Spinal Cord; so that, if this portion or segment be com-
pletely separated from the rest, it may still execute the reflex
movements of the parts to which its nerves are distributed ;
— just as a single segment of a Centipede will continue to
move its legs, provided its own ganglion be entire (§ 443).

470. But in other instances it happens that we can more
clearly distinguish between the excitor and the motor nerves,
from their being distributed separately, and being connected
with distinct portions of the spinal cord. Thus in the act of
deglutition (§ 195), the chief excitor nerve is the glosso-pha-
ryngeal (§ 459) ; this conveys the impression made by the
contact of food with the pharynx, to the Medulla Oblongata ;
but it does not convey the motor influence to the muscles,
this being accomplished by branches of another nerve, the

378 REFLEX ACTIONS OF THE SPINAL CORD.

pneumogastric. If the trunk of the glosso-pharyngeal nerve
be pinched, an act of deglutition is made to take place ; but
if it be separated from the Medulla Oblongata, or the pneumo-
gastric nerve be divided, or the Medulla Oblongata itself be
destroyed, the movement can no longer be thus excited.
Hence we see the necessity of the completeness of this
nervous chain or circle — consisting of the nerve proceeding
from the part stimulated to the ganglion, the ganglion itself,
and the nerve proceeding from the ganglion to the muscles
acted-on — in order that any such reflex movements may be
produced.

471. The functions of the Spinal Cord appear to be wholly
restricted to the performance of movements of this character.
The proportion they bear to the motions which are de-
pendent upon sensation and will, varies greatly in different
animals ; and it may be judged-of with tolerable accuracy, by
comparing the relative sizes of the spinal cord and the brain.
Thus in the lowest Fishes, the spinal cord seems the principal
organ, and the brain an insignificant appendage to it. In
Man, on the contrary, the spinal cord is so small in com-
parison with the brain, as to have been regarded (though
incorrectly, as we have seen) in the light of a mere bundle
of nerves proceeding from it. In the former, the ordinary
movements of the body seem principally to depend upon the
spinal cord, being only controlled and directed by the brain ;
just as those of Articulated animals are chiefly dependent
upon the ganglia of the trunk, being only guided by those of .
the head (§ 442). But in Man, those only are left to the
spinal cord which are necessary for the maintenance of life ;
the ordinary motions of the body being for the most part
voluntary. Still, as we have just now seen (§ 468), reflex
movements may be excited through the spinal cord, even in
Man, when the influence of the will is cut off; and it is
curious to observe, that the stimulus is most powerful when
it acts upon the soles of the feet, and that it ceases to produce
the same effect, when, by the restoration of the functions of
the injured part of the cord, the power of the will over the
limbs, and also their sensibility, are regained. There is much
reason to believe that, when we are walking steadily onwards,
and the mind is intently occupied with some train of thought
which engrosses its whole attention, the individual movements

REFLEX ACTIONS OF THE SPINAL CORD. 379

of the limbs may be kept-up by reflex action, while their
general direction is guided by visual sensation (§ 479). ^And
even when the mind is sufficiently on the alert to guide, direct,
and control the motions of the limbs, their separate actions
appear to be performed without any immediate exertion of the
will ; and probably depend, therefore, rather upon the reflex
function of the spinal cord, than upon the continual influence
of the brain.

472. Besides the reflex movements of deglutition and re-
spiration, which have been formerly considered (§§ 195 and
340), and those of locomotion, on which we have now dwelt
sufficiently, there are several others of a similar character, all
of which have for their object the supply of the wants of the
body, or its preservation from injury. Of these the only one
which it is desirable here to notice is that of sucking, as per-
formed by the young Mammiferous animal. In this opera-
tion there is a very complex union of the actions of different
muscles, — those of respiration, together with those of the
tongue and lips. So beautifully adapted is this combination
to its designed purpose, that it could not be better contrived
by the longest experience or the most careful study. Yet we
find that the young Mammal commences to perform it without
any experience or study, the instant that its lips touch the
nipple of its parent. And that it is a reflex action, dependent
upon the spinal cord alone for its performance, and requiring
a stimulus to excite it, is proved by these remarkable facts ; —
that it has been performed by human infants which have
been born destitute of brain, and which have lived for some
hours ; and also by puppies whose brain had been removed.
These last not only sucked a moistened finger, when it was
introduced between their lips, but also pushed out their feet,
as the young animal naturally does against the dugs of the
parent.

473. There are many irregular actions of the Spinal Cord,
however, the careful study of which is of the highest impor-
tance to the Medical Man. It is probable that all convulsive
movements are produced through its agency ; these being for
the most part of a reflex character, that is, dependent upon
some stimulus or irritation which acts through the nervous
circle described in § 470. Thus, convulsions are not unfre-
quent in children during the period of teething ; and are then

380 CONVULSIVE FORMS OF REFLEX ACTION.

excited by the irritation which results from the pressure of
the tooth as it rises against the unyielding gum (§ 174).
They are often occasioned, too, by the presence of indigestible
or injurious substances, or of intestinal worms, in the alimen-
tary canal ; and will cease as soon as this is properly cleared
out. Again, in Tetanus or "lockjaw" resulting from a lace-
rated wound, the irritation of the injured nerve is the first
cause of the convulsive action ; and a similar local irritation
is often the origin of Epileptic fits, in which the convulsion
is accompanied by loss of consciousness. When these com-
plaints prove fatal, it is usually by suffocation, — the muscles
of respiration being fixed by the convulsive action, in such a
manner that air cannot pass either in or out.

474. In other forms of convulsive disorders, however, the
cause of irritation may directly affect the Spinal Cord, instead
of being conveyed to it by the nerves from a distance. This
seems to be the case, for example, in Hydrophobia ; which
terrible complaint is probably due to a poison introduced into
the blood by the bite of the rabid animal, and conveyed by
the circulating current to the nervous centres. So, when the
poison termed Strychnia has found its way into the circula-
tion, the whole Spinal Cord is thrown into such an excitable
state, that the slightest stimulus produces the most violent
convulsive movements, which succeed one another in extra-
ordinary variety. And the teething-convulsions of infants
often depend more upon a peculiar excitable state of the
spinal cord, which results from atmospheric impurity, and is
removed by change of air, than they do upon the irritation
of the gums. — By knowing, as he now does, the part of the
nervous system on which these convulsive disorders depend,
the Physician is enabled to apply his remedies with much
greater precision than heretofore, and to form a much more
accurate estimate of the danger which attends them.

Functions of the Ganglia of Special Sense. — Consensual Actions.

475. It has been seen that the nerves of special sense —
those of smell, sight, and hearing — terminate in ganglionic
centres peculiar to themselves, which are lodged within the
skull, and form part of what is commonly termed the brain,
though distinct both from the Cerebrum and the Cerebellum.
These Sensory Ganglia are almost the only representatives of

GANGLIA OF SPECIAL SENSE : — INSTINCTIVE ACTIONS. 381

the brain in the Invertebrated animals ; and in Fishes they
bear a very large proportion to the other parts, their relative
size gradually diminishing as we ascend the scale towards
Man. JSTow when we study the actions of these lower tribes
of animals, we find that those which evidently depend upon
sensation, especially the sense of sight, are very far from
being of the same spontaneous or voluntary character as those
which we perform. We judge of this by their unvarying
nature, — the different individuals of the same species execut-
ing precisely the same movements, when the circumstances
are the same, — and this evidently without any choice, or
intention to fulfil a given purpose, but in direct respondence
to an internal impulse. Of this we cannot have a more
remarkable example than is to be found in the operations
of Bees, "Wasps, and other social Insects ; which construct
habitations for themselves upon plans which the most enlight-
ened human intelligence could not surpass ; yet which do so
without hesitation, confusion, or interruption, — the different
individuals of a community all labouring effectively for one
common purpose, because their impulses are the same (Chap-
ter XIV.)

476. In higher animals we may often notice the effect of
similar promptings, by which the various species are guided
in their choice of food, in the construction of their habitations,
in their migrations, &c. : but these are frequently modified
to a certain degree by the intelligence which they possess.
The closure of the pupil when the eye is exposed to a strong
light, and its dilatation when the light diminishes (§ 534),
afford a very marked example of this " consensual" class of
movements, which differ from the simply-reflex in requiring the
stimulus of sensations, but which are, like them, quite indepen-
dent both of the reason and of the will. A still more striking
illustration, however, is furnished by the mode in which a
little Fish, termed the Chcetodon restrains, obtains its food.
Its mouth is prolonged into a kind of beak or snout, through
which it shoots drops of liquid at insects that may be hover-
ing near the surface of the water, and rarely fails in bringing
them down. Now, according to the laws of Optics, the insect,
being above the water whilst the eye of the fish is beneath,
it, is not seen by it in its proper place ; since the rays do not
pass from the insect to the fish's eye in a straight line (§ 528).

382 CONSENSUAL ACTIONS IN MAN.

The insect will in fact appear to the fish a little above the
place which it really occupies ; and the difference is not con-
stant, but varies with every change in the relative positions
of the fish and the insect. Yet the wonderful instinct with
which the fish is endowed, leads it to make the due allowance
in every case ; doing that at once, for which a long course of
experience would be required by the most skilful Human
marksman, under similar circumstances.

477. Though the Intelligence and Will of Man in a great
degree supersede his consensual impulses, in the same man-
ner as they hold in subordination his reflex movements
(§ 471), yet we have many indications of the direct operation
of sensations in determining respondent movements. Of this
kind are the start produced by a loud sound, particularly if
unexpected ; the closure of the eyes to a dazzling light, or on
the sudden approach of a body that might injure them ; the
production of sneezing by a dazzling light ; the provocation
of laughter by tickling, or by some sight or sound to which
no distinct ludicrous idea or emotion attaches itself ; and the
excitement of vomiting by highly disagreeable sensations, as
the sight of a loathsome object, an offensive smell, a nauseous
taste, or by tickling the back of the mouth with a feather.1
None of these " consensual " movements can be excited with-
out the consciousness of the subject of them; and this
circumstance marks them out as belonging to a different
category from the "reflex" movements performed through
the instrumentality of the Spinal Cord. — In some convulsive
disorders, the attacks are excited by causes that act through
the organs of sense : thus, in Hydrophobia we observe the
immediate influence of the sight or sound of liquids ; and in
many Hysteric subjects, the sight of a paroxysm in another
individual is the most certain means of its induction in them-
selves.

478. But we may trace the agency of the Sensory Ganglia

1 This is the most ready way of exciting vomiting, when it is desired
to free the stomach from poisons or unwholesome articles of food ;
but care must be taken not to apply the feather so low down as to
cause it to be grasped by the muscles concerned in the act of swallow-
ing ; for its irritation, instead of producing vomiting, will then occasion
an act of deglutition (§ 195), which may draw the feather from the
hand of the operator, and carry it down into the stomach of the
patient.

IMPORTANCE OF GUIDING SENSATIONS. 383

in Man and the highest Vertebrata, not merely in their direct
and independent operation on the Muscles, but also in the
manner in which they participate in all voluntary action.
For it is now well established, that the Will cannot bring
about any definite movement, except under the guidance of
sensations, derived either from the muscles themselves, or
through some channel of information which indicates what
the muscles are doing. It is for want of the guiding sensa-
tions afforded by the ear, that persons who are born deaf are
also dumb, the will not being able to make use of the muscles
concerned in vocalization ; and where, by long training, some
imperfect power of speech has been acquired, it has been
gained by attention to the sensations arising from the mus-
cular exertion of the organs themselves. It is by the guiding
influence of the visual sensations, that the movements of the
two eye-balls are made to correspond ; and, in children born
completely blind, it may be observed that the eyes roll about
without any harmony, though a very slight perception of light
is sufficient to bring their motions into consent. So, again,
if the arm or the leg be so paralysed that its sensibility is
lost whilst its muscles are still under the power of the will,
that power can only be exerted to occasion movement by the
assistance of the sight ; a mother, for example, so affected,
being only able to hold her infant upon her arm so long as
she looks at it ; and a man, whose legs are thus paralysed,
being only able to sustain himself in standing or walking by
constantly looking at his legs.

479. It seems to be obviously through the shorter channel
afforded by the Sensory Ganglia, that those actions are per-
formed, which, though originally directed by Intelligence and
Will, come by frequent repetition to be so completely auto-
matic as to resemble the instinctive actions of the lower
animals. Thus it is within the experience of almost every
one, that he occasionally walks through the streets with his
mind intently and continuously engaged on some train of
thought, without the least attention to, or even consciousness
of, the direction he is taking; yet he avoids obstacles, and
follows his accustomed course, obviously under the guidance
of his visual sense, whilst the movements of his limbs are
kept-up by reflex action (§471); and on awaking, as it were,
from his reverie, he may find that he has thus been automa-

384 HABITUAL ACTIONS : — FUNCTION OF CEREBELLUM.

tically conducted to a place very different from that to
which, he had intended going. So, again, we may read
aloud, or play on a musical instrument, without being at all
aware of what we are about, the whole attention being ab-
sorbed by some engrossing thoughts or feelings within. And
it seems to be in this manner that the movements of Som-
nambulists are guided; their Cerebrum being, as it were,
cut-off from communication with the outer world, and their
Sensory Ganglia acting independently of it.

Function of the Cerebellum. — Combination of Muscular Actions.

480. Much discussion has taken place of late years respect-
ing the uses of the Cerebellum ; and many experiments have
been made to determine them. That it is in some way con-
nected with the powers of motion, is now generally admitted.
Its size in the different tribes of Yertebrated animals bears a
pretty close correspondence with the variety and energy of
the movements performed by them ; being greatest in those
animals which require the constant united effort of a large
number of muscles to maintain their usual position, whilst it
is least in those which require no muscular exertion for this
purpose. Thus in animals that habitually rest and move upon
four legs, there is but little occasion for any organ to combine
and harmonize the actions of their several muscles ; and in
these the Cerebellum is small. But among the more active
predaceous Fishes (as the Shark), — Birds of most powerful
and varied flight (as the Swallow, which not only flies rapidly,
but executes the most complicated evolutions in pursuit of its
Insect prey with the greatest facility), — and Mammals which
can maintain the erect position and use their extremities for
other purposes than support and motion, — we find the Cere-
bellum of much greater size : whilst in Man, who surpasses
all other animals in the number and variety of the combina-
tions of muscular movement which he is capable of executing,
it attains its largest dimensions and its greatest complexity of
structure.

481. From experiments upon all classes of Vertebrated
Animals, it has been found that, when the Cerebellum was
removed, the power of walking, springing, flying, standing,
or maintaining the equilibrium of the body, was destroyed.
It did not seem that the animal had in any degree lost volun-

FUNCTIONS OP THE CEREBELLUM AND CEREBRUM. 385

tary power over its individual muscles ; but it could not
combine their actions for any general movement of the body.
The reflex movements, such as those of respiration, remained
unimpaired. When an animal in this state was laid on its
back, it could not recover its former posture ; but it moved
its limbs or fluttered its wings, and evidently was not in a
state of stupor. When placed in the erect position, it stag-
gered and fell like a drunken man ; not, however, without
making efforts to maintain its balance. — Phrenologists, who
attribute a different function to the Cerebellum, have attempted
to put aside these results, on the ground that the severity of
the operation was alone sufficient to produce them j but (as
we have already seen, § 465) after a much more severe opera-
tion— the removal of the Cerebral Hemispheres, the Cere-
bellum being left untouched — the animal could stand, walk,
fly, maintain its balance, and recover it when disturbed.

482. The motions of the body in the Invertebrated classes,
being simple in their nature, and probably all of a reflex
character (§ 442), do not require a Cerebellum ; and we do
not find in them any nervous mass which clearly represents
this organ,

Functions of the Cerebrum. — Intelligence and Will.

483. From the facts already stated, it is tolerably clear that
the Cerebrum is the organ by which we reason upon the ideas
that are excited by sensations, — by which we judge and de-
cide upon our course of action, — and by which we put that
decision into practice, by issuing a mandate (as it were), which,
being conveyed by the nervous trunks proceeding from the
brain to the muscles, excites the latter to contract. It is a
common, but entirely erroneous idea, that Reason or Intelli-
gence is peculiar to Man ; and that the actions of the lower
classes of Animals are entirely due to Instinct. There can be
no doubt, however, that reasoning processes exactly resem-
bling those of Man are performed by many Mammals, such
as the Dog, the Horse, and the Elephant ; and it is probable
that although we are best acquainted with these animals,
on account of their tendency to associate with Man, there
are others which have powers yet higher. We must admit
that an animal reasons, when it profits by experience, and
obviously adapts its actions to the ends it desires to gain,

0 0

386 SUPERIOR INTELLIGENCE OP HIGHER VERTEBRATA.

especially when it departs from its natural instincts to do
this. Such is continually the case with the animals just
mentioned, as will appear from some striking examples to be
mentioned hereafter (Chap. xiv.). We perceive the presence
of Intelligence also in the differences of character which we
encounter among the various individuals of the same species ;
thus every one knows that there are stupid Dogs and clever
Dogs, ill-tempered Dogs and good-tempered Dogs, as there
are stupid Men and clever Men, ill-tempered Men and good-
tempered Men. But no one could distinguish between a
stupid Bee and a clever Bee, or between a good-tempered
Wasp and an ill-tempered Wasp ; simply because all the
actions of these animals are prompted by an unvarying instinct.
484. Among Birds, too, there are many manifestations of
Intelligence, which constitute a remarkable distinction between
their actions and those of Insects ; though the instinctive
tendencies of the two classes bear a close correspondence with
each other. Their mode of life is nearly the same, so that
Birds may be called the Insects of the Vertebrated series,
whilst 'Insects may be regarded as the Birds of the Arti-
culated ; and there are several curious points of analogy in
the structure of their bodies. The usual arts which Birds
exhibit in the construction of their habitations, in pro-
curing their food, and in escaping from danger, must be
regarded (like those of Insects) as instinctive; on account
of the unrformity with which they are practised by different
individuals of the same species, and the perfection with which
they are exercised on the very first occasion, independently of
all experience. But in the adaptation of their operations to
particular circumstances, Birds display a variety and fertility
of resource far surpassing that which is manifested by Insects j
— as for instance, when they make trial of several means, and
select that one which best answers the purpose ; or when they
make an obvious improvement from year to year in the com-
forts of their dwelling ; or when they are influenced in the
choice of a situation by peculiar conditions, such as in a
state of nature can scarcely be supposed to affect them. All
these are obvious indications of an Intelligence which Insects
do not possess ; that which is most wonderful in the actions
of the latter (and there are none more wonderful) being the
same in all the individuals of one species, being uninfluenced

LOW INTELLIGENCE OP REPTILES AND FISHES. 387

by education, and being performed under the direction of
an Intelligence much higher than the boasted reasoning
power of Man.

485. In the classes of EEPTILES and FISHES, the manifesta-
tions of Intelligence are so slight as to be scarcely distin-
guishable. We find them capable of such an amount of
education as enables them to recognise individuals from whom
they have been accustomed to receive food ; but they seem to
have very little further power of profiting by experience ; and
we do not find that individuals ever shape-out for themselves
a new course which can be regarded as purely rational. This
very low grade of Intelligence obviously corresponds with
the very rudimentary development of the Cerebrum in these
classes (§§ 453, 454).

The contrast between Instinct and Intelligence will be more
fully displayed in a future Chapter ; in which also a general
account will be given of the Mental Operations to which the
Cerebrum of Man is subservient.

CHAPTEE XL

ON SENSATION, AND THE ORGANS OF THE SENSES.

486. ALL save the very lowest kinds of Animals possess,
there is good reason to believe, a consciousness of their own
existence, first derived from & feeling of some of the changes
taking place within themselves; and also a greater or less
amount of sensibility to the condition of external things.
How far any such endowment can be possessed by creatures
which are destitute of a nervous system, and which are little
else than particles of animated jelly, may be questioned. But
there can be no reasonable doubt that where a nervous
system exists, whatever consciousness any Animal may pos-
sess of that which is taking place within or around itself, is
all derived from impressions made upon the extremities of
certain of its nervous fibres ; which, being conveyed by them
to the central sensorium, are there felt (§ 430). Of the mode
in which the impression, hitherto a change of a material cha-
racter, is there made to act upon the mind, we are absolutely
ignorant ; we only know the fact. Hence, although we com-

c c 2

388 SENSATION IN GENERAL.

monly refer our various sensations to the parts at which the
impressions are made, — as, for instance, when we say that we
have a pain in the hand, or an ache in the leg, — we really
use incorrect language ; for, though we may refer our sensa-
tions to the points where the impression was made on the
nerve, they are really felt in the brain. This is evident from
two facts ; first, that if the nervous communication of the
part with the brain be interrupted, no impressions, however
violent, can make themselves felt; and, second, that if the
trunk of the nerve be irritated or pinched anywhere in its
course, the pain which is felt is referred, not to the point
injured, but to the surface to which these nerves are distri-
buted. Hence the well-known fact that, for some time after
the amputation of a limb, the patient feels pains which he
refers to the fingers or toes that have been removed ; this con-
tinues until the irritation of the cut extremities of the nervous
trunks has subsided.

487. Among the lower tribes of Animals, it would seem
probable that there is no other kind of sensibility than
that which is termed general or common, and which exists, in
a greater or less degree, in almost every part of the bodies of
the higher. It is by this that we feel those impressions,
made upon our bodies by the objects around us, or by actions
taking place within, which produce the various modifications
of pain, the sense of contact or resistance, the sense of varia-
tions of temperature, and others of a similar character. From
what was formerly stated (§ 63) of the dependence of im-
pressions made on the sensory nerves upon the action of the
blood-vessels, it is obvious that no parts destitute of the latter
can receive such impressions, or (in common language) can
possess sensibility. Accordingly we find that the hair, nails,
teeth, tendons, ligaments, fibrous membranes, cartilages, and
bones, whose substance either contains no vessels, or but very-
few, are either completely incapable of receiving painful
impressions, or have but very dull sensibility to them. On
the other hand, the skin and other parts which usually receive
such impressions, are extremely vascular ; and it is interesting
to observe that some of the tissues just mentioned, when new
vessels form in them in consequence of diseased action,
become acutely sensible. It does not necessarily follow, how-
ever, that parts should be sensible in a degree proportional to

NERVES OF SPECIAL SENSIBILITY. 389

the amount of blood they contain ; since this blood may be
sent to them for other purposes. Thus, it is a condition
necessary to the action of Muscles, that they should be
copiously supplied with blood (§ 591) ; but they are not
acutely sensible : and Glands, also, the substance of which
has very little sensibility, receive a large amount of blood for
their peculiar purposes.

488. But besides the general or common sensibility which is
diffused over the greater part of the body of most animals,
there are certain parts which are endowed with the property
of receiving impressions of a peculiar or special kind, such as
sounds or odours, which would have no influence upon the
rest ; and the sensations which these impressions excite, being of
a kind very different from those already mentioned, arouse ideas
in our minds such as we should never have formed without
them. Thus, although we can gain a knowledge of the shape
and position of objects by the touch, we could form no notion
of their colour without sight, of their sounds without hearing,
or of their odours without smell.

489. The nerves which convey these special impressions
are not able to receive those of a " common " kind : thus, the
Eye, however well fitted for seeing, would not feel the touch
of the finger, if it were not supplied with branches from the
5th pair, as well as by the optic nerve. .Nor can the different
nerves of special sensation be affected by impressions that are
adapted to operate on others : thus, the ear cannot distinguish
the slightest difference between a luminous and a dark object;
nor could the eye distinguish a sounding body from a silent
one, except by seeing its vibrations. But Electricity possesses
the remarkable power, when transmitted along the several
nerves of special sense, of exciting the sensations peculiar to
each; and thus, by proper management, this single agent
may be made to produce flashes of light, distinct sounds,
a phosphoric odour, a peculiar taste, and a pricking feeling,
in the same individual at one time. The inference which
might hence be drawn — that Electricity and Nervous agency
are identical— is nevertheless premature, as will be shown
hereafter (§ 585).

390 TACTILE SENSIBILITY OP THE SKIN.

Sense of Touch.

490. By the sense of Touch is usually understood that
modification of the common sensibility (§ 487) of the body, of
which the surface of the skin is the especial seat. In some
animals, as in Man, nearly the whole exterior of the body is
endowed with it in no inconsiderable degree ; but in others,
as in the larger number of Mammals, most Birds and Eep-
tiles, and many Fishes, the greater part of the body is so
covered by hairs, scales, or bony plates, as to be nearly insen-
sible ; and the faculty is restricted to particular portions of
the surface, which often possess it in a very high degree.
The sensory impressions, by which we receive the sensation of
Touch, are made by the objects themselves upon the nerves
which are distributed to the Skin ; the general structure of
which has been already described (§§ 36 — 38). Of the papillce
which are thickly set upon many parts of its surface, some
contain looped tufts of blood-vessels without nerves ; and as
these are found to be largest where the Epidermis is thickest
(as, for example, in the pads on the under side of the Dog's
foot), it seems obvious that they minister, not to sensation,
but to the nutrition of that protective coating (§ 492). But
in other papillae the blood-vessels are comparatively scanty,
their interior being chiefly occupied by little cushions of con-
densed areolar substance to which the sensory nerves proceed ;
and as their Epidermic coating is thin, and as the degree
of sensibility of any part of the skin bears a close correspond-
ence to the number of these papillae which are included
within a given area of its surface, it can scarcely be doubted
that they are the special instruments of the sense of Touch.

491. The true skin, or Cutis (§ 37), from which alone
leather is prepared, is thicker in most Mammals than in
Man '} but the thickness of the skin does not by any means
involve (as is commonly supposed) deficient sensibility. Thus,
in the Spermaceti Whale it has been observed that the
sensory nerves, which are destined to be distributed on the
skin, pass through the blubber without giving off any con-
siderable branches, but spread out into a network of extreme
minuteness as soon as they arrive near the surface. It is
a fact well known to Whale-fishers, especially to those who
pursue this species, that these animals have the power of

EPIDERMIC PROTECTION. — OTHER ORGANS OF TOUCH. 391

communicating with each other at great distances. It has
often been observed, for instance, that, when a straggler is
attacked, at the distance of several miles from a " school," a
number of its fellows bear down to its assistance in an' almost
incredibly short space of time. It can scarcely be doubted
that the communication is made through the medium of the
vibrations of water, excited by the struggles of the animal, or
perhaps by some peculiar movements specially adapted for
this purpose, and propagated through the liquid to the
immense surface of the skin of the distant Whales.

492. The sensibility of the true skin would be too great, if
it were not protected by the Epidermis (§ 38), the thickness
of which varies considerably, according as the part is to be
endowed with acute sensibility, or to be protected from impres-
sions of too strong a nature. Thus it is particularly thin at the
ends of the fingers, and on the surface of the lips, which are
used for feeling; but is thick on the palm of the hand, which
is used for firmly grasping, and which would be continually
suffering pain if its sensibility were too acute ; and it is still
thicker on the sole of the foot, especially on the heel and the
ball of the great toe, where pressure has to be sustained.

493. Although the fingers of Man and of the Quadrumana,
being endowed with peculiar sensibility, are their special organs
of touch, yet we find that they cease to be so in most of the
other Mammalia, whose extremities are adapted only for sup-
port and locomotion, and are terminated by hard claws or
hoofs that completely envelop them. In many of these, we
find the lips and tongue employed as the chief organs of touch ;
in the Elephant, this sense is evidently possessed very acutely
by the little finger-like projection at the end of its trunk ; and
in several other cases the mbrissce or whiskers are its special
instruments, the bulbs of their long stiff hairs being copiously
supplied with sensory nerves.

494. A curious modification of the sense of Touch appears
to exist in Bats. It has been found that these animals, when
deprived of sight and (as far as possible) of hearing and smelling
also, still flew about with equal certainty and safety, avoiding
every obstacle, passing through passages only just large enough
to admit them, and flying through places with which they
were previously unacquainted, without striking against the
objects near which they passed. The same result happened

392 IMPROVEMENT OF TOUCH BY EXERCISE.

when threads were stretched in various directions across the
apartment. Hence some Naturalists were inclined to attribute
to the Bat the possession of a sixth sense unknown to Man ;
but Cuvier correctly pointed out that this idea becomes un-
necessary, if we attribute to the delicate membrane of the
wings (as we are justified in doing) a high degree of tactile
sensibility, so as to receive impressions from the pulses of the
air that are produced by the action of the wings and modified
by the neighbourhood of solid bodies.

495. The only idea communicated to our minds by the sense
of Touch, when this is exercised in its simplest form, is that of
resistance; and we cannot form a notion either of the size or
shape of an object, or of the nature of its surface, by feeling
it, unless we move the object over our own sensory organ, or
move the latter over the former. By the various degrees of
resistance which we encounter, we estimate the hardness or
softness of the body ; and by the impressions made upon the
papillae, when they are moved over its surface, we form our
idea of its smoothness or roughness. It is by attention to the
muscular movements we execute, in passing our hands or
fingers over its surface, that we acquire our ideas of its size
and figure; and hence we perceive that the sense of touch,
without the power of moving the tactile organ over the object,
would have been- of comparatively little use.

496. This sense is capable of improvement to a remarkable
degree ; as we see in persons who have become more dependent
upon it in consequence of the loss of their sight. This doubt-
less results, in part, from the increased attention which is
given to the sensations ; and partly from the greater acuteness
or impressibility of the organ itself, arising from the frequent
use of it. Amongst other remarkable instances of this kind
was that of Saunderson, who, though he lost his sight at two
years old, acquired such a reputation as a mathematician, that
he obtained a Professorship at Cambridge. He exhibited, in
several ways, an extraordinary acuteness in his touch ; but
one of his most remarkable faculties was the power of distin-
guishing genuine .medals from imitations, which he could do
more accurately than many connoisseurs in full possession of
their senses.

497. The sense of temperature is of a different character
from common tactile sensibility; and either may be lost,

SENSE OF TEMPERATURE. ANTENNA OF INSECTS. 393

without the other being affected. It is rather of a comparative
than of a positive kind ; that is, we form our estimate of tem-
perature rather by comparing it with that to which our body
(or the part of it employed to test the heat or cold) has been
previously exposed, than by any knowledge which we derive
through the sensation as to the actual degree of heat or cold
to which the organ is exposed. Thus, if we plunge one hand
into a basin of hot water, and the other into cold, and then
transfer both of, them to a basin of tepid water, this will feel
cold to the hand which has been previously accustomed to
the heat, and warm to the other. In the same manner, the
temperature of Quito, which is situated half-way up a lofty
mountain, is felt to be chilly by a person who has ascended
from the burning plains at its base, whilst it seems intensely
hot to another who has descended from its snow-capped sum-
mit ; the residents in the town at the same time regarding it
as moderate, — neither hot nor cold. It is a curious circum-
stance, that a weak impression made on a large surface seems
more powerful than a stronger impression made on a small
surface ; thus, if the fore-finger of one hand be immersed in
water at 104°, and the whole of the other hand be plunged in
water at 102°, the cooler water will be thought the warmer ;
whence the well-known fact, that water in which a finger can
be held will scald the whole hand.

498. Where any special organs of Touch exist in Inverte-
brated Animals, they are for
the most part prolongations
from the portion of the head
near the mouth. This is
the case with the arms of
the Cuttle-fish, and with the
tentacula of the lower Mol-
lusca which are similar in
position. Among Crustacea
and Insects, the antennae or
feelers (fig. 198, a, a) appear
to be the special organs of
touch. These are frequently Fig- ^.-CAPRICORN-BEETLE.
very long, and present an extraordinary variety in their forms,
of which some are depicted in fig. 199. They contain,
for the most part, a large number of joints (in the Mole-

394

ANTENNAE OF INSECTS.

Cricket above 100), and are very flexible. This flexibility
enables them to be turned towards any object under examina-
tion by the Insect ; and when the animal is walking, we see
them constantly being applied to the surfaces of the bodies
which it approaches, in a manner which leaves little doubt
that they are used as organs of touch. It is no objection to
this view, to say that, as their surfaces are hard, no delicate
sensations can be received through them; for the slightest

Fig. 199. — VARIOUSLY-FORMED ANTENNJE of INSECTS.

contact of their firmest points with a hard substance, may
produce a sense of resistance which will afford to the animal
the information which it requires. The stick used by the
blind man in feeling his way, serves a very similar purpose.
— It appears to be by sensations received through their
antennae, that Bees and other Insects which naturally work
in the dark, are enabled to carry-on their labours without
confusion or inaccuracy ; and to be by the same means, that
they communicate with each other. When the antennae are
cut off, the Bee at once ceases to work, and seems unable to
direct its movements in any other way than towards the light.
When any important event has happened in a community,
such as the loss of the Queen, the spreading of the intelligence
through the whole hive may be watched by a close observer.
The working bees which were near her are observed to run
about restlessly, applying their antennae to those of the others
they may meet, crossing them and striking lightly with them ;
these in their turn become agitated and do the same ; and
thus the intelligence is speedily propagated throughout the
hive. In the same manner, when two bees meet each other
out of their hives, they seem to reconnoitre one another for
some time by the movements of their antennae; and often

SENSE OP TASTE I PAPILLA OF TONGUE. 395

keep up these movements for a considerable period, as if
carrying on a close conversation. That the Antennae are
delicate organs of Touch can, therefore, be scarcely questioned.

Sense of Taste.

499. The sense of Taste, like that of touch, is excited by
the direct contact of particular substances with certain parts
of the body ; but it is of a much more refined nature than
touch, inasmuch as it communicates to us a knowledge of
properties which that sense would not reveal to us. All sub-
stances, however, do not make an impression on the organ of
taste. Some of them have a strong savour, others a slight
one, and others again are altogether insipid. The cause of
these differences is not understood ; but it may be remarked
that, in general, bodies which cannot be dissolved in water
have no savour ; whilst most of those which are soluble have
a taste more or less strong. Their solubility, in fact, seems
to be one of the conditions requisite for their action on the
organ of taste ; for when that organ is completely dry, it does
not receive any sensation from solid bodies brought into con-
tact with it, which may have the most powerful taste if re-
duced to a fluid form ; and there are substances known, which,
being perfectly insoluble in water, are insipid if applied to
the tongue when it is covered as usual with a watery secre-
tion ; but which have a strong taste when they are dissolved
in some other liquid, spirit of wine for instance.

500. The sense of Taste has for its chief purpose, to direct
animals in their choice of food ; hence its organ is always
placed at the entrance to the digestive canal. In the higher
animals, the tongue is the principal seat of it ; but other
parts of the mouth are also capable of receiving the impres-
sions of certain savours. The mucous membrane which covers
the tongue is copiously supplied with blood-vessels ; and is
thickly set, especially upon its upper surface, and towards the
tip, with papillae, resembling in structure those of the skin,
but larger. These papillae, however, are not all sensory ; for
some, which are of conical form, are covered with a firm horny
epithelium, and their function seems to be chiefly mechanical.
These " conical " papillae are very strongly developed in the
tongues of many of the lower Mammalia, to which they impart
a particular roughness ; thus it is by their means that a Dog

396 PAPILLA OF TONGUE : — NATURE OF SENSE OF TASTE.

cleans of all its flesh the bone he licks, and that the Lion, by
a single stroke of his tongue, can take off" the skin from any
part of the Human body. The tongue itself is made-up of
muscular substance, which accomplishes the varied move-
ments that are required in the acts of mastication and in the
production of articulate sounds. It is supplied with nerves
from the third division of the fifth pair, from the glosso-
pharyngeal, and from the hypoglossal (§ 459). The last is
the motor nerve of the tongue ; the first is the one chiefly
concerned in the conveyance of sensory impressions from the
front and sides of the tongue ; and the other (the glosso-
pharyngeal) seems, to have for its office to convey those im-
pressions from the back of .the tongue which excite the muscles
of swallowing to action (§ 470), as well as those which produce
the sensation of nausea and excite the act of vomiting. The
gustative papillae, which have a very thin epithelial covering,
are for the most part supplied from the fifth pair ; and the
branch of this proceeding to the tongue is known as the
lingual nerve. When they are called into action by the con-
tact of substances having a pleasant savour, they not unfre-
quently become very turgid, and rise-up from the surface of
the mucous membrane ; in this manner is produced the rough-
ness that is felt on the surface of any portion of the tongue or
inside of the cheek, against which a piece of barley-sugar or
other similar substance has lain for some little time.

501. A considerable part of the impression produced by
many substances, is received through the sense of Smell rather
than by that of Taste. Of 4his any one may convince him-
self by closing the nostrils and breathing through the mouth
only, whilst holding in the mouth, or even rubbing between
the tongue and the palate, some aromatic substance ; its taste
is then scarcely recognised, although it is immediately per-
ceived when the nasal passages are re-opened, and its effluvia
are drawn into them. There are many substances, however,
whose taste, though not in the least dependent upon the
action of the nose, is nevertheless of a powerful character ;
such are sugar, salt, quinine, and vinegar. Others, again,
by irritating the mucous membrane, produce a sense of pun-
gency allied to that which the same substances (strong acids,
for instance, pepper, or mustard) will produce when applied
to the skin for a sufficient length of time. Such sensations,

USES OF SENSE OF TASTE. 397

therefore, are evidently of the same kind with those of Touch,
differing from them only in the degree of sensibility of the
organ through which they are received ; and through these the
sense of Taste is more nearly related to that of Touch, than is
either of the other forms of special sensibility.

502. This sense has a very important function in most
animals which possess it, — that of directing them in their
choice of food. Most of the lower animals will instinctively
reject the articles of food that would be pernicious to them ;
thus even the voracious Monkey will seldom touch fruits of a
poisonous character, though their taste may be agreeable ; and
animals whose digestive apparatus is adapted to one kind of
food, will reject all others. It may be stated, as a general
rule, that substances of which the taste is agreeable to us
are useful and wholesome articles of food, and vice versd; but
there are many signal exceptions to this. It is interesting to
remark that in Man, when the reasoning powers are obscured
by disease, his instincts in regard to food often manifest them-
selves strongly, and frequently constitute the best guide in its
administration ; thus, there are many cases of fever in which
the physician is in doubt whether wine will be injurious or
beneficial, and in which he will usually find the disposition of
the patient to reject it, or his readiness to receive it, to be tiis
best guide. And in general it may be remarked that, in ill-
ness, the desire of the patient for food, or his disposition to
take it, pretty certainly indicate the fitness or unfitness of the
system to digest and appropriate it.

503. The tongue presents nearly the same structure among
the Mammalia in general, as in Man ; but in Birds it is
usually cartilaginous or horny in its texture, and destitute of
nervous papillae, so that the sense of taste cannot be very
acute in any of those animals. Several of them use their
tongues for other purposes, — the Woodpecker, for instance,
to transfix insects, and the Parrot to keep steady the nut or
seed which is being crushed between the jaws. In some
Reptiles the tongue is large and fleshy; in others long and
slender ; in others, again, it is almost entirely deficient : but
in no instance does it seem to minister to any acute sense of
taste. In Fishes it is for the most part absent. Many In-
vertebrated animals possess a tongue ; but its uses are for the
most part mechanical. Thus the tongue of the Limpet is a

398

SENSE OF SMELL : — ODOROUS SUBSTANCES.

powerful rasp (resembling that in fig. 107), by which it rubs
down the sea- weeds on which it feeds ; whilst the tongue of
the Bee (fig. 289) forms a channel through which it draws-up
the juices of flowers. In most Insects, the palpi, small jointed
appendages in the neighbourhood of the mouth (§ 172), seem
to answer the purpose of an organ of taste ; being observed
to be in incessant motion whilst the animal is taking food,
touching and examining it before it is introduced into the
mouth.

Sense of Smell.

504. Certain bodies possess the property of exciting in us
sensations of a ^peculiar nature, which cannot be perceived by
the organs of taste or touch, and which seem to depend upon
emanations that spread from them through the air, pro-
ducing what we term odours. It appears probable that odours
are, in reality, very finely-divided particles of the odoriferous
substance ; and this idea derives support from the fact that
most volatile bodies are more or less odorous, whilst those
which do not readily transform themselves into vapour, have
little or no fragrancy in their natural state, but possess strong
odorous properties as soon as they are converted into vapour —
by the aid of heat, for instance. The most powerful and
penetrating odours are for the most part those of bodies
already in a gaseous state, — such as sulphuretted and carbu-
retted hydrogen; or of fluids which readily pass into the
state of vapour, as the vegetable essential oils. But there are
some solid substances, as musk, which are very strongly
odorous ; and which yet do not appear to diffuse themselves
through the air in the state of vapour. The odoriferous
particles of these must be of extreme minuteness ; for the
substances do not seem to lose weight by freely imparting
their peculiar scent to an unlimited quantity of air. Thus the
experiment has been tried, of keeping a grain of musk freely
exposed to the air of a room of which the doors and windows
were constantly open, for a period of ten years ; the air, thus
continually changed, was completely impregnated with the
odour of musk ; and yet at the end of that time, the particle
was not found to have suffered any perceptible diminution in
weight.

505. In order that we should become sensible of odours, it

ODOEOUS SUBSTANCES : — STRUCTURE OF NOSE.

399

seems necessary that the odoriferous particles should come into
actual contact with the membrane on which the nerve of smell
is spread out. In this respect, the sense of Smell agrees with
the senses of taste and touch ; whilst it differs from those of
sight and hearing, which take cognisance of changes that are pro-
duced by vibrations or undulations in the surrounding medium.
It is, moreover, desirable that these odoriferous particles should
be conveyed by the air, and not be diffused through fluid ;
for though it is necessary to the perfection of the sense of
smell that the olfactory membrane should be kept moist, too
great a quantity of fluid upon its surface deadens its peculiar
sensibility, — as we find to be the case when we are suffering
under an ordinary " cold." Hence it is only in air-breathing
animals, that the sense of Smell can possess any considerable
acuteness.

506. The most advantageous position of this organ is evi-
dently at the commencement of the respiratory passages j so
that the air which is being re-
ceived into the lungs may pass
through it and be tested (as it
were) by its peculiar sensibility.
In all the air-breathing Verte-
brata we find a pair of cavi-
ties, the nasal fossae (fig. 200),
which are situated between the
mouth and the orbits. They
possess two orifices, — the ante-
rior nares, or nostrils (6), usually
opening upon the front of the
face, — and the posterior nares,
which open into the upper part
of the pharynx (c). The two
cavities are separated from each
other by a vertical partition,
which passes backwards and
forwards on the middle line ; their sides are formed by the
various bones of the face, and by the cartilages of the nose ;
their extent is very considerable, especially in animals that
have a prolonged muzzle. The interior of these cavities is
lined by a delicate mucous membrane, whereon the Olfactory
nerves, which enter through a multitude of minute orifices in

Fig. 200.— VERTICAL SECTION OP THE
NASAL CAVITY.

a, mouth; b, nostril; c, posterior open-
ing ; d, portion of the base of the
skull ; e, forehead ; /, h, passages be-
tween the spongy bones, g, i, k; I,
frontal sinus; m," sphenoidal sinus;
w.opening of Eustachian tube ; o, cur-
tain of the palate.

400 SENSE OF SMELL.

the roof of the cavity, are distributed ; and the extent of its
surface is increased, by its being folded over certain projec-
tions from the walls of the cavity, which are termed spongy
bones. Of these there are three in Man (<jr, i, k). Prolonga-
tions of this membrane are carried also into cavities hollowed
out in the neighbouring bones, which are termed sinuses.
Thus we have the frontal sinuses I, situated just above the
nose, between the eyebrows ; and the sphenoidal sinuses m,
situated further back. There is also a large cavity hollowed
out in the bone of the upper jaw on either side. The mem-
brane which lines these is kept moist by its own secretion ;
and it is covered with vibratile cilia, the office of which seems
to be to prevent that secretion from unduly accumulating, by
conveying it over the surface of the membrane to the outlet.

507. The mechanism of the sense of Smell is very simple.
When air charged with odoriferous particles passes over the
membrane that lines the nose, some of these particles are
delayed by the mucous secretion that covers it, and act upon
the delicate sensory extremities of the olfactory nerve with
which it is thickly set. The highest part of the nasal cavity
appears to be that in which there is the most acute sensibility
to odours ; and hence it is that when we snuff the air, so as
to direct it into the upper part of the nose, instead of allowing
it to pass simply along the lower portion from the anterior to
the posterior nares, we perceive delicate odours which would
have otherwise escaped us. The acuteness of the sense of
smell depends, in no small degree, upon the extent of surface
exposed by the membrane lining the nasal cavity; and in this
respect Man is far surpassed by many of the lower Mammalia,
especially among the Carnivora, Euminantia, and some Pa-
chydermata. The extreme delicacy of this sense in Deer,
Antelopes, &c., is well known, from its being a source of great
difficulty to the hunter, who cannot advance near enough to
attack them, except by stealing upon them in the direction
contrary to that of the wind. In these animals it serves as
the chief means by which they are warned of the proximity
of their enemies j in the Carnivora, on the other hand, it
serves to direct them to their prey. In general, however, it
seems to have for its office to assist in directing animals to
their food, and in warning them of the presence of noxious
vapours.

SENSE OF SMELL. 401

508. Besides receiving the Olfactory nerve, the mucous mem-
brane of the nose is supplied by branches of the Fifth pair ;
this nerve endows it with common sensibility, and also receives
the impressions produced by acrid or pungent vapours, which
act upon it in the same way as the corresponding fluids do
upon the tongue. Such vapours are felt by the irritation they
produce, rather than smelt; and the impression they occasion
gives rise to the reflex action of sneezing, by which they are
driven from the nose (§ 342). Hence this action may be
excited by an irritating agent (such as snuff) after the olfac-
tory nerve has been divided, if the branches of the fifth pair
be entire : whilst it does not take place when the fifth pair is
paralysed, even though the sense of smell may be retained.
This sense loses much of its acuteness, however, when the
branches of the fifth pair supplying its organ can no longer
discharge their functions ; for the membrane then becomes
dry from the want of its proper secretion, and the odoriferous
particles are consequently not properly applied to it.

509. Among animals that live in water, the olfactory organs
cannot act to the like advantage ; and we do not find much
provision made for this sense. In the Whale tribe, the nostrils
serve as the channels by which the water is expelled, that has
been drawn-in through the mouth (§ 185) ; they are situated
at the top of the head, and are known as blow-holes. In
Fishes, the nasal cavity has no posterior opening; but the
surface of its lining membrane is very much extended by its
arrangement in folds, which are sometimes disposed in a
radiated manner around a centre, and sometimes parallel like
the teeth of a comb. There are many Invertebrated Animals,
from whose actions it may be judged that they possess a deli-
cate sense of smell, although the precise seat of it cannot be
assigned. This is the case especially with Insects, Crustacea,
and the higher Mollusca. The lining membrane of the air-
tubes of Insects appears to be delicately sensitive to irritating
vapours (§ 443) ; but we have no evidence that it ministers
to the sense of Smell properly so called.

Sense of Hearing.

510. By this sense we become acquainted with the Sounds
produced by bodies in a certain state of vibration. The vibra-
tions which sonorous bodies undergo, are communicated by

D D

402

TRANSMISSION OF SONOROUS VIBRATIONS.

them to the air, producing in it a series of undulations or
waves, by which the sound is conveyed to a great distance.
These undulations spread more widely as they become more
distant from the sounding body, just like the ripples produced
on the surface of the water when we throw a stone into it ;
and in proportion as they spread, they become less powerful.
This is the reason why Sound becomes less intense as the
sounding body is more distant. Although: air is the usual
conducting medium for the sonorous undulations, liquids or
solids may answer the same purpose. Thus if a person hold
his head under water, whilst two stones are struck together,
also under water, even at a considerable distance, he will hear
the sound produced by the blow with extreme distinctness,
and even with painful force. Or if the ear be laid against
one end of a long piece of timber, whilst a scratch with a pin
be made on the other, or a watch be laid upon it, even the
faint sounds thus produced will be heard very distinctly.
That a medium of some kind is necessary to convey the
sonorous vibrations, is proved by the fact, that if a bell be
made to ring in the receiver of an air-pump from which the air
has been exhausted, no sound is heard, though ..its ringing
becomes audible as soon as the air is allowed to re-enter.

511. It, is a fact of much importance, in regard to the
action of the organ of Hearing, that sonorous vibrations which
have been excited and are being transmitted in a medium of
one kind, are not imparted with the same readiness to others.
The following conclusions have been drawn from experi-
mental inquiries on this subject. I. Vibrations excited in
solid bodies may be transmitted to water without much loss
of their intensity, although not with the same readiness that
they would be communicated to another solid, n. On the
other hand, vibrations excited in water lose something of
their intensity in being propagated to solids ; but they are
returned, as it were, by these solids to the liquid, so that the
sound is more loudly heard in the neighbourhood of those
bodies, than it would otherwise have been. in. The sonorous
vibrations of solid bodies are much more weakened by trans-
mission to air; and those of air make but little, impression
on solids, iv. Lastly, sonorous vibrations in water are trans-
mitted but feebly to air ; and those which are taking place in
air are with difficulty communicated to water ; but the com-

TRANSMISSION OP SONOROUS VIBRATIONS. 403

munication is rendered muck more easy by the intervention
of a membrane extended between them.

512. The Auditory nerve, or nerve of Hearing, is adapted
to receive and transmit to the brain the sonorous undulations
produced in the surrounding medium by vibrating bodies.
Now, it is obvious that it may be affected by these in various
ways, especially in animals that inhabit the water. The
vibrations excited in the liquid will be transmitted to the
solid parts of the head, and thence to the nerve contained in
it, without much interruption ; and this independently of any
special apparatus of hearing. Indeed, the simplest form of
this apparatus is only designed to give increased effect to the
vibrations thus excited in the solid parts of the head ; for it
consists merely of a cavity excavated in their thickness, which
cavity is filled with fluid, and is lined by a membrane whereon
the auditory nerve is minutely distributed. This is the con-
dition of the organ of hearing in the Mollusca, where any such
exists j and also in many of the Crustacea. In those of the
latter class which chiefly inhabit air, however, this cavity is
excavated in the surface of the shell covering the head, and is
shut-in by a membrane which is exposed to the surrounding
medium. According to the principle (iv.) mentioned in the
last paragraph, the liquid contained in the chamber will be
thrown into undulation by vibrations in air, as well as by
those of water; so that those animals which possess this kind
of apparatus are able to hear much better in air, than are those
in which the cavity is completely shut-in by stony walls. Of
the degree in which sonorous vibrations may be communicated
to our own auditory nerves through the solid parts of the
skull, we may easily satisfy ourselves by closing the ears
carefully, and placing any part of the head against a solid
body which communicates with the one in vibration. In this
manner we may hear the sounds produced by the latter with
considerable distinctness, though accompanied by an unpleasant
jarring. A deaf gentleman was once agreeably surprised
to find that, when smoking his pipe, with the bowl resting on
his daughter's pianoforte, he could distinctly hear the music
she was producing from it ; and many deaf persons may be
made to hear conversation, by holding a piece of stick
between their own teeth, and placing it against the teeth of
the person speaking.

DD 2

404 STRUCTURE OF ORGAN OF HEARING.

513. In animals which have the organ of hearing con-
structed upon the simple plan just described, the force of the
vibrations of the fluid contained in the cavity is increased by
several minute stony concretions suspended in it : these act
according to the second principle just stated (§ 511). They
are termed otolithes, or ear-stones ; and some traces of them
may be found even in Man and the higher animals.1

514. We see, then, that a cavity excavated in the solid
walls of the head, covered-in externally by a membrane,
having the auditory nerve distributed upon its walls, and
filled with fluid, is the simplest form of the organ of hearing ;
and may be regarded as including all that is essential to the
exercise of this function. No more complicated apparatus is
to be found in any of the Invertebrata ; and even in the
lowest Fishes there is but little variation from this type. On
the other hand, in Man and the higher Vertebrata we find a
very complex structure, adapted to render the faculty much
more perfect ; enabling us to receive impressions which make
us. aware, not only of the presence of a sounding body, but of
its nature, its direction, the pitch and peculiar quality of the
sound ; and also, it is probable, taking cognizance of sounds
much fainter than those which would be perceptible to the
lower animals. Yet even in the most complicated forms of
the organ of hearing, we shall find that the essential part is
still the same as that which forms the whole organ in the
lower tribes ; and also that the faculty is retained, though in
an inferior degree, when by disease or injury the accessory
parts are prevented from acting. — To the structure of the Ear
of Man we shall now proceed.

515. The organ of hearing in Man may be divided into
three parts — the external, the middle, and the internal ear. The
former is the fibro-cartilaginous appendage placed on the out-
side of the head, to receive and collect the sounds which are
to be transmitted to the interior ; the two latter divisions are
excavated in a bone of remarkable solidity, the petrous (stony)
portion of the temporal bone. The uses of the different

1 Vesicles containing otoliths which are kept in rapid movement
within them by ciliary action, are found in immediate contiguity with
the cephalic ganglia of the lower Mollusks, or are even imbedded in
their substance; and these seem to constitute the most rudimentary
form of an organ of hearing.

EXTERNAL EAR : — TYMPANIC CAVITY. 405

hollows and elevations on the surface of the external ear of
Man are not very apparent; but it is probable that they
direct the sonorous undulations towards the entrance of the
canal which leads to the middle ear. The form of the external
ear in many Quadrupeds evidently adapts it to this purpose ;
and there are several which have the power of changing its
direction by muscular action, in such a manner as to enable it
to catch most advantageously the faintest sounds from any
quarter. This is especially the case with animals that are
naturally timorous, such as the Hare or the Deer ; these have
usually very large external ears. But it is among the Bat
tribe — whose residence in the dark recesses of caverns and
excavations makes their eyes of comparatively little use to
them, and causes them to depend greatly for guidance in their
movements upon the sense of hearing — that we find the
greatest development of the external ear (fig. 201).

Fig. 201. -LONG-EARED BAT.

516. The canal hollowed -out in the temporal bone (d,
fig. 204) into which the external ear collects the sonorous
vibrations, passes inwards until it is terminated by a mem-
brane stretched across it, which is called the membrana tym-
panij or membrane of the drum of the ear (g). This forms
the outside wall of a cavity (h) which constitutes the middle
ear, and which is bounded on the inside by a bony wall that
separates it from the internal ear. The cavity of the tym*
panum is not one of the essential parts of the organ ; for
nothing analogous to it exists either in Fishes or in the lower
Eeptiles. It contains air ; and communicates with the back

406 TYMPANIC CAVITY AND CHAIN OF BONES.

of the nasal cavity (n, fig. 200) by a canal termed the Eusta-
chian tube (k, fig. 204). The partial or complete closure of
this tube, occasioned either by swelling of its lining membrane
or by the viscid secretion from it, produces the slight deafness
common among those who are suffering from " colds." Within
the cavity of the tympanum, there is a very curious apparatus
of small bones and muscles, which serves to establish a con-
nexion between the membrane of the drum and the small
membrane covering the entrance to the internal ear. These
bones are four in number ; and are termed the malleus or

Fig. 202.— BONES OF Fig. 203.— CAVITY OF THE TYMPANUM, WITH

THE EAR. THE BONES IN THEIR PLACES.

hammer (a, fig. 202) ; the incus, or anvil (b) ; the os orbicu-
lare, a minute globular bone (c); and the stapes, or stirrup-
bone (d). These bones are connected together in the manner
represented in fig. 203 ; where a a represents the wall of the
tympanic cavity ; 5, the membrana tympani ; c, one of the
long processes of the malleus, which is attached to the mem-
brane ; d, the head of the malleus, which articulates with the
incus ; e, the other long process of the malleus, which is
acted-on by the minute muscle f, that serves to tighten the
tympanum ; g, the incus, of which one leg is in contact with
the wall of the cavity, whilst the other is connected with the
orbicular bone h ; i, the stapes, of which the upper end is
connected with the orbicular bone, whilst the lower (which is
of an oval form) is attached to the membrane that covers the
entrance to the internal ear ; and Tc is a small muscle which

ACTION OF THE TYMPANIC APPARATUS.

407

acts upon this bone in such a manner as to relax the tym-
panum.

517. The us£ of this apparatus is evidently to receive the
sonorous vibrations from the air, and to transmit them to the
membrane forming the entrance to the internal or essential
part of the organ of hearing ; in such a manner, that the
sonorous vibrations excited in the latter may be much more
powerful than they would be if the air acted immediately
upon it. The usual state of the membrane of the tympanum
appears to be rather lax or slack ; and when in this condition,

Je h g q r f s b

Fig. 204. — VERTICAL SECTION OP THE ORGAN OP HEARING IN MAN.

The internal portions are proportionately enlarged to make them more evident:
a, b, c, the external ear ; d, entrance to the auditory canal /; e, e, petrous portion
of the temporal bone, in which the internal ear is excavated ; g, membrane of the
tympanum ; h, cavity of the tympanum, the chain of bones being removed ;
», openings from the cavity into the cells j excavated in the bone ; on the side
opposite themembrana tympani are seen the fenestra ovalis and rotunda;"*, Eusta-
chian tube ; I, vestibule ; m, semicircular canals ; n, cochlea ; o, auditory nerve ;
p, canal by which the carotid artery enters the skull ; g, part of the glenoid fossa
which receives the head of the lower jaw ; r, styloid process of the temporal bone.

it vibrates in accordance with grave or deep tones. By the
action of a small muscle lodged within the Eustachian tube, it

408 TYMPANIC APPARATUS : INTERNAL EAR.

may be tightened, so as to vibrate in accordance with sharper
or higher tones ; but it will then be less able to receive the
impressions of deeper sounds. This state we may artificially
produce either by holding the breath and forcing air into the
Eustachian tube, so as to make the membrane bulge-out by
pressure from within ; or by exhausting the cavity by an effort
at inspiration with the mouth and nostrils closed, which will
cause the membrane to be pressed inwards by the external
air. In either case the hearing is immediately found to be
imperfect ; but it will be observed that while the experimenter
thus renders himself deaf to grave sounds, acute sounds are
heard even more distinctly than before. There is a different
limit to the acuteness of the sounds of which the ear can
naturally take cognizance, in different persons. If the sound
be so acute (or high in pitch) that the membrana tympani will
not vibrate in unison with it, the individual will not hear it,
although it may be loud ; and it has been noticed that some
persons cannot hear the very shrill tones produced by par-
ticular Insects, or even by Birds, which are distinctly audible
to others. There is good reason to think that the two
muscles which have been mentioned (§ 516) as tightening
and relaxing the tympanum, exert a regulative influence upon
its tension analogous to that which the contractile fibres of
the iris possess in regard to the diameter of the pupil (§ 534) ;
preparing it to be acted-on by faint sonorous undulations
when we are listening, and moderating the concussion of very
loud sounds which are anticipated.

518. The internal ear is composed of various cavities that
communicate with each other; of these the vestibule (I, fig. 204)
may be regarded as the centre, whilst from it there pass-off
on one side the three semicircular canals, m, and on the other
the cochlea, n. The vestibule is the part which corresponds
with the simple cavity that constitutes the whole organ of
hearing in the lower animals (§ 512), and the others may be
regarded as extensions of it for particular purposes. It com-
municates with the cavity of the tympanum by a small orifice
in the bony wall that separates them, termed the fenestra ovalis
(oval window) ; but this orifice is closed by a membrane, to
which the lower end of the stapes is attached. The three
semicircular canals are passages excavated in the solid bone,
and lined by a continuation of the same membrane as that

STRUCTURE OF THE INTERNAL EAR. 409

which lines the vestibule ; each passes-off from the vestibule
and returns to it again. The cochlea, n, also is a cavity exca-
vated in the hard bone, and lined by a continuation of the
same membrane ; its form is almost precisely that of the in-
terior of a snail-shell (whence its name), being a spiral canal
which makes about two turns and a half round a central pillar.
This canal is divided into two, however, by a partition that
runs along its whole length ; which partition is partly formed
by a very thin lamina of bone, and partly (in the living state)
by a delicate membrane. The two passages do not communi-
cate with each other except at the top or centre; at their
lower end (corresponding to the mouth of the snail-shell) they
terminate differently; for whilst one freely opens into the
vestibule, the other communicates with the cavity of the
tympanum, by an aperture termed the fenestra rotunda (round
window), which is closed by a membrane.1 Thus the internal
ear communicates with the cavity of the tympanum by two
minute orifices only, — the fenestra ovalis and the fenestra
rotunda, — both of them closed by membranes, against the
former of which the stapes abuts, whilst the latter is free.

519. The whole internal ear is lined by a delicate mem-
brane, on which the auditory nerve (o, fig. 204) is very
minutely distributed, especially on the membranous portion
of the partition between the two passages of the cochlea.
The cavities are completely filled with fluid, which is set in
vibration by the movements of the stapes, communicated
through the membrane of the fenestra ovalis; and these vibra-
tions are probably rendered more free by the existence of the
second aperture — the fenestra rotunda. It is by the influence
of these undulations upon the expanded fibrils of the auditory
nerve, that the sensation of sound is produced ; but in what
way the different parts of the labyrinth (as this complex
series of cavities is not unaptly called) contribute to the per-
formance of this function, is not yet known. In all Fishes
but the lowest, the three semicircular canals exist ; they have,
however, no vestige of a cochlea. In the true Eeptiles, a
rudiment of the cochlea may be generally discovered. In
Birds, this cavity is more completely formed, though the
passage is not spiral, but is nearly straight; of its real

1 There is a double spiral staircase constructed exactly on this plaa
in Tamworth church.

410 FUNCTIONS OF THE INTERNAL EAR.

character, however, there can. be no doubt, from its being
divided, like the cochlea of Mammals and of Man, by a
membranous partition on which the nerve is spread out.

520. From the circumstance that in almost every instance
in which the semicircular canals exist at all, they are three in
number, and lie in three different directions, corresponding to
those of the bottom and two adjoining sides of 'a cube, it has
been supposed (and with much probability) that they assist
in producing the idea of the direction of sounds. It has been
also supposed that the cochlea is the organ by which we judge
of the pitch of sounds ; and this would seem to be not im-
probable, especially when we compare the development of the
cochlea in different animals, with the variety in the pitch of
the sounds which it is important they should hear distinctly,
especially the voices of their own kind. The compass of the
voice (that is, the distance between its highest and its lowest
tones) is much greater in Mammals than in Birds; as is also
the length of the cochlea. In Reptiles, which have little true
vocal power, the cochlea is reduced to its lowest form ; and in
the Amphibia, it disappears altogether.

521. That the Vestibule, and the passages proceeding from
it, constitute even in Man the essential part of the organ of
hearing, is evident from the fact, that when (as happens not
unfrequently) the membrana tympani has been destroyed by
disease, and the chain of bones has been lost, the faculty is
not by any means abolished, though it is deadened. In this
state, the vibrations of the air must act "at once upon the
membrane of the fenestra ovalis, as in the lower animals which
possess no external or middle ear; instead of striking the
membrane of the tympanum, and being transmitted along the
chain of bones.

522. It- has been stated (§ 510) that the sensation of
hearing is produced by the successive undulations or vibra-
tions communicated to the Ear from the sonorous body, either
by the air, or by a liquid or solid medium. This is the case
with all continuous sounds or tones; but single momentary
sounds, such as those produced by the discharge of a pistol,
the blow of a hammer, the ticking of a watch, or the beat of
a clock, make their impression on the ear by a single shock.
All continuous tones are in fact caused by a succession of
such shocks, communicated to the ear with sufficient rapidity

NATURE OP CONTINUOUS TONES. 411

for the interval between them not to be distinguished. Thus,
if we cause a tight string to vibrate by pulling or striking it,
we occasion, not one vibration only, but a long succession of
vibrations (MEGHAN. PHILOS. § 187) ; every one of which
gives a new impulse to the air, and produces a new impression
on the organ of hearing. These vibrations we can see, when
they are sufficiently extensive ; and we can always feel them,
by placing the finger on the string. In the same manner,
the vibrations of a bell or of a tuning-fork continue long after
the first blow ; and these, though we cannot see them, may
be readily felt with the finger. It is, in fact, in their power
of continuing to vibrate after they have been struck, that the
peculiarity of these resonant bodies consists. In other instances
in which continuous tones are produced, the vibrations are
kept-up by the continued application of the original cause,
and cease as soon as it is withdrawn : this is the case, for
instance, in the string of the violin when set in vibration by
the bow, and in the flute and organ-pipe when caused to
sound by the passage of air through them.

523. In all these cases, then, the continuous tones are due
to a succession of impulses given by the sounding body to the
air ; and according to the rapidity with which the impulses
succeed one another, will be the pitch of the sound. It is
not difficult to ascertain by experiment the number of such
impulses required to produce particular tones. The lowest
note (C) given by any musical instrument (that which is
produced by an open organ-pipe of 32 feet long, or by a
stopped pipe of 16 feet) requires 16 impulses per second;1
but continuous tones have been produced by impulses occur-
ring at the rate of only 7 or 8 per second; so that the
impression produced upon the ear by each must have lasted
at least l-7th or l-8th of a second. On the other hand, it
has been ascertained that the ear can appreciate tones pro-
duced by 24,000 impulses in a second; so that the limit
already adverted to (§ 517) must be above this tone, the
pitch of which is about 4 octaves above the highest F of the
pianoforte.

524. The strength or loudness of musical tones depends

1 A backward as well as forward vibration must take place with
every impulse ; consequently the number of the vibrations is twice that
of the impulses.

4:12 DIFFERENCES IN TONES, AND IN SENSE OF HEARING.

upon the force and extent of the vibrations communicated by
the sounding body to the air. Thus, when we draw the
middle of a tight string far out of the straight line, and then
let it go, a loud sound is produced, and we can see that the
space through which the string passes from side to , side is
considerable. As the extent of the vibrations of the string
diminishes^ the sound becomes less powerful ; and when we
can no longer see the vibrations, but can only feel them, the
sound is faint. The length of the undulations in the air
corresponds with that of the vibrations in the sounding
body ; and consequently they will strike upon the tympanum
with more or less force, according as these are longer or
shorter. The cause of the differences in the timbre or quality
of musical tones, — such, for instance, as those which exist
between the tones of a flute, a violin, and a trumpet, all
sounding a note of the same pitch, — are unknown ; but they
probably depend upon the different form of the vibrations.

525. The faculty of hearing, like that of sight, may be very
much increased in acuteness by cultivation ; but this increase
depends rather upon the habit of attention to the faintest
impressions made upon the organ, than upon any change in
the organ itself. This habit may be cultivated in regard to
sounds of some one particular class ; all others being heard as
by an ordinary person. Thus the watchful North American
Indian recognises footsteps, and can even distinguish between
the tread of friends or foes, whilst his companion who lives
amid the busy hum of cities is unconscious of the slightest
sound. Yet the latter may be a Musician, capable of distin-
guishing the tones of all the different instruments in a large
orchestra, of following any one of them through the part
which it performs, and of detecting the least discord in the
blended effects of the whole, — effects which would be, to his
coloured companion, but an indistinct mass of sound. In the
same manner, a person who has lived much in the country is
able to distinguish the note of every species of bird which
lends its voice to the general concert of .Nature ; whilst the
inhabitant of a town hears only a confused assemblage of
shrill sounds, which may impart to him a disagreeable rather
than a pleasurable sensation. — Of the direction and distance
of sounds, our ideas are for the most part formed by habit.
Of the former we probably judge, in great degree, by the

APPEECIATION OF SOUNDS I — TKANSMISSION OF LIGHT. 413

relative intensity of the impressions received by the two
ears ; though we may form some notion of it by either
singly (§ 520). Of the distance we judge by the intensity of
the sound, comparing it with that which we know the same
body to produce when nearer to us. The Ear may be deceived
in this respect as well as the eye (§ 566) ; thus the effect of
a full band at a distance may be given by the subdued tones
of a concealed orchestra close to us ; and the Ventriloquist
produces his deceptions by imitating, as closely as possible,
not the sounds themselves, but the manner in which they
would strike the ear.

Sense of Sight.

526. By the faculty of Sight we are made acquainted, in.
the first place, with the presence of light ; and by the medium
of that agent we take cognizance of the forms of surrounding
bodies, their colours, dimensions, and positions. It is desira-
ble that a short account should be here given of the laws of
the transmission of light; since, without the knowledge of
them, the beautiful action of the Eye cannot be understood.

527. The rays of light uniformly travel in straight lines,
so long as they traverse the same medium (air, water, or
glass, for instance), without obstruction. When issuing from
a single luminous point into space, they diverge or separate,
in such a manner as to cover a larger and larger surface as
they proceed ; and the intensity of the light diminishes in the
same proportion. But when the rays pass from one medium to
another either more or less dense,

they are bent out of their straight
course, or refracted; unless they
should happen to pass from the one
to the other in a direction perpendi-
cular to the plane which separates
them. This maybe made evident
by a very simple experiment. Place
a com or any heavy body (a, fig. 205)
at the bottom of a basin, and then
retreat from it until the coin is hidden by the edge of the basin ;
if water be then poured-in, up to the level c, the coin will
again become visible, although neither its own place nor that
of the observer has undergone any change. The reason of this

414 COURSE OP REFRACTED RAYS.

is, that the rays of light, as they pass out of the water, are
bent downwards, or from the perpendicular ; so that they
reach the eye of the observer when situated at a lower point
than that at which the rays would have arrived if they had
proceeded in a straight line. Thus the eye, situated at the
end of the line a c, could not see the coin in a straight line,
because rays passing in that line would be interrupted by the
opaque sides of the basin ; but it receives the ray which was
passing through the water in the direction a d, and which
was bent downwards at the moment of quitting it. If the
eye had been placed directly over the coin, however, so that
the ray passing through the latter to it would have emerged
from the water in a direction perpendicular to its surface, no
change in the apparent place of the object would have been
made by pouring-in the water ; since a ray that passes from
one medium to another, however different, in a direction
perpendicular to the surface which separates them, is not
refracted. Those rays which pass-out most nearly in this
direction are refracted least, whilst those which pass-out most
nearly in the horizontal direction are refracted most.

528. The general law of refraction then is, — that all rays
passing from a dense to a rare medium are refracted from the
perpendicular, the degree of change being less as they are
near the perpendicular, and greater as they depart from it.
On the other hand, when rays pass from a rare medium into
a dense one, they are bent towards the perpendicular ; and
this in a greater or less degree, according as their direction is
more distant from the perpendicular, or nearer to it. Thus,
in fig. 205, we will suppose the point a to be the position of
the eye of a Fish ; and the end of the line a c (previously
occupied by the eye of the observer) to be the position of an
Insect in the air. Now this insect will not be seen by the
fish in its true place ; for a ray passing from it to c would be
so bent out of its course as not to reach the point a. The
direction in which it is really seen is a d ; for the ray pro-
ceeding from the object to the surface of the water, there
undergoes such a refraction that it is bent downwards to a ;
and, as we always judge of the place of an object by the
direction in which the rays last come to the eye, the insect is
seen by the fish at d, that is, considerably above its real place
(§ 476).

REFRACTION OP RAYS THROUGH CURVED SURFACES. 415

529. When the surface which separates the two media is
not flat, but is either convex or concave (bulging or hollowed-
out), a very important alteration is produced in the direction

of the rays that fall upon it. Thus we shall suppose that
three diverging rays, issuing from a point, a (fig. 206), and
traversing the air, strike upon a convex surface of glass, bb'.
The central ray a c falls upon the glass in a direction perpen-
dicular to its surface at that point, and passes-on unchanged
in its course. But the ray a d falls upon the surface very
obliquely ; and consequently in entering the glass it will be
bent towards the line e, which is perpendicular to the surface
at the point where it enters, and will pass onwards in the
direction / In the same manner, the ray a g will be refracted
into the direction i. Hence these rays, now converging, would
be found, if prolonged, to meet each other again ; and the
point at which they meet is termed the focus. To this point
all the other rays which fall upon the convex surface, at a
moderate distance from the central ray, will also be conducted.
530. On the other hand, if the surface of the glass, instead
of being convex, is concave, the diverging rays which fall upon
it will be made to diverge still more. Thus in fig. 207, let a
be the point whence the rays issue, and b b the surface of the
lens ; the central ray a c will pass-on unchanged as before ;
but the ray a d will be bent towards the perpendicular e, so
as to pass-on in the direction/; and the ray a j will be bent
towards the perpendicular h, into the direction i. It is easy
to understand that the change of direction will be greater, as

416 REFRACTION BY LENSES: — FORMATION OF IMAGES.

the curvature of the lens is more considerable. Thus a convex
lens has a long focus or a short focus (that is, brings rays to a

Fig. 207.

focus at a greater or less distance from itself) according as the
curvature of its surface is less or more considerable.

531. The rays issuing from every point in an object, and
falling upon a convex lens, are brought to a focus on the
other side of the lens ; and thus a distinct image or picture
is formed upon any screen placed at the proper distance to
receive it (as is seen in a Camera Obscura or a Magic Lantern),
every point in that image being the representative of the
corresponding point in the object, but this image being
inverted.

532. JSTow, the Eye, in its most perfect form — such as it
possesses in Man and the higher animals — is an optical
instrument of wonderful completeness, designed to form an
exact picture of surrounding objects upon the expanded sur-
face of the optic nerve, by which its impression is conveyed
to the brain. As it is in the most perfect form of this instru-
ment that we are best able to judge of the uses of its different
parts, it will be preferable to consider this in the first instance,
and then to advert to the less complete forms which we meet
with in the lower animals.

533. The Eye of Man, like that of all Vertebrata, has a
nearly globular form. The walls of the sphere are composed
of three coats; whilst in its interior are found three humors
of a more or less fluid character. The outer coat, named the
Sclerotic (s s, fig. 208), is tough and fibrous, and is destined
to support and protect the delicate parts which it contains.
It does not cover the whole globe, however ; but gives place
in the front of the eye to a transparent lamina of cartilaginous

COATS OP THE EYE.

417

Fig. 208.— INTERIOR OF THE EYE.

c, cornea; s, sclerotic; *', portion of
the sclerotic turned back to show
the parts beneath; cA, choroid;
r, retina; n, optic nerve; ca, an-
terior chamber; i, iris; p, pupil;
cr, crystalline lens ; pc, ciliary pro-
cesses ; v, vitreous humor ; bb, con-
junctiva.

structure c, which is termed the Cornea. The manner in which
this cornea is set upon the sclerotic coat, so as to serve as the
continuation of it, may be compared to that in which a watch-
glass is made to serve as the con-
tinuation of the watch-case over
the dial. The cornea is rather
more convex than the rest of
the sphere of the eye ; so that
the globe makes a slight addi-
tional projection in that part.
— When the sclerotic coat is
removed, we come upon the
second coat ch, which is termed
the Choroid; this is much more
delicate in its structure, con-
sisting almost entirely of blood-
vessels and nerves ; and it has
a deep black hue, owing to its
being lined with a thick layer
of black pigment, which consists
of cells that have the power of
secreting a black granular matter in their interior.1 This coat
also changes its character in the front of the eye ; being there
continuous with the Iris, or coloured portion, i, which forms
a sort of curtain that hangs-down behind the cornea. The
surface of the iris is flat, or nearly so ; and there is con-
sequently a space between it and the cornea, like that which
intervenes between the dial-plate and the glass of a watch ; this
space is termed the anterior chamber of the eye. The iris is
perforated in its middle by an aperture p, termed the Pupil.
This aperture is always round in Man ; but in animals whose
range of vision is required to extend widely in a horizontal
direction (such as the Euminants, and others which feed
upon herbage), it is an ellipse with the long diameter hori-
zontal ; whilst in animals which rather seek their food above
or below them (such as the Cat and other Carnivora which
naturally live among trees and high places), the pupil is an
ellipse whose long diameter is vertical.

1 Similar pigment-cells, having great variety in their form, are to be
found composing the black spots on the skin of the Frog, Water
Newt, &c.

E E

418 CONTRACTION AND DILATATION OP THE PUPIL : — RETINA.

534. By the contraction and relaxation of certain fibres in
the Iris, the size of the Pupil is changed according to the
degree of light to which the eye is exposed ; the aperture being
made to diminish in a strong light, in such a manner as to
exclude the rays that would be superfluous, and to prevent
too many from falling upon the expansion of the optic nerve ;
whilst it dilates in a faint light, so as to admit as many rays
as possible. If we notice the pupil of a Man who is looking
towards the mid-day sun, we shall see that it is contracted to
a small round speck ; but the pupil of a Sheep would be con-
tracted, in similar circumstances, into a horizontal slit ; and
the pupil of a Cat into a vertical one. The alteration in the
size of the pupil in accordance with the degree of light, may
be easily observed by stationing oneself at a window pro-
vided with shutters, and holding a looking-glass in the hand :
if the light be at first strong, the pupil will be seen in a con-
tracted state ; but if the shutters be gradually closed, so as to
diminish the amount of light that falls upon the eye, the
pupil will be seen to enlarge; and it will diminish again
when the shutters are re-opened. The blackness which the
pupil always presents, in the healthy state of the eye, is due
to our seeing the black lining of the back of the eye through
it. In many Quadrupeds, the black pigment is replaced, in
one portion of the eye, by a layer of a blueish colour, having
an almost metallic lustre ; and from this we see the light
brilliantly reflected, when we look at their eyes in certain
directions.

535. On turning back the choroid coat, we come to the third,
r, of the layers of which the wall of the eye is composed : this
is an extremely delicate film, chiefly consisting of nerve-cells
and nerve-fibres that spread-out from the optic-nerve, n ; and
it is called the Retina (or net). It advances nearly as far as
the iris ; but it is deficient in the front of the eye. The part
of the retina which lines the globe at the point exactly
opposite to the centre of the pupil, is distinguished from the
rest by a peculiar yellow opacity, which causes it to be
designated " the yellow spot/' In this spot, visual sensibility
is more acute than elsewhere. On the other hand, the part
of the retina that covers the entrance of the optic-nerve
(which is below " the yellow spot," and nearer to the nose) is
so much less sensitive to light than the rest, that under

HUMOES OF THE EYE : — CONJUNCTIVA. 419

certain circumstances the image that falls upon it may not be
perceived at all (§ 554).

536. The cavity of the globe is occupied by three humors
of different consistence — the Aqu^pus, Vitreous, and Crystal-
line. The aqueous humor is nearly pure water, being nothing
else than the serum of the blood very much diluted : it occu-
pies the anterior chamber ca, and a small space behind the
iris, in front of the crystalline lens. The vitreous humor v
resembles thin jelly in consistence, and occupies the greater
part of the globe of the eye behind the iris. The crystal-
line humor cr is of much firmer consistence, resembling
very thick jelly or soft gristle ; it has the form of a double-
convex lens, the posterior surface of which is more convex
than its anterior j and hence it is commonly known as the
crystalline lens. It is suspended in its place by a set of little
bands pc, proceeding from the choroid coat, and known as
the ciliary processes.

537. The cornea is covered externally by a membrane bb,
termed the Conjunctiva. This membrane is perfectly transparent
where it covers the cornea, and seems like an outer layer of
it ; the front of the sclerotic also is covered by it, but it is
there semi-opaque, having a whitish colour. The membrane
does not pass back over the globe of the eye, however, but
bends forward again, as seen at bb, so as to form the lining of
the eyelids, at the edge of which it becomes continuous with
the skin. Thus the smooth surfaces of the eye and of the
under side of the lids are both formed by this membrane ;
the mucous secretion from which serves to diminish the
friction of one upon the other. But the smallest particle of
any hard substance, which may become interposed between
these surfaces, produces great irritation. It cannot pass far
backwards, however, on account of the bend of the membrane
at bb; and thus it may be always removed (if loose) with
little difficulty. The lower lid can be easily drawn down, so
as to expose the membrane as far as this bend ; and any loose
particle that is lying upon its surface may thus be detected
and removed. But the upper lid, being longer, cannot be
drawn out sufficiently for this purpose ; and it is necessary to
evert it, or turn it inside-out. As the knowledge of the mode
of performing this very simple operation will often save a
good deal of suffering, it will be here described. Nothing

E E 2

420

MUSCLES OF THE EYE.

more is necessary than to close the upper lid — not forcibly,
however ; next to make pressure upon its upper part with a
pencil, bodkin, knitting-needle, or other hard body of small
diameter; and then, taking hold of the eyelashes, to draw
the lower edge of the lid forwards and upwards. By a dex-
terous movement of this kind, the lid may be everted without
any pain, a little temporary discomfort being all that the dis-
placement occasions ; its lining membrane is then exposed,
and any offending particle may be readily removed.

538. The globe of the eye is moved by six muscles, which
are lodged within the bony cavity or orbit, hollowed-out in the
skull. All these muscles, except one, originate at the back of

the orbit, and are inserted
into the sclerotic coat, near its
front, by broad thin tendons.
Four of them are termed recti
or straight muscles. One of
these, the superior rectus(e, fig.
209), is inserted at the upper
part of the eye, and conse-
quently by its contraction rolls
the globe upwards; another,
d, the inferior rectus, pro-
duces a corresponding move-
ment downwards. A third, the

internal rectm (which could

nerve; d, inferior rectus muscle; e, supe- not be shown in this figure),

rior rectus ;/, cut extremity of the ex- 11 fi o-ln"hp inwarrlc m-

ternal rectus; g, end of the inferior * inwards, C

obU^ue ; h, superior oblique ; i, elevator towards the nOS6 I whilst a

of the upper lid ; A, lachrymal gland. /> ,1 ji /,i

fourth, the external rectus (the

cut extremity of which is seen at/), turns it outwards. Besides
these, there is a remarkable muscle, h, the superior oblique,
which originates at the back of the orbit, comes forwards to
the front, where its tendon passes through a pulley, and then
turns backwards to be inserted into the sclerotic coat, at a point
considerably behind the pulley. The sixth muscle, g, termed
the inferior oblique, does not arise, like the rest, from the
back of the orbit, but from its lower border. The action of
the two oblique muscles (which act in antagonism the one to
the other) appears to be to rotate the eyeball upon its axis ;
as is done when the eyes are kept steadily fixed upon any

Fig. 209.— VERTICAL SECTION OF THE

MOVEMENTS OF THE EYE : — EYELIDS, ETC. 421

object, whilst the head is inclined to one side or the other. —
Of these muscles, the superior, inferior, and internal recti,
together with the inferior oblique, and also the elevator
muscle, i, of the upper eyelid, are supplied with motor nerves
by the third pair (§ 459) ; whilst the superior oblique has a
nerve to itself, the fourth; and the external rectus has another
nerve to itself, the sixth.

539. There is this very peculiar circumstance attending
the movements of the two eyes, — that although they are
harmonious, they are seldom symmetrical. Thus, when we
direct our eyes towards an object on one side of us, they move
harmoniously, that is, with a common purpose ; but their
movement is not symmetrical, for one globe is rolled inwards
by the internal rectus, whilst the other is rolled outwards by
the external rectus. These two different actions seem to be
instinctively connected, and to be guided by the sensations
which are received through the two eyes respectively (§ 478).
They are performed without any consciousness on our own
part, when, having fixed our gaze upon any object, we
rotate the head from side to side in the horizontal plane, the
eyeballs executing a corresponding rotatioin in the opposite
direction.

540. The eyebrows, eyelids, and eyelashes, serve in various
ways for the protection of the eyes. In Birds and Reptiles
there is a third eyelid, which is drawn across the eye by a
muscle that passes through a loop in it. This nictitating
membrane, as it is termed, is semi-transparent ; and it serves
to protect the eye from the too-powerful rays of light, without
destroying the power of vision. The upper and lower eyelids
of Mammals, and the nictitating membrane of Birds and
Reptiles, are very frequently drawn over the front of the
globe during the waking state, for the purpose of sweeping
from it dust and other accidental impurities which would
otherwise be injurious.

541. Beneath the upper eyelid, in the upper and outer
portion of the orbit, is situated the lachrymal gland (k, fig.
209) ; this is continually pouring-out a watery secretion over
the globe of the eye, which serves to wash from it these
impurities and to keep it moist. It is only, however, when
the quantity of this secretion is increased by mental emotion
or by irritation in the eye itself, so as to produce a flow of

LACHRYMAL APPARATUS I — SENSE OF VISION.

tears, that we become conscious of its existence. It is ordi-
narily drawn-off as fast as it is formed, by a curious apparatus
situated at the inner border of the eye. If the edges of the
lids be carefully examined, there will be seen upon each of
them, close to the inner corner of the eye, a minute spot
which is the entrance to a small canal termed the lachrymal
duct. The two ducts, one commencing at the corner of the
upper lid and the other at that of the lower, soon unite into one
canal, which swells into a sort of reservoir, the lachrymal sac,
that lies upon the side of the upper part of the nose ; and
from this reservoir a canal passes down through the bones of
the nose into its cavity. By this apparatus, the fluid which
is poured by the lachrymal gland over the exterior of the eye,
is drawn-off at the interior after washing its surface ; whence
it is carried into the nose, to be got rid of by the current of
air that passes through its cavity in- breathing. The edges
of the lids meet in such a manner, when they are closed, as
to form a sort of gutter or channel, along which the lachrymal
secretion flows from their outer to
their inner corner during sleep.

54:2. Having thus described the
structure of the Eye, and the general
actions of the parts by which it
is adapted to the performance of
its remarkable function, we shall
examine into the details of this
function ; in other words, into the
nature of vision.

543. The rays of light which
diverge from the several points of
any object, and fall upon the front
of the cornea, are refracted by its
convex surface whilst passing
through it to the eye, and are made
to converge slightly. They are
brought more closely together by
the crystalline lens, which they
reach after passing through the
pupil ; and its refracting influence,

together with that produced by the vitreous humor, is such
as to cause the rays that issued from each point to meet

FORMATION OF IMAGE ON THE RETINA. 423

in a focus on the retina. As every point is thus repre-
sented in its proper position relatively to others (except that
those which were above are now below, and vice versd), a
complete inverted image or picture of the object is formed
upon the retina. This is shown in fig. 210 ; where, for the
sake of convenience, three rays only are represented as issuing
from the centre and the two extremities of an object a b. These
rays cross each other at h, in the middle of the eye ; so that
those from a being brought to a focus at c, and those from
b at d, and all the other rays being refracted in the same
manner, a complete inverted picture of the object is formed at
the back of the eye.

544. That this is really the case, may not only be inferred,
but proved. If the eye of a Rabbit be removed from its
socket, and cleansed of the muscles, fat, &c., adherent to its
back part, and a candle be then brought in front of it, the
transparency of the sclerotic coat will allow the image of the
candle that is formed upon the retina to be distinctly seen.
Or, if we take the eye of a Sheep or an Ox, and after cleans-
ing it in the same manner, we cut out from the back of it- a
portion of the sclerotic and choroid coats, covering the part
of the retina thus left bare with a piece of tissue-paper (for
the purpose of keeping -in the vitreous humor, without
interrupting our view of the image), a distinct but inverted
miniature picture of all the objects in front of the eye will
be seen at its back. It is necessary in these experiments
that the eyes should be taken from animals recently killed ;
as the cornea and humors soon lose their transparency, and
the distinctness of the picture is consequently impaired.

545. The black pigment, which is situated immediately
behind the retina, — that is, in contact Avith its external
surface,— is destined to absorb the rays of light immediately
that they have passefl through the retina j so as to prevent
them from being reflected from one part of the interior of
the globe of the eye to another, which would cause a great
confusion and indistinctness in the picture. Hence it is that
in those individuals (both among Man and the lower animals)
in whose eyes this pigment is deficient, vision is extremely
imperfect. The eyes of such individuals (termed Albinos)
derive from the absence of pigment a very peculiar appear-
ance. The iris does not possess its ordinary colour ; but,

424

SPHERICAL ABERRATION.

owing to the large quantity of minute blood-vessels which it
contains, it presents a bright red hue. The choroid coat, seen
through the pupil, has exactly the same aspect ; so that the
pupil is not readily distinguished. During the day the vision
of these Albinos is very indistinct, and the glare of light is
painful to them ; and it is only when twilight comes-on, that
they can see clearly and without discomfort.

546. The eye is much more remarkable for its perfection
as an optical instrument, than we might be led to suppose
from the cursory view we have hitherto taken of its functions ;
for by the peculiarities of its construction certain faults and
defects are avoided, to which all ordinary optical instruments
are liable. One of these, termed spherical aberration, results
from the fact, that rays falling upon the central and the outer
portions of an ordinary convex lens, whose surfaces form part of
a sphere, are not brought to meet in one point, — the focus of
the central portion being rather more distant than that of the
outer part. This is shown in fig. 211, where L L is the lens,

Fig. 211.

R L, R L, are rays falling upon its circumference, and R' L',
R' L', are rays falling near its centre. The former set of rays
meet in /; whilst the latter pass-on to F, before they meet in
a focus. This may be shown by covering the central and
the outer portions of the lens, alternately, with some opaque
substance, which shall stop all the rays of light proceeding
through either. When the central portion is covered, a distinct
image will be formed at/ by the rays falling upon the outer
portion; and when the outer portion is covered, a distinct
image will be formed at F by the rays that have passed
through the central portion. But when the whole lens is
employed, no distinct image is formed anywhere; for if a
screen be held at /, it will receive, not only the rays which
are brought to a focus at that point, but also the rays which
are going-on to meet at F ; whilst, on the other hand, if the

SPHERICAL AND CHROMATIC ABERRATION. 425

screen be held jat" r, it will receive, not only the rays which
are brought to a focus there, but also those which, having
met at/, have crossed and passed-on to G and H.

547. Now this indistinctness is ordinarily got over in
practice, by employing only the central portion of the lens ;
so that only those rays which correspond to R' I/, R' i/, shall
pass- through it. This we observe in ordinary Microscopes
and Telescopes ; — a stop or perforated partition being inter-
posed behind the lenses, so as to allow the light to pass
through only a small aperture in their centre. By this plan
a great deal of light is cut off, so that the image is rendered
dark. The spherical aberration may be completely corrected,
however, by a certain adaptation of two or more lenses whose
surfaces have different curvatures ; the effect of which is, to
bring all the rays that have passed through every part of this
compound lens to the same focus. Now this very adjustment
is made in the eye, by the arrangement of the curvatures of
the cornea and of the two surfaces of the crystalline lens ;
and in the well-formed eye it is so perfect as to produce
complete distinctness in the image or picture thrown upon
the retina. The only case in which this would not occur, is
when an object is brought very near the eye ; for the rays
then diverge from each other at so wide an angle, that those
which fall upon different parts of the lens would not be all
brought to the same focus. This error is corrected by the
contraction of the pupil, which takes place involuntarily
when an object is brought very near the eye, and thus
cuts-off the rays that would otherwise render the picture
indistinct.

548. But there is another imperfection to which ordinary
optical instruments are liable, that is completely corrected in
the eye. If we look through a common Microscope, especially
when a high power is employed, by the light of a lamp or
candle, we see that the edges of the image are bordered by
coloured fringes, which very much impair its distinctness,
and prevent it from being seen in its true aspect. This is
the result of what is termed chromatic aberration; and it
results from the fact, that the rays of different colours, which
are all blended in ordinary colourless light, are refracted by
the same lens in different degrees, so as to be brought to a
focus at different points. Thus we will suppose that the lens

426 CHROMATIC ABERRATION I ACHROMATISM OP THE EYE.

L L (fig. 211) has been corrected for spherical aberration ; and
that R L, R L, are violet rays falling upon it, whilst R' L', R' L',
are red rays. The former are capable of being refracted in a
much higher degree than the latter ; so that they are brought
to a focus at/, whilst the others do not meet until F. Hence
if a screen be placed to receive the image at fy the picture
will be formed by the violet rays only, and will be surrounded
by red fringes occasioned by the red rays which are passing
on to their focus at P ; whilst, on the other hand, if the screen
be placed at F, the picture will be chiefly formed by the red
rays, and will be surrounded by violet fringes produced by
the violet rays, which, having met in /, have crossed and
passed-on to G and H. Now as from each point of almost
every object proceed rays in which the different colours are
blended, the refraction of an ordinary lens produces a sepa-
ration of these, and a consequent indistinctness and false
colouring in the picture. This is particularly the case with
regard to the rays that pass through the outer portion of the
lens ; for, as these are subject to greater change in their
direction than are those which pass through its centre, the
separation of the differently-coloured rays of which they are
composed is more considerable.

549. In practice, this error is got over, like the preceding,
by very much contracting the aperture of the lens ; so that
only the central rays, in which the colours are but little
separated, are allowed to pass. But it may be perfectly cor-
rected by combining lenses formed out of different materials,
which possess a different refracting power; the errors of
these being made to counterbalance one another. Such
lenses, which are termed achromatic, are now employed in
all superior Telescopes and Microscopes ; but no artificial
combination can surpass that which exists in the Eye, the
different density of whose humors is adjusted in such a
manner as completely to answer this purpose. The contrac-
tion of the pupil which takes place when we look at a very
near object, prevents the only imperfection which could occur ;
and thus the picture on the retina, in a healthy eye, is always
rendered free from false colours. It is said that the first idea
of uniting glasses of different kinds, so as to produce an
achromatic lens, was taken from the Eye ; and this is not at
all improbable. In this, as in many other instances, Nature

ADJUSTMENT OF THE EYE FOR VARYING DISTANCES. 427

has served as a guide to Art ; or, in other words, the Divine
Artificer has thus condescended to teach the human workman.

550. There is another wonderful arrangement in the
healthy Eye, which the optician can only imitate in his
instruments in a very bungling manner. It is that by which
the eye adapts itself to view objects at different distances
from it, with equal distinctness. If we look at a near object
with a Telescope, adjusting the instrument so as to see it dis-
tinctly, and then turn it towards a remote object, we shall
not see the latter with equal clearness until the instrument
has been again adjusted. If we then turn it back to the
nearer object, we shall find that the change in the adjustment
occasions the representation of it to be now indistinct ; and
in order to bring back the image to its former clearness, it is
requisite to re-adjust the instrument to its first condition.
This is a necessary consequence of the optical law, that the
distance of the image from the lens which forms it, varies
with that of the object, — being increased as the object is
brought nearer, and diminished as it recedes. If the Eye
were constructed in the same manner, we should not be able
to see distinctly, without the aid of artificial assistance, at any
other distance than that for which it is adjusted. Hence if a
perfect picture of an object situated at twelve inches' distance
from the eye, were formed upon the retina, we should not be
able to see it clearly when brought to the distance of six inches,
nor when removed to the distance of six feet ; because in the
first of these cases the rays would not be brought to a focus
upon the retina, but at a point behind it (if they were
allowed to pass on unchecked) ; whilst in the second, they
would be brought to a focus at a point nearer than the retina,
and would consequently begin to separate again before they
reach it.

551. But the healthy eye possesses a power of perfect
adjustment to the viewing of objects situated at different
distances ; and this without any effort or intention on our
parts, but, as it were, by an instinctive operation. That such
a change really takes place, we may readily convince ourselves,
by looking at a near and at a distant object placed in the
same line, — a pencil-case, for instance, held up at a few inches
from the eye, and a chimney half a mile off. We shall find
that no effort of attention will enable us to see them both

428 LONG AND SHORT SIGHT: SPECTACLES.

distinctly at the same time ; but that, on whichever of the
two objects we fix our eyes, we shall see it clearly, whilst the
other will become indistinct. Recent observations have con-
clusively shown that this adjustment is brought about by an
alteration in the curvature of the crystalline lens ; its con-
vexity being increased when a near object is looked at, so as
to act more powerfully in bringing its diverging rays into
convergence ; and being diminished when the gaze is turned
towards a distant object.

552. In advanced life, however, from the diminution in
the convexity of the cornea and in the refracting power of the
humors, the eye can no longer accommodate itself to near
objects ; their rays not being brought to a focus by the time
that they reach the retina. But as it is still able to see dis-
tant objects clearly, it is said to be long-sighted. By the use
of a convex glass, however, adapted to supply that additional
amount of refraction which is required, near objects may be
distinctly seen. — A contrary state of the eye not unfrequently
exists, in which the cornea is too convex, and the refracting
power of the humors is too high ; from which it happens
that the rays proceeding from distant objects are brought to a
focus too soon, so as to cross each other before they reach the
retina. But as such an eye can form a very distinct picture
of a near object, it is said to be near-sighted. This imper-
fection is remedied by interposing a concave lens between the
object and the eye, by which its refracting power is dimi-
nished to the necessary degree.

553. In the choice of spectacles or eye-glasses for these
purposes, particular care should be taken that they are not too
powerful ; since great mischief is frequently done to the eye, by
the employment of lenses of too great a curvature. A person
who in youth and middle age has enjoyed good sight, very
commonly finds it necessary to employ spectacles for assist-
ance in reading and writing, as his age advances towards fifty
years; and he should be very cautious, when first availing
himself of their assistance, to employ those of the longest
focus. As his age advances, it will be necessary to substitute
more powerful lenses for these ; but this should be done very
gradually; and in no instance should a glass be employed
that produces any apparent enlargement in the object, its
proper use being only to render the object distinct. The evil

SPECTACLES : SENSITIVE SPOT OF RETINA. 429

influence of using spectacles of too high a power, soon mani-
fests itself in the strained feeling which the eyes experience
for some time ; but this feeling at last subsides, in conse-
quence of the eye having adapted itself to the glasses, and
having thus undergone a change which it might otherwise
take years to produce. In this manner the eyes of a person
at sixty may be brought to the state which, under more
careful management, might have been deferred ten or fifteen
years longer. — Similar remarks apply to the use of concave
lenses by short-sighted persons. They should never be em-
ployed of a higher power than is requisite to see objects with
distinctness, when at a moderate distance ; and on no account
should any glasses be used that diminish their apparent size.
As age advances, the eyes of short-sighted persons usually
become more flattened, and are then able to adapt themselves
to objects at a variety of distances ; so that persons who have
been short-sighted when young, are not unfrequently able to
see distinctly at an advanced age, without the assistance of
convex glasses.

554. The power of receiving and transmitting visual im-
pressions is by no means uniform over the whole retina. In.
the whole field of vision which at any time lies before the
eye, we only see with perfect distinctness that part to which
its axis (namely, that diameter of the sphere which passes
through the centre of the pupil) is directed, and of which the
image, therefore, is formed upon "the yellow spot" (§ 535)
which lies at the posterior pole of the axis. Nevertheless we
have a sufficiently distinct perception of the remainder of the
field, to enable us to judge of the general relations of its
objects to each other and to those which we distinctly see :
thus, whilst reading or writing, we can only recognise letters
and words at any one moment within a spot which a sixpence
or a shilling would cover, but we may distinguish the lines
over the whole area of the page, and can plainly see the
position of the book or paper upon the desk or table, together
with the position of this in the apartment. In the act of
reading or writing, as in surveying the different parts of a
landscape or a picture, or in examining any solid object that
is brought under our notice, we direct the axis of the eye
successively to one point after another, until we have satisfied
ourselves that we have gained a distinct view of every part,

430 INSENSIBLE SPOT OF RETINA : — VISUAL ATTENTION.

as well as of its relations to the rest. It will be presently
shown that when we employ both eyes at once, their axes
meet in the object, and that the degree of their convergence
affords us a very important means of judgment as to their
distance (§§ 563, 564). — The part of the retinal surface which
lies over the entrance of the optic nerve, is remarkable for the
imperfection of its power of receiving impressions ; as is made
apparent by the following experiment. Let two black spots
be made upon a piece of paper, about four or five inches apart ;
then let the left eye be closed, and the right eye be strongly
fixed upon the left-hand spot ; if the paper be then moved
backwards and forwards, so as to change its distance from the
eye, a point will be found at which the right-hand spot dis-
appears, though it is clearly seen when the paper is brought
nearer or removed further ; and it can be shown that in this
position of the eye and the object, the rays from the right-
hand spot fall upon the point in question.

555. The degree in which the attention is directed to them,
has a great influence on the readiness with which very minute
or distant objects can be perceived; and there is a much
greater variation in this respect amongst different individuals,
than there is in regard to the absolute limits of vision. Many
persons can distinctly see such objects, when their situation
is exactly pointed-out to them, who cannot otherwise distin-
guish them. There must be few who have not experienced
this, in regard to a balloon or a faint star in a clear sky, or a
ship in the horizon ; we easily see them after they have been
pointed-out to us ; but if we withdraw our eyes for a few
minutes we search in vain for them, until they are again
pointed-out to us by some one who has been watching in the
interval. The faculty of rapidly descrying such objects much
depends upon the habit of using the eyes in search of them ;
thus a seaman will distinguish land, when to the landsman
there is no appearance more distinct than that of a faint cloud
on the horizon presenting no definite outline ; or he will
recognise the course and rig of a distant ship, which to the
landsman appears but as a white speck on the ocean : yet the
landsman, placed before a piece of delicate machinery, might
be able to astonish the seaman by distinguishing the forms
and movements of minute parts, which to the latter appear
only as a confused mass.

INTERPRETATION OF VISUAL SENSATIONS. 431

556. The picture formed upon the retina closely resembles
that which we see in a camera obscura. It represents the
outlines, colours, lights and shades, and relative positions, of
the objects before us ; but these do not necessarily convey
to the mind the knowledge of their real forms, characters, or
distances. It would appear, from the actions of the lower
animals, that many of them have the power of intuitively or
instinctively determining the latter from the former, from the
moment when they come into the world ; thus a Fly-catcher
just come out of its egg, has been seen to make a successful
stroke with its bill at an insect which was near it. If it were
not so, those animals which are thrown from the first upon their
own resources, would perish almost inevitably ; being starved
by want of food during the time required for them to learn
how to obtain it. But this is well known not to be the case
in regard to Man. The infant is educating his senses long
before any indications of mind present themselves. By the
combination, especially, of the sensations of sight and touch,
he is learning to judge of the shapes and surfaces of objects, as
they would be felt, by the appearance they present, — to form
an idea of their distance, by the mode in which his eyes are
directed towards them (§ 563), — and to estimate their size, by
combining the notions obtained through the picture on the
retina, with those he acquires by the movement of his hands
over their different parts. A simple illustration will show
how closely the ideas excited by the two sets of sensations
are blended in our minds. The idea of smoothness is one
which has reference to the touch, and yet it constantly occurs
to us on looking at a surface which reflects light in a particular
manner. On lie other hand, the idea of polish is essentially
visual, having reference to the reflection of light from the
surface of the object ; and yet it would occur to us from the
sensation conveyed through the touch, even in the dark.

557. That this combination is not formed intuitively in
Man, but is the result of experience, is particularly evident
from cases in which the sense of sight has been wanting
during the first years of life, and has then been obtained by
an operation. Several such cases are now on record. The
earliest, which still remains the most interesting, is one which
occurred to Cheselden, a celebrated surgeon at the beginning
of the last century. The youth (about twelve years of age), for

432 INTERPRETATION OP VISUAL SENSATIONS.

some time after tolerably distinct vision had been obtained,
saw everything flat as in a picture, the impression made upon
his retina being simply transferred to his mind ; and it was
some time before he acquired the power of judging, by his
sight, of the real forms, characters, and distances of objects
around him. Thus, among other interesting circumstances,
it is mentioned that he was well acquainted with a Dog and
a Cat by feeling, but could not remember their respective
characters when he saw them ; one day, when thus puzzled,
he took up the Cat in his arms and felt her attentively, at
the same time looking steadfastly at her, so as to associate
the two sets of ideas ; and then, setting her down, said, " So,
puss, I shall know you another time." A similar instance
has come under the Author's own knowledge ; but the subject
of it was scarcely old enough to present facts of so striking a
character. One curious circumstance, however, may be men-
tioned, as fully bearing out the view here given. The lad had
been accustomed to find his way readily about his father's
house by the use of his hands, and he continued to do the
same for some time after his sight was tolerably clear, being
evidently puzzled, rather than assisted, by the impressions
conveyed through his new sense ; but, when learning a new
locality, he employed his sight, and evidently perceived the
increase of facility which he derived from it. Hence, we can
have little hesitation in deciding upon the question which has
perplexed many able reasoners, whether a person born blind,
who was able by the sense of touch to distinguish a cube from
a sphere, would, on suddenly obtaining his sight, be able to
recognise these bodies by the latter sense. This question was
answered in the negative by the celebrated mental philosopher,
Locke, and with perfect justice.

558. We shall now inquire into the mode in which we
form our notions of the nature, sizes, distances, &c., of external
objects, from their pictures impressed upon our retina. The
first question is one on which a vast amount of discussion has
taken place, with very little satisfactory result. It is, — why
are the objects which we see, represented to our minds in
their true erect position, their images upon the retina being
inverted? Various replies to this question have been pro-
posed at different times; and, amongst others, it has been
actually assumed that the Infant really does see objects

SENSE OF DIRECTION : — SINGLE VISION. 433

inverted, and that this error is only corrected by experience.
The cases alluded-to in the last paragraph, however, satisfac-
torily prove this assumption to be incorrect ; since, although
the individuals saw everything upon the same plane, as in a
picture, the representation was erect from the first. The fact
now appears certainly to be, that we have an intuitive sense
of direction, which guides us in our appreciation of the actual
situations of objects and parts of objects ; so that, when a visual
impression is made upon any part of the retina, we see the
point from which the rays proceed, in the direction of a line
drawn from the affected spot of the retina through the common
centre (fig. 210, h) through which all the rays pass, this line
serving as a true guide to the actual place of the object.

559. The same may be said of the cause of single vision,
that is, of our seeing but one object, although its picture is
double, being formed upon both retinse. In animals which,
like Man, look straight forwards, the field of vision of the
two eyes is nearly the same ; so that most of the objects seen
with one eye will be seen with the other also. The objects
at the right and left sides of the field of vision, however, are
seen with the right and left eyes singly ; yet we perceive no
difference in the aspect of these from that of the objects
towards which both our eyes are directed. It is evident,
then, that the pictures formed on the two retinae are blended,
as it were, by the mind, into a single perception. This seems
to be, in part at least, the effect of habit. "When the images
do not fall upon parts of the two retinas which are accustomed
to act together, double vision is the result. Thus if, when
looking steadily at an object, we press one of the eyeballs
sideways with the finger, the object is seen double. In the
same manner, if an affection of the nerves or muscles of one
eye (as happens temporarily in intoxication) prevent it from
being directed to the same point with its fellow, double vision
of all objects is the result. This, when it does not soon pass
away, is a not unfrequent symptom of serious disorder within
the brain. If it continue long enough, however, the indivi-
dual becomes accustomed to the double images, or rather
ceases to perceive that they are double, probably because the
mind becomes habituated to receive them, or rather perceives
only one of the impressions on the two parts of the retinae
which now act together. For if, after the double vision has

F F

434 COMBINATION OP RETINAL PICTURES : — STEREOSCOPE.

passed away, the conformity of the two eyes be restored (as
by the operation for the cure of squinting), there is double
vision for some little time, although the two parts of the
retinae, which originally acted together, are now brought to
do so again.

560. That the combination of the two images must be
effected by an operation of the mind, is evident from another
circumstance. It is easy to show that no near object is seen
by the two eyes in exactly the same manner. Thus, let the
reader hold up a thin book, in such a manner that its back
shall be exactly in front of his nose, and at a moderate
distance from it ; he will observe, by closing first one eye and
then the other, that his view of it is very different, according
to the eye. with which he sees it. With the right eye he will
see its back and right side, the latter very much foreshortened,
but none of the left side ; whilst with the left eye he will see
its back and left side, the latter also foreshortened, but none
of the right side. Hence if he were to draw a perspective
view of the object as seen by each eye, the two delineations
would be very different. But on looking at the object with
the two eyes conjointly, there is no confusion between these
pictures, nor does the mind dwell upon either of them singly ;
the union of the two gives us the idea of a solid projecting
body — such an idea as we could have only acquired otherwise
by the exercise of the sense of touch.

561. That this is really the case, has been proved by ex-
periments with the very ingenious instrument (invented by
Professor Wheatstone) known as the Stereoscope. In its
original form this consisted of two plane mirrors, inclined with
their backs to one another at an angle of 90°, the point of
meeting being opposite to the middle of the forehead. Two
drawings representing the different perspective views of any
solid object, as seen by the two eyes, being placed before
these mirrors, in such a manner that their images are re-
flected to the two eyes respectively, and are made to fall
on corresponding parts of the two retinae as the two images
formed by the solid object itself would have done, so
that their apparent places are the same, — the mind perceives
not one or other of the single representations of the object,
nor a confused union of the two, but a body projecting in
relief, the exact counterpart of that from which the drawings

MENTAL APPBECIATION OP PROJECTION. 435

were made. In the small portable instrument which has of
late become so extensively popular, the like effect is produced
by a particular arrangement of convex lenses, devised by
Sir David Brewster, which also has the advantage of magnify-
ing the pictures.

562. It is, then, by the combination which is effected
through a mental process, based on the consentaneous percep-
tion of the two dissimilar pictures formed on the two retinas,
that these are made to blend into one representation, which
gives the idea of projection. When we look at a distant
object, our judgment is based on less positive data, the two
pictures being then almost precisely the same ; and hence it
is impossible (without moving the head) to distinguish with
certainty between a well-painted picture, in which the pro-
portions, lights and shades, &c. are well preserved, and the
objects it is intended to represent, if we are prevented from
knowing that it is a picture. Some admirable illusions of
this kind have been effected in the Diorama. But a slight
movement of the head suffices to dispel the doubt; since
by this movement a great change would be effected in the
perspective view of a solid object, — a little of the side of a
projecting buttress or column being seen, for instance, where
only the front was perceived before, — whilst the image formed
by a picture is but slightly affected. The same indecision is
experienced when we look with a single eye at certain near
objects, which the mind can apprehend either as projecting or
as receding, with equal, or nearly equal, readiness ; such, for
example, as a metal plate stamped-out into a figure which
stands-forth in relief on one side and is counter-sunk on the
other. And the idea of the object which is the reverse of
the reality may present itself most forcibly, if it should
happen to be the one most familiar to the mind ; thus if we
look with one eye at the interior of a mask that has been
coloured to the semblance of a human face, it will seem to
rise into the likeness of the exterior ; whilst the actual pro-
jecting surface of the mask will never seem to exhibit the
concavity of the interior.1 In looking with a single eye,

1 In making these and similar experiments, it is necessary to take
care that the whole of the projecting or receding surface is equally
illuminated; since the presence of any shadow proceeding from a
known source of light, destroys the illusion, by forcing the mind to
recognise the real figure of the object.

F P 2

436 ESTIMATION OF DISTANCE.

moreover, we are deprived of that power of measuring the
relative distances of near objects, which we derive from the
conjoint use of both eyes (§ 563) ; and thus a well-painted
picture, still more a photograph, may so strongly suggest the
idea of projection, in virtue of its exact perspective and its
contrast of light and shadow, that it is difficult to believe it
to be a flat surface, even though it be within but arm's
length of the eye.

563. Our idea of the distance of objects is evidently
acquired by experience. An infant, when a bright object
is held before its eyes, attempts to grasp it with its little
hands, but obviously has no certain idea of its situation;
and the same is observed in persons who have but recently
acquired sight. Here, then, the impressions made upon the
eyes have to be corrected by those received through the touch,
before the power of judging of distances is acquired; but
when it has been once acquired, we can accurately estimate
the relative distances of near objects without using our hands.
This we do chiefly by the interpretation we have learned to
make, of the sensations which are occasioned in the muscles
of the eyes by their various actions. When we fix both our
eyes upon a distant object, their axes are nearly parallel to
each other ; but when we direct them to a near object, the
axes of the eyes meet in the point at which we are looking.
This is very easily seen by watching the eyes of another
person, when fixed upon an object, first held at arm's length,
and then brought nearer and nearer to the middle point
between the eyes ; the two cornese are at first directed nearly
straightforwards ; but they gradually turn inwards as the
object is brought nearer, and at last a very decided inward
squint is produced, which disappears as soon as the object is
removed. Thus, for objects within a moderate distance, the
degree of convergence of the axes of the eyes, and the mus-
cular sensations thereby produced, afford us sufficient means
of judgment.

564. We perceive this in another, as well as in ourselves ;
for by observing his eyes, we can judge, not only of the
direction, but of the distance, of the object he is looking at.
Thus when a person A sees a friend B looking towards
him, he can at once tell, by the appearance of B's eyes,
whether he is looking at Mm, or at an object nearer or more

ESTIMATION OP DISTANCE AND SIZE. 437

remote ; or whether, being in a contemplative mood, his eyes
are fixed upon no definite object, but are looking into space.
In the latter case, as in the case of blind persons in whose
eyes there is no other indication of loss of sight, the peculiar
vacant expression is due to the want of any convergence
between the axes of the eyes, such as would indicate that
they are fixed upon an object. The assistance which the
joint use of both eyes affords in the estimation of distance, is
evident from the fact, that, if we close one eye, we are unable
to execute with certainty many actions which require a
precise appreciation of the distance of near objects, — such as
threading a needle, or snuffing a candle. Instances are not
unfrequent in which persons have first become aware, by
experiencing this difficulty, that they had lost the sight of'
one of their eyes.

565. In regard to distant objects, our judgment is chiefly
founded upon their apparent size, if their actual size be
known to us, and also upon the extent of ground which we
see to intervene between ourselves and the object. But if
we do not know their actual size, and are so situated that we
cannot estimate the intervening space, we principally form
our judgment from the greater or less distinctness of their
colour and outline. Hence, this estimate is liable to be very
much affected by varying states of the atmosphere ; a distant
ridge of hills, for example, sometimes appearing to be more
remote, at other times to be comparatively near, according as
the air is hazy or peculiarly clear.

566. Our notion of the size of an object is closely con-
nected with that of its distance. It is founded on the
dimensions of the picture formed by the object upon the
retina ; but it is corrected by the known or supposed distance
of the object itself. Thus, I hold up a book at a certain
distance from my eye, and it covers the whole of the opposite
window ; the apparent size of both pictures, therefore, is just
the same ; but knowing that the book is much nearer than
the window, I infer that it is much smaller. When we know
their respective distances, the estimate of their real sizes is
very easy : but this is not the case when we only guess-at
their distances. Hence our estimation of the size of objects
even moderately distant, is much influenced by states of the
atmosphere. Thus, if we walk across a common in a fog,

438 DURATION OF LUMINOUS IMPRESSIONS.

a child approaching us appears to have the size of a man, and
a man seems like a giant ; for the indistinctness of the outline
excites in the mind the idea of distance ; and the same pic-
ture, if supposed to be that of a more remote object, will give
rise to the idea of greater size. The want of innate power in
Man to form a true conception of either size or distance, is
well shown by the effect produced on the mind unprepared
for such illusions, by a skilfully-painted picture, the view of
which is so contrived that its distance from the eye cannot be
estimated in the ordinary manner ; the objects it represents
being invested by the mind with their real sizes and respective
distances, as if their real images were formed upon the
retina. This illusion has been extremely complete in some
of those who have seen the panoramic view of London in the
Colosseum.

567. When a number of luminous impressions are made
upon the retina at short intervals, they become blended into
one, — the intervals being undistinguishable. Thus, when we
rapidly move the end of a lighted stick in a straight line or
circle, the impression produced is that of a band or ring of
light ; for the impression made by the light, as it passes each
point, remains for some time subsequently. If the stick be
whirled round with sufficient rapidity for it to reach any
point a second time, before the impression made by its pre-
vious passage has departed, an unbroken circle of light is
produced. By experiments made in this manner, we may
determine the longest interval during which visual im-
pressions remain on the sensorium ; for if we find that a hot
coal, whirling round at the rate of six times in a second,
produces a continued circle of light, but that the circle is
broken when it turns round only five times in a second, we
know that the length of the impression is l-6th of a second.
By experiments of this kind, it has been found that the
duration varies in different individuals, and in the same
individual at different times, from l-4th to 1-1 Oth of a
second. On this principle several very ingenious toys have
been constructed, in which two or more images are com-
bined, by the rapid revolution of a wheel on which they are
painted.

568. Some persons, whose sight is perfectly good for forms,
distances, &c., are unable to discriminate colours. This is

COMPLEMENTARY COLOURS I — COLOURED SHADOWS. 439

particularly noticed in regard to the complementary^ colours,
especially red and green ; so that such persons are not able to
distinguish ripe cherries amongst the leaves of the tree, except
by their form.

569. When the retina has been exposed for some time to a
strong impression of some particular kind, it seems less sus-
ceptible of feebler impressions of the same kind ; just as the
ear, when it has been exposed to a series of very loud sounds
(as the discharge of cannon in a naval engagement), is for
some time deaf to fainter ones. Hence several curious visual
phenomena result. If we look at any brightly luminous
object, and then turn our eyes on a sheet of white paper, we
shall perceive a dark spot upon it ; the portion of the retina
which had been affected by the bright image, not being affected
by the fainter rays reflected by that part of the paper. If the
eye has received a strong impression from a coloured object,
the spot afterwards seen exhibits the complementary colour ;
thus, if the eye be fixed for any length of time upon a bright
red spot on a white ground, and then be suddenly turned so
as to rest upon the white surface, we see a green image of the
spot. The same explanation applies to the curious pheno-
menon of coloured shadows. It may be not unfrequently
observed at sunset, that, when the light of the sun acquires
a bright orange colour from the hue of the clouds through
which it passes, the shadows cast by it have a blue tint.
Again, in a room with red curtains, the light which passes
through these produces green shadows. In both instances, a
strong impression of one colour is made upon the general
surface of the retina ; and at any particular spots from which
the coloured light is excluded, that particular hue is not per-
ceived in the faint light that remains, and its complement
only is visible. The correctness of this explanation is proved
by the fact, that, if the shadow be viewed through a tube, in
such a manner that the coloured ground is excluded, it seems
like an ordinary shadow.

1 White, or colourless light, is spoken of as composed of three primary
colours, red, blue, and yellow. By the complementary colour is meant
that which would be required to make white light, when mixed with
the original. Thus, red is the complement of green (which is composed
of yellow and blue) ; blue is the complement of orange (red and
yellow); yellow of purple (red and blue); and vice versd, in all
instances.

440 COMPLEMENTARY COLOURS. — DIRECTION OF MOVEMENTS.

570. Upon these properties of the eye are founded the laws
of harmonious colouring ; a full knowledge of which should
be possessed by artists of every kind who are concerned with
contrasts of colour, whether in pictures, architectural decora-
tions, or even in dress. All complementary colours have an
agreeable effect when judiciously disposed in combination ;
and all bright colours which are not complementary have
a disagreeable effect, if they are predominant : this is espe-
cially the case in regard to the simple colours (red, blue, and
yellow), strong combinations of any two of which, without
any colour that is complementary to either of them, are ex-
tremely offensive. Painters who are ignorant of these laws,
introduce a large quantity of dull grey into their pictures, in
order to diminish the glaring effects which they would other-
wise produce ; but this benefit is obtained by a sacrifice of the
vividness and force which may be secured in combination
with the richest harmony, by proper attention to physiological
principles.

571. The Eye is endowed with common sensibility (§ 487)
.by the fifth pair of nerves ; and when this is paralysed, all
parts of it are completely insensible to the touch, although
the power of vision may remain unimpaired. It seldom pre-

. serves its healthy condition in this state, however ; for the
. lachrymal and mucous secretions which protect its surface, are
no longer formed as they should be ; and inflammation, often
, terminating in the destruction of the eye, is the result.

572. The visual sensations obtained through the Eye have
numerous and varied purposes among the lower animals.
That they chiefly serve to direct their movements, is evident
jrom observation of these movements ; and from the fact,
.that, when the eyes are covered or destroyed, most animals
make little attempt at determinate motions, though they fre-
quently exhibit a great deal of restlessness. There are a few
Vertebrata, however, which do not possess perfectly-formed
eyes, and which are consequently guided in their movements
by other senses. This is the case with the Mole, which
spends its whole life in burrowing beneath the ground ; and
also with the Proteus, and some others of the lower Am-
phibia, which inhabit the dark recesses of subterranean lakes
and channels.

£73. In the Articulated series of animals, we meet with

COMPOUND EYES OP ARTICULATA. 441

eyes of a kind entirely different from those which, have been
previously described. In most Insects we notice a large black
or dark-brown hemispherical body, situated on either side of
the head (fig. 212) ; and in Crabs, Lobsters, &c., we find
spherical bodies of similar appearance mounted on short
footstalks, which are capable of some degree of motion.
When these are examined with the microscope, their surface

Fig. 212. — HEAD AND EYES OF THE BEE, SHOWING THE DIVISION INTO FACETS.
a, a, antennae; A, facets enlarged ; B, the same with hairs growing between them.

is seen to be divided into a vast multitude of hexagonal (six-
sided) facets. In a species of Beetle (Mordella) upwards of
25,000 of these have been counted ; in a Butterfly, above
17,000; in a Dragon-fly, more than 12,500; and in the
common House-fly, 4,000. Every one of these facets may be
regarded as the front of a distinct eye, which, however, instead
of being globular, is conical in its form ; the front being the
base of the cone, and the apex or point being directed towards
the optic nerve, which swells-out into a bulbous expansion
that fills a large part of the interior of the hemisphere. Each
individual eye consists, therefore, of its facet, which (being-
convex on both surfaces) acts as a lens ; of the transparent
cone behind this, which may be compared to the vitreous
humour; and of the fibre which passes from the bulbous
expansion of the optic nerve to the point of this cone. The
several fibres are separated from one another by a considerable
quantity of black pigment, which also fills up the spaces
between the cones ; and it is to this that the black appearance
of the whole compound eye is due.

574. We must thus regard each of the cones, which, united
together, constitute the hemispherical or globular mass, in
the light of a distinct eye ; but the entire aggregate seems to

442 COMPOUND EYES OF ARTICULATA.

correspond in function with the single eye of the Vertebrate
animal. For no rays except those which correspond in direc-
tion with the axis of each cone, can reach the fibre of the
optic nerve at its apex ; all others being stopped by the layer
of black pigment which surrounds it. Hence it is evident
that each separate eye must have an extremely limited range
of vision, being adapted to receive but a very small collection
of rays proceeding from a single point in any object ; and as
these eyes are usually immoveable, animals with but a small
number of them would be very insufficiently informed of the
position of external things. But by the vast multiplication in
the number of the eyes, and the direction of their axes to
every point in the hemisphere, their defects are compensated j
a separate eye being provided, as it were, for every point to
be viewed. And it is quite certain, from observation of the
movements of Insects, that their vision must be very perfect
and acute.1

575. Although these Compound Eyes exist in all Insects
and in most Crustaceans, Spiders and Centipedes, they are in
general not the only organs of vision which these animals
possess. Most of them are also furnished with several simple
eyes, analogous in their structure to those of higher animals,
but les? complex and perfect in their organization ; these,
which are for the most part disposed on the back of the head,
are largest in Spiders. The larvas of some Insects possess the
simple eyes without the compound; the latter being only
developed at the time of the last metamorphosis. The simple
eyes of Insects do not appear to be nearly so efficient as
instruments of vision, as are their compound ones ; for when
the latter are covered, the animals seem almost as perplexed
as if they were perfectly blinded. Simple eyes, closely re-
sembling those of Insects in structure, are found in most of

1 It is commonly believed that each of these compound eyes pro-
duces its own image of the same external object, as do our two eyes;
but from the description here given of their separate directions when
united, it is evident that in no two of them can an image of the same
object be formed at the same time. The membrane formed of all the
lens-like cornese united together, when separated from the other parts
of the eye, and flattened-out, has the properties of a multiplying-glass,
each lens forming a distinct image of the same object ; but this is not
the case when they are arranged in their natural position, because no
two of them have the same direction.

EYES OP MOLLUSKS, ETC. ANIMAL MOTION. 443

the Mollusca which possess a head — namely, in the Gastero-
pods, Pteropods, and Cephalopods ; those of the last class
present an evident approach to the eyes of Fishes, in the
greater completeness of their structure, and in their adapta-
tion for distinct vision. In many of the lower Mollusca, as in
the Eotifera and several Annelida, and also at the end of the
arms of the Star-fish, red spots may be seen, which appear to
be rudiments of eyes ; but no distinct organs of vision can be
seen in the Zoophytes and lowest Mollusca ; although many
of them appear very sensible to the action of light.

CHAPTER XII.

ANIMAL MOTION, AND ITS INSTRUMENTS.

576. THE different modifications of the faculty of Sensation
which have been described in the preceding chapter, enable
Man and other Animals to become acquainted with what is
going-on around them. But their connexion with the external
world is not confined to this faculty ; for if they possessed it
alone, they would be nearly as passive as are Plants, — expe-
riencing, it is true, pain and pleasure from their sensations,
but not having the power of avoiding the one or of procuring
the other. They are endowed, however, with another faculty,
that of spontaneous movement ; which serves the double
purpose of enabling them to act upon the inanimate world
around them, and of communicating to each other their feel-
ings and ideas. Thus, if we find ourselves scorched by a flame,
we either withdraw our bodies from it, under the direction of
the instinct which leads us to avoid suffering, or we set about
to extinguish the fire by an act of the will, founded upon our
rational knowledge of its injurious tendency. The Plant,
even if it had sensation (which some naturalists have sup-
posed), could do neither of these things. Again, it is entirely
by the movements concerned in speech, by those giving
expression to the countenance, and by the gestures of the
body, that we convey to beings like ourselves a knowledge of
what is passing in our own minds ; of this power we know
that plants are entirely destitute, and it is possessed in a very
limited degree by the lower Animals.

444 MOVEMENTS NOT DEPENDENT ON MENTAL DIRECTION.

Contractile Tissues : — Muscular Contractility.

577. When we examine into the nature of the movements
of the lower tribes of Animals, however, we find that they
bear a much closer analogy to those of Plants, than they do
to those which are the expressions of the self-determining
power of higher Animals. Among the simplest Protozoa, it
seems as if the change of form of the single cell of which each
individual is composed, were the sole means of movement
which it possesses (§ 129) ; and this change of form often
appears rather to be due to the nutrient actions taking place
within the cell, than to occur in respondence to any im-
pression made upon its exterior. In such movements it is
impossible to suppose with any probability that consciousness
can participate. So, again, among Infusory Animalcules, all
the movements of the body are effected by the action of cilia
(§ 133), which we know in our own experience to be entirely
removed from any mental direction, and which we see to be
exhibited under a no less remarkable aspect by the " zoo-
spores " or motile buds of the Algae (BOTANY, § 767).

578. Although the movements of the Hydra (§ 121) and
other Zoophytes may appear to indicate the existence of a self-
determining power, yet it is very doubtful whether such an
endowment is really possessed by these animals. For their
contractile tissue is of the simplest possible character, resem-
bling that which is found in the early state of newly-forming
parts of higher Animals ; and when the movements executed
by it are carefully compared with our own, it becomes obvious
that they are analogous, not to those of the Human body and
limbs generally, but to those of the muscular coat of the
alimentary canal and of the muscles concerned in deglutition,
which not only take place without any voluntary determina-
tion on our parts, but may even be performed without our
consciousness. In like manner, the rhythmical movements of
the umbrella-like discs of the Medusce (§ 120), by which many
species of them are propelled through the water, bear a much
closer analogy to the rhythmical movements of the heart of
higher Animals, than they do to any other of their actions ;
and are probably performed, like these, without any exercise
of will, and even without the guidance of consciousness.

579. In proportion, however, as we ascend the scale, we

MOVEMENTS PRODUCED BY MUSCULAR CONTRACTION. 445

find a peculiar tissue, the Muscular (§§ 55 — 59), distin-
guished from the rest ; in which the general contractility of
the body becomes, as it were, concentrated. In proportion to
the development and complexity of this muscular apparatus,
it supersedes, the more feeble contractility diffused through
the fabric of the lower tribes. It now, moreover, becomes in
great degree subjected to the Nervous System ; by which all
those parts of it which are not connected with the functions
of Organic life merely, are rendered subservient to the Will,
:md thus become its instruments in determining the place
and the various actions of the body. Still we find that the
ordinary actions of those portions of the muscular apparatus
which are most immediately subservient to the functions of
organic life, are essentially independent of nervous influence,
and are very little under its control ; as we see in the case of
the alternate contraction and relaxation of the heart, and the
peristaltic movements of the alimentary canal.

580. The peculiar contractility of muscular fibre may be
called into action by various means. As in certain vegetable
tissues (VEGET. PHYS. § 390), contraction may be excited by
a mechanical stimulus directly applied to the muscle itself
Thus, if the heart of an animal recently killed be touched
with a pointed instrument, it will contract and then dilate, as
if performing its ordinary action ; and this may be repeated
several times. In the same manner, if the walls of the intes-
tinal canal be pricked or pinched, they will re-commence and
continue for a short time their peristaltic movement. And if
any part of an ordinary muscle be irritated in the same
manner, that particular bundle will contract, but the rest will
not be affected. Now these actions are analogous to those
performed by the Sensitive Plant, Venus' s Fly-trap, and many
other plants, some part of whose tissues contracts in like
manner when an irritation is applied to it, causing — it may
be — extensive and important motions. It appears to be in
this manner that the contractions of the heart, and of the
alimentary tube from the stomach to the rectum, are ordinarily
excited in the living body.

581. But there must be some other cause for the con-
tinuance of the rhythmical movements of the heart, as well as
of some other organs; for the heart of many cold-blooded
animals will continue to contract and dilate many hours after

446 STIMULANTS TO CONTRACTION OF MUSCLES.

it has been removed from the body, when it neither receives
nor propels blood.1 In the same manner, the peristaltic
motions of the intestine continue to propel its contents for
some time after the general death of the body; and may even
take place when the whole tube has been removed from it,
and has been completely emptied. There is strong reason, in
fact, for attributing to certain kinds of muscular tissue an
inherent motiltiy, in virtue of which it moves of itself without
any external stimulation; the changes which are concerned
in its nutrition developing a force which must manifest itself
in action; just as a Leyden jar, which is receiving a con-
tinuous charge from an electrical-machine, discharges itself
whenever its electricity attains a certain tension.

582. But the muscles of the trunk, limbs, &c., are not
called into action in this manner; for, as just now stated,
a stimulus applied to any one part of these does not excite
contraction in the whole muscle (as it does in the case of the
heart), but only in the individual bundle of fibres irritated.
These muscles are all of them supplied with nerves (§ 63)
from the Cerebro-spinal system, or the nervous centres that
correspond to them in Invertebrated animals ; and it is only
by a stimulus transmitted to them along these trunks, that all
the bundles of which the muscle is composed can be called
into action at once.

583. When the trunk of a nerve supplying a muscle is
irritated by pricking, pinching, &c., in the body of a living
animal, or in one recently dead, the whole muscle is thrown
into contraction; and this contraction is peculiarly strong
when a current of Electricity is transmitted along the nerve.
In cold-blooded animals, whose muscular fibre retains its vital
properties for a much longer time after death than that of
warm-blooded, this contraction may be excited by a very feeble
current of electricity, even in a limb which has been separated
from the body for twenty-four hours or more ; and it was
owing, in fact, to this circumstance, that the peculiar form of
electricity which is now termed Galvanic or Voltaic was dis-
covered. The wife of Galvani, who was Professor of Medicine

1 There is an instance on record, in which the heart of a sturgeon,
that had been removed from the body and had been inflated with air,
continued to beat until the auricle had become so dry as to rustle
during its movements.

INFLUENCE OF ELECTRICITY ON MUSCLES. 447

at Bologna, being about to prepare some soup from frogs, and
having taken off their skins, laid them on a table in his study,
near the conductor of an electrical machine which had been
recently charged ; and she was much surprised, upon touching
them with the scalpel (which must have received a spark
from the machine), to observe the muscles of the frog strongly
convulsed. Her husband, on being informed of the circum-
stance, repeated the experiment ; and found that the muscles
were called into action by electricity communicated through
the metallic substance with which the limb was touched.

584. The experiment was repeated in various ways by
Volta, who was Professor of Natural Philosophy at Pavia;
and he found that the effects were much stronger when the
connecting medium through which the electricity was sent,
consisted of two metals instead of one ; and from this circum-
stance he was led to the discovery that electricity is produced
by the contact of two different metals — a discovery which has
since been so fruitful in most important results. The follow-
ing simple experiment puts this in a striking point of view.
If the skin of the legs of a Frog recently killed be removed,
and the body be cut across, above the origin of the great
(sciatic) nerve going to the legs, — if the spine and nerves be
then enveloped in tin-foil, and the legs be laid upon a plate
of silver or copper, — convulsive movements in the muscles
will be excited every time that the metals are made to touch
each other so as to complete the electric circuit.

585. Similar experiments have been tried with the Voltaic
battery, upon the dead bodies of criminals recently executed.
If one wire be placed upon the muscles which it is desired to
call into action, and the other upon the part of the spine from
which the nerves proceed, movements of every kind may be
produced. No agent more effectually imitates the natural
action of the nerves, in exciting the contractility of muscles,
than Electricity thus transmitted along their trunks ; and we
have already seen (§ 489) that Electricity, transmitted along
the sensory nerves, excites the peculiar changes in the brain
by which sensations are produced. Hence it has been, sup-
posed by some philosophers, that electricity is the. real force
by which the nerves act upon the muscles ; more especially
since it is certain that, in those animals which generate large
quantities of electricity, the nerves have a great share in this

448 INFLUENCE OF NERVES ON MUSCLES.

peculiar operation (§ 423). But there are many objections to
such a view ; and it appears more correct to regard Electricity
and Nerve-force as correlated, — that is, as each capable, under
certain conditions, of exciting an equivalent measure of the
other, — than to consider them as identical.

586. The power, whatever be its nature, by which the
Nerves act upon the Muscles in the living body, originates
in the central organs, or ganglionic masses, of the nervous
system ; and is propagated from these, through the nervous
fibres, to the muscles, in a mode precisely analogous to that
in which the electric power, called-forth by the action of an
electrical machine or galvanic battery, is transmitted to any
distance through conducting wires. If the conductor be
divided, no action at the centre, however powerful, can pro-
duce any change at its extremities ; and in this manner, by
division of the nervous trunk, the muscle supplied by it is
palsied. The muscle itself does not thereby lose its contrac-
tility; for it may still be made to contract by a stimulus
transmitted through the part of the trunk that remains
attached to it, — as, for instance, by pricking or pinching the
cut extremity, or by passing an electric current along it ; but
it is completely withdrawn from the dominion of the nervous
centres under which it previously was ; and cannot be called
into action either by the will, by an emotion, or by a reflex
impulse. The part of the trunk in connexion with it soon
loses its power of conveying irritations ; and the muscle itself
being thrown into disuse, in time loses its contractility.

587. From this last fact it has been supposed that the
contractility of muscular fibre depends upon its connexion
with the nervous system, and is not an endowment peculiar
to itself. But this idea is disproved by a number of circum-
stances. Thus the contractility of the heart and intestinal
tube is exhibited, long after these parts have been separated
from their nerves. The contractility of other muscles may be
exhausted by repeated excitement, so that even the stimulus
of galvanism will not produce movement in them ; and yet it
may be recovered after the nervous trunks have been divided.
And it has been ascertained that if the muscles be frequently
exercised, as by the application of galvanism once or twice a
day, they will retain their contractility for any length of time.
This exercise is further found to have the effect of preventing

EFFECTS OF WITHDRAWAL OF NERVOUS POWER. 449

the wasting-away of the muscles which otherwise takes place ;
and thus we see that the preservation of this peculiar property
is dependent upon the due nutrition of the muscle, whilst the
loss of the property results from its want of nutrition, — as we
find to be the case in regard to other tissues. Further, the
activity of the nutrition of muscles depends in great part
upon the use that is made of them ; and thus we find that
any set of muscles in continual employment undergoes a great
increase in size and vigour ; whilst those that are disused,
even though their nervous connexions remain entire, lose
their firmness and diminish in bulk, until, if the inaction be
continued long enough, almost all trace of proper muscular
substance disappears, and the contractility of the part is lost.

588. But a muscle may be palsied by some change taking
place in the central organs, which shall prevent the nervous
influence from being excited there. Thus by an effusion of
blood in a certain part of the brain, the arm, leg, or the whole
of one side may be paralysed to the influence of the will.
But the muscles which are thus withdrawn from the power of
the will, may sometimes be moved by an emotional or instinctive
impulse, or by reflex action ; their connexion with the parts of
the nervous centres, in which these actions respectively origi-
nate, remaining unimpaired (Chap. x.). Thus a completely
paralytic arm has been seen to be violently shaken, when the
emotions of the patient were strongly excited by the approach
of a friend. The muscles of the shoulder, in a case of com-
plete paralysis of one side, were called into contraction in the
reflex movement of yawning (§ 341). And the muscles of
the legs, when their communication with the brain, — and
consequently the control of the will over them, — has been
completely cut off", have been made to act energetically when
the feet were tickled, although the patient was not conscious
either of the irritation or of the motion. When the muscles
are thus aroused to occasional activity, their nutrition is not
so much impaired, and their contractility does not depart
nearly as completely as when they are thrown into entire
disuse by division of their nerves.

589. Muscles are commonly divided into voluntary and
involuntary, according as they act in obedience to the will,
or are not under its dominion. But this is not a correct
division ; since, whilst nearly all the muscles of the body are

G G

450 VOLUNTARY AND INVOLUNTARY ACTIONS OF MUSCLES.

more or less under the control of the will, they may all at
times have an involuntary action. The heart and the muscular
coat of the alimentary canal, with the muscles concerned in
swallowing and in one or two other actions of a similar
character, are the only muscles which the will cannot either
set in action, or control when in action. There are several
muscles whose usual movements are of a reflex and therefore
involuntary character, and are yet capable of being, to a
certain extent, controlled and governed by the will. Such
are the movements of respiration ; which will continue to
take place after the brain has been removed, and which go on
regularly during the profoundest sleep and the most complete
withdrawal of the attention from them. In the Invertebrated
animals these motions are probably not influenced by the
will ; but in the air-breathing Vertebrata they are placed in a
certain degree under the dominion of the will, in order that
they may be made to contribute to the production of the vocal
actions of speaking, singing, &c., which are restricted to these
classes. We can hold the breath for a certain time by a
voluntary effort, or we can expel or draw it in more quickly
tli an usual ; but no voluntary effort can cause the breath to
be held for more than a few moments; for the uneasiness
which is then felt, and which is continually increasing, causes
an involuntary action of the muscles, by which action it is
relieved.

590. But again, there are other muscles, whose ordinary
actions are voluntary ; but which are occasionally made to
act independently of the will, or even against its direction.
Such are those which are excited by the emotions, as in
laughing, crying, sobbing, &c. We may have the strongest
desire to check these actions, owing to the unfitness of the
time and place for their manifestation ; and yet we may be
unable to do so. And lastly, muscles whose action is usually
voluntary may be occasionally called into powerful contrac-
tion, which the will cannot in the least degree control or
prevent ; this is the case in cramps, convulsions, &c., of
various kinds. — All these facts are readily accounted-for by
the knowledge we now possess, as to the functions of the
different parts of the nervous centres from which the muscles
receive their stimulus to action (Chap. x.).

591. The vigorous action of the muscular structure is de-

CONDITIONS OP MUSCULAR ACTIVITY. 451

pendent upon several conditions. In the first place, it requires
an active nutrition of the muscles themselves. Firm, plump,
and high-coloured muscles act with greater force than those
which are pale and flabby, even though the size of the latter
may be greater. Again, in all those animals whose activity is
greatest, a constant supply of oxygen is requisite for muscular
vigour. This, like the nutritive material, is conveyed, in
Birds and Mammals, by the blood (§ 235) ; in Insects, on the
other hand, it actually enters the muscular tissue in the state
of atmospheric air (§ 321). In Eeptiles, again, the blood
goes to the tissues very imperfectly oxygenated ; and their
movements are comparatively slow and feeble. But it is a
remarkable circumstance, that in the dead bodies of the latter,
or in parts separated from the living body, the property of
contractility does not depart nearly so soon as it does in similar
parts of warm-blooded animals. By experiments on Mammals
it has been found that the muscles of the trunk cannot be
caused to contract by galvanism for more than two or three
hours after death, though the auricles of the heart retain their
contractility for some hours later. The muscles of Birds
(whose respiration is more active, and whose temperature is
higher) lose their contractility yet sooner ; but those of Rep-
tiles sometimes retain the power of contracting for several
days. When venous or imperfectly-aerated blood is made to
circulate through the vessels of warm-blooded animals, it acts
like a poison upon them, diminishing or even destroying their
contractility.1

592. Further, the energy of muscular contraction depends
in great degree upon the power of the stimulus which is trans-
mitted to it through the nervous system. We often have the
opportunity of observing this, in the case of persons who are
under the excitement of violent passion or of insanity; a
delicate female becoming a match for three or four strong
men, and even breaking cords and bands that would hold the
most powerful man in his ordinary state. The strength in
such circumstances seems almost preternatural ; but it is not

1 Other substances do this with even greater rapidity ; thus a strong
solution of nitrate of potass (nitre) injected into the blood-vessels, and
conveyed by them to the heart, causes the immediate cessation of its
action, — the poison finding its way, through the vessels of the organ
itself, into the capillaries of its muscular structure.
GG2

452 FATIGUE. ENERGY AND KAPIDITY OF MOVEMENT.

greater than that which we see manifested in convulsive
actions, where the movements depend only upon the reflex
activity of the spinal cord. Thus a slender girl affected with
a spasmodic affection of the muscles of the spine, which threw
the back into an arch of which the head and the heels were
the two resting-points, has been known to raise a weight of
9001bs. laid on the abdomen with the absurd intention of
straightening the body.

593. The sense of fatigue, which comes-on after prolonged
muscular exertion, is really dependent upon a change in the
brain, though usually referred by us to the muscles that have
been exercised. For it is felt after voluntary motions only ;
and the very same muscles may be kept in reflex action for
a much longer time, without any fatigue being experienced.
Thus, we never feel tired of breathing; and yet a forced
voluntary action of the muscles of respiration soon causes
fatigue. The voluntary use of the muscles of our limbs, in
walking or running, soon occasions weariness; but similar
muscles are used in Birds and Insects, for very prolonged
nights, without apparent fatigue; and as we find that the
actions of flight may be performed, after the brain, or the
ganglia that correspond to it in Insects, have been removed
(§§ 444, 465), we may regard them as of a reflex character ;
and the absence of fatigue is thus accounted-for.

594. The energy of muscular contraction appears to be
greater in Insects, in proportion to their size, than it is in any
other animals. Thus a Flea has been known to leap sixty
times its own length, and to move as many times its own
weight. The short-limbed Beetles that inhabit the ground
have an enormous power, which is manifested both in their
movement of heavy weights, and in the resistance they
overcome with their jaws. Thus the Dung- or " shard-borne "
Beetle can support uninjured, and even elevate, a weight equal
to at least 500 times that of its body. And the Stag-Beetle
has been known to gnaw a hole of an inch diameter, in the
side of an iron canister in which it had been confined. The
rapidity of the movements of Insects is also most extraordi-
narily great, and is especially seen in the vibrations of their
wings. It would be impossible to form an estimate of the time
occupied by these, were it not for the musical tones they pro-
duce ; and it may be calculated from these that the wings of

APPLICATIONS OF MUSCULAR POWER. 453

many Insects strike the air several hundred times, — and
those of some of the smaller Insects many thousand times, —
in a second of time.

Applications of Muscular Power : — Bones and Joints.

595. Muscular contraction performs an important part in
nearly every one of the functions of which we have already
treated. Thus the reception of the food, and its propulsion
along the alimentary canal, forming part of the function of
Digestion, are accomplished through its means. The Circula-
tion of the blood, again, depends mainly on the agency of a
contractile organ, the heart. The Eespiration cannot be kept
up, in the higher animals at least, without the aid of certain
movements which are accomplished by the muscles. With
the processes of Nutrition and Secretion it is not so closely
connected ; but the latter is dependent upon it so far as this,
that its products are carried out of the body by the aid of
muscular contraction. And even in Sensation, the peculiar
endowment of muscular tissue comes into use ; by giving to
the organs of sense those movements which enable them to
take a wider range, and to apply themselves most perfectly to
the objects before them. But we have now to study its appli-
cations in those general and partial movements of the body,
on which depend the locomotion (or change of place) of
animals, their attitudes, and a number of other important
actions, entirely of a mechanical nature.

596. The organs by which these are effected, may be con-
veniently divided into the active and passive. The active are
those which have peculiar vital powers within themselves, and
which exert these in giving motion to other parts. To this
class, therefore, we refer the Muscles, whose peculiar endow-
ments have been just considered. The passive organs, on the-
other hand, are those which perform no action of themselves,,
which have no power but that of yielding a simply mechanical
support, and which consequently perform no movement but
such as they are made to do by the muscles. Of this kind
are the hard parts which form the skeleton or solid frame-
work of the body, whether this be internal or external.

597. In the lower tribes of animals, the muscles are all
inserted in the soft and flexible membrane which covers the
body ; and it is by acting upon this, that they can change the

454 APPLICATIONS OF MUSCULAR POWER I — SKELETON.

form of the body in such a manner as to cause it to move,
either altogether or in part. This is the case, for example, in
the Leech, Earth-worm, and other Annelids ; which are fur-
nished with two sets of muscular fibres, one running along
the body, and the other passing round it in rings. By the
contraction of the former, the two ends are drawn together,
so that the body is shortened ; whilst by that of the latter,
its diameter is lessened, so that it is necessarily lengthened.
By these two movements, which take place alternately, the
progression of the animal is accomplished ; and by varying
the contractions of one part or another, almost any form and
direction can be given to the soft and flexible body.

598. But in the higher animals we find the apparatus of
movement to consist, not only of muscles, but also of a frame-
work of solid pieces, which serves to augment the precision,
the force, and the extent of the movements ; whilst, at the
same time, it determines the general form of the body, and
protects the viscera against injury from without. This solid
framework, or skeleton, to which the muscles are attached,
may be, as we have seen, either internal or external. In the
Vertebrated classes, the hard skeleton is internal; in the
Articulated series it is external; in the Mollusks it is
external, but does not afford fixed attachments to muscles,
except to such as draw together its valves, or connect it with
the soft body of the animal ; and in the Radiata its position is
variable, being sometimes external as in the Echinodermata,
and sometimes internal as in the stony Corals.

599. The skeleton of Vertebrata differs from that of all the
Invertebrated classes in the much higher character of its or-
ganization, which enables it to grow with the growth of the
body generally, not merely in virtue of the additions it
receives, but by the successive removal of its previously
formed parts as occasion may require ; so that the skeleton of
the adult has been entirely substituted for that of the child,
probably no part of the latter being contained in the former.
The skeletons of the Invertebrata, where they are not formed
of horny matter alone, are consolidated by carbonate of lime,
which in some instances (as the shells of many Mollusks,
and the Stony Corals) bears so large a proportion to the
animal basis, that the latter can scarcely be detected. The
growth of these skeletons takes place entirely by additions to

CONNEXION OF SEPARATE PIECES OP SKELETON. 455

the parts already formed ; there is nothing like that " inter-
stitial " change which we see in bone, and which is performed
by the agency of the blood conveyed through the Haversian
canals that traverse its substance j and where the skeleton is
external, it must either be adapted by such additions to the
augmenting bulk of the body it incloses, or must be cast-off
and replaced by another. The latter method is that which
is followed in the Crustacea ; of the former we have examples
in the shell-bearing Mollusks, whose shells receive successive
additions at their free margins, and in the Echinodermata,
whose box-like envelopes are made to increase equally in all
directions, by additions to the edges of the numerous separate
pieces of which they are composed (§ 118).

600. The different portions of the skeleton are articulated,
or united by joints to one another, in such a manner that
they can move with greater or less freedom. This we see
both in the Vertebrated and in the Articulated classes. In the
latter, the joints are for the most part very simple in their
construction. The different rings or pieces are held together
by a flexible membrane passing from one to the other j this
seems to be little else than a portion of the integument
originally covering the body, which has remained uncon-
solidated whilst the rest has been hardened. And sometimes
they are made to adhere to each other by a kind of " solder-
ing," so as to be altogether immovable. But in the internal
skeletons of the Vertebrata we find a more complex mode of
union, fitted to afford scope for the greater variety of motions
which their parts perform. Here, too, we find some parts
immovably united to each other, where support and protec-
tion alone are required. These immovable articulations, of
which there are several kinds, will be first considered.

601. All the bones of the head and face (with the excep-
tion of the lower jaw), in Man and the higher Vertebrata,
have their edges in immediate contact with each other; so
that they hold together in the dry skull, as well as during
life. Those bones of the skull, which inclose and protect the
brain, are very firmly united by what are termed sutures,1
which are mostly formed by the interlocking of the jagged
edges of one bone into corresponding notches of the adjoining
one : though in some this kind of union is incomplete, while

1 From the Latin sutura, a seam.

456 SUTUEES : — MOVABLE ARTICULATIONS.

in others it is replaced by a bevelling of the edges that are in
contact, or by the reception of a ridge of one bone into a
groove in the other. So firmly are the bones united in this
manner, that it is difficult to separate them without breaking
away some of their projecting parts ; and in the skulls of
old persons, the sutures are almost obliterated by the complete
union between the adjacent bones. In the infant, on the
other hand, the bones of the skull are only united to each
other by a membranous substance ; and there is a point at
the top of the head, which is not even covered by a bony
layer for some time after birth. It is only as the age ad-
vances, and ossification becomes more complete (§ 52), that
firm bony union is effected.

602. In several other articulations, the bones do not come
into direct contact with each other, but are connected by an
intervening layer of cartilage, and also by ligaments and
other fibrous membranes encircling the articulations. The
adjacent surfaces of the bones are flat, and have a slight
gliding movement over one another; but the extent of
motion permitted is very small. This kind of articulation
exists between the bodies of the vertebrae of Man and the
higher Vertebrata, between the bones of the pelvis, and some
other parts.

603. The proper movable articulations, by which the limbs
are connected with the trunk, and their different divisions to
each other, are those to which we commonly give the name of
joints. In these, the surfaces of the adjacent bones are not
united in any other way than by the ligaments and muscles
which surround them ; and they have a free gliding move-
ment over each other. They are covered, it is true, by car-
tilage; this, however, does not pass from one bone to the
other, as in the previous case, but forms a thin layer over the
end of each, and presents a very smooth surface, which is
secreted from the synovial membrane that envelops the joint
lubricated by the fluid (§ 44). The beautiful smoothness of the
surfaces of the joints, and the manner in which the bones are
held together by the muscles and ligaments, is well seen by
examining the knuckle-joint at the lower end of a leg of
mutton (before being cooked), and the joint which connects
it -with the bones of the haunch. These two joints are
examples of the two principal varieties of freely-movable

HINGE- AND BALL-AND-SOCKET JOINTS; — DISLOCATIONS. 457

articulations, — the hinge-joint, and the ball-and-socket joint.
In the first of these, the surfaces of the bones are so formed
that the movement, though free as regards its extent, is very
limited in its direction; being in fact restricted to a back-
ward and forward action in the same line, just like that given
by a common hinge. In the second, the end of one bone is
formed into a rounded head or ball, and this is received into
a corresponding socket or cup in the other, the edge of which
is usually deepened by cartilage ; in this manner the bone
which carries the ball is enabled to move upon the other in
any direction, unless restrained by external checks. Of the
hinge-joint we have examples in the elbow, the knee, and the
joints of the fingers and toes. Of the perfect ball-and-socket
joint we have in Man only two examples, — the shoulder, and
the hip. In the former the socket is much shallower than in
the latter, and the motions of the arm are consequently more
extensive than those of the thigh : both, however, are un-
checked in regard to their range and direction, except when
the limb is brought against the body or against its fellow.
The wrist and the ankle-joint are of an intermediate cha-
racter; the former more resembling the ball-and-socket, and
the latter the hinge-joint.

604. All these joints are more or less subject to dislocation,
by violence of different kinds. This takes place by the
slipping-away from each other of the two surfaces, which
ought to be in contact. Thus the head of the humerus (or
arm-bone) may slip over the edge of its socket, so as to lie
entirely on the outside of it ; and this, in consequence of the
shallowness of the cup, happens not unfrequently. The head
of the thigh-bone, also, may slip out of its socket ; but this
accident is more rare, on account of the deepness of its cup.
The elbow and knee-joints, as also those of the wrists, ankle,
fingers, and toes, may be dislocated by the slipping of one
surface on the other, either forwards or backwards, to one side
or to the other. One of the most common dislocations is that
of the thumb, the lowest articulation of which has rather the
character of the ball-and-socket (with a very shallow cup),
than of the hinge-joint. But in proportion to the liability of
any joint to dislocation, is usually the ease with which it may
be brought into place again.

605. Of the attachments of muscles to the skeleton, one is

458 ORIGIN AND INSERTION OF MUSCLES.

usually called the origin, and the other the insertion, — the
origin being in the part that is most fixed, and the insertion
in that which moves upon it. Thus the muscle chiefly con-
cerned in bending the elbow, has its origin at the shoulder
and its insertion in the bones of the fore-arm ; its general
dction being to move the latter, while the former is fixed or
nearly so. But its attachment to the fore-arm may really
become its origin, and its other attachment its insertion ; for,
when a person is hanging by his hands from a beam or cord,
and raises his body by bending his elbows, the fore-arm is
the fixed point, and the shoulder is moved upon it. In like
manner, the muscular mass at the bottom of the back, having
one attachment to the bones of the pelvis and another to the
thigh-bone, serves to draw the latter backwards when the
former is made the fixed point, as when we rise-up from the
sitting posture ; but it is also continually serving to keep
the body upright upon the thighs, the latter being the fixed
point, and brings it back into this position when it has been
bent forwards as in stooping. — Muscles are seldom directly
attached to the hard parts, but are united with them by
means of the fibrous bands which are called tendons (§ 29).
Sometimes the tendon is long and round ; this is the case with
several of those that move the hand and fingers (fig.^28), which
arise from the muscles forming the fleshy part of the fore-
arm, and may be felt at the wrists as hard round cords. In
other instances, however, the tendon is a broad flat band ; of
such we find several within the shell of the body and limbs
of the Crab or Lobster, when we have removed the muscle
or flesh.

606. The action of any muscle, in producing a change in
the position of a movable bone on which it acts, is deter-
mined in the first place by the nature of the movement of
which the bone is capable ; and in the second, by the direc-
tion in which the power of the muscle is applied to it.
Having now considered the former of these conditions, we
proceed to the latter. The contraction of a straight muscle,
which is attached to a fixed point at one end, and to a
movable point at the other, will obviously tend to draw the
latter towards the former. Thus the muscles which bend
the fingers lie in the palm of the hand and on the correspond-
ing side of the fore-arm; whilst those that straighten the

ACTION OF MUSCLES ON BONES. 459

fingers are situated on the opposite side. But we often find
that the direction of a muscle's action is changed, by the
passing of its tendon through a pulley-like groove or loop ;
so that it draws the movable bone in a direction different
from that of its fixed attachment. This is the case, for ex-
ample, with some of the muscles that bend the toes ; these
being situated in the calf of the leg, would draw the toes
upwards, were it not that their tendons are carried beneath
the bones of the heel, working in smooth pulley-like channels
hollowed-out in them (fig. 233) ; hence, when the muscle con-
tracts, the tendons draw the ends of the toes towards the heel,
and thus bend them.

607. We generally find that even movements of a simple
character are performed by the combined action of several
muscles ; of which some may be considered as the principal,
and others as assistants. Those which are principals in one
movement may become assistants in another ; and vice versd.
Thus, if we wish to bend the wrist directly downwards upon
the fore-arm, we put in action, not only certain muscles whose
tendency would be to produce this movement, but others
which, acting by themselves, would produce a different motion.
One of these would draw the wrist towards the thumb-side
of the fore-arm, and the other towards the little-finger-side,
and they become the principal muscles in these movements
respectively ; but when they act together, their several ten-
dencies to draw the wrist to opposite sides counterbalance
one another, and they simply assist the principal muscles in
bending the wrist downwards upon the fore-arm.

608. Almost every muscle in the Human body has its
antagonist, which performs an action precisely opposite to its
own. Thus by one set of muscles, termed flexors, the joints
are bent ; by a contrary set, the extensors, they are straightened.
One set of muscles draws the arm or leg outwards, or away from
the central line of the body; another draws the limbs inwards.
One set, again, closes the jaws ; and another opens it. But
we find an economy of muscular substance in some of the
lower animals, where parts are to be usually kept in a parti-
cular position, which has only to be changed occasionally and
for a short time; the antagonism being then supplied by
yellow elastic tissue (§ 29).

609. We commonly find that, in order to preserve the

460 ACTION OP MUSCLES ON BONES.

necessary form of the animal body, Muscles are applied at a
great mechanical disadvantage as regards the exercise of their
power ; that is, a much larger force is employed than would
suffice, if differently applied, to overcome the resistance. But
we generally find that, in this as in other forms of lever action,
what is lost in power is gained in time ; and thus a very slight
change in the length of a muscle is sufficient to produce a
considerable movement.

610. The first source of disadvantage results from the
direction in which the muscle is attached to the bone. This
is rarely at right angles to it ; and consequently a considerable
part of the power is lost (see MEGHAN. PHILOS., § 299). Thus
if the muscle m (fig. 213), whose force we shall suppose equal

to 10, be fixed at right angles
to the bone /, whose extremity
a is movable upon the point of
support r, its force of contrac-
tion will be most advantageously
applied to overcome the resist-
ance, and will draw the bone
from the position a b into the
direction a c, making it traverse
a space which we shall also
represent by 10. But if this muscle act obliquely on the
bone, in the direction of the line n b for example, it will
be quite otherwise ; for it will then tend to draw the bone
in the direction b n, and will consequently make it approach
the articular surface r. But as this bears upon an immovable
socket, and as the bone can move in no other way than by
turning upon the point r as upon a pivot, the contraction of
the muscle to the same amount as before will carry the bone
no further than into the direction a d ; three-quarters of the
force employed will thus be lost, and the resulting effect will
be no more than one-fourth of that which the same power
applied perpendicularly to the bone would have produced.

611. We usually find that the muscles are inserted so
obliquely, that their power is applied at a great disadvantage ;
but this disadvantage is rendered much less than it would
have otherwise been, by a very simple contrivance, — that very
enlargement of the bones at the joints which is necessary to
give them the required extent of surface for working over

LEVERAGE OP BONES. 461

each other. Thus, let r and o (fig. 214) be two bones con-
nected by a joint ; and let the muscle m, which moves the
lower bone upon the upper, be attached to the former at i.
Now as this muscle acts almost precisely in the line of the
bones themselves, almost all its
power will be expended in draw- ™
ing the lower bone against the
upper. But by the enlargement
of the ends of the bones, as seen 1
in fig. 215, the direction of the
tendon of the muscle m is so
changed, near its insertion i, that

the contraction of the muscle will cause the lower bone to
turn upon the upper one with comparatively little loss of
power. In the knee we find a still greater change of direction
effected, by the interposition of a movable bone, the patella or
knee-pan, in the substance of the tendon.

612. But the advantage or disadvantage with which the
muscles act upon the bones, depends in great degree upon the
relative distances of their point of attachment from the fulcrum
on which the bone moves, and from the point at which the
resistance is applied. Every bone acted-on by muscles may
be regarded as a lever, having its fulcrum or point of support
in the joint, its power where the muscle is attached to it, and
its weight where the resistance is to be overcome ; and the
distances of the fulcrum from the power and the weight
respectively are termed the two arms of the lever. Now, on
the mechanical principles fully explained elsewhere (MEGHAN.
PHILOS., § 287), the relative length of
these two arms determines the force y

which is necessary to overcome a I ...1M......

given resistance. Thus in the Steel- j J""" ^^ P

yard (fig. 216), the beam is divided T
into two arms of unequal length O
at the point of support or fulcrum a ;

at the end of the short arm r, hangs the body whose down-
ward pressure we wish to determine ; and on the other p
there slides a weight, which will balance a greater or less
amount of pressure at the opposite extremity r, according as
it is made to hang from a point which is more distant from
the fulcrum or nearer to it, — that is, according as the length

462

LEVEEAGE OP BONES.

P

P

£

m

c

W* v,

7

2

»

Fig. 217.

of the power-arm of the lever is increased or diminished, that
of the weight-arm remaining the same.

613. Now in order that there may be an equilibrium, or
balancing between the power and the weight, it is necessary
that they should be inversely proportional to the lengths of
their respective arms ; that is, the power multiplied by the
length of its arm, should be always equal to the weight
multiplied by the length of its arm. Thus, to balance a

certain resistance r, equal
to 10, and applied at the
end of a lever a b (fig.
217), whose length we
shall call 20, it is neces-
sary that a force p, ap-
plied at the same point,
and consequently at the
same distance from the
fulcrum a, should also be equal to 10 ; but, if the power be
applied at the point c, which is at only half the distance from
the fulcrum a, it must be doubled in amount, or equal to 20, —
since it must be sufficient, when multiplied by its distance 10
from the fulcrum, to make 200, which is the product of the
resistance 10 and its distance from the fulcrum 20 ; and in
like manner, if the power be applied at d, where its distance
from the fulcrum is only 2, its amount must be 100, in order
that its product with the distance at which it is applied may
be equal to 200. Hence, when a muscle is applied near the

fulcrum, while the resist-
ance is at a distance from

rt ^ c t r it, so that the bone be-
comes a lever of the
"third order," its force
must be proportionably

jr.

614. But this arrange-
ment greatly increases the
rapidity of the motion
which is the consequence
Fig<218< of the muscular action.

For let us suppose that the muscle p (fig. 218) acts upon the
lever a r, in such a manner that its point of insertion c tra-

LEVERAGE OF BOtfES. 463

verses a space equal to 5 in one second ; the extremity r of
the lever will traverse a space equal to 25 in the same time,
its distance from the fulcrum a being five times as great as
that of the point c from the fulcrum. Hence, although, to
raise a given weight at r, a power more than five times its
amount must be applied at c, that power will raise the weight
through a space five times as great as that through which
itself passes in the same time. Thus, what is lost in power is
gained in time ; and the shortening of a muscle, small in
amount, but effected with sufficient power, causes the raising
of a weight through a considerable space.

615. We shall find that this is the case in regard to most
of the muscular actions in the animal economy. Thus, the
fore-arm (fig. 219, b, c) is bent upon the arm a by a muscle d,

Fig. 219.

which arises from the top of the latter, and which is inserted
at e, a short distance from the elbow-joint. Hence its con-
traction to a very slight extent will raise the hand through a
considerable space ; but a proportional increase in its power
will be required to overcome any resisting force in the hand.
— The arm is straightened again by an antagonist muscle,
which lies on the back of the arm, and which is attached to a
short projection made by one of the bones of the fore-arm
behind the elbow : this muscle also operates at a similar dis-
advantage in regard to power, and advantage in point of time,
in consequence of its point of attachment being so near to the
fulcrum. In responding to its action, however, the bones of
the fore-arm constitute a lever of the " first order ;" the elbow-
joint, which serves as the fulcrum, being now between the
power and the resistance.

464

BONES OF THE SKULL.

Motor Apparatus of Man : — Skeleton and Muscles.

616. Before entering upon the examination of the various
movements of the lower animals, and of the means by which
these are effected, it will be useful to acquire a general know-
ledge of the structure of the Human Skeleton, and of the
uses of its several parts. The skeleton, which is formed by
the union of about 200 bones, is divided like the body into head.,
trunk, and members. The bones of these parts will now be
separately described.

617. The Head is composed of two parts, the cranium or
skull, and the face. The cranium (fig. 220) is a bony case of
oval form, occupying the upper and back part of the head,

and serving for the protection of
the brain, which is lodged in its
cavity. Its walls are made-up of
eight bones : the frontal/ in the
region of the forehead; the two
parietal bones p, which occupy
the top and sides of the skull;
the two temporal bones t, which
form the walls of the temporal
region ; the occipital bone o at the
back of the head ; and the sphe-
noid s, and the ethmoid, which
assist in forming the floor of the
cavity. These bones are firmly
united to each other by sutures,
the character of which varies in
different parts of the cranium,
so that they are the better able to resist external violence.
Thus, a blow upon the top of the arch formed by the parietal
bones will tend to separate them from each other and from
the frontal bone, and to force asunder their lower borders.
Both these effects are resisted by the peculiarity of the suture
which unites different parts of the parietal bone to its neigh-
bours ; for at the top of the skull the bones are firmly held
together by the interlocking of the projections of each, whilst the
lower edge of the parietal bone is prevented from being driven
outwards by the overlapping edge of the temporal bones, which
form, as it were, a buttress to the arch. This same contrivance

Fig. 220.— HUMAN SKULL.

/, frontal bone ; p, parietal ; t, tem-
poral ; o, occipital ; «, sphenoid ;
n, nasal ; ms, superior maxillary ;
j, malar or cheek bone ; mi, in-
ferior maxilla; no, anterior open-
ing of the nose ; ta, auditory aper-
ture; az.zygpmatic arch ; a,b,c,d,
lines indicating the facial angle.

BONES OF THE SKULL. 465

prevents the temporal bone from being driven inwards, as it
might have otherwise been, by a blow on the side of the head.

618. In the base or floor of the cavity of the cranium are
seen a number of apertures, which serve for the passage of
the blood-vessels that supply the brain, and of the nerves that
issue from it. One of these apertures, much larger than the
rest, and situated in the occipital bone, gives passage to
the Spinal Cord ; and on each side of this aperture there is
a large bony projection from the under surface, termed the
condyle, by which the skull rests on the vertebral column, and
is enabled to move forwards or backwards upon it. The head
is nearly balanced upon this pivot ; nevertheless, the portion
situated in front of the joint is more heavy than that which
is situated behind it, and is consequently not altogether
counterpoised by the latter. Hence the muscles which, arising
from the back and being attached to the occipital bone, tend
to draw the head backwards, and thus to keep it upright, are
more numerous and powerful than those which are situated in
front of the vertebral column, and which tend to draw the head
downwards and forwards ; and when the former are relaxed,
as in a person sleeping upright, the head has a tendency to
fall forwards upon the chest. In no other animal is this joint
situated so far forwards as in Man. As we descend the scale,
we find it nearer and nearer to the back of the skull ; and
consequently the whole weight of the head bears, not directly
upon the spine, but upon the muscles and ligaments by which
it is attached to the vertebral column.

619. On each side of the base of the cranium, we observe
a large rounded projection, termed the mastoid. To this pro-
jection (which we feel behind the lower part of the ear) is-
attached on either side a powerful muscle, the sterno-mastoid
(28, fig. 227), which passes downwards and towards the central
line ; so that the two muscles nearly meet at the bottom of the
neck, where they are attached to the upper edge of the breast-
bone. These muscles, acting together, serve to draw the head
forwards ; but either of them acting separately will turn it to
one side or the other. In front of these two projections of
the skull, we notice the opening t a of the external ear ;
which, like the different chambers of the internal ear, is
excavated in a portion of the temporal bone which is termed
petrous from its very dense and stony character.

H H

466 BONES OP THE FACE.

620. The face is formed by the union of fourteen bones ;
and presents five large cavities, which serve for the lodgment
and protection of the organs of sight, smell, and taste. All
the bones of the face, with the exception of the lower jaw, are
completely immovable, and are firmly united to each other
and to the bones of the cranium (§ 617). The two principal
are the superior maxillary (m s, fig. 220), which, form nearly
the whole of the upper jaw, and are connected with the
frontal bone in such a manner as to contribute to the forma-
tion of the orbital cavities in which the eye is lodged, and of
the nasal cavities which form the interior of the nose ; they
also constitute the front of the roof of the mouth ; on the
sides of the face, they articulate with the malar or cAee^-bones
j \ whilst they are united behind with the palate-bones which
form the back part of the roof of the mouth, and which in
their turn are united to the sphenoid.

621. The orbits, as we have already seen (§ 538), are two
deep cavities, of a conical form, — the base of the cone being
directed forwards, and its apex or point backwards ; the roof
of these cavities is formed by a portion of the frontal bone,
and their floor chiefly by the superior maxillary. Their
inside wall is formed by the ethmoid bone, and by the small
bone termed the lachrymal, in which is the canal for the
passage of the tears into the nose (§ 541) ; and the outside
wall is formed partly by the cheek-bone and partly by the
sphenoid, — the latter also bounding the cavity at its deepest
part, and containing the apertures which serve for the passage
of the optic and other nerves that enter the orbit from the
cranium. In the roof of the orbit, on its outer side, there is
a broad shallow pit or depression, in which the lachrymal
gland is lodged.

622. The greater part of the nose is formed by cartilages ;
so that, in the bony skull, the anterior opening of the nasal
cavity (n a, fig. 220) is very large ; and the bony portion of
the nose, formed by the two small bones (n) termed nasal,
projects but slightly. The nasal cavity, divided in the middle
by a vertical partition into two fossce or excavations, is very
extensive (§ 506) ; at the upper part it is hollowed-out into
the ethmoid bone, the whole interior of which is made-up of
large cells ; its floor is formed by the arch of the palate,
which separates it from the mouth ; behind it extends as far

BONES OF THE FACE. 467

as the back of the mouth, and communicates with the
pharynx by two apertures termed the posterior nares (fig.
200, c). The partition between the fossse is formed at the
upper part by a plate that projects downwards from the eth-
moid bone, and at the lower by a distinct bone called the
vomer (or ploughshare) from its peculiar form ; to the front
edge of this last is attached a cartilage, which continues the
partition forwards into the soft projecting portion of the nose.
It is through the thin horizontal plate of the ethmoid bone,
which separates the nasal cavity from that of the skull, that
the olfactory nerves make their way out from the former into
the latter : they descend in numerous branches, for the passage
of which through the roof of the nose this plate is perforated
by a number of small apertures, which give it a sieve-like
aspect; whence it is called the cribriform^- plate of the
ethmoid.

623. It is in the superior maxillary bone that all the teeth
of the upper jaw are implanted in Man ; but in the embryo
this bone is composed of several pieces ; and one of these
pieces, termed the intermaxillary bone (im, fig. 221), remains

mi mo c

Fig. 221. — SKULL op HORSE.

OC, occipital bone ; t, temporal ; /, frontal ; », nasal ; tn, superior maxillary ; im,
intermaxillary; mi, inferior maxillary; o, orbit; », incisor teeth; c, canines; mo,
molars.

permanently separate in most of the lower animals. The

lower jaw of adult Man, also, is composed but of a single

piece ; though this is divided in the infant on the central line,

and the two halves remain separate in many of the lower

animals. This bone has a general resemblance in form to a

horse-shoe with its extremities turned up considerably ; it is

1 From the Latin, crilrum, a sieve.

H H 2

468 BONES AND MUSCLES OF THE FACE.

articulated with, the temporal bones by a condyle or projecting
head with which each of these extremities is furnished ; and
this head is received into what is called the glenoid l cavity
on the under side of the temporal bone. In front of the
condyle is another projection, or process, termed the coronoid
(a, fig. 92), which serves for the attachment of one of the
principal muscles that raise the jaw. These muscles are all
attached near the angle of the jaw (or the point at which it
bends upwards), and they consequently act at a small distance
from its fulcrum, whilst the resistance is applied at the
furthest point (§ 180). We are continually reminded of the
loss of mechanical power which results -from this, by our in-
ability to exercise the same force with our front teeth that we
can employ with the back. Thus, when we wish to crack
a nut, or to crush any hard substance between the teeth,, we
almost instinctively carry it to the back of the jaws, so as to
place it nearer the joint, where it may receive more of the
power of the muscle.

624. The general arrangement of the chief muscles of the
face is seen in fig. 222. The largest is the temporal muscle, t,
the fibres of which arise from an extensive
surface of the parietal and temporal bones,
and then converge or approach each other,
passing under the bony arch or zygoma, z
(which is partly formed by a process from
the temporal bone, and partly by the malar
or cheek bone), to be attached to the
j m c a coronoid process of the lower jaw. This
Fig. 222. muscle is of extraordinary power in those

beasts of prey which lift and drag heavy carcases in their
jaws; and in those which (like the Hyaena) obtain their
support by crushing the bones that others have left. It is
assisted by the masseter muscle m, which passes from the
zygomatic arch and cheek-bone to the angle of the lower jaw,
and also by other muscles. Besides these, the figure shows
the ring-like muscle or sphincter o, which surrounds the
opening of the eye, and serves by its contraction to close the
lids ; and also the similar muscle 6 b, which surrounds the

1 The term condyle is applied to most of the projecting surfaces of
articulation, in different parts of the body; and the term glenoid to
the cavities into which these are received.

Frontal bone

Parietal bone

Orbit

Lower Jaw

Cervical Vertebrae

Scapula

Humerus

LumbarVertebrae

Ilium

Ulna
Radius

Carpus
Metacarpus

Fig. 223. — SKELETON OF MAN.

470 STRUCTURE OP THE VERTEBRAL COLUMN.

mouth and draws together the lips. The antagonists to these
are several small muscles which form the fleshy part of the
face, and produce the various changes "by which its expression
is given. These muscles are more numerous in Man and the
Monkey tribe than in any other animals.

625. Besides the twenty-two bones of which the skull is
properly composed, we may reckon as belonging to it the four
small bones which form part of the apparatus of hearing
(§ 516); and also the hyoid bone, which lies at the root of
the tongue and at the top of the larynx (fig. 107). This last
bone, in Man and the Mammalia generally, is connected with
other parts of the skeleton by ligaments and muscles only ;
but in Birds it is connected with the temporal bone on each
side by a set of bony pieces jointed together like links in
a chain.

626. The most important part of the Trunk, and even of
the whole skeleton, — that which serves to sustain the rest,
and which varies the least in the different classes of Verte-
brated animals, — is the spinal or vertebral column. The
general conformation of this has been already described (§71).

In Man it consists of 33 vertebra (fig. 224),
which are arranged under five divisions ; — r. The
Cervical vertebrae c, or vertebrae of the neck, of
which there are 7 \ n. The dorsal vertebrae d, or
vertebrae of the back, of which there are 12 ;
— in. The lumbar vertebrae I, or vertebrae of the
loins, of which there are 5 ; — iv. The sacral ver-
tebrae s, of which also there are 5 ; — and v. The
coccygeal vertebrae co, of which there are 4. All
these vertebrae are separate at the time of birth ;
but the 5 sacral vertebrae are soon afterwards united
into one piece, forming the bone which is termed
\s the sacrum : and the coccygeal vertebrae are also
) commonly united into one piece, the coccyx, which
is not unfrequently united in old age to the sacrum.

Fig. 224. In old persons, too, it is not uncommon for the
VERTEBRAL lumbar vertebrae to be united together by bony

COLUMN. ma^.er deposited in their cartilages and ligaments.

627. The dorsal vertebrae are distinguished from the cervi-
cal and lumbar, as being those to which the ribs are attached.
It is remarkable that the number of the cervical vertebrae

STRUCTURE OF THE VERTEBRAL COLUMN.

471

should be the same in all the Mammalia ; even the long-
necked Giraffe having only seven, while the Whale, whose
head seems to be joined to its body without the intervention
of any neck, also has seven cervical vertebrae, although they
are almost as thin as a sheet of paper. It is owing to the
small number of joints in its neck, that the movements of the
head of the Giraffe are far less graceful than those of the
Swan and other long-necked Birds, in which the number of
cervical vertebrae is much greater. The following table shows
the number of vertebrae in animals of different groups.

MAMMALIA.

Cervical.

Dorsal.

Lumbar.

Sacral.

Coccygeal

Total.

Man ... .

Long-tailed Monkey
Lion

7

7

12
13

7

7

3
3

31
2fi

60

Long-tailed Opossum ...
Long-tailed Ant- Eater ...
Elephant

7
7
7

16
16
20

6
3
3

2
6
4

36
40
27

67
72
61

Giraffe

7

48

Whale

7

BIRDS.
Vulture

15

7

g

41

Swallow ... ...

Turkey

14

j

Q

42

Ostrich

18

Crane .

17

Swan

REPTILES.
Tortoise

Monitor (Lizard)
Python (Boa)

6

21

2

2

115

146

Rattle-Snake
Land Salamander
Axolotl .

1

171
14

—

1

36
26

207
42

FISHES.
Perch

21

21

42

Mackerel

15

16

31

Trichiurus

60

100

160

Salmon . . ..

34

22

56

Cod

19

34

53

Conger Eel

60

102

162

Electric Eel

236

Shark

95

270

365

We see from the above table, that it is by the multiplica-
tion of the coccygeal vertebrae, that the tail is prolonged in
those animals which possess it. In fact, it is only in Man,
and in those of the Ape tribe which approach nearest to him,
that the number of these vertebrae is as low as 4.

472 STRUCTURE AND CONNEXIONS OF VERTEBRA.

628. It has been already noticed (§71) that an ordinary
character of the vertebra consists in their being perforated by
an aperture (fig. 225), which, when several vertebrae are united.

together, forms a continuous tube or canal for
the lodgment of the spinal cord. This cha-
racter is usually lost, however, in the coccy-
geal vertebrae ; which are so much contracted
and simplified as to contain no aperture.
The purpose of the division of the spinal
column into so large a number of separate
Fig. 225.— SINGLE bones, is obviously to allow of considerable
freedom of motion by a slight shifting amongst
the individual parts ; whilst any such sudden bend as would
be injurious to the spinal cord, is avoided. Each vertebra con-
sists of a solid " body " a, which is situated in front of the
spinal canal in Man, but below it in animals whose back has
a horizontal position, and which serves to give solidity to the
structure, — and of "processes" or projections, b and c, that
serve to form the spinal canal, and to unite the vertebra to
each other. In Man and other warm-blooded animals, the
two surfaces of the "body" are nearly flat and are parallel to
each other ; and they are united to the corresponding surfaces
of the neighbouring vertebrae by a disk of fibro-cartilage (§47),
which extends through the whole space that intervenes be-
tween them, and which, being firmly adherent to both, prevents
them from being far separated from each other.

629. But in Reptiles and Fishes, a different plan is adopted.
In the animals of the former class, particularly in Serpents,
we find one surface of each vertebra convex or projecting, and
the other concave or hollowed-out ; and the convex surface of
each vertebra fits into the concave surface of the next, in such
a manner that the whole spinal column becomes a series of
ball-and-socket joints, and is thus endowed with that flexibi-
lity which is essential to the peculiar movements of these
animals. In Fishes both surfaces are concave, and between
each vertebra there is interposed a bag containing fluid, and
having two convex surfaces, over which those of the vertebrae
can freely play. Extreme facility of movement is thus given
to the spinal column ; but its strength is proportionally dimi-
nished. It is to be remembered, however, that strength is
not required in the bony framework of animals, whose bodies,

MUTUAL CONNEXIONS OP VERTEBRA. 473

instead of being supported upon foui fixed points, are buoyed
up in every part by a liquid of nearly the same density with
themselves. The extreme flexibility of the spine of Fishes,
enables them to propel their bodies by the movements of the
hinder portion and tail from side to side ; their members, or
pectoral and ventral fins (fig. 243), being but little used
except for influencing the direction of their motion. And
thus we see that in the lowest Vertebrate, as in the lower
Articulata (such as the Leech and Earth-worm), the propul-
sion of the body being accomplished by the movements of
the trunk itself, its skeleton (internal in the one case, external
in the other) is left in the soft condition which it has in all
at an early period : whilst in the higher classes of both series,
— Birds and Insects for example, — the extremities being so
developed, and being furnished with muscles so powerful
that the function of locomotion is entirely committed to them,
the skeleton of the body undergoes great consolidation, its
various pieces being so knit together as to make the trunk
almost immovable.

630. This knitting-together is partly accomplished by
means of projections or processes from^the several vertebrae,
which are united to one another by muscles and ligaments.
Of these processes there are seven in Man from each vertebra.
One of these, termed the spinous process (b, fig. 225), projects
directly backwards ; and thus is formed the prominent ridge
on the back, in which the ends of these projections can be
distinguished. The spinous processes serve in Man to give
attachment to the muscles, by which the trunk and head are
kept erect ; in Animals whose spine is horizontal, they are
generally much longer, in order to give firm attachment to the
muscles and ligaments which support the head (fig. 229, vc, vd).
And in Fishes they are greatly prolonged (fig. 243), so as to in-
crease the surface by the stroke of which from side to side the
body is propelled through the water. On each side of the
vertebra is a process (c, fig. 225) which is called transverse;
this serves for the attachment of the ribs to the vertebra.
And lastly, from the upper and under side of each vertebra,
two articulating processes project, which lock against each
other in such a manner as to prevent the movements of the
vertebrse from being carried to an injurious extent. These
processes are peculiarly long in Birds, where they almost

474 UNION OF VERTEBRAE IXTO SPINAL COLUMN.

completely check the movements of the dorsal vertebrae ;
thereby giving to the trunk that firmness which is required
for the attachment of the muscles of the wings. The portions
of bone which pass backwards from the body of each vertebra
to its transverse processes, and thus form the side-wall of the
spinal canal, are called the arches of the vertebrae. These are
the parts first formed. On the under edge of each there is
a notch which corresponds with one in the upper side of the
next, in such a manner that, when two vertebras are placed
together, a complete foramen or aperture is formed, which
serves for the passage of the nerves that are given-off from
the Spinal Cord (§ 457).

631. The vertebral column of Man is disposed in a double
curve, as seen in fig. 224 ; the effect of this is to diminish
the shock that would be produced by a sudden "jar," — such
as when a man jumps from a height upon his feet. If the
vertebral column had been quite straight, this jar would have
been propagated directly upwards from the pelvis to the head,
and would have produced very injurious effects upon the
brain ; but by means of the double curvature, and the elasti-
city of the ligaments%&c. which hold together the vertebrae,
it is chiefly expended in increasing for a moment the curves
of the spine, which thus acts the part of a spring. The
constant pressure of the head and upper part of the trunk
has a tendency to increase these curves permanently, and
thus to diminish the height of the body. The elasticity of
the intervertebral substance, however, causes it to recover,
during the time when the body is in the horizontal posture,
the form it had lost by pressure in the upright position ; and
thus a man is taller by half an inch or more when he rises
in the morning, than he was when he lay down the night
before.

632. The first vertebra of the neck, termed the atlas, is
much more movable than the rest, and differs considerably
from them in its form. It is destitute of body ; but it has a
broad smooth surface on either side, on which rest the " con-
dyles" of the occipital bone of the skull (§ 618), in such a
manner that the head is free to nod backwards and forwards.
The atlas itself turns upon a sort of pivot, formed by an
upward projection from, the next vertebra, which is termed
the axis; this projection, called from its form the processus

ARTICULATION OF HEAD WITH SPINAL COLUMN. — RIBS. 475

dentatus (or tooth-like process), occupies the place of the body
of the atlas ; and by the rotation of the atlas around it, the
movements of the head from side to side are accomplished.
Wherever great freedom of motion is permitted, displacement
or dislocation is necessarily more easy ; and accordingly we
find that the atlas and axis can be more easily separated from
each other, than can any other two vertebrae. This dislocation
may be produced by violence of different kinds ; thus if the
head be suddenly forced forwards while the neck is held back,
the tooth of the axis may be caused to press against the
spinal cord, and thus to interrupt or completely check its
functions. Or, again, if the weight of the body be suspended
from the head, and especially if it be thrown upon it with a
jerk, the two vertebras are liable to be dragged asunder, and
the spinal cord to be stretched or broken. This is sometimes
the immediate cause of death in hanging ; and it has not
unfrequently occurred when children have been held in the
air by the hands applied to the head, — a thing often done in
play, but of which the extreme danger should prevent its
ever being practised. Any serious injury of the spinal cord
in this region must be immediately fatal, for the reason for-
merly stated (§ 470), — that it causes the suspension of the
motions of respiration.

633. The number of the ribs which are attached to the
bodies and transverse processes of the dorsal vertebra, is, in
the Human species, twelve on each side.1 The number in
different animals may be judged-of by that of the dorsal ver-
tebras in the table already given (§ 627) ; since it is the attach-
ment of the ribs that makes the essential difference between
the dorsal vertebras and the cervical or lumbar. The other
extremity of each rib is connected with a cartilage, which is
a sort of continuation of it ; in Birds, the cartilages of the
ribs are ossified or converted into bone. The cartilages of
the first seven ribs (in Man), which are termed the true ribs,
are united to the sternum or breast-bone, which forms the
front wall of the thorax (fig. 163). The cartilages of the five
lower ribs are not directly connected with this, and they are
hence called false ribs ; those of three of them, however, are

1 It is scarcely necessary here to state, that the common notion
respecting the deficiency of a rib on one side of the body of Man is a
popular error.

476 RIBS AND STERNUM. BONES OF SHOULDER.

connected with the cartilage of the seventh rib ; while the
other two ribs, being completely unattached at the anterior
ends, are termed floating ribs. The sternum or breast-bone
is flat and of simple form in Man ; but it is much larger in
many other animals. In those which have need of great
strength in the upper limbs, such as Birds, Bats, and Moles,
it is not only increased in breadth, but is furnished with a
projecting keel or ridge for the attachment of powerful muscles
(fig. 250). In the Turtle tribe, on the contrary, it is very
much extended on the sides, so as to afford, with the ribs, a
complete protection to the contained parts (§ 83).

634. We have next to consider the structure of the members
or appendages which are attached to this central framework.
These are spoken-of as superior and inferior, when we are
treating of Man, whose erect posture places one pair above
the other : but when the ordinary Quadrupeds are alluded to,
they are termed anterior and posterior, one pair being in front
of the other. Each member consists of a set of movable
bones, which serve as levers ; but the socket in which the
first of these works, is formed by a bony framework, which is
connected more or less closely with the spinal column. This
framework, in the upper extremity, consists of the Scapula
or blade-bone, and the Clavicle or collar-bone. In the lower
extremity, it is formed by a set of bones, the union of which
with the sacrum completes the Pelvis or bason at the bottom
of the spinal column (fig. 223).

635. The Scapula is a large flat bone, which occupies the
upper and external1 part of the back. Its form is somewhat
triangular ; and at its upper and outer angle is a broad but
shallow cavity, destined to receive the head of the humerus
or arm-bone. Above this cavity is a large projection, termed
the acromion-process, which is united by ligaments, &c , with
the external end of the clavicle, and thus forms the bony
eminence that we feel at the top of the shoulder. A little
internally to this we find another process, the coracoid, which
only serves in Man for the attachment of certain muscles, but
which in Birds is developed into a distinct bone (§ 668).
The hinder surface of the scapula is divided into two by a
projecting ridge or keel, which gives a more extensive and

1 The term external is continually used in Anatomy, to describe the
parts furthest removed from the central or median line of the body.

MUSCLES OF THE BACK OF THE TRUNK,

477

Fig. 226 — MUSCLES OF THE BACK OF THE TRUNK, THE SUPERFICIAL LATER BEING

SHOWN ON THE LEFT SIDE, THE Mll>DLE LAYER ON THE RIGHT.

I. Trapezius ; 2, occipital bone; 3, spine of the scapula; 4, acromipn process;
5, deltoid; 6, infraspinatus ; 7, teres minor; 8, teres major; 9, latissimus dorsi;
10, its aponeurosis ; 11, glutaeus ma^nus ; 12, space between the latissimus dorsi
and external oblique; 13, rhomboideus major ; 14, splenius; 15, angularis scapulae ;
16, rhomboideus minor; 17, external oblique; 18, supraspinatus ; 19, serratus
magnus ; 22, inferior serratus minor; 23, internal ob'ique; 24, crest of the iliac
bone; 25, glutasus medius; 26, pyramidalis; 27, gemellus superior; 28, obturator
internus; 29, gemellus inferior; 30, quadratus femoris; 31, semi-membranosus ;
32, tuberosity of the ischium ; 33, insertion of the glutaeus maximus ; 38, long
head of the triceps extensor of the arm.

478 BONES OF SHOULDER I SCAPULA AND CLAVICLE.

firmer attachment to the muscles that arise from it (fig. 226, 3).
The scapula is never deficient in animals that possess a superior
extremity, though sometimes it is very narrow. The muscles
attached to it are chiefly those which draw the arm upwards,
and which turn it on its axis. In Man, their actions are
very numerous and varied ; but in animals that only use
their extremities for giving motion to the body, the muscular
apparatus is much simpler, and the scapula is narrower (fig.
229, o). This is particularly the case in Birds (fig. 250, o),
the raising of whose wings in flight is an action that requires
very little power, though for their depression or pulling-down
great muscular force is needed.

636. The Clavicle is a rounded bone, attached at one ex-
tremity to the acromion-process of the scapula, and at the
other to the top of the sternum. Its principal use is to keep
the shoulders separate ; and we accordingly find it strongest
in those animals, the actions of whose superior extremities
tend to draw them together ; whilst it is comparatively weak
or altogether deficient in animals, the actions of whose limbs
naturally tend to keep them asunder. In Birds, the violent
drawing-down of whose wings in flight would tend to bring
the shoulders together if they were not prevented, there is
not only a strong clavicle, but usually a second bone having
a similar function (§ 668). In the Horse and other animals,
on the contrary, the bearing of whose weight on their fore-
legs tends rather to separate the shoulders than to bring them
together, the clavicle is deficient.

637. The Scapula is connected with the central framework
of the skeleton by various muscles (fig. 226), which pass
towards it from the spinal column and ribs, and which serve
alike to fix it, and to assist in sustaining the weight which it
sometimes has to bear. In Man these are numerous, and their
actions are various ; since the scapula is left very movable in
him, that the actions of the arm may be more free. In
Quadrupeds it is generally more fixed ; and the trunk is slung
from it, as it were, by a muscle (the serratus magnus, 9) of mode-
rate thickness in Man, but in these animals of great strength,
which passes from the scapula to be attached to the ribs.

638. The superior or anterior member itself is divided into
three principal portions, — the arm, fore-arm, and hand. The
arm is supported by a single long and cylindrical bone, which

MUSCLES OF THE FRONT OF THE TRUNK. 479

is called the kumerus ; this has a large rounded head, which
is received into the glenoid cavity of the scapula ; whilst its
lower end is rather flattened, so as to articulate with the two
bones of the fore-arm in the hinge-joint of the elhow. The

Fig. 227.— MUSCLES OF THE FRONT OF THE TRUNK, THE SUPERFICIAL LATER

BEING SHOWN ON THE LEFT SIDE OF THE FIGURE, THE MIDDLE LAYER ON THE
RIGHT.

1, sternum; 2, sternal portion of the pectoralis major; 3, clavicular portion of the
same muscle; 4, space between the deltoid and pectoral muscles; 5, deltoid; 6, 6,
clavicles; 7, external oblique; 8, digitations of the serratus magnus ; 9, 9, latissi-
musdorsi; 10, aponeurosis of the external oblique; 11, linea alba; 12, umbilicus;
13, pectoralis minor; 14, 14, crests of the iliac bone; 15, symphysis of the pubis ;
16, crural arches; 17, rectus abdominis; 18, inferior oblique; 20, internal inter-
costal; 21, external intercostal; 22, infra-clavicular; 23, coracoid process; 21, in-
ferior border of the internal oblique and transversalis muscles; 25, short head
of the biceps ; 26, long head of the biceps ; 27, biceps ; 28, sternomastoid ; 32,
adductor of the thigh; 33, rectus femoris.

480 BONES AND MUSCLES OF UPPER EXTREMITY.

muscles which move it are for the most part attached to its upper
third ; and the chief of them are the pectoralis major (fig. 227,
2, 3) which rises from the sternum and cartilages of the ribs,
and consequently draws the arm forwards, inwards, and down-
wards,— the latissimus dorsi (fig. 226, 9), which rises from the
spinal column and hinder part of the ribs, and consequently
draws the arm backwards, inwards, and downwards, — and the
deltoid (fig. 226, 5), which arises from the upper edge of the
clavicle, and from the ridge of the scapula, and is the chief
muscle concerned in raising the arm. The first of these forms
the principal part of the fleshy mass upon the front of the
chest, and is the muscle which is so remarkably developed in
Birds. It forms also the front boundary of the axillary space,
or hollow of the arm-pit, the hinder boundary of which is
formed by the second muscle. This space, of which we can
distinctly feel the front and back walls when we raise the
arm a little from the side, contains the large vessels and
nerves proceeding to the arm, and also a number of lymphatic
glands (§ 219). The deltoid muscle forms the thick fleshy
mass on the top of the shoulder and on the upper part of the
outside of the arm.

639. In the fore-arm of Man there are two long bones,
termed the Radius and the Ulna, which lie nearly parallel to
each other ; the radius being on the outer or thumb side of the
fore-arm, and the ulna on the inner. They are connected
with one another, not only by ligaments at their extremities,
but by a strong fibrous membrane that passes between their
adjacent edges, along their entire length. Nevertheless they
have considerable freedom of motion, not only upon the
humerus, but upon each other; so as to give to the fore -arm
the power of rotation on its own axis, by which either the
palm or the back of the hand may be turned upwards. The
ulna is connected with the humerus, at the elbow, by means
of a hinge-joint, into which the radius does not enter ; but it
is the radius with which the hand is connected at the wrist,
by a kind of ball-and-socket joint, the ulna having no direct
share in this articulation : hence, while both bones move
together in bending or straightening the elbow, we can make
the radius roll round the ulna, carrying the hand with it.
This movement is one of very great importance, in rendering
the hand capable of a great variety of uses to which it would

BONES AND MUSCLES OF ARM AND HAND. 481

be otherwise inapplicable. It is only among the higher
orders of Quadrupeds, however, that it can possibly be exe-
cuted ; for in the lower, the two bones are united more or less
completely into one, or are articulated in such a manner as to
be incapable of rotation.

640. The fore-arm is bent upon the arm, chiefly by muscles
that lie upon the front of the latter ; of these the principal is
the biceps or two-headed muscle (7, fig. 227), which arises from
the coracoid process of the scapula, and from the top of the
glenoid cavity, and is inserted into the radius a little in front
of the elbow, forming a great part of the fleshy mass in front
of the arm (fig. 2 1 9). The arm is straightened again by a large
muscle, the triceps or three-headed muscle, which arises from
the back of the humerus and scapula, and passes down to be
inserted into a projection of the ulna behind the elbow-joint,
forming the fleshy mass at the back of the arm. The muscles
which rotate the fore-arm arise from the lower end of the
humerus, or from one of its own bones, and pass obliquely
across to the other.

641. The Hand is anatomically divided into three portions,
— the carpus^ metacarpus, and phalanges (fig. 223). The
carpus, which is the portion nearest the wrist-joint, is com-
posed of eight small short bones, which are firmly united to
each other by ligaments, but yet have a certain degree of
motion permitted them ; these are arranged in two rows, of
which one has a rounded surface, and enters into the forma-
tion of the wrist-joint ; whilst the other has a series of shal-
low pits, to receive the rounded heads of the metacarpal
bones. These last almost precisely resemble the bones of the
fingers, and in the skeleton might be mistaken for their first
joints ; but with the exception of that of the thumb they are
all united to each other by ligaments and muscles, so as to
form the compact framework which gives support to the palm
of the hand. The metacarpal bone of the thumb is much
more free in its movements ; and it is chiefly by an alteration
in its direction, that the thumb can be opposed to the fingers.
The thumb and fingers are formed by a series of small bones
which are termed the phalanges; of these there are only two
in the thumb, whilst there are three in the fingers. They are
bent on each other chiefly by the action of the muscles that
occupy the front of the fore-arm ; and they are extended or

i i

482

MUSCLES OF THE HAND.

straightened by others that lie along its back. These termi-
nate in long tendons, which are bound down at the wrist by
a fibrous band that stretches between the bony projections on

either side, and is termed
the annular (or ring-like)
ligament (fig. 228). The
tendons then spread asunder
in the hand, and pass-on to
be inserted into the bones
of the several fingers, being
reinforced by a set of small
muscles that arise from the
hand itself.

642. When we consider
the superior extremity of
Man as a whole, we remark
that the several levers which
are joined end-to-end to form
it, diminish progressively in
length. Thus the arm is
longer than the fore-arm;
the latter is longer than the
wrist ; and each of the pha-

Fig. 228.— MUSCLES OP THE PALM OF THE ,

HAND (SUPERFICIAL LAYER) lailgCS IS longer than the

1, anterior annular ligament of the carpus; one which SUCCCeds it. The
2, 2, extremities of the short abductor of f ,-1 • -> rrn-m n-n4-

the thumb, the intermediate body of the purpose of this arrangement

muscle having been removed; 3, opposing ig very evident. The nume-
muscle of the thumb; 4, short flexor of . J. . . -,,

the thumb; 5, adductor of the thumb ; 6, roUS joints, in the neighDOUr-

lower border of the same muscle; 7, 7, "U^^rl nf oar>Ti nfhpr wliiph

lumbricales; 8, one of the tendons of the h°0(i Ol eacn Otner> Wm

deep flexor of the fingers, passing-on to we S66 towards the extremity
insert itself in the bone of the third pha- f ,, i- "U ^ ^U Uc, O^TTQT-Q!

lanx, after perforating the tendon of the of the limb, permit its Several

superficial flexor ; 9, tendon of the long portions to change their place

flexor of the thumb; 10, adductor of the f

little finger; 11, short flexor of the little in VariOUS Ways, SO as to aC-

themselves to

the form of the body which
it is desired to grasp ; whilst the long levers formed by the
arm and fore-arm, allow the place of the entire hand to be
rapidly changed to a considerable extent. It is principally
by the movements of the humerus upon the scapula, that
the direction of the limb is given ; the bending or straight-
ening of the limb regulates its length; whilst the move-

PECULIAR ENDOWMENTS OP HUMAN HAND. 483

ments of the thumb and fingers are concerned in its particular
applications.

643. The hand of Man is distinguished from the extremity
of most Quadrupeds by its possession of an opposable thumb,
— that is, of a finger which can be made to act in a direction
opposite to that of the rest. But among the Apes and
Monkeys, we find this peculiarity not only in the superior
extremity, but also in the inferior ; whence these animals are
said to be quadrumanous or four-handed, whilst Man is
bimanous, possessing two hands only. It must not be sup-
posed, however, that Apes and Monkeys are superior in this
respect to Man ; for they possess this distinguishing character
in a much less striking degree than he does. All the four
extremities of Apes and Monkeys possess the power of grasp-
ing, but they are all used also for support ; and we find that
in consequence of the shortness of the thumb and great toe, the
grasping power is very inferior to that which Man possesses.
But of the four extremities of Man, one pair is specially adapted
for support, and the other for prehension or grasping ; and this
by the length and mobility of the thumb, which is capable of
being brought into exact opposition to the extremities of
all the fingers, whether singly or in combination. But even
in those Quadrumana which most nearly approach Man,
the thumb is so short and weak, and the fingers so long
and slender, that their tips can scarcely be opposed to each
other, and then with only a slight degree of force ; hence,
although completely adapted for clinging round bodies of
a certain size, — such as the small branches of trees, &c. — the
extremities of the Quadrumana can neither seize very minute
objects with that precision, nor support large ones with that
firmness, which is essential to the dexterous performance of a
variety of actions for which the hand of Man is admirably
suited. Hence they may be more appropriately termed
claspers than hands.

644. In many of the inferior Mammalia, whose extremities
are adapted for support only, we find each row of phalanges
consolidated into two bones, or even into one. This is the
case, for example, in the Ruminant Quadrupeds, as the Camel
(fig. 229), and in the Horse (§ 652). Such an arrangement
obviously increases the firmness of the limb, though it
altogether deprives it of prehensile power. In other in-

i I 2

484

EXTREMITIES OF LOWER ANIMALS.

stances, we find the number of bones in the hand increased,
but all of them enclosed in one envelope, so that the fingers
are not separate. This is the case with many aquatic animals

Fig. 229.— SKELETON OF THE CAMEL.

vc, cervical vertebrae ; v d, dorsal vertebrae ; vl, lumbar vertebras ; vs. sacral vertebrae ;
vq, caudal vertebras; s, scapula; h, humerus; cu, ulna; ca, carpus; me, meta-
carpus ; ph, phalanges ; fe, femur ; ro, patella ; ti, tibia ; ta, tarsus ; nut, metatarsus.

— such as the Whale tribe among Mammals, Turtles among
Reptiles, and Fishes in general, — in which the hand is made
to serve as a fin or paddle. In most of these, the bones of
the arm are very short ; and the movements of the extremity
are chiefly confined to the wrist-joint.

645. The structure of the lower extremities has a very great
analogy to that of the upper \ and the principal differences to
be remarked between them, are such as are necessary to give
to the former more solidity at the expense of freedom of
motion, and to make them organs of locomotion instead of
organs of prehension. Here, too, we have a bony framework,
for the purpose of connecting the limb itself with the spine ;
and as the weight of the body is constantly thrown upon the

BONES AND MUSCLES OF LOWER EXTREMITY. 485

lower extremities, this framework is much more firmly at-
tached to that of the trunk, than is the case with that which
supports the arms. It consists, on each side, of a bone which
in the adult state is single, though at an early age it is com-
posed of three distinct pieces ; and this is closely connected
with the sacrum behind, while it meets with its fellow in
front in such a manner as to form a sort of bason termed the
Pelvis. The spreading sides of this, formed by the iliac bones
(Fig. 213), afford support above to the viscera contained in
the abdomen ; and they give attachment by both surfaces to
large muscles by which the thigh-bone is moved, and by their
edges to large expanded muscles that pass upwards to the ribs
and sternum, and form the walls of the abdomen. Below this
spreading portion, we find the articular cavity of the thigh-
bone, which is so deep as almost to form a hemispheric cup
when it is completed by its cartilaginous border. The move-
ments of the thigh-bone are consequently more limited than
those of the arm ; but it is much less liable to displacement.

646. The thigh, like the arm, contains but a single bone,
which is named the Femur. Its upper extremity is bent at
an angle ; and its rounded head is separated from the rest by
a narrow portion which is termed its neck. At the point
where this neck joins the shaft of the bone, there are two
large projections termed trochanters, one on the outer side and
the other on the inner ; these serve to give attachment to the
muscles by which the thigh is moved. Of these muscles, one
descends from the lumbar vertebrae, and passes-down with
another that rises from the upper expanded surface of the
pelvis, over the front border of the pelvis, to be attached
to the smaller and interior of the projections just mentioned ;
these with the assistance of other muscles raise or draw
forwards the thigh, — an action which does not require in Man
to be performed with any great force. The muscles which
draw back the thigh, on the other hand, arise from the under
surface and back of the pelvis, where they form a very thick
fleshy mass (u, 25, fig. 226) ; and they pass to the larger and
external projection, and to a ridge which runs from it down
the thigh-bone. Other muscles which arise from the lower
border of the pelvis, serve to rotate the thigh upon its axis.
The lower end of the thigh-bone spreads into two large condyles,
on which the principal bone of the leg moves backwards and

486 BONES AND MUSCLES OF THE LOWER EXTREMITY.

forwards. The knee is a good example of a pure hinge-joint,
all its movements being restricted to one plane.

647. The leg, although containing two bones like the fore-
arm, does not in Man possess the peculiar movement which
characterises it. One of these bones, called the Tibia, is much
larger than the other which is called the Fibula ; and it is
the former alone on which the thigh-bone rests, and which
itself rests upon the foot, so that no movement of rota-
tion is permitted in the leg. In fact, the fibula, which is
a long slender bone running nearly parallel with the tibia
(fig. 223), looks like a mere appendage or rudiment, and
serves only for the attachment of muscles. The upper end of
the tibia is broad, and has two shallow excavations, in which
the condyles of the femur are received. Upon the front of
the knee-joint we find a small separate bone, the patella or
knee-pan ; the purpose of this is to change the direction of
the tendons that come down from the front of the thigh to be
attached to the tibia ; in such a manner as to enable them to
act more advantageously, upon the principle formerly stated
(§. 611). In the elbow-joint, this change was not required ;
since the ulna projects sufficiently far backwards to afford ad-
vantageous attachment to the tendon of the extensor muscle.
— The very powerful muscles which tend to straighten the
knee-joint, • arise from the front of the pelvis and from the
femur itself j and they form the fleshy mass of the front
of the thigh. On the other hand, those which bend the knee
arise from the lower border of the pelvis and from the back of
the thigh-bone, and pass downwards to be inserted into the
sides of the tibia and fibula a little below the knee, then-
tendons forming the two strong cords known as the hamstrings.
The articulating surface at the lower extremity of the leg,
which enters into the ankle-joint, is principally formed by the
tibia j but its outer border is formed by the fibula, which
there makes a considerable projection that can be felt through
the skin. — In the Quadrumana, and in a less degree in some
other Mammals, the two bones of the leg resemble those
of the fore-arm ; and are so articulated as to give to the foot
a power of rotation corresponding with that of the hand.

648. The Foot is composed, like the hand, of three distinct
portions, which are called the tarsus, metatarsus, and. phalanges.
There are seven bones in the tarsus, all of which are larger

BONES AND MUSCLES OF THE FOOT.

487

than those of the carpus, and some of them of considerable
size. The articulation with the leg is formed by one of these
only, the astragalus, which projects above the rest, and is im-
bedded between the projecting extremity of the tibia (which
forms the inner boundary of the ankle-joint) and that of the
fibula. The astragalus rests on the os calcis or bone of the
heel, which projects considera-
bly backwards, and is connected
in front with the other bones of
the tarsus. In front of the
tarsus we find the metatarsus,
composed of five long bones,
which in man are all attached
to each other, but of which one
is separate in the Quadrumana,
in order to give freer play to
the great toe, the action of which
resembles that of the thumb.
The toes, like the fingers, are
composed of three phalanges
(with the exception of the
great toe, which has only two) ;
these are in Man much shorter
than those of the hand, and
are evidently not adapted for
prehension • but in many of the
Quadrumana, their length is
nearly equal to that of the
fingers, and the great toe is as
opposable as the thumb. The
foot is far from being thus con-
verted, however, into a perfect
hand ; but it becomes a very
useful instrument for clasping
the small branches and twigs
of the trees among which these
animals live. The foot of Man
is distinguished from theirs, by
its power of being planted flat upon the ground, and thus
affording a firm basis of support. Even the Chimpanzee
and the Orang, when they attempt to walk erect, rest upon

Fig. 230, — MUSCLES OF THE SOLE OF

THE FOOT (MIDDLE LAYER).

1, accessory of the long flexor of the
toes ; 2, tendon of the long flexor
issuing from its sheath ; 3, tendon
of the long flexor of the great toe;
4, first lumbricalis ; 5, tendon of
the superficial flexor, divided be-
hind its perforation ; 6, short flexor
of the little toe ; 7, short flexor of the
great toe; 8, portion of the oblique
abductor of the great toe ; 9, poste-
rior extremity of the fifth metatar-
sal bone ; 10, sheath of the long pero-
neal; 11, os calcis, or bone of the
heel.

488 BONES AND MUSCLES OF THE FOOT.

the side of the foot ; and the absence of a projecting heel
causes them to be very deficient in the power of keeping the
leg upright upon it. For it is to this projection that the
strong muscles of the calf of the leg are fixed, by which the
heel is drawn upwards or the leg drawn back upon it. Other
muscles at the side and back of the leg, the direction of whose
tendons is changed by a sort of pulley at the ankle-joint,
aided by the muscles of the foot itself, serve to bend the toes,
— an action which gives great assistance in walking, running,
leaping, &c. And the toes are straightened by an extensor
muscle, which lies on the front of the leg, and of which
the tendon runs under an annular ligament that encircles
the ankle, and is then divided and spread -out to the
toes, over the upper surface of the foot. The great toe is
a very important instrument in the act of walking, since
much of the spring forwards is given by the bending of
its phalanges ; and it is provided with two flexor muscles
of its own.

649. On the internal side of the foot, the bones of the
tarsus and metatarsus form a kind of vault or arch, which
serves to lodge and protect the vessels and nerves that
descend from the leg towards the toes. This arch further
serves the important purpose of deadening the shock that
would otherwise be experienced every time that the foot is
put to the ground ; for, by the elasticity of the ligaments
which hold together the bones that compose it, a sort of
spring is formed, which yields for a moment to the shock,
and then recovers itself. We feel the difference which this
makes, when we jump from a height upon our heels ; the jar
is then propagated directly upwards from the heel to the leg,
thence to the thigh, and thence to the spinal column , and if
it were not from the peculiar manner in which this is con-
structed (§ 631), a severe shock of this kind might produce
fatal effects by concussion (or shaking) of the brain. In
animals which walk upon four extremities, the difference of
direction in which the legs are connected with the spine
prevents a jar from being propagated along the latter to a
similar degree. But in those which are destined to obtain
their food by sudden and extensive leaps, such as the animals
of the Cat tribe (the Lion, Tiger, &c.), we find an arrange-
ment of the bones of the foot, well adapted to diminish the

STANDING POSTURE : EQUILIBRIUM. 489

shock produced by the sudden descent of the body upon the
ground.

Of the Attitudes of the body, and the various kinds of Locomotion.

650. A small number of Vertebrated animals, — Serpents,
for instance, — bear habitually on the whole length of their
bodies, which rest entirely on the ground ; and their only
movements are effected by undulations of the spinal column.
But the rest are supported upon their extremities ; and we
give the name of standing to that position in which the
animal rests supported by its limbs upon the ground or on
any firm horizontal basis. In maintaining this position, the
extensor muscles, by which the joints are straightened, must be
in continual action, since the limbs would otherwise bend
beneath the weight of the body. Now as the sense of
fatigue, in any set of muscles, depends in great degree upon
the length of time during which they have been in continuous
action, the maintenance of the standing posture for a long
period is, in most animals, more fatiguing than walking ;
since in the latter exercise the action of the flexors alternates
with that of the extensors.

651. But this condition is not the only one essential to
steadiness in the standing posture ; for in order that the
body may rest firmly upon the members, it must be in equi-
librium. It has been shown (MEGHAN. PHILOS. Chap, iv.)
that equilibrium exists, — or in other words, that a body
remains at rest in its position, — not only when it bears upon
the whole of a broad sur-
face, but also when it is

so placed that the tenden-
cies of its different parts
to descend or gravitate
towards the earth counter-
balance each other. This
is the case when its centre
of gravity is supported, —
that is, when a line drawn

perpendicularly from that centre falls within the base. In
order, then, that an animal may rest in equilibrium on its legs,
it is necessary that the vertical line from its centre of
gravity (or line of direction) should fall within the space

490 EQUILIBRIUM OF ANIMALS : BASE OF SUPPORT.

which its feet cover and inclose between them ; and the
wider this space, in proportion to the height of the centre of
gravity, the more stable will the equilibrium be, since the
body may be more displaced without being upset. Thus in
fig. 231 the table a must be upset ; because the line of direc-
tion e from the centre of gravity c falls outside the base of
support d ; whilst the table b, although equally inclined, will
not be upset but will return to its proper place, because the
line of direction e from its centre of gravity c falls within its
base d. Hence an animal which is supported upon four legs
will stand much more firmly than one which rests on two
only ; since its real base is the whole space included between
its four points of support. And again, an animal is more
firm when standing upon two legs, than when resting upon
one only.

652. Moreover when an animal rests upon four legs, the
extent of its base is but little influenced by the size of the

feet ; and thus to render them
broad would be to increase
their weight without adding
much to their use as supports.
This is easily understood by
comparing a quadruped to a
four-legged table ; if the legs
are sufficiently strong to support
the weight that rests upon
them, it matters little in regard
to the steadiness of the table,
whether they bear upon the
ground by mere points or by flat
surfaces j since it is the large sur-
face that would be enclosed by
lines joining them, which consti-
tutes the real base. Hence we
SE. find that, in most quadrupeds,
the limbs only touch the ground
by slightly-dilated extremities ; and the number of fingers is
reduced more and more, without diminishing their effect as
instruments of locomotion. Thus in Ruminant animals, as
the Deer, the number of toes is reduced to two in each foot, as
•seen in fig. 232, where £ represents the tibia, ta the bones of

Fig. 232.
FOOT OF DEEB

233

EQUILIBRIUM OF ANIMALS : - BASE OF SUPPORT. 491

the tarsus, c the bone of the metatarsus termed the canon (in
which the trace of a division into two pieces can be seen),
and p, pi, pt, the three phalanges of the toes, of which the
last is enveloped in the hoof, which is nothing else than a
large nail inclosing the whole extremity of the toe. In the
Horse this consolidation is carried still further than in the
Ruminants, for it has only one toe in each foot (fig. 233); but
we see the rudiment of an additional bone in the metatarsus
5, which is commonly termed the splint bone.

653. But when an animal is supported upon two feet only,
whatever may be their degree of separation from each other,
the base of support cannot have sufficient extent, unless the
extremities touch the ground by a considerable surface. This
is the case with the foot of Man, and still more with that of
many Birds which habitually stand
upon one leg (fig. 234). In. order
that an animal may hold itself in
equilibrium upon a single limb, it
is necessary that the foot should
be placed vertically beneath the
centre of gravity of the body ;
and that its muscles should be so
arranged as to permit it to keep
this limb inflexible and immov-
able. Man can accomplish this,
for the centre of gravity of his
body is at about the middle of the
pelvis ; and to place this vertically
over one foot, it is sufficient for
him to bend himself a little from
the side which is not supported.
But the greater number of Qua-
drupeds are destitute of the power
of doing this ; and a large part
of them cannot even raise them-
selves on their hind legs, on
account of the direction of these members relatively to the
trunk; or if they can do so for an instant, they cannot
maintain themselves in this position. The reason of this is
very simple. The base of support, on account of the small-
ness of the feet, is very narrpw, and the centre of gravity of

492 MUSCULAR EXERTION TO MAINTAIN EQUILIBRIUM.

the body is placed near the front ; hence the body must be
entirely changed in its position by a violent and not sustain-
able action of the muscles which connect it with the hind
legs ; and, when thus reared up, it cannot rest with firmness
on account of the narrowness of the base.

654. There are some Quadrupeds, however, which are able
to raise themselves occasionally into this position ; this is the
case, not only with the Quadrumana, but also with the Bear,
Squirrel, and other animals whose habits require them to
ascend and live among trees, — as well as in the Kangaroo,
and animals constructed upon the same plan, whose peculiar
organisation will be presently considered (§661). In standing
upright, the muscles of the back part of the neck are kept in
a contracted state, to retain the head in equilibrium on the
vertebral column ; and the extensor muscles of that column
must also be kept in action, to prevent it from bending
forwards under the weight of the head, upper extremities,
and viscera of the trunk. The whole weight of the upper
part of the body is thus transmitted to the sacrum, and thence
to the other bones of the pelvis, by which it is brought to
bear on the femur. If left to themselves, the thigh-bones
would bend beneath the pelvis, and the trunk would fall
forwards ; but the contraction of their extensor muscles keeps
them firm. In the same manner, the extensor muscles of the
knee and ankle keep these joints from yielding beneath the
weight of the body, which is thus at last transmitted to the
ground. The sitting posture is less fatiguing than the stand-
ing position, because the weight of the body is then directly
transmitted from the pelvis to the base of support, so that it
is not requisite for the extensor muscles of the lower limbs
to keep-up a sustained action. But the lying posture is that
of the most complete rest ; because the weight of every part
of the body is at once transmitted to the surface on which it
bears, and no muscular movement is requisite to keep it in its
position.

655. This difference in muscular effort, is the cause of a
well-marked variation in the pulse, according to the position
in which the body is at the time. From a considerable
number of observations it has been found that the average
pulse of an adult man is about 81 when standing, 71 when
sitting, and 66 when lying ; so, that the difference between

PULSE IN DIFFERENT POSTURES : ACT OF WALKING. 493

standing and sitting is 10 beats of l-8th of the whole,
whilst the difference between sitting and lying is 5 beats or
1-1 3th of the whole. In the female, the pulse is quicker in
each position by from 10 to 14 beats per minute ; but the
differences occasioned by position are nearly the same. It
will be observed that the difference between standing and
sitting is greater than that between sitting and lying ; and
this closely corresponds with the relative amounts of muscular
exertion required in these positions respectively. At the
moment when the posture is changed, the pulse is considerably
quickened, in consequence of the muscular effort required for
the purpose, which acts especially on the veins, and forces the
blood more rapidly back to the heart (§ 279); but this
increase in rapidity is temporary only.

656. All that has been said of the positions of Vertebrated
animals applies equally well to those of the Invertebrata,
which like them have the body raised from the ground upon
extremities. This is the case in the higher Articulata, such
as Insects, Crustacea, Arachnida, and Myriapoda. But the
lower Articulata crawl, like Serpents, upon the whole length
of their bodies ; or, being aquatic, are buoyed-up by the
element they inhabit. And among the Mollusca and Eadiata,
there are none that have members upon which they can be
said to stand.

657. The progressive movements by which the bodies of
Man and other animals are made to change their places, are
accomplished by means of the alternate contractions and
extensions of those limbs, which we have hitherto considered
only as supporting them in a rigid position. It is easy to see
that when a joint is straightened after being bent, the two
ends of the levers which form it must be separated from each
other, and that motion must thus be given to the parts against
which one or both of them bear. Now in the ordinary move-
ments of progression, one of these levers bears against the
ground, which is immovable ; and the whole motion produced
by straightening the joint must consequently be communicated
to the body. In the ordinary act of walking, one of the feet
is planted in front, whilst the other is extended or carried
backwards beneath the leg, by the action of the muscles of
the calf aided by those of the toes (§. 648). Its length is
thus increased j and as it bears upon the resisting soil, this

494 ACT OF WALKING : — OTHER MODES OF LOCOMOTION.

elongation acts through the thigh upon the pelvis, and thus
carries forward the whole body. At the same time, the pelvis
makes a slight turn upon the femur of the other side on
which it is resting ; and the limb which was at first behind
the other, is now drawn forward by a flexion of its joints,
and is planted on the ground in front of the other, so as to
serve for the support of the body in its turn ; whilst the
other, by extending itself, gives a fresh forward impulse to
the body. Thus each limb is alternately made to support the
whole weight of the body, just as it would do in standing on
one leg ; while at the same time the other is engaged in
urging it forwards. Hence the centre, of gravity must vibrate
a little from side to side in the act of walking, so that it may
be brought alternately over each foot ; and this movement
from side to side is the more obvious, in proportion as the
pelvis is wider, and the limbs more separated from each other.
Hence it is more seen in women than in men, on account of
the greater proportional breadth of the hips in the former.

658. In all the higher animals, as in Man, there are
members which serve for locomotion ; but the nature of these
movements varies greatly ; and there is a corresponding differ-
ence in the structure of the instruments by which they are
performed. The manner in which the Creator has made the
same organs answer a variety of different purposes, in accord-
ance with the habits of the animals to which they belong, is
a most interesting object of study ; for we see the most
varied results attained, without the least departure from the
general plan which has been adopted in the construction of
the various species of the same group ; and this solely by
slight changes in the forms and proportions of some of the
instruments whose union makes-up the entire body. The
organs of locomotion in the Mammalia furnish us with
obvious examples of this principle. This class includes not
only the quadrupeds which run or bound along the surface of
the ground, — but animals which are destined to live solely in
water like fishes, — others which sometimes swim through that
element and sometimes inhabit the land, — others which
possess wings that enable them to fly through the air like
birds, — and others which only employ their anterior members
for grasping or feeling ; yet in all these animals, these organs
are constructed of the same parts. In the paddles of a Seal

ACTS OF WALKING AND KUNNING. 495

(fig. 240), the wing of a Bat (fig. 251), and the fore-paw of a
Squirrel or a Mole, we find the same bones as in the arm of
Man (fig. 223). And even in the fore-legs of the Ruminant
Quadrupeds, and in those of the Solidungula, or single-toed
animals (such as the Horse), we can usually perceive traces of
the existence of three or four toes, whose bones are more or
less completely united.

659. From what has already been stated as to the influence
of the length of the levers on the quickness of the movement
of the extremities (§. 614), it is easy to see that animals
which have the most rapid progression must necessarily have
long members ; since, the quickness with which the extensor
muscles act remaining the same, the change of place in the
free extremity of the lever will be greater, in proportion as
that extremity is more distant from the point of insertion of
the muscles that move it, and from the fulcrum on which the
tever works. But in proportion to the elongation of this arm
of the lever, must be the increase in the power of the muscles
that move it, in order to overcome the same resistance ;
according to the general principle that what is gained in
velocity is lost in power. Hence, in order to endow an animal
with great agility, it is only necessary to lengthen its limbs,
and to render its muscles capable of exerting a proportional
power.

660. We have seen that in walking, the body is sustained
upon one limb (in quadrupeds, upon one pair of limbs),
whilst it is pushed onwards by the other ; so that it never
ceases to bear upon the ground. In running r, however, the
body of Man momentarily quits its support at intervals ; the
foot in advance not being planted on the ground by the time
that the hinder one quits it. In this action, the Ostrich and
its allies probably surpass all other animals ; as they can out-
strip the fleetest horse at full gallop, or the swiftest greyhound
at its greatest speed. The amble of Quadrupeds is a pace
which resembles the walk or run of bipeds, the two legs on
one side being moved together, whilst the body rests upon the
other. This pace is peculiar to the Giraffe, and to horses
which have been trained to execute it. The trot, however, is
a step of a different and much more secure nature. The fore-
foot of one side is raised and advanced with the hind foot on
the other side ; and when these are set down, the other fore

496 ACTS OF RUNNING AND LEAPING.

and hind feet are raised and advanced together. Now, if we
consider the fore-feet of a horse as constituting the four angles
of a parallelogram, it is easy to see that the base of support,
when the feet are thus raised, will be one of its diagonals ;
and as the feet are alternately advanced, the weight will
alternately be thrown upon these two lines. But the centre
of gravity in the horse, especially when carrying a rider, is in
a point almost exactly above that at which the two diagonals
cross ; so that it is always supported either by the one or the
other. The gallop of greatest speed is a run performed on
the same plan as the trot ; — that is, the right fore and left
hind feet leave and reach the ground together, and then the
left fore and right hind feet are advanced. The canter is a
kind of step altogether different. The four legs strike the
ground successively, the left hind foot reaching it first, the
right hind foot second, the left fore foot third, and the right
fore foot fourth. — The celebrated race-horse Eclipse, when
galloping at liberty and with his greatest speed, passed over
the space of 25 feet at each stride or leap ; this he repeated
2jL times in a second, so as to pass over 58 feet in that time,
which was at the rate of nearly 4 miles in six minutes and
two seconds. But this performance was completely surpassed
by that of Flying Childers, who was computed to have
passed over 82| feet in a second, or nearly a mile in a
minute.

661. In leaping, the body is projected into the air by the
sudden extension of the joints, especially those of the hinder
part of the body which had been previously bent ; and having
traversed a greater or less distance, the body comes again to
the ground and may be again projected. This is a kind of
motion usually practised by many animals whose structure is
expressly adapted to it. Thus among Mammals we find se-
veral in which the hind legs are enormously elongated, for
the purpose of giving greater quickness to the motion of the
body ; and their muscles are developed to an extraordinary
degree in order to supply the necessary force. This is the
case among most of the animals of the order Rodentia, such
as the Hare, Rabbit, Squirrel, &c. ; but particularly in the
Jerboa or Jumping Rat, and in the Kangaroo and its allies.
In these animals the fore feet, which are little used for pro-
gression, are comparatively small j and in the last they are

ACTS OF RUNNING AND LEAPING : KANGAROO.

497

less than half the length of the hinder limbs (fig. 235). The,
feet, as well as the legs, of the Kangaroo are very long (fig.

Fig. 235.— KANGAROOS.

236), so as to afford (in conjunction with the tail) a firm sup-
port to the animal when preparing to leap. Quadrupeds in

Fig. 236.— SKELETON OF KANGAROO.

which the length of the posterior extremities greatly predomi-
nates over that of the anterior, are observed to descend hills
with difficulty at a rapid pace, since the forward inclination

K K

498

ACTS OF LEAPING : — PLEA : CRICKET.

of their bodies places them in continual danger of oversetting ;
they therefore take a zig-zag course. In ascending a hill,
however, their progression is greatly favoured by the length
of their posterior extremities (fig. 237). The Rabbit, when

Fig. 237. — HARE ASCENDING A HILL.

moving slowly, advances the fore-feet two or three steps
alternately, the posterior limbs remaining inactive ; and the
body having been lengthened by these means, the posterior
legs are suddenly extended together, and then drawn for-
wards : thus the rabbit walks with the fore and leaps with
the hind pair of legs. The Frog moves in a very similar
manner.

662. It is among Insects that we find the most extraordi-
nary powers of leaping, considered with reference to the size

of the animals that
possess them. Thus
the Flea will spring
to a height equal to
200 times the length
of its body. Let us
imagine a Kangaroo
or a Tiger doing the
same ! In many of

Fig.238.-CRiCKET. ^ leaping ingectg>

the hind legs are of great length, as in the Grasshopper
and Cricket tribe (fig. 238); and in one curious family,
that of the Poduras or spring-tails, the leap is accomplished
by the sudden extension of the tail, which is ordinarily bent
under the body (fig. 239). A very remarkable kind of leap is

LEAPING INSECTS. SWIMMING AND PLYING. 499

executed by the Beetles of the family of Elateridce ; the larva
of one species of which devours the roots of wheat, and is
known under the name of the
"wire -worm;" whilst other
species inhabiting tropical cli-
mates, and having the power
of emitting light, are termed
"fire-flies" (§ 397). The legs
of these insects are very short :
so that when they are laid on Fig" 239-

their backs they cannot by means of them recover their
natural position. This they are enabled to do, however, by
their power of jerking backwards the head and upper part
of the thorax, which causes the body to be projected verti-
cally into the air, whence it usually descends with the feet
towards the ground. The leap of the Crickets, Locusts, Frog-
hoppers, &c. is executed more in a horizontal direction ; and
it is assisted by the wings, which bear-up the body whilst it
is moving onwards through the air. In this manner a Locust
can traverse 200 times its length, and a Frog-hopper 250
times ; which is as if a Man were to take a quarter of a mile
at one leap.

663. Swimming and Flying are movements which have
much resemblance to each other ; both being executed in a
fluid medium, which to a certain extent buoys-up the body,
which offers resistance to its progress, and which also offers
something resembling a fixed point against which the mem-
bers may act to propel it. The chief differences between
them depend upon the nature of the medium ; this being
liquid in the one case, and aeriform or gaseous in the other.
The liquid medium affords more support to the body, and a
firmer surface for the action of its propelling organs ; but at
the same time it offers more resistance to its progress. The
movement of a body through the atmosphere, as in flight,
requires a considerable expenditure of power to keep it up ;
and the yielding nature of the element prevents the propelling
organs from acting against a firm surface ; but the onward
movement, in consequence of the slight resistance, is easily
accomplished.

664. When the feet of a Quadruped are to serve both as
walking and swimming organs, the end is accomplished by

K K 2

500 ADAPTATION OP EXTREMITIES FOR SWIMMING.

the spreading-out of the fingers, and their union by means of a
fold of skin which is stretched over them ; as the web of a
swimming Bird is stretched over its toes, so as to make an
oar or paddle of sufficiently wide surface. This is the ease,
for example, in the Ornithorhyncus of Australia, and in the
Otter of our own country. When the members are intended

Fig. 240.— SKELETON OF SEAL.

vc, cervical vertebrae, vd, dorsal vertebrae; vl, lumbar vertebrae; vs, sacral vertebras;
vq, caudal vertebrae ; b, pelvis ; s, sternum; h, humerus; r, radius ; ca, carpus ;
me, metacarpus ; ph, phalanges ; o, scapula ; c, ribs ; /, femur ; r, patella ; t , tibia ;
ta, tarsus ; mt, metatarsus ; ph, phalanges.

exclusively for swimming, however, they undergo more con-
siderable modifications in structure. The parts corresponding
with the arm and fore-arm are very short, and the movements
of the hand are thus limited, but they can be accomplished
with all the more force. But the bones of the hand are large
and spread asunder, and are enclosed in a firm integument
which may even cover their extremities. Sometimes the
number and arrangement of these bones are precisely the
same as in the hand of Man ; this we see in the Seal (fig. 240),
where their extremities are furnished with separate claws that
project beyond the integument. Sometimes the number of
phalanges in the fingers is considerably increased, as in the
Whale ; and in other instances, the fingers are replaced by a
multitude of small rods of bone, enclosed within a continuous
skin, such as we see in the fins of Fishes (fig. 243).

665. In the Seal, which does not depart widely in its
general construction from land quadrupeds, the hind feet are
formed upon the same plan as the fore ; but they are carried

ADAPTATION OF EXTREMITIES FOR SWIMMING. 501

far backwards, so as almost to occupy the position of the tail.
In the Whale and its allies, on the other hand, the posterior
extremities are almost entirely wanting, and the tail is- greatly
prolonged and expanded at its extremity (fig. 241), This

Fig. 241. — SKELETON OF DUGONG.

expansion, however (which is in the horizontal direction, fig.
242), is not supported by bones, except in its centre ; but it
consists internally of cartilages and tendons, which last are
prolonged from a set of very powerful muscles that are at-
tached to the spine, and give to this organ an enormous force

Fig. 242.— TAIL-FIN OF WHALE.

and great variety of motion. The texture of the portion of it
by which the blow is usually given, is such that it can hardly
be injured ; it is so tough that it cannot be torn, and so free
from feeling, that a stroke of it against a hard substance gives
no pain to the animal. If it strike a boat across the middle
with its edge, the boat is cut asunder as clean and suddenly
as if by one stroke of a giant axe ; whereas, if it strike with

502 PROPULSION OF WHALES AND FISHES BY TAIL.

the flat surface, the boat is driven to the depth of many fa-
thoms with the swiftness of an arrow. Hence this tail is a
most efficient instrument for the propulsion of the bulky
body of the Whale through the water ; and it is, in fact, its
principal organ of locomotion. The paddles formed by the
fore-feet are placed near the centre of gravity of the whole
mass ; and thus can readily exert their peculiar action, which
is that of changing the direction of the movement, and espe-
cially of raising and lowering the body.

666. The propulsion of the body by the stroke of the tail
in Whales and Fishes, is effected precisely in the same manner
as the urging-forwards a boat through the water, by the

Fig. 243.— SKELETON OF PERCH.

lateral strokes of an oar at the stern, in the mode commonly
termed sculling. The expansion of the Whale's tail-fin being
horizontal, its stroke is vertical, and may thus readily bring
the animal to the surface of the water for occasional respira-
tion, as well as propel it forwards ; but that of the Fish's
body and tail being vertical, its stroke is horizontal, and its
action will simply be to urge the body through the water.
The power of ascending and descending, as well as of changing
the direction of the motion, is principally due to the side-fins,
which represent the arms and legs. The direction of the
surface and stroke of these side-fins varies in different species.
In the Cod, Halibut, and others, their action appears to be
principally directed towards keeping the body in its right
position in the water; since, without such an action, the body
would be liable to turn over, in consequence of the position

ACTION OF THE FINS OF FISHES : FLYING FISH. 503

of its centre of gravity. In other instances, the pectoral and
ventral fins move in such a manner as to assist the action of
the tail. In the Kays, the pectoral fins are developed to an
enormous extent (fig. 244) ; and being
directed horizontally, their action is vertical
like that of the wings of a bird. They are
furnished with a great number of joints,
by which they are rendered very flexible \
and their surface may be thus increased
during the down-stroke of the fin, and
diminished during the -^p-stroke. If this
were not done, the action of the fins in
elevation would exactly counterbalance the
effect of their depression ; and no movement Flgt 244*~ RAT'
would be produced. The great power of the pectoral fins of
these Fishes seems connected with their want of an air-
bladder, which causes them to require a constant exercise of
force to keep them up in the water. Their propulsion forwards
is chiefly accomplished, as in other Fishes, by the action of the
tail. But sometimes the Rays change their position and swim
sideways, making horizontal strokes with the pectoral fins
(whose surface is then vertical), by which they are moved
through the water, and sustaining themselves by vertical
strokes of the tail, whose surface is then horizontal.

667. The structure of the organs adapted for movement in
air bears great analogy to that of such expanded fins ; and
there are instances in which the same instruments may serve
both purposes. Thus there are Fishes which are able to quit
the water, and execute leaps of considerable length, supported

Fig. 245. — FLYING-FISH.

upon their wing-like pectoral fins. These are known as
Fly ing- Fish (fig. 245) ; but it is not correct to speak of their

504 ORGANS OF FLIGHT : FLYING FISH, PENGUIN, ETC.

movement as one of flight, since it does not appear that they
have any power of propelling themselves in the air ; the
impulse being given at the moment of their quitting the
water, in the manner of a leap. From 50 to 100 yards, how-
ever, are sometimes traversed by the Fish at one leap ; and
the height to which it rises from
the surface of the water is occa-
sionally such as to carry it over the
deck of a ship. On the other hand,
there are several among the diving
Birds which use their wings as
instruments of progression beneath
the water — in other words, as fins.
The most remarkably constructed
of all these is the Penguin (fig.
246), in which the wings are so
short as to be incapable of answer-
ing any other purpose ; but there
are several species in which they
Fig. 246.-PENGUIN. may fce used Qs, organs of flight

in the air, without losing their fin-like power in the water. —
There are several animals that can sustain themselves for a
short time in the air, by the aid of
an expanded surface formed by an
extension of the skin and serving
as a parachute. This is the case,
for instance, with the Galeopithecus,
or Flying Lemur (fig. 247), the
Flying Squirrel, and the Petaurus,
or Flying Phalanger (ZooL. § 314),
which have the skin stretched out
on either side like a cloak, sup-
ported by the anterior and pos-
terior extremities and by the tail.
By this parachute-like surface they
are sustained in extensive leaps
from bough to bough; though it
does not enable them to support

Fig. 247.-GA«oPi.HKCU.. themselves in the air for any length

6f time. In the Draco Volans (fig. 248), a little animal which
lives among the trees of tropical forests, the body is furnished

DRACO VOLANS. WINGS OP BIRDS. 505

with a wing-like appendage on either side, formed by an
expansion of the skin over six lengthened ribs. These
appendages serve as a kind of parachute, on which this little

Fig. 248.— DRACO VOLANS.

animal, not more than a few inches long, flutters from branch
to branch in search of its insect prey, or shoots, like the flying
squirrel, from tree to tree. They cannot be made to strike the
air, and therefore are not true wings ; but they can be folded
up and extended at the will of the animal.

668. True wings, or instruments of propulsion as well
as of support in the air, are found in some members of all
classes of air-breathing Vertebrata; but they are especially
characteristic of the class of Birds, in which the absence of
them is the exception to the general rule, whilst in Mam-
mals and Reptiles it is their presence which constitutes the
exception. These wings are universally formed by some
modification of the anterior extremities, which renders them
unfit to be used as instruments of progression on the ground ;
but the nature of this modification varies considerably. In
the Bird, the required extent of surface is chiefly given by
the feathers ; these are supported upon an anterior member, of
which the arm and fore-arm (especially the latter) constitute
the largest part, the hand being contracted and consolidated.
The general structure of the Bird's skeleton, the whole of
which is modified with special reference to the actions of
flight, is shown in fig. 249, which represents that of the
Vulture. The head is supported upon a very flexible neck, of
which the vertebrae vc are often very numerous. The ver-
tebrae of the back and loins, however, are usually few in

506

SKELETON OF BIRDS.

number, and are connected together very firmly, so as to form
a nearly inflexible column ; and this, again, is closely united
to the sacrum vs. The vertebrae of the tail v% are few in

ta

Fig. 249.— SKELETON OF VULTURE.

vc, cervical vertebrae; vs, sacral vertebrae; vq, caudal vertebrae; cl, clavicle;
h, humerus ; o, fore-arm; ca, carpus; ph, phalanges; st, sternum; /, femur;
t, tibia ; ta, tarsus.

number, and possess little motion. The ribs are very strongly
connected to each other and to the vertebras, and are united
to the sternum st by bony instead of cartilaginous prolonga-
tions. Thus the whole bony apparatus of the trunk is very
strongly knit together ; and the purpose of this is evidently
to give as firm an attachment as possible to the muscles which
move the wings. The sternum is raised into a high keel or
ridge (as is better seen in fig. 250, 6), for the attachment of
the powerful pectoral muscles which draw down the wings ;
and the degree of this projection is proportioned to the power
of flight which the species possesses, — the sternum being flat
(as in Mammals) in birds which, like the Ostrich, have the
wings undeveloped. The scapula (fig. 250, o), to which are

SKELETON OF BIRDS. 507

attached the muscles that raise the wings, is very narrow in

Birds, in accordance with their small demand for muscular

power in this direction. This narrow scapula forms one part

of what is known as the " side-bone ;" the other part c of

which is formed by a bone termed the coracoid, that is only

represented in Man and other Mammals by the short coracoid

process of the scapula

(§ 635). The two clavi-

cles// are united together

where they join the ster-

num, to form the fork-

like bone known as the

" merry - thought/' the

strength of which, like

the projection of the keel

of the sternum, serves

to indicate the power of

flight, by the degree of

resistance which it is ca-

,, „ ~, ,. Fig. 250.— BONES OF THE SHOULDER AND

pable 01 attorning to the BREAST OF BIRDS.

drawing-together of the 0;Scapu]a; c, coracoid bone; /? claviclesunited

Shoulder- JOintS by the at their junction with the summit of the keel

nf fho vkonfnval b of the sternum s, which is connected with
Ot tne ectOial

the ribs by the ossified costal cartilages co.

muscles. The bones of

the pinion consist of the humerus (fig. 249, A), the two bones
of the fore-arm o, the bones of the wrist ca (which are here
scarcely developed), and the bones of the fingers ph, each
joint of which shows indications of being made up of two or
three separate bones united together. In no bird are these
bones ever separated into distinct fingers, since they are never
required for any other purpose than that of supporting the
wing-feathers. — The leg is connected with the spinal column
by a pelvis, of which the iliac bones are greatly lengthened
and firmly attached to the spine, but which is not completed
into a ring by the junction of the bones in front, as in Mam-
mals ; such a completion would have prevented the passage
of the bulky eggs deposited by these animals (§ 755). In the
hinder extremity we find the femur or thigh-bone (principally
concealed in the figure by the bones of the wing), the two
bones of the leg t, which are commonly united in part of their
length, the shank or ancle-bones ta, which are peculiarly

508

SKELETON OF BATS AND PTERODACTYLS.

elongated in the wading birds, and the four separate toes, by
the spread of which the body is firmly supported, though
resting only on two feet.

Fig. 25!.— SKELETON OF BAT. (References as in Fig. 229.)

669. In the Bat (fig. 251), however, the plan is very dif-
ferent. We have here no long stiif feathers, by the projection
of which from the limb itself the surface may be increased to
almost any extent ; but the wing is formed by an expansion

Fig. 252. — SKELETON OF PTERODACTYLS.

of soft and delicate skin over a framework of bones, which
must consequently be made to support it to its very edge.

PTERODACTYLE. WINGS OF INSECTS. 509

This is accomplished by the enormous extension of the bones
of the hand, especially the metacarpal me, which are here
separate ; and the membrane is further sustained by the legs
and tail. The thumb po is not included in the wing, but
serves as a hook by which the animal can suspend itself. —
The only true flying Eeptile is (or rather was) the Pterodac-
tyle,, a kind of winged lizard, which does not now exist, but
of whose character the skeletons that are found imbedded in
the earth afford most convincing proof. The structure of its
wing differed from that of either Birds or Bats ; for it appears,
from the conformation of its anterior member (fig. 252), that
the animal could have used it for resting or walking, the
framework of the wing being formed by the enormous elonga-
tion of one finger only.

Fig. 253.— DRAGON FLY.

670. The wings of Insects (fig. 253) have no correspon-
dence whatever with those of Vertebrata, except in serving
for the like use, and in being composed of an expanded sur-
face of membrane, stretched upon a firm framework. This
framework is not composed of solid pieces jointed together,
but is merely an extension of the air- tubes and vessels within
the body, which are strengthened by a continuation of its
hard envelope. Their only action is a hinge-like movement
at the point where they are united to the body ; and this
is accomplished by powerful muscles contained within the
thorax.

671. In all instances, the action of the wings must be such,
that the air is struck with less force during the up-stroke than

510 POWER OF FLIGHT POSSESSED BY BIRDS.

during the down-stroke ; otherwise the effect of the former
would neutralise that of the latter. This is partly accom-
plished by the great velocity of the down- stroke compared
with the up-stroke, which causes the resistance of the air to be
much greater against the former than against the latter.1 But
it is by the alteration in the surface of the wing, as it acts upon
the air, that the chief difference is made in Birds ; the arrange-
ment of their great feathers being such, that they strike the air
with their flat sides, but present only their edges in rising.
What is called " feathering the oar " in rowing, is a similar
operation, performed with the same intention, and deriving
its name from this resemblance.

672. The degree in which the wings act in raising the body
or in propelling it through the air, varies considerably in
different species, according to the way in which they are set.
Thus in Birds of Prey, which require a rapid horizontal
motion, the surface of the wings is very oblique, so that they
strike backwards as well as downwards, and thus impel the
body forwards whilst sustaining it in the air. Such birds find
a difficulty in rising perpendicularly ; and can in fact only do
so by flying against the wind, which then acts upon the
inclined surface of the wings just as it does upon that of a
kite. On the other hand, the Lark, Quail, and such other
birds as rise to great heights in a direction nearly vertical,
have the wings so disposed as to strike almost directly dowTi-
wards. It has been estimated that a Swallow, when simply
sustaining itself in the air, is obliged to use as much force to
prevent its fall, as would raise its own weight to a height of
about twenty-six feet in a second. Hence, we may form some
idea of the enormous expenditure of force which must take
place, when the body is not only supported, but raised and
propelled through the air. The Eider-duck is said to fly
90 miles in an hour, and the Hawk 150. The Swallow
and Swift pass nearly the whole of the long summer days
upon the wing, in search of food for themselves and their

1 This resistance varies as the square of the velocity of the stroke.
Hence, if the down-stroke be made three times as fast as the up-stroke,
the resistance it experiences will be nine times as great. But as this
only operates during one-third of the time, it is in effect equal to three
times that which operates against the up-stroke, and which would tend
to lower the Bird in the air.

IMPOSSIBILITY OF HUMAN FLIGHT. 511

helpless offspring ; and the rapidity of their flight is such,
that they can scarcely traverse less than seven or eight hun-
dred miles in that time, although they go but a short distance
from home. The flight of Insects is even more remarkable
for its velocity in proportion to their size ; thus a Swallow,
which is one of the swiftest-flying of Birds, has been seen to
chase a Dragon-fly for some time without success ; the Insect
always keeping about six feet in advance of the Bird, and
turning to one side and the other so instantaneously, that the
Swallow, with all its powers of flight, and its tact in chasing
Insects, was unable to capture it.

673. If the preceding estimate of the power expended by a
Bird in sustaining itself in the air be correct, it may be easily
proved that it would be impossible for a Man to sustain him-
self in the air by means of his muscular strength alone, in
any manner that he is capable of applying it. It is calculated
that a man of ordinary strength can raise 13 J Ibs. to a height
of 3^ feet per second; and can continue this exertion for
eight hours in the day. He will then exert a force capable of
raising (13| X 60 X 60 X 8) 381,600 Ibs. to a height of
3J feet ; or one-eighth that amount, namely 47,700 Ibs., to the
height of twenty-six feet, which, as we have seen, is that to
which the Bird would raise itself in one second, by the force it
is obliged to exert in order to sustain itself in the air. Now if
we suppose it possible that a Man could by any means concen-
trate the whole muscular power required for such a day's
labour, into as short a period as the accomplishment of this
object requires, we might find the time during which it would
support him in the air, by simply dividing this amount by his
weight, which we may take to be 150 Ibs. The quotient is
318, which is the number of seconds, during which the ex-
penditure of a force that would raise 47,700 Ibs. to a height
of twenty-six feet, will keep his body supported in the air ;
and this is but little more than five minutes. There is no
possible means, however, by which a Man could thus concen-
trate the force of eight hours' labour, into the short interval in
which he would have to expend it while supporting himself
in the air. And we have elsewhere seen (MECHANICS, § 285),
that by no combination of mechanical powers can force be
created; as these only enable force to be more advantageously
applied. Hence, the problem of human flight will never be

512 USE OF PREHENSILE ORGANS IN LOCOMOTION.

solved, until some source of power shall be discovered, far
surpassing that which his muscular strength affords, and so
portable in its nature as not materially to add to his weight.

674. The only other organs of locomotion which we have
to c6nsider, are those of prehension. Of these, the principal
have been elsewhere noticed, with reference to their use in
laying hold of food and conveying it to the mouth (§ 172),
and with regard to the differences between the hand of Man
and the claspers of the Quadrumana (§ 643). The hand of
Man is seldom employed to assist in his locomotion, except
in swimming (where it serves the purpose of a fin), and in
climbing ; neither of which kinds of movement can be said
to be natural to him. But the claspers of the Quadrumana

(fig. 254) are most efficient instru-
ments of locomotion ; enabling
them not only to grasp the branches
of the trees which they climb to
despoil them of their fruit, but
also to catch hold of them at the
end of a long leap. This they do
with the most wonderful agility ;
as all who have seen Monkeys in
circumstances at all like those of
their natural habitations, must
have observed. The Gibbons, or
long-armed Apes of the East Indies,
are probably the most remarkable
in this respect. The Author has
seen the Unglcaputi leap round and
round a room of about fifteen feet
square, catching at each side by
some small support attached to the
wall ; and taking its next leap (if
such it could be called) by merely

Fig. 254.-^Ti«A«EE. swinging itself from this, without
touching anything solid with, its
feet. There are many of the Monkey tribe, however, espe-
cially in the New World, whose hands are less efficient as
instruments of prehension ; and these are furnished with a
prehensile tail ; that is, a tail which can be coiled round the
branch of a tree, and by which the animal can suspend itself

PRODUCTION OP SOUNDS BY ANIMALS.

513

(fig. 255). A similar tail is possessed by some of the Opos-
sum tribe ; and by the Chameleon among Reptiles.

Fig. 255.— WHITE-THROATED SAJOU.

CHAPTER XIII.

OF THE PRODUCTION OF SOUNDS : VOICE AND SPEECH.

675. IT is not by their movements alone, that Animals are
enabled to influence one another. Were it so, their commu-
nication would be restricted to the small amount which can
be effected by signs and gestures. This, however, is necessa-
rily the case amongst aquatic animals in general ; since they
are prevented by the nature of the medium they breathe
from producing sounds through its means. Some of them
appear to have the power of communicating with each
other by the vibrations which they can excite in the water ;
of this we have already noticed an example among the
Whale tribe (§ 491) ; and there is reason to believe that certain
Mollusks possess a similar means of communication.

L L

514 PRODUCTION OF SOUNDS BY INSECTS.

676. Many Insects have the power of prodacing a conti-
nuous sound, which probably serves the purpose of intimating
to each other the neighbourhood of their own kind ; and
even, in some instances, of expressing their feelings : some
of these sounds are produced only during flight. Of this
kind is the sharp hum of the Gnat, Mosquito, Gad-fly, &c.,
which, though often a source of extreme annoyance to man
and beast, serves to give warning of the proximity of these
blood-thirsty Insects, and is therefore of real service to the
animals they attack. From recent experiments, however, it
appears that in Bees and Flies, at least, the sound is not
produced so much by the vibrations of the wings (to which it

is commonly attributed), as
by those of a little mem-
branous plate, situated in
one of the spiracles or stig-
mata (§ 321) of the thorax;
for if the apertures of these
be stopped, no sound is heard,
though the wings remain in
movement. But in Cock-
chafers, and other noisy
Beetles, Butterflies, &c., no
such apparatus can be dis-
Fig. 256.-BoMBYi.ius. covered. Other sounds are

produced while the insect is feeding ; that occasioned by the
armies of Locusts, when incalculable millions of powerful jaws
are in action at the same time, has been compared to the crack-
ling of a flame of fire driven by the wind. Certain two-winged
Flies, distinguished by a long proboscis (fig. 256), make a
humming sound whilst sucking honey from flowers ; and the
same is the case with some of the Hawk-moths.

677. Some Insects are remarkable for a peculiar mode of
calling, commanding, or giving an alarm. The neuters or
soldiers among the White Ants make a vibrating sound,
rather shriller and quicker than the ticking of a watch, by
striking against hard substances with their mandibles ; this
seems intended to keep the labourers, who answer it by a
hiss, upon the alert and at their work. The well-known
sound termed the "death-watch" is produced by a small beetle
termed Anobium (fig. 257), that burrows in old timber ; and

PRODUCTION OF SOUNDS BY INSECTS.

515

it is occasioned by the striking of its mandibles upon the
wood. The sound is evidently intended by the animal as a
means of communication with its fellows ; for if it be an-
swered it is continually repeated, whilst if no answer be
returned the animal repeats the signal in another place. The
noise exactly resembles that pro-
duced by tapping moderately
with the nail upon the table ;
and, when familiarised, the insect
will very readily answer this imi-
tation.— The most remarkable
example of the production of
sounds for the purpose of autho-
rity, is that of the Queen-Bee ; which has the power of
influencing the whole hive, especially about the time of
swarming, by the peculiar notes she produces.

678. Many Insects have the power of expressing their
passions, also, — as fear, anger, sorrow, joy, or love, — by the
sounds they can generate. The most curious of those given
out under the influence of alarm is that produced by the
Sphinx Atropos or Death's-head Hawk-moth (fig. 258); which

Fig. 257.— ANOBIUM.
Natural size and magnified.

Fig. 258. — SPHINX ATHOPOS.

when confined, or taken into the hand, sends forth a strong
and sharp cry, resembling, some say, that of a mouse, but
more plaintive and even lamentable. The means by wliich
this cry is produced, have not yet been certainly ascertained.
The influence of anger, sorrow, and joy, in modifying the tone
of the hum of Bees, is well known to those who have studied
their habits ; the first is particularly evident in the sharp
angry tone which is heard when the hive has been disturbed,
especially if some of the Bees have been killed ; the second

LL2

516 SOUNDS PRODUCED BY INSECTS.

is manifested in a low plaintive tone which is given-out when
the queen has been taken away ; and the cheerful humming
which is immediately heard when the sovereign is restored, is
an evident indication of the last. Of all the Insects inha-
biting this country, the most noisy are the Crickets; whose

Fig. 259. — HOUSE-CRICKET.

sound, which seems to be their expression of love, is produced
by the rubbing of the elytra or wing-covers one against the
other. In several species it may be distinctly seen that a
very strong nervure on one of these has a jagged surface like
that of a file ; and that this works against a collection of
smaller nervures, which resemble so many strings.

679. The Cicada (fig. 260) was a very favourite insect
among the ancient Greeks ; and was frequently mentioned
by their poets with the most endearing epithets. Its song
was considered particularly musical ; and it was regarded as
the happiest as well as the most innocent of animals. The
Cicadae of other countries produce an extremely shrill and
disagreeable sound, which can be heard at a great distance.
In the warmer parts of the United States, there is a species
which, in the hotter months of summer, is a very troublesome
and impertinent neighbour. The Cicadae of Brazil are said
to be audible at the distance of a mile : this is as if a man of
ordinary stature, supposing his powers of voice increased in
the ratio of his size, could be heard all over the world. The
organs by which the sound is produced are placed on the
under side of the body, between the base of the hind legs
and the abdomen, and consist externally of a pair of large
flattened plates of a horny texture, varying in form in the
different species. When these are raised, they are found to
conceal a large cavity partially covered with a membrane of a

SOUNDS OF INSECTS. VOICE OF VEKTEBRATA.

517

much, more delicate nature than the external covering, with a
horny plate in the middle, which lies along the bottom.
Still more internally are two bun-
dles of muscles, which are the real
agents in producing the sound ;
for, when they are pulled and sud-
denly let go, even in a dead speci-
men, the sound is produced as
well as though the insect were
alive. They draw-in and force-out,
by their alternate and rapid con-
traction, a horny drum or mem-
brane, stretched in such a manner
as to vibrate readily ; the sound
occasioned by the movements of
which passes out through an aper-
ture resembling the sound-holes
of a violin. The Fulgorce, also,
have considerable sound-producing
powers, but exert them in the
night, whilst the Cicadae perform
in the day. The Great Lantern-
fly of Guiana (§ 400, fig. 175) begins regularly at sunset;
and its noise, resembling that of a razor-grinder at work, is
so loud, that the insect is called " scare-sleep" by the Dutch
colonists.

680. In all air-breathing Vertebrata, the production of
sound depends upon the passage of air through a certain,
portion of the respiratory tube, which is so constructed as to
set the air in vibration. In Reptiles and Mammals, it is at-
the point where the windpipe opens into the front of the
pharynx, that this vibrating apparatus is situated. Few of
the animals of the former class, however, can produce any
other sound than a hiss, occasioned by the passage of air
through the narrow chink by which the trachea communicates
with the pharynx ; but this sound, owing to the great capa-
city of their lungs (§ 325), is often very much prolonged.
Among Mammals, on the other hand, there are few, if any,
which have not some vocal sound ; but the variety and
expressiveness which can be given to it differ considerably in
the several tribes of this class, being by far the greatest in

Fig. 260.— CICADA.

518

STRUCTURE OF THE LARYNX.

Man. This sound is produced by the apparatus termed the
larynx, which is situated beneath the base of the tongue, and
in front of the pharynx (§ 192, fig. 107). It is suspended,
as it were, from the hyoid bone (h, fig. 261), — a bone of a
horse-shoe form, detached from the rest of the skeleton;
from two projections (I) on the upper side of which, several
of the muscles of the tongue originate. The sides of the
larynx are formed by two large cartilages (t, fig. 261), which

VERTICAL SECTION op
THE LARYNX.

ar, arytenoid cartilages;
v, ventricle of tlie glot-
tis ; e, ep:glottis; — the
other references as be-
fore.

Fig. 263.

FRONT VIEW OF THE
LARYNX.

The interior wall is mark-
ed by the lines a, a,b,b;
— li, inferior ligaments
of the glottis, or vocal
cords ; Is, superior liga-
ments;— the other re-
ferences as before.

tr

Fig. 261.

HUMANLARYNX,VIEWED
SIDEWAYS.

h, hyoid bone ; I, point of
origin of muscles of the
tongue ; t, thyroid car-
tilage ; a, projection in
front, commonly called
Adam's apple ; c,cricoid
cartilage ; tr, trachea ;
o, posterior side of the
larnyx, in contact with
the oesophagus.

are termed the thyroid cartilages ; where these meet on the
middle line a projection is formed, which is particularly
prominent in Man, and has received the name of Pomum
Adami, or Adam's apple (a). The thyroid cartilages rest
upon another, termed the cricoid (c); this has the form of a
ring, much deeper behind than in front, and surmounts the
trachea, with the upper ring of which its lower edge is con-
nected by a membrane. Upon the upper surface of the back
of the cricoid cartilage, where there is an open space left
between the two thyroid cartilages, are mounted two small
cartilaginous bodies, the arytenoid (ar, fig. 262). These are
movable to a certain extent ; and their position may be
changed in various directions by several muscles which act
upon them.

STRUCTUEE OF THE LARYNX. 519

681. To these arytenoid cartilages are attached two ligaments
of elastic fibrous substance (§ 23), which pass forwards to be
attached to the front of the thyroid cartilage, where they meet
in the same point. These are the instruments concerned in the
production of sound, and also in the regulation of the aperture
by which air passes into the trachea; and they are termed
the vocal cords or ligaments (fig. 263, li). By the meeting of
these ligaments in front and their separation behind, the usual
aperture has the form of a V ; but it may be narrowed by the
drawing-together of the arytenoid cartilages, until the two
vocal ligaments touch each other along their whole length,
and the aperture is completely closed. In this manner, the
amount of air permitted to pass through the larynx is regu-
lated ; and a protection is afforded against the entrance of
solid substances. An additional guard is afforded by the
doubling of the lining membrane, in such a manner as to form
a second pair of folds (I s, fig. 263), above the preceding ; and
over the space between these (which is much wider than that
between the vocal cords) there is a valve-like flap, the epi-
glottis (e, fig. 262), which is pushed-down upon it in the act
of swallowing, so as to prevent the entrance of solid or fluid
particles into the space beneath, which is called the glottis.
From the causes formerly mentioned (§ 193), such particles
are occasionally drawn into the glottis ; and they excite, by a
reflex action, an involuntary and extremely violent cough,
which tends to expel them again. Sometimes, however, solid
bodies of no inconsiderable size find a lodgment in the wide
spaces (v, fig. 263) between the upper and lower pair of liga-
ments, which are termed the ventricles of the larynx ; and
occasionally they pass through the opening between the vocal
cords, which is termed the rima glottidis or fissure of the
glottis, into the wind-pipe.

• 682. In the ordinary acts of inspiration and expiration, the
arytenoid cartilages are wide apart, so that the aperture is as
large as possible ; but for the production of vocal sounds, it
is necessary that the aperture should be narrowed, and that
the flat sides, rather than the edges, of the vocal ligaments
should be opposed to one another. This is accomplished by
a peculiar movement of the arytenoid cartilages, occasioned
by the contraction of certain muscles. When these ligaments
are thus brought into position, the air in passing through the

520 ACTIONS OF THE LARNYX : VOICE.

larynx sets them in vibration, in a manner very much resem-
bling that in which the reed of a Hautboy or Clarionet, or the
tongue of an Accordion or Harmoninm, is set in vibration by
the current of air that is made to pass beneath them. The
rapidity of the vibrations, and consequently the pitch of the
sound (§ 523), depends on the degree of tension or tightness
of the vocal ligaments ; and this is regulated by muscles which
act upon the thyroid and arytenoid cartilages. If the thyroid
cartilage be drawn forwards, and the arytenoid cartilages back-
wards, the two ends of the vocal cords will be further sepa-
rated from each other, and they will consequently be tightened ;
by the contrary movements they will be relaxed.

683. It is on account of the greater length of the vocal
cords, that the pitch of the voice is much lower in Man than
in Woman ; but this difference does not arise until the end
of the period of childhood, the size of the larynx being about
the same in the Boy and Girl up to the age of 14 or 15 years,
but then undergoing a rapid increase in the former, whilst it
remains nearly stationary in the latter. Hence it is that Boys,
as well as Girls and Women, sing treble ; whilst Men sing
tenor which is about an octave lower than the treble, or bass
which is lower still.

684. The cause of the variations of timbre or quality in
different voices, is not certainly known ; but it appears to be
due, in part, to differences in the degree of flexibility and
smoothness in the cartilages of the larynx. In women and
children these cartilages are usually soft and flexible, and
their voices clear and smooth ; whilst in men, and in women
whose voices have a masculine roughness, the cartilages are
harder, and are sometimes almost completely ossified. The loud-
ness of the voice depends in part upon the force with which the
air is expelled from the lungs ; but the variations in strength
of voice which exist among different individuals, are in some
measure due to the degree in which its resonance is increased
by the vibration of the other parts of the larynx and of the
neighbouring cavities. In the Howling Monkeys of America,
there are several pouches which open from the larynx, and are
destined to increase the volume of tone that issues from it ;
one of these i§ excavated in the substance of the hyoid bone
itself, which is very greatly enlarged ; and to this bony drum
seems due the mournful plaintiveness which characterises the

LARYNX OF BIRDS.

521

tone of these animals. Although the largest of the American
Monkeys, these Howlers are of inconsiderable size ; yet their
voices are louder than the roaring of lions, being distinctly
audible at the distance of two miles ; and, when a number are
congregated together, the effect is terrific.

685. In Birds, the situation of the vocal organ is very
different. The trachea opens into the pharynx, as in Reptiles,
by a mere slit ; the borders of which have no other movement
than that of approaching one another, so as to close the aper-
ture when necessary. But at the lower extremity of the
trachea, just where it subdivides into the bronchial tubes,
there is a sort of larynx or vocal organ, which is of very
complex construction, especially in the singing-birds. The
external surface of this larynx is represented in fig. 264; its
muscles, m m', being left i

in their places on one side,

and removed on the other. ,

At 1 1, is seen the trachea ;

at the lower extremity of

which, ^, is a sort of bony *

drum, I, divided at its ,

lower part by a partition of

the same material (o, fig. b

265), which is surmounted l

by a semilunar membrane

(c). This drum communi- c

cates below with the two Fig. ZBI.-LAHYHX OF

bronchial tubes, b b' (fig.

264), each of which has its own glottis and vocal cords;

the inner lip of one of these is seen at a (fig. 265) ; and at me

is shown a drum-like membrane, forming the inner wall of

the bronchial tube, which probably increases the resonance

of the voice. These parts are acted-on by several muscles,

the number of which varies according to the compass and

flexibility of the voice in the different species ; being very

considerable in the most esteemed of the singing-birds, and

being reduced to a small amount in those which have no vocal

powers. Iii some, indeed, they are altogether absent; and

the state of the glottis can be influenced only by those muscles

which raise and lower the whole trachea.

686. The vocal sounds produced by the action of the larynx

Fig. 265.— VERTICAI
SECTION OF SAME.

522 DIFFERENT KINDS OF VOICE.

are of very different characters ; and may be distinguished
into the cry, the song, and the ordinary or acquired voice. —
The cry is generally a sharp sound, having little modulation
or accuracy of pitch, and being usually disagreeable in its
timbre or quality. It is that by which animals express their
unpleasing emotions, especially pain or terror ; and the Hu-
man infant, like many of the lower animals, can utter no other
sound. — In song, by the regulation of the vocal cords, definite
and sustained musical tones are produced, which can be
changed or modulated at the will of the individual. Different
species of Birds have their respective songs ; which are partly
instinctive, depending upon the construction of their larynx ;
and are partly governed by their education. In Man, the
power of song is entirely acquired ; but, when once acquired,
it is far more susceptible of variety and expression than that
of any other animal. In fact, the larynx of Man may be
said to be the most perfect musical instrument ever con-
structed.— The voice is a sound more resembling the cry, in
so far as it does not consist of sustained musical tones ; but
it differs from the cry, both in the quality of its tone, and in
the modulation of which it is capable by the will. In ordi-
nary conversation, the voice passes through a great variety of
musical tones, in the course of a single sentence, or even a
single word, — sliding imperceptibly from one to another;
and it is when we attempt to fix it definitely to a certain
pitch, that we change it from the speaking to the singing
tone.

687. It is to the wonderful power that Man possesses, of
producing articulate sounds, which form a medium whereby
he can communicate ideas of any kind to his fellows, — that
much of his superiority to other animals is due. Neverthe-
less, it is not to this alone that we must attribute it ; for many
animals, especially Birds, can produce, by imitation, sounds
as articulate as those of Man ; but the mind which originates
them, and which uses them as expressions of its ideas and
desires, is deficient.

688. All spoken language is made up of a certain number
of elementary sounds, which are combined into syllables, words,
and sentences. It may be easily shown, upon arithmetical
principles, that from 20 or more of these elementary sounds,
an almost infinite variety of combinations may be produced ;

ELEMENTARY ARTICULATE SOUNDS. 523

and from such an inexhaustible store there is no difficulty in
deriving new combinations, to represent any new ideas that
we may desire to express. These simple or elementary sounds
ought to be represented by an equal number of single letters ;
this is the case, however, in but few languages. Our own is
particularly faulty in this respect ; for there are many simple
sounds that can be only represented by a combination of
letters, whilst others may be represented by more than one
single letter, and in some instances a single letter represents
a composite sound. Thus the sounds of au and th are really
simple ones, and ought to be represented by single letters.
Again, the sound of k is represented also by the hard c, as in
the first syllable of concert ; and the sound of s by the soft c,
as in the second syllable of the same word, where the c is
sounded exactly as the s in consent. And the letter i (as
usually pronounced in English) does not represent a simple
sound, but a combination of two, as will be presently shown.
Most of the Continental languages are superior to the English
in this respect.

689. Vocal sounds are divided into Vowels and Consonants ;
the true distinction between which appears to be, that the
Vowel sounds are continuous tones, modified by the form of
the aperture through which they pass out ; whilst in giving
utterance to Consonants, there is a partial or complete inter-
ruption to the breath in its passage through the organs in
front of the larynx. Hence all true Vowels may be prolonged
for any length of time that the breath is supplied from the
lungs ; whilst the sound of many Consonants is momentary
only. It is easy for any one to convince himself that the
Vowel sounds are governed simply by the form of the cavity
of the mouth, and by that of the aperture of the lips ; by
passing, in one continued tone, from one of the following
Vowel sounds to another : —

English a
English a
English a
English e
English 0
English oo

as in ah
as in all
as in name
as in theme
as in cold
as in cool

. Continental a
Diphthong au
. Continental e
. Continental i
. Continental o
. Continental u

The short Vowel sounds, as a in fat, e in met, o in pot, &c.,
are not capable of being prolonged ; as they are formed in

524 VOWELS AND CONSONANTS STAMMERING.

the act of preparation for sounding the succeeding consonant.
The sound of the English i is a compound one, being formed
in the act of transition from that of a in ah, to that of e as in
theme ; hence it cannot be prolonged ; and it is the very worst
vowel sound upon which to sing a long note, since it is impos-
sible to prevent its being heard as one of the sounds of which
it is composed. Much discussion has taken place withr efer-
ence to the true characters of the letters w and y, when
employed to commence words, as wall, yawl, wet, yet. A little
attention to the state of the mouth in pronouncing them will
show, however, that they are then really vowel sounds, rapidly
transformed into the succeeding ones ; for the sound of w in
this situation is oo ; and that of y is the long e ; so that wall
might be spelt ooall, and yawl eaul.

690. Consonants are naturally divided into those which
require a total stoppage of the breath at the moment previous
to their being pronounced, and which cannot therefore be
prolonged ; and those in pronouncing which the interruption
is partial, and which can be prolonged like the vowels. The
former are termed explosive consonants ; the latter continuous.
The explosive consonants are 6 and p, d and t, the hard g and
k. All the others are continuous ; but the sound is modified
by the position of the tongue, palate, lips, and teeth ; and also
by the degree in which the air is permitted to pass through
the nose.

691. The study of the mode in which the different conso-
nants are produced, is of particular importance to those who
labour under defective speech, especially that difficulty which
is known as stammering. This very annoying impediment is
occasioned by a want of proper control over the muscles con-
cerned in articulation ; which are sometimes affected with a
kind of spasmodic action. It is in the pronunciation of the
consonants of the explosive class, that the stammerer usually
experiences the greatest difficulty ; for the total interruption
to the breath which they occasion is frequently continued
involuntarily, so that either the expiration is entirely checked,
or the sound comes out in jerks. Sometimes, on the other
hand, in pronouncing vowels and continuous consonants, the
stammerer prolongs his expiration, without being able to
check it. The best method of curing this defect (where there
is no malformation of the organs of speech, but merely a want

REFLEX AND INSTINCTIVE ACTIONS. 525

of power to use them aright), is to study the particular defect
under which the individual suffers ; and then to make him
practise systematically the various movements concerned in
the production of the sounds in question, at first separately,
and afterwards in combination, until he feels that his volun-
tary control over them is complete.

CHAPTER XIV.

OF DISTINCT AND INTELLIGENCE.

692. IT will be remembered that, when the Nervous System
was described (Chap. XL), it was shown to be the instrument
of three classes of operations, each of which seems to be per-
formed by a distinct portion of the apparatus. — I. The first of
these is the class of simply Reflex actions, which are executed
only in respondence or answer to impressions made upon the
nerves proceeding to a ganglionic centre ; as when a Dytiscus,
whose head has been cut off, executes swimming movements
immediately that its feet come in contact with water. These
movements evidently take place without any choice or direc-
tion on the part of the animal, which, in , executing them,
seems like a mere machine adapted to perform certain actions
when certain springs are touched ; and it has been shown
that they may be called-forth even without its consciousness.
Of these reflex movements, the Spinal Cord of Vertebrata, and
in Invertebrata the ganglia corresponding to it (in regard to
their connexions with the organs of locomotion, respiration,
&c.), are the instruments. — II. The second class comprehends
those Instinctive actions, which differ from the preceding in
being dependent on the sensations received by the animal,
and in being, therefore, never performed without its conscious-
ness. Nevertheless, the animal in executing them is not
guided by any perception of the object to be attained, or by
any choice of the means by which it is to be accomplished ;
but acts blindly and involuntarily, in accordance with an
irresistible impulse, implanted in it by its Creator for the
purpose of causing it to do that, without or even against its
Will, which it would not have chosen or devised by its very
imperfect intelligence. The actions of this class are most

526 RELATION OF INSTINCTIVE TO INTELLIGENTIAL ACTIONS.

wonderful in the Invertebrata, which possess the least Intel-
ligence ; and, on the contrary, they are fewest and least
remarkable in Man, whose Intelligence is highest. From
the constant proportion they bear to the size of the ganglia
of sensation, which form nearly the whole nervous mass in
the head of Insects, &c., and a large part of that of the lower
Vertebrata, but which are comparatively small in the Mam-
malia and especially so in Man, there seems good reason to
regard these organs as their chief instruments. — III. The
third and highest class of actions, is that in which Intelligence
is the guide, and the Will the immediate agent. The animal
receives sensations, forms a notion of their cause, reasons
upon the ideas thus excited, perceives the end to be attained,
chooses or devises the means of accomplishing it, and volun-
tarily puts those means into execution. These actions are
seen, in their highest and most complete form, in Man ; but
they are not confined to him ; for, as will be shown hereafter,
true reasoning processes are performed by many of the lower
animals. There can be no doubt that the Cerebral Hemi-
spheres, which form the Brain properly so called, constitute
the instrument by which these actions are executed; for
we find that their size and development bear a very regular
proportion to the degree of Intelligence which the animal
possesses.

693. It follows, then, that the lower we descend in the
scale of Animal life, the larger is the proportion of the move-
ments of any particular species which we are to attribute to
the Reflex and the Instinctive classes ; whilst the proportion
which is due to Intelligence and Will diminishes in a like
degree. Thus we have seen that the ordinary movements of
locomotion, which Man performs in the first instance by volun-
tary effort, are reflex in Insects (§ 445) • and there can be no
reasonable doubt that the movements of the tentacula of the
Hydra, by which it entraps its prey and draws it to the entrance
of its stomach (§ 121), are of a reflex, rather than a voluntary
or instinctive character, since they are obviously analogous to
those movements of the pharyngeal muscles, by which the food
is grasped and carried into the alimentary tube of the highest
animals (§ 195). There is one curious fact, which would
seem to indicate a difference between them, but which is
really a strong argument in favour of their analogy. It is

CHARACTERISTICS OF REFLEX AND INSTINCTIVE ACTIONS. 527

continually observed that when the stomach of the Polype is
full, its arms do not make any attempt to seize objects that
touch them ; so that small worms, insects, &c., which would
at other times be entrapped, may now come near them with
impunity. It has been supposed that this results from an act
of choice on the part of the animal, and that its choice is
influenced by its consciousness that its stomach is supplied
with food. It must seem improbable that an Animal which
so nearly resembles Plants in its general habits, and in which
the nervous system is so obscure that it has not yet been dis-
covered, should possess mental endowments of so high a
character ; and we may find, in studying our own functions,
a circumstance exactly parallel to that just mentioned. For
when we commence eating, with a good appetite, we may
notice that the muscles of Deglutition act very readily ; but
when we are completely satisfied, it is often difficult to excite
these muscles to contraction, so as to swallow another morsel,
even though, for the gratification of our palate, we may desire
to do so. Thus we see how much better a guide we find in
Nature, for the amount of food we require, than in our own
pampered tastes.

694. The first class, that of Reflex movements, has been
already considered in sufficient detail ; but it is intended, in
the present chapter, to offer some examples of those of the
second and third classes, — those actions, namely, which are
guided by Instinct and Intelligence respectively. These actions
may be usually distinguished by the two following tests : —
1. Although, in most cases, experience is required to give the
Will command over the muscles concerned in its operations,
no experience or education is required, in order that the dif-
ferent actions which result from an Instinctive impulse may
follow one another with unerring precision. 2. Instinctive
actions are performed by the different individuals of the same
species, nearly, if not exactly, in the same manner ; present-
ing no such variation of the means applied to the objects in
view, and admitting of no such improvements in the progress
of life, or in the succession of ages, as we observe in the
habits of individual Men, or in the manners and customs of
nations, which are for the most part adapted to the attainment
of particular ends, by voluntary efforts guided and directed
by reason. — Where, as in the examples hereafter to be men-

528 DISTINCTIVE CHARACTERS OF INTELLIGENTIAL ACTIONS.

tioned (§ 717), we find individual animals "learning wisdom
by experience," and acquiring the power of performing actions
which do not correspond with their natural instincts, we
cannot do otherwise than regard them as possessed of a certain
degree of Intelligence, by which they are rendered susceptible
of education.

695. The amount of Intelligence displayed in such acquire-
ments, can only be judged-of, however, by carefully examining
the circumstances under which they are made. If the new
habits are gained — like the talking of a Parrot — by imitation
simply, no great degree of intelligence is manifested ; but if
it spontaneously result from a reasoning process on the part
of the animal, our idea of its sagacity is raised. There may
be a combination of both these conditions ; as in the following
curious circumstance, related to the Author by a friend who
has repeatedly witnessed it. Some horses kept in a paddock
were supplied with water by a trough, which was occasionally
filled from a pump, — not, however, as often as the horses
seem to have wished ; for one of them learned, of his own
accord, to supply himself and his companions, by taking the
pump-handle between his teeth, and working it with his
head. The others, however, appear to have been less clever,
or more lazy ; and finding that this one had the power of sup-
plying their wants, they would teaze him, by biting, kicking,
&c., until he had pumped for them, and would not allow him
to drink until they were satisfied. That this was not a mere
act of imitation, appears from the circumstance that the horse
did not attempt to imitate the movement of the man, but
performed the same action in a different manner, — evidently
because it had associated in its mind the motion of the pump-
handle with the supply of water.

696. The Instincts of Animals may be shown to have
immediate reference, probably in every instance, to the supply
of the wants of the individual, or to the continuance of the
race. Thus we have Instincts which guide in the selection
and acquirement of food ; others which govern the construc-
tion of a habitation for the individual, and of a receptacle for
the eggs, — and these may influence a number at once, in such
a manner as to unite them into a society ; and others which
direct their migrations, whether in search of food, for the
deposit of their eggs, or for other purposes. Of these, some
examples will now be given.

INSTINCT OF THE ANT-LION.

529

697. Among the instincts which direct animals in the
acquirement of their food, few are more remarkable than those
possessed by the larva of the Ant-lion (fig. 266), a small insect
allied to the Dragon-fly. This larva (fig. 267) is destined to feed

Fig. 206.— ANT LION IN PERFECT STATE.

upon ants and other small insects, whose juices it sucks ; but
it moves slowly and with difficulty, so that it could scarcely
have obtained the requisite supply of food, if Nature had not
guided it in the construction of a remarkable snare, which
entraps the prey it could not acquire by pursuit. It digs in
fine sand a little funnel-shaped pit (fig. 268), and conceals

Fig. 267. LARVA or
THE ANT-LION.

Fig. 268. — PITFALL OF THE ANT-LI-. x.

itself at the bottom of this, until an insect falls over its edge ;
and if its victim seeks to escape, or stops in its fall to the
bottom, it throws over it, by means of its head and mandibles,
a quantity of sand, by which the insect is caused to roll down
the steep, within reach of its captor. The manner in which

M M

530

PITFALL OF ANT-LION : WEB OF SPIDER.

the Ant-lion digs this pit is extremely curious. After having
examined the spot where it purposes to establish itself, it
traces a circle of the dimensions of the mouth of its pit ; then,
placing itself within this line, and making use of one of its
legs as a spade, it digs out a quantity of sand, which it heaps
upon its head, and then, by a sudden jerk, throws this some
inches beyond its circle. In this manner it digs a trench,
which serves as the border of its intended excavation, moving
backwards along the circle, until it comes to the same point
again ; it then changes sides, and moves in the contrary
direction, and so continues until its work is completed. If,
in the course of its labours, it meets with a little stone, the
presence of which would injure the perfection of its snare, it
neglects it at first, but returns to it after finishing the rest of
its work, and uses all its efforts to get this upon its back, and
carry it out of the excavation ; but if it cannot succeed in
doing so, it abandons its work and commences anew else-
where. When the inclination of the walls of the pit has
been altered by any slip, as almost always happens when
an insect has fallen-in, the Ant-lion hastens to repair the
damage.

698. Snares of a still more singular character are con-
structed by many Spiders, which spin webs of the finest silk,
for the purpose of entrapping their prey. The arrangement

Fig. 269. — ETEIRA DIATEMA.

of these toils varies according to the species, and sometimes
does ijot present any regularity ; but in several instances it is

BURROWS OF HAMSTER AND MYGALE. 531

of extreme elegance ; and no one can watch the labours of a
common garden spider, as, for instance, the Epeira diadema
(fig. 269), without being struck with the marvellous sagacity
which it displays in the execution of its work, and the per-
fection with which its web is constructed.

699. An equally curious instinct is often displayed in the
construction of the habitations which the animal designs for
its abode. Thus the

Hamster (fig. 270), a
small rodent animal
allied to the Eat, which
is met with in most
cultivated districts on
the Continent from
Alsace to Siberia, and
which is very injurious
to agriculture, con- Fig. 27o.-iiAMSTEn.

structs a burro win the

soil which has always two openings, — one in an oblique direc-
tion, which serves the animal for casting out the earth it has
dug away, — the other perpendicular, which is the passage by
which it enters and makes its exit. These galleries lead to
a regular series of circular excavations, which communicate
with each other by horizontal passages ; one of these cavities,
furnished with a bed of dried herbage, is the abode of the
Hamster; while the others serve as magazines for the pro-
visions which it collects in large quantities.

700. There are certain Spiders known to Zoologists under
the name of Mygale, which perform operations analogous to
those of the Hamster, but still more complicated ; for not
only do they excavate in the ground a large and commodious
habitation, but they line it with a silken tapestry, and
furnish it with a door regularly hung upon a hinge (fig. 271).
For this purpose, the Mygale digs, in a clayey soil, >a sort of
cylindrical pit, about 3 or 4 inches in length ; and plasters

-its walls with a kind of very consistent mortar. It then
constructs, of alternate layers of earth, and of threads woven
into a web, a trap-door exactly adapted to the orifice of its
hole, and only capable of opening outwards ; and it attaches
this by a hinge of the same thread to the tapestried lining of
its chamber. The outside of this trap-door is covered with

532

HABITATIONS OF SPIDERS AND INSECTS.

materials resembling the soil around ; and so little does it
differ from this, as to be with difficulty distinguished, even

by a person seeking to discover
the Spider's habitation. If an
attempt is made to lift it, when
the animal is within its excava-
tion, the movement is resisted
>S?& by the whole force of the Spider,
ich holds down the door, by

^^^^•liirP^^^'Sc*^ fixing its claws into small holes
on its under surface at the
point most distant from the hinge,
r-*»p*r where its force may be most ad-
*s2a^. vantageously applied.

Fig. 271. NEST OF MYGALE. 701' Am°nS IllSects> We find

a great number of very curious

processes instinctively performed in the construction of
their habitations. Many Caterpillars form for themselves a
protection, by rolling together portions of leaves, and attach-
ing these by threads. In almost every garden, we may
observe (at the proper season) nests of this kind, on the
leaves of the Lilac or Gooseberry ; and a similar one, repre-
sented in fig. 272, is constructed in the leaves of the oak, by
the caterpillar of a small nocturnal Butterfly, the Tortrix viri-
dissima. The Larva of the little Clothes-moth, again, forms a
sort of tubular sheath, composed of the filaments it detaches

from the stuff through which
it excavates its galleries ; this
sheath it is continually prolong-
ing at one extremity ; and
when, in consequence of the
growth of the larva, its tube
becomes too small for its com-
fortable residence, it slits it
down and lets-in a piece. The
aquatic Larvae of the Caddice-
flies (fig. 273, c), which are
commonly known as Caddice-
worms, house themselves in straws, pieces of hollow stick,
rushes, &c. ; and those of some species glue together a
number of minute stones, pieces of stick, small shells, &c.,

Fig. 272. — NiiST OF TORTRIX.

HABITATION OP CADDICE-WORM.

>33

so as to make a tube (A), in which the animal creeps along
the bottom and sides of the brook it inhabits, and sometimes
rows itself on the surface
of the water. When full-
grown, the larva attaches
its case by threads to some
large stone ; and then
covers its mouth with an
open net-work of threads
(B), sufficiently close to
prevent the entrance of
insects, but with meshes
permitting the water to
pass through. In this way

it Undergoes its metamor- Fig 273.-C, PHRYGANF.A OR CADDICE-FLY
phosis into the Pupa State ; A, tube formed by its larva ; B, network at

and a short time before its the entrance of the tube-

last change it cuts the threads of the network, by means

of two hooks with which its head is furnished, and creeps

out of the water ; soon after which it changes into the perfect

insect.

702. It is scarcely possible to point to any actions better
fitted to give an idea of the nature of Instinct, than those
which are performed by various Insects when they deposit
their eggs. These animals never behold their progeny, and
cannot acquire any notion from experience, therefore, of that
which their eggs will produce ; nevertheless they have the
remarkable habit of placing, in the neighbourhood of each of
these bodies, a supply of aliment fitted for the nourishment
of the larva that is to proceed from it ; and this they do,
even when they are themselves living on food of an entirely
different nature, such as would not be adapted for the larva.
They cannot be guided in such actions by anything like
reason, since the data on which alone they could reason
correctly are wanting to them ; so that they would be led to
conclusions altogether erroneous, if they were not prompted
by an unerring instinct, to adopt the means best adapted for
the attainment of the required end.

703. Of this kind of instinct, the Necrophorus (fig. 274), a
kind of Beetle not uncommon in our fields, offers a good
example. When the female is about to lay her eggs, she

534

PREPARATION OF FOOD FOR INSECT-LARVAE.

seeks for the dead body of a mole, shrew, or such other
quadruped ; and having found one, she excavates beneath it
a hole of sufficient dimensions to contain the body, which she
gradually drags into it; she then de-
posits her eggs in the carcase, so that
the larvse, when they come forth, find
themselves in the midst of a supply of
carrion, on which they feed like their
parents. — This instinct is still more
remarkable, when an Insect whose diet
is exclusively vegetable prepares for its
larva a supply of animal food. Such is
the case with the Pompilus, an Insect
allied to the wasp. In its perfect state
it lives entirely on the juices of flowers ; but the larvae are
carnivorous ; and the mother provides for them the requisite
supply of the food they require, by placing in the nest, by
the side of the eggs, the body of a spider or caterpillar which
she had previously killed by means of her sting. The Xylocopa,

Fig. 274. — NECROPHORUS.

Fig. 275.— XYLOCOPA.

Fig. 276. — NEST OP XYLOCOPA.

or Carpenter-bee (fig. 275), has very analogous habits ; the
female makes long burrows in wood, palings, &c., in which
she excavates a series of cells (fig. 276) ; and in every one of
these she deposits an egg, with a supply of pollen-paste.

704. The instinct of support and protection to the young
and helpless offspring, is seen in all animals in which it is
needed; and it is particularly observable in Birds. The nests

NESTS OF BIRDS. 535

which they construct are destined much more for the recep-
tion of their eggs, and for the protection of the young, than
for their own residence ; for there are few Birds which pass
much time in their nests, except at night, and during the
period of incubation. It is impossible to watch the process
of their construction, without admiring the perseverance with
which the materials are brought together that are destined for
their erection, and the art with which these are arranged. The
form and structure of the habitations are always nearly the
same among the individuals of the same species ; but there
is necessarily a certain latitude in regard to the materials of
which they are composed, since the same could not be every-
where procured. The nests of different species vary greatly,
however, both as to form, structure, and materials; and these
are admirably adapted to the particular circumstances in which
the young families are respectively destined to live. Some-
times these habitations are constructed of earth, the particles
of which are united by the viscid saliva of the Bird into a
tenacious mortar ; and they are then commonly built against
the sides of a rock or wall. But, in general, they are corn-

Fig. 277.— NEST OF GOLDFINCH.

posed of sticks, straws, and other vegetable substances ; and
are placed either on the ground, or among the branches of

536

NESTS OF BAYA AND TAILOR-BIRD.

trees. The greater number of them have a somewhat hemi-
spherical form, resembling a little round basket; and their
interior is lined with moss and down (fig. 277).

705. But sometimes the arrangement is much more com-
plicated, so as to avert some particular danger, or to answer
some special purpose. Thus the nest of the Baya, a little
Indian bird allied to our Bulfinch, has the form of a bottle ;
and it is suspended from a twig of such slenderness and flexi-
bility, that neither monkeys, serpents, nor squirrels can reach
it (fig. 278). It is rendered still more secure against the
attacks of its numerous enemies, by the formation of the
entrance of the nest on its under side, so as to be only reached
by even the bird itself with the aid of its wings. This curious
habitation is constructed of long grass ; and several chambers
are found in its interior, of which one serves for the female

Fig. 278.— NEST t.F THE BAYA. Fig. 279.— NEST OF THE TAILOR-BIRD.

to sit on her eggs, whilst another is occupied by the male,
who solaces his companion with his song, whilst she is occu-
pied in maternal cares. Another curious nest is that of the

BUILDING INSTINCT OF BEAVER. 537

Sylvia sutoria, or Tailor-bird, a little eastern bird allied to
our linnet ; which, by the aid of filaments of cotton drawn
from the cotton-plant, sews leaves together with its beak and
feet, in such a manner as to conceal the nest which they
enclose from the observation of its enemies (fig. 279).

706. The association of a number of individuals of a certain
species, for the performance of labours in which they all unite
to one common end, is another most remarkable example of
the operation of instinct. Several Mammals exhibit this
tendency in a greater or less degree ; but the most interesting
of all, in this point of view, is the Beaver (fig. 280), which is

Fig. 280. — BEAVER.

now chiefly found in Canada, though it formerly abounded
on the Continent of Europe. During the summer it lives
solitarily in burrows, which it excavates for itself on the
borders of lakes and streams ; but as the cold season ap-
proaches, it quits its retreat, and unites itself with its fellows,
to construct, in common with them, a winter residence. It is
only in the most solitary places that their architectural in-
stinct fully developes itself. Having associated in troops of
from two to three hundred each, they choose a lake or river
which is deep enough to prevent its being frozen to the
bottom ; and they generally prefer running streams, for the
sake of the convenience which these afford in the transporta-
tion of the materials of their erection. They begin by
constructing a sloping dam, whereby the water is kept-up to

538 BUILDING INSTINCT OF BEAVER.

a uniform height ; this they form of branches interlaced one
with another, the intervals between them being filled with
stones and mud, with which materials they give a coat of
rough-cast to the exterior also. When the dam passes across
a running stream, they make it convex towards the current ;
by which it is caused to possess much greater strength than
if it were straight. This dam is usually eleven or twelve feet
across at its base, and it is enlarged every year ; and it fre-
quently becomes covered with vegetation, so as to form a kind
of hedge.

707. When the dam is completed, the community separates
into a certain number of families ; and the Beavers then em-
ploy themselves in constructing huts, or in repairing those of
a preceding year. These cabins are built on the margin of
the water ; they have usually an oval form, and an internal
diameter of six or seven feet. Their walls are constructed,
like the dam, of branches of trees ; and they are covered on
two of their sides with a coating of mud. Each has two
chambers, one above the other, separated by a floor ; the
upper one serves as the habitation of the Beavers ; and the
lower one as the magazine for the store of bark which they
lay up for their provision. These chambers have no other
opening than one by which they pass out into the water. It
has been said that the flat oval tail of the Beavers serves them
as a trowel, and is used by them in laying-on the mud of
which their erections are partly composed ; but it does not
appear that they use any other implements than their incisor
teeth and fore-feet. With their strong incisors they cut-down
the branches and even the trunks of trees which may be
suitable ; and by the aid of their mouths and fore-feet they
drag these from one place to another. When they establish
themselves on the banks of a running stream, they cut-down
trees above the point where they intend to construct their
dwellings, set them afloat, and, profiting by the current, direct
them to the required spot. It is also with their feet that
they dig-up the earth they require for mortar, from the banks
or from the bottom of the water. These operations are exe-
cuted with extraordinary rapidity, although they are only
carried-on during the night. When the neighbourhood of
Man prevents the Beavers from multiplying to the degree
necessary to form such associations, and from possessing the

BUILDING INSTINCT OF BEAVER. 539

tranquillity which, they require for the construction of the
works now described, they no longer build huts, but live in
excavations in the banks of the water.

708. The building instinct shows itself, even when the
Beaver is in captivity, and under circumstances in which it
could be of no use. A half-domesticated individual, in the
possession of Mr. Broderip, began to build as soon as it was
let out of its cage and materials were placed in its way. Even
when it was only half-grown, it would drag along a large
sweeping-brush or warming-pan, grasping the handle with its
teeth, so that the load came over its shoulder ; and would
endeavour to lay this with other materials, in the mode em-
ployed by the Beaver when in a state of nature. " The long
and large materials were always taken first ; and two of the
longest were generally laid cross-wise, with one of the ends
of each touching the wall, and the other ends projecting out
into the room. The area formed by the cross-brushes and
the wall, he would fill up with hand-brushes, rush-baskets,
books, boots, sticks, cloths, dried turf, or anything portable.
As the work grew high, he supported himself upon his tail,
which propped him up admirably ; and he would often, after
laying-on one of his building materials, sit up over against it,
appearing to consider his work, or, as the country people say,
'judge it.' This pause was sometimes followed by changing
the position of the material judged ; and sometimes it was
left in its place. After he had piled up his materials in one
part of the room (for he generally chose the same place), he
proceeded to wall-up the space between the feet of a chest of
drawers which stood at a little distance from it, high enough
on his legs to make the bottom a roof for him ; using for this
purpose dried turf and sticks, which he laid very even, and
filling up the interstices with bits of coal, hay, cloth, or any-
thing he could pick up. This last place he seemed to appro-
priate for his dwelling ; the former work seemed to be intended
for a dam. When he had walled-up the space between the
feet of the chest of drawers, he proceeded to carry-in sticks,
cloths, hay, cotton, &c., and to make a nest; and when he
had done, he would sit up under the drawers, and comb him-
self with the nails of his hind feet."

709. "We see, in this account, a very interesting example
of the irrational character of Instinct. If the animal were

540

IRRATIONALITY OF INSTINCT.

guided in its ordinary building operations by such an amount
of intelligence as would lead it to choose and execute its
various movements with a design to accomplish certain ends,
the same intelligence would direct it to leave these actions
unperformed when the purpose no longer required it ; instead
of which, we see that the animal is impelled by an internal
impulse to construct erections as nearly resembling those
which it would build-up on the banks of its native streams,
as the materials and circumstances will permit. — Other ani-
mals are, in like manner, occasionally conducted by their
natural instincts to the performance of actions equally irra-
tional, and quite incapable of answering the purpose which
the particular instinct is destined to serve. Thus the Hen
will sit upon an egg-shaped piece of chalk, as readily as upon
her own egg, being deceived without difficulty by the general
similarity of its appearance ; and the Flesh-fly lays its eggs
in the petals of the carrion-flower, whose odour so much
resembles that of tainted meat as evidently to furnish the
same attraction to the insect.

Fig. 281.— NEST OF REPUBLICAN GKOSBEAK.

710. Societies like those of the Beaver are rare among
Birds, whose associations are usually less perfect. There is

SOCIETIES OP BIRDS AND INSECTS.

541

a small species, however, termed the Republican Grosbeak
(Loxia Socia), which lives in numerous societies in the neigh-
bourhood of the Cape of Good Hope, and constructs its nest
tinder a sort of roof which is common to the whole colony
(fig. 281). t

711. It is among Insects that we find the most remarkable
examples of this kind of social instinct ; and the structures
which are produced by the united labours of a large number,
working together in harmony, are extremely interesting. The
nests of Wasps are constructed in this manner. In order to

Fig. 282.— NEST OF WASP.

form the materials for building them, these Insects detach
with their mandibles the fibres of old wood, which they convert
by mastication into a sort of pulp that hardens into the con-
sistence of pasteboard ; of this substance they construct several
ranges of hexagonal cells ; and the combs thus formed are

542 SOCIETIES OP INSECTS 1 HIVE-BEE.

arranged parallel to each other at a regular distance, and are
united at intervals by little columns which serve to suspend
them (fig. 272). The whole is either hung in the air, lodged
in the hollow of a tree, or buried in the ground ; and it is
sometimes enclosed in a general envelope, sometimes left un-
covered, according to the species.

712. The same community of labour is observed in the
ordinary Hive-Bees, which present to the intelligent observer
a source of interesting occupation that scarcely ever fails.
The number and variety of instincts, each of them most per-
fectly adapted to the end in view, which these Insects exhibit,
is most wonderful; and many volumes have been written
upon them, without by any means exhausting the subject.
Nothing more than a very general sketch of these can be
attempted in the present treatise ; but the illustrations they
afford of the general remarks heretofore made upon the nature
of Instinct, are too valuable to be passed-by. Each Hive
contains but a single queen ; and she is the only individual
ordinarily capable of laying eggs. There are usually from 6 to
800 males or drones ; and from 10,000 to 30,000 neuters or
" working-bees " (fig. 283). In their
natural condition Bees live in the
hollows of trees ; but they appear
equally ready to avail themselves of
the habitations prepared for them
by Man. The cells of which their
combs are composed, are built-up of
Fig. 283.- WORKING BEE. the material that we- term wax. Of this
a part may be obtained direct from Plants, since it is secreted
in greater or less abundance by several species ; but there seems
to be no doubt, that Bees can elaborate it for themselves from
the saccharine materials of their aliment (§ 155). The wax is
separated in little scales, from between the segments of the
abdomen ; these scales are kneaded-together by the mandibles
of the Insect, and are then applied to the construction of the
cells. It is easy to understand that the hexagonal form is
that which enables the cells to be best adapted to the purposes
for which they are built, whilst the least amount of material
is expended. As they are intended not only to contain a
store of honey, but also to serve as the residence for the larvae
(fig. 284) and pupce (fig. 285), it is evident that their form.

ARCHITECTURE OF HIVE-BEE.

543

must approach near to that of the cylinder, in order that
there may be the greatest economy of space ; but it is also
evident that if their walls were circular, a large quantity of

Fig. 284.— LARVJE OF BEE.
(Natural size and Magnified.)

Fig. 285.— PUPA. OF BEE.
(Magnified.)

Fig. 286.— HEXAGONAL CELLS.

(Showing the manner of their
union at the Base.)

material would be required to fill up the interspaces left
between them; whilst, by giving the cells the hexagonal
form, their walls everywhere have the same thickness, and
their cavity is sufficiently well adapted to the forms of the
larva and the pupa.

713. Every comb contains two sets of cells, one opening on
each of its faces. The cells of one side, however, are not
exactly opposite to those of the other, for the middle of each
cell abuts against the point where
the walls of three cells meet on the
opposite side ; and thus the partition
that separates the cells of the op-
posite sides is greatly strengthened.
This partition is not flat, but con-
sists of three planes, which meet
each other at a particular angle, so as to make the centre
of the cell its deepest part. Of the three planes which form
the bottom of each cell, one forms part of the bottom of each
of the three cells against which it abuts on the opposite side,
as shown in the accompanying figure. Now it can be proved,
by the aid of mathematical calculation of a very high order,
that, in order to combine the greatest strength with the least
expenditure of material, the edges of these planes should have
a certain fixed inclination ; and- the angles formed by them
were ascertained by the measurement of Maraldi to be
109° 28', and 70° 32' respectively. By the very intricate
mathematical calculations of Koenig, it was determined that
the angles should be 109° 26', and 70° 34', — a coincidence
between the theory of the Mathematician and the practice of
the Bee (untaught, save by its Creator^ which has been ever

544

ARCHITECTURE OF HIVE-BEE.

regarded as truly marvellous, and as affording one of the most
remarkable examples of the operation of instinct. The very
small discrepancy, amounting to only two minutes of a degree
(or 1-1 0,800th part of the whole circle), was usually sup-
posed to result from a slight error in the observation of the
angle employed by the Bees ; until Lord Brougham, not being
satisfied with this explanation, applied himself to a fresh
mathematical investigation of the question ; and he showed
that, owing to the neglect of certain small quantities, the
result previously obtained was erroneous to the exact amount
of two minutes ; so that the Bees proved to be right, and the
Mathematician wrong.1

714. The ordinary cells of the comb are of two sizes ; one
for the larvee of the working-bees, and the other for those of

Fig. 287. — APIARY.

the drones. Both of these may be used for layiiig-up a store
of food, either for themselves or their progeny ; but it is ob-
served that in the breeding season
the central portion only of each comb
is tenanted by the young Bees, this
being the part of the hive where
they will most constantly obtain the
warmth requisite for their develop-

Fig. 288.-ROYAL CELLS. ment ^ 4ny The deposition of

the eggs in these cells only, therefore, is another remark-
1 See h:s Supplement to New Edition of Paley's Natural Theology.

COLLECTION OF FOOD BY BEES.

545

able instinct on the part of the Queen ; and this is further
manifested in the fact, that she never deposits eggs in the
comb which fills the glasses that are sometimes placed on
the top of a hive, as in fig. 287, the temperature of these
glasses being necessarily lower than that of the interior of
the hive. — The " royal cells," as they are termed, in which
the larvae of the young queens are reared, are different in
form from the rest (fig. 288); sometimes they lie in the
midst of them ; but most commonly they project from the
sides or edges of the comb.

715. The food which the Bees collect is of two kinds, —
the honey of flowers for themselves, and the pollen for their
larvse. The honey, which they suck-up by means of their
proboscis-like tongues (fig. 289), seems to undergo some change

Fig. 289.— BEE'S MOUTH.

Fig. 290.
HIND LEG OF WORKER.

in their digestive cavity ; and the part not required for nourish-
ment is afterwards returned from the stomach, and deposited
in one of the cells, which, when filled, is sealed with a cover-
ing of wax. The pollen is gathered by rubbing the body either
against the anthers, or against other parts of the flower over
which it may have been scattered by their bursting ; and
when the surface of the body has been sufficiently dusted
with its fine particles, these are collected from it by little
brushes with which the feet of the Bee are furnished, and are
worked-up into small pellets, which the Insect carries home
in basket-shaped hollows, of which there is one on each
hind- thigh (fig. 290). The pollen or farina thus collected is
worked-up with honey in a mass, to which the name of
" bee bread " has been given ; and with this the larvae are

N N

546 ARTIFICIAL PRODUCTION OF QUEENS.

nourished, until the time when they are about to pass into
the pupa state. The mouth of the cell is then sealed by a
waxen cover ; and the larva spins a delicate silken cocoon,
within which it undergoes its metamorphosis. In the
chrysalis state it remains quite inactive for some days ; and
during the latter part of this period, when it is rapidly ap-
proaching the condition of the perfect Insect, its development
is aided by the heat supplied by the "nurse-bees," whose
remarkable instinct has been already described (§ 411).

716. One of the most curious features in the whole
economy of Bees is the manner in which they manufacture
new Queens, when from any cause (as by the intentional
removal of her from the hive) their sovereign has been lost.
In order to understand the process, it is necessary to be
aware that the ordinary working-bees may be regarded as
females, with the reproductive organs undeveloped ; and it
appears to depend on the manner in which they are treated
in the larva state, whether the egg shall be made ultimately
to produce a queen or a working-bee. For if, when the
queen has been removed, the royal cells (which are usually
among the last constructed) be not sufficiently forward, and
contain no eggs, the bees select one or more worker- eggs or
larvae, remove the egg or larva on either side of it, and throw
the three cells into one. The larva thus promoted is liberally
fed with "royal jelly," a pungent food prepared by the
working-bees' for the exclusive nourishment of the queen
larvae; and in due time it comes forth a perfect queen.
This change is doubtless owing to the peculiar effect of the
food ; and it is remarkable that it should operate, not only in
developing the reproductive organs, but also in altering the
shape of her tongue, jaws, and sting, in depriving her of the
power of producing wax, and in obliterating the hollows just
referred-to, which would otherwise have been formed upon
her thighs.

Manifestations of Intelligence.

717. The amount of reasoning power possessed by some
among the lower animals, may be considered as very much
upon a par with that exhibited by an intelligent child, about
the time when it is learning to speak. One of its first exer-
cises is in the connexion or association of ideas, which is the

INTELLIGENCE OF LOWER ANIMALS. 547

source of the faculty of Memory, and thus becomes the
foundation of that power of profiting by experience, which is
manifested in the actions of animals that are distinguished
for Intelligence. Such a power is well shown in the following
instance, related to the Author by an eye-witness. A "Wren
built its nest in the slate- quarries at Penrhyn, in such a
situation as to be liable to great disturbance from the occa-
sional explosions. It soon, however, learned to quit its nest
and fly to a little distance, on the ringing of the bell which
warned the workmen. This action, having been noticed, was
frequently shown to visitors, the bell being rung when there
was not to be an explosion ; so that the poor bird suffered
many needless alarms. It seems gradually to have learned,
however, that the first notion it had formed, by the associa-
tion of the ringing of the bell with the explosion, was liable
to exceptions, and to have formed another more correct ; for
it was observed, after a time, that the wren did not leave its
nest, unless the ringing of the bell was followed by the
moving-away of the workmen. — A similar process of associa-
tion, carried rather further, but still quite simple enough to be
readily believed, is shown in two Dogs, which have been
taught by their master to play at dominoes, and which go
through the game with another person (under circumstances
which render the idea of collusion with their master impos-
sible) with the utmost regularity and correctness ; not only
playing rightly themselves, but watching to see that their
adversary does so too. This, also, is a feat which a very
young child might be taught to perform. — A third instance
has reference to the patient endurance of bodily pain, in
opposition to the instinctive tendency to struggle against the
infliction of it, and evidently occasioned by a voluntary effort
on the part of the animal, made by it in obedience to the
dictates of its reason. Dr. Davy mentions having seen an
Elephant, in India, that was suffering under a deep abscess in
its back, which it was necessary to lay open in order to effect
a cure. " He was kneeling down, for the convenience of the
operator, not tied ; his keeper was at his head. He did not
flinch, but rather inclined towards the surgeon, uttering a low
suppressed groan. He seemed conscious that what was doing
was intended for his good ; no human being could have be-
haved better; and so confident were the natives that he

NN2

548 RELATION OF INTELLIGENCE TO CEREBRUM.

would behave as lie did, that they never thought of tying
him." It were much to be wished, that all human beings
would imitate this docile Elephant's self-control. It is some-
times manifested, however, even in Infancy ; the painful
operation of lancing the gums being often sustained without
a cry, from the consciousness of the benefit derived from it.

718. It has been stated that the relative amount of Intelli-
gence in different animals bears a pretty constant proportion
to the size and development of the Cerebral hemispheres
(§ 452). That size alone, however, does not produce the dif-
ference, is evident from a number of facts. As we advance
from the lower to the higher Vertebrata, we observe an obvious
advance in the complexity of the structure of the brain. In
proportion to the increase in the number and depth of the
convolutions by which its surface is extended (§ 456), do we
find an increase in the thickness of the layer of grey or
vesicular matter (§ 61), which seems to be the real centre of
all the operations of the organ. The arrangement of the
white or tubular tissue (§ 60), which forms the interior of
the mass, also increases in complexity ; and as we ascend
from the lower Mammalia up to Man, we trace a great in-
crease in the number of the fibres which establish communi-
cations between different parts of the surface. Still there can
be no doubt that the size of the Cerebrum, compared with that
of the Spinal Cord and of the Sensory Ganglia at its summit,
usually affords a tolerably correct measure of the intelligence
of the animal ; and that, even in comparing together different
Men, we shall find the same rule to hold good, when due
allowance has been made for the comparative activity of their
general functions, such as is expressed by the word tempera-
ment. Thus, two men, having brains of the same size and
general conformation, may differ greatly in mental vigour,
because the general system of one performs its functions
much more actively and energetically than that of the other.
For the same reason, a man of small brain, but whose general
habit is active, may have a more powerful mind than another
whose brain is much larger, yet whose system is inert, his
perceptions dull, and his movements languid. But of two
men alike in these respects, and having the same general con-
figuration of head, it cannot be doubted that the one with the
larger brain will surpass the other. It is a striking fact, that

SIZE OF BRAIN : — FACIAL ANGLE.

549

Fig. 291. — SKULL OF EUROPEAN.

almost all those persons who have been eminent for the
amount of their acquirements, or for the influence they have
obtained by their talents for command over their fellow-men,
have had large brains : this was the case, for example, with
Newton, Cuvier, and Napoleon.

719. The size of the brain, and especially of its anterior
lobes (which seem particularly connected with the higher
reasoning powers), as compared with that of the face, may be
estimated pretty correctly by the measurement of the facial
angle; as proposed by Camper, an eminent Dutch naturalist.
This is done by drawing a horizontal line (cd, figs. 291 and
292), between the entrance to the

ear and the floor of the nose, so as
to pass in the direction of the base
of the skull ; this is met by another
line (a, b) which passes from the
most prominent part of the forehead
to the front of the upper jaw. It is
evident that this last will be more
inclined to the former, so as to make
a more acute angle with it, in pro-
portion as the face is more developed and the forehead more
retreating ; whilst it will approach more nearly to a right angle,
if the forehead be prominent, and the muzzle project but little.
Hence this facial angle will indicate, with tolerable correct-
ness, the proportion which the brain bears to the face, — the
instrument of intelligence, to the receptacle of the organs
of sense.

720. Of all animals, there are none in which the facial
angle is so open as in Man ; and great variations- exist
in this respect, even among the . a

different human races. Thus, in
European heads, the angle is usually
about 80° (fig. 291). The ancient
Greeks, in those statues of Deities
and Heroes to which they wished
to give the appearance of the greatest
intellectual power, made it 90°, or
even more, by the projection they
gave to the forehead. On the other
hand, in the Negro races, it is commonly about 70° (fig. 292) ;

Fig. 292.— SKULL OF NEGRO.

550 FACIAL ANGLE I SPECIAL ENDOWMENTS OF MAN.

in the different species of the Monkey tribe, it varies from
about 65° to 30° (fig. 293) ; and as we descend still lower, we
find it still more acute. In the Horse and Boar, for example,
it becomes impossible to draw a straight line from the fore-

Fig. 293.— SKULL OP MACACOS. Fig. 294.— SKULL OF BOAR.

head to the upper jaw ; in consequence of the retreating
character of the former, and the projection of the nose ; this
will be evident from an examination of fig. 294. In Birds,
Reptiles, and Fishes, the facial angle, when it can be mea-
sured, is found to be still further diminished.

721. It appears, then, that the mind of Man differs from,
that of the lower animals, rather as to the degree in which the
reasoning faculties are developed in him, than by anything
peculiar in their kind. Among the more sagacious Quadrupeds,
it is easy to discover instances of reasoning as close and pro-
longed as that which usually takes place in early childhood ;
and it is only with the advance of age and the maturity of
the powers, that the superiority of Man becomes evident. The
foundation of this superiority lies in the power of self-direc-
tion and self-improvement which Man possesses. No race
among the lower animals ever exhibits a spontaneous ten-
dency to the elevation of its mental powers. When placed
under new circumstances, and especially when subjected to
Human training, the domesticable races acquire new capa-
cities ; and individuals frequently display a very extra-
ordinary degree of sagacious appreciation of matters quite
foreign to their natural habits of life. But neither in races
nor in individuals are these powers transmitted from one
generation to another, when, left to themselves, they return
to anything like a state of nature. In Man, on the other
hand, the power which every rightly-constituted and rightly-

SPECIAL ENDOWMENTS OF MAN. 551

trained individual possesses (§ 525) of fixing his attention
upon any particular object of consciousness, to the exclusion
of all others, becomes the source of the highest and most
enduring intellectual advancement, and of all moral improve-
ment. It is in virtue of this power, that he is not only
enabled to profit largely by the acquired knowledge of others,
but that he comes to possess a moral responsibility for the
use he makes of his faculties, which cannot be predicated
of beings whose succession of ideas is entirely determined
by impressions made from without.

722. There is another attribute, moreover, by which Man
seems to be distinguished from all other animals ; namely,
that disposition to believe in the existence of an unseen but
powerful Being, which seems never to be wanting (under
some form or other) in any race or nation, although (like
other natural tendencies) it may be defective in individuals.
It requires a higher mental cultivation than is commonly to
be met with among savage races, to conceive of this Power as
having a spiritual existence ; but it appears, from the reports
of Missionaries who have laboured to spread Christianity
amongst the Heathen, that an aptitude or readiness to receive
this idea is rarely wanting ; so that the faculty is obviously
present, though it has not been called into operation. — Closely
connected with this tendency to believe in a Great unseen
Power, is the desire to share in His spiritual existence, which
seems to have been implanted by the Creator in the mind of
Man, and the existence of which is one of the chief natural
arguments for the immortality of the soul, since it could
scarcely be supposed that such a desire would have been
implanted, if it were not in some way to be gratified. Such
views tend to show us the true nobility of Man's rational
and moral nature, and the mode in which he may most
effectually fulfil the ends for which his Creator designed him.
"We learn from them the evil of yielding to those merely
animal tendencies, — those "fleshly lusts which war against
the soul," — that are characteristic of beings far below him
in the scale of existence, and tend to degrade him to their
level ; and the dignity of those pursuits, which, by exercising
his intellect, and by expanding and strengthening the higher
part of his moral nature, tend to raise him towards the per-
fection of the Divine Being.

552 TWO PRINCIPAL MODES OF REPRODUCTION.

CHAPTEE XV.

OF REPRODUCTION.

723. THERE is no one of the functions of living beings, that
distinguishes them in a more striking and evident manner
from the inert bodies which surround them, than the process
of Eeproduction. By this function, each race of Plants and
Animals is perpetuated ; whilst the individuals composing it
successively disappear from the face of the earth, by that
death and decay which is the common lot of all. — A very un-
necessary degree of mystery has been spread around this pro-
cess. It has been regarded as one altogether inscrutable,
whose real nature could not be unveiled, even by the scientific
inquirer, and whose secrets the uninitiated should never seek
to comprehend. But so much light has been thrown upon it
by recent investigations, that we now know at least as much
of this, as of almost any other function ; and the Author's ex-
perience has led him to believe that such knowledge may be
communicated to the general reader, without the least in-
fringement of the purest delicacy of feeling. In his own
judgment, indeed, it is far better to afford a legitimate satis-
faction to the curiosity which naturally exists upon the sub-
ject, than, by refusing all information, to drive the inquirer
into objectionable methods of gratifying it.

724. It has been elsewhere shown (YEGET. PHYS., Chaps,
ix., XIL), that, in the Vegetable Kingdom, there are two
distinct modes by which the propagation of Plants may
take place ; — the extension of the parent structure into new
portions, which, being independent of it and of each other,
can maintain their lives when separated from it ; — and the
origination of a new being by the concurrent action of two
sets of cells set apart for this special function, and desig-
nated " sperm-cells " and " germ-cells." The bodies of the
first class are known as leaf-buds or gemmae in the Flowering
Plants, and sometimes also among Cryptogamia, some of which
last, as the Marchantia (VEGET. PHYS. § 757), are fur-
nished with a peculiar means of producing them ; and it
appears from recent investigations that the " spores " of Ferns,

TWO PRINCIPAL MODES OF REPRODUCTION.

553

Mosses, and Hepaticae, as well also the " zoospores " of Algae,
belong to the same class of reproductive bodies. The gemmae
of Phanerogamia may be developed in connexion with the
parent .structure, and may continue to form a part of it ; or
they may be removed from it (as in the processes of budding,
grafting, &c.), and may be developed into new individuals. —
On the other hand, the bodies of the second class are known
as seeds among Flowering Plants ; among the Cryptogamia
they present a variety of forms. From the very first, these
are destined to produce new individuals ; and although they
are often assisted in the early stage of their development by
the parent, they are its true offspring, rather than (like
gemmae) extensions of itself. Both these modes of Keproduc-
tion, namely, gemmation and sexual generation, exist in the
Animal Kingdom ; but the former is confined to its lower-
tribes, among which we often find it exercised in very remark-
able modes.

Gemmiparous or Non-Sexual Reproduction.

725. Among Infusoria (§ 133) we find the process of gem-
mation, or of fission, which is a modification of it, to be almost
the only ostensible means of propagation which the beings
composing that wonderful group possess. The former may be
continually witnessed by the microscopic observer in the
common Vorticella, a bell-shaped animalcule attached by a stalk

Fig. 295.— VARIOUS FORMS OF ANIMALCULES, some of them undergoing spon-
taneous fission.

(fig. 295, a, a), and abundant in almost every pool in which
aquatic vegetables grow, especially clustering around the stems
of Duckweed ; and its various stages closely resemble those

554 GEMM1PAROUS REPRODUCTION OP LOWEST ANIMALS.

which have been already described (§122) in the Hydra. But
not unfrequently in this species, and ordinarily in many others,
the body divides into two equal parts, in each of which we see
a mouth and other parts resembling those of the original. This
division is gradual. A narrowing of the body along or across
its middle (for the fission or cleavage sometimes takes place
lengthways, as at 6, sometimes transversely, as at c), is first
seen j the indentation at the edge becomes gradually deeper,
and at last the two parts hold together by but a narrow band,
which finally breaks, and they become free. — The same
method of multiplication is observed among the simple
Rhizopoda (§ 129); but when the gemmae remain connected
with each other, as in Zoophytes, we have such composite
fabrics as are presented to us in the classes of Foraminifera
(§ 131) and Sponges (§ 136).

726. Reproduction by Gemmation is most characteristically
seen among the Radiated classes j and in none better than in
the Hydra already so frequently referred-to. Although this
interesting little animal sometimes reproduces itself by true
sexual generation (§ 734), yet its usual mode of propagation
is by buds (§ 122), as shown on the left hand side of the ac-
companying figure (fig. 296). And, as already explained, it

is by this same process of gemma-
tion that the arborescent struc-
tures of the Composite Zoophytes
are formed ; the gemmae not de-
taching themselves, but remaining
as parts of the common stock (§§
124,127). In some of those, how-
ever, which are formed upon the
plan of the Sea Anemone (§ 126),
the multiplication (fig. 297) is ef-
fected rather by fission or division
into two equal parts (as among In-
fusoria), than by the out-growth of
buds. We have already had occa-
sion to notice (§ 125) the very re-
markable form of gemmation that
takes place among Zoophytes, giv-
ing origin to independent beings
which seem to belong to a class altogether different, but which

Fig. 296.— HYDIUE (attached to duck-
weed): one of them developing buds.

GEMMATION OF EADIATA AND ABTICULATA.

555

are in reality the representatives of the flower-buds of Plants,
distinguished by their capability, not only of living and en-
during, but of obtaining their own nu-
triment, after their spontaneous detach-
ment from the stock that bore them.
Among the Medusae we occasionally
meet with instances of propagation
by buds that resemble the stock from
which they proceed, and that are
thrown off in due time so as to lead
independent lives ; but this kind of
gemmation seems limited to , -ie lower
members of the group. In a large
proportion of it, however, a very extra-
ordinary kind of multiplication by
gemmation takes place at an early
period of development (§ 740). — In
the highest Eadiata, the class of
Echinodermata, we take leave of multiplication by gemmation
altogether ; for although the bodies of these creatures possess
a very extraordinary reproductive power, so that the result of
very severe injuries may be repaired (§ 389), we do not find
that they either spontaneously produce independent buds, or
that they have the capacity for being multiplied by artificial
division.

727. Among several of the lower Articulata, detached
segments of the body appear to be capable of reproducing the
whole ; and there are some whose ordinary propagation is

Fig. 297.— POLYPES OP

ASTRJEA,

Undergoing fission ; a, b, c,
d, successive stages.

Fig. 298.— NEREIS PROLIFERA.

accomplished by an exercise of this power. Thus in the Nais,
an aquatic worm allied to the Earth-worm, the last joint of the
body gradually extends and increases to the size of the rest of
the animal ; and a separation is made by a narrowing of the

556 GEMMATION OF ARTICULATA AND MOLLUSCA.

preceding joint, which at last divides. Previously to its separa-
tion, however, the young one often shoots out another from
its own last joint, in a similar manner ; and three successions
have thus been seen united. In some species of Nereis, the sepa-
ration takes place nearer the middle of the body (fig. 298). In
the greater number of cases, however, in which such a detach-
ment of the posterior part of the body of Annelids takes place,
the separated gemma does not contain the structure of the entire
animal, but consists of little else than the generative apparatus,
endowed with locomotive organs ; so that this process of mul-
tiplication does not so much correspond with the ordinary pro-
pagation by buds, as with the peculiar development and throw-
ing-off of generative buds to be presently described. — Among
the higher Articulata, we do not meet with any instances of
ordinary gemmation ; but the non-sexual production, which
is now known to take place not only in the Aphides (§ 746)
but in many other Insects, as well as in Rotifera (Wheel-
Animalcules) in Entomostracous Crustacea (Water Fleas, &c.),
and probably in some higher Crustacea, must be regarded as
a peculiar form of the same process ; the offspring being pro-
duced from eggs, which have the power of self-development
without sexual fertilization, and which must therefore be
accounted internal gemmce.

728. In the Molluscous series, the power of multiplying by
gemmation appears to be limited to the Tunicata (§ 114)
and the Polyzoa (§ 115); being restricted in the first of
these classes to a section of the group ; whilst in the second,
which closely follows the habit of Zoophytes, it seems to
be universal. The bud arises in some instances directly
from the body ; but in other cases it is put forth by a stolon
or creeping stem that connects all the bodies together
(fig. 63). Among the Polyzoa the buds usually remain in
connexion with the parent-stock, so as to form composite
fabrics so closely resembling those of Zoophytes as to be com-
monly ranked with them. And the like happens also among
the Compound Ascidians. But where gemmation takes place
among the solitary Tunicata, the bud becomes detached, and
maintains a perfectly independent existence. There is a very
curious case of internal gemmation among the Salpce (a tribe
of Tunicata which are not attached, but float over the waves) ;
for the buds are developed, not from the exterior of the

GEMMATION OF MOLLUSCA AND VERTEBRA! A. 557

body, but from a kind of stolon within it ; and they
differ from their parent-stock in having organs of attach-
ment to each other, whereby they hold together in long

Fig. 299.— AGGREGATE SALP^E. Fig. 300.— SOLITARY SALPA.

chains (fig. 299). These, in their turn, being furnished with
true generative organs, give origin to the solitary SalpaB (fig.
300) by ordinary sexual reproduction ; whilst the solitary
Salpa never comes to possess any sexual apparatus, but merely
continues the race by gemmation from its internal stolon.

729. In the Vertebrated classes, as in the higher Mollusca,
we lose all trace of propagation by gemmation as an ordinary
method of multiplication. Yet there is evidence that this
power is not altogether extinguished, even in Man. For we
not unfrequently hear of " monstrosities by excess," that is,
of cases in which the body possesses a superfluity of some of
its parts ; the simplest cases being those of double thumbs or
of six fingers on each hand, and the gradation being so conti-
nuous from these to cases like that of the Siamese twins (in
which there are two complete bodies united only by a cross
band), as to make it evident that they are all referable to one
common principle. And although it has been commonly
believed that monsters with two heads and one body, or with
two bodies and one head, or with supernumerary legs or arms,
are results of the partial " fusion" of two distinct germs at an
early period, yet there is now far stronger reason to believe that
they proceed from a kind of attempt at multiplication by
fission or gemmation, that is sometimes made by a single germ
at a time when its grade of development corresponds with
that of a Hydra or Planaria ; which attempt, under peculiarly
favourable circumstances, may proceed to the full length of
production of two complete bodies. (See § 390.)

Sexual Reproduction, or Generation.

730. We now have to notice the most important features
of the proper Generative process, which differs from the

558

SEXUAL GENERATION : — SPERM-CELLS.

preceding in exactly the same manner as the flowering and
fruiting of Plants differ from their extension and propagation
by leaf-buds. — In all save the very lowest tribes of Animals,
we meet at particular seasons with two peculiar sets of cells,
termed sperm-cells and germ-cells ; these are sometimes borne
by the same individuals, which then correspond as regards
their reproductive apparatus with the generality of Flowering
Plants ; but they are more commonly separated, as in dioecious
Plants (VEGET. PHYSIOL., § 409) ; the individual bearing the
"sperm-cells" being then designated as the male, and the
individual bearing the "germ-cells" as the female.

731. The " sperm-cells " very closely resemble those con-
tained within the antheridia of Cryptogamia (VEGET. PHYSIOL.
§ 399 ; BOTANY, §§ 737, 776). When mature, each cell is

found to contain one or more spirally
coiled filaments (fig. 301), which,
when set free by the bursting of
the cell, have an active spontaneous
movement lasting for some time like
ciliary action. These filaments were
formerly regarded as true Animal-
cules ; but since other examples of
independent movement have been
discovered in what are certainly
nothing else than detached parts of
the organism, and more especially
since moving filaments of a precisely
analogous character have been dis-
covered in Plants, all idea of their
independent animality has been laid
aside, and they are now known as
spermatozoids. — The use of their
motor activity is obviously to bring
them into contact with the germ-cells, when both have been
set free from the interior of the bodies within which they
were formed. When the sperm-cells are developed in a
special or distinct organ, as happens in all save the lowest
types of Animal structure, this organ usually more or less
resembles the ordinary glands in structure (§§ 356, 357), and
is termed the Testis.

732. The "germ-cells" are not so clearly distinguished

Fig. 301. — SPERMATOZOIDS :
e, immature sperm-cells.

GERM-CELLS : FERTILIZATION OF GERM. 559

from other cells by the nature of their contents, though they are
usually recognisable by the peculiar nuclei they present ; each
cell is known as the germinal vesicle (fig. 302, d), whilst its
nucleus (e) is designated the germinal spot. — The act of fertiliza-
tion appears to consist in the contact of one or more spermato-
zoids with the exterior of the germinal vesicle ; the sperma-
tozoids, ceasing to move, undergo a sort of liquefaction ; and
the product of their dissolution, being received by absorp-
tion into the interior of the germinal vesicle, mingles with its
contents, to form with them the basis of the new structure. —
When, as usually happens, the germ-cells are developed in a

Fig. 302. — SECTION OF OVAB.IUM OF FOWL:

a, fibrous substance of the ovary; b, yolk; c, yolk-bag; d, germinal vesicle; e,
germinal spot.

special and distinct organ, this organ, which is termed the
Ovary, has very commonly among the lower animals a glan-
dular character, the mature ova being discharged by the ovi-
duct, just as the products of secretion pass-off through the
ducts of their respective glands : but among the Vertebrata
the ovary has a much more solid texture, and the germ-cells,
developed in the very midst of its fibrous tissue (fig. 302, d),
have to find their way to its surface, and to burst forth from it ;
being then received into an oviduct, whose trumpet-shaped
mouth embraces the ovary, so as to prevent the liberated
germs from falling (as they would otherwise do) into the
visceral cavity of the body.

733. "With the "germ-cell" there is always associated in
Animals, as in Flowering Plants, a store of nutriment that
serves for the early development of the germ ; this consists
of a mixture of albuminous and oily matter, known as the
yolk (fig. 302, b) ; and it is inclosed in a membranous envelope c,

560 SIMPLEST FOKM O7 GENERATIVE PROCESS.

termed the yolk-bag. The yolk-bag and its contents, namely
the yolk and the germinal vesicle, constitute the ovum. The
eggs of many animals (as of Birds) contain an additional
store of liquid albumen, the " white," enveloping the yolk-bag
and destined to be gradually drawn into" it, so as to replace
the albumen of the yolk as it is progressively used-up in the
development of the embryo ; and the " shell " is a subsequent
formation around this (§ 755).

734. The Hydra presents us with a very apposite illustra-
tion of the simplest mode in which the generative function is
performed. Sperm-cells are developed at certain periods in
the substance of its body near the origin of the arms ; whilst
ovules are evolved in the wall of the stomach nearer to the
foot or base. By the rupture of the sperm-cells, their con-
tained spermatozoids are set free in the surrounding water,
and they penetrate to the ovules, which are exposed to their
influence by the thinning-awa'y of their exterior covering.
From what has been observed in higher animals, there seems
no reasonable doubt that the spermatozoids make their way
through the germinal membrane, and penetrate into absolute
contact with the germinal vesicle, which then lies near the
surface of the ovule. What is the precise change effected by
fertilization, has not yet been fully ascertained ; the germinal
vesicle, however, disappears ; and it would seem as if its
contents, with the product of the liquefaction of the sperma-
tozoids, were diffused through the yolk, which soon begins to
undergo changes of a very remarkable nature.

735. Before going further, it may be well to notice the
remarkable antagonism which exists between the processes of
Gemmation and Generation, as regards the conditions by which
they are respectively favoured. For we see that in the Hydra,
as in Plants, the extension of the body into buds is promoted
by warmth and a copious supply of food ; so that, as it would
appear, if these be afforded, this mode of multiplication may
be protracted indefinitely. On the other hand, if the supply
of food be limited and the temperature lowered, the pro-
duction of buds ceases, and the formation of sperm-cells and
of germ-cells begins. The result is that ova or eggs are
produced, which have a very firm horny covering, and possess
a great power of resistance to cold; and thus provision is
made for the continuance of the race through a winter tern-

EARLIEST STAGES OF DEVELOPMENT OF OVUM. 561

perature that might be fatal to the Hydrse themselves. The
same thing is observable among the Rotifer a; for, as has
long been known, two kinds of eggs are produced by them,
the ordinary and the "winter eggs ;" and it now appears that
the ordinary eggs, being evolved without any generative pro-
cess, and with a rapidity proportional to the favouring
influences of food and warmth, are really to be regarded as
internal gemmae ; whilst the " winter eggs," which are pro-
duced in the autumn by the concurrent action of males and
females, and have a peculiarly dense horny investment, are
the only true ova. Among the Aphides (§ 746), again, it has
been experimentally shown that the non-sexual multiplication
may be indefinitely protracted by warmth and food ; whilst a
reduction in the temperature and in the supply of nutriment
causes this at any time to give place to sexual generation.

736. The first obvious change that presents itself in the
Ovum, after its fertilization, is the " segmentation," or division
.of the yolk-mass into two halves, by the formation of a sort
of hour-glass contraction, which gradually deepens, until it
produces a complete separation. Another segmentation of
these two halves soon follows in the opposite direction, so
that the yolk-mass becomes divided into four segments ; each
of these in its turn undergoes the like subdivision ; and this
duplicating process is repeated, forming successively 8, !(>„
32, 64, &c., segments, until a " mulberry-mass " is produced,,
which is composed of an aggregation of an immense number
of minute yolk-spherules. Up to this stage, the develop-
mental process takes place on essentially the same plan in all
animals, save that in some the process of segmentation does
not extend to the entire mass of the yolk, but only to a small
proportion of it, which is distinguished as the "germ-yolk,"
whilst the remainder, which is applied to the nourishment of"
the more advanced embryo through an entirely different chan-
nel, is known as the "food-yolk" (§ 754).

737. It appears, among some of the simplest Worms, as if
the " mulberry-mass " gradually shaped itself into the body
of the animal, without the intervention of any intermediate
structure ; but in almost all Animals, the first stages in deve-
lopment tend to the production of a membranous expansion
that may be likened to the " cotyledon " of Flowering Plants, —
with this important difference, however, that whilst the latter

o o

562 GERMINAL MEMBRANE. — DEVELOPMENT OP POLYPES.

spreads itself out so as to come into contact with the " albu-
men" of the seed by its external surface, the "germinal
membrane " of the Animal forms itself around the yolk, and
thus constitutes as it were a temporary stomach, within which
the nutrient material is stored-up, and through the walls of
which it is drawn into the embryo. This is accomplished in
the following manner. The spherules of the outer layer of
the mulberry-mass which are in immediate contact with the
yolk-bag become invested with walls of their own, and thus
become converted into proper cells ; these are somewhat flat-
tened and of a polygonal shape, very much resembling those
of the epithelium of serous membrane (fig. 10). Another
layer is afterwards formed within this, the cells of which
retain more of their original globular form. But the spherules
of the internal portion of the mulberry-mass, instead of be-
coming converted into cells, undergo dissolution and return
to the condition of a liquid yolk ; so that the ovum, in this
stage of its development, consists of two layers of cells, con-
stituting what are known as the " serous " and the " mucous "
layers of the germinal membrane, enclosing a mass of nutritious
matter on which a change has been worked that seems to
predispose it to become organized.

738. The development of the Polypes seems to advance
but little beyond this point. The covering of the ovum
bursts, and the contained embryo is set at liberty as soon as
the germinal membrane has been formed around the yolk.
In this state it becomes clothed with cilia, and is termed a
gemmule ; and it swims about freely in water for some time.
Its form gradually becomes more elongated (fig. 303),
tapering away at one end, which attaches itself after
a time to some solid body \ and its development
into the polype-form soon commences. In the group
of which the Hydra is an example, this change takes
place in the following simple manner. The germinal
membrane gradually thins away at the point furthest
removed from the attached base, and at last an aper-
ture is formed, which becomes the mouth ; from around this
aperture the tentacula or arms shoot forth, a single row being
first formed, and others being afterwards added in those species
in which they are numerous. Thus the two layers of the
germinal membrane enter into the permanent structure of

DEVELOPMENT OF POLYPES AND MEDUSAE. 563

the animal ; the outer one constituting the external integu-
ment, and the inner becoming the lining of the stomach.
The arborescent fabric of the composite Hydrozoa (§ 124)
is gradually evolved by continuous gemmation from the original
Polype ; and whilst in some of them the sperm-cells and
ova are developed within peculiar capsules not ostensibly
differing (except in size) from the ordinary polype-cells,
there are others in which they are the product of peculiar
buds having the form and structure of Medusce, which buds
in many instances become detached, and henceforth live as
independent zooids, their sexual apparatus being only evolved
after they have separated themselves from the parent stock.
The sperm-cells and the ova are developed within different
Medusa-buds ; but both kinds of buds may (in many cases at
least) be put forth from the same Polype-stock, as in moncecious
Flowering-Plants.

739. Although the two layers of the germinal membrane
remain united in the Hydra and other Zoophytes formed upon
its simple plan, they separate from each other at certain points
in the Sea Anemone and its allies, so that a series of chambers
is formed between them ; and these chambers are afterwards
set apart for the production of sperm-cells and germ-cells
(§ 126). We do not meet in this group of Anthozoa with
any example of that detachment of the sexual apparatus in
the form of separate zooids, which is so remarkable a feature
of the Hydrozoa.

740. The development of the Medusce, as elucidated by
recent discoveries, presents several features of extraordinary
interest. The sexes are distinct in these animals ; sperm-
cells being developed in some individuals, and ova in others,
within the fxrar chambers that surround the stomach (§ 120).
When the ova have received the fertilizing influence, their
first products are ciliated gemmules resembling those of
Hydraform Polypes (fig. 304, a). These, after moving about
for some time in the ovarial chambers of their parent, make
their exit by the orifices of these, and then swim freely
through the water. Gradually, however, they undergo the
usual elongation, and fix themselves by one extremity (e) ; at
the opposite extremity a depression appears in the middle,
which is to become the mouth, as seen at 6, and an elongation
of the four corners (c,f) gives origin to the first tentacula,

o o 2

564

DEVELOPMENT OF MEDUSAE.

Fig. 304.— DEVELOPMENT OF MEDUSJE.

which are afterwards increased by the addition of many others
(g). In this manner a true polype is formed, which leads the
life of a Hydra, and, like it, propagates its kind by the forma-
tion of polype-buds, which detach themselves and lead inde-
pendent lives ; and thus from a single Medusan egg there may
arise a whole colony of polypes multiplied by gemmation. These
differ entirely from Hydras, however, in regard to their sexual
apparatus, which is detached (as in the" composite Hydrozoa)
under an entirely different form, that of a Medusa. The body of

the polype undergoes a great
lengthening, and seems as if
divided by transverse bands,
which gradually deepen, so as
to make the whole body almost
resemble a pile of saucers
with divided edges (h) ; for
beneath the lowest of these
saucer-like disks, a new set
of polype -arms makes its
appearance ; and after the
detachment of the whole pile of disks, the polype-body
remains at their base, and may continue to lead its former
life, and to propagate itself in the polype-form. The disks
progressively enlarge, those at the summit of the pile in-
creasing most rapidly, and then detaching themselves from
the pile (i) ; when thus detached, they swim about freely in
the water after the manner of the smaller and simpler Me-
dusas, to which they closely correspond in form (d) ; and they
gradually enlarge and acquire the structure of their original
parents (&). It is not correct to represent (which is commonly
done) the pile of Medusa-disks as being formed1 by the sub-
division of the polype-body. The Medusa-disks are in reality
sexual buds, resembling those of the composite Hydrozoa
(§ 125); and the only essential difference between the two
cases lies in the fact, that among the latter it is the Zoophytic
form which ostensibly constitutes the animal (the Medusan
buds being thrown-off only at certain times for a special pur-
pose), whilst the former are only known (save to such as
search-out the history of their polypoid development) in the
Medusan stage of their lives.

741. The recent researches of Professor Muller and others

DEVELOPMENT OF ECHINODEKMATA. 565

have brought to light a most remarkable set of facts in regard
to the developmental history of the Echinodermata. The
details of this history vary so greatly in the different sections
of the group, that all which can be here attempted is a
general notice of its most important features. The em-
bryonic mass of cells, which is produced in the ordinary way
from the egg, is usually converted, not (as in Insects) into a
larva which is subsequently to attain the perfect form by a
process of metamorphosis, but into a peculiar being, destined
to a merely temporary existence, whose function seems to be
to give origin to the real Echinoderm by internal gemmation,
to obtain and prepare for it the materials of its development,
and to carry it to a distance from its fellows, so as to prevent
the spots inhabited by the several species from being over-
crowded by the accumulation of their progeny. These larval
zooids present many points of resemblance to the larvae of
certain Annelids ; their bodies have a bilateral not a radial
symmetry, the two sides being exactly alike ; each side

Fig. 305.— PENTACRI.XOID LARVA OF COMATULA. Fig. 306.— COMATULA.

is furnished with, a ciliated fringe along the whole or the
greater part of ifs length ; and the two fringes are united
by an upper and a lower transverse ciliated band, between
which the, mouth of the zooid is situated. Although the
adult Star-fish and Sand-stars have neither intestinal tube nor

566 DEVELOPMENT OF ECHINODERM AT A AND ENTOZOA.

anal orifice, their larval zooids, like those of other Echino-
derms, always possess both. It is from the side of the intes-
tinal canal, that the young Echinoderm is usually budded-off.
In some instances it separates itself completely from the
zooid, when it has attained a certain stage of development,
no part of the latter entering into its composition ; this seems
to be the case in the Comatula, which, present this further
remarkable feature, that the young Echinoderm at first
attaches itself to some fixed object by a footstalk (fig. 305),
so as to resemble the fossil Encrinites in every essential
particular, but afterwards becomes detached, and henceforth
remains free (fig. 306). In the Starfish and Echinus, the only
part of the larval zooid which is retained in the Echinoderm,
is a portion that is (as it were) pinched-off from the stomach
and intestines. In the Holothuria (fig. 67), on the other hand,
which has a much closer conformity to the type of the Annelids,
a much larger part of the larva is retained in the adult, and
the process of development more nearly resembles an ordinary
metamorphosis.

742. Passing-on now to the Articulated series, we find
that the developmental history of its lower forms presents
phenomena not less remarkable than those already noticed,
feecent studies on the propagation of the Entozoa have re-
moved many of the difficulties previously felt -in regard to
their mode of passage from one animal to another ; by
showing that the same creature may exist under two or more
forms, which may differ so greatly from each other as appa-
rently to belong to separate orders. This has now been
ascertained to be the case, for example, in regard to the
Tcenia or Tape- worm (fig. 53) and the Cysticercus (fig. 307).
The segments of which the body of the Tape-worm is composed,
are in reality repetitions (like the medusa-buds of a Hydroid
zoophyte) of its generative apparatus ; with this difference,
however, that each segment contains both kinds of sexual
organs, so that the eggs it contains are fertilized without any
extraneous assistance. These segments, when mature, detach
themselves one by one ; and being voided from the intestine,
fall to the ground, over which the eggs they contain become
disseminated by various agencies. Being swallowed with
the herbage or the water ingested by herbivorous animals,
the eggs are conveyed into their stomachs, where* the little

DEVELOPMENT OF ENTOZOA I OLENIA AND CYSTICERCUS. 567

embryos escape from them. These embryos, which are small
vesicles furnished with six minute hooks or spines, make
their way through the walls of the stomach into the substance
of other viscera ; and by getting into the current of the cir-
culation, they are sometimes carried to remote parts of the
body. Nourished by the juices which it absorbs, the vesicle
swells : and a head resembling that of the Tape-worm (fig.
307, a), begins to bud from its wall into its cavity. In this
condition the product of the egg of the Tape-worm has long
been known as the Cysticercus (its presence in large numbers
in the flesh of the Pig giving to it that diseased appearance
which is known as " measly "), without its relationship to its
parent being in the least suspected ; and it undergoes no
further change until the flesh of the animal it inhabits is
devoured by some other, so that the Cysticercus is conveyed
into the intestinal canal of the latter. The head which was
previously turned into the vesicle, now protrudes from its
exterior (fig. 307), and attaches
itself to the intestine of its new
host by means of the hooks and
suckers with which it is furnished
(a) ; the vesicle is then cast off,
and its place is taken by the
series of generative segments suc-
cessively budded-forth from the
head, which constitutes the body
of a new Tape-worm ; and from
the ova which these produce there
springs a new generation, which

repeats the same curious cycle. — Numerous other parasites
present a history that resembles the preceding in
all its essential features, whilst varying in details
(see ZOOLOGY, §§ 925, 926).

743. The Trematode Entozoa, of which the Dis-
torna (known under the name of fluke) that infests
the livers of Sheep is a characteristic example,
undergo a yet longer succession of changes j these
have been especially studied in a species which
infests the Lymnceus, one of the Water-snails, and
which is represented in fig. 308. From the egg
deposited by this Distoma is produced a long flat-bodied embryo

Fig. 307.— CYSTICERCUS ; a, head
greatly enlarged.

568

DEVELOPMENT OF ENTOZOA.

(fig. 309) having a sac-like body clothed with cilia and capable
of motion ; within the anterior part of which is to be observed
an elongated stomach s, whilst the posterior portion of the
cavity is occupied by a number of bodies a, having a general
resemblance to itself, which are produced from it by a process
of internal gemmation. These, when mature, burst forth
from the containing cyst, and develope themselves into
worms (fig. 310) not very unlike the preceding in general
form and organization, though differing from them in some
important particulars; and in their turn they produce, by
internal gemmation, a fresh brood of bodies much more
dissimilar to themselves. These, when set free in due time,

Fig. 309.
GRAND-NURSE OF CEHCARIA.

Fig. 310.
NURSE OF CERCARIA.

develope themselves into a form which has long been known
as the Cercaria (fig. 311), and which has a tadpole-like body
with a large sucker a in its middle, a triangular head, a long
tail by the motion of which it swims, and various viscera c
in its interior. The Cercarise attach themselves by means of
their sucker to the bodies of the Lymnseus, and then begin to
undergo an important metamorphosis. The tail, being now
useless, falls off ; the animal becomes invested with a mucous
substance, within which it lies encysted like a chrysalis
within its cocoon ; and, on its emergence from this it presents
itself as a Distoma, which is ready by the performan.ee of the
true generative process, to set in renewed operation the whole

DEVELOPMENT OF ENTOZOA AND ANNELIDA. 569

succession of these changes. — Thus between every act of
generation there intervene two sets of gemmations, by which
a single embryo may produce a multitude of Cercarise ; and
the conversion of the Cercaria into the Distoma involves, in
addition, a metamorphosis not less complete than that of
Insects. The body (fig. 310) within which the Cercariae are
developed, and which is the second remove from the Dis-
toma, has been called their "nurse ;" and that (fig. 309) from
which the nurses themselves are developed, and which is the
first remove from the Distoma, has been called their " grand-
nurse."

744. Among the Annelida, or Worms properly so called,
there is considerable variety in the history of development ;
some of them, as the Leech and Earth-worm, coming forth
from the egg in a nearly perfect state ; whilst in most of the
marine worms that state is not attained until long after the
embryo has begun to lead an independent life. This embryo,
on its first emersion from the egg, very commonly has an oval
or roundish body, furnished with one or more bands of cilia,
by the agency of which it swims freely in the water ; the
body then gradually becomes elongated, and additional bands
of cilia make their appearance ; and after a time a mouth and
intestinal canal are formed, indications of eyes and of a seg-
mental division show themselves, and the cilia disappear, their
place being usually taken by bristly appendages. The body is
gradually elongated by the production of additional segments,
sometimes to the number of several hundred ; each new seg-
ment being formed between those which were previously the
last and the last but one. Thus the formation of the body of
the Worm is really accomplished by a process of continuous
gemmation ; and it is therefore the less surprising that some
of these worms should be capable of producing independent
buds (§ 727), which buds, however, not unfrequently resem-
ble the segments of the Tape-worm, in containing nothing
else than the generative apparatus, save locomotive organs for
the purpose of dispersing its products.

745. It was in the class of INSECTS that the phenomena of
metamorphosis were first studied ; and notwithstanding the
familiarity of the leading facts of the case, it is desirable to
recapitulate them here, for the sake of showing their relation-
ship to those we have been already considering. There are

570 METAMORPHOSIS OF INSECTS.

some Insects (such, as the Grasshopper and the Cricket) which
come-forth from the egg in a form so nearly resembling that
which they are ultimately to present, that the deficiency of
wings is their principal difference. Such are said to undergo
an incomplete metamorphosis; the fact being, however, not
that these finally attain a less elevated condition than other
Insects, but that they make a much nearer approach to it in
that part of their embryonic state which they pass within the
egg. In the tribes of Beetles, Butterflies, Bees, and Flies, on
the other hand, the embryo comes-forth from the egg in the
condition of a Worm ; and only acquires either the form or
structure of an Insect after a complete metamorphosis, in which
every part of its organization undergoes important modifica-
tions. The larva, sometimes known as a " maggot," sometimes
as a " caterpillar " or " grub," is in many instances completely
destitute of legs ; and where it does possess feet by which it
can crawl, these are not jointed members, but mere fleshy
protuberances. The segments are all nearly equal and similar,
both externally and internally; they are never more than
thirteen in number, counting the head as one • and having
been all formed in the first instance by the subdivision of the
original yolk-mass, they undergo no subsequent augmentation
but that of size. The voracity of the larva is its most extra-
ordinary characteristic ; and its increase in bulk is propor-
tional, the full-sized larva being estimated in some instances
to weigh no less than 72,000 times as much as it did when it
came-forth from the egg (§141). During this rapid increase, its
skin is several times thrown off; a new one being first formed
within, this, better adapted to its augmented size. Very little
change takes place in the structure of the larva, until after
the completion of its growth ; it then ceases to eat, and fre-
quently forms some protection to itself, either by spinning a
silken cocoon, or by gluing bits of stick, straw, &c. into a
case, or it may bury itself in the ground. The last larva-skin
hardens into a firm case around the body, which, diminishing
in size, shrinks away from its interior. The creature, now
known as a chrysalis or pupa, remains for some time without
food and apparently inert ; important changes, however, are
taking place within its body, which tend towards the forma-
tion of the organs of the perfect insect ; and these are pro-
duced at the expense of the mass of nutrient material that

METAMORPHOSIS OP INSECTS. 571

had been stored-up within the body of the larva. "When the
development of the wings, legs, &c. has been completed, the
Imago or perfect insect bursts-forth from' its pupa-case, and
enters upon the life of activity for which it is destined. In
this condition alone does it possess proper generative organs ;
and the business of rearing the larvae, or of preparing a habi-
tation in which they shall find a store of food laid-up for
them, seems generally one of the principal objects of its
existence. In many instances, indeed, as in the Silkworm,
the Imago takes no nourishment whatever, and dies as soon
as the generative act has been completed, and the fertilized
eggs have been deposited. — It is scarcely possible to find a
greater contrast than that which exists between the footless
Maggot, almost destitute of the power of movement, and
having no capacity but that of gorging itself with the nou-
rishment provided for its sustenance, and the active Bee,
almost constantly on the wing, darting from flower to flower
in search of the honied sweets which it is now content to
sip, and coming home to toil in the construction of that
wonderful edifice which human skill could have scarcely
rivalled, certainly not surpassed. And it is evident that the
true way of looking at the metamorphoses of Insects, is, to
consider the chrysalis condition as a continuation of the de-
velopmental process which takes place within the egg ; the
larva being adapted to come forth into the world for a time
in a very immature condition, that it may obtain for itself
such a supply of nutrient material as could not have been
stored up within the egg, without adding so greatly to its
bulk, as to render impossible that enormous multiplication in
the number of eggs which is so characteristic a phenomenon
in the history of this class.

746. A remarkable departure from the method of sexual
propagation common among Insects, has long been known to
occur in the tribe of Aphides, or "plant-lice" (ZooL. § 785);
which multiply themselves while yet in a state of develop-
ment that may be considered as larval, without any proper
generative act. No distinction exists between males and
females, but every individual is formed upon the ordinary
female type ; and eggs are produced from an ovarium, which
are hatched within the body, so that the young come forth
alive. These in their turn repeat the same process within a

572 AGAMIC REPRODUCTION OF APHIDES, &C.

very short time ; arid one viviparous brood succeeds another,
so long as adequate warmth and food are supplied. Ten or
twelve are thus commonly put-forth in a single season ; and
as each brood may consist of a hundred individuals, it may
be easily calculated that no fewer than ten thousand million
million of Aphides may thus be produced in one summer from
a single individual. With the advance of autumn, however,
the last brood of larval Aphides, instead of continuing to
propagate after this fashion, is developed into the perfect
sexual form ; distinct males and females present themselves •
the true generative process is performed ; and, as its result,
eggs are deposited, that are capable of resisting the cold of
winter, which would be fatal to the viviparous larvae. From
these eggs, larval Aphides are hatched in the spring, which
repeat the same curious series of phenomena. — Recent in-
quiries have shown that this method of propagation is by no
means confined to the Aphides, but is common to many other
Insects. Thus, it appears that the various species of Cynips
or gall-fly (ZooL. § 755) are for the most part known only
under the female type, and that they can propagate without
any male ; the eggs which they deposit producing larvas, which
are developed into the likeness of their parents without re-
ceiving any fertilization. It may be surmised that, as among
Aphides, males make their appearance under certain condi-
tions, and that a proper generative act occasionally intervenes
between the successive productions of non-sexual broods.
Comparing these phenomena with those of the gemmation of
Salpa3 (§ 728), and with other cases of like nature, it might
be supposed that the non-sexual or agamic1 production of
Aphides, Cynipidce, &c. is only another case of the same kind.
But there is this peculiarity about it, — that the young are
produced, not from buds, but from bodies having all the
characters of ordinary eggs. And it would seem, from the
facts next to be mentioned, that in certain cases the same
ovum may develope itself either with or without fertilization j
its product in the two cases, however, being different.

747. Among Bees, Wasps, Ants, and other social Insects,

the generative process is performed in a very peculiar manner.

By far the larger proportion of their communities are neuters,

that is, are incapable of reproduction ; the continuance of the

1 From o, not ; and 7o(uoy, marriage.

AGAMIC REPRODUCTION OF BEES. 573

race being effected by a comparatively small number of indi-
viduals. Thus, among the common Hive-Bees, the queen is
the only perfect female ; the drones are the males ; and the
workers are neuters. But these neuters are undeveloped
females, as is shown by the curious fact already mentioned
with regard to the capacity of their larvae for being developed
under certain conditions into queens (§ 716). In the case of
the " queen-bee," — as in the still more remarkable case of the
queen Termes (ZooL. § 740), which will lay 80,000 eggs in
twenty-four hours, and will continue to do so at the same
rate for many weeks, — a single generative act suffices for the
fertilization of a long succession of ova, though a large pro-
portion of these may have been undeveloped at the time of
its occurrence ; for the spermatic fluid is stored-up in a little
receptacle opening-off from the oviduct of the female, so that
a minute portion of it may come into contact with the eggs,
as they descend one after the other. But it appears from
recent inquiries, that the worker-eggs alone undergo this
fertilization, and that the drone-eggs are deposited without
receiving it ; and there is strong reason to believe that the
very same eggs may be developed either into workers or into
drones, according as they do or do not receive the influence
of the spermatic fluid. When engaged in depositing her
eggs, the queen moves over the cells of the comb, apparently
without any order, dropping an egg into each ; and it seems
to be determined by the size of the cells (those prepared for
the drone-eggs being of larger diameter than those destined
for the worker-eggs), whether or not this fertilizing act shall
be performed as the eggs descend. It has long been observed,
that queen-bees will occasionally deposit eggs without having
left the hive for the "nuptial flight" with the male ; and the
eggs thus deposited always prove to be drones. It has also
been observed that eggs are occasionally laid by workers ;
and of these also the products are always drones. Hence, it
seems certain that the drones are always developed from
agamic or unfertilized eggs ; and it would appear that these
very eggs, if fertilized by the male spermatic fluid, would
produce workers. This is one of the most curious discoveries
yet made in the physiology of Bees; and it remains to be
determined how far the same thing is true among other tribes
of Insects.

574

METAMORPHOSIS OP CRUSTACEA.

748. In the class of Crustacea there is great variety as
to the degree of metamorphosis undergone by the young after
their emersion from the egg ; for whilst there are several in-
stances in which either the mature form, or one closely re-
sembling it, is presented from the first, the more common fact
is that the early or larval condition is extremely dissimilar to
that of the parent, and that a succession of changes has to
be gone-through before the latter is attained. This is in no
instance more remarkable than in that of the common Crab,
whose larva (fig. 48) was long known under the name of
Zoea, and was supposed to belong to a type altogether dif-
ferent. No instance of agamic reproduction by eggs is
as yet known among the higher Crustacea ; but in the
Entomostracous division there are probably many examples
of it. Thus in the little Daphnia (ZooL. § 879), one of the
commonest of the " water fleas," the ordinary eggs seem to be
always " agamic ; " whilst the eggs which are formed within
the peculiar case termed the epkippium, and which seem
enabled by its protection to endure a degree of cold that is
fatal to the ordinary eggs as well as to the parents, are the
products of sexual action. The Cyclops, again, has been
found, like the Aphis, to produce many
successive broods, which broods repeat the
like mode of propagation, without the appear-
ance of a male. — Among the examples pre-
sented by this class, of entire change of form
in the progress of development, none are
more remarkable than those which are met
with among the suctorial tribes, which live
as parasites upon the exterior of other
animals, especially Fishes. As an example
of this change we may refer to the Lerncea
(fig. 312), an animal which is not unfre-
quently found clinging to the eyes and gills
of fish, the anterior part of its body being
commonly imbedded in the substance of the
part to which it attaches itself. This creature
is characterized by the size of its large suc-
torial trunk a, and by that of its single pair
of legs c, which terminate together in the sucker/; and by the
immense development of the abdominal portion of its body d,

Fig. 312.— LERNJEA.

METAMOEPHOSES OF CRUSTACEA AND CIRRHIPEDS. 575

as well as of the two large egg-capsules e, which, are attached to
it. Yet in its larval condition (fig. 313) it is an active little
creature, resembling in all essential particulars
the larvas of Entomostraca generally ; and from
this type the males do not depart nearly so much
as the females, the former retaining the general
plan of structure, as well as the activity of habits,
that prevail among the Entomostraca, whilst the
latter lose their instruments of movement, acquir-
ing the apparatus of suction and prehension.

749. A still more remarkable example of meta-
morphosis is presented by the tribe of Cirrhipeds
(§ 102), which, notwithstanding that they were
long ranked as Mollusks, on account of their shelly invest-
ment and their immovable attachment to solid bodies, are
now known to be so closely related to Crustacea, as in
the opinion of many naturalists to rank merely as a sub-
division of that group. The young, alike of the Lepas or
" barnacle," and of the Balanus or " acorn-shell," very much re-
semble those of the ordinary Entomostraca ; they possess eyes
and several pairs of motor appendages in this state ; and they
swim freely through the water, after the manner of water-fleas.
Before their last change, they are enclosed in a bivalve cara-
pace like that of Cypris ; and they then possess a pair of
large four-jointed antenna, which are well furnished with
muscles ; and it is by these antenna that the animal finally
attaches itself, by means of a peculiar cement which is poured
out from ducts running up into them from the body. In the
anterior of the recently-attached larva, the young Cirrhiped
may' be detected with its valves and cirrhi, like the embryo
insect in its pupa-case ; and the carapace and integuments of
the larva being thrown-off like a pupa-case, the perfect form
is disclosed. In most of these animals, as in many mollusks,
the sexes are united in the same individuals ; but when they
are separate, the males are minute imperfectly-formed crea-
tures, which lead a sort of parasitic life upon the surface of the
females ; and in some of the hermaphrodite species " supple-
mental males" are also provided.

750. Among the Rotifera or Wheel- Animalcules, the double
mode of reproduction by agamic and by sexual eggs seems to
be the ordinary rule. The former may go on at a prodigious

576 DEVELOPMENT OF ROTIFERA AND ARACHNIDA.

rate ; so that from the known rate of propagation in a
Hydatina, it is calculated that nearly seventeen millions might
be produced from a single individual within twenty-four days,''
although not more than three or four ova are being brought
forward at once. The latter takes place only at certain
seasons, most frequently before the winter ; the sexual eggs,
in these as in other cases, being endowed with a special power
of resisting cold. It is only occasionally, therefore, that
males are to be met with ; and it is a remarkable circumstance
that they are in many instances destined for so brief an exist- "
ence, as not even to be furnished with any digestive apparatus ;
so that their development is completed, and their reproduc-
tive function performed, at the sole expense of the nutriment
which was furnished by the egg.

751. In the class of Arachnida there is nothing that corre-
sponds with the metamorphosis of Insects ; for Spiders and
Scorpions attain to the full development of their organs within
the egg, so that the young come-forth from it differing in
scarcely any respect from their parents, except in size ; and
among the Acaridce or " mites," the only important difference
lies in the deficiency of one of the four pairs of legs, which is
supplied after the first moult. This completion of the process
of development within the egg could scarcely take place, but
for the large supply of nutriment afforded to the embryo
by the yolk. The Arachnida deposit a far smaller number of
eggs than Insects do ; and thus, as each egg can be made of
much greater size, there is no necessity for that early emersion
of the embryo in search of the material for its continued de-
velopment, which has been shown to be the real purpose of
the larva-life of Insects (§ 745).

752. In the Molluscous series the generative function pre-
sents few such peculiarities as have been noticed among Articu-
lata. Save in the lowest members of the series, the Tunicata
and the Polyzoa (§§ 114, 115), we have no example of repro-
duction by gemmation ; and no instance of reproduction by
agamic ova is yet known. The union of the two sexes in the
same individual is much more common in this series than
among Articulata; and thus the fertilization of the eggs is
secured without the exercise of locomotive power. It is
remarkable, however, that the embryos even of such Mollusks
as are destined to remain almost motionless when they attain

DEVELOPMENT OF MOLLUSCA. 577

their adult condition, are adapted to move actively through,
the water : those of the Polyzoa swimming forth as ciliated
gemmules ; those of the Tunicata having a tadpole-like tail,
formed by an outgrowth from the " mulberry mass," which
propels them by its lateral strokes ; and those of the Gastero-
pods having two large ciliated lobes, much resembling the
"wheels" of Eotifera, placed one on either side of the mouth.
It is further remarkable that all Gasteropods possess a shell
in their early condition, whether or not they are to possess
one ultimately. Among some of the Pectinibranchiata, which
constitute the highest order of this class (ZooL. § 985), a very
remarkable provision has been observed for carrying-on the
development of the embryo to a more advanced stage than is
attained in other instances within the egg. In addition to the
" germ-yolk," which undergoes the usual processes of fission
and of conversion into the mulberry-mass (§ 736), by the sub-
sequent metamorphoses of which the body of the embryo
is formed, we find a quantity of " food-yolk " stored-up with
each embryo, which may be likened to the supply that is
provided by many Insects for the nutrition of their larvas on
their first emersion frpm the egg (§ 703) ; this store is greedily
devoured by the embryos, as soon as they have a mouth to
swallow it and a stomach to hold it ; and it is at the expense
of this, that all their later development is carried on.

753. The class of Cephalopods, in which the sexes are
always separate, presents us with some extremely curious
provisions for bringing the products of the sperm-cells into
contact with the eggs. "Whilst passing through the duct that
conveys them forth from the glandular organ within which
they are formed, the spermatozoids cluster together ; and these
clusters become invested with peculiar casings, which, when
immersed in water, have a peculiar movement that enables
them to advance through it, and causes them, when they meet
with an obstacle, to rupture and set free their contents. In
this mode it is that the spermatozoids find their way into the
midst of the large grape-like clusters of eggs which have been
deposited by the females, and which thus receive the fertili-
zing influence of the male. — In the Argonaut or "paper-nau-
tilus " (ZooL. § 962) there is a still more remarkable provision
for the same end. All the individuals of this species that
form the beautiful paper-like shell from which it derives

p P

578 KEPRODUCTION OF ARGONAUT.

its name, are females ; and it now seems clear that the
essential purpose of the shell is the protection, not of the
animal (which is not in any way attached to it), but of the
eggs. The male is of such comparatively insignificant size,
that, not being provided with a shell, it was not recognised
until recently as belonging to the same species. The sper-
matic duct passes through one of its arms, which is extended
into a long whip-like appendage j and in this duct are found
bundles of spermatozoids, all contained within one casing,
which does not possess any self-moving power. At a certain
epoch, this arm detaches itself entirely from the body, and
moves freely through the water by means of the apparatus of
nerves and muscles with which it is endowed, until it comes
into contact with a female of its own species, whose eggs are
fertilized by its contents which are then set free. In this de-
tached condition, the arm was long since observed within the
shell of the Argonaut, and was supposed to be a parasitic
"Worm ; subsequent inquiry showed it to possess, in its suckers
and its nervo-muscular apparatus, the characteristic structure
of the Cephalopod, and it was at first supposed to be itself the
male, destitute (like the male of some Rotifera, § 750) of any
nutritive apparatus. Its true history, as now elucidated, is
one of the most curiously-exceptional phenomena in the whole
of this department of physiology.

754. Having at last arrived at the Vertebrated series, we
shall take a general survey of the history of Development as
presented to us in the case most familiar to every one, the
formation of the Chick, within the egg of the Bird ; pointing
out, as occasion arises, the principal points of difference be-
tween the mode in which the process is there carried-on, and
the corresponding phenomena presented by other classes. —
The ovum, as formed within the ovary, has neither " white "
nor "shell," but consists of the yolk-bag and its contents
alone. Under the influence of domestication, which affords
a more constant supply of food and warmth than the Fowl
would obtain in its natural condition, a much larger number
of eggs are produced by the hen than she would produce in
her wild state ; so that, instead of laying four or five at a
time, with long intervals between each deposit, she is conti-
nually evolving them. An enormous quantity of "food-
yolk" is prepared, in addition to the "germ -yolk;" and thus
the Bird's egg comes to acquire a far larger size in proportion

DEVELOPMENT OP OVA OP BIRD. 579

to that of the parent, than we see in any other animals. The
appearance of the ovary of the Fowl in process of laying is
shown in fig. 314 ; its surface is rendered uneven or knobby

Fig. 314. — OVAHY OF THE FOWL, with ova in various stages of development:

a, mature ovum within its calyx, which is about to rupture along the non-vascular
streak bb; c c, less advanced ova ; d, a calyx from which the ovum has escaped ;
e, still younger ova.

by the protrusion of the ova in various stages of enlargement,
those which are most mature (a, c) forming pear-shaped
projections which only hang-on by a narrow stalk ; but each
ovum is still included within an extension of the fibrous sub-
stance of the ovary (fig. 302), termed the calyx, through which
blood-vessels are conveyed over its surface ; and when the
ovum escapes by the rupture of this, along a line (6) from
which the vessels have previously retreated, the calyx remains
as an open cup (d). The ovarium of such Mammals as pro-
duce several young at once, presents a corresponding appear-
ance as each set of ova is approaching maturity ; save that, as
the mass of yolk is comparatively small (no "food-yolk" being
provided), the ova do not project so much from its surface.
In Fishes, on the other hand, we find an enormous number
of ova produced at once ; the whole ovarium, when they are
approaching maturity, being so crowded with them that its
p p 2

580 STRUCTURE OF THE BIRD'S EGG.

own substance is scarcely distinguishable. Not only thousands
but tens of thousands of eggs are often produced by a single
individual, their aggregate forming what is known as the
"hard roe;" whilst the "soft roe" or "milt" is the corre-
sponding mass of sperm-cells produced by the male.

755. After the ovum of the Bird has quitted the ovarium,
and is passing through the oviduct towards its outlet, it
receives layer after layer of albumen poured out in a viscid
condition from the lining membrane of the oviduct, forming
the "white" of the egg (fig. 315, g) ; and this is inclosed in

Fig. 315.— SECTION OF FOWL'S EGG :

o, cicatricula; b, yolk-bag; c, membrane lining shell; d, attachment of chalazag;
e, chalazse ; /, air-space ; g, albumen.

a double membrane composed of a network of fibres, which
is formed by the consolidation of a, plastic exudation (§ 391),
poured out after the albuminous exudation has been com-
pleted. The outer layer of this membrane is consolidated by
the deposit of calcareous particles in the interspaces of its
fibrous matting, so as to form the " shell " of the egg ; an
arrangement that gives the necessary protection, without cut-
ting-off the contents of the shell from that communication
with the atmosphere which is requisite for the development
of the embryo. The inner layer, which forms a lining to the
shell, separates into two laminae at the large end of the egg ;
and, inclosed between these, there is a bubble of air (/), which
serves to give the young bird, just before it is hatched, the

EARLIEST STAGES OF EMBRYONIC DEVELOPMENT. 581

power of filling its lungs with air. The yolk-bag floats within
the albumen, and always tends to ta,ke the highest place, being
the lighter of the two ; but it is kept nearly to one place by
two cords (e, e) termed the chalazce, which seem formed of
peculiarly viscid albumen, and connect the yolk-bag with the
lining membrane at the two ends of the shell (d, d). In this
manner the yolk-bag is always kept at the part of the shell
where it can most favourably receive the warmth imparted to
it by the mother ; and the cicatricula or germ-spot (which is
the mass of cells first developed from the germ-yolk) is made,
by a similar contrivance, always to rise to the highest point. —
In the eggs of Fishes there is no additional albumen ; and
in those of Frogs the albumen is common to the general mass
of ova, constituting the peculiar "gelatinous envelope," which
forms long necklace-like strings of " spawn," within which the
black yolk-bags are disposed at tolerably regular intervals. In
Mammals, each ovum receives a separate investment of a jelly-
like substance in its passage along the oviduct into the uterus ;
and around this there is formed a fibrous membrane termed
the Chorion, which is destined to take a very important share
in the subsequent nutrition of the embryo (§ 761).

756. In the eggs of Frogs, as in those of Mammals, the
whole of the yolk undergoes the process of segmentation
already described (§ 736), and takes a share in the formation
of the " mulberry mass." But in the eggs of Fishes, Birds,
and the higher Reptiles, this process is limited to that small
portion of the yolk which is distinguished as the germ-yolk ;
and the formation of the germinal membrane takes place after
a different fashion. The mass of cells that immediately results
from segmentation, flattens itself out on the surface of the
yolk, forming the minute semi-opaque whitish spot, which is
known as the cicatricula, germ-spot, or "tread;" and by a
further extension it constitutes the "germinal membrane,"
which gradually spreads itself over the food-yolk ; at the same
time dividing itself into two layers, between which a third
is afterwards interposed. Thus the " germinal membrane,"
which may be compared to the seed-leaves or cotyledons of
Plants, forms a sort of temporary stomach round the mass of
nutriment prepared for the sustenance of the embryo ; the
whole of which nutriment, as will be presently seen, is absorbed
into the body of the embryo through its instrumentality.

582 PRIMITIVE TRACE : DEVELOPMENT OF VERTEBRAL COLUMN.

757. The first indication of the permanent fabric in all
Vertebrated animals, consists of a delicate longitudinal streak,
termed the "primitive trace" (fig. 316, 6), that is observable
in the midst of a pellucid area, which is again surrounded by

a ring of more opaque as-
pect (c). This primitive trace
is the foundation of the
Vertebral Column. It is in
the first instance a mere
furrow in the outer layer of
the germinal membrane ;
but the sides of this furrow,
known as the dorsal lamince,
rise up and arch-over, so as
gradually to meet and con-
vert the furrow into a canal.
The meeting first takes place
in what is afterwards to be-
come the middle of the back;
and here we find the first
distinct rudiments of the
vertebral column, in the
condition of a series of small square plates (figs. 317, c, c,
325, I, I) on either side, which are the representatives of the
arches of as many vertebras. The furrow widens-out in the
situation of the head, so as to form the receptacle (d) for the
series of large ganglionic masses that is to constitute the brain
(fig. 323, d, e,f) ; and though its sides do not there close-in
for some time longer, it receives a special hood-like covering
from a peculiar fold of the germinal membrane, the edge of
which is seen at e, fig. 317. The cells, of which the parts of these
lamines that bound the bottom and sides of this furrow are
composed, appear to furnish the rudiments of the nervous
centres that are afterwards to occupy the canal ; but beneath
its deepest part there lies a continuous rod of peculiar nucleated
cells (/), the chorda dorsalis, which marks-out the situation
afterwards to be taken by the bodies of the vertebrae. This
remains the only representative of the vertebral column in
the Lamprey and other Fishes of a low grade, the develop-
ment of whose bony skeleton is checked so early that it never
advances beyond this simple embryonic type (§ 53).

Fig. 316. — YOLK-BAG OF FOWL'S EGG,
after twelve hours' incubation :

a, yolk; b, primitive trace surrounded by
pellucid area; c, more opaque ring, the
commencement of the vascular area.

DEVELOPMENT OF CIRCULATING APPARATUS. 583

758. During the progress of this change, another very im-
portant one is taking place, which is destined for the nourish-
ment of the embryo during its further development. This
is the formation of vessels in the substance of the germinal
membrane ; which vessels serve to take up the nourishment
supplied by the yolk, and to convey it through the tissues
of the embryo. The space over which these vessels spread

Fig. 317.— MORE ADVANCED EM-
BRYO OF FOWL, much enlarged :

a, pellucid area; b b, dorsal
laminae ; c c, rudiments of ver-
tebral arches; d, dilatation for
brain ; e e, cephalic hood ; /,
chorda dorsalis.

Fig. 318. — YOLK-BAG OF FOWL'S EGG, at
the beginning of the third day of incu-
bation :

a, yolk; 6, embryo; c c, arteries of vascu-
lar area; d d, veins ; e e, terminal sinus.

themselves, is called the Vascular Area ; it makes its ap-
pearance during the second day of incubation in the Fowl's egg
(fig. 316, c), and soon spreads itself over the surface of the yolk
(figs. 318, 319). Islets or points of a dark colour first appear
in it ; these unite in rows ; and at last continuous vessels are
formed. The heart makes its appearance at the twenty-seventh
hour of incubation, as a simple dilatation of the trunk into
which the blood-vessels unite .(fig. 32Q, h). Its wall is at

584 DEVELOPMENT OF VESSELS AND DIGESTIVE CAVITY.

first formed by a layer of cells ; and no muscular structure is
seen in it, until after its regular pulsations have commenced.
It is in these vessels that the first blood is formed ; and the
same process appears to be continued through the whole period
of incubation, the yolk being progressively converted into

blood, and this blood being
conveyed by the great
trunks which collect it into
the body of the embryo.
Looking at the yolk-bag
in the light of a temporary
stomach, its vessels may
be likened to those which
take so large a share in the
act of absorption from the
digestive cavity of the
adult (§ 218).

759. During the same
early period of incubation,
the layers of the germinal
membrane begin to exhibit
various folds, which after-
wards serve for the forma-
tion of the several cavities

Fig. 319.— EMBRYO OF BIRD, WITH THE of the body. The points of
VESSELS, i, of the vascular area, after four ., -I--IT v i j/u

days' incubation. it which lie beyond the

extremities, and which

spread-out from the sides of the embryo, are double d-in so
as to make a depression upon the yolk ; and their folded edges

gradually approach one an-
other under the abdomen,
which lies next the in-
terior of the egg. In this
manner is formed the per-
manent digestive cavity ;
which is first a simple
pouch communicating with
the yolk-bag, by a wide
opening, as seen at s, fig. 320 ; but which is gradually separated
from it by the narrowing of this orifice (fig. 322), the connecting
portion being elongated into a duct (fig. 321, b). Thus we may

Fig. 320. — DIAGRAM OF THE FORMATION
OF THE DIGESTIVE CAVITY:

e, embryo ; /, g, layers of germinal mem-
brane; h, heart; s, stomach.

DEVELOPMENT OF DIGESTIVE CAVITY AND ALLANTOIS. 585

say that the digestive cavity in Vertebrata is formed by the
pinching-off (as it were) of a small portion of the general sac
of the yolk. In the Mammalia, the remainder of the yolk-bag-
is completely separated from this by the closure of its narrow
orifice, and is afterwards thrown off; so that only a very
small portion of the germinal membrane is received into the
permanent structure. But in
Birds and other oviparous
animals, the whole of the yolk-
bag is ultimately drawn into
the abdomen of the embryo ;
the former gradually shrinking
as its contents are exhausted ;
and the latter enlarging, so as
to receive it as a little pouch
or appendage. In Fishes, the
hatching of the egg very com-
monly takes place before this
process has been completed ;
so that the little Fish swims
about with the yolk-bag hang-
ing from its body.

760. The embryo, like the
adult, has need of Respiration ;
partly that its own heat may be
kept up; and partly that the
carbonic acid liberated in the various processes of nutrition,
may be set free. Owing to the peculiar structure of the
membrane covering the albumen and forming the basis of the

Fig. 321. — A, MORE ADVANCED EM-
BRYO OF FOWL, connected only by
the vitelline duct b, with the yolk-
bag a a, over which are distributed
the blood-vessels cc; B, early form
of the anterior extremity a, and of
the posterior extremity b.

Fig 322. — DIAGRAM OF THE FORMATION OF THE ALLANTOIS, ».
(The other references as in fig. 320.)

shell (§ 755), the outer air is enabled to gain access to the
interior of the egg; and at first its action upon the blood,

586 RESPIRATION OF THE EMBRYO : ALLANTOIS.

whilst circulating in the vascular area, is sufficient. In Fishes,
no further provision is made for this process ; since, by the
time it would be required, the egg is hatched ; the young
animal comes forth into the medium it is permanently to
inhabit, its own gills come into play, and the air contained in
the water can act directly upon the blood circulating in the
vascular area. But in the higher oviparous animals, whose

development proceeds further
before they leave the egg, a
special provision is made for
this purpose. On the third day
of incubation, in the Fowl, a
bag termed the Allantois (fig.
322, i) begins to sprout (as it
were) from the lower end of the
body ; and gradually enlarges
(fig. 323), passing round the
y embryo, and beneath its en-
Fig. 323.-EMBKTO OP Fowz.Twith the vel°Ping membranes, so as

Allantois, a, over which ramify the almost Completely to inclose

umbilical vessels, b ; at c is seen the •+ npi a11T.fonp nf fhie

indication of the place of the external lt;-

ear ; d, e, f, rudiments of the cere- bag is plentifully Supplied
bellum, optic ganglia, and cerebrum -ii. ui j i r it

respectively. with blood-vessels from the

embryo ; and as one side of

it lies in close proximity with the membrane of the shell,
it is very advantageously situated for receiving the in-
fluence of the air. It thus serves as the temporary respi-
ratory apparatus of the Chick, up to the time when it is pre-
paring to quit the egg.1 There is reason to believe that the
bird then receives air into its lungs, from the air-space formerly
mentioned (§ 755), which increases in size as the contents of
the egg diminish in bulk by the evaporation of their watery
part. By the increased vigour which it thus acquires, it is
enabled to perform the movements requisite for extricating

1 If the respiration of the embryo be prevented by rendering the
shell impermeable to air, its development is completely checked. No
means of accomplishing this is so effectual, as smearing the shell with
oil or grease of any kind. Hence the effect of the well-known practice
of buttering the surface of the egg, in preventing the chick from being
reared ; and the same operation, if performed when the egg is quite
fresh, will preserve it for some time fit for eating, its decomposition
being prevented by the complete exclusion of the air.

DEVELOPMENT AND NUTKITION OF MAMMALIAN EMBRYO. 587

itself from its shell ; which it does entirely by its own exer-
tions. When it thus becomes independent of the allantois,
the circulation through the latter diminishes ; and almost the
whole sac is separated from the body by the contraction of
the connecting foot-stalk, which at last gives way.

761. The formation of the yolk-bag and the allantois takes
place in Mammals (fig. 324) almost exactly on the same plan

Fig. 324.— EMBRYO OF MOLE :

A, entire ; B, with the abdomen laid open :— a, chorion ; b, footstalk of allantois and
umbilical vessels; c, yolk-bag; d, vitelline duct; e, upper portion of intestinal
canal; /.lower portion; ff, eye; h, indication of branchial arches; f, auditory
vesicle; k, anterior, and I, posterior extremity, m, n, Wolffian bodies or rudi-
mentary kidneys ; o, rudimentary lung ; p, trachea ; q, ventricle of heart ; r,
atrium of heart.

as in Birds j but on account of the absence of food-yolk, these
sacs are comparatively small ; and the function of both is su-
perseded, at an early period of the development of the embryo,
by a new and remarkable contrivance. The ovum, in passing
through the oviduct, has been already stated to receive a new
envelope, analogous to that which forms the membrane of the
shell in Birds ; this is termed the Chorion. It is then received
into the cavity of the Uterus, a receptacle within which it is
delayed for a considerable period, and continually supplied
with nourishment drawn from the blood of the parent. From

588

FORMATION AND USES OP PLACENTA.

the whole surface of the chorion, a number of little tufts shoot
out (fig. 325), which come into contact with the lining mem-
brane of the uterus ; and this is furnished with a multitude
of glandular follicles, which secrete a nutritious fluid that is
absorbed by the tufts of the chorion, and by them communi-
cated to the embryo. When the allantois is formed, it serves
to carry the blood-vessels of the embryo to the inner surface
of one part of the chorion ; and they shoot through this, so as

Fig. 325. — INTERIOR OF HUMAN UTERUS at the seventh week of Pregnancy :
b, outlet of the uterus, of which the walls c, c, c, c, laid open by incision, are turned
back to display its contents; d d, its lining membrane ; g, tufted surface of the
chorion ; g%, its internal aspect ; h, h, amnion ; i, yolk-bag ; k, umbilical cord ;
/, embryo.

to dip-down, as it were, into large expanded vessels that extend
outwards from the walls of the uterus. In this manner is
formed, in all the higher Mammalia, the important organ
termed the Placenta; which essentially consists of the ramifi-
cations of the foetal blood-vessels contained in the Umbilical
Cord or " navel-string," ensheathed by prolongations of the
large vessels of the maternal uterus. In the Marsupials and
Monotremes (ZOOLOGY, §§ 309 — 320), no placenta is ever
formed, the embryo coming into the world in a stage scarcely
more advanced than that represented in fig. 325. In either
case, the vessels of the embryo are enabled to absorb from
the blood contained in those of the parent, through the thin

DEVELOPMENT OF CIRCULATING APPARATUS.

589

walls of both, the materials requisite for its growth j but there
is no direct communication between the two. The same means
serve for the aeration of the blood of the embryo ; for this,
being brought from its body in the venous condition, is exposed
to the influence of the arterial blood of the parent, through
the thin walls of its vessels, — just as the venous blood of aquatic
animals is aerated in their gill-tufts, — and passes back to the
embryo in the arterial condition, having imparted its carbonic
acid to the blood of the parent,
and received from it oxygen. —
Thus all but the very early stages
of development are performed in
Mammals, by means of which we
scarcely find a trace in Oviparous
animals ; yet the ova of both are
originally formed on the same plan,
and the first changes which they
undergo are exactly analogous.

762. It would not be consistent
either with the design or with the
limits of this work, to enter in
much detail into the considera-
tion of the processes of develop-
ment, although they present many
points of the highest interest.
The general history of the evolu-
tion of the Circulating apparatus
and of the Nervous centres may,
however, be noticed, as character-
istic examples of the mode in
which the evolution of the several
organs of the body takes place. —
The Heart, in Man and other
Mammals, as in the Bird (§ 758),
is at first a simple tube, resembling
the pulsatile trunk that remains as

the sole organ of impulsion in the lowest forms of circulating
apparatus. After a time this tube is doubled upon itself, and
two cavities are formed, an auricle and a ventricle ; in this con-
dition, it strongly resembles the heart of the Fish (§ 286). The
circulation too is, at an early period, that of the Fish ; for the

Fig. 326. — EMBRYO OP THE FOWL,
from the Ovum shown in fig.
318, greatly enlarged :

a, b, folds of irerminal membrane,
enveloping head and tail; c, la-
teral folds; d, e, rudiments of
optic ganglia and cerebrum ; f,
heart ; g, dilated termination of
venous trunk, forming atrium of
heart; A, aorta; 1, 2, 3, 4, bran-
chial arches; i, i, vessels of
vascular area; k, k, dorsal lami-
nse; I, I, rudiments of vertebral
arches.

590 DEVELOPMENT OF CIRCULATING APPARATUS.

arterial trunk that springs from the ventricle, divides into a <
set of arches on each side (fig. 326), which closely resemble
the branchial arches of Fishes and Tadpoles. Although
no gills are present, yet there is a series of clefts on
each side of the neck, passing through to the pharynx,
which are analogous to the branchial apertures of the
cartilaginous Fishes (§ 317). After a time, however, the
auricle and ventricle of the heart are each divided by a
vertical partition, so that four cavities are formed, out of
the two which previously existed ; and at the same period,
the arrangement of the vessels undergoes a change, by the
division of some trunks, and the obliteration of others, so
that they gradually assume the distribution which is charac-
teristic of warm-blooded animals (§ 281). But even up to
the time of the birth of the Mammalia, there is a communica-
tion between the two sides of the heart, and between the
pulmonary and systemic vessels, which is closely analogous to
that which permanently exists in the Crocodile (§ 283).

763. Again, the space within the head of the embryo, into
which the vertebral canal widens-out (§ 757), is occupied in
the first instance by a succession of vesicles or bags, arranged
in a linear series (fig. 323, d, e, /) ; each of which is the rudi-
ment of one of those principal ganglionic masses, that col-
lectively make-up the brain of the Fish (§ 453), in which
they present a very similar aspect (fig. 192). As in many
Fishes, too, the Cerebrum is inferior in size to the Optic
ganglia, and only comes to surpass and finally (as it were) to
overpower them (§§ 455, 456) in the later periods of embry-
onic development.

764. The true representation of these and similar facts is
not, as was maintained when they were first brought into view,
that the several organs of the higher animals go through a
series of forms which remain permanent in the lower; but
that the development of all animals formed upon the same
general plan commences in the same manner, their special
differences manifesting themselves as development proceeds.
Thus, as we have seen, the foundation of the Vertebral column
is laid in all Vertebrata in precisely the same method (§ 757) ;
in some of the lowest Fishes, the evolution of this structure
is checked at so early a period that it never advances beyond
the embryonic type (§ 53) ; but the fully-formed spine has a

VON BAER'S LAW OP DEVELOPMENT. 591

characteristically-different structure in each of the classes of
Vertebrata, which is not presented at any period in the history
of the others. So, the evolution of the Circulating apparatus
commences in all Vertebrata upon the same original plan ; and
from this plan there is but little departure in the Fish ; but
the circulating apparatus of the early Human embryo, how-
ever like that of the adult Fish, differs from it in this essential
particular, — the absence of gill-tufts receiving capillary
vessels from the branchial arches (§ 286). The like is true
in regard to the Nervous centres ; for although the earliest
condition of the Human brain very closely resembles that of
the brain of the foetal Fish, it never bears any exact analogy
to that of the adult Fish.

765. Hence the principal facts of Organic Development
admit of being stated in this general formula, which we owe
to the sagacity of Von Baer, — that the more special forms of
structure arise progressively out of the more general, — a prin-
ciple than which there is none more comprehensive or more
important in the whole range of Physiological Science.

766. THE Unity of Plan which is visible through the whole
Animal Kingdom, is nowhere more remarkable than in the
function of which an outline has now been given. We have
seen that, however apparently different, the essential character
of the Eeproductive process is the same in the highest Animal
as in the lowest. It has been shown that the development of
the highly-organized body of Man, — though it is to serve as the
instrument of those exalted faculties, by the right employment
of which he is made " but a little lower than the Angels," —
commences from the same starting-point with that of the
meanest creature living : for even Man, in all the pride of his
philosophy, and all the splendour of his luxury, was once but a
single cell, undistinguishable, by all human means of observa-
tion, from that which constitutes the entire fabric of the
simplest Protozoon. And when the Physiologist is inclined
to dwell unduly upon his capacity for penetrating the secrets
of Nature, it may be salutary for him to reflect that, — even
when he has attained the furthest limits of his Science, by-
advancing to those general principles which tend to place it

592 CONCLUSION.

on the elevation which others have already reached, — he yet
knows nothing of those wondrous operations, which are the
essential parts of every one of those complicated functions by
which the life of the body is sustained. Why one cell should
absorb, — why another, that seems exactly to resemble it,
should assimilate, — why a third should secrete, — why a fourth
should prepare the reproductive germs, — and why, of the two
germs that seem exactly similar, one should be developed into
the simplest Zoophyte, and another into the complex fabric of
Man, — are questions that Physiology is not likely ever to
answer. All our science is but the investigation of the mode
or plan on which the Creator acts ; the Power which operates'
is Infinite, and therefore inscrutable to our limited eompre-1
hension. But when Man shall have passed through this
embryo state, and shall have undergone that metamorphosis
by which everything whose purpose was temporary shall be
thrown aside, and his permanent or immortal essence shall
alone remain, then, we are encouraged to believe, his finite
mind shall be raised more nearly to the character of the Infi-
nite, all his highest aspirations shall be gratified, and never-
ending sources of delightful contemplation shall be continually
opening to his view. The Philosopher who has attained the
highest summit of mortal wisdom, is he who, if he use his
mind aright, has the clearest perception of the limits of human
knowledge, and the most earnest desires for the lifting of the
veil that separates him from the Unseen. He, then, has the
strongest motives for that humility of spirit and purity of
heart, without which, we are assured, none shall see God.

INDEX.

N.B. — The numbers refer to the paragraphs.

A.

ABERRATION, chromatic, 548, 549;
spherical, 546, 547; avoided in the
eye, 549.

ABSORPTION, nutritive, 171 ; in Verte-
brata, through lacteals, 217 ; through
blood-vessels, 218: in Invertebrata,
225; in non-vascular animals, 225;
interstitial, performed by lymphatics,
219; from general surface, by veins,
220.

Abstinence, power of, 140, 141, 152.

ACALEPHJE, 120 ; luminousness of, 395 ;
(see Medusa).

Acephalous Mollusca, 111, 113.

Actinophrys, 130.

Adipose tissue, 46.

Aeration of blood, 253, 303.

Aerating surface, 311, 312.

Agamic Reproduction, of Insects, 746,
747 ; of Entomostraca, 748.

Air, atmospheric, composition of, 300;
need of in animals, 207; change of, by
respiration, 300 — 302; effects of de-
privation of, 297, 298, 310, 338.

bladder of Fishes, 324.

cells of Birds, 80, 326, 327.

tubes and sacs of Insects, 320 — 322.

Albinos, 545.

Albumen, chemical composition of, 13,
14; use of in blood, 240.

Albuminous principles of food, 153; des-
tination of, 158—164.

Aliment, definition of, 132 (see Food).

Allantois, 760, 761.

Ammonite, 111.

Amphibia, 85, 87; circulation in, 287 —
289 ; (see Batrachia and Frog}.

Amphioxus, 53.

Anabas, respiratory organs of, 318.

Anastomosis, of arteries, 262 — 265 ; of
nerves, 428.

Anemone, Sea, 126, 127; reproduction
of, 726, 739.

Aneurism, 263.

Animal Life, functions of, 6, 426, 427 ;
nervous system of, 461.

Animals, distinctive characters of, 6—
12; dependent on Plants for support,
144--147.

Animalcules, structure of, 13, 136, 137;
heat produced by, 404 ; movements of,
577; reproduction of, 135, 725.

ANNELIDA, general characters of, 104;
circulation in, 294 ; respiration of,
314; luminosity of, 396; nervous sys-
tem of, 440; muscular motion of, 597;
reproduction of, 727, 744.

Annular ligament, of wrist, 641 ; of
ancle, 648.

Anobium, sound of, 677.

Ant-eaters, 186.

Antennae of Insects, uses of, 498.

Anthpzoa, 126; development of, 736.

Ant-lion, instinct of, 697.

Aorta, 258 ; valves of, 273.

Aphides, agamic reproduction of, 727,
735, 746.

Aplysia, nervous system of, 438.

Aquatic animals, respiration in, 298.

Aqueous humor, 536.

ARACHNIDA, general characters of, 106;
circulation in, 293; respiration in,
323 ; development of, 751.

Arachnoid membrane, 43.

Area pellucida, 757.

vasculosa, 758.

Areas, comparative, of arteries, 247.

Arenicola, 314.

Areolar tissue, structure and properties
of, 24—27.

Argonaut, reproduction of, 753.

Arm, bones and muscles of, 638 — 640.

Arteries, 246; comparative areas of, 247;
walls of, 248; pressure of blood in,
249 ; protection given to, 250 ; division
of into capillaries, 251 ; general distri-
bution of, in Man, 258, 259 ; in Bird,
260, 261 ; peculiarities of distribution »f,
262, 264, 265 ; flow of blood through,
274—276 ; wounds of, 277.

ARTICULATA, general structure of, 93 —
96; skeleton of, 597—600; nervous
system of, 440—447; eyes of, 573—575.

Articulate sounds, 687 — 691.
Q Q

594

INDEX.

Articulation of bones, different modes
of, 600—604.

Arytenoid cartilages, 680, 681.

Ascidia, 114; nervous system of, 435.

Asphyxia, 280, 338 ; treatment of, 339.

Assimilating glands, 223, 224.

Astraea, fission of, 726.

Atlas-vertebra, 632.

Attention, effect of, on sense of touch,
494; on hearing, 525; on sight, 555;
on mental power, 721.

Auditory nerve, 512.

Auricles of heart, 257 ; action of, 270.

Automatic movements, 479.

Axis vertebra, 632.

Axolotl, 87.

Azotized principles of food, 153, 154;
destination of, 158—165; effects of ex-
cess of, 348.

Azotized compounds, excretion of, 346
—348.

B.

Balancing of body, 480, 481.

Balanus, 102; development of, 749.

Ball-and-socket joint, 603.

Barnacle, 102; development of, 749.

Basement-membrane, 31.

Bat, wings of, 669; ear of, 515; hyber-
nation of, 407 ; peculiar sensibility of,
494.

Satrachia, 86, 87; reparative power of,
390; development of egg of, 756.

Baya, nest of, 705.

Beat of heart, 269, 271.

Beaumont, Dr., his experiments upon
gastric fluid, 207, 208.

Beaver, operations of, 706 — 708.

Bees, formation of wax by, 156; heat
produced by, 410, 41 ]; use of antennae
in, 498; sounds of, 678; instincts of,
712—716; reproduction of, 747.

Beetle, digestive apparatus of, 202.

Bell, Sir C., his discoveries, 429, 451.

Bicuspid teeth, 187.

Bile, chemical characters of, 364 ; use of,
in digestion, 213; formed from venous
blood, 266, 366 ; purposes of excretion
of, 346—351, 365 ; effect of suspension
of, 351.

Bilious complaints, 350.

Binary subdivision of cells, 33.

BIRDS, general characters of, 78 — 80;
digestive apparatus of, 200 ; digestive
powers of, 201 ; blood-discs in, 230 ;
arterial system in, 260; circulation in,
281, 282; respiration in, 326, 327; heat
of, 407, 413; nervous system of, 455 ;
intelligence of, 484, 717; wings of,
669; flight of, 672; larynx of, 685;
nests of, 704, 705; social instinct in,
710; ovarium of, 754; structure of
eggs of, 755 ; development of embryo
of, 757—760.

Bladder, gall, 362 ; urinary, 362.

Blastema. 33; production of cells from, 34.

Blood of Vertebrata, general characters
of, 226; arterial and venous, 227; ge-
neral purposes of, 228, 239; composed
of liquor sanguinis and corpuscles,
229; coagulation of, 236 — 239; serum
of, 238; buffy coat of, 236 ; flow of,
means of checking, 277, 278; changes
of, in disease, 233 ; assimilating power
of, 242.

Blood of Invertebrata, character of, 235.

Blood Corpuscles, colourless, 35, 234.

red, 35, 229— 234; ac-
tion of endosmose on, 231 ; composi-
tion of, 232 ; variety of proportion of,
in different animals, 233; functions
of, 235, 241 ; change of, in respiration,
310 ; connexion of, with heat libe-
rated, 413.

Blood-vessels, 244; formation of in new
tissues, 393; in embryo, 758.

Bombylius, sound of, 676.

Bone, structure of, 49, 50; chemical
composition of, 51 ; formation of, from
cartilage, 52, 53 ; reparation of, 390.

Bones of head, 617— 693; of ear, 516; of
spine, 626—632; of trunk, 623; of
upper extremity, 635 — 644; of lower
extremity, 645—648.

Bowerbankia, 115, 356.

Brain of Vertebrata, 72; not the only
source of action, 465, 466 ; comparative
size of, 718,719; development of, 763;
(see Cerebrum and Cerebellum).

Branchial arches, 286—289 ; of embryo,
762.

Bronchial tubes, 326, 328.

Bri/ozoa, 115; (see Polyzod).

Buds, reproduction by (see Gemmation).

Buffy coat of blood, 236.

Bulk required in food, 205.

Butter, 377.

C.

Caddice-worms, 701.

Calamary, 111.

Camel, stomach of, 198; skeleton of, 644.

Campanularia, 124.

Cancelli of bone, 49 ; formation of, 52.

Canine teeth, 181, 183.

Cantering, 660.

Capillary vessels, 251; movement of
blood in, 275, 280.

Carapace of Turtles, 83.

Carbon, modes of excretion of, 345 — 351,
365; combustion of, 157, 305, 306,
412, 413..

Carbonic acid, set free in respiration,
301, 303; mode of its production, 305,
306 ; quantity of, proportional to ac-
tivity of animal, 307—309 ; amount of,
disengaged by Man, 334 ; deterioration
of atmosphere by, 335—338.

INDEX.

595

Carnivorous tribes of animals, 148 — 151 ;
nutrition of, 161; teeth of, 179,
182.

Carpenter-Bee, 703.

Cartilage, structure of, 47 ; nutrition of,
48 ; transformation of, into bone, 52.

Casein, 15.

Cat, electricity of, 418.

Cauda equina, 460.

Cells, nature of, 32 ; multiplication of,
33; new production of, 34, 393 ; inde-
pendent condition of, 35 ; origin of all
organisms from, 379 ; differentiation of
functions of, 380.

Cells of Bee, 712, 713; royal, 714, 716.

Cementum of Teeth, 54.

Centipede, 93, 103; reflex actions of,
443.

CEPHALOPODA, general characters of,
121 ; circulation in, 291 ; respiration
in, 316; nervous system in, 448; re-
production of, 753.

Cercaria-larva of distoma, 743.

Cerebellum, 449; development of, in
different classes, 452—456; in Man,
458; functions of, 480, 481.

Cerebro-spinal nerves, 458—461.

Cerebrum, 449 ; development of, in
different classes of Vertebrata, 452—
456; in Man, 458; development of,
correspondent with intelligence, 452,
718 ; functions of, 483—485 ; effects
of removal of, 465.

Cerumen of ear, 375.

Chaetodon rostratus, 476.

Chalaza? of egg, 755.

Cheese, 377.

Chemical constitution of organized
bodies, 4; of albumen, 13, 14; of casein,
15; of syntonin, 16; of fibrin, 17, 18;
of gelatin, 19; of chondrin, 20; of
bone, 51; of teeth, 54.

Cheselden's case, 507.

Childers, speed of, 660.

Chimpanzee, 173, 674.

Cholesterin, 364.

Cholic acid, 364.

Chondrin, 20.

Chorda dorsalis, 53, 757.

Chorion, 761.

Choroid coat, 533.

Chromatic aberration, 548, 549.

Chrysalis, 97, 309, 745.

Chyle, 171, 213, 222; change of, in lac-
teals, 222, 223 ; delivery of into blood-
vessels, 221.

Chylification, 171, 212—215.

Chyme, 171, 211.

Cicada, sound of, 679.

Cicatricula, 755, 756.

Cilia, 45, 143, 319, 329.

Ciliary processes, 536.

CIRCULATION, 253 ; purposes of, 228,
253, 254-. complete double, 282;
greater, 253; lesser, 253, 268; pecu-

liarity of, in liver, 267; mechanism of,
269, 270.

Circulation, course of, in warm-blooded
animals, 282; in foetus, 283,762; in
Reptiles, 284,285; in Fishes, 286; in Am-
phibia, 287, 288 ; in Invertebrata, 289 ;
in Mollusca, 290 ; in Cephalopoda, 291 ;
in Crustacea, 292 : in Insects, 293 ; in
Spiders, 293; in Worms, 294; inTuni-
cata and Echinodermata, 295 ; in Zoo-
phytes and Sponges, 295.

Cirrhipeda, general structure of, 102; de-
velopment of, 74!).

Claspers of monkeys, 643, 674.

Clavicle, 634, 636.

Climbing perch, 318.

Coagulation of albumen, 14; of fibrin,
17 ; of blood, 236-238 ; of chyle, 222.

Coagulable lymph, 391.

Cochlea of ear, 5 1 8—520.

Cockchafer, digestive apparatus of, 358.

Cod, brain of, 453.

Cod-liver oil, use of, 386.

Coscum, 214.

Cold sustained by animals, Introd., 405.

Cold-blooded animals, temperature of,
403—406.

Coloured shadows, 569.

Colouring matter, formed by cells, 359,
360, 533.

Colourless corpuscles of blood, 234.

Colours, want of power to distinguish,
468; complementary, 568 — 570.

Comatula, development of, 741.

Combustion in animal body, 157, 305,
306,412,413.

Commissures of nervous system, 434,
458.

Complementary colours, 568 — 570.

Compound eyes of Articulata, 572—575.

Polypes, 124, 127.

Tunicata, 114.

CONCHIFERA, 113; respiration in, 316;
luminosity of, 396; nervous system
of, 437.

Conjunctival membrane, 537-

Consonants, 689, 690.

Consumption, nature and treatment of,
386.

Contractility of muscular fibre, 579,
590 ; dependent on its nutrition, 591.

Contractions of heart, 269, 271, 581, 587.

of muscles, energy of, 592

—594; use of, in organic functions,
595 ; in locomotion, 595.

Convex surfaces, influence of, on light.
529—531.

Convolutions of brain, 456, 458.

Convulsive movements, 473, 474; energy
of, 592.

Cooling effects of evaporation, 372, 373.

Coral-forming animals, 131—134.

Cornea, 533 ; structure of, 46.

Corpora striata, 458.

Corpus callosum, 458.

QQ2

596

INDEX.

Corpuscles of blood, 35 (see Blood-cor-
puscles).

chyle, 222.

Cortical substance, of brain, 430.
kidney, 357.

Coughing, act of, 342.

Crab, anatomy of, 9!) ; metamorphosis
of, 101 ; nervous system of, 447.

Cranium, bones of, 617 — 623.

Crassamentum of blood, 236.

Cray-fish, 95.

Cricket, leaping powers of, 662; sound
produced by, 678.

Cricoid cartilage, 680.

Crinoidea, 118.

Crocodile, 93.

Crusta petrosa of teeth, 54.

CRUSTACEA, general characters of, 99 —
101; formation of shell of, 170; teeth
of, in stomach, 202 ; circulation in,
292 ; respiration in, 315 ; liver of, 356 ;
luminousness of, 396; reproduction of
claws in, 389 ; generation of, 748 ;
development of, 101, 748.

Crystalline lens, 536 ; reproduction of,
390; change of form of, 551.

Cutis, structure of, 37 ; sensibility of,
490, 491.

Cuttle-fish, 111; ink of, 359.

Cynips, agamic reproduction of, 746.

Cysticercus, development of, 742.

D.

Daphnia, reproduction of, 748.

Death, of the body, 67 ; apparent, 66.

of parts, continually taking place,

65, 68.

Death-watch, 677.

Death's-head Moth, 678.

Decay of dead animal matter, 54, 160.

Decay continually taking place in living
body, 65, 68.

Deer, foot of, 652.

Defecation, 171, 216.

Degeneration of tissues, from want of
use, 30.

Deglutition, 171, 192—196, 470.

Dentine, 54.

Development, first stages of in ovum,
736, 737; of Polypes, 738, 739 ; of Me-
dusae, 740 ; of Echinodermata, 741 ; of
Entozoa, 742, 743 ; of Annelida, 744 ;
of Insects, 745—747 ; of Crustacea, 748;
of Cirrhipeda, 749 ; of Rotifera, 750 ; of
Arachnida, 751; of Mollusca, 752; of
Vertebrata, 756—762.

Diaphragm, 328; deficiency of, in Birds
and Reptiles, 327 ; peculiar to Mam-
mals, 330; action of, in respiration,
331—333.

Diet, natural, of Man, 163 — 165.

Differentiation of structure and function,
380.

DIGESTION, several stages of, 171; gas-
tric, 204—211 ; intestinal, 212—215.

Digestive cavity, characteristic of Ani-
mals, 8; different forms of, 197—203;
formation of, in embryo, 759.

Direction of action of muscles, 606 — 611.
visual objects, 558.

Dislocations, 604.

Distance, adaptation of the eye to, 550,
551 ; mode of estimating, 563—565.

Distoma, development of, 743.

Division of labour in living organisms, 2.

Dogs, intelligence of, 717.

Doris, circulation in, 290; respiration in,
316.

Dorsal vessel of Articulata, 293.

Double vision, 559.

Draco volans, 668.

Drowning, 338 ; treatment of, 339.

Drum of the ear, 516.

Duration of luminous impressions, 567.

Dytiscus, 444.

Ear, simplest forms of, 512, 514; of Man,
external, 515; middle, 516, 517; in-
ternal, 518—521 ; bones of, 516.

Earthworm, 104, 142, 389, 400, 404, 597.

ECHINODERMATA, 118,119; circulation
in, 295; luminousness of, 396; deve-
lopment of, 741.

Echinus, 118; teeth of, 189.

Egg of Birds, structure of, 755 ; shell of,
its permeability to gases, 760.

Elastic fibrous tissue, 23, 29.

Elasticity of arteries, 274, 275; of foot,
649; of vertebral column, 631.

Elateridce, luminousness of, 397; leap-
ing power of, 662.

Electricity, animal, sources of, 416, 417;
in Cat, 418; in Fishes, 419—423; of
muscle and nerve, 424; analogy of to
nervous agency, 488, 585.

Electric organs, structure of, 421, 422;
copiously supplied with nerves, 423.

Elephant, trunk of, 172, 4f!3; molar
teeth of, 102; tusks of, 177; intelli-
gence of, 717.

Embryo, origin of, 737, 757; develop-
ment of, 757 — 762; circulation in, 283,
758, 762 ; respiration in, 760.

Emotions, 477, 478 ; influence of on
muscles, 590; on organic functions,
461.

Enamel, structure and composition of,
54; arrangement of, 182.

Encrinites, 118.

Endosmose, action of, on blood cor-
puscles, 232.

Entomostraca, agamic reproduction of,
727, 748.

Entozoa, 105; development of, 742, 743.

Ephemera, 315.

Ephippial eggs of Daphnia, 748.

Epidermis, structure of, 38; use of, 492.

Epidermic appendages, 38.

Epiglottis, 193, 681.

INDEX.

597

•Epileptic fits, 473.

Epithelium, structure of, 40 ; action of,
in secretion, 41, 42, 355.

Eunice, 314.

Eustachian tube, 516, 517.

Evaporation from surface, 370—373;
cooling effects of, 372, 373.

EXCRETION, objects of, 345—351 ; mate-
rials of, formed in the blood, 351; effects
of retention of, 351.

Exhalation of moisture from the lungs,
343 ; from the skin, 371 — 374; amount
of, 374.

Eye, an optical instrument, 532, 543 —
553; structure of, in Man, 533 — 536;
muscles of, 538 ; motions of, 538, 539 ;
aberration corrected in, 547, 549 ;
adaptation of to distance, 550, 551;
limits of vision by, 554 ; common sen-
sibility of, 571 ; long and near-sighted,
£62, 553; peculiar structure of, in
Articulata, 573 — 575; deficient in seme
Vertebrata, 572; rudiments of, in the
lower animals, 575.

Eyelids, uses of, 537.

Flying Phalanger, 668.

Squirrel, 668.

Follicles, of mucous membrane, 41.
of glands, 42, 355, 356.

Food of Animals, 7, 8 ; derived from
Plants, 144— 147; from Animals, 148—
150; chemical nature of, 153, 154, 164;
mineral ingredients of, 166, 167.

demand for, 140, 141 ; economy

of, 165.

Food-yolk, 736, 752, 754.

Foot, structure of, 648, 649.

Foraminifera, 128, 131.

Freezing of animal bodies, 67, 405.

Frog tribe, 86, 87 ; blood-discs of, 230 ;
metamorphosis of, 86, 87; change of
circulating system in, 287 — 289 ; respi-
ration of, 325 ; experiments on nervous
system of, 466, 468 ; eggs of, 755 ; de-
velopment of, 756.

Fulgorid£B, luminousness of, 400; sound
emitted by, 679.

Functions of living beings, 2 ; nutritive,
6 ; animal, 6 ; relation of organic and
animal, 425—427.

F.

Face, bones of, 690 — 623; muscles of,
624.

Facial angle, 719, 720.

Fat, structure and uses of, 46,412; de-
position of, 157, 162; of blood, 232,
241.

Fatigue, sense of, 595.

Fertilization of ovum, 732, 734, 736.

Fibre, muscular (see Muscular Fibre).

Fibres, of nerves (see Nerves).

Fibrin, composition and properties of,
17,18; uses of in blood, 236—240.

Fibrous membranes, 29.

tissues, general uses of, 12 ; for-
mation of, 22 ; general characters of,
22—30 ; nutrition of, 384, 390.

Fibro-cartilages, 47.

Fins of fishes, 666.

Fire-flies, 397.

FISHES, general structure of, 88 — 91;
teeth of, 188; circulation in, 286 ; re-
spiration in, 317, 318; air-bladder of,
324; luminousness of, 396; heat of,
405 ; electricity of, 419—424; nervous
system of, 453 ; organs of smell of,
509 ; vertebral column of, 629, 630 ;
movements of, 666, 667; reproduction
of, 754, 755, 759.

Fission, multiplication of cells by, 33;
of Infusoria by, 135, 725.

Flea, leaping powers of, 594, 662.

Flesh-fly, voracity of larva of, 141.

Flight, action of, 667—672; impossible
iu Man, 673.

Flying Fish, 667.

Lemur, 668.

G.

Gall-bladder, 362.

Galloping, 660.

Galvanic electricity, discovery of, 583.

Ganglia, 61 (see Nervous System).

of special sense, functions of,

475—479.

olfactive and optic, 453—456.

Gases, poisonous, 335, 344.
GASTEROPODA, 107, 108,112; palate of,

189 ; circulation in, 290; respiration in,
316; nervous system of, 438; develop-
ment of, 752.
Gastric follicles, 204.

juice, 204; properties of, 207—

210; artificial, 210.

Gavial, teeth of, 1 86.

Gelatin, chemical compostion of, 19 ;
use of as food, 159 ; present in blood,
380.

Gelatinous nerve-fibres, 60.

principles of food, 153 ; des-
tination of, 159.

Gemmation, multiplication by, of Plants,
724; of Infusoria, 725 ; of Zoophytes,
726; of Medusae, 726, 741 ; of Echiro-
dermata, 726, 741; of Articulata, 7/.7,
744 ; of Mollusca, 728 ; of Vertebrata,
729 ; antagonism of, to Generation,
735; (see Agamic Reproduction.)

Gemmules of polypes, 738.

GENERATION, sexual, essential nature
of, 730 — 733; antagonism of, to gem-
mation, 735; simplest form of, 734.

Germ-cells of plants, 724; of animals,
732.

Germ -yolk, 736, 752, 754.

598

INDEX.

Germinal membrane, 737, 756.

spot, 732.

vesicle, 732.

Gizzard, of Birds, 200, 201 ; of Insects,

202 ; of Bryozoa, 202; of Rotifera, 202.

Glands, secreting, structure of, 355—358.

— mesenteric, 218; lymphatic, 219.

Glaucus, 316.

Globules of Blood (see Blood).
Globulin, 232.

Glosso-pharyngeal nerve, 459, 470.
Glottis, 193, 681.
Glow-worms, 398—401.
Gluten of bread, composition of, 153.
Glycine, 364.
Glycocholic acid, 364.
Gnat, larva of, 321.
Goat-moth, larva of, 141.
Goldfinch, nest of, 704.
Gout, nature and cure of, 348, 349.
Granulation, repair of wounds by, 392.
Gravel, 348, 367.
Gregory, Dr., case of, 349.
Grey substance of nerves, 430, 431.
Guiding Sensations, importance of, 478.
Gymnotus, electricity of, 419 — 424.

H.

Habitual actions, 479.

Haematin, 232.

Hairs, structure of, 38.

Hall, Dr. Marshall, his treatment of
asphyxia, 339.

Hamster, instinct of, 699.

Hands, use of, in prehension, 172, 173 ; in
locomotion, 674; structure of, 641 —
644.

Hare, leaps of the, 661.

Hartz forest, devastated by Insects, 147.

Haversian canals of bone, 49, 50 ; forma-
tion of, 52.

Head, definition of, 111 ; bones of, 617 —
623 ; muscles of, 624.

Healing of wounds, 391, 392.

Hearing, sense of, 510—524; improved
by cultivation, 525.

Heart, 245 ; structure of, in Man, &c.,
256, 257 ; respiratory and systemic,
281; action of, 269, 270, 581, 583;
valves of, 272, 273 ; number of pulsa-
tions of, 271; development of, 753,
762.

structure of, in Reptiles, 284 ; in

Fishes, 286 ; in Mollusca, 290 ; in
Cephalopoda, 291; in Crustacea, 292 ;
in Insects, 293.

Heat, sustainable by Animals, Introd.,
372, 373.

generated by Animals, 403 — 415 ;

of Invertebrata, 404; of Fishes, 405;
of Reptiles, 406; of Birds, 407; of
Mammals, 407 ; of Man, 407 ; of young
animals, 408, 409 ; of Insects, 410,
411.

Heat, animal, dependent on combustion
of carbon and hydrogen, 412, 413; on
supply of oxygen, 413 ; maintained by
respiration, 414; influence of nervous
system on, 415.

Hemispheres of Brain, 449.

Herbivorous animals, 144 — 147; nutri-
tion of, 162 ; teeth of, 179, 182.

Hiccup, 341.

Hinge-joint, 603.

Hippuric acid, 367.

Holothuria, 118, 119; reparative power
of, 389; development of, 741.

Horse, foot of, 652 ; intelligence of, 695.

Howling Monkeys, 684.

Humble-bee, heat produced by, 410.

Hunger, sense of, 140, 205.

Hyalaea, 112.

Hybernation, 309.

Hydatina, multiplication of, 750.

Hydra, 121; referred to, 131, 296, 577;
propagation of, by buds, 122, 726; by
artificial division, 122; by eggs, 123,
734 ; development of ovum of, 738.

Hydrogen, combustion of, in animal body,
343.

Hydropathic system, 374.

Hydrophobia, 474.

Hydrozoa, 124, 125; development of,
726, 738.

Hyoid bone, 625, 680.

Hysteric disorder, 474.

I.

Iliac bones, 645.

Images, formation of, by lenses, 531 ; on
retina, 543, 544.

Imago or perfect insect, 745.

Immortality of the soul of man, 721,
722.

Impressions on nervous system, 432,486.

Incisor teeth, 181, 183.

Infants, necessity of warmth to, 408, 409.

Infusoria, 133 — 135; multiplication of,
725.

Ink, of cuttle-fish, 359.

Insalivation, 179, 190, 191.

INSECTS, general characters of, 97 ; diges-
tive apparatus of, 202 ; circulation in,
293 ; respiration in, 308, 321, 322 ; repa-
rative powers of, 389 ; secreting appa-
ratus in, 358 ; luminousness of, 3U7 —
401 ; heat of, 410, 411 ; nervous system
of, 440—446 ; instincts of, 483, 484, 667
— 716; antennae of, 498. 499; eyes of,
573—575 ; muscular power of, 594, 662 ;
wings of, 670 ; production of sounds by,
676, 679 ; reproduction of, 745, 746.

Instinctive actions, 692 ; predominance
of, in Articulata, 96 ; characters of,
694; examples of, 696 — 716; corre-
spondence of, with ganglia of special
sense, 475—479; irrationality of, 709.

INDEX.

599

Intelligence of Vertebrata, 73, 483—485,
694; examples of, 695, 717; corre-
spondence of, with development of the
Cerebrum, 452, 692, 718—720.

Intervertebral substance, 631.

Intestinal juice, 213.

tube, motion of aliment

through, 215; digestion continued in,
213, 214; relative length of, 213.

Worms, 105 ; development of,

742, 743.

Invertebrata, 92 ; absorption in, 225 ;

nature of circulating fluid in, 225, 235 ;

heat of, 404 ; skeletons of, 598, 599.
Involuntary movements, 589, 590.
Iris, 533, 534.
Iron, a constituent of animal bodies, 166,

167; of red corpuscles of blood, 232.
Ivory, structure and composition of, 54.
lulus, 103.

J.

Jaw, motion of, in Quadrupeds, 138 ; in

Man, 189; articulation of, 623.
Jelly-fish, 120.
Joints, 603; dislocation of, 604.

K.

Kangaroo, leaping powers of, 661 ; skele-

ton of, 661.
Kidneys, structure of, 357, 358, 368, 369 ;

purposes of their excretion, 346 — 348,

367.

L.

Labyrinth of ear, 519.
Lachrymal apparatus, 540,

gland, 540.

• sac, 540.

Lacteals, 217, 218.

Lactic acid, 349, 367.

Lacunae of bone, 50.

Lamprey, 317; chorda dorsalis of, 53,

757.

Lampyridae, 397—399.
Land-crabs, 315.
Lantern-flies, 400.
Larva, of Cirrhipeds, 749; of Crab, 101 ;

of Echinodermata, 741 ; of Entozoa,

742 ; of Insects,97, 141 ,745 ; of Medusae,

740.
Larynx, 192; structure of, 680, 681 ;

action of, (582—684 ; in Birds, 685.
Laughing, act of, 341.
Leaping, 661, 662.
Leech, 104, 105.
Leg, bones and muscles of, 647.
Lepidosiren, 81; blood-discs of, 230;

respiration of, 324.
Leverage of bones, 612 — 615.
Life, maintained by continual change,

68.
Ligaments, structure of, 29.

vocal, 681—684.

Light, emitted by living animals, 394 —
402 ; by dead bodies, 402.

propagation of, 526 ; refraction of

527—532.

Lirne, amount of, in bones, 49; in teeth,
54; in egg-shell, 169; in shells of
Mollusks, 169; in shells of Crustacea,
170.

sources of, in animal bodies, 166 —

170.

Limulus, 100.

Lingual nerve, 500.

Liquids, reception of, 173.

Liquor sanguinis, 229, 232, 236 — 241, 385,
387, 391.

Lithic acid, 346, 348, 367.

Liver, structure of, 356, 358, 363; circu-
lation in, 267, 363 ; assimilating action
of, 224 ; secreting action of, 364—366 ;
formation of sugar by, 366.

objects of its excretion, 346, 350.

Living beings, distinctive characters of,

1—5.
Lizard tribe, 84; reparative powers of,

390.

Lobster. 100 ; circulation in, 292.
Lock-jaw, 473.
Locomotion, reflex movements of, 471 ;

organs of, 596.
Locusts, voracity of, 146 ; multiplication

of prevented, 149.
Long-sighted eyes, 552, 553.
Luminousness, animal, 394-400 ; uses of,

401; from decomposition, 402.
Lungs, rudimentary in Fishes, 324 ; in

Reptiles, 325 ; in Birds, 326, 327 ; in

Mammals, 326—333.
Lymnaeus, parasites of, 743.
Lymph, coagulable, 391.
Lymphatics, 219, 220.

M.

Mactra, 113,

Madrepore, 127.

Malapterurus, electric, 419, 422.

Malpighian bodies of kidney, 369.

MAMMALS, general structure of, 77 ; di-
gestive apparatus in, 197 — 199; blood-
discs in, 229; circulation in, 281, 282;
respiration in, 328—333 ; heat of, 407 ;
nervous system of, 456 ; reproduction
in, 756, 761.

Mammary glands, structure of, 376.

Man, food of, 163 ; stomach of, 197 ;
heart of, 256, 257 ; arterial system of,
258; quantity of air required by, 334
— 337 ; reproduction of lost parts in,
390; repair of injuries in, 391—393;
heat of, 407; nervous system of, 456 —
462 ; peculiar characters of soul of,
721, 722.

Mantis, actions of, 444.

Mantle of Mollusca, 107.

Marmot, hybernation of, 309.

Marrow of bones, 49.

Spinal (see Spinal Cord).

600

INDEX.

Mastication, 171, 174—189.

Mastodon, teeth of, 182.

Measles of pork, 742.

Medulla oblongata, 450, 460.

Medusa, 120 ; development of, 125, 740 ;
circulation in, 296; rhythmical move-
ments of, 578; gemmation of, 726.

Membrana Tympani, 516.

Membrane, basement or primary, 31.

Membranes, fibrous, 29 ; serous, 28, 43 ;
mucous, 39 — 41.

Mesentery, 217.

Mesenteric glands, 217.

Metamorphosis of Frog tribe, 86, 87.

Insects, 97, 745.

Crustacea, 101.

Milk, different classes of aliment con-
tained in, 158; chemical composition
of, 377 ; influence of mind on, 353.

Milk-teeth, 184.

Mineral ingredients required by Ani-
mals, 166—170.

Mitral valve, 272.

Molar teeth, 181—183.

MOLLUSCA, general characters of, 106 —
110; circulation in, 290; respiration
in, 316, 320 ; structure of liver in, 356;
of kidneys, 358 ; luminousness of, 396 ;
nervous system of, 435 — 439 ; develop-
ment of, 752.

Monkey, interior of, 77.

Monstrosities by excess, 729.

Mortality under different circumstances,
Introd.

Mucous membranes, general structure
of, 39—41.

Mulberry mass of ovum, 736, 737.

Muscles, general purposes of, 10 ; general
mode of action of, 605 — 615; of eye,
538; of face, 624; of trunk, 637; of
arm, 638, 640 ; of hand, 641 ; of leg,
646,647; of foot, 648.

Muscular Contraction, 58, 59, 579; con-
ditions of, 591; stimuli to, 580—586;
influence of electricity on, 583 — 585 ;
relation of, to nervous power, 586 —
588, 592, 593; voluntary and involun-
tary, 589, 590; energy of, 592—594;
use of, in organic functions, 595 ; in
locomotion, 605—615.

Muscular Fibre, structure of, 55 — 57 ;
contraction of, 58; alternates with re-
laxation, 58, 59.

Musk, odour of, 504.

Mygale, nest of, 700.

MYKIAPODA, general structure of, 103;
nervous system of, 440.

N.

Nails, structure of, 38.
Nais, spontaneous fission of, 727.
Near-sighted eyes, 552, 553.
Necrophorus, instinct of, 703.
Negro, skin of, 375.
Nemestrina, trunk of, 173.

Nepa, tracheal system of, 322.

Nereis, 104, 314, 727.

Nerita, palate of, 189.

NERVOUS SYSTEM, general structure of,
483; general objects of, 9, 10, 429 —
432; form of, in Vertebrata, 72; in
Articulata, 94; in Mollusca, 110; in
Radiata, 116.

particular structure

and actions of, in Radiata, 434; in
Mollusca, 435—439 ; in Articulata, 440
— 446; in highest Invertebrata, 447,
448; in Vertebrata, 449— 452; in Fishes,
453; in Reptiles, 454; in Birds, 455;
in Mammalia, 456; in Man, 457 — 462.

Sympathetic, 461,462.

the instrument of the

mind, 427; influence of, on secretion,
190, 353 ; on muscular contraction,
584, 585 ; on animal heat, 415.

Nervous Tissue, white or fibrous sub-
stance of, 62, 63 ; distribution of, 63 ;
grey or vesicular substance of, 61.

Nests of Insects and Birds, 700—705,
710—714.

Newt, 87.

Nictitating membrane, 540.

Nitrogen, absorption and exhalation of,
302.

Non-azotized constituents of food, 154;
destination of, 155—157, 162—165 ;
effects of excess of, 350.

Nose, structure of, 506, 507; common
sensibility of, 508.

Nurse-bees, 411.

Nurses of Cercariae, 743.

NUTRITION of tissues, increased by use,
242, 589; dependent on liquor san-
guinis, 240, 241, 385; mode of, in dif-
ferent tissues, 384, 385 ; share of blood
in, 387 ; share of blood-vessels in. 388;
share of tissues in, 387 ; imperfect
forms of, 386.

O.

Oak, caterpillars supported on, 145.

Octopus, 121.

Odours, 504, 505.

(Esophagus, 192.

Oleaginous principles, 153; destination

of, 154—157, 162.
Olfactive ganglia, 453 — 456, 458.

nerve, 507.

Optic ganglia, 453—456, 458.

— nerve, 459.
Organic Life, 6, 425, 426; nervous system

of, 461.

Functions, relation of, to ani-
mal, 425 — 427; influenced by emo-
tions, 461.

Organized bodies, distinctive form of,
1 ; structure of, 2 ; consistence of, 3 ;
chemical constitution of, 4; actions
of, 5.

INDEX.

601

Organs of Sense (see Sensation, Organs
of).

Ornithorhyncus, 186, 664.

Otolithes, 513.

Ovarium, 732 ; of Bird, 754.

Ovum, structure of, 732, 733.

Oxygen, carried by blood-corpuscles,
235 ; by liquor sanguinis, 241 ; ab-
sorbed in respiration, 300—306, 343,
346 ; consumption of, dependent on

• muscular action, 307 — 309.

Oyster, 113, 316, 437.

P.

Palates of Gasteropods, 189.

Palpi of Insects, 172, 503.

Palsy of muscles, 586— 58S.

Paludina, 112.

Pancreatic fluid, use of in digestion, 213.

Papillae of skin, 37, 490; of tongue, 500.

Parotid gland, 356.

Paxy-waxy, 29.

Pecten, nervous system of, 110, 437.

Pectinibranchiata, development of, 752.

Pedal ganglia, of Mollusks, 437, 438; of
Articulata, 446.

Pelvis, 645.

Penguin, G67.

Pentacrinoid-larva of Comatula, 741.

Perch, skeleton of, 666.

Pericardium, 43.

Peristaltic movement of intestines, 215,
579.

Peritoneum, 43.

Perspiration, 371—374.

Pharyngeal ganglia, of Mollusks, 438;
of Articulata, 446.

Pharnyx, 192.

Phosphorescence of the sea, 394, 395.

Phosphorus in animal bodies, 166;
sources of, 167; light produced by.
402.

Pia mater, 458.

Pigment, black, of eye, 533; use of, 545

Pigment-cells, 533.

Pitch of sound, dependent on number
of vibrations, 523, 682.

Placenta of Mammals, 761.

Planaria, reparative power of, 389.

PLANTS, general comparison of with
Animals, 6 — 12 ; afford food to Ani-
mals, M4— 147; resemblance of their
life to organic life in Animals, 425,
426.

Plastron, of Turtles, 83.

Plethoric state of body, 233.

Pleura, 328.

Pneumogastric nerve, 459, 470.

Podura, leaping power of, 662.

Poisonous gases, 344.

Polijcystina, 132.

Polype, fresh-water (see Hydra).

POLYPIFEUA, 121—125 (see Zoophytes).

Polyzoa, 115; gizzard of, 202; circula-

tion of, 295 ; gemmation of, 728 ; de-
velopment of, 752.

Pompilus, nest of, 703.

PORJFERA, 136, 137; circulation in, 296.

Portal system of blood-vessels, 267, 366.

Poulp, 316, 448.

Primitive trace, 757.

Prehension, act of, 171—173, 643, 674.

Projection, idea of, to what due, 560 — 562.

Proteus, blood-discs of, 230, 231.

PROTOZOA, 128 ; movements of, 577.

Pseudopodia, 130, 131.

Pterodactylus, 669.

PTEROPODA, 122.

Pulmonary circulation, 268.

Pulse, 276; influence of posture on, 655.

Pupa, of insect, 97.

Pupil, 553 ; dilatation and contraction
of, 534.

Pus, 393.

Q.

Quadrumana, extremities of, 643, 648,

674.
Queen-bee, 712, 714, 716, 747.

R.

Rabbit, teeth of, 177; movements of, 661.

RADIATA, general characters of, 116,
117; stomach of, 203 ; nervous system
of, 434.

Radius, 639.

Ray, pecuuar swimming of, 666.
— electric, 419.

Red Corpuscles of blood (see Blood-
Corpuscles).

Reflex actions, 195, 340, 430, 692; in
Mollusca, 436. 439 ; in Articulata, 442
— 445, 693; In Vertebrata, 451; the
spinal cord their instrument, 464 —
474 ; dependent on stimuli, 466 ; not
dependent on sensation, 467 — 469.

Refraction of light, 527 — 532.

Relief, perception of, 560—562.

Rennet, action of, 15; nature of, 199.

Repair of injuries, 389—393.

Reproduction (see Development, Gemma-
tion, and Generation).

REPTILES, general characters of, 81 —
87; teeth of, 187; blood-discs in, 430;
circulation in, 284, 285 ; respiration in,
325; importance'of skin, as respiratory
organ in, 325 ; heat of, 406 ; nervous
system of, 454; vertebral column of,
629 ; reproduction of, 756 — 760.

Republican Grosbeak, 710.

Resistance, sense of, 496.

RESPIRATION, 299; use of blood-cor-
puscles in, 235 ; necessity for, 297 ; in
aquatic animals, 298 ; changes pro-
duced by, in air, 300—302, 334—336;
in blood, 303—306; related to nervo-
muscular activity, 307 ; energy of, in

602

INDEX.

Birds, Mammalia, and Insects, 308 ;
small amount of, in cold-blooded ani-
mals, 309, 310; no special provision
for in lowest, 311 ; uses of cilia in,
319, 329; an excreting process, 345,
346 ; subservient to maintenance of
heat, 412 — 414; in embryo, 760, 761.
Respiratory apparatus of Annelida, 314 ;
of aquatic Insects, 315; of Crustacea,
315 ; of Mollusca, 316, 320; of Fishes,
317, 318, 324; of Myriapoda, 320; of
Insects, 321— 322; of Arachnida, 323;
of Reptiles, 325; of Birds, 326, 327;
of Mammals, 328—333; of embryo,
760, 761.

movements, 331—334, 340—

342.

surface, extension of, 312; pro-
longation of, externally, into gills, 313;
internally into lungs, 313.

system of nerves, in Articu-

lata, 446; in Mollusca, 437, 438; in
Vertebrata, 450.

Rete mucosum, 38.

Retina, 535; yellow spot of, 554; in-
sensible spot of, 554.

Rhizopods, 129 ; substance of, 64.

Rhythmical movements, 578, 581.

Ribs, 633.

Rodentia, teeth of, 177.

Rooks, benefit of, 148.

Rotifera, 105; reproduction of, 750;
drying up of, 66.

Ruminating Animals, stomach of, 198;
foot of, 652.

Runningj act of, 66C.

Saccharine aliments, 153 ; destination of,
155 — 157, 162; conversion of, into olea-
ginous, 156.

Sacrum, 624, 645.

Salamander, 732.

Saliva, secretion of, 190; union of with
food, 191.

Salpa, reproduction of, 728.

Salt, use of, 166, 167.

Sandhopper, 100.

Sanguification, 222—224.

Sarcode, 128.

Saunderson, case of, 496.

Scapula, 634, 635, 637.

Sclerotic coat, 533.

Scurvy, 165.

Seal, 664, 665.

Sebaceous follicles, 38, 375.

SECRETION, general nature of, 245; act
of, performed by cells, 42, 354 ; distin-
guished from excretion, 352 ; influence
of mind upon, 353 ; transference of,
361.

Secreting follicles, 355, 356.

membranes, 355.

tubes, 357, 358.

Segmentation of yolk, 736, 756.

Semicircular canals., 518, 520.

Semilunar valves, 273.

Sensation, 432, 486 ; organs of, 9 ; general,
487 ; special, 488, 489 ; dependent on
supply of blood, 63, 487; modes of
exciting, 487, 488.

Sensori-motor actions, 430.

Sensorium, 429, 486.

Sensory ganglia, 452 ; functions of, 475—
479.

Serous membranes, structure of, 28;
arrangement of, 43.

Serpents, 85, 203; lung of, 325; verte-
bral column of, 629.

Serpula, 314.

Serum of blood, 238.

Shark, teeth of, 188; brain of, 453.

Shell of Mollusca, 106 ; of Crustacea,
99, 170; of Bird's egg, 755.

Siamese twins, 729.

Sighing, 341.

Sight, sense of, 526—575 (see Vision).

Silk-worm, voracity of larva of, 141.

Single Vision, 559.

Sitting posture, 654, 655.

Size, visual estimate of, 566.

Skeleton, position of, in different animals,
598; internal, of Vertebrata, 71, 599;
external, of Articulata, 93, 598; of
Mollusca, 106, 598; of Radiata, 118,
124, 127, 131, 132, 598; of Man, 616;
of Camel, 644; of Bird, 669; of Perch,
666 ; of Kangaroo, 661 ; of Seal, 664;
of Dugong, 664 ; of Bat, 669 ; of Pteio-
dactylus, 669.
articulation of pieces of, 601 — 605.

Skin, structure of, 36 — 38; exhalation
from, 370 — 374 ; secretions from, 375 ;
sensory papillae of, 37, 490 ; sensibility
of, 491—495.

Skull, bones of, 617—619.

Sloth, peculiar arterial distribution in,
264.

Slug, 106, 107.

Smell, sense of, 504—509; concerned in
taste, 501.

Snail, 106, 107; torpidity of, 67; respi-
ration of, 320 ; reparative power of, 389.

Sneezing, 342, 508.

Sobbing, 341,

Societies of animals, 706 — 711.

Song of animals, 686.

Soul of Man, 721, 722.

Sounds, propagated by vibrations, 510 —
512; produced by insects, 676—679; by
larynx, 682 ; pitch of, dependent on
number of vibrations, 523, 682.

Spatangus, 142.

Spectacles, choice of, 553.

Speech, articulate, 686—691.

Spermatozoids, 731.

Sperm-cells, of plants, 724 ; of animals,
730,731.

Spherical aberration, 546, 547.

Sphinx atropos, sound produced by, 678.
ligustri, nervous system of, 441.

INDEX.

603

Spiders, 98; circulation in, 293; respi-
ration in, 323 ; nervous system of, 447;
instincts of, 698, 700.

Spinal column, 71, 626—632.

Spinal cord, 72, 451, 460; independent
powers of, 464 — 474; nerves, 451, 457,
460.

Spiracles of Insects, 320—322.

Spleen, uses of, 224.

Sponge, 136, 137; circulation in, 296.

Stammering, 691.

Standing posture, 650 — 654.

Star-fish, 116—119; reparative power of,
389; nervous system of, 434; develop-
ment of. 741.

Sternum, 633.

Stereoscope, 561.

Stomach, need of in animals, 8 ; form of,
197; in Ruminants, 198, 199; move-
ments of, 206.

Stomato-gastric system of nerves, 447 —
450.

Stork, 653,

Strychnia, action of, 474.

Sturgeon, continued action of heart of,
583.

Sucking, act of, 172,472.

Suffocation, 338, 339.

Sugar, formation of by liver, 366.

Sulphur in animal bodies, 166; sources
of, 167.

Supra-renal capsules, 224.

Sutures, 602.

Swallowing, act of, 192—196.

Swimming, act of, 663 — 666.

Symmetry of disease, 380.

Sympathetic system of nerves, 60, 61,
461, 462.

Syncope, 271.

Synovial membranes, 44.

Syntonin, 16.

T.

Tadpole, 95—97; circulation in, 287—
288.

Tailor-bird, nest of, 705.

Tape-worm, 105 ; development of, 742.

Tardigrada, drying-up of, 66.

Taste, sense of, 499, 503.

Taurine, 364.

Tauro-cholic acid, 368.

Teeth, structure of, 54 ; development of,
174; cutting of, 175; cessation of
growth of> 176; continued growth of,
177; structure of, 178—180; different
kinds of, 181—183; first set of, 184;
motion of, in mastication, 178 — 180.

Teething, convulsions of, 174, 473, 474.

Tellina, 316.

Temperament, 718

Temperature, sense of, 497.

Tendons, structure of, 29; attachment
of muscles by, 605.

Testacella, 106.

Testis, 731.

Tetanus, 380.

Thalami optici, 458.

Thigh, bone and muscles of, 646.

Thoracic duct, 221.

Thorax, 328; movements of, 332.

Thumb, uses of, 643; reproduction of,
390.

Thunny, temperature of, 405.

Thymus gland, 224.

Thyroid cartilage, 680.
gland, 224.

Timbre of Sounds, 524.

Tissues, of Animals, distinctive pecu-
liarities of, 10 — 12; chemical composi-
tion of, 13—21; fibrous, 22—30; mem-
branous, 37 — 45; osseous, 49 — 54; cel-
lular, 32—36, 46—48 ; muscular, 55—
59; nervous, 60 — 63; degeneration of,
from want of use, 58 ; continual decay
and renewal of, 67, 68; self-formative
power of, 382, 387; reproduction of,
390.

Tongue, nerves of, 501 ; mechanical
uses of, 503.

Torpedo, electricity of, 419, 421.

Torpidity of animals, 66, 309.

Tortoise-shell, 92.

Tortrix, nest of, 701.

Touch, sense of, 490—499.

Tracheae of Insects, 321, 322.

Tranference of secretion, 361.

Transfusion of blood, 239.

Trematode Entozoa, 743.

Tricuspid valve, 272.

Tridacne, 119.

Tritonia, 316.

Trunk of Elephant, 172.
Insects, 173.

Tubercle, nature of, 386.

TUNICATA, 114; circulation in, 295 ; re-
spiration in, 316 ; luminousness of, 396 ;
nervous system of, 435, 436 ; gemmi-
parous reproduction of, 728 ; develop-
ment of, 752.

Turbo, anatomy of, 108.

Turnip-fly, voracity of, 147.

Turtle tribe, 83.

Tympanum, 516, 517.

U.

Ulna, 639.

Ungkaputi, 674.

Unity of Design, 261, 763.

Urea, 346, 367.

Ureters, 362.

Uric acid, 346, 348, 367.

Urinary apparatus, 368, 369.

bladder, 362.

excretion, purposes of, 346 —

348; effects of suspension of, 351;
composition of, 367 ; water discharged
by, 369.

Uterus, 761.

604

INDEX.

V.

Valves of heart, 272, 273; of veins, 279.

Vascular area, 758.

Vegetative repetition of parts, 2.

Veins, 246 ; structure of, 248 ; pressure
of blood in, 249; arrangement of, 250,
266 ; flow of blood through, 277, 278 ;
valves in, 279.

Vena cava, 266.

Venaportae, 267, 366.

Ventilation, importance of, 336, 337.

Ventricles of brain, 458.

of heart, 257; action of, 270.

Ventriloquism, 525.

Vertebrae, structure of, 71, 628; classifi-
cation of, 626 ; number of, 627 ; con-
nexion of, in Reptiles and Fishes,
629 ; in Man, 630; in Birds, 630.

Vertebral column, 70, 71, 626—632 ; first
development of, 757.

VERTEBRATA, general characters of, 70
— 76; nervous system of, 449 — 452;
skeleton of, 599 ; gemmation in, 729 ;
embryonic development of, 757 — 762.

Vessels, origin of, 393.

Vestibule of ear, 518, 521.

Vibrations, sonorous, 510 — 512; pitch
determined by number of, 523, 682.

Villi of mucous membrane. 41 ; absorp-
tion performed through, 41, 217.

Vision, dependent on light, 526, 542 ;
adaptation of eye to distinct, 543 — 553 ;
influence of attention on, 555 ; infe-
rences drawn from, 556 — 566 ; duration
of impressions, 567 ; distinction of
colours by, 568 — 570; erect, though
picture inverted, 558 ; single, with two
eyes; 559; double, 559.

Vitality, independent, of parts of organ-
ism, 65.

Vitreous humour, 536.

Vocal cords, 681.

Voice, confined to Vertebrata, 680 ; how
produced in larynx, 682; differences in
pitch and quality of, 683, 684.

Voltaic electricity, discovery of, 583, 584.

Voluntary movements, 589, 590.
Vorticella, reproduction of, 725.
Vowel sounds, 689, 690.

Vulture, skeleton of, 668.

W.

Walking, act of, 657.

Warm-blooded animals, 407—409.

Wasps, nest of, 711.

Waste of the system, 160, 307, 345.

Water, passed off" by kidneys, 369 ; ex-
haled from lungs, 343, 344 ; from skin,
370—374.

Water-Newt, 87.

Wax formed from sugar only, 155.

Webs of Spiders, 698.

Whale, mouth of, 185; peculiar arterial
distribution in, 265; sensibility of
surface in, 491; blow-holes of, 509;
propulsion of, in water, 665, 666.

Whalebone, 185.

Wheel-animalcules, (see Rotifer a}. '

White fibrous tissue, 23—29.

White of egg, 14, 755.

Wings, of Birds, 78, 668 ; of Bat, 669 ;
of Pterodactylus, 669; of Insects, 670;
action of, 667 — 672.

Winter eggs of Hydra and Rotifera, 735.

Wounds, healing of, 391 — 393; of ar-
teries, treatment of, 277.

Wren, intelligence of, 717.

X.

Xylocopa, nest of, 703.

Y.

Yawning, 341.

Yellow Fibrous tissue, 23—29.

Yellow spot of retina, 554.

Yolk of egg, 733, 736, 754.

Yolk-bag, 733, 754.

Young animals, heat of, 408, 409.

Z.

Zoea, larva of Crab, 101.

Zoophytes, 117, 121; tissues of, 64, 577;
circulation in, 296; gemmiparous pro-
duction of, 726; development of Me-
dusae from, 738, 740; generation of,
738, 739.

THE END.

I. CLAY, PRINTER, BREAD STREET JlILL.

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