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THE BRIDGEWATER TREATISES

ON THE POWER WISDOM AND GOODNESS OF GOD
AS MANIFESTED IN THE CREATION

TREATISE V

ANIMAL AND VEGETABLE PHYSIOLOGY CONSIDERED

WITH REFERENCE TO NATURAL THEOLOGY

BY PETER MARK ROGET, M. D.

SEC. R. S. ETC.

, IN TWO VOLUMES

VOL II
[SECOND EDITION]

"And there are diversities of operations, but it is the same God
WHICH worketh all in all." 1 Cor. xii. 6.

3

ANIMAL AND VEGETABLE PHYSIOLOGY

CONSIDERED WITH REFERENCE TO

NATURAL THEOLOGY

BY

PETER MARK ROGET, M. D.

SECRETART TO THB ROYAL SOCIETY, FULLERIAN PROFESSOR OF PHYSIOIXKJY- IN THF. ROYAI,

INSTITUTION OF GREAT BRITAIN, VICE PRESIDENT OF THE SOCIETY OP ARTS,
I'ELLOW OF THE ROYAL COLLEGE OF PHYSICIANS, CONSULTING PHYSICIAN TO THE QUEEN
CHARLOITE's LYING-IN HOSPITAL, AND TO THE NORTHERN
DISPENSARY, ETC. ETC,

VOL II

LONDON

WILLIAM PICKERING

1834

C. WHITTINUHAM, TOOKS COHRT, CUANCEHY LAMG.

CONTENTS
OF THE SECOND VOLUME.

PART II.— THE VITAL FUNCTIONS.

Page

Chapter I — Objects of Nutrition 1

Chapter II. — Nutrition in Vegetables 15

§ I. Food of Plants 15

2. Absorption of Nutriment by Plants 19

3. Exhalation 27

4. Aeration of the Sap 29

5. Return of the Sap 36

6. Secretion in Vegetables 45

7. Excretion in Vegetables 51

Chapter III. — Animal Nutrition in general 57

§ 1 . Food of Animals 57

2. Series of Vital Functions 69

Chapter IV. — Nutrition in the lower orders of
Animals 74

Chapter V. — Nutrition in the higher orders of
Animals 104

Chapter VI. — Preparation of Food 113

§ 1 . Prehension of Liquid Food 113

2. Prehension of Solid Food 117

3. Mastication by means of Teeth 140

VI CONTENTS.

Page

4. Formation and Developement of the Teeth . . 155

5. Trituration of Food in Internal Cavities 167

6. Deglutition 174

7. Receptacles for retaining Food , 178

Chapter VII. — Digestion , 180

Chapter VIII. — Chylification 203

Chapter IX. — Lacteal Absorption 226

Chapter X. — Circulation , . 229

§ 1. Diffused Circulation 229

2. Vascular Circulation , . 235

3. Respiratory Circulation 265

4. Distribution of Blood Vessels 281

Chapter XI. — Respiration 290

§ 1 . Respiration in general 290

2. Aquatic Respiration , , . . 293

3. Atmospheric Respiration 310

4. Chemical Changes effected by Respiration . . 333

Chapter XII. — Secretion 342

Chapter XIII. — Absorption 351

Chapter XIV. — Nervous Power 354

PART III.— THE SENSORIAL FUNCTIONS.

Chapter I. — Sensation 362

Chapter II. — Touch 377

Chapter HI. — Taste 393

CONTENTS. Vll

Page

Chapter IV. — Smell 396

Chapter V. — Hearing 414

§ 1 , Acoustic Principles 414

2. Physiology of Hearing in Man 420

3. Comparative Physiology of Hearing 434

Chapter VI,"— Vision 444

§ 1 . Object of the Sense of Vision 444

2. Modes of accomplishing the objects of Vision . 449

3. Structure of the Eye 460

4. Physiology of Perfect Vision 469

5. Comparative Physiology of Vision 477

Chapter VII. — Perception 508

Chapter VIII. — Comparative Physiology of thl

Nervous System. 537

§1. Nervous System of Invertebrated Animals. .. . 537

2. Nervous System of Vertebrated Animals .... 553

3. Functions of the Brain 561

4. Comparative Physiology of Perception 566

PART IV.— THE REPRODUCTIVE FUNCTIONS.

Chapter I. — Reproduction 581

Chapter II. — Organic Developement 599

Chapter III. — Decline of the System 619

Chapter IV. — Unity of Design 625

Index 643

ANIMAL AND VEGETABLE
PHYSIOLOGY.

PART II.

THE VITAL FUNCTIONS.

Chapter I.

OBJECTS OF NUTRITION.

The mechanical structure and properties of the
organized fabric, which have occupied our atten-
tion in the preceding volume, are necessary for
the maintenance of life, and the exercise of the
vital powers. But however artificially that fabric
may have been constructed, and however admi-
rable the skill and the foresight which have been
displayed in ensuring the safety of its elaborate
mechanism, and in preserving the harmony of
its complicated movements, it yet of necessity
contains within itself the elements of its own dis-
solution. The animal machine, in common with

VOL. II. B

2 THE VITAL FUNCTIONS.

every other mechanical contrivance, is subject
to wear and deteriorate by constant use. Not
only in the greater movements of the limbs, but
also in the more delicate actions of the internal
organs, we may trace the operation of many
causes inevitably leading to their ultimate des-
truction. Continued friction must necessarily
occasion a loss of substance in the harder parts
of the frame ; and evaporation is constantly tend-
ing to exhaust the fluids. The repeated actions
of the muscles induce certain changes in these
organs, both in their mechanical properties and
chemical composition, which impair their powers
of contraction, and which, if suffered to continue,
would, in no long time, render them incapable
of exercising their proper functions; and the
same observation applies also to the nerves, and
to all the other systems of organs. Provision
must accordingly be made for remedying these
constant causes of decay by the supply of those
peculiar materials, which the organs require for
recruiting their declining energies.

It is obvious that the developement of the
organs, and general growth of the body, must
imply the continual addition of new particles
from foreign sources. Organic increase consists
not in the mere expansion of a texture previously
condensed, and the filling up of its interstices
by inorganic matter ; but the new materials that
are added must, for this purpose, be incorporated
with those which previously existed, and become

OBJECTS OF NUTRITION. 3

identified with the living substance. Thus we
often find structures forming in the bodies of
animals, of a nature totally different from that of
the part from which they arise.

In addition to these demands, a store of mate-
rials is also wanted for the reparation of occa-
sional injuries, to which, in the course of its long
career, the body is unavoidably exposed. Like
a ship fitted out for a long voyage, and fortified
against the various dangers of tempests, of ice-
bergs, and of shoals, the animal system, when
launched into existence, should be provided with
a store of such materials as may be wanted for
the repair of accidental losses, and should also
contain within itself the latent source of those
energies, which may be called into action when
demanded by the exigencies of the occasion.

Any one of the circumstances above enume-
rated would of itself be sufficient to establish the
necessity of supplies of nourishment for the
maintenance of life. But there are other consi-
derations, equally important in a physiological
point of view, and derived from the essential
nature of organization, which also produce a
continual demand for these supplies ; and these
I shall now endeavour briefly to explain.

Constant and progressive change appears to
be one of the leading characteristics of life ; and
the materials, which are to be endowed with vi-
tality, must therefore be selected and arranged
with a view to their continual modification, cor-

4 THE VITAL FUNCTIONS.

responding to these ever varying changes of con-
dition. The artificer, whose aim is to construct
a machine for permanent use, and to secure it as
much as possible from the deterioration arising
from friction or other cases of injury, wouhi, of
course, make choice for that purpose of the most
hard and durable materials, such as the metals,
or the denser stones. In constructing a watch, for
instance, he would form the wheels of brass, the
spring and the barrel-chain of steel ; and for the
pivot, where the motion is to be incessant, he
would employ the hardest of all materials,— the
diamond. Such a machine, once finished, being
exempt from almost every natural cause of decay,
might remain for an indefinite period in the
same state. Far different are the objects which
must be had in view in the formation of organized
structures. In order that these may be qualified
for exercising the functions of life, they must be
capable of continual alterations, displacements,
and adjustments, varying perpetually, both in
kind and in degree, according to the progressive
stages of their internal developement, and to the
different circumstances which may arise in their
external condition. The materials which nature
has employed in their construction, are, there-
fore, neither the elementary bodies, nor even
their simpler and more permanent combinations ;
but such of their compounds as are of a more
plastic quality, and which allow of a variable

ORGANIC CHEMISTRY. O

proportion of ingredients, and of great diversity
in the modes of their combination. So great
is the complexity of these arrangements, that
although chemistry is fully competent to the
analysis of organized substances into their ulti-
mate elements, no human art is adequate to
effect their reunion in the same state as that in
which they had existed in those substances ; for
it was by the refined operations of vitality, the
only power which could produce this adjustment,
that they have been brought into that condition.
We may take as an example one of the simplest
of organic products, namely Sugar ; a substance
which has been analysed with the greatest accu-
racy by modern chemists : yet to reproduce this
sugar, by the artificial combination of its simple
elements, is a problem which has hitherto baffled
all the efforts of philosophy. Chemistry, not-
withstanding the proud rank it justly holds among
the physical sciences, and the noble discoveries
with which it has enriched the arts; notwith-
standing it has unveiled to us many of the secret
operations of nature, and placed in our hands
some of her most powerful instruments for acting
upon matter; and notwithstanding it is armed
with full powers to destroy, cannot, in any one
organic product, rejoin that which has been once
dissevered. Through the medium of chemistry
we are enabled, perhaps, to form some estimate
of the value of what we find executed by other

6 THE VITAL FUNCTIONS.

agencies ; but the imitation of the model, even
in the smallest part, is far beyond our power.
No means which the laboratory can supply, no
process, which the most inventive chemist can
devise, have ever yet approached those delicate
and refined operations which nature silently con-
ducts in the organized texture of living plants
and animals.

The elements of organic substances are not
very numerous ; the principal of them being
oxygen, carbon, hydrogen, nitrogen, sulphur,
and phosphorus, together with a few of the alka-
line, earthy, and metallic bases. These sub-
stances are variously united, so as to form cer-
tain specific compounds, which, although they
are susceptible, in different instances, of endless
modifications, yet possess such a general cha-
racter of uniformity, as to allow of their being
arranged in certain classes ; the most character-
istic substance in each class constituting what is
called a proximate orga?iic principle. Thus in
the vegetable kingdom we have Lignin,2^amiin,
Mucilage, Oil, Sugar, Fecula, &c. The animal
kingdom, in like manner, furnishes Gelatin,
Albumen, Fibrin, Mucus, EntomoUne, Elearin^
Stearin, and many others.

The chemical constitution of these organic
products, formed, as they are, of but few pri-
mary elements, is strikingly contrasted with
that of the bodies belonging to the mineral

ORGANIC CHEMISTRY. 7

kingdom. The catalogue of elementary, or
simple bodies, existing in nature, is, indeed,
more extensive than the list of those which
enter into the composition of animal or vege-
table substances. But in the mineral world
they occur in simpler combinations, resolvable,
for the most part, into a few definite ingredients,
which rarely comprise more than two or three
elements. In organized products, on the other
hand, although the total number of existing
elements may be smaller, yet the mode of com-
bination in each separate compound is infinitely
more complex, and presents incalculable diver-
sity. Simple binary compounds are rarely ever
met with ; but, in place of these, we find three,
four, five, or even a greater number of consti-
tuent elements existing in very complicated
states of union.

This peculiar mode of combination gives rise
to a remarkable condition, which attaches to
the chemical properties of organic compounds.
The attractive forces, by which their several
ingredients are held together, being very nume-
rous, require to be much more nicely balanced,
in order to retain them in combination. Slight
causes are sufficient to disturb, or even overset,
this equipoise of affinities, and often produce
rapid changes of form, or even complete decom-
position. The principles, thus retained in a
kind of forced union, have a constant tendency

8 THE VITAL FUNCTIONS.

to react upon one another, and to produce, from
slight variations of circumstances, a totally new
order of combinations. Thus a degree of heat,
which would occasion no change in most mineral
substances, will at once effect the complete dis-
union of the elements of an animal or vegetable
body. Organic substances are, in like manner,
unable to resist the slower, but equally destruc-
tive agency of water and atmospheric air ; and
they are also liable to various spontaneous
changes, such as those constituting fermentation
and putrefaction, which occur when their vitality
is extinct, and when they are consequently
abandoned to the vmcontrolled operation of their
natural chemical affinities. This tendency to
decomposition may, indeed, be regarded as
inherent in all organized substances, and as
requiring for its counteraction, in the living
system, that perpetual renovation of materials
which is supplied by the powers of nutrition.

It would appear that during the continuance
of life, the progress of decay is arrested at its
very commencement ; and that the particles,
which first undergo changes unfitting them for
the exercise of their functions, and which, if suf-
fered to remain, would accelerate the destruction
of the adjoining parts, are immediately removed,
and their place supplied by particles which have
been modified for that purpose, and which,
when they afterwards lose these salutary pro-

ORGANIC CHEMISTRY. ;9

perties, are in their turn discarded, and replaced
by others. Hence the continued interchange
and renewal of particles, which take place in
the more active organs of the system, especially
in the higher classes of animals. In the fabric
of those animals which possess an extensive
system of circulating and absorbing vessels, the
changes which are effected are so considerable
and so rapid, that even in the densest textures,
such as the bones, scarcely any portion of the
substance which originally composed them is
permanently retained in their structure. To so
great an extent is this renovation of materials
carried on in the human system, that doubts
may very reasonably be entertained as to the
identity of any portion of the body after the
lapse of a certain time. The period assigned by
the ancients for this entire change of the sub-
stance of the body was seven or eight years ; but
modern inquiries, which show us the rapid re-
paration that takes place in injured parts, and
the quick renewal of the bones themselves, tend
to prove that even a shorter time than this is
adequate to the complete renovation of every
portion of the living fabric*

Imperfect as is our knowledge of organic
chemistry, we see enough to convince us that a

* See the article " Age" in the Cyclopsedia of Practical
Medicine, where I have enlarged upon this subject.

10 THE VITAL FUNCTIONS.

series of the most refined and artificial opera-
tions is required in order to bring about the com-
plicated and elaborate arrangements of elements
which constitute both animal and vegetable
products. Thus in the very outset of this, as of
every other inquiry in Physiology, we meet with
evidences of profound intention and consummate
art, infinitely surpassing not only the power and
resources, but even the imagination of man.

Much as the elaborate and harmonious me-
chanism of an animal body is fitted to excite our
admiration, there can be no doubt that a more
extended knowledge of that series of subtle pro-
cesses, consisting of chemical combinations and
decompositions, which are continually going on
in the organic laboratory of living beings, would
reveal still greater wonders, and would fill us
with a more fervent admiration of the infinite
art and prescience, which are even now mani-
fested to us in every department both of the
vegetable and animal economy.

The processes, by which all these important
purposes are fulfilled, comprise a distinct class of
functions, the final object of which may be
termed Nutrition, that is, the reparation of the
waste of the substance of the organs, their
maintenance in the state fitting them for the
exercise of their respective offices, and the appli-
cation of properly prepared materials to their
developement and growth.

PROCESSES OF NUTRITION. 1 1

The functions subservient to nutrition may be
distinguished, according as the processes they
comprise relate to seven principal periods in the
natural order of their succession. The first
series of processes has for its objects the re-
ception of the materials from without, and their
preparation and gradual conversion into proper
nutriment, that is, into matter having the same
chemical properties with the substance of the
organs with which it is to be incorporated ; and
their purpose being to assimilate the food as
much as possible to the nature of the organic
body it is to nourish, all these functions have
been included under the term Assimilation.

The second series of vital functions com-
prises those which are designed to convey the
nutritive fluids thus elaborated, to all the organs
that are to be nourished by them. In the more
developed systems of organization this purpose
is accomplished by means of canals, called vessels,
through which the nutritive fluids move in a
kind of circuit : in this case the function is de-
nominated the Circulation.

It is not enough that the nutritive juices are
assimilated ; another chemical process is still
required to perfect their animalization, and to
retain them in their proper chemical condition
for the purposes of the system. This third object
is accomplished by the function of Respiration.

Fourthly, several chemical products which are

12 THE VITAL FUNCTIONS.

wanted in different parts of the economy, are
required to be formed by a peculiar set of organs,
of which the intimate structure eludes observa-
tion ; although we may perceive that in many
instances, among the higher orders of beings, a
special apparatus of vessels, sometimes spread
over the surface of a membrane, at other times
collected into distinct masses, is provided for
that purpose. These specific organs are termed
glands; and the office performed by them, as
well as by the simpler forms of structure above
mentioned, is termed Secretion.

Fifthly, similar processes of secretion are also
employed to carry off from the blood such animal
.products as may have been formed or introduced
into it, and may possess, or have acquired noxious
properties. The elimination of these materials,
which is the office of the excretoiies, constitutes
the function of Excretion.

Sixthly, changes may take place in various
parts of the body, both solid and fluid, rendering
them unfit to remain in their present situation ;
and measures are taken for the removal of
these useless or noxious materials, by transferring
them to the general mass of circulating blood,
so as either to be again usefully employed,
or altogether discarded by excretion from the
system. This object is accomplished by a
peculiar set of vessels; and the function they
perform is termed Absorption.

POWERS OF ASSIMILATION. 13

Lastly, the conversion of the fluid nutriment
into the solids of the body, and its immediate
application to the purposes of the developement
of the organs, of their preservation in the state of
health and activity, and of the repair of such
injuries as they may chance to sustain, as far as
the powers of the system are adequate to such
reparation, are the objects of a seventh set of
functions, more especially comprised under the
title of Nutrition, which closes this long series of
chemical changes, and this intricate but har-
monious system of operations.

Although the order in which the constituent
elements of organized products are arranged, and
the mode in which they are combined, are
entirely unknown to us, we can nevertheless
perceive that in following them successively
from the simplest vegetables to the higher orders
of the animal kingdom, they acquire continually
increasing degrees of complexity, corresponding,
in some measure, to the greater refinement and
complication of the structures by which they have
been elaborated, and of the bodies to which they
are ultimately assimilated. Thus plants derive
their nourishment from the crude and simple
materials which they absorb from the earth, the
waters, and the air that surround them ; mate-
rials which consist almost wholly of water, with
a small proportion of carbonic acid, and a few
saline ingredients, of which that water is the

14 THE VITAL FUNCTIONS.

vehicle. But these, after having been converted
by the powers of vegetable assimilation, into the
substance of the plant, acquire the charac-
teristic properties of organized products, though
they are still the simplest of that class. In this
state, and when the fabric they had composed is
destroyed, and they are scattered over the soil,
they are fitted to become more highly nutritive
to other plants, which absorb them, and with
more facility adapt them to the purposes of their
own systems. Here they receive a still higher
degree of elaboration ; and thus the same mate-
rials may pass through several successive series
of modifications, till they become the food of ani-
mals, and are then made to undergo still further
changes. New elements, and in particular
nitrogen, is added to the oxygen, hydrogen and
carbon, which are the chief constituents of
vegetable substances:* and new properties are
acquired, from the varied combinations into
which their elements are made to enter by the
more energetic powers of assimilation apper-
taining to the animal system. The products
which result are still more removed from their
original state of inorganic matter; and in this
condition they serve as the appropriate food of

* Nitrogen, however, frequently enters into the composition
of vegetables ; though m general, in a much smaller proportion
than into the substance of animals, of which last it always ap-
pears to be an essential constituent.

VEGETABLE NUTRITION. 15

carnivorous animals, which generally hold a
higher rank in the scale of organization, than
those that subsist only on vegetables.

Thus has each created being been formed with
reference, not merely to its own welfare, but
also to that of multitudes of others which are
dependent on it for their support, their preser-
vation,— nay, even for their existence. In con-
templating this mutual relationship, this suc-
cessive subordination of the different races to one
another, and this continual tendency to increased
refinement, we cannot shut our eyes to the mag-
nificent unfolding of the great scheme of nature
for the progressive attainment of higher objects ;
until, in the perfect system, and exalted endow-
ments of man, we behold the last result which
has been manifested to us of creative power.

Chapter II.

NUTRITION IN VEGETABLES.

§ t . Food of Plants.

The simplest kind of nutrition is that presented
to us by the vegetable kingdom, where water
may be considered as the general vehicle of the
nutriment received. Before the discoveries of

16 THE VITAL FUNCTIONS.

modern chemistry it was very generally believed
that plants could subsist on water alone; and
Boyle and Van Helmont, in particular, endea-
voured to establish by experiment the truth of
this opinion. The latter of these physiologists
planted a willow in a certain quantity of earth,
the weight of which he had previously ascer-
tained with great care ; and during five years, he
kept it moistened with rain water alone, which
he imagined was perfectly pure. At the end of
this period he found that the earth had scarcely
diminished in weight, while the willow had
grown into a tree, and had acquired an addi-
tional weight of one hundred and fifty pounds :
whence he concluded that the water had been
the only source of its nourishment. But it does
not seem to have been at that time known that
rain water always contains atmospheric air, and
frequently also other substances, and that it
cannot, therefore, be regarded as absolutely pure
water : nor does it appear that any precautions
were taken to ascertain that the water actually
employed was wholly free from foreign matter,
which it is easy to conceive it might have held
in solution. In an experiment of Duhamel, on
the other hand, a horse-chestnut tree and an oak,
exposed to the open air, and watered with
distilled water alone, the former for three, and
the latter for eight years, were kept alive, indeed,
but they were exceedingly stinted in their growth,

FOOD OF PLANTS. 17

and evidently derived little or no sustenance
from the water with which they were supplied.
Experiments of a similar nature were made by
Bonnet, and with the like result. When plants
are contained in closed vessels, and regularly
supplied with water, but denied all access to
carbonic acid gas, they are developed only to a
very limited extent, determined by the store of
nutritious matter which had been already col-
lected in each plant when the experiment com-
menced, and which, by combining with the
water, may have afforded a temporary supply of
nourishment.

But the water which nature furnishes to the
vegetable organs is never perfectly pure ; for, be-
sides containing air, in which there is constantly
a certain proportion of carbonic acid gas, it has
always acquired, by percolation through the soil,
various earthy and saline particles, together with
materials derived from decayed vegetable or
animal remains. Most of these substances are
soluble, in however minute a quantity, in water:
and others, finely pulverized, may be suspended
in that fluid, and carried along with it into the
vegetable system. It does not appear, however,
that pure carbon is ever admitted ; for Sir H.
Davy, on mixing charcoal, ground to an im-
palpable powder, with the water into which the
roots of mint were immersed, could not discover
that the smallest quantity of that substance had

VOL. II. c

18 THE VITAL FUNCTIONS.

been, in any case, absorbed.* But in the form
of carbonic acid, this element is received in
great abundance, through the medium of water,
which readily absorbs it ; and a considerable
quantity of carbon is also introduced into the
fluids of the plant, derived from the decomposed
animal and vegetable materials, which the water
generally contains. The peculiar fertility of
each kind of soil depends principally on the
quantity of these organic products it contains in
a state capable of being absorbed by the plant,
and of contributing to its nourishment.

The soil is also the source whence plants derive
their saline, earthy, and metallic ingredients.
The silica they often contain is, in like manner,
conveyed to them by the water, which it is now
well ascertained, by the researches of Berzelius,
is capable of dissolving a very minute quantity
of this dense and hard substance. It is evident
that, however small this quantity may be, if it
continue to accumulate in the plant, it may in
time constitute the whole amount of that which
is found to be so copiously deposited on the sur-
face, or collected in the interior of many plants,
such as the bamboo, and various species of
grasses. The small degree of solubility of many
substances thus required for the construction of
the solid vegetable fabric, is, probably, one of the
reasons why plants require so large a supply of
water for their subsistence.

* Elements of Agricultural Chemistry, Lect. VI. p. 234.

VEGETABLE ABSORPTION. 19

§ 2. Absorption of Nutriment by Plants.

The greater number of cellular plants absorb
water with nearly equal facility from every part
of their surface : this is the case with the AlgcB, for
instance, which are aquatic plants. In Lichens,
on the other hand, absorption takes place more
partially ; but the particular parts of the surface
where it occurs are not constantly the same, and
appear to be determined more by mechanical-
causes than by any peculiarity of structure :
some, however, are found to be provided in cer-
tain parts of the surface with stomata, which De
Candolle supposes may act as sucking orifices.
Many mushrooms appear to be capable of ab-
sorbing fluids from all parts of their surface
indiscriminately ; and some species, again, are
furnished at their base with a kind of radical
fibrils for that purpose.

In plants having a vascular structure, which
is the case with by far the greater number, the
roots are the special organs to which this office
of absorbing nourishment is assigned : but it
occasionally happens that, under certain cir-
cumstances, the leaves, or the stems of plants
are found to absorb moisture ; which they have
been supposed to do by the stomata interspersed
on their surface. This, however, is not their
natural action ; and they assume it only in forced

20 THE VITAL FUNCTIONS.

situations, when they procure no water by means
of the roots, either from having been deprived of
these organs, or from their being left totally dry.
Thus a branch, separated from the trunk, may
be preserved from withering for a long time, if
the leaves be immersed in water ; and when the
soil has been parched by a long drought, the
drooping plants will be very quickly revived by
a shower of rain, or by artificial watering, even
before any moisture can be supposed to have
penetrated to the roots.

It is by the extremities of the roots alone, or
rather by the spongioles which are there situ-
ated, that absorption takes place; for the surface
of the root, being covered in every other part by
a layer of epidermis, is incapable of performing
this office. It was long ago remarked by Du-
hamel, that trees exhaust the soil only in those
parts which surround the extremities of the
roots ; but the fact, that absorption is effected
only at those points, has been placed beyond a
doubt by the direct experiments of Sennebier,
who, taking two carrots of equal size, immersed
in water the whole root of the one, while only
the extremity of the other was made to dip into
the water, and found that equal quantities were
absorbed in both cases ; while on immersing the
whole surface of another carrot in the fluid, with
the exception of the extremity of the root, which
was raised so as to be above the surface, no ab-

VEGETABLE ABSORPTION. 21

sorption whatever took place. Plants having a
fusiform^ or spindle-shaped root, such as the
carrot and the radish, are the best for these ex-
periments.

In the natural progress of growth, the roots
are constantly shooting forwards in the direction
they have first taken, whether horizontally, or
downwards, or at any other inclination. Thus
they continually arrive at new portions of soil,
of which the nutritive matter has not yet been
exhausted ; and as a constant relation is pre-
served between their lateral extension and the
horizontal spreading of the branches, the greater
part of the rain which falls upon the tree, is
made to drop from the leaves at the exact dis-
tance from the trunk, where, after it has soaked
through the earth, it will be received by the ex-
tremities of the roots, and readily sucked in by
the spongioles. We have here a striking instance
of that beautiful correspondence, which has been
established between processes belonging to diffe-
rent departments of nature, and which are made
to concur in the production of such remote effects,
as could never have been accomplished without
these preconcerted and harmonious adjustments.

The spongioles, or absorbing extremities of
the roots, are constructed of ordinary cellular or
spongy tissue ; and they imbibe the fluids, which
are in contact with them, partly by capillary
action, and jjartly, also, by what has been termed

22 THE VITAL FUNCTIONS.

a hygroscopic power. But though these principles
may sufficiently account for the simple entrance
of the fluids, they are inadequate to explain its
continued ascent through the substance of the
root, or along the stem of the plant. The most
probable explanation of this phenomenon is that
the progressive movement of the fluid is produced
by alternate contractions and dilatations of the
cells themselves, which compose the texture of
the plant ; these actions being themselves refer-
able to the vitality of the organs.

The absorbent power of the spongioles is
limited by the diameter of their pores, so that
fluids which are of too viscid or glutinous a con-
sistence to pass readily through them are liable
to obstruct or entirely block up these passages.
Thus if the spongioles be surrounded by a thick
solution of gum, or even of sugar, its pores will
be clogged up, scarcely any portion of the fluid
will be absorbed, and the plant will wither and
perish ; but if the same liquids be more largely
diluted, the watery portion will find its way
through the spongioles, and become available
for the sustenance of the plant, while the greater
part of the thicker material will be left behind.
The same apparent power of selection is exhibited
when saline solutions of a certain strength are
presented to the roots ; the water of the solution,
with only a small proportion of the salts, being
taken up ; and the remaining part of the fluid

VEGETABLE ABSORPTION. 23

being found to be more strongly impregnated
with tlie salts than before this absorption had
taken place. It would appear, however, that all
this is merely the result of a mechanical opera-
tion, and that it furnishes no evidence of any
discriminating faculty in the spongiole ; for it is
found that, provided the material presented be
in a state of perfect solution and limpidity, it is
sucked in with equal avidity, whether its qualities
be deleterious or salubrious. Solutions of sul-
phate of copper, which is a deadly poison, are
absorbed in large quantities by the roots of plants,
which are immersed in them ; and water which
drains from a bed of manure, and is consequently
loaded with carbonaceous particles, proves ex-
ceedingly injurious when admitted into the system
of the plant, from the excess of nutriment it con-
tains. But in the ordinary course of vegetation,
no danger can arise from this general power of
absorption, since the fluids which nature supplies
are always such as are suitable to the organs
that are to receive them.

The fluid, which is taken up by the roots, and
which, as we have seen, consists chiefly of water,
holding in solution atmospheric air, together
with various saline and earthy ingredients neces-
sary for the nourishment of the plant, is in a
perfectly crude state. It rises in the stem of
the plant, undergoing scarcely any perceptible
change in its ascent ; and is in this state conducted

24 THE VITAL FUNCTIONS.

to the leaves, where it is to experience various
important modifications. By causing the roots
to imbibe coloured liquids, the general course of
the sap has been traced with tolerable accuracy,
and it is found to traverse principally the ligneous
substance of the stem : in trees, its passage is
chiefly through the alburnum, or more recently
formed wood, and not through the bark, as was
at one time believed.

The course of the sap, however, varies under
different circumstances, and at different epochs
of vegetation. At the period when the young
buds are preparing for their developement, which
usually takes place when the genial warmth of
spring has penetrated beyond the surface, and
expanded the fibres and vessels of the plant,
there arises an urgent demand for nourishment,
which the roots are actively employed in supply-
ing. As the leaves are not yet completed, the
sap is at first applied to purposes somewhat
different from those it is destined to fulfil at a
more advanced period, when it has to nourish
the fully expanded organs : this fluid has, ac-
cordingly, received a distinct appellation, being
termed the nursling sap. Instead of rising
through the alburnum, the nursling sap ascends
through the innermost circle of wood, or that
which is immediately contiguous to the pith, and
is thence transmitted, by unknown channels,
through the several layers of wood, till it reaches

ASCENT OF THE SAP. 25

the buds, which it is to supply with nourishment.
During this circuitous passage, it probably un-
dergoes a certain degree of elaboration, fitting it
for the office which it has to perform : it appa-
rently combines with some nutriment, which had
been previously deposited in the plant, and which
it again dissolves ; and thus becoming assimilated,
is in a state proper to be incorporated with the
new organization that is developing. This nurs-
ling sap, provided for the nourishment of the
young buds, has been compared to the milk of
animals, which is prepared for a similar purpose
at those times only when nutriment is required
for the rearing of their young.

Several opinions have been entertained with
regard to the channels tiiT-ough which the sap is
conveyed in its ascent along the stem, and in
its passage to its ultimate destination. Many
observations tend to show, that, in ordinary cir-
cumstances, it is not transmitted through any
of the distinguishable vessels of the plant : for
most of these, in their natural state, are found to
contain only air. The sap must, therefore, either
traverse the cells themselves, or pass along the
intercellular spaces. That the latter is the
course it takes is the opinion of De Candolle,
who adduces a variety of arguments in its sup-
port. The sap, he observes, is found to rise
equally well in plants whose structure is wholly
cellular; a fact which proves that vessels arc not

26 THE VITAL FUNCTIONS.

in all cases necessary for its conveyance. In
many instances the sap is known to deviate from
its usual rectilinear path, and to pursue a cir-
cuitous course, very different from that of any of
the known vessels of the plant. The diffusion
of the sap in different directions, and its sub-
sidence in the lowest parts, on certain occasions,
are facts irreconcileable with the supposition
that it is confined in these vessels.

Numerous experiments have been made to
discover the velocity with which the sap rises in
plants, and the force it exerts in its ascent.
Those of Hales are well known : by lopping off
the top of a young vine, and applying to the
truncated extremity a glass tube, which closed
round it, he found that the fluid in the tube rose
to a height, which, taking into account the spe-
cific gravity of the fluid, was equivalent to a per-
pendicular column of water of more than forty-
three feet ; and consequently exerted a force of
propulsion considerably greater than the pressure
of an additional atmosphere. The velocity, as
w^ell as the force of ascent, must, however, be
liable to great variation ; being much influenced
by evaporation, and other changes, which the sap
imdergoes in the leaves. Various opinions have
been entertained as to the agency by which the
motion of the sap is effected ; but although it
seems likely to be resolved into the vital move-
ments of the cellular structure already mentioned,

VEGETABLE EXHALATION. 27

the question is still enveloped in considerable
obscurity. There is certainly no evidence to
prove that it has any analogy to a muscular
power; and the simplest supposition we can
make is that these actions take place by means
of a contractile property belonging to the vege-
table tissue, and exerted, under certain circum-
stances, and in conformity to certain laws, which
we have not yet succeeded in determining.

<^ 3. Exhalatio7i.

The nutrient sap, which, as we have seen, rises
in the stem, and is transmitted to the leaves
without any change in its qualities or compo-
sition, is immediately, by the medium of the
stomata, or orifices which abound in the surface
of those organs, subjected to the process of
exhalation. The proportion of water which the
sap loses by exhalation in the leaves is generally
about two-thirds of the whole quantity received ;
so that it is only the remaining third that returns
to nourish the organs of the plant. It has been
ascertained that the water thus evaporated is
perfectly pure ; or at least does not contain more
than a 10,000,0()0th part of the foreign matter
with which it was impregnated when first ab-
sorbed by the roots. The water thus exhaled,

28 THE VITAL FUNCTIONS.

being dissolved by the air the moment it escapes,
passes off in the form of invisible vapour. Hales
made an experiment with a sun-flower, three
feet high, enclosed in a vessel, which he kept for
fifteen days ; and inferred from it that the weight
of the fluid daily exhaled by the plant was twenty
ounces ; and this he computes is a quantity
seventeen times greater than that lost by insen-
sible perspiration from an equal portion of the
surface of the human body.

The comparative quantities of fluid exhaled
by the same plant at different times are regu-
lated, not so much by temperature, as by the
intensity of the light to which the leaves are
exposed. It is only during the day, therefore,
that this function is in activity. De Candolle
has found that the artificial light of lamps pro-
duces on the leaves an effect similar to that of
the solar rays, and in a degree proportionate to
its intensity.* As it is only through the stomata
that exhalation proceeds, the number of these
pores in a given surface must considerably in-
fluence the quantity of fluid exhaled.

By the loss of so large a portion of the water
which, in the rising sap, had held in solution
various foreign materials, these substances are
rendered more disposed to separate from the
fluid, and to become consolidated on the sides

* Physiologic Vegetale, i. 112.

AERATION OF THE SAP. 29

of the cells or vessels, to which they are con-
ducted from the leaves. This, then, is the first
modification in the qualities of the sap which it
undergoes in those organs.

§ 4. Aeration of the Sap.

A CHEMICAL change much more considerable
and important than the preceding is next effected
on the sap by the leaves, when they are sub-
jected to the action of light. It consists in the
decomposition of the carbonic acid gas, which
is either brought to them by the sap itself, or
obtained directly from the surrounding atmo-
sphere. In either case its oxygen is separated,
and disengaged in the form of gas ; while its
carbon is retained, and composes an essential
ingredient of the altered sap, which, as it now
possesses one of the principal elements of vege-
table structures, may be considered as having
made a near approach to its complete assimi-
Intion, using this term in the physiological sense
already pointed out.

The remarkable discovery that oxygen gas is
exhaled from the leaves of plants during the
day time, was made by the great founder of
pneumatic chemistry. Dr. Priestley : to Senne-
bier we are indebted for the first observation

30 THE VITAL FUNCTIONS.

that the presence of carbonic acid is required
for the disengagement of oxygen in this process,
and that the oxygen is derived from the decom-
position of the carbonic acid ; and these latter
facts have since been fidly established by the
researches of Mr. Woodhouse, of Pensylvania,
M. Theodore de Saussure, and Mr. Palmer.
They are proved in a very satisfactory manner
by the following experiment of De Candolle.

Two glass jars were inverted over the same
water-bath ; the one filled with carbonic acid
gas, the other filled with water, containing a sprig
of mint ; the jars communicating below by means
of the water-bath, on the surface of which some
oil was poured, so as to intercept all communi-
cation between the water and the atmosphere.
The sprig of mint was exposed to the light of the
sun for twelve days consecutively : at the end
of each day the carbonic acid was seen to dimi-
nish in quantity, the water rising in the jar to
supply the place of what was lost, and at the
same time the plant exhaled a quantity of
oxygen exactly equal to that of the carbonic
acid which had disappeared. A similar sprig of
mint, placed in a jar of the same size, full of dis-
tilled water, but without having access to carbonic
acid, gave out no oxygen gas, and soon perished.
When, in another experiment, conducted by
means of the same apparatus as was used in the
first, oxygen gas was substituted in the first jar

AERATION OF THE SAP. 31

instead of carbonic acid gas, no gas was disen-
gaged in the other jar, which contained a sprig
of mint. It is evident, therefore, that the oxygen
gas obtained from the mint in the first experi-
ment was derived from the decomposition, by
the leaves of the mint, of the carbonic acid,
which the plant had absorbed from the water.

Solar light is an essential agent in effecting
this chemical change ; for it is never found to
take place at night, nor while the plant is kept
in the dark. The experiments of Sennebier
would tend to show that the violet, or most re-
frangible of the solar rays have the greatest
power in determining this decomposition of car-
bonic acid ; but the experiments are of so deli-
cate a nature, that this result requires to be con-
firmed by a more rigid investigation, before it
can be admitted as satisfactorily established.

That the carbon resulting from this decompo-
sition of carbonic acid is retained by the plant,
has been amply proved by the experiments of
M. Theodore de Saussure, who found that this
process is attended with a sensible increase in
the quantity of carbon which the plant had pre-
viously contained.

It is in the green substance of the leaves alone
that this process is conducted ; a process, which,
from the strong analogy that it bears to a similar
function in animals, may be considered as the
respiration of vegetables. The effect appears to

32 THE VITAL FUNCTIONS.

be proportionate to the number of stomata which
the plant contains. It is a process which takes
place only in a living plant ; for if a leaf be
bruised so as to destroy its organization, and
consequently its vitality, its substance is no longer
capable either of decomposing carbonic acid gas
under the influence of solar light, or of absorbing
oxygen in the dark. Neither the roots, nor the
flowers, nor any other parts of the plant, which
have not this green substance at their surface,
are capable of decomposing carbonic acid gas :
they produce, indeed, an effect which is in some
respects the opposite of this ; for they have a
tendency to absorb oxygen, and to convert it
into carbonic acid, by uniting it with the carbon
they themselves contain. This is also the case
with the leaves themselves, whenever they are
not under the influence of light : thus, during
the whole of the night, the same leaves, which
had been exhaling oxygen during the day, ab-
sorb a portion of that element. The oxygen
thus absorbed enters immediately into combina-
tion with the carbonaceous matter in the plant,
forming with it carbonic acid : this carbonic acid
is in part exhaled ; but the greater portion either
remains attached to the substance of the leaf, or
combines with the fluids which constitute the
sap : in the latter case, it is ready to be again
presented to the leaf, when daylight returns,
and when a fresh decomposition is again effected.

AERATION OF THE SAP. i^.'i

This reversal at night of what was done in the
day may, at first sight, appear to be at variance
with the unity of plan, which we should ex-
pect to find preserved in the vegetable economy ;
but a more attentive examination of the process
will show that the whole is in perfect harmony^
and that these contrary processes are both of
them necessary, in order to produce the result
intended.

The water which is absorbed by the roots
generally carries with it a certain quantity of
soluble animal or vegetable materials, which
contain carbon. This carbon is transmitted to
the leaves, where, during the night, it is made to
combine with the oxygen they have absorbed.
It is thus converted into carbonic acid, which,
when daylight prevails, is decomposed ; the
oxygen being dissipated, and the carbon retained.
It is evident that the object of the whole process
is to obtain carbon in that precise state of disin-
tegration, to which it is reduced at the moment
of its separation from carbonic acid by the action
of solar light on the green substance of the
leaves ; for it is in this state alone that it is avail-
able in promoting the nourishment of the plant,
and not in the crude condition in which it exists
when it is pumped up from the earth, along with
the water which conveys it into the interior of
the plant. Hence the necessity of its having to
undergo this double operation of first combining

Vol. II. D

34 THE VITAL FUNCTIONS.

with oxygen, and then being precipitated from
its combination in the manner above described.
It is not the whole of the carbon introduced into
the vegetable system, in the form of carbonic
acid, which has to undergo the first of these
changes, a part of that carbon being already in
the condition to which that operation would re-
duce it, and consequently in a state fit to receive
the decomposing action of the leaves. The whole
of these chemical changes may be included under
the general term Aeration.

Thus the great object to be answered by this
vegetable aeration is exactly the converse of
that which we shall afterwards see is effected by
the respiration of animals : in the former it is
that of adding carbon, in an assimilated state, to
the vegetable organization ; in the latter, it is
that of discharging the superfluous quantity of
carbon from the animal system. The absorption
of oxygen, and the partial disengagement of
carbonic acid, which constitute the nocturnal
changes effected by plants, must have a tendency
to deteriorate the atmosphere with respect to its
capability of supporting animal life ; but this
effect is much more than compensated by the
greater quantity of oxygen given out by the same
plants during the day. On the whole, therefore,
the atmosphere is continually receiving from the
vegetable kingdom a large accession of oxygen,
and is, at the same time, freed from an equal
portion of carbonic acid gas ; both of which

AERATION OF THE SAP. 35

effects tend to its purification and to its remaining
adapted to the respiration of animals. Nearly
the whole of the carbon accumulated by vege-
tables is so much taken from the atmosphere,
which is the primary source from which they
derive that element. At the season of the year
when vegetation is most active, the days are
longer than the nights ; so that the diurnal pro-
cess of purification goes on for a greater number
of hours than the nocturnal process by which the
air is vitiated.

The oxygen given out by plants, and the car-
bonic acid resulting from animal respiration, and
from the various processes of combustion, which
are going on in every part of the world, are
quickly spread through the atmosphere, not only
from the tendency of all gases to uniform diffu-
sion, but also from the action of the winds, which
are continually agitating the whole mass, and
promoting the thorough mingling of its different
portions, so as to render it perfectly homogeneous
in every region of the globe, and at every eleva-
tion above the surface.

Thus are the two great organized kingdoms of
the creation made to co-operate in the execution
of the same design : each ministering to the
other, and preserving that due balance in the
constitution of the atmosphere, which adapts it
to the welfare and activity of every order of
beings, and which would soon be destroyed,
were the operations of any one of them to be

.36 THE VITAL FUNCTIONS.

suspended. It is impossible to contemplate so
special an adjustment of opposite effects without
admiring this beautiful dispensation of Provi-
dence, extending over so vast a scale of being,
and demonstrating the unity of plan on which
the whole system of organized creation has been
devised.

§ 5. Return of the Sap.

The sap, which, during its ascent from the roots,
contains but a small proportion of nutritious par-
ticles, diluted with a large quantity of water,
after undergoing in the leaves, as in a chemical
laboratory, the double processes of exhalation
and aeration, has become much more highly
charged with nutriment ; and that nutriment has
been reduced to those particular forms and states
of composition which render it applicable to the
growth of the organs, and the other purposes of
vegetable life. This fluid, therefore, corresponds
to the blood of animals, which, like the elaborated
sap, may be regarded as fluid nutriment, per-
fectly assimilated to that particular kind of or-
ganization, with which it is to be afterwards in-
corporated. From the circumstance of its being
sent back from the leaves for distribution to the
several organs where its presence is required, it
has received the name of the returning sap, that
it might be distinguished from the crude fluid

RETURN OF THE SAP. 37

which arrives at the leaves, and which is termed
the ascending sap.

The returning sap still contains a considerable
quantity of water, in its simple liquid form; which
was necessary in order that it might still be the
vehicle of various nutritive materials that are
dissolved in it. It appears, however, that a large
proportion of the water, which was not ex-
haled by the leaves, has been actually decom-
posed, and that its separated elements, the oxygen
and the hydrogen, have been combined with
certain proportions of carbon, hydrogen, nitrogen,
and various earths, metals, and salts, so as to
form the proximate vegetable products, which
are found in the returning sap.

The simplest, and generally the most abundant
of these products, is that which is called Gum*
From the universal presence of this substance
in the vegetable juices, and more especially
in the returning sap, of all known plants, from
its bland and unirritating qualities, from its great
solubility in water, and from the facility with
which other vegetable products are convertible
into this product. Gum may be fairly assumed

* According to the investigations of Dr. Prout, 1000 grains
of gum are composed of 586 grains of the elements of water, that
is, of oxygen and hydrogen, in the exact proportions in which
they would have united to form 586 grains of water ; together
with 414 of carbon, or the base of carbonic acid. This, accord-
ing to the doctrine of chemical equivalents, corresponds to one
molecule of water, and one molecule of carbon. Phil. Trans,
for 1827, p. 584.

.18 THE VITAL FUNCTIONS.

to be the principal basis of vegetable nutriment ;
and its simple and definite composition points
it out as being the immediate result of the che-
mical changes which the sap experiences in the
leaves. During the descent of the sap, however,
this fluid undergoes, in various parts of the plant,
a further elaboration, which gives rise to other
products. We are now, therefore, to follow it in
its progress through the rest of the vegetable
system.

The returning sap descends from the leaves
through two different structures : in exogenous
plants the greater portion finds a ready passage
through the liber, or innermost layer of bark,
and another portion descends through the albur-
num, or outermost layer of the wood. With re-
gard to the exact channels through which it
passes, the same degree of uncertainty prevails
as with regard to those which transmit the as-
cending sap. De Candolle maintains that, in
either case, the fluids find their way through the
intercellular spaces : other physiologists, how-
ever, are of opinion, that particular vessels are
appropriated to the office of transmitting the des-
cending sap. The extreme minuteness of the
organs of vegetables has hitherto presented
insuperable obstacles to the investigation of this
important question ; and consequently our rea-
sonings respecting it can be founded only on
indirect evidence. The processes of the animal

RETURN OF THE SAP. 'W

economy, where the channels of distribution,
and the organs of propulsion are plainly obser-
vable, afford but imperfect analogies to guide us
in this intricate inquiry ; for although it is true
that in the higher classes of animals the circula-
tion of the nutrient fluid, or blood, through dis-
tinct vessels, is sufficiently obvious, yet in the
lower departments of the animal kingdom, and
in the embryo condition even of the more perfect
species, the nutritious juices are distributed with-
out being confined within any visible vessels ;
and they either permeate extensive cavities in
the interior of the body, or penetrate through the
interstices of a cellular tissue. That this latter is
the mode of transmission adopted in the vegetable
system has been considered probable, from the
circumstance that the nutritious juices are diffused
throughout those plants which contain no vessels
whatsoever with the same facility as through-
out those which possess vessels ; from which it
has been concluded that vessels are not absolutely
necessary for the performance of this function.
The nature of the forces which actuate the sap
in its descent from the leaves, and its distribu-
tion to different parts, is involved in equal ob-
scurity with the nature of the powers which
contribute to its motion upwards along the stem,
from the roots to the leaves. In endogenous
plants the passage of the sap in its descent, is,
in like manner, through those parts which have

40 THE VITAL FUNCTIONS.

been latest formed ; that is, through the inner-
most layers of their structure.

The returning sap, while traversing these se-
veral parts of the plant, deposits in each the par-
ticular materials which are requisite for their
growth, and for their maintenance in a healthy
condition. That portion which flows along the
liber, not meeting with any ascending stream of
fluid, descends without impediment to the roots,
to the extension of which, after it has nourished
the inner layer of bark, it particularly contri-
butes : that portion, on the other hand, which
descends along the alburnum, meets with the
stream of ascending sap, which, during the day
at least, is rising with considerable force. A
certain mixture of these fluids probably now
takes place, and new modifications are in con-
sequence produced, which, from the intricacy of
the chemical processes thus conducted in the
inner recesses of vegetable organization, we are
utterly baffled in our attempts to follow. All
that we are permitted to see are the general re-
sults, namely the gradual deposition of the mate-
rials of the future alburnum and liber. These
materials are first deposited in the form of a
layer of glutinous substance, termed the Cam-
bium; a substance which appears to consist of
the solid portion of the sap, precipitated from it
by the separation of the greater part of the water
that held it in solution. The cambium becomes
in process of time more and more consolidated,

RETURN OF THE SAP. 41

and acquires the organization proper to the plant
of which it now forms an integrant part : it con-
stitutes two layers ; the one, belonging to the
wood, being the alburnum ; the other, belonging
to the bark, being the liber.

The alburnum and the liber, which have been
thus constructed, perform an important part
in inducing ulterior changes on the nutrient
materials which the returning sap continues to
supply. Their cells absorb the gummy sub-
stance from the surrounding fluid, and by their
vital powers effect a still further elaboration in
its composition ; converting it either into starch,
or sugar, or lignin, according to the mode in
which its constituent elements are arranged.
Although these several principles possess very
different sensible properties, yet they are found
to differ but very slightly in the proportions of
their ingredients ; and we may infer that the
real chemical alterations, which are required in
order to effect these conversions, are compara-
tively slight, and may readily take place in the
simple cellular tissue.*

In the series of decompositions which are arti-

* According to the analyses of Dr. Prout, the following is the
composition of these substances : 1000 parts of

Pure Gum Arabic consist of 586 of oxygen and hydrogen,

united in the proportions in which they exist in water, and

414 of carbon.
Dried Starch or Fecula of 560 water, and 440 carbon.

Pure crystallized Sugar . . 572 428

Lignin from Boxwood . . . 500 ------ 500

42 THE VITAL FUNCTIONS.

ficially isfFected in the laboratory of the chemist,
it has been found that gum and sugar are inter-
mediate products, or states of transition between
various others ; and they appear to be peculiarly
calculated, from their great solubility, for being
easily conveyed from one organ to another.
Starch, and lignin, on the other hand, are com
pounds of a more permanent character, and
especially adapted for being retained in the
organs. Starch which, though solid, still pos-
sesses considerable solubility, is peculiarly fitted
for being applied to the purposes of nourish-
ment: it is accordingly hoarded in magazines,
with a view to future employment, being to
vegetables, what the fat is to animals, a resource
for exigencies which may subsequently arise.
With this intention, it is carefully stored in small
cells, the coats of which protect it from the im-
mediate dissolving action of the surrounding-
watery sap, but allow of the penetration of this
fluid, and of its solution, when required by the
demands of the system. The tuberous root of
the potatoe, that invaluable gift of Providence
to the human race, is a remarkable example of
a magazine of nutritive matter of this kind.

The lignin, on the contrary, is deposited with
the intention of forming a permanent part of the
vegetable structure, constituting the basis of the
woody fibre, and giving mechanical support and
strength to the whole fabric of the plant. These

RETURN OF THE SAP. 43

latter structures may be compared to the bones
of animals ; composing by their union the solid
frame work, or skeleton of the organized system.
The woody fibres do not seem to be capable of
further alteration in the living vegetable ; and
they are never, under any circumstances, taken
up and removed to other parts of the system, as
is the case with nutritive matter of a more con-
vertible kind.

The sap holds in solution, besides carbona-
ceous matter, some saline compounds, and a few
earthy and metallic bases ; bodies which, in how-
ever minute a quantity they may be present,
have unquestionably a powerful influence in
determining certain chemical changes among
the elements of organic products, and in im-
parting to them peculiar properties ; for it is now
a well ascertained fact that a scarcely sensible
portion of any one ingredient is capable of pro-
ducing important differences in the properties of
the whole compound. An example occurs in
the case of gold, the ductility of which is totally
destroyed by the presence of a quantity of either
antimony or lead, so minute as barely to amount
to the two thousandth part of the mass ; and even
the fumes of antimony, when in the neighbour-
hood of melted gold, have the power of destroy-
ing its ductility.* In the experiments made by

* Hatchett.

44 THE VITAL FUNCTIONS.

Sir John Herschel on some remarkable motions
excited in fluid conductors by the transmission
of electric currents, it was found that minute
portions of calcareous matter, in some instances
less than the millionth part of the whole com-
pound, are sufficient to communicate sensible
mechanical motions, and definite properties, to
the bodies with which they are mixed .*

As Silica is among the densest and least soluble
of the earths, we might naturally expect that
any quantity of it taken into the vegetable
system in a state of solution, would very early
be precipitated from the sap, after the exhala-
tion of the water which held it dissolved ; and it
is found, accordingly, that the greater portion of
this silica is actually deposited in the leaves,
and the parts adjacent to them. When once
deposited, it seems incapable of being again
taken up, and transferred to other parts, or
ejected from the system ; and hence, in course
of time, a considerable accumulation of silicious
particles takes place, and by clogging up the cells
and vessels of the plant, tends more and more to
obstruct the passage of nourishment into these
organs. This change has been assigned as a
principal cause of the decay and ultimate de-
struction of the leaves : their foot-stalks, more
especially suffering from this obstruction, perish,

* Philosophical Transactions for 1824, p. 162.

VEGETABLE SECRETION. 45

and occasion the detachment of the leaves,
which thus fall off at the end of each season,
making way for those that are to succeed them
in the next.

^ 6. Secretion in Vegetables.

While the powers of the simpler kinds of cells
are adequate to produce in the returning sap the
modifications above described, by which it is
converted into gummy, saccharine, amylaceous,
or ligneous products ; there are other cellular
organs, endowed with more extensive powers of
chemical action, which effect still greater changes.
The nature of the agents by which these changes
are produced are unknown, and are therefore
referred generally to the vital energies of vege-
tation ; but the process itself has been termed
Secretion; and the organs in which it is con-
ducted, and which are frequently very distin-
guishable as separate and peculiar structures, are
called Glands. When the products of secretion
are chemically analysed, the greater number are
found to contain a large quantity of hydrogen,
in addition to that which is retained in combi-
nation with oxygen as the representative of
water : this is the case with all the oily secre-
tions, whether they be fixed or volatile, and also
with those secretions which are of a resinous

46 THE VITAL FUNCTIONS.

quality. Some, on the contrary, are found to
have an excess of oxygen ; and this is the con-
dition of most of the acid secretions; while
others, again, appear to have acquired an addi-
tion of nitrogen.

All these substances have their respective uses,
although it may frequently be difficult to assign
them correctly. Some are intended to remain
permanently inclosed in the vesicles where they
were produced ; others are retained for the pur-
pose of being employed at some other time ; while
those belonging to a third class are destined to be
thrown off from the system, as being superfluous
or noxious : these latter substances, which are
presently to be noticed, are specially designated
as excretions. Many of these fluids find their
way from one part of the plant to another, with-
out appearing to be conducted along any definite
channels; and others are conveyed by vessels,
which appear to be specially appropriated to this
office.

The following are examples of the uses to
which the peculiar secretions of plants are ap-
plied. Many lichens, which fix themselves on
calcareous rocks, such as the Patellaria immersa,
are observed, in process of time, to sink deeper
and deeper beneath the surface of the rock, as if
they had some mode of penetrating into its sub-
stance, analogous to that which many marine
worms are known to possess. The agent appears

VEGETABLE SECRETIONS. 47

in both instances to be an acid, which here is
probably the oxalic, acting upon the carbonate
of lime, and producing the gradual excavation
of the rock. This view is confirmed by the ob-
servation that the same species of lichen, when
attached to rocks which are not calcareous, re-|
mains always at the surface, and does not pene-
trate below it.

A caustic liquor is sometimes collected in
vesicles, situated at the base of slender hairs,
having a canal which conducts the fluid to the
point. This is the case with the Nettle. The
slightest pressure made by the hand on the hairs
growing on the leaves of this plant, causes the
fluid in their vesicles to pass out from their
points, so as to be instilled into the skin, and
occasion the well known irritation which ensues.
M. De Candolle junior has ascertained by che-
mical tests that the stinging fluid of the nettle is
of an alkaline nature. In some species of this
genus of plants, the hairs are so large that the
whole mechanism above described is visible to
the naked eye. This apparatus bears a striking
resemblance to that which exists in the poisonous
teeth of serpents, and which is hereafter to be
described.

As the resinous secretions resist the action of
water, we find them often employed by nature
as a means of effectually defending the young
buds from the injurious effects of moisture ; and

48 THE VITAL FUNCTIONS.

for a similar purpose we find the surface of many
plants covered with a varnish of wax, which is
another secretion belonging to the same class :
thus the Ceroxylon, and the Iriartea have a thick
coating of wax, covering the whole of their stems.
Sometimes the plant is strewed over with a bluish
powder, possessing the same property of repelling
water : the leaves of the Mesembt^i/anthemum, or
Fig-marigold, of the Atriplex, or Orache, and of
the Srassica, or Cabbage, may be given as ex-
amples of this curious provision. Such plants,
if completely immersed in water, may be taken
out without being wetted in the slightest degree ;
thus presenting us with an analogy to the plu-
mage of the Cygnet, and other aquatic birds,
which are rendered completely water-proof by
an oily secretion spread over their surface.
Many aquatic plants, as the Satrachospermum,
are, in like manner, protected by a viscid layer,
which renders the leaves slippery to the touch,
and which is impermeable to water.

Several tribes of plants contain liquids which
are opaque, and of a white milky appearance ;
this is the case with the Poppy, the Fig-tree, the
Convolvulus, and a multitude of other genera ;
and a similar kind of juice, but of a yellow
colour, is met with in the Chelidonimn, or Celan-
dine. All these juices are of a resinous nature,
usually highly acrid, and even poisonous in
their qualities ; and their opacity is occasioned

CIRCULATION IN PLANTS. 49

by the presence of a great number of minute
globules, visible with the microscope. The vessels
in which these fluids are contained are of a pe-
culiar kind, and exhibit ramifications and junc-
tions, resembling those of the blood vessels of
animals. We may also discover, by the aid of
the microscope, that the fluids contained in these
vessels are moving in currents with considerable
rapidity, as appears from the visible motions of
their globules ; and they present, therefore, a re-
markable analogy with the circulation of the
blood in some of the inferior tribes of animals.
This curious phenomenon was first observed in
the Chelidonium by Schultz, in the year 1820 ;
and he designated it by the term Cyclosis, in
order to distinguish it from a real circulation^ if,
on further inquiry, it should be found not to be
entitled to the latter appellation.*

The circular movements, which have been
thus observed in the milky juices of plants, have
lately attracted much attention among botanists :
but considerable doubt still prevails whether these
appearances afford sufficient evidence of the
existence of a general circulation of nutrient
juices in the vegetable systems of those plants
which exhibit them ; for it would appear that, in
reality, the observed motions of the fluid are, in
every case, partial ; and the extent of the circuit

* " Die Natur der lebendigen Pflanze." See also Annales
des Sciences Naturelles, xxiii, 75.

VOL. II. E

50

THE VITAL FUNCTIONS.

very limited. The cause of these motions is not
yet known ; but probably they are ultimately
referable to a vital contraction of the vessels ; for
they cease the moment that the plant has re-
ceived an injury, and are more active in pro-
portion as the temperature of the atmosphere is
higher.

These phenomena are universally met with in
all plants that contain milky juices; but they
have also been observed in many plants, of which
the juices are nearly transparent, and contain
only a few floating globules, such as the Chara,
or stone- wort, the Caulinia fragilis, &c.,* where
the double currents are beautifully seen under
the microscope, performing a complete circulation

within the spaces of the
stem that lie between two
adjacent knots or joints ;
and where, by the pro-
per adjustment of the
object, it is easy to see
at one view both the
ascending and descend-
ing streams passing on
opposite sides of the
stem. Fig. 239 shows
this circulation in the
cells of theCaulinia fragilis very highlymagnified,

Amici, Annales des Sciences Naturelles, ii. p. 41,

CIRCULATION IN PLANTS. 51

the direction of the streams being indicated by
the arrows. Fig. 240 represents the circulation
in one of the jointed hairs, projecting from the
cuticle of the calyx of the jTradescantia vir-
ginica* in each cell of which the same circu-
latory motion of the fluids is perceptible.

§ 7. Excretion in Vegetables.

It had long been conjectured by De Candolle,
that the superfluous or noxious particles contained
in the returning sap are excreted or thrown out by
the roots. It is evident that if such a process takes
place, it will readily explain why plants render
the soil where they have long been cultivated
less suitable to their continuance in a vigorous
condition, than the soil in the same spot was origi-
nally ; and also why plants of a different species
are frequently found to flourish remarkably well
in the same situation where this apparent dete-
rioration of the soil has taken place. The truth
of this sagacious conjecture has been established
in a very satisfactory manner by the recent ex-
periments of M. Macaire.t The roots of the

* Fig. 239 is taken from Amici, and Fig. 240 from that given
by Mr. Slack, Trans. Soc. Arts, vol. xlix.

t An account of these experiments was first published in the
fifth volume of the " Memoires de la Societe de Physique et
d'Histoire Naturelle de Geneve," and repeated in the " Annales
des Sciences Naturelles," xxviii, 402.

I

52 THE VITAL FUNCTIONS.

Chondrilla muralis were carefully cleaned, and
immersed in filtered rain water : the water was
changed every two days, and the plant continued
to flourish, and put forth its blossoms : at the
end of eight days, the water had acquired a
yellow tinge, and indicated, both by the smell
and taste, the presence of a bitter narcotic sub-
stance, analogous to that of opium ; a result
which was farther confirmed by the application
of chemical tests, and by the reddish brown re-
siduum obtained from the water by evaporation.
M. Macaire ascertained that neither the roots
nor the stems of the same plants, when com-
pletely detached, and immersed in water, could
produce this effect, which he therefore concludes
is the result of an exudation from the roots, con-
tinually going on while the plant is in a state of
healthy vegetation . By comparative experi ments
on the quantity of matter thus excreted by the
roots of the French bean (Phaseolns vulgaris)
during the night and the day, he found it to be
much more considerable at night ; an effect
which it is natural to ascribe to the interruption
in the action of the leaves when they are deprived
of light, and when the corresponding absorption
by the roots is also suspended. This was con-
firmed by the result of some experiments he
made on the same plants by placing them, during
day time, in the dark ; under which circumstances,
the excretion from the roots was found to be

VKGETAIJLE EXCRETIONS. 5'}

immediately much augmented : but, even when
exposed to the light, there is always some exu-
dation, though in small quantity, going on from
the roots.

That plants are able to free themselves, by
means of this excretory process, from noxious
materials, which they may happen to have im-
bibed through the roots, was also proved by ano-
ther set of experiments on the Mercurialis annua^
the Senecio vulgaris^ and Brassica campestris, or
common cabbage. The roots of each specimen,
after being thoroughly washed and cleaned, were
separated into two bunches, one of which was
put into a diluted solution of acetate of lead, and
the other into pure water, contained in a sepa-
rate vessel. After some days, during which the
plants continued to vegetate tolerably well, the
water in the latter vessel being examined, was
found to contain a very perceptible quantity of
the acetate of lead. The experiment was varied
by first allowing the plant to remain with its
roots immersed in a similar solution, and then
removing it, (after careful washing, in order to
free the roots from any portion of the salt that
might have adhered to their surface,) into a
vessel with rain water ; after two days, distinct
traces of the acetate of lead were afforded by
the water. Similar experiments were made with
lime-water, and with a solution of common salt,
instead of the acetate of lead, and were attended

54 THE VITAL FUNCTIONS.

with the like results. De CandoUe has ascer-
tained, that certain maritime plants which yield
soda, and which flourish in situations very distant
from the coast, provided they occasionally re-
ceive breezes from the sea, communicate a saline
impregnation to the soil in their immediate vi-
cinity, derived from the salt which they doubt-
less had imbibed by the leaves.

Although the materials which are thus excreted
by the roots are noxious to the plant which rejects
them, and would consequently be injurious to
other individuals of the same species, it does not
therefore follow that they are incapable of sup-
plying salutary nourishment to other kinds of
plants : thus it has been observed that the Sali-
caria flourishes particularly in the vicinity of the
willow ; and the Orohanche, or broom-rape, in
that of hemp. This fact has also been established
experimentally by M. Macaire, who found that
the water in which certain plants had been kept
was noxious to other specimens of the same
species ; while, on the other hand, it produced a
more luxuriant vegetation in plants of a difterent
kind.

This fact is of great importance in the theory
of agriculture, since it perfectly explains the
advantage derived from a continued rotation of
different crops in the same field, in increasing
the productiveness of the soil. It also gives a
satisfactory explanation of the curious pheno-

VEGETABLE EXCRETIONS. Oi)

meuoii oi fairy rings, as they are called ; that is,
of circles of dark green grass, occurring in old
pastures : these Dr. WoUaston has traced to the
growth of successive generations of certain^Mwo*,
or mushrooms, spreading from a central point.*
The soil, which has once contributed to the sup-
port of these fungi, becomes exhausted or dete-
riorated with respect to the future crops of the
same species, and the plants, therefore, cease
to be produced on those spots ; the second year's
crop consequently appears in the space of a
small ring, surrounding the original centre of
vegetation ; and in every succeeding year, the
deficiency of nutriment on one side necessarily
causes the new roots to extend themselves solely
in the opposite direction, and occasions the circle
of fungi continually to proceed by annual en-
largement from the centre outwards. An ap-
pearance of luxuriance of the grass follows as a
natural consequence ; for the soil of an interior
circle will always be enriched and fertilized with
respect to the culture of grass, by the decayed
roots of fungi of the preceding years' growth.
It often happens, indeed, during the growth of
these fungi, that they so completely absorb all
nutriment from the soil beneath, that the her-
bage is for a time totally destroyed, giving rise
to the appearance of a ring bare of grass, sur-

* Phil. Trans, for 1807, p. 133.

66 fHK VITAL FUNCJ^IONS.

rounding the dark ring ; but after the fungi have
ceased to appear, the soil where they had grown
becomes darker, and the grass soon vegetates
again with peculiar vigour. When two adjacent
circles meet, and interfere with each other's pro-
gress, they not only do not cross each other, but
both circles are invariably obliterated between
the points of contact ; for the exhaustion occa-
sioned by each obstructs the progress of the
other, and both are starved. It would appear
that different species of fungi often require the
same kind of nutriment ; for, in cases of the in-
terference of a circle of mushrooms with another
of puff-balls, still the circles do not intersect one
another ; the exhaustion produced by the one
being equally detrimental to the growth of the
other, as if it had been occasioned by the pre-
vious vegetation of its own species.

The only final cause we can assign for the
series of phenomena constituting the nutritive
functions of vegetables is the formation of cer-
tain organic products calculated to supply suste-
nance to a higher order of beings. The animal
kingdom is altogether dependent for its support,
and even existence, on the vegetable world.
Plants appear formed to bring together a certain
number of elements derived from the mineral
kingdom, in order to subject them to the opera-
tions of vital chemistry, a power too subtle for
human science to detect, or for human art to

VEGETABLE EXCRETIONS. 57

imitate ; and by which these materials are com-
bined into a variety of nutritive substances. Of
these substances, so prepared, one portion is con-
sumed by the plants themselves in maintaining
their own structures, and in developing the em-
bryos of those which are to replace them; another
portion serves directly as food to various races of
animals ; and the remainder is either employed
in fertilizing the soil, and preparing it for subse-
quent and more extended vegetation, or else,
buried in the bosom of the earth, it forms part of
that vast magazine of combustible matter, des-
tined to benefit future communities of mankind,
when the arts of civilization shall have developed
the mighty energies of human power.

Chapter III.

ANIMAL NUTRITION IN GENERAL.

§ 1 . Food of Animals.

Nutrition constitutes no less important a part
of the animal, than of the vegetable economy.
Endowed with more energetic powers, and en-
joying a wider range of action, animals, com-
pared with plants, require a considerably larger
supply of nutritive materials for their sustenance,

58 THE VITAL FUNCTIONS.

and for the exercise of their various and higher
faculties. The materials of animal nutrition
must, in all cases, have previously been combined
in a peculiar mode ; which combination the
powers of organization alone can effect. In the
conversion of vegetable into animal matter, the
principal changes in chemical composition which
the former undergoes, are, first, the abstraction of
a certain proportion of carbon ; and secondly, the
addition of nitrogen.* Other changes, however,
less easily appreciable, though perhaps as im-
portant as the former, take place to a great
extent with regard to the proportions of saline,
earthy, and metallic ingredients ; all of which,
and more especially iron, exist in greater quantity
in animal than in vegetable bodies. The former
also contain a larger proportion of sulphur and
phosphorus than the latter.

The equitable mode in which nature dispenses
to her innumerable offspring the food she has
provided for their subsistence, apportioning to

* The recent researches of Messrs. Macaire and Marcet tend
to establish the important fact that both the chyle and the blood
of herbivorous and of carnivorous quadrupeds are identical in their
chemical composition, in as far, at least, as concerns their ulti-
mate analysis. They found, in particular, the same proportion
of nitrogen in the chyle, whatever kind of food the animal habi-
tually consumed ; and it was also the same in the blood, whether
of carnivorous or herbivorous animals ; although this last fluid
contains more nitrogen than the chyle. {Memoires de la Socicte
de Physique et d'Histoire Naturelle de Geneve, v. 389.)

ANIMAL NUTRITION. 59

each the quantity and the kind most consonant
to enlarged views of prospective beneficence, is
calculated to excite our highest wonder and
admiration. While the waste is the smallest
possible, we find that nothing which can afford
nutriment is wholly lost. There is no part of the
organized structure of an animal or vegetable,
however dense its texture, or acrid its qualities,
that may not, under certain circumstances, be-
come the food of some species of insect, or con-
tribute in some mode to the support of animal
life. The more succulent parts of plants, such
as the leaves, or softer stems, are the principal
sources of nourishment to the greater number of
larger quadrupeds, to multitudes of insects, as
well as to numerous tribes of other animals^
Some plants are more particularly designed as
the appropriate nutriment of particular species,
which would perish if these ceased to grow : thus
the silkworm subsists almost exclusively upon the
leaves of the mulberry tree ; and many species of
caterpillars are respectively attached to a parti*-
cular plant which they prefer to all others. There
are at least fifty different species of insects that
feed upon the common nettle ; and plants, of which
the juices are most acrid and poisonous to the
generality of animals, such as Euphorhiumy Hen-
bane ^ and Nightshade, afford a wholesome and
delicious food to others. Innumerable tribes of
animals subsist upon fruits and seeds; while others

00 THE VITAL FUNCTIONS.

feast upon the juices which they extract from
flowers, or other parts of plants ; and others,
again, derive their principal nourishment from
the hard fibres of the bark or wood.

Still more general is the consumption of animal
matter by various animals. Every class has
its carnivorous tribes, which consume living prey
of every denomination ; some being formed to
devour the flesh of the larger species, whether
quadrupeds, birds, or fish ; others feeding on
reptiles or mollusca, and some satisfying their
appetite with insects alone. The habits of the
more diminutive tribes are not less predatory
and voracious than those of the larger quad-
rupeds; for the spiders on the land, and the
Crustacea in the sea, are but representatives of
the lions and tigers of the forest, displaying an
equally ferocious and insatiable rapacity. Other
families, again, generally of still smaller size, are
designed for a parasitic existence ; their organs
being fitted only for imbibing the blood or juices
of other animals.

No sooner is the signal given, on the death of
any large animal, than multitudes of every class
hasten to the spot, eager to partake of the repast
which nature has prepared. If the carcass be
not rapidly devoured by rapacious birds, or car-
nivorous quadrupeds, it never fails to be soon
attacked by swarms of insects, which speedily
consume its softer textures, leaving only the

ECONOMY OF NUTRITIVE MATTER. Gl

bones.* These, again, are the favourite repast
of the Hyaena, whose powerful jaws are pecu-
liarly formed for grinding them into powder,
and whose stomach can extract from them an
abundant portion of nutriment. No less speedy
is the work of demolition among the inha-
bitants of the waters, where innumerable fishes,
Crustacea, annelida, and mollusca are on the
watch to devour all dead animal matter which
may come within their reach. The consumption
of decayed vegetables is not quite so speedily
accomplished ; yet these also afford an ample
store of nourishment to hosts of minuter beings,
less conspicuous, perhaps, but performing a no
less important part in the economy of the creation.
It may be observed that most of the insects which
feed on decomposing materials, whether animal
or vegetable, consume a much larger quantity
than they appear to require for the purposes of
nutrition. We may hence infer that in their
formation other ends were contemplated, besides

* So strongly was Linnaeus impressed with the immensity of
the scale on which these works of demolition by insects are car-
ried on in nature, that he used to maintain that the carcass of a
dead horse would not be devoured with the same celerity by a
lion, as it would be by three flesh flies (Musca vomitoria) and
their immediate progeny ; for it is known that one female fly will
give birth to at least 20,000 young larvee, each of which will, in
the course of a day, devour so much food, and grow so rapidly,
as to acquire an increase of two hundred times its weight ; and
a few days are sufficient for the production of a third generation.

62 THE VITAL FUNCTIONS.

their own individual existence. They seem as if
commissioned to act as the scavengers of organic
matter, destined to clear away all those particles,
of which the continued accumulation would have
tainted the atmosphere or the waters with infec-
tion, and spread a wide extent of desolation and
of death.

In taking these general surveys of the plans
adopted by nature for the universal subsistence
of the objects of her bounty, we cannot help ad-
miring how carefully she has provided the means
for turning to the best account every particle of
each product of organic life ; whether the material
be consumed as food by animals, or whether it
be bestowed upon the soil, reappearing in the
substance of some plant, and being in this way
made to contribute eventually to the same ulti-
mate object, namely, the support of animal life.

But we may carry these views still farther,
and following the ulterior destination of the
minuter and unheeded fragments of decomposed
organizations, which we might conceive had been
cast away, and lost to all useful purposes, we
may trace them as they are swept down by the
rains, and deposited in pools and lakes, amidst
waters collected from the soil on every side.
Here we find them, under favourable circum-
stances, again partaking of animation, and in-
vested with various forms of infusory animalcules,

ECONOMY OF NUTRITIVE MATTER. 63

which sport in countless myriads their ephemeral
existence within the ample regions of every drop.
Yet even these are still qualified to fulfil other
objects in a more distant and far wider sphere ;
for, borne along, in the course of time, by the
rivers into which they pass, they are at length
conveyed into the sea, the great receptacle of all
the particles that are detached from the objects
on land. Here also they float not uselessly in
the vast abyss; but contribute to maintain in
existence incalculable hosts of animal beings,
which people every portion of the wide expanse
of ocean, and which rise in regular gradation
from the microscopic monad, and scarcely visible
medusa,* through endless tribes of mollusca, and
of fishes, up to the huge Leviathan of the deep.
Even those portions of organic matter, which,
in the course of decomposition, escape in the form
of gases, and are widely diffused through the at-
mosphere, are not wholly lost for the uses of living
nature ; for, in course of time, they, also, as we
have seen, re-enter into the vegetable system,
resuming the solid form, and reappearing as
organic products, destined again to run through

* The immensity of the numbers of these microscopic medusae,
which people every region of the ocean, may be judged of from
the phenomenon of the phosphorescent light which is so fre-
quently exhibited by the sea, when agitated, and which, as I have
already observed, is found to arise from the presence of an incal-
culable multitude of these minute animals.

64 THE VITAL FUNCTIONS.

the same never ending cycle of vicissitudes and
transmutations.

The diffusion of animals over wide regions of
the globe is a consequence of the necessity which
prompts them to search for subsistence wherever
food is to be met with. Thus while the vegetation
of each different climate is regulated by the sea-
sons, herbivorous animals are in the winter forced
to migrate from the colder to the milder regions,
where they may find the pasturage they require ;
and these migrations occasion corresponding
movements among the predaceous tribes which
subsist upon them. Thus are continual inter-
changes produced, contributing to colonise the
earth, and extend its animal population over
every habitable district. But in all these changes
we may discern the ultimate relation they ever
bear to the condition of the vegetable world,
which is placed as an intermediate and necessary
link between the mineral and the animal king-
doms. All those regions, which are incapable of
supporting an extensive vegetation, are, on that
account, unfitted for the habitation of animals.
Such are the vast continents of ice, which spread
around the poles ; such are the immense tracts
of snow and of glaciers, which occupy the sum-
mits of the highest mountain chains ; and such
is the wide expanse of sand, which covers the
largest portions both of Africa and of Asia : and
often have we heard of the sunken spirits of the

INFLUENCE OF THE DEMAND FOR FOOD. 65

traveller through the weary desert, from the
appalling silence that reigns over those regions
of eternal desolation ; but no sooner is his eye
refreshed by the reappearance of vegetation,
than he again traces the footsteps and haunts
of animals, and welcomes the cheering sound
of sensitive beings.

The kind of food which nature has assigned
to each particular race of animals has an impor-
tant influence, not merely on its internal organ-
ization, but also on its active powers and dispo-
sition ; for the faculties of animals, as well as
their structure, have a close relation to the cir-
cumstances connected with their subsistence,
such as the abundance of its supply, the facility
of procuring it, the dangers incurred in its search,
and the opposition to be overcome before it can
be obtained. In those animals whose food lies
generally within their reach, the active powers
acquire but little developement : such, for in-
stance, is the condition of herbivorous quad-
rupeds, whose repast is spread every where in
rich profusion beneath their feet ; and it is the
chief business of their lives to crop the flowery
mead, and repose on the same spot which affords
them the means of support. Predaceous animals,
on the contrary, being prompted by the calls of
appetite to wage warwith living beings, are formed
for a more active and martial career ; their mus-
cles are more vigorous, their bones are stronger,

VOL. II. F

66 THE VITAL FUNCTIONS.

their limbs more robust, their senses more deli-
cate and acute. What sight can compare with
that of the eagle and the lynx ; what scent can
be more exquisite than that of the wolf and the
jackall ? All the perceptions of carnivorous ani-
mals are more accurate, their sagacity embraces
a greater variety of objects, and in feats of
strength and agility they far surpass the herbi-
vorous tribes. A tiger will take a spring of fif-
teen or twenty feet, and seizing upon a buffalo,
will carry it with ease on its back through a
dense and tangled thicket: with a single blow
of its paw it will break the back of a bull, or
tear open the flanks of an elephant.

While herbivorous animals are almost con-
stantly employed in eating, carnivorous animals
are able to endure abstinence for a great length
of time, without any apparent diminution of their
strength : a horse or an ox would sink under
the exhaustion consequent upon fasting for two
or three days, whereas the wolf and tlie martin
have been known to live fifteen days without
food, and a single meal will suffice them for a
whole week. The calls of hunger produce on
each of these classes of animals the most opposite
effects. Herbivorous animals are rendered weak
and faint by the want of food, but the tiger is
roused to the full energy of his powers by the
cravings of appetite; his strength and courage
are never so great as when he is nearly famished,

INFLUENCE OF THE DEMAND FOR FOOD. 67

and he rushes to the attack, reckless of conse-
quences, and undismayed by the number or
force of his opponents. From the time he has
tasted blood, no education can soften the native
ferocity of his disposition : he is neither to be
reclaimed by kindness, nor subdued by the fear
of punishment. On the other hand, the elephant,
subsisting upon the vegetable productions of the
forest, superior in size and even in strength to
the tiger, and armed with as powerful weapons
of offence, which it wants not the courage to
employ when necessary, is capable of being
tamed with the greatest ease, is readily brought
to submit to the authority of man, and requites
with affection the benefits he receives.

On first contemplating this extensive destruc-
tion of animal life by modes the most cruel and
revolting to all our feelings, we naturally recoil
with horror from the sanguinary scene ; and
cannot refrain from asking how all this is consis-
tent with the wisdom and benevolence so conspi-
cuously manifested in all other parts of the crea-
tion. The best theologians have been obliged
to confess that a difficulty does here exist,* and
that the only plausible solution which it admits
of, is to consider the pain and suffering thus
created, as one of the necessary consequences of
those general laws which secure, on the whole,

* See, in particular, Paley's Natural Theology, chap. xxvi.

68 THE VITAL FUNCTIONS.

the greatest and most permanent good. There
can be no doubt that the scheme, by which one
animal is made directly conducive to the sub-
sistence of another, leads to the extension of
the benefits of existence to an infinitely greater
number of beings than could otherwise have en-
joyed them. This system, besides, is the spring
of motion and activity in every part of nature.
While the pursuit of its prey forms the occupa-
tion, and constitutes the pleasure of a considerable
part of the animal creation, the employment of
the means they possess of defence, of flight, and
of precaution is also the business of a still larger
part. These means are, in a great proportion of
instances, successful ; for wherever nature has
inspired sagacity in the perception of danger, she
has generally bestowed a proportionate degree of
ingenuity in devising the means of safety. Some
are taught to deceive the enemy, and to employ
stratagem where force or swiftness would have
been unavailing : many insects, when in danger,
counterfeit death to avoid destruction ; others,
among the myriapoda, fold themselves into the
smallest possible compass, so as to escape detec-
tion. The tortoise, as we have already seen,
retreats within its shell, as within a fortress ; the
hedge-hog rolls itself into a ball, presenting
bristles on every side ; the diodon inflates its
globular body for the same purpose, and floats
on the sea, armed at all the points of its surface ;

SERIES OF VITAL FUNCTIONS. 69

the cuttle-fish screens itself from pursuit by effu-
sing an intensely dark coloured ink, which renders
the surrounding waters so black and turbid as to
conceal the animal, and favour its escape ; the
torpedo defends itself from molestation by reite-
rated discharges from its electric battery ; the
butterfly avoids capture by its irregular move-
ments in the air, and the hare puts the hounds
at fault by her mazy doublings. Thus does
the animated creation present a busy scene
of activity and employment : thus are a variety
of powers called forth, and an infinite diversity
of pleasures derived from their exercise ; and
existence is on the whole rendered the source of
incomparably higher degrees, as well as of a larger
amount of enjoyment, than appears to have been
compatible with any other imaginable system.

§ 2. Series of Vital Functions.

In the animal economy, as in the vegetable, the
vital, or nutritive functions are divisible into seven
kinds, namely, Assimilation, Circulation, Respi-
ration, Secretion, Excretion, Absorption, and
Nutrition ; some of which even admit of further
subdivision. This is the case more particularly
with the processes of assimilation, which are
generally numerous, and require a very compli-
cated apparatus for acting on the food in all the

70 THE VITAL FUNCTIONS.

Stages of its conversion into blood ; a fluid which,
like the returning sap of plants, consists of nutri-
ment in its completely assimilated state. It will
be necessary, therefore, to enter into a more par-
ticular examination of the objects of these diflfe-
rent processes.

In the more perfect structures belonging to
the higher orders of animals, contrivances must
be adopted, and organs provided for seizing the
appropriate food, and conveying it to the mouth.
A mechanical apparatus must there be placed
for effecting that minute subdivision, which is
necessary to prepare it for the action of the che-
mical agents to which it is afterwards to be sub-
jected. From the mouth, after it has been
sufficiently masticated, and softened by fluid
secretions prepared by neighbouring glands, the
food must be conveyed into an interior cavity,
called the Stomach, where, as in a chemical
laboratory, it is made to undergo the particular
change which results from the operation termed
Digestion. The digested food must thence be
conducted into other chambers, composing the
intestinal tube, where it is converted into Chyle,
which is a milky fluid, consisting wholly of
nutritious matter. Vessels are then provided,
which, like the roots of plants, drink up this
prepared fluid, and convey it to other cavities,
capable of imparting to it a powerful impulsive
force, and of distributing it through appropriate

RECEPTACLES OF FOOD. 71

channels of circulation, not only to the respi-
ratory organs, where its elaboration is completed
by the influence of atmospheric air, but also to
all other parts of the system, where such a supply
is required for their maintenance in the living
state. The objects of these subsequent functions,
many of which are peculiar to animal life, have
already been detailed.*

This subdivision of the assimilatory processes
occurs only in the higher classes of animals ; for
in proportion as we descend in the scale, we
find them more and more simplified, by the con-
centration of organs, and the union of many offices
in a single organ, till we arrive, in the very lowest
orders, at little more than a simple digestive
cavity, performing at once the functions of the
stomach and of the heart ; without any distinct
circulation of nutrient juices, without vessels, —
nay without any apparent blood. Long after
all the other organs, such as the skeleton, whe-
ther internal or external, the muscular and ner-
vous systems, the glands, vessels, and organs of
sense, have one after another disappeared, we
still continue to find the digestive cavity retained,
as if it constituted the most important, and only
indispensable organ of the whole system.

The possession of a stomach, then, is the pecu-
liar characteristic of the animal system, as con-

* See the first chapter of this volume, p. 11.

72 THE VITAL FUNCTIONS.

trasted with that of vegetables. It is a distinctive
criterion that applies even to the lowest orders
of zoophytes, which, in other respects, are so
nearly allied to plants. It extends to all insects,
however diminutive ; and even to the minutest
of the microscopic animalcules.*

The mode in which the food is received into
the body is, in general, very different in the two
organized kingdoms of nature. Plants receive
their nourishment by a slow, but nearly constant
supply, and have no receptacle for collecting it
at its immediate entry; the sap, as we have
seen, passing at once into the cellular tissue of
the plant, where the process of its gradual elabo-
ration is commenced. Animals, on the other
hand, are capable of receiving at once large
supplies of food, in consequence of having an in-
ternal cavity, adapted for the immediate recep-
tion of a considerable quantity. A vegetable
may be said to belong to the spot from which it
imbibes its nourishment; and the surrounding
soil, into which its absorbing roots are spread
on every side, may almost be considered as a

part of its system. But an animal has all its

#

* In some species of animals belonging to the tribe of Medusas,
as the Eudora, Berenice, Oryihia, Favonia, Lymnoria, and
Geryonia, no central cavity corresponding to a stomach has been
discovered : they appear, therefore, to constitute an exception to
the general rule. See Peron, Annales de Museum, xiv, 227 and
326.

INFLUENCE OF THE DEMAND FOR FOOD. 73

organs of assimilation within itself; and having
receptacles in which it can lay in a store of
provisions, it may be said to be nourished from
within ; for it is from these interior receptacles
that the lacteals, or absorbing vessels, corres-
ponding in their office to the roots of vege-
tables, imbibe nourishment. Important conse-
quences flow from this plan of structure ; for since
animals are thus enabled to subsist for a certain
interval without needing any fresh supply, they
are independent of local situation, and may enjoy
the privilege of moving from place to place.
Such a power of locomotion was, indeed, abso-
lutely necessary to beings which have their sub-
sistence to seek. It is this necessity, again,
that calls for the continued exercise of their
senses, intelligence, and more active energies ;
and that leads, in a word, to the possession of all
those higher powers, which raise them so far
above the level of the vegetable creation.

74

Chapter IV.

Nutrition in the lower Orders of Animals.

The animals which belong to the order of Polypi
present us with the simplest of all possible forms
of nutritive organs. The Hydra, for instance^
which may be taken as the type of this formation,
consists of a mere stomach, provided with the
simplest instruments for catching food, — and no-
thing more. A simple sac, or tube, adapted to
receive and digest food, is the only visible organ
of its body. It exhibits not a trace of either
brain, nerves, or organs of sense, nor any part
corresponding to lungs, heart, or even vessels
of any sort ; all these organs, so essential to
the maintenance of life in other animals, being
here dispensed with. In the magnified view of
the hydra, exhibited in Fig. 241,
the cavity into which the food is
received and digested is laid open
by a longitudinal section, so as
to show the comparative thick-
ness of the walls of this cavity.
The structure of these walls must
be adapted, not only to prepare
and pour out the fluids by which the food is
digested, but also to allow of the transudation

NUTRITION IN POLYPI. 75

through its substance, probably by means of in-
visible pores, of the nutritious particles thus ex-
tracted from the food, for the purpose of its being
incorporated and identified with the gelatinous
pulp, of which the body appears wholly to consist.
The thinness and transparency of the walls of
this cavity allow of our distinctly following these
changes by the aid of the microscope. Trembley
watched them with unwearied perseverance for
days together, and has given the following ac-
count of his observations. The hydra, though it
does not pursue the animals on which it feeds,
yet devours with avidity all kinds of living prey
that come within the reach of its tentacula, and
which it can overcome, and introduce into its
mouth. The larvae of insects, naides, and other
aquatic worms, minute Crustacea, and even small
fishes, are indiscriminately laid hold of, if they
happen but to touch any part of the long fila-
ments which the animal spreads out, in different
directions, like a net, in search of food. The
struggles of the captive, which finds itself en-
tangled in the folds of these tentacula, are gene-
rally ineiFectual ; and the hydra, like the boa
constrictor, contrives, by enormously expanding
its mouth, slowly to draw into its cavity ani-
mals much larger than its own body. Worms
longer than itself are easily swallowed by being
previously doubled together by the tentacula.
Fig. 242 shows a hydra in the act of devouring

76 THE VITAL FUNCTIONS.

the vermiform larva of a Tipula, which it has
encircled with its tentacula, to which it has

applied its expanded mouth, and of which it is
absorbing the juice, before swallowing it. Fig.
243 shows the same animal after it has suc-
ceeded, though not without a severe contest, in
swallowing a minnow, or other small fish, the
form of which is still visible through the trans-
parent sides of the body, which are stretched to
the utmost. It occasionally happens, when two
of these animals have both seized the same object
by its different ends, that a struggle between
them ensues, and that the stronger, having ob-
tained the victory, swallows at a single gulp, not
only the object of contention, but its antagonist
also. This scene is represented in Fig. 244,
where the tail of the hydra, of which the body
has been swallowed by the victor, is seen pro-
truding from the mouth of the latter. It soon,
however, extricates itself from this situation,

NUTRITION IN POLYPI. 77

apparently without having suffered the smallest
injury. The voracity of the hydra is very great,
especially after long fasting ; and it will then
devour a great number of insects, one after ano-
ther, at one meal, gorging itself till it can hold
no more ; its body becoming dilated to an extra-
ordinary size ; and yet the same animal can
continue to live for more than four months with-
out any visible supply of food.

On attentively observing the changes induced
upon the food by the action of the stomach of
these animals, they appear to consist of a gradual
melting down of the softer parts, which are re-
solved into a kind of jelly ; leaving unaltered only
a few fragments of the harder and less digestible
parts. These changes are accompanied by a
kind of undulation of the contents of the stomach,
backwards and forwards, throughout the whole
tube ; apparently produced by the contraction
and dilatation of its different portions. The un-
digested materials being collected together and
rejected by the mouth, the I'emaining fluid is
seen to contain opaque globules of various sizes,
some of which are observed to penetrate through
the sides of the stomach, and enter into the gra-
nular structure which composes the flesh of the
animal. Some portion of this opaque fluid is
distributed to the tentacula ; into the tubular
cavities of which it may be seen entering by
passages of communication with the stomach.

78 THE VITAL FUNCTIONS.

By watching attentively the motions of the glo-
bules, it will be perceived that they pass back-
wards and forwards through these passages, like
ebbing and flowing tides.

All these phenomena may be observed with
greater distinctness when the food of the animal
contains colouring matter, capable of giving a
tinge to the nutritious fluid, and allowing of its
progress being traced into the granules which are
dispersed throughout the substance of the body.
Trembley is of opinion that these granules are
vesicular, and that they assume the colour they
are observed to have, from their becoming filled
with the coloured particles contained in the nou-
rishment. The granules which are nearest to
the cavity of the stomach are those which are
first tinged, and which therefore first imbibe the
nutritious juices : the others are coloured succes-
sively, in an order determined by their distance
from the surface of the stomach. Trembley
ascertained that a living hydra introduced into
the stomach of another hydra, was not in any
degree acted upon by the fluid secretions of that
organ, but came out uninjured. It often happens
that a hydra in its eagerness to transfer its victim
into its stomach, swallows several of its own ten-
tacula, which had encircled it ; but these tenta-
cula always ultimately come out of the stomach,
sometimes after having remained there twenty-
four hours, without the least detriment.

NUTRITION IN POLYPI. 79

The researches of Trembley have brought to
Ught the extraordinary fact that not only the
internal surface of the stomach of the polypus is
endowed with the power of digesting food, but
that the same property belongs also to the ex-
ternal surface, or what we might call the skin of
the animal. He found that by a dexterous ma-
nipulation, the hydra may be completely turned
inside out, like the finger of a glove ; and that
the animal, after having undergone this singular
operation, will very soon resume all its ordinary
functions, just as if nothing had happened. It
accommodates itself in the course of a day or
two to the transformation, and resumes all its
natural habits, eagerly seizing animalcules with
its tentacula, and introducing them into its Dewly
formed stomach, which has for its interior sur-
face what before was the exterior skin, and
which digests them with perfect ease. When the
discovery of this curious phenomenon was first
made known to the world, it excited great asto-
nishment, and many naturalists were incredulous
as to the correctness of the observations. But
the researches of Bonnet and of Spallanzani,
who repeated the experiments of Trembley,
have borne ample testimony to their accuracy,
which those of every subsequent observer have
farther contributed to confirm.

The experiments of Trembley have also proved
that every portion of the hydra possesses a won-

80 THE VITAL FUNCTIONS.

derful power of repairing all sorts of injuries,
and of restoring parts which have been removed.
These animals are found to bear with impunity
all sorts of mutilations. If the tentacula be cut
off, they grow again in a very short time : the
whole of the fore part of the body is, in like
manner, reproduced, if the animal be cut asun-
der ; and from the head which has been removed
there soon sprouts forth a new tail. If the head
of the hydra be divided by a longitudinal section,
extending only half way down the body, the cut
portions will unite at their edges, so as to form
two heads, each having its separate mouth, and
set of tentacula. If it be split into six or seven
parts, it will become a monster with six or seven
heads ; if each of these be again divided, ano-
ther will be formed with double that number.
If any of the parts of this compoimd polypus be
cut off, as many new ones will spring up to re-
place them ; the mutilated heads at the same
time acquiring fresh bodies, and becoming as
many entire polypi. Fig. 245 represents a hydra
with seven heads, the result of several operations
of this kind. The hydra will sometimes of its
own accord split into two ; each division be-
coming independent of the other, and growing
to the same size as the original hydra. Trembley
found that different portions of one polype might
be engrafted on another, by cutting their sur-
faces, and pressing them together ; for by this

NUTRITION IN POLYPI. 81

means they quickly unite, and become a com-
pound animal. When the body of one hydra is
introduced into the mouth of another, so that
their heads are kept in contact for a sufficient
length of time, they unite and form but one in-
dividual. A number of heads and bodies may
thus be joined together artificially, so as to com-
pose living monsters more complicated than the
wildest fancy has conceived.

Still more complicated are the forms and eco-
nomy of those many-headed monsters, which
prolific nature has spread in countless multitudes
over the rocky shores of the ocean in every part
of the globe. These aggregated polypi grow, in
imitation of plants, from a common stem, with
widely extended flowering branches. Myriads
of mouths open upon the surface of the animated
mass; each mouth being surrounded with one
or more circular rows of tentacula, which are
extended to catch their prey : but as the station-
ary condition of these polypes prevents them
from moving in search of food, their tentacula
are generally furnished with a multitude of cilia,
which, by their incessant vibrations, determine
currents of water to flow towards the mouth,
carrying with them the floating animalcules on
which the entire polypus subsists.

Each mouth leads into a separate stomach ;
whence the food, after its digestion, passes into
several channels, generally five in number, which

VOL. II. G

82 THE VITAL FUNCTIONS.

proceed in different directions from the cavity of
each stomach, dividing into many branches, and
being distributed over all the surrounding portions
of the flesh. These branches communicate with
similar channels proceeding from the neigh-
bouring stomachs : so that the food, which has
been taken in by one of the mouths, contributes
to the general nourishment of the whole mass of
aggregated polypi. Cuvier discovered this struc-
ture in theVeretilla,which belongs to this order of
polypi : he also found it in the Pennatula, and it
is probably similar in all the others. Fig. 246
represents three of the polypes of the Veretilla,
with their communicating vessels seen below.
The prevailing opinion among naturalists is, that
each polypus is an individual animal, associated
with the rest in a sort of republic, where the
labours of all are exerted for the common benefit
of the whole society. But it is perhaps more con-
sonant with our ideas of the nature of vitality to
consider the extent of the distribution of nutritive
fluid in any organic system as the criterion of
the individuality of that system, a view which
would lead us to consider the entire polypus, or
mass composed of numerous polypes, as a single
individual animal ; for there is no more incon-
sistency in supposing that an individual animal
may possess any number of mouths, than that it
may be provided with a multitude of distinct
stomachs, as we shall presently find is actually
exemplified in many of the lower animals.

NUTRITION IN THE ENTOZOA.

83

Some of the Entozoa, or parasitic worms, ex-
hibit a general difFiision, or circulation of nou-
rishment through numerous channels of commu-

nication, into which certain absorbing vessels
convey it from a great number of external ori-
fices, or mouths, as they may be called. This is
the case with the TcBnia, or tape worm, which is
composed of a series of flat jointed portions, of
which two contiguous segments are seen, highly
magnified, in Fig. 247, exhibiting round the
margin of each portion, a circle of vessels (v),
which communicate with those of the adjoining
segments ; each circle being provided with a
tube (o), having external openings for imbibing
nourishment from the surrounding fluids. Al-
though each segment is thus provided with a
nutritive apparatus complete within itself, and
so far, therefore, independent of the rest, the
individuality of the whole animal is sufficiently
determined by its having a distinct head at one

84 THE VITAL FUNCTIONS.

extremity, provided with instruments for its at-
tachment to the surfaces it inhabits.

The Hydatid (Fig. 248) is another parasitic
vrorm of the simplest possible construction. It
has a head (o), (of which h is a magnified repre-
sentation,) furnished with four suckers, and a
tubular neck, which terminates in a globular
sac. When this sac, which is the stomach, is
fully distended with fluid, its sides are stretched,
so as to be reduced to a very thin transparent
membrane, having a perfectly spherical shape :
after this globe has become swollen to a very
large size, the neck yields to the distension, and
disappears ; and the head can then be distin-
guished only as a small point on the surface of
the globular sac. It is impossible to conceive a
more simple organic structure than this, which
may, in fact, be considered as an isolated living
stomach. The Ccenurus, which is found in the
brain of sheep, has a structure a little more com-
plicated ; for instead of a single head, there are
a great number spread over the surface, opening
into the same general cavity ; and when the sac
is distended, appearing only as opaque spots on
its surface.

The structure of the Sponge has been already
fully described ; and the course of the minute
channels pointed out, in which a kind of circu-
lation of sea water is carried on for the nourish-
ment of the animal. The mode by which nutri-

NUTRITION IN MEDUSA. 85

ment is extracted from this circulating fluid, and
made to contribute to the growth of these plant-
like structures, is entirely unknown.

The apparatus for nutrition possessed by
animals belonging to the tribe of Medus(B is of
a peculiar kind. I have already described the
more ordinary form of these singular animals,
which resemble a mushroom, from the hemi-
spherical form of their bodies, and their central
foot-stalk, or pedicle. In the greater number
of species there exists, at the extremity of this
pedicle, a single aperture, which is the begin-
ning of a tube leading into a large central cavity
in the interior of the body, and which may there-
fore be regarded as the mouth of the animal ;
but in those species which have no pedicle, as
the JEquorea, the mouth is situated at the centre
of the under surface. The aperture is of suffi-
cient width to admit of the entrance of prey of
considerable size, as appears from the circum-
stance that fishes, of some inches in length, are
occasionally found entire in the stomachs of those
medusae which have a single mouth. The central
cavity, which is the stomach of the animal, does
not appear to possess any proper coats, but to
be simply scooped out of the soft structure of
the body. Its form varies in different species;
having generally, however, more or less of a
star-like shape, composed of four curved rays,
which might almost be considered as consti-

$d T|IE VITAL FUNCTIONS.

tuting four stomachs, joined at a common centre.
Such, indeed, is the actual structure in the
Medusa aurita, in which Gaede found the
stomach to consist of four spherical sacs, com-
pletely separated by partitions. These arched
cavities, or sacs, taper as they radiate towards
the circumference, and are continued into a
canal, from which a great number of other
canals proceed; generally at first by successive
bifurcations of the larger trunks, but afterwards
branching off more irregularly, and again uniting
by lateral communications, so as to compose a
complicated net-work of vessels. These rami-
fications at length unite to form an annular
vessel, which encircles the margin of the disk.
It appears also, from the observations of Gaede,
that a further communication is established
between this latter vessel and others, which
permeate the slender filaments, or tentacula, that
hang like a fringe all round the edge of the
disk, and which, in the living animal, are in
perpetual motion. It is supposed that the elon-
gations and contractions of these filaments are
effected by the injection or recession of the
fluids contained in those vessels.* Here, then,
we see not only a more complex stomach, but
also the commencement of a vascular system,
taking its rise from that cavity, and calculated

* Journal de Physique, Ixxxix, 146.

NUTRITION IN MEDUSiE. 87

to distribute the nutritious juices to every part
of the organization.

There are other species of Medusae, com-
posing the genus Rhizostoma of Cuvier, which,
instead of having only one mouth, are provided
with a great number of tubes which serve that
office, and which bear a great analogy to the
roots of a plant.* The pedicle terminates below
in a great number of fringed processes, which,
on examination, are found to contain ramified
tubes, with orifices opening at the extremity of
each process. In this singular tribe of animals
there is properly no mouth or central orifice ;
the only avenues to the stomach being these
elongated canals, which collect food from every
quarter where they extend, and which, uniting
into larger and larger trunks as they proceed
towards the body, form one central tube, or
oesophagus, terminating in the general cavity
of the stomach. The Medusa pulmo, of which
a figure was given in Vol. i., page 192, belongs
to this modern genus, and is now termed the
JRhizostoma Cuvieri.

The course of these absorbent vessels is most
conveniently traced after they have been filled
with a dark coloured liquid. The appearances
they present in the Rhizostoma Cuvieri, after

* It is from this circumstance that the genus has received the
name it now bears, and which is derived from two Greek words,
signifying root-like mouths.

88

THE VITAL FUNCTIONS.

being thus injected, are represented in the
annexed figures ; the first of which (Fig. 249),
shows the under surface of that animal, after the
pedicle has been removed by a horizontal section,
at its origin from the hemispherical body, or

cupola, as it niay be termed, where it has a
square prismatic form, so that its section presents
the square surface, Q, Q. Fig. 252 is a vertical
section of the same specimen ; both figures being
reduced to about one-half of the natural size.
The dotted line, h, h, in the latter figure, shows
the plane where the section of the pedicle was
made in order to give the view of the base of the
hemisphere presented in Fig. 249. On the
other hand, the dotted line v, v, in Fig. 249, is
that along which the vertical section of the same

NUTRITION IN MEDUSjE. 89

animal, represented in Fig. 252, was made ; four
of the arms (a, a, a, a), descending from the
pedicle, being left attached to it. In these arms,

or tentacula, may be seen the canals, (marked
by the dark lines, c, c, c), which arise from
numerous orifices in the extremities and fringed
surface of the tentacula, and which, gradually
uniting, like the roots of a plant, converge
towards the centre of the pedicle, and terminate
by a common tube, which may be considered
as the oesophagus (o), in one large central cavity,

90 THE VITAL FUNCTIONS.

or stomach (s), situated in the upper part of the
cupola. The section of this oesophagus is visible
at the centre of Fig. 249, where its cavity has
the form of a cross. The stomach has a quad-
rangular shape, as in the ordinary medusae ; and
from each of its four corners there proceed
vessels, which are continuous with its cavity,
and are distributed by endless ramifications over
the substance of the cupola, extending even to
the fringed margin all round its circumference.
The mode of their distribution, and their nume-
rous communications by lateral vessels, forming
a complete vascular net-work, is seen in Fig. 251,
which represents, on a larger scale, a portion of
the marginal part of the disk. The two large
figures (249 and 252) also show the four lateral
cavities (r, r, Fig. 252), which are contiguous
to the stomach, but separated from it by mem-
branous partitions : these cavities have by some
been supposed to perform an office in the system
of the Medusa corresponding to respiration ; an
opinion, however, which is founded rather on
analogy than on any direct experimental evi-
dence. The entrances into these cavities are
seen open at e, in Fig. 249, and at e, e, in the
section Fig. 252. A transverse section of one of
the arms is given in Fig. 253, showing the form
of the absorbent tube in the centre ; and a similar
section of the extremity of one of the tentacula
is seen in Fig. 254, in which, besides the central

NUTRITION IN MEDVSM. 9t

tube, the cavities of some of the smaller branches
(b, b), which are proceeding to join it, are also
visible.

The regular gradation which nature has ob-
served in the complexity of the digestive cavities
and other organs, of the various species of this
extensive tribe, is exceedingly remarkable : for
while some, as the Eudora, have, to all appear-
ance, no internal cavity corresponding to a sto-
mach, and are totally unprovided with either
pedicle, arms, or tentacula ; others, furnished
with these latter appendages, are equally desti-
tute of such a cavity ; and those belonging to
a third family possess a kind of pouch, or false
stomach, at the upper part of the pedicle, appa-
rently formed by the mere folding in of the
integument. This is the case with the Geronia^
depicted in Fig. 250, whose structure, in this
respect, approaches that of the Hydra, already
described, where the stomach consists of an
open sac, apparently formed by the integuments
alone. Thence a regular progression may be
followed, through various species, in which the
aperture of this pouch is more and more com-
pletely closed, and where the tube which enters
it branches out into ramifications more or less
numerous, as we have seen in the Rhizostoma.*
It is difficult to conceive in what mode nutrition

* See Peron, Annales du Museum, xiv. 330.

92 THE VITAL FUNCTIONS.

is performed in the agastric tribes, or those
destitute of any visible stomach ; unless we sup-
pose that their nourishment is imbibed by direct
absorption from the surface.

Ever since the discovery of the animalcula of
infusions, naturalists have been extremely de-
sirous of ascertaining the nature of the organi-
zation of these curious beings ; but as no mode
presented itself of dissecting objects of such
extreme minuteness, it was only from the ex-
ternal appearances they present under the
microscope, that any inferences could be drawn
with regard to the existence and form of their
internal organs. In most of the larger species,
the opaque globules, seen in various parts of the
interior, were generally supposed to be either
the ova, or the future young, lodged within the
body of the parent. In the Rotifer, or wheel
animalcule of Spallanzani,* a large central
organ is plainly perceptible, which was by some
imagined to be the heart ; but which has been
clearly ascertained by Bonnet to be a receptacle
for food. Muller, and several other observers,
have witnessed the larger animalcules devouring
the smaller ; and the inference was obvious that,
in common with all other animals, they also
must possess a stomach. But as no such struc-
ture had been rendered visible in the smallest
species of infusoria, such as monads, it was

• Vol. i. p. 62, Fig. 1.

NUTRITION IN THE INFUSORIA. 93

too hastily concluded that these species were
formed upon a different and a simpler model.
Lamarck characterized them as being throughout
of a liomogeneous substance, destitute of mouth
and digestive cavity, and nourished simply by
means of the absorption of particles through the
external surface of their bodies.

The nature and functions of these singular
beings long remained involved in an obscurity,
which appeared to be impenetrable ; but at
length a new light has been thrown on the
subject by Professor Ehrenberg, whose re-
searches have recently disclosed fresh scenes of
interest and of wonder in microscopic worlds,
peopled with hosts of animated beings, almost
infinite in number as in minuteness.* In en-
deavouring to render the digestive organs of the
infusoria more conspicuous, he hit upon the for-
tunate expedient of supplying them with coloured
food, which might communicate its tinge to the
cavities into which it passed, and exhibit their

* The results of Ehrenberg's labours were first communicated
to the Berlin Academy; they have since been published in two
works in German : the first of which appeared at Berlin in
1830, under the title of ^^Organisation, Systematik und Geo-
graphisches Verhdltniss der Infusionsthierchen." The second
work appeared in 1832, and is entitled " Zur Erkenntniss der
Organisation in der Richtung des kleinsten Raumes." Both are
in folio, with plates. An able analysis of the contents of the
former of these works, by Dr. Gairdner, is given in The Edin-
burgh New Philosophical Journal for 1831, p. 201, of which 1
have availed myself largely in the account which follows.

94 THE VITAL FUNCTIONS.

situation and course. Obvious as this method
may appear, it was not till after a labour of ten
years that Ehrenberg succeeded in discovering
the fittest substances, and in applying them in
the manner best suited to exhibit the pheno-
mena satisfactorily. We have already seen that
Trembley had adopted the same plan for the
elucidation of the structure of the hydra.
Gleichen also had made similar attempts with
regard to the infusoria ; but, in consequence of
his having employed metallic or earthy colour-
ing materials, which acted as poisons, instead of
those which might serve as food, he failed in his
endeavours. Equally unsuccessful were the trials
made by Ehrenberg with the indigo and gum-lac
of commerce, which are always contaminated
with a certain quantity of white lead, a sub-
stance highly deleterious to all animals ; but, at
length, by employing an indigo which was quite
pure, he succeeded perfectly.* The moment a
minute particle of a highly attenuated solution
of this substance is applied to a drop of m ater
in which are some pedunculated Vorticellae, oc-
cupying the field of the microscope, the most

* The colouring matters proper for these experiments are such
as do not chemically combine with water, but yet are capable of
being diffused in a state of very minute division. Indigo, sap
green, and carmine, answer these conditions, and being also
easily recognised under the microscope, are well adapted for
these observations. Great care should be taken, however, that
the substance employed is free from all admixture of lead, or
other metallic impurity.

NUTRITION IN THE INFUSORIA. 9»5

beautiful phenomena present themselves to the
eye. Currents are excited in all directions by
the vibrations of the cilia, situated round the
mouths of these animalcules, and are readily dis-
tinguished by the motions of the minute particles
of indigo which are carried along with them ;
the currents generally all converging towards
the orifice of the mouth. Presently the body
of the vorticella, which had been hitherto quite
transparent, becomes dotted with a number of
distinctly circular spots, of a dark blue colour,
evidently produced by particles of indigo accu-
mulated in those situations. In some species,
particularly those which have a contracted part,
or neck, between the head and the body, as the
Rotifer vulgaris, these particles may be traced in a
continuous line in their progress from the mouth,
through the neck, into the internal cavities.

In this way, by the employment of colouring
matters, Ehrenberg succeeded in ascertaining
the existence of a system of digestive cavities
in all the known genera of this tribe of animals.
There is now, therefore, no reason for admitting
that cuticular absorption of nutritive matter ever
takes place among this order of beings. Whole
generations of these transparent gelatinous ani-
malcules may remain immersed for weeks in an
indigo solution, without presenting any coloured
points in their tissue, except the circumscribed
cavities above described.

Great variety is found to exist in the forms,

96

THE VITAL FUNCTIONS.

situations, and arrangement of the organs of
digestion in the Infusoria. They differ also in
their degree of complication ; but without any
obvious relation to the magnitude of the ani-
malcule. The Monas atomus, the minutest of
the whole tribe, exhibits a number of sacs,
opening by as many separate orifices, from a
circumscribed part of the surface. In others, as
in the Leucophi^a patiila, of which Fig. 255
represents the appearance under the micro-

scope, there is a long alimentary canal, tra-
versing the greater part of the body, taking
several spiral turns, and furnished with a great
number of c(Bca; a term which denotes blind
pouches, proceeding laterally from any in-
ternal canal, and having no other outlet.
These cavities become filled with coloured
particles, immediately after their entrance
into the alimentary canal ; and must there-
fore be considered as so many stomachs

NUTRITION IN THE INFUSORIA. 97

provided for the digestion of the food which they
receive.* But they are not all filled at the
same time ; for some continue long in a con-
tracted state, so as not to be visible ; while, at
another time, they readily admit the coloured
food. It is, therefore, only by dint of patient
watching that the whole extent of the alimentary
tube, and its apparatus of stomachs, can be
fully made out. Fig. 2e55, above referred to,
exhibits the Leiicophra patula of Ehrenberg,'|"
with a few of its stomachs filled with the opaque
particles ; but Fig. 256 shows the whole series of
organs, as they would appear if they could be
taken out of the body, and placed in the same rela-
tive situation with the eye of the observer as they
are in the first figure. In some species, from
one to two hundred of these sacs may be
counted, connected with the intestinal tube.
Many of the larger species, as the Hydatina
senta, exhibit a greater concentration of organs,
having only a single oval cavity of considerable
size, situated in the fore part of the body. In
the Rotifer vulgaris, the alimentary canal is a
slender tube, considerably dilated near its termi-
nation. In some Vorticellce, the intestine, from
which proceed numerous caeca, makes a complete
circular turn, ending close to its commencement :

^ Ehrenberg terms these Polygastric infvsoria, from the
Greek, signifying with many stomachs.
I Trichoda patula. Muller.
VOL. II. H

98 THE VITAL FUNCTIONS.

Ehrenberg forms of these the tribe of Cycloccela,
of which the Vorticella citrina, and the Stentor
jiolymorphus, are examples. Thus do we dis-
cover the same diversity in the structure of the
digestive organs of the several races of these
diminutive beings, as is found in the other classes
of animals.

The Ilydatina setita, one of the largest of the
Infusoria, was found by Ehrenberg to possess a
highly developed structure with respect to many
systems of organs, which we should never have
expected to meet with in animals situated so low
in the scale. As connected with the nutritive
functions, it may here be mentioned that the
head of this animalcule is provided with a regular
apparatus for mastication, consisting of serrated
jaws ; each having from two to six teeth. These
jaws are seen actively opening and shutting when
the animal is taking its food, which consists of
particles brought within reach of the mouth by
means of currents excited by the motions of the
cilia.

Such are the simple forms assumed by the
organs of assimilation among the lowest orders
of the animal creation ; namely, digesting cavities,
whence proceed various canals, which form a
system for the transmission of the prepared nou-
rishment to different parts ; but all these cavities
and canals being simply hollowed out of the
solid substance of the body. As we ascend a

NUTRITION IN THE ACTINIA. 99

step higher in the scale, we find that the stomach
and intestinal tube, together with their appen-
dages, are distinct organs, formed by membranes
and coats proper to each ; and that they are
themselves contained in an outer cavity, which
surrounds them, and which receives and collects
the nutritious juices after their elaboration in
these organs. The Actinia, or Sea Anemone, for
example, resembles a polypus in its general
form, having a mouth, which is surrounded with
tentacula, and which leads into a capacious
stomach, or sac, open below, and occupying

the greater part of the
bulk of the animal ; but
while, in the Polypus,
the sides of the stomach
constitute also those of
the body, the whole
being one simple sac ; in
the Actinia, spaces inter-
vene between the coats
of the stomach, and the skin of the animal. As
the stomach is not a closed sac, but is open below,
these cavities are, in fact, continuous with that
of the stomach : they are divided by numerous
membranous partitions passing vertically between
the skin, and the membrane of the stomach, and
giving support to that organ. Fig. 257, repre-
senting a vertical section of the Actinia coriacea,
displays this internal structure, b is the base

100

THE VITAL FUNCTIONS.

or disk, by which the animal adheres to rocks :
I is the section of the coriaceous integument,
showing its thickness : m is the central aperture
of the upper surface, which performs the office
of a mouth, leading to the stomach (s), of which
the lower orifice is open, and which is suspended
in the general cavity by means of vertical par-
titions, of which the cut edges are seen below^
uniting at a central point (c), and passing between
the stomach and the integument. These mus-
cular partitions are connected above with three
rows of tentacula, of which the points are seen
at T. The ovaries (o) are seen attached to the
partition ; and the apertures in the lower part of
the stomach, by which they communicate with
its cavity, may also be perceived.

If we considered the Medusa as having four sto-
machs, we might in like manner regard the Aste-
rias, or star-fish, as having ten, or even a greater
number. The mouth of this radiated animal is

at the centre of the under surface ; it leads into
a capacious bag, situated immediately above it,

NUTRITION IN THE ASTERIAS. 101

and which is properly the stomach. From this
central sac there proceed ten prolongations, or
canals, which occupy in pairs the centre of
each ray, or division of the body, and subdivide
into numerous minute ramifications. These
canals, with their branches, are exhibited at c, c,
Fig. 258, which represents one of the rays of the
Asterias, laid open from the upper side. The
canals are supported in their positions by mem-
branes, connecting them with the sides of the
cavity in which they are suspended.

In the various species of Echini, we find that
the alimentary tube has attained a more perfect
developement ; for instead of constituting merely
a blind pouch, it passes entirely through the body
of the animal. We here find an oesophagus, or
narrow tube, leading from the mouth to the sto-
mach ; and the stomach is continued into a
regular intestine, which takes two turns in the
cavity of the body, before it terminates.

The alimentary tube in the lower animals fre-
quently exhibits dilatations in different parts :
these, if situated in the beginning of the canal,
may be considered as a succession of stomachs ;
while those that occur in the advanced portions
are more properly denominated the great intes-
tine, by way of distinction from the middle por-
tions of the tube, wdiich are generally narrower,
and are termed the small intestine. We often
see blind pouches, or c(Eca, projecting from dif-^

102 THE VITAL FUNCTIONS.

ferent parts of the canal ; this is the case with the
intestine of the Aphrodita aculeata, or sea-mouse.
The intestine, being generally longer than the
body, is obliged to be folded many times within
the cavity it occupies, and to take a winding
course. In some cases, on the other hand, the
alimentary tube passes in nearly a straight line
through the body, with scarcely any variation in
its diameter : this is the case with the Ascaris,
which is a long cylindric worm ; and nearly so
with the Lufiibricus terrestris, or earth-worm. In

the Nais, on the contrary,
as shown in Fig. 259, the
alimentary tube presents a
series of dilatations, which,
from the transparency of
the skin, may be easily
seen in the living animal. The food taken in
by these worms is observed to be transferred
from the one to the other of its numerous sto-
machs, backwards and forwards many times,
before its digestion is accomplished.

The stomach of the Leech is very peculiar in
its structure : its form, when dissected off, and
removed from the body, is shown in Fig. 260.
It is of great capacity, occupying the larger part
of the interior of the body ; and its cavity is
expanded, by folds of its internal membrane,
into several pouches (c, c, c). Mr. Newport, who
has lately examined its structure with great care,

NUTRITION IN THE ANNELIDA.

103

261

260

262

finds that each of the ten portions into which it
is divided sends out, on the part most remote
from tlie oesophagus (o), two lateral pouches, or
caeca ; which, as they are traced along the
canal, become both wider and
longer, so that the tenth pair
of caeca (a) extends to the
hinder extremity of the animal;
the intestine (i), which is very
short, lying between them.*
It has long been known, that
if, after the leech has fas-
tened on the skin, a portion
of the tail be cut off, the ani-
mal will continue to suck
blood for an indefinite time :
this arises from the circum-
stance that the caecal portions
of the stomach are laid open,
so that the blood received into
that cavity flows out as fast
as it is swallowed.
A structure very similar to that of the leech is

* This figure was engraved from a drawing made, at my re-
quest, by Mr. Newport, from a specimen which he dissected,
and which he was so obliging as to show me. Fig. 261 repre-
sents the mouth, within which are seen the three teeth; and
Fig. 262, one of the teeth detached. A paper, descriptive of
the structure of the stomach of the leech, by Mr. Newport, was
lately read at a meeting of the Royal Society. See the Abstracts
of the proceedings of the Society, for June, 1833.

104

THE VITAL FUNCTIONS.

263

264

met with in the digestive organs of the GlossO'
pora tuberc?ilata, (Hirudo complanata, Linn.) of

which Fig. 263 represents
a magnified view from the
upper side. When seen
from the under side, as
is shown in Fig. 264, the
cavity of the stomach
is distinctly seen, pro-
longed into several cells,
divided by partitions, and
directed towards the tail. The two last of these
cells (c c) are much longer than the rest, and
terminate in two blind sacs, between which is
situated a tortuous intestinal tube.*

Chapter V.

Nutrition in the higher orders of Animals.

In proportion as we rise in the animal scale, we
find that the operations of Nutrition become
still farther multiplied, and that the organs which
perform them are more numerous, and more com-

* In both these figures, t is the tubular tongue, projected
from the mouth. In Fig. 263, e are the six eyes, situated on
the extremity which corresponds to the head ; and a double lon-
gitudinal row of white tubercles is also visible, extending along
the back of the animal, e, in Fig. 264, is the entrance into a
cavity, or pouch, provided for the reception of the young. See
Johnson, Phil. Trans, for 1817, p. 343.

COMPLEX APPARATUS FOR NUTRITION. 105

plicated in their structure. The long series of
processes requisite for the perfect elaboration of
nutriment, is divided into different stages ; each
process is the work of a separate apparatus, and
requires the influence of different agents. We
no longer find that extreme simplicity which we
noticed as so remarkable in the Hydra and the
Medusa, where the same cavity performs at once
the functions of the stomach and of the heart.
The manufacture of nutriment, if we may so
express it, is, in these lower zoophytes, con*
ducted upon a small scale, by less refined
methods, and with the strictest economy of
means : the apparatus is the simplest, the
agents the fewest possible, and many different
operations are carried on in one and the same
place.

As we follow the extension of the plan in more
elevated stages of organic developement, we find
a further division of labour introduced. Of this
we have already seen the commencement in the
multiplication of the digesting cavities of the
Leech and other Annelida ; but, in animals
which occupy a still higher rank, we observe
a more complete separation of offices, and a still
greater complication of organs ; the principle of
the division of labour being carried to a much
greater extent than in the inferior departments
of the animal creation. Besides the stomach, or
receptacle for the unassimilated food, another
organ, the heart, is provided for the uniform dis-

106

THE VITAL FUNCTIONS.

tribution of the nutritious fluids elaborated by
the organs of digestion. This separation of
functions, again, leads to the introduction of
another system of canals or vessels, for trans-
mitting the fluids from the organs which prepare
them to the heart, as into a general reservoir.
In the higher orders of the animal kingdom,
all these processes are again subdivided and
varied, according to the species of food, the
habits, and mode of life, assigned by nature to
each individual species. For the purpose of
conveying clearer notions of the arrangement of
this extensive system of vital organs, I have
drawn the annexed plan (Fig. 265), which ex-

hibits them in their natural order of connexion,
and as they might be supposed to appear in a
side view of the interior of a quadruped. To

COMPLEX APPARATUS FOR NUTRITION. 107

this diagram I shall make frequent reference in
the following description of this system.

The food is, in the first place, prepared for
digestion by several mechanical operations, which
loosen its texture and destroy its cohesion.
It is torn asunder and broken down by the
action of the jaws and of the teeth ; and it
is, at the same time, softened by an admixture
with the fluid secretions of the mouth. It is
then collected into a mass, by the action of the
muscles of the cheek and tongue, and swallowed
by the regulated contractions of the different
parts of the throat. It now passes along a mus-
cular tube, called the CEsophagus, (represented
in the diagram by the letter o,) into the stomach
(s), of which the entrance (c) is called the
cardia.

In the stomach the food is made to undergo
various chemical changes ; after which it is con-
ducted through the aperture, termed the pylorus
(p), into the canal of the intestine (i i), where it
is farther subjected to the action of several fluid
secretions, derived from large glandular organs
situated in the neighbourhood, as the Liver (l)
and the Pancreas ; and elaborated into the fluid
which is termed Chyle.

The Chyle is taken up by a particular set of
vessels, called the Lacteals, which transmit it to
the heart (h). These vessels are exceedingly
numerous, and arise by open orifices from the

108 THE VITAL FUNCTIONS.

inner surface of the intestines, whence they
absorb, or drink up the chyle. They may be
compared to internal roots, which unite as they
ascend along the mesentery (m), or membrane
connecting the intestines with the back ; forming
larger and larger trunks, till they terminate in
an intermediate reservoir (u), which has been
named the Receptacle of the Chyle. From this
receptacle there proceeds a tube, which, from its
passing through the thorax, is called the Tho-
racic duct (t) : it ascends along the side of the
spine, which protects it from compression, and
opens, at v, into the large veins which are pour-
ing their contents into the auricle^ or first cavity
of the heart (u) ; whence it immediately passes
into the ventricle^ or second cavity of that
organ (h). Such, in the more perfect animals,
is the circuitous and guarded route, which every
particle of nourishment must take before it can
be added to the general mass of circulating
fluid.

By its admixture with the blood already con-
tained in these vessels, and its purification by
the action of the air in the respiratory organs (b),
the chyle becomes assimilated, and is distri-
buted by the heart through appropriate chan-
nels of circulation called arteries (of which the
common trunk, or Aorta, is seen at a), to every
part of the system ; thence returning by the veins
(v, V, V,) to the heart. The various modes in

COMPLEX APPARATUS FOR NUTRITION. 109

which these functions are conducted in the seve-
ral tribes of animals will be described hereafter.
It will be sufficient for our present purpose to
state, by way of completing the outline of this
class of functions, that, like the returning sap
of plants, the blood is made to undergo further
modifications in the minute vessels through
which it circulates : new arrangements of its
elements take place during its passage through
the subtle organization of the glands, which no
microscope has yet unravelled : new products
are here formed, and new properties acquired,
adapted to the respective purposes which they
are to serve in the animal economy. The whole
is one vast Laboratory, where mechanism is sub-
servient to Chemistry, where Chemistry is the
agent of the higher powers of Vitality, and where
these powers themselves minister to the more
exalted faculties of Sensation and of Intellect.

The digestive functions of animals, however
complex and varied, and however exquisitely
contrived to answer their particular objects, yet
afford less favourable opportunities of tracing
distinctly the adaptation of means to the re-
spective ends, than the mechanical functions.
This arises from the circumstance that the pro-
cesses they effect imply a refined chemistry,
of which we have as yet but a very imperfect
knowledge ; and that we are also ignorant of the
nature of the vital agents concerned in pro-

110 THE VITAL FUNCTIONS.

ducing each of the chemical changes which the
food must necessarily undergo during its assimi-
lation. We only know that all these changes
are slowly and gradually effected ; the materials
having to pass through a great number of inter-
mediate stages before they can attain their final
state of elaboration.

Hence, whenever we can ascertain the degrees
of difference existing between the chemical con-
dition of the substance taken into the body, and
that of the product derived from it, we are fur-
nished with a kind of scale whereby we may
estimate the length of the process required, and
the amount of power necessary for its conversion
into that product. It is obvious, for example,
that the chemical changes which vegetable food
must be made to undergo, in order to assimilate
it to blood, must be considerably greater than
those required to convert animal food into the
same fluid ; because the latter is itself derived,
with only slight modification, immediately from
the blood. We accordingly find it to be an esta-
blished rule, that the digestive organs of animals
which feed on vegetable materials are remark-
able for their size, their length, and their com-
plication, when compared with those of car-
nivorous animals of the same class. This rule
applies, indeed, universally to Mammalia, Birds,
Reptiles, Fishes, and also to Insects ; and below
these we can scarcely draw the comparison,

COMPLEX ArPARATUS FOR NUTRITION. Ill

because nearly all the inferior tribes subsist
wholly upon animal substances. Many of these
latter animals have organs capable of extracting
nourishment from substances, which we should
hardly imagine contained any sensible portion
of it. Thus, on examining the stomach of the
earth-worm, we find it always filled with moist
earth, which is devoured in large quantities, for
the sake of the minute portion of vegetable and
animal materials that happen to be intermixed
with the soil ; and this slender nutriment is suf-
ficient for the subsistence of that animal. Many
marine worms, in like manner, feed apparently
on sand alone ; but that sand is generally in-
termixed with fragments of shells, which have
been pulverized by the continual rolling of the
tide and the surge ; and the animal matter con-
tained in these fragments, affords them a supply
of nutriment adequate to their wants. It is evi-
dent, that when, as in the preceding instances,
large quantities of indigestible materials are
taken in along with such as are nutritious, the
stomach and other digestive cavities must be
rendered more than usually capacious. It is
obvious also that the structure of the digestive
organs must bear a relation to the mechanical
texture, as well as to the chemical qualities of the
food ; and this we find to be the case in a variety
of instances, which will hereafter be specified.
The activity of the digestive functions, and the

112 THE VITAL FUNCTIONS.

structure of the organs, will also be regulated by
a great variety of other circumstances in the
condition of the animal, independent of the me-
chanical or chemical nature of the food. The
greater the energy with which the more pecu-
liarly animal functions of sensation and muscular
action are exercised, the greater must be the
demand for nourishment, in order to supply the
expenditure of vital force created by these exer-
tions. Compared with the torpid and sluggish
reptile, the active and vivacious bird or quadruped
requires and consumes a much larger quantity
of nutriment. The tortoise, the turtle, the toad,
the frog, and the chamelion, will, indeed, live
for months without taking any food. Fishes,
which, like reptiles, are cold-blooded animals,
although at all times exceedingly voracious when
supplied with food, can yet endure long fasts
with impunity.

The rapidity of developement has also great
influence on the quantity of food which an ani-
mal requires. Thus the caterpillar, which grows
very quickly, and must repeatedly throw off its
integuments, during its continuance in the larva
state, consumes a vast quantity of food compared
with the size of its body ; and hence we find it
provided with a digestive apparatus of consi-
derable size.

113

Chapter VI.

PREPARATION OF FOOD.

^ 1 . Prehension of Liquid Food.

In studying the series of processes which con-
stitute assimilation, our attention is first to be
directed to the mode in which the food is in-
troduced into the body, and to the mechanical
changes it is made to undergo before it is sub-
jected to the chemical action of the digestive
organs. The nature of these preliminary pro-
cesses will, of course, vary according to the tex-
ture and mechanical condition of the food. Where
it is already in a fluid state, mastication is unne-
cessary, and the receiving organs consist simply
of an apparatus for suction. This is the case
very generally with the Entozoa, which subsist
upon the juices of other animals, and which are all
provided with one or more sucking orifices, often
extended in the form of a tube or proboscis.*
The Hydatid, for instance, has four sucking
apertures disposed round the head of the animal :

* Some species of FasciolcB, or flukes, are furnished with two,
three, six, or more sucking disks, by which they adhere to sur-
faces : to these animals the names Distoma, Tristoma, Hexas-
toma, and Polystorna have been given ; but these denominations,
implying a plurality of mouths, are evidently incorrect, since the

VOL. II. I

114 THE VITAL FUNCTIONS.

the Tcenia has orifices of this kind in each of its
jointed segments: the Ascaris and the Earth-
worm have each a simple mouth. The margin
of the mouth is often divided, so as to compose
lips ; of these there are generally two, and in
the Leech there are three. In some rare cases,
as in the Plmiaria, there is, besides the ordinary
mouth, a tube also provided for suction, in a dif-
ferent part of the body, but leading into the
same stomach.*

When the instrument for suction extends for
some length from the mouth, it is generally termed
^proboscis : such is the apparatus of the butterfly,
the moth, the gnat, the house fly, and other
insects that subsist on fluid aliment. The pro-
boscis of the Lepidoptera, (Fig. '266), is a double
tube, constructed by the two
edges being rolled longitudi-
nally till they meet in the
middle of the lower surface ;
thus forming a tube on each
side, but leaving also another
tube, intermediate to the two
lateral ones. This middle
tube is formed by the junction

sucking disks are not perforated, and do not perform the office
of mouths ; and the true mouth for the reception of food is single.
Cuvier discovered an animal of this class furnished with above a
hundred of these cup-shaped sucking organs. See Edinburgh
Philos. Journal, xx. 101.

* Phil. Trans, for 1822, 442.

PREHENSION OF LIQUID FOOD. 1 15

of two grooves, which, by the aid of a curious
apparatus of hooks, resembling those of the la-
minae of a feather already described,* lock into
each other, and can be either united into an air
tight canal, or be instantly separated at the
pleasure of the animal. Reaumur conceives that
the lateral tubes are intended for the reception
of air, while the central canal conveys the
honey, which the insect sucks from flowers, by
suddenly unrolling the spiral coil, into which
the proboscis is usually folded, and darting it
into the nectary.-f

In the Hemiptera, the proboscis is a tube,
either straight or jointed, guarded by a sheath,
and acting like a pump. The Diptera have a
more complicated instrument for suction, con-
sisting of a tube, of which the sides are strong
and fleshy, and moveable in every direction,
like the trunk of an elephant : it has, at its ex-
tremity, a double fold, resembling lips, which
are well adapted for suction. The Gnat, and
other insects which pierce the skin of animals,
have, for this purpose, instruments termed, from
their shape and office, la?icets.\ In the gnat they
are five or six in number, finer than a hair, ex-
ceedingly sharp., and generally barbed on one
side : in the Tabamis, or horse-fly, they are 'flat,

* Volume i. page 570.

t Kirby and Spence's Entomology, vol. ii. p. 390.

X Ibid, vol. iii. p. 467.

116 TH^ VITAL FUNCTIONS.

like the blade of a knife. These instruments
are sometimes constructed so as to form, by their
union, a tube adapted for suction. In the flesh-
fly, the proboscis is folded like the letter Z ; the
upper angle pointing to the breast, and the lower
one to the mouth : in other flies there is a single
fold only.

Those insects of the order Hymenoptera,
which, like the Bee, suck the honey of flowers,
have, together with regular jaws, a proboscis
formed by the prolongation of the lower lip,
which is folded so as to constitute a tube : this
tube is protected by the mandibles, and is pro-
jected forwards by being carried on a pedicle,
which can be folded back when the tube is not
in use. The mouths of the Acephalous Mollusca
are merely sucking apertures, with folds like
lips, and without either jaws, tongue, or teeth ;
but having often tentacula arising from their
margins.

Among fishes, we meet with the family of
Cyclostomata, so called from their having a cir-
cular mouth, formed for suction. The margin
of this mouth is supported by a ring of cartilage,
and is furnished with appropriate muscles for
producing adhesion to the surfaces to which it is
applied ; the mechanism and mode of its attach-
ment being similar to that of the leech. To this
family belong the Myxine and the Lamprey.
So great is the force of adhesion exerted by this

PREHENSION OF LIQUID FOOD. 117

sucking apparatus, that a lamprey has been
raised out of the water with a stone, weighing
ten or twelve pounds, adhering to its mouth.

Humming birds have a long and slender
tongue, which can assume the tubular form, like
that of the butterfly or the bee, and for a similar
purpose, namely, sucking the juices of flowers.
Among the mammalia, the Vampire Bat affords
another instance of suction by means of the
tongue, which is folded into a tubular shape
for that purpose. But suction among the mam-
malia is generally performed by the muscles of
the lips and cheeks, aided by the movements
of the tongue, which, when withdrawn to the
back of the cavity, acts like the piston of a
pump. In the Lamprey, this hydraulic action
of the tongue is particularly remarkable. Many
quadrupeds, however, drink by repeatedly dip-
ping their tongue into the fluid, and quickly
drawing it into the mouth.

§ 2. Prehension of Solid Food.

When the food consists of solid substances,
organs must be provided ; first, for their pre-
hension and introduction into the mouth ; se-
condly, for their detention when so introduced ;
and thirdly, for their mechanical division into
smaller fragments.

118 THE VITAL FUNCTIONS.

Of those instruments of prehension which are
not portions of the mouth itself, and which form
a series of variously constructed organs, extend-
ing from the tentacula of the polypus to the
proboscis of the elephant, and to the human
arm and hand, some account has already been
given in the history of the mechanical functions ;
butj in a great number of instances, prehension
is performed by the mouth, or the parts which
are extended from it, and may be considered as
its appendices. The prehensile power of the
mouth is derived principally from the mecha-
nical form and action of the jaws, which open to
receive, and close to detain the bodies intended
as food; and to this latter purpose, the teeth,
when the mouth is furnished with them, likewise
materially contribute ; although their primary
and more usual office is the mechanical division
of the food by means of mastication, an action
in which the jaws, in their turn, co-operate.
Another principal purpose effected by the jaws
is that of giving mechanical power to the
muscles, which, by acting upon the sides of the
cavity of the mouth, tend to compress and
propel the contained food. We find, accord-
ingly, that all animals of a highly developed
structure are provided with jaws.

Among the animals which are ranked in the
class of Zoophytes, the highest degrees of deve-
lopement are exhibited by the Echinodermata,

JAWS OF THE ECHINUS. 119

and in them we find a remarkable perfection in
the organs of mastication. The mouth of the
Echinus is surrounded by a frame-work of shell,
consisting of five converging pieces, each armed
with a long tooth ; and for the movement of
each part there are provided separate muscles,
of which the anatomy has been minutely de-
scribed by Cuvier. In the shells of the echini
which are cast on the shore, this calcareous frame
is usually found entire in the inside of the outer
case ; and Aristotle having noticed its resem-
blance to a lantern, it has often gone by the
whimsical name of the lantern of Aristotle.

In all articulated animals which subsist on
solid aliment, the apparatus for the prehension
and mastication of the food, situated in the
mouth, is exceedingly complicated, and admits
of great diversity in the different tribes; and,
indeed, the number and variety of the parts of
which it consists is so great, as hardly to admit
of being comprehended in any general descrip-
tion. In most insects, also, their minuteness is
an additional obstacle to the accurate obser-
vation of their anatomy, and of the mechanism
of their action. The researches, however, of
Savigny* and other modern entomologists have
gone far to prove, that amidst the infinite vari-

* See his " Theorie des Organes de la bouche des Anitnaux
invertebres et articules," which forms the first part of the *' Me-
moires sur les Animaux sans vert^bres." Paris, 1816.

120

THE VITAL FUNCTIONS.

atioiis observable in the form and arrangement
of the several parts of these organs, there is still
preserved, in the general plan of their con-
struction, a degree of uniformity quite as great
as that which has been remarked in the fabric
of the vertebrated classes. Not only may we
recognise in every instance the same elements
of structure, but we may also trace regular
chains of gradation, connecting forms appa-
rently most remote, and organs destined for
widely different uses^: so that even when there
has been a complete change of purpose, we still
perceive the same design followed, the same model
copied, and the same uniformity of plan pre-
served in the construction of the organs of every
kind of mastication ; and there prevails in them
the same unity of system as is displayed in so
marked a manner in the conformation of the
organs of progressive motion. The jaws, which
in one tribe of insects are formed for breaking
down and grinding the harder kinds of food,
are, in another, fitted for tearing asunder the
more tough and fibrous textures : they are fa-
shioned, in a third, into instruments for taking
up the semi-fluid honey prepared by flowers;
while, again, in a fourth, they are prolonged
and folded into a tubular proboscis, capable of
suction, and adapted to the drinking of fluid
aliment. Pursuing the examination of these
organs in another series of articulated animals,

JAWS OF ARTICU LATA. 121

we find them gradually assuming the characters,
as well as the uses of instruments of prehension,
of weapons for warfare, of pillars for support, of
levers for motion, or of limbs for quick pro-
gression. Some of these remarkable metamor-
phoses of organs have already attracted our
attention in a former part of this treatise.* Jaws
pass into feet, and feet into jaws, through every
intermediate form ; and the same individual
often exhibits several steps of these transitions,
and is sometimes provided also with super-
numerary organs of each description. In the
Arachnida, in particular, we frequently meet
with supernumerary jaws, together with various
appendices, which present remarkable analogies
of form with the antennae, and the legs and feet
of the Crustacea.

The principal elementary parts which enter
into the composition of the mouth of an insect,
when in its most perfect state of developement,
are the seven following ; a pair of upper jaws,
a pair of lower jaws, an upper and a lower lip,
and a tongue. t These parts in the Locusta

* Vol. i. p. 289.

t All these parts, taken together, were termed by Fabricius
instrumenta cibaria ; and upon their varieties of structure he
founded his celebrated system of entomological classification.
Kirby and Spence have denominated them tropin. See their
Introduction to Entomology, vol. iii. p. 417. To the seven
elements above enumerated Savigny adds, in the Hemiptera, an
eighth, which he terms the Epiglossa.

122

THE VITAL FUNCTIONS.

viridissima, or common grasshopper, are deli-
neated in their relative situations, but detached
from one another, in Fig. 267. The upper jaws
(m), which are termed the mandibles, are those

principally employed for the mastication of hard'
substances; they are accordingly of greater
strength than the lower jaws, and their edges
are generally deeply serrated, so as to act like
teeth in dividing and bruising the food. Some of
these teeth are pointed, others wedge-shaped, and
others broad, like grinders ; their form being in
each particular case adapted to the mechanical
texture of the substances to which they are
designed to be applied. Thus the mandibles
of some MelolonthcB have a projection, rendered
rough by numerous deep transverse furrows,
converting it into a file for wearing down the

JAWS OF INSECTS. 123

dry leaves, which are their natural food.* In
most cases, indeed, we are, in like manner,
enabled, from a simple inspection of the shape
of the teeth, to form tolerably accurate ideas of
the kind of food on which the insect naturally
subsists, t ^

Above, or rather in front of the mandibles, is
situated the lahrum, or upper lip (u). It is
usually of a hard or horny texture, and admits
of some degree of motion ; but its form and
direction are exceedingly various in different
tribes of insects. The lower pair of jaws (j), or
maxillce, as they have been termed, are behind
the mandibles, and between them is situated the
labium, or lower lip (l), which closes the mouth
below, as the lahrum does above. In the grass-
hopper, each maxilla consists of an outer and
an inner plate (o and i). The jaws of insects
are confined, by their articulations with the
head, to motions in a horizontal plane only, so
that they open and close by lateral movements,
and not upwards and downwards, as is the case
with the jaws of vertebrated animals. The
maxillae are, in most cases, employed principally
for holding the substances on which the dividing
or grinding apparatus of the mandibles is exerted.

* Knoch, quoted by Kirby.

t See a memoir by Marcel des Serres, in the Annales du
Museum d'Hist. Nat. xiv. 56.

124 THE VITAL FUNCTIONS.

A similar use may be assigned also to the organs
denominated Palpi, or Antenmdce (p, q), which
are jointed filaments, or processes, attached to
different parts of the mouth, and most usually
to the maxillae and the labium ; the former (p)
being termed the maxillary, and the latter (q)
the labial palpi. In addition to these parts,
another, which, from its supposed use, has been
denominated Glossa, or tongue (g), is also gene-
rally found.

For an account of the various modifications
which these parts receive in different tribes and
species, I must refer to works which treat pro-
fessedly of this branch of comparative anatomy.
I shall content myself with giving a single
example of the conversion of structure here
alluded to, in that of the rostrmn, or proboscis of
the Cimex nigricornis. This insect belongs to
the order Hemiptera, which has been usually
characterised as being destitute of both man-
dibles and jaws; and as having, instead of these
parts, an apparatus of very different construc-
tion, designed to pierce the skin of animals and
suck their juices. But Savigny, on applying
the principles of his theory, has recognised, in
the proboscis of the Cimex, the existence of all
the constituent elements which are found in the
mouth of insects formed for the mastication of
solid food. This proboscis consists of four elon-
gated filaments, contained in a kind of sheath :

JAWS OF INSECTS.

125

268

269

the filaments are represented in Fig. 268,
separated to a little distance
from each other, in order that
their respective origins may
be distinctly seen ; the one
set (q) being prolongations of
the mandibles (j), and the
other set (p) being, in like
manner, prolongations of the
maxillae (m). Between these
filaments, and near their com-
mencement, is seen a pointed
cartilaginous body (g), which
is the glossa, or tongue; and
the aperture seen at its root is
the passage into the cesopha-
gus. The sheath is merely
the elongated labium, of which
the base is seen at l, in Fig.
268 ; but is represented in its whole length in
Fig. 269, where the groove for containing the
filaments above described, is apparent.

In the mouths of the Annelida we often meet
with hard bodies, which serve the purposes of
jaws and of teeth. The retractile proboscis of
the Aphrodite, or sea-mouse, is furnished with
four teeth of this description. The Leech has,
immediately within its lips, three semi-circular
teeth, with round and sharp cutting edges : they
are delineated in Fig. 261, (p. 103), in their rela-

126 THE VITAL FUNCTIONS.

tive positions ; and Fig. 262 represents one of
the teeth detached from the rest. It is with these
teeth that the leech pierces the skin of the
animals whose blood it sucks ; and as soon as
the wound is inflicted, the teeth, being moveable
at their base, fall back, leaving the opening of
the mouth free for sucking. The wound thus
made is of a peculiar form ; being composed of
three lines, radiating from a centre, where the
three teeth had penetrated.

Most of the Mollusca which inhabit univalve
shells are provided with a tubular organ, of a
cylindric or conical shape, capable of elongation
and contraction, by circular and longitudinal
muscular fibres, and serving the purpose of a
proboscis, or organ of prehension for seizing and
conveying food into the mouth. These tubes
are of great size in the JBuccinum, the Murex^
and the Voluta ; as also in the Doris, which,
though it has no shell, is likewise a gasteropode.
In those mollusca of this order which have not a
proboscis, as the Limax, or slug, the Helix, or
snail, and the Aplysia, or sea-hare, the mouth
is furnished with broad lips, and is supported by
an internal cartilage, having several tooth -like

270 projections, which assist in laying hold
of the substances taken as food. That
of the snail is represented in Fig. 270.

All the Sepice, or cuttle fish tribe, are fur-
nished, at the entrance of the mouth, with two

JAWS OF FISHES. 127

horny jaws, having a remarkable resemblance
to the bill of a parrot ; excepting that the lower
piece is the larger of the two, and covers the
upper one, which is the reverse of what takes
place in the parrot. These constitute a powerful
instrument for breaking the shells of the mol-
lusca and Crustacea, which compose the usual
prey of these animals.

Fishes almost always swallow their food entire ;
so that their jaws and teeth are employed prin-
cipally as organs of prehension and detention ;
and the upper jaw, as well as the lower one,
being moveable upon the cranium, they are
capable of opening to a great width. The bony
pieces which compose the jaws are more nume-
rous than the corresponding bones in the higher
classes of vertebrata ; and they appear, therefore,
as if their developement had not proceeded suf-
ficiently far to effect their consolidation into
more compact structures.*

Fishes which live upon other animals of the
same class having a soft texture, are furnished
with teeth constructed merely for seizing their
prey, and perhaps also for slightly dividing it,
so as to adapt it to being swallowed. These
teeth are of various shapes, though usually sharp

* Attempts have been made to trace analogies between the
different segments of the jaws of fishes and corresponding parts
of the mouths of Crustacea and of insects ; but the justness of
these analogies is yet far from being satisfactorily proved.

128 THE VITAL FUNCTIONS.

at the points ; and either conical or hooked at
the extremity, with the points always directed
backwards, in order to prevent the escape of the
animal which has been seized. Fishes which
subsist on testaceous moUusca have teeth with
grinding surfaces, and their jaws are also adapted
for mastication. Every part of the mouth, tongue,
and even throat, may afford lodgement for teeth
in this class of animals. Almost the whole
cavity of the mouth of the Anarrhichas lupus, or
wolf-fish, may be said to be paved with teeth, a
triple row being implanted on each side ; so that
this fish exerts great power in breaking shells.
The Shark has numerous rows of sharp teeth,
with serrated margins : these at first sight ap-
pear to be formidable instruments ; but as the
teeth in the opposite jaws do not meet, it is
evident that they are not intended for cutting,
like the incisors of mammalia.

Among Reptiles, we find the Batrachia almost
wholly destitute of teeth. Frogs, indeed, exhibit
two rows of very fine points ; the one in the upper
jaw, and the other passing transversely across the
palate : they may be considered as teeth existing
in a rudimental state ; for whatever may be their
uses, they are not sufiiciently developed to be
useful in mastication. There are about forty of
these minute teeth on each side in the frog. In
the Salamander, there are sixty above and be-
low ; and also thirty on each side of the palate.

TONGUES OF REPTILES. 129

The tongue of the frog is of great length ; its
root is attached close to the fore part of the
lower jaw, while its point, which is cloven, is
turned backwards, extending into the throat, and
acting like a valve in closing the air passage
into the lungs. If, when this animal has ap-
proached within a certain distance of the insect
it is about to seize, we watch it with attention,
we are surprised to observe the insect suddenly
disappear, without our being able to perceive
what has become of it. This arises from the
frog having darted out its tongue upon its victim
with such extreme quickness, and withdrawn it,
with the insect adhering to it, so rapidly, that it
is scarcely possible for the eye to follow it in its
motion. The Chameleo7i also has a very long and
slender tongue, the extremity of which is dilated
into a kind of club, or spoon, and covered with a
glutinous matter : with this instrument the animal
catches insects at a considerable distance, by a
similar manoeuvre to that practised by the frog. *

As Serpents swallow their prey entire, so the
bones of their jaws and face are formed to admit
of great expansion, and freedom of motion upon
one another. Serpents and Lizards have gene-
rally curved or conical teeth, calculated rather

* Mr. Houston has given a descriptipn of the structure of this
organ, and of the muscles by which it is moved, in a paper con-^
tained in the Transactions of the Royal Irish Academy, vol. xv.
p. 177. ; . . i ;-/ '.

VOL. II. . K

130 THE VITAL FUNCTIONS.

for tearing and holding the food, than for masti-
cating it : like those of fishes, they are affixed
partly to the jaws, and partly to the palate.
The Chelonian reptiles have no teeth ; their ofiice
being supplied by the sharp cutting edges of the
horny portion of the jaws.

Birds, as well as serpents, have a moveable
upper jaw ; but they are also provided with
beaks of various forms, in which we may trace
an exact adaptation to the kind of food appro-
priated to each tribe : thus predaceous birds, as
the eagle and the hawk tribe, have an exceed-
ingly strong hooked beak, for tearing and di-
viding the flesh of the animals on which they
prey; while those that feed on insects, or on
grain, have pointed bills, adapted to picking up
minute objects. Aquatic birds have generally
flattened bills, by which they can best select
their food among the sand, the mud, or the
weeds at the bottom of the water; and their
edges are frequently serrated, to allow the fluid
to filter through, while the solid portions are
retained in the mouth. The Duck affords an
instance of this structure ; which is, however,
still more strongly marked in the genus MerguSy
or Merganser, where the whole length of the
margin of the bill is beset with small sharp
pointed teeth, directed backwards : they are par-
ticularly conspicuous in the Mergus serrator^ or
red-breasted Merganser. The object of the
barbs and fringed processes, which are appended

JAWS OF BIRDS. 1 .'3 1

to the tongue in many birds, such as that of the
Toucan and the Parrakeet, appears, in like
manner, to be the detention of substances intro-
duced into the mouth.

The beak of the Hcematopus, or Oyster-catcher,
has a wedge shape, and acts like an oyster-
knife for opening bivalve shells.

In the Loxia curvirostra, or Cross-bill, the
upper and lower mandibles cross each other
when the mouth is closed, a structure which
enables this bird to tear open the cones of the
pine and fir, and pick out the seeds, by insi-
nuating the bill between the scales. It can split
cherry stones with the utmost ease, and in a
very short time, by means of this peculiarly
shaped bill.*

Birds which dive for the purpose of catching
fish have often a bill of considerable length,
which enables them to secure their prey, and
change its position till it is adapted for swal-
lowing.

The Rhynchops, or black Skimmer, has a very
singularly formed beak : it is very slender, but
the lower mandible very much exceeds in length
the upper one; so that while skimming the
waves in its flight, it cuts the water like a
plough-share, catching the prey which is on the
surface of the sea.

The Woodpecker is furnished with a singular

* See a paper on the mechanism of the bill of this bird, by
Mr. Yanell, in the Zoological Journal, iv. 459.

132 THE VITAL FUNCTIONS.

apparatus for enabling it to dart out with great
velocity its long and pointed tongue, and transfix
the insects on which it principally feeds; and
these motions are performed so quickly that the
eye can scarcely follow them. This remarkable
mechanism is delineated in Fig. 271, which
represents the head of the woodpecker, with the
skin removed, and the parts dissected. The
tongue itself (t) is a slender sharp-pointed
horny cylinder, having its extremity (b) beset
with barbs, of which the points are directed
backwards: it is supported on a slender Os
Hyoides^ or lingual bone, to the posterior end
of which the extremities of two very long and
narrow cartilaginous processes are articulated.*
The one on the right side is shown in the figure,

* These cartilages correspond in situation, at the part, at
least, where they are joined to the os hyoides, to what are called
the cornua, or horns of that bone, in other animals.

TONGUE OF THE WOODPECKER. 133

nearly in the whole extent of its course, at c, d,
E, F, and a small portion of the left cartilage is
seen at l. The two cartilages form, at their
junction with the tongue, a very acute angle,
slightly diverging as they proceed backwards ;
until, bending downwards (at c), they pass ob-
liquely round the sides of the neck, connected
by a membrane (m) ; then, being again inflected
upwards, they converge towards the back of the
head, where they meet, and, being enclosed in a
common sheath, are conducted together along a
groove, which extends forwards, along the middle
line of the cranium (e), till it arrives between
the eyes. From this point, the groove and the
two cartilages it contains, which are now more
closely conjoined, are deflected towards the
right side, and terminate at the edge of the
aperture of the right nostril (f), into which the
united cartilages are finally inserted. In order
that their course may be seen more distinctly,
these cartilages are represented in the figure
(at d), drawn out of the groove provided to
receive and protect them.* A long and slender
muscle is attached to the inner margin of each
of these cartilages ; and their actions conspire to
raise the lower and most bent parts of the car-
tilages, so that their curvature is diminished,
and the tongue protruded to a considerable dis-

* S is the large salivary gland on the right sido.

134 THE VITAL FUNCTIONS.

tance, for the purpose of catching insects. As
soon as this has been accomplished, these
muscles being suddenly relaxed, another set of
fibres, passing in front of the anterior portion of
the cartilages nearly parallel to them, are thrown
into action, and as suddenly retract the tongue
into the mouth, with the insect adhering to its
barbed extremity. This muscular effort is, how-
ever, very materially assisted by the long and
tortuous course of these arched cartilages, which
are nearly as elastic as steel springs, and effect
a considerable saving of muscular power.* This
was the more necessary, because, while the bird
is on the tree, it repeats these motions almost
incessantly, boring holes in the bark, and pick-
ing up the minutest insects, with the utmost
celerity and precision. On meeting with an ant-
hill, the woodpecker easily lays it open by the
combined efforts of its feet and bill, and soon
makes a plentiful meal of the ants and their
eggs.

Among the Mammalia which have no teeth,
the Myrmecophagaj or Ant-eater, practises a re-
markable manoeuvre for catching its prey. The
tongue of this animal is very long and slender,
and has a great resemblance to an earth-worm :
that of the two-toed ant-eater is very nearly
one-third of the length of the whole body ; and

* An account of this mechanism is given by Mr. Waller, in
the Phil. Trans, for 1716, p. 509.

TONGUE OF THE ANT-EATER. 135

at its base is scarcely thicker than a crow-quill.
It is furnished with a long and powerful muscle,
which arises from the sternum, and is continued
into its substance, affording the means of a quick
retraction, as well as lateral motion ; while its
elongation and other movements are effected by
circular fibres, which are exterior to the former.
When laid on the ground in the usual track of
ants, it is soon covered with these insects, and
being suddenly retracted, transfers them into
the mouth ; and as, from their minuteness, they
require no mastication, they are swallowed un-
divided, and without there being any necessity
for teeth.

The lips of quadrupeds are often elongated for
the more ready prehension of food, as we see
exemplified in the Rhinoceros, whose upper lip
is so extensible as to be capable of performing
the office of a small proboscis. The Sorex
moschatus, or musk shrew, whose favourite food
is leeches, has likewise a very moveable snout,
by which it gropes for, and seizes its prey from
the bottom of the mud. More frequently, how-
ever, this office of prehension is performed by
the tongue, which for that purpose is very
flexible and much elongated ; as we see in the
Camelopard, where it acts like a hand in grasp-
ing and bringing down the branches of a
tree.*

* Home, Lectures, &c. vi. Plate 32.

13() THE VITAL FUNCTIONS.

In the animals belonging to the genus Felis, the
papillae in the fore part of the tongue are each
armed with a horny sheath terminating in a sharp
point, which is directed backwards, so as to detain
the food and prevent its escape. These prickles
are of great size and strength in the larger beasts
of prey, as the Lion and the Tiger ; they are met
with also in the Opossum, and in many species
of Bats, more especially those belonging to the
genus Pteropus : all these horny productions
have been regarded as analogous to the lingual
teeth of fishes, already noticed.

The mouth of the Oimilhorliynchus has a form
of construction intermediate between that of
quadrupeds and birds ; being furnished, like
the former, with grinding teeth at the posterior
part of both the upper and lower jaws, but
they are of a horny substance ; and the mouth
is terminated in front by a horny bill, greatly
resembUng that of the duck, or the spoon-
bill.

The Whale is furnished with a singular appa-
ratus designed for filtration on a large scale.
The palate has the form of a concave dome, and
from its sides there descends perpendicularly into
the mouth, a multitude of thin plates, set parallel
to each other, with one of their edges directed
towards the circumference, and the other towards
tlie middle of the palate. These plates are known
by the name of ^vhalehone; and their general form

MOUTH OF THE WHALE.

137

and appearance, as they hang from the roof of
the palate, are shown in Fig. 272, which repre-
sents only six of these plates.* They are con-
nected with the bone by means of a white liga-
mentous substance, to which they are imme-
diately attached, and from which they appear to
grow : at their inner margins,
the fibres, of which their tex-
ture is throughout composed,
cease to adhere together ; but,
being loose and detached,
form a kind of fringe, calcu-
lated to intercept, as in a sieve,
all solid or even gelatinous
substances that may have been
admitted into the cavity of the
mouth, which is exceedingly
capacious; for as the plates
of whalebone grow only from
the margins of the upper jaw,
they leave a large space with-
in, which, though narrow an-
teriorly, is wider as it extends
backwards, and is capable of
holding a large quantity of water. Thus the
whale is enabled to collect a whole shoal of mol-

* In the Piked Whale the plates of whalebone are placed
very near together, not being a quarter of an inch asunder; and
there are above three hundred plates in the outer rows on each
side of the mouth.

138 THE VITAL FUNCTIONS.

lusca, and other small prey, by taking into its
mouth the sea water which contains these ani-
mals, and allowing it to drain off through the
sides, after passing through the interstices of
the net work formed by the filaments of the
whalebone. Some contrivance of this kind was
necessary to this animal, because the entrance
into its oesophagus is too narrow to admit of the
passage of any prey of considerable size ; and it
is not furnished with teeth to reduce the food
into smaller parts. The principal food of the
BalcBua Mysticetus, or great whalebone whale of
the Arctic Seas, is the small Clio Borealisy
which swarms in immense numbers in those
regions of the ocean ; and which has been al-
ready delineated in Fig. 120.*

These remarkable organs for filtration entirely
supersede the use of ordinary teeth ; and ac-
cordingly no traces of teeth are to be discovered
either in the upper or lower jaw. Yet a ten-
dency to conform to the type of the mammalia
is manifested in the early conformation of the
whale ; for rudiments of teeth exist in the in-
terior of the lower jaw before birth, lodged in
deep sockets, and forming a row on each side.
The developement of these imperfect teeth pro-
ceeds no farther ; they even disappear at a very
early period, and the groove which contained

* Vol. i. p. 258.

MOUTH OF THE WHALE. 139

them closes over, and after a short time can no
longer be seen. For the discovery of this
curious fact we are indebted to GeofFroy St.
Hilaire.* In connexion with this subject, an
analogous fact which has been noticed in the
Parrot may here be mentioned. The young of
the parrot, while still in the egg, presents a row
of tubercles along the edge of the jaw, in ex-
ternal appearance exactly resembling the rudi-
ments of teeth, but without being implanted
into regular sockets in the maxillary bones :
they are formed, however, by a process precisely
similar to that of dentition ; that is, by deposi-
tion from a vascular pulp, connected with the
jaw. These tubercles are afterwards consoli-
dated into one piece in each jaw, forming by
their union the beak of the parrot, in a manner
perfectly analogous to that which leads to the
construction of the compound tooth of the ele-
phant, and which I shall presently describe.
The original indentations are obliterated as the
beak advances in growth ; but they are per-
manent in the bill of the duck, where the
structure is very similar to that above described
in the embryo of the parrot.

• Cuvier, Ossemens Fossiles, 3me edition, torn. v. p. 360.

140 THE VITAL FUNCTIONS.

§ 3. Mastication by means of Teeth.

The teeth, being essential instruments for seizing
and holding the food, and effecting that degree
of mechanical division necessary to prepare it
for the chemical action of the stomach, perform,
of course, a very important part in the economy
of most animals; and in none more so than in
the Mammalia, the food of which generally re-
quires considerable preparation previously to its
digestion. There exist, accordingly, the most
intimate relations between the kind of food
upon which each animal of this class is intended
by nature to subsist, and the form, structure,
and position of the teeth ; and similar relations
may also be traced in the shape of the jaw,
in the mode of its articulation with the head,
in the proportional size and distribution of
the muscles which move the jaw, in the form of
the head itself, in the length of the neck, and its
position on the trunk, and indeed in the whole
conformation of the skeleton. But since the
nature of the appropriate food is at once indi-
cated by the structure and arrangement of the
teeth, it is evident that these latter organs, in
particular, will afford to the naturalist most im-
portant characters for establishing a systematic
classification of animals, and more especially of
quadrupeds, where the difterences among the

OFFICES OF THE TEETH. 141

teeth are very considerable ; and these differ-
ences have, accordingly, been the object of much
careful study. To the physiologist they present
views of still higher interest, by exhibiting most
striking evidences of the provident care with
which every part of the organization of animals
has been constructed in exact reference to their
respective wants and destinations.

The purposes answered by the teeth are prin-
cipally those of seizing and detaining whatever
is introduced into the mouth, of cutting it
asunder, and dividing it into smaller pieces, of
loosening its fibrous structure, and of breaking
down and grinding its harder portions. Occa-
sionally some particular teeth are much enlarged,
in order to serve as weapons of attack or of
defence ; for which purpose they extend beyond
the mouth, and are then generally denominated
tusks; this we see exemplified in the Elephant,
the Narwhal, the Walrus, the Hippopotamus, the
Soar, and the Bahiroussa.

Four principal forms have been given to teeth,
which accordingly may be distinguished into
the conical, the sharp-edged, the flat, and the
tuberculated teeth ; though we occasionally find
a few intermediate modifications of these forms.
It is easy to infer the particular functions of
each class of teeth, from the obvious mechanical
actions to which, by their form, they are espe-
cially adapted. The conical teeth, which are

142 THE VITAL FUNCTIONS.

generally also sharp-pointed, are principally em-
ployed in seizing, piercing, and holding objects :
such are the offices which they perform in the
Crocodile, and other Saurian reptiles, where all
the teeth are of this structure ; and such are also
their uses in most of the Cetacea, where similar
forms and arrangements of teeth prevail. All
the Dolphin tribe, such as the Porpus, the
Grampus, and the Dolphin, are furnished with
a uniform row of conical teeth, set round both
jaws, in number amounting frequently to two
hundred. Fig. 273, which represents the jaws
of the Porpus, shows the form of these simply

prehensile teeth. The Cachalot has a similar
row of teeth, which are, however, confined to the
lower jaw. All these animals subsist upon fish,
and their teeth are therefore constructed very
much on the model of those of fish ; while those
Cetacea, on the other hand, which are her-
bivorous, as the Manatus and the Dugong, or
Indian Walrus, have teeth very differently
formed. The tusks of animals must necessarily,
as respects their shape, be classed among the
conical teeth.

TEETH OF CETACEA. 143

The sharp-edged teeth perform the office of
cutting and dividing the yielding textures pre-
sented to them : they act individually as wedges
or chisels ; but when co-operating with similar
teeth in the opposite jaw, they have the power
of cutting like shears or scissors. The flat teeth,
of which the surfaces are generally rough, are
used, in conjunction with those meeting them in
the opposite jaw, for grinding down the food by
a lateral motion ; in a manner analogous to the
operation of mill-stones in a mill. The tuber-
culated teeth, of which the surfaces present a
number of rounded eminences, corresponding to
depressions in the teeth opposed to them in the
other jaw, act more by their direct pressure in
breaking down hard substances, and pounding
them, as in a mortar.

The position of the teeth in the jaws is
another ground of distinction. In those Mam-
malia which exhibit the most complete set of
teeth, the foremost in the row have the sharp-
edged or chisel shape, constituting the blades of
a cutting instrument ; and they are accordingly
denominated incisors. The incisors of the upper
jaw are always implanted in a bone, intermediate
between the two upper jaw bones, and called
the intermaxillary bones.* The conical teeth

* Those teeth of the lower jaw which correspond with the
incisors of the upper jaw, are also considered as incisors. In
Man, and in the species of Quadrumana that most nearly re-

144 THE VITAL FUNCTIONS.

immediately following the incisors, are called
cuspidate, or canine teeth, from their being par-
ticularly conspicuous in dogs; as they are, in-
deed, in all the purely carnivorous tribes. In the
larger beasts of prey, as the Lion and the Tiger,
they become most powerful weapons of destruc-
tion : in the Boar they are likewise of great
size, and constitute the tusks of the animal. All
the teeth that are placed farther back in the
jaw are designated by the general name oi molar
teeth, ov grinders, but it is a class which includes
several different forms of teeth. Those teeth
which are situated next to the canine teeth,
partake of the conical form, having pointed emi-
nences : these are called the false molar teeth,
and also, from their having generally two points,
or cusps, the bicuspidate teeth. The posterior
iriolar teeth are differently shaped in carnivorous
animals ; for they are raised into sharp and often
serrated ridges, having many of the properties
of cutting teeth. In insectivorous and fru-
givorous animals, their surface presents pro-
minent tubercles, either pointed or rounded, for
pounding the food ; while in quadrupeds that
feed on grass or grain, they are fiat and rough,
for the purpose simply of grinding.

The apparatus for giving motion to the jaws

semble him, the sutures which divide the intermaxillary from the
maxillary bones are obliterated before birth, and leave in the
adult no trace of their former existence.

MOVEMENTS OF THE JAWS. 145

is likewise varied according to the particular
movements required to act upon the food in the
different tribes. The articulation of the lower
jaw with the temporal bone of the skull ap-
proaches to a hinge joint ; but considerable lati-
tude is allowed to its motions by the interposi-
tion of a moveable cartilage between the two
surfaces of articulation, a contrivance admirably
answering the intended purpose. Hence, in ad-
dition to the principal movements of opening
and shutting, which are made in a vertical
direction, the lower jaw has also some degree of
mobility in a horizontal or lateral direction, and
is likewise capable of being moved backwards
and forwards to a certain extent. The muscles
which effect the closing of the jaw are princi-
pally the temporal and the masseter muscles ;
the former occupying the hollow of the temples ;
the latter connecting the lower angle of the jaw
with the zygomatic arch. The lateral motions
of the jaw are effected by muscles placed inter-
nally, between the sides of the jaw and the basis
of the skull.

In the conformation of the teeth and jaws, a
remarkable contrast is presented between car-
nivorous and herbivorous animals. In the for-
mer, of which the Tiger, Fig. 274, may be taken
as an example, the whole apparatus for masti-
cation is calculated for the destruction of life,
and for tearing and dividing the fleshy fibres.

VOL. II. L

146 THE VltAL FUNCTIONS.

The molar teeth are armed with pointed emi-
nences, which correspond in the opposite jaws,

so as exactly to lock into one another, like
wheelwork, when the mouth is closed. All the
muscles which close the jaw are of enormous
size and strength ; and they imprint the bones
of the skull with deep hollows, in which we
trace marks of the most powerful action. The
temporal muscles occupy the whole of the sides
of the skull (t, t) ; and by the continuance of
their vigorous exertions, during the growth of
the animal, alter so considerably the form of the
bones, that the skulls of the young and the old
animals are often with difficulty recognised as
belonging to the same species.* The process of
the lower jaw (seen between t and t), to which
this temporal muscle is attached, is large and
prominent ; and the arch of bone (z), from which

* This is remarkably the case with the Bear, the skull of
which exhibits in old animals a large vertical crest, not met with
at an early period of life.

JAWS AND TEETH OF HEKBIVORA. 147

the masseter arises, takes a wide span outwards,
so as to give great strength to the muscle. The
condyle, or articulating surface of the jaw (c), is
received into a deep cavity, constituting a strictly
hinge joint, and admitting simply the motions of
opening and shutting.

In herbivorous animals, on the contrary, as
may be seen in the skull of the Antelope, Fig.
275, the greatest force is bestowed, not so much

on the motions of opening and shutting, as on
those which are necessary for grinding, and
which act in a lateral direction. The temporal
muscles, (occupying the space t,) are compara-
tively small and feeble ; the condyles of the jaw
are broad and rounded, and more loosely con-
nected with the skull by ligaments ; the muscles
in the interior of the jaw, which move it from
side to side, are very strong and thick ; and the
bone itself is extended downwards, so as to afford
them a broad basis of attachment. The surfaces

148 THE VITAL FUNCTIONS.

of the molar teeth are flattened and of great ex-
tent ; and they are at the same time, by a provi-
sion which will be hereafter explained, kept
rough, like those of mill-stones ; their office being
in fact very similar to that performed by these im-
plements for grinding. All these circumstances of
difference are exemplified in the most marked
manner, in comparing together the skulls of the
larger beasts of prey, as the tiger, the wolf, or
the bear, with those of the antelope, the horse,
or the ox.

The Rodentia, or gnawing quadrupeds, which
I have already had occasion to notice, compose
a well-marked family of Mammalia. These
animals are formed for subsisting on dry and
tough materials, from which but little nutriment
can be extracted ; such as the bark, and roots,
and even the woody fibres of trees, and the
harder animal textures, which would appear
to be most difficult of digestion. They are
all animals of diminutive size, whose teeth are

expressly formed for
gnawing, nibbling,
and wearing away by
continued attrition,
the harder textures
of organized bodies.
The Rat, whose skull
is delineated in Fig. 276, belongs to this tribe.
They are all furnished with two incisor teeth in

TEETH OF QUADRUMANA. 149

each jaw, generally very long, and having the
exact shape of a chisel ; and the molar teeth
have surfaces, irregularly marked with raised
zig-zag lines, rendering them very perfect in-
struments of trituration. The zygomatic arch is
exceedingly slender and feeble ; and the condyle
is lengthened longitudinally to allow of the jaw
being freely moved forwards and backwards,
which is the motion for which the muscles are
particularly adapted, and by which the grinding
operation is performed. The Beaver, the Rat^
the Marmot, and the Porcupine, present examples
of this structure, among the omnivorous rodentia :
and the Hare, the Rabbit, the Squirrel, among
those which are principally herbivorous.

The Quadrumana, or Monkey tribes, approach
nearest to the human structure in the confor-
mation of their teeth, which appear formed for
a mixed kind of food ; but are especially
adapted to the consumption of the more esculent
fruits. The other orders of Mammalia exhibit
intermediate gradations in the structure of their
teeth to those above described, corresponding to
greater varieties in the nature of their food. Thus
the teeth and jaws of the Hycena are formed
more especially for breaking down bones, and
in so doing exert prodigious force ; and those of
the Sea Otter have rounded eminences, which
peculiarly fit them for breaking shells.

The teeth, though composed of the same

150 THE VITAL FUNCTIONS.

chemical ingredients as the ordinary bones,
differ from them by having a greater density
and compactness of texture ; whence they derive
that extraordinary degree of hardness which
they require for the performance of their peculiar
office. The substances of which they are com-
posed are of three different kinds ; the first,
which is the basis of the rest, constituting the
solid nucleus of the tooth, has been considered
as the part most analogous in its nature to bone ;
but from its much greater density, and from its
differing from bone in the mode of its formation,
the name of ivory has been generally given to it.
Its earthy ingredient consists almost entirely of
phosphate of lime ; the proportion of the car-
bonate of that earth entering into its composition
being very small ; and the animal portion is
albumen, with a small quantity of gelatin.

A layer of a still harder substance, termed the
etmmel, usually covers the ivory, and, in teeth of
the simplest structure, forms the whole of their
outer surface : this is the case with the teeth of
man and of carnivorous quadimpeds. These two
substances, and the direction of their layers, are
seen in Fig. 277, which is the section of a simple
tooth. E is the outer case of enamel, o the
osseous portion, and p the cavity where the
vascular pulp which formed it was lodged. The
enamel is composed almost wholly of phosphate
of lime, containing no albumen, and scarcely a

STRUCTURE OF TEETH.

151

trace of gelatin : it is the hardest of all animal
substances, and is capable of striking fire with

steel. It exhibits a fibrous structure, approach-
ing to a crystalline arrangement ; and the direc-
tion of its fibres, as shown by the form of its
fragments when broken, is every where perpen-
dicular to the surface of the ivory to which it is
applied. The ends of the fibres are thus alone
exposed to the friction of the substances on
which the teeth are made to act ; and the effect
of that friction in wearing the enamel is thus
rendered the least possible.

In the teeth of some quadrupeds, as of the
Rhinoceros^ the Hippopotamus^ and most of the
Rodentia, the enamel is intermixed with the
ivory ; and the two so disposed as to form jointly
the surface for mastication. In the progress of
life, the layers of enamel, being the hardest, are
less worn down by friction than those of the
ivory, and therefore form prominent ridges on

152 THE VITAL FUNCTIONS.

the grinding surface ; preserving it always in that
rough condition, which best adapts it for the
bruising and comminuting of hard substances.

The incisors of the rodentia are guarded by a
plate of enamel on their anterior convex sur-
faces only ; so that by the wearing down of the
ivory behind this plate, a wedge-like form, of
which the enamel constitutes the fine cutting
edge, is soon given to the tooth, and is constantly
retained as long as the tooth lasts (Fig. 280).
This mode of growth is admirably calculated to
preserve these chisel teeth fit for use during the
whole life-time of the animal ; an object of greater
consequence in this description of teeth than in
others, which continue to grow only during a
limited period. The same arrangement, attended
with similar advantages, is adopted in the struc-
ture of the tusks of the Hippopotamus.

In teeth of a more complex structure, a third
substance is found, uniting the vertical plates of
ivory and enamel, and performing the office of
an external cement. This substance has re-
ceived various names, but it is most commonly
known by that of the Crusta petrosa : it resem-
bles ivory both in its composition and its extreme
hardness ; but is generally more opaque and
yellow than that substance.

Other herbivorous quadrupeds, as the horse,
and animals belonging to the ruminant tribe,
have also complex teeth composed of these three

STRUCTURE OF TEETH. 153

substances ; and their grinding surfaces present
ridges of enamel intermixed in a more irregular
manner with the ivory and crusta petrosa ; but
still giving the advantage of a very rough surface
for trituration. Fig. 278 represents the grinding
surface of the tooth of a horse, worn down by
long mastication, e is the enamel, marked by
transverse lines, showing the direction of its
fibres, and enclosing the osseous portion (o),
which is shaded by interrupted lines. An outer
coating of enamel {e) is also visible; and between
that and the inner coat, the substance called
crnsta petrosa (c), marked by waving lines, is
seen : on the outside of all there is a plate of
bone, which has been left white. In ruminants,
the plates of enamel form crescents, which are
convex outwardly in the lower, and inwardly in
the upper jaw ; thus providing for the crossing
of the ridges of the two surfaces ; an arrange-
ment similar to that which is practised in con-
structing those of mill-stones. The teeth of the
lower jaw fall within those of the upper jaw; so
that a lateral motion is required in order to bring
their surfaces opposite to each other alternately
on both sides. Fig. 2^7.9 shows the grinding sur-
face of the tooth of a Sheep^ where the layers of
bone are not apparent ; there being only two layers
of enamel (e), and one of crusta petrosa (c).

These three component parts are seen to most
advantage in a vertical and longitudinal section

154

THE VITAL FUNCTIONS.

of the grinding tooth of the elephant, in which
they are more completely and equally inter-
mixed than in that of any other animal. Fig.
281 presents a vertical section of the grinding
tooth of the Asiatic Elephant, in the early stage

of its growth, and highly polished ; so as to
exhibit more perfectly its three component
structures. The enamel, marked e, is formed
of transverse fibres ; the osseous, or innermost
structure is composed of longitudinal plates : the
general covering of crusta petrosa, c, is less
regularly deposited : p is the cavity which had
been occupied by the pulp. In this tooth, which
is still in a growing state, the fangs are not yet
added ; but they are, at one part, beginning to
be formed. The same tooth in its usual state,
as worn by mastication, gives us a natural and

DENTITION. 155

horizontal section of its interior structure, in
which the plates of white enamel are seen
forming waved ridges. These constitute, in the
Asiatic Elephant, a series of narrow transverse
bands (Fig. 283) ; and in the African Elephant,
a series of lozenge- shaped lines (Fig. 282), having
the ivory on their interior, and the yellow crusta
petrosa on their outer sides ; which latter sub-
stance also composes the whole circumference
of the section.

'§ 4. Fonnation and Dev elopement of the Teeth.

Few processes in animal developement are more
remarkable than those which are employed to
form the teeth ; for they are by no means the
same as those by which ordinary bone is con-
structed ; and being commenced at a very early
period, they afford a signal instance of Nature's
provident anticipation of the future necessities of
the animal. The teeth, being the hardest parts
of the body, require a peculiar system of opera-
tions for giving them this extraordinary density,
which no gradual consolidation could have im-
parted. The formation of the teeth is in some
respects analogous to that of shell ; inasmuch as
all their parts, when once deposited, remain as
permanent structures, hardly ever admitting of
removal or of renewal by the vital powers.

1.56 THE VITAL FUNCTIONS.

Unlike the bones, which contain within their
solid substance vessels of different kinds, by
which they are nourished, modified, and occa-
sionally removed, the closeness of the texture of
the teeth is such as to exclude all vessels what-
soever. This circumstance renders it necessary
that they should originally be formed of the
exact size and shape Avhich they are ever after
to possess : accordingly the foundation of the
teeth, in the young animal, are laid at a very
early period of its evolution ; and considerable
progress has been made in their growth even prior
to birth, and long before they can come into use.
A tooth of the simplest construction is formed
from blood-vessels, which ramify through small
masses of a gelatinous appearance ; and each of
these pulpy masses is itself enclosed in a delicate
transparent vesicle, within which it grows till it
has acquired the exact size and shape of the
future tooth. Each vascular pulp is farther
protected by an investing membrane of greater
strength, termed its capsule, which is lodged in a
small cavity between the two bony plates of the
jaw. The vessels of the pulp begin at an early
period to deposit the calcareous substance, which
is to compose the ivory, at the most prominent
points of that part of the vesicle, which corres-
ponds in situation to the outer layer of the crown
of the tooth. The thin scales of ivory thus
formed increase by further depositions made on

DENTITION. 157

their surfaces next to the pulp, till the whole has
formed the first, or outer layer of ivory : in the
mean time, the inner surface of the capsule,
which is in immediate contact with this layer,
secretes the substance that is to compose the
enamel, and deposits it in layers on the surface
of the ivory. This double operation proceeds
step by step ; fresh layers of ivory being depo-
sited, and building up the body of the tooth,
and in the same proportion encroaching upon
the cavity occupied by the pulp, which retires
before it, until it is shrunk into a small compass,
and fills only the small cavity which remains in
the centre of the tooth. The ivory has by this
time received from the capsule a complete coat-
ing of enamel, which constitutes the whole outer
surface of the crown ; after which no more is
deposited ; and the function of the capsule
having ceased, it shrivels and disappears. But
the formation of ivory still continuing at the part
most remote from the crown, the fangs are gra-
dually formed by a similar process from the
pulp ; and a pressure being thereby directed
against the bone of the socket, at the part where
it is the thinnest, that portion of the jaw is ab-
sorbed, and the progress of the tooth is only
resisted by the gum; and the gum, in its turn,
soon yielding to the increasing pressure, the
tooth cuts its way to the surface. This process
of successive deposition is beautifully illustrated

158 THE VITAL FUNCTIONS.

by feeding a young animal at different times
with madder ; the teeth which are formed at
that period exhibiting, in consequence, alternate
layers of red and of white ivory.*

The formation of the teeth of herbivorous
quadrupeds, which have three kinds of substance,
is conducted in a still more artificial and com-
plicated manner. Thus in the Elephant, the
pulp which deposits the ivory is extended in the
form of a number of parallel plates ; while the
capsule which invests it, accompanies it in all
its parts, sending down duplicatures of mem-
brane in the intervals between the plates.
Hence the ivory constructed by the pulp, and
the enamel deposited over it, are variously inter-
mixed ; but besides this, the crusta petrosa is
deposited on the outside of the enamel. Cuvier
asserts that this deposition is made by the same
capsule which has formed the enamel, and which,
previously to this change of function, has become
more spongy and vascular than before. But
his brother, M. Frederic Cuvier represents the
deposit of crusta petrosa, as performed by a third
membrane, wholly distinct from the two others,
and exterior to them all, although it follows them
in all their folds. In the Horse and the Ox, the
projecting processes of the pulp, have more of a
conical form, with undulating sides ; and hence

* Cuvier. Dictionnaire des Sciences Medicales, t. viii. p. 320.

DENTITION. 159

the waved appearance presented by the enamel,
on making sections of the teeth of these animals.

The tusks of the elephant are composed of
ivory, and are formed precisely in the same
manner as the simple conical teeth already des-
cribed, excepting that there is no outer capsule,
and therefore no outer crust of enamel. The
whole of the substance of the tusk is constructed
by successive deposits of layers, having a conical
shape, from the pulp which occupies the axis of
the growing tusk ; just as happens in the forma-
tion of a univalve shell which is not turbinated,
as, for instance, the Patella. Hence any foreign
substance, a bullet, for example, which may
happen to get within the cavity occupied by the
pulp, becomes, in process of time, encrusted
with ivory, and remains embedded in the solid
substance of the tusk. The pulp, as the growth
of the tusk advances, retires in proportion as its
place is occupied by the fresh deposits of ivory.

The young animal requires teeth long before
it has attained its full stature ; and these teeth
must be formed of dimensions adapted to that of
the jaw, while it is yet of small size. But as the
jaw enlarges, and the teeth it contains admit not
of any corresponding increase, it becomes neces-
sary that they should be shed, to make room for
others of larger dimensions, formed in a more
capacious mould. Provision is made for this
necessary change at a very early period of the

160 THE VITAL FUNCTIONS.

growth of the embryo. The rudiments of the
human teeth begin to form four or five months
before birth : they are contained in the same
sockets with the temporary teeth, the capsules
of both being connected together. As the jaw
enlarges, the second set of teeth gradually ac-
quire their full dimensions ; and then, by their
outward pressure, occasion the absorption of the
fangs of the temporary teeth, and, pushing them
out, occupy their places.*

As the jaw-bone, during its growth, extends
principally backwards, the posterior portion,
being later in forming, is comparatively of a
larger size than either the fore or the lateral
parts; and it admits, therefore, of teeth of the
full size, which consequently are permanent.
The molar teeth, which are last formed, are, for
want of space, rather smaller than the others,
and are called the wisdoin-teeth ; because they
do not usually make their appearance above
the gum till the person has attained the age of
twenty. In the Negro, however, where the jaw
is of greater length, these teeth have sufficient
room to come into their places, and are, in gene-
ral, fully as large as the other molares.

The teeth of carnivorous animals are, from

* It is stated by Rousseau that the shedding of the first molar
tooth both of the Guinea-pig, and the Capibara, and its re-
placement by the permanent tooth, take place a few days before
birth. (Anatomic Comparee du Systfeme Dentaire, p. 164.)

DENTITION. 101

the nature of their food, less liable to be worn,
than those of animals living on grain, or on the
harder kinds of vegetable substances ; so that
the simple plating of enamel is sufficient to pre-
serve them, even during a long life. But in
many herbivorous quadrupeds we find that, in
proportion as the front teeth are worn away in
mastication, other teeth are formed, and advance
from the back of the jaw to replace them. This
happens in a most remarkable manner in the
Elephant, and is the cause of the curved form
which the roots assume ; for in proportion as the
front teeth are worn away, those immediately
behind them are pushed forwards by the growth
of a new tooth at the back of the jaw ; and this
process ^oes on continually, giving rise to a suc-
cession of teeth, each of which is larger than
that which has preceded it, during the whole
period that the animal lives. A similar suc-
cession of teeth takes place in the Wild Boar,
and also, though to a less extent, in the Sm
^thiopicus* This mode of dentition appears
to be peculiar to animals of great longevity,
and which subsist on vegetable substances con-
taining a large proportion of tough fibres, or
other materials of great hardness ; and requiring
for their mastication teeth so large as not to
admit of both the old and new tooth being

* Home, Phil. Trans, for 1799, p. 237 ; and 1801, p. 319.
VOL. II. M

16*2 THE VITAL FUNCTIONS.

contained at the same time in the alveolar por-
tion of the jaw.

An expedient of a different kind has been
resorted to in the Rodeiitia, for the purpose of
preserving the long chisel-shaped incisors in a
state fit for use. By the constant and severe
attrition to which they are exposed, they wear
away very rapidly, and would soon be entirely
lost, and the animal would perish in conse-
quence, were it not that nature has provided for
their continued growth, by elongation from their
roots, during the whole of life. This growth
proceeds in the same manner, and is conducted
on the same principles, as the original formation
of the simple teeth already described ; but, in
order to effect this object, the roots of these
teeth are of great size and length, and are
deeply imbedded in the jaw, in a large bony
socket provided for that purpose ; and their
cavity is always filled with the vascular pulp,
from which a continued secretion and deposition
of fresh layers, both of ivory and enamel, take
place. The tusks of the Elephant and of the
Hippopotamus exhibit the same phenomenon of
constant and uninterrupted growth.

In the Shark, and some other fishes, the same
object is attained in a different manner. Several
rows of teeth are lodged in each jaw; but only
one of these rows projects and is in use at the
same time ; the rest lying flat, but ready to rise

DENTITION.

163

284

in order to replace those which have been
broken or worn down. In some fishes the teeth
advance in proportion as the jaw lengthens, and
as the fore teeth are worn away : in other cases
they rise from the substance of the jaw, which
presents on its surface an assemblage of teeth in
different stages of growth ; so that in this class
of animals the greatest variety occurs in the
mode of the succession of the teeth.

The teeth of the Crocodile, which are sharp-
pointed hollow cones, composed of ivory and
enamel, are renewed by the new tooth (as is
shown at a, in Fig. 284), being formed in the
cavity of the one (b) which it
is to replace, and not being
inclosed in any separate cavity
of the jaw bone (c). As this
new tooth increases in size, it
presses against the base of
the old one, and entering its
cavity, acquires the same co-
nical form ; so that, when the
latter is shed, it is already in
its place, and fit for immediate use. This suc-
cession of teeth takes place several times during
the life of the animal ; so that they are sharp
and perfect at all ages.

The fangs of serpents are furnished, like the
stings of nettles, with a receptacle at their base
for a poisonous liquor, which is squeezed out by

1(J4 THE VITAL FUNCTIONS.

the pressure of the tooth, at the moment it
inflicts the wound, and conducted along a canal,
opening near the extremity of the tooth. Each
fang is lodged in a strong bony socket, and is,
by the intervention of a connecting bone, pressed
forwards whenever the jaw is opened sufficiently
wide ; and the fang is thus made to assume an
erect position. As these sharp teeth are very
liable to accidents, others are ready to supply
their places when wanted : for which purpose
there are commonly provided two or three half-
grown fangs, which are connected only by soft
parts with the jaw, and are successively moved
forwards into the socket to replace those that
were lost.*

The tube through which the poison flows is
formed by the folding in of the edges of a deep
longitudinal groove, extending along the greater
part of the tooth ; an interval being left between
these edges, both at the base and extremity of
the fang, by which means there remain apertures
at both ends for the passage of the fluid poison.
This structure was discovered by Mr. T. Smith
in the Coluber naia, or Cobra de Capello ;-\ and
is shown in Fig. 285, which represents the full
grown tooth, where the slight furrow, indicating
the junction of the two sides of the original
groove, may be plainly seen ; as also the two

* Home, Lectures, &c. I. 333.

t Philosophical Transactions, 1818, p. 471.

FANGS OF SERPENTS.

165

apertures (a and b) above mentioned. This
mode of formation of the tube is farther illus-
trated by Fig. 280, which shows a transverse

section of the same tooth, exhibiting the cavity
(p) which contains the pulp of the tooth, and
which surrounds that of the central tube in the
form of a crescent. Figures 287 and 288 are
delineations of the same tooth in different stages
of growth ; the bases of which, respectively, are
shown in Figures 289 and 290. Figures 291
and 292 are magnified representations of sections
of the fangs of another species of serpent, resem-
bling the rattle-snake. Fig. 291 is a section of
the young fang taken about the middle : in this
stage of growth, the cavity which contains the
pulp, almost entirely surrounds the poison tube ;
and the edges of the depression, which form the
suture, are seen to be angular, and present so
large a surface to each other, that the suture is
completely filled up, even in this early stage of

166

THE VITAL FUNCTIONS.

growth. Fig. 292 is a section of a full-grown
fang of the same species of serpent, at the same
part as the preceding ; and here the cavity of
the pulp is seen much contracted from the more
advanced stage of growth.

It is a remarkable circumstance, noticed by
Mr. Smith, that a similar longitudinal furrow
is perceptible on every one of the teeth of the
same serpent ; and that this appearance is most
marked on those which are nearest to the
poisonous fangs : these furrows, however, in the
teeth that are not venomous, are confined en-
tirely to the surface, and do not influence the
form of the internal cavity. No trace of these
furrows is discernible in the teeth of those
serpents which are not armed with venomous
fangs.

Among the many instances in which teeth are
converted to uses widely different from mastica*
tion, may be noticed that of the Squalus pristis.

or Saw-fish, where the teeth are set horizontally

GASTRIC TEETH. 167

on the two lateral edges of the upper jaw, which
is prolonged in the form of a snout (seen in a,
Fig. 293), obviously constituting a most formid-
able weapon of offence, b is a more enlarged
view of a portion of this instrument, seen from
the under side.

§ 5 . Trituration of Food in Internal Cavities.

The mechanical apparatus, provided for tritu-
rating the harder kinds of food, does not belong
exclusively to the mouth, or entrance into the
alimentary canal ; for in many animals we find
this office performed by interior organs. Among
the inferior classes, we meet with examples of
this conformation in the Crustacea, the Mollusca,
and above all in Insects. Thus there is found
in the stomach of the Lobster^ a cartilaginous
fame-work, in which are implanted hard cal-
careous bodies, having the
form, and performing the
functions of teeth. They
are delineated in Fig. 294,
which presents a view of
the interior of the sto-
mach of that animal . The
tooth A is situated in the
middle of this frame, has a rounded conical
shape, and is smaller than the others (b, c),

168 THE VITAL FUNCTIONS.

which are placed one on each side, and which
resemble in their form broad molar teeth. When
these three teeth are brought together by the
action of the surrounding muscles, they fit
exactly into each other, and are capable of
grinding and completely pulverizing the shells
of the moUusca introduced into the stomach.
These teeth are the result of a secretion of cal-
careous matter from the inner coat of that organ,
just as the outer shell of the animal is a pro-
duction of the integument ; and at each casting
of the shell, these teeth, together with the whole
cuticular lining of the stomach to which they
adhere, are thrown off, and afterwards renewed
by a fresh growth of the same material. In the
Craw-fish, the gastric teeth are of a different
shape, and are more adapted to divide than to
grind the food.

Among the gasteropodous MoUusca, several
species of J^ullce have stomachs armed with
calcareous plates, which act as cutting or grind-
ing teeth. The Bulla aperta has
three instruments of this descrip-
tion, as may be seen in Fig. 295,
which shows the interior of the
stomach of that species. Similar
organs are found in the Bulla
lignaria. The Aplysia has a con-
siderable number of these gastric teeth. An
apparatus of a still more complicated kind is

GIZZARDS OF BIRDS.

109

provided in most of the insects belonging to the
order of Orthoptera ; but I shall not enter at
present into a description of them, as it will be
more convenient to include them in the general
account of the alimentary canal of insects, which
will be the subject of future consideration.

The internal machinery for grinding is exem-
plified on the largest scale in granivorpus birds;
where it forms part of the stomach itself, and is

termed a Gizzard. It is
shown in Fig. 298, repre-
senting the interior of the
stomach of a Swan. Both
the structure and the mode
of operation of this organ
bear a striking analogy to
a mill for grinding corn ;
for it consists of two power-
ful muscles (g), of a hemis-
pherical shape, with their
flat sides applied to each other, and their edges
united by a strong tendon, which leaves a vacant
space, of an oval or quadrangular form, between
their two surfaces. These surfaces are covered
by a thick and dense horny substance, which,
when the gizzard is in action, performs an office
similar to that of mill-stones. In most birds,
there is likewise a sac, or receptacle, termed the
CraWy (represented laid open at c), in which the
food is collected for the purpose of its being

170 THE VITAL FUNCTIONS.

dropped, in small quantities at a time, into the
gizzard, in proportion as the latter gradually
becomes emptied.* Thus the analogy between
this natural process and the artificial operation
of a corn-mill is preserved even in the minuter
details ; for while the two flat surfaces of the
gizzard act as mill-stones, the craw supplies the
place of the hopper, the office of which is to
allow the grain to pass out in small quantities
into the aperture of the upper mill-stone, which
brings it within the sphere of their action.

Innumerable are the experiments which have
been made, particularly by Reaumur and Spal-
lanzani, with a view to ascertain the force of
compression exerted by the gizzard on its con-
tents. Balls of glass, which the bird was made
to swallow with its food, were soon ground to
powder ; tin tubes, introduced into the stomach,
were flattened, and then bent into a variety of
shapes ; and it was even found that the points of
needles and of lancets, fixed in a ball of lead,
were blunted and broken off" by the power of the
gizzard, while its internal coat did not appear to
be in the slightest degree injured. These results
were long the subject of admiration to physio-
logists ; and being echoed from mouth to mouth,
were received with a sort of passive astonishment,

* The gastric glands, which are spread over the greater part of
the internal surface of the craw, and which prepare a secretion
for macerating the grain, are also seen in this part of the figure.

ACTION OF THE GIZZARD. 171

till John Hunter directed the powers of his mind
to the inquiry, and gave the first rational expla-
nation of the mechanism by which they are pro-
duced. He found that the motion of tlie sides of
the gizzard, when actuated by its muscles, is
lateral, and at the same time circular; so that
the pressure it exerts, though extremely great, is
directed nearly in the plane of the grinding sur-
faces, and never perpendicularly to them ; and
thus the edges and points of sharp instruments
are either bent or broken off by the lateral pres-
sure, without their having an opportunity of
acting directly upon those surfaces. Still, how-
ever, it is evident that the effects produced upon
sharp metallic points and edges, could not be
accomplished by the gizzard without some as-
sistance from other sources ; and this assistance
is procured in a very singular, and, at the same
time, very effectual manner.

On opening the gizzard of a bird, it is con-
stantly found to contain a certain quantity of
small particles of gravel, which must have been
swallowed by the animal. The most natural
reason that can be assigned for the presence of
these stones, is, that they aid the gizzard in tri-
turating the contained food, and that they, in
fact, supply the office of teeth in that operation.
Spallanzani, however, has called in question the
soundness of this explanation, and has contended
that the pebbles found in the gizzard are swal-

172 THE VITAL FUNCTIONS.

lowed merely by accident, or in consequence of
the stupidity of the bird, which mistakes them
for grain. But this opinion has been fully and
satisfactorily refuted both by Fordyce and by
Hunter, whose observations concur in establishing
the truth of the common opinion, that in all birds
possessing gizzards, the presence of these stones
is essential to perfect digestion. A greater or less
number of them is contained in every gizzard,
when the bird has been able to meet with the
requisite supply ; and they are never swallowed
but in order to assist digestion. Several hun-
dred were found in the gizzard of a turkey, and
two thousand in that of a goose : so great an
accumulation could never have been the result
of mere accident. If the alleged mistake could
ever occur, we should expect it to take place
to the greatest extent in those birds which are
starving for want of food ; but this is far from
being the case. It is found that even chickens,
which have been hatched by artificial heat,
and which could never have been instructed
by the parent, are yet guided by a natural in-
stinct in the choice of the proper materials for
food, and for assisting its digestion ; and if a
mixture of a large quantity of stones with a
small proportion of grain be set before them,
they will at once pick out the grain, and swallow
along with it only the proper proportion of stones.
The best proof of the utility of these substances

GIZZARDS OF BIRDS. 173

may be derived from the experiments of Spal-
lanzani himself, who ascertained that grain is
not digested in the stomachs of birds, when it is
protected from the effects of trituration. 'i

Thus the gizzard may, as Hunter remarks, be
regarded as a pair of jaws, whose teeth are taken
in occasionally to assist in this internal mastica-
tion. The lower part of the gizzard consists of
a thin muscular bag, of which the office is to
digest the food that has been thus triturated.

Considerable differences are met with in the
structure of the gizzards of various kinds of
birds, corresponding to differences in the texture
of their natural food. In the Turkey, the two
muscles which compose the gizzard are of un-
equal strength, that on the left side being consi-
derably larger than that on the right ; so that
while the principal effort is made by the former,
a smaller force is used by the latter to restore
the parts to their situation. These muscles pro-
duce, by their alternate action, two effects ; the
one a constant trituration, by a rotatory motion ;
the other a continued, but oblique, pressure of
the contents of the cavity. As this cavity is of
an oval form, and the muscle swells inwards, the
opposite sides never come into contact ; and the
interposed materials are triturated by their being
intermixed with hard bodies. In the Goose and
Swan, on the contrary, the cavity is flattened,
and its lateral edges are very thin. The surfaces

174 THE VITAL FUNCTIONS.

applied to each other are mutually adapted in
their curvatures ; a concave surface being every
where applied to one which is convex : on the
left side, the concavity is above ; but on the
right side, it is below. The horny covering is
much stronger, and more rough than in the
turkey ; so that the food is ground by a sliding,
instead of a rotatory motion of the parts opposed ;
and they do not require the aid of any inter-
vening hard substances of a large size. This
motion bears a great resemblance to that of
the grinding teeth of ruminating animals, in
which the teeth of the under jaw slide upwards,
within those of the upper, pressing the food be-
tween them, and fitting it, by this peculiar kind
of trituration, for being digested.*

§ 6. Deglutition.

The great object of the apparatus which is to
prepare the food for digestion, is to reduce it
into a soft pulpy state, so as to facilitate the
chemical action of the stomach upon it: for
this purpose, solid food must not only be sub-
jected to mechanical trituration, but it must
also be mixed with a certain proportion of fluid.
Hence all animals that masticate their food are

* Home, Phil. Trans, for 1810, p. 188.

SALIVARY APPARATUS. 175

provided with organs which secrete a fluid, called
the Saliva, and which pour this fluid into the
mouth as near as possible to the grinding sur-
faces of the teeth. These organs are glands,
placed in such a situation as to be compressed
by the action of the muscles which move the
jaw, and to pour out the fluid they secrete in
greatest quantity, just at the time when the food
is undergoing mastication. Saliva contains a
large quantity of water, together with some salts
and a little animal matter. Its use is not only
to soften the food, but also to lubricate the pas-
sage through which it is to be conveyed into the
stomach ; and the quantity secreted has always
a relation to the nature of the food, the degree
of mastication it requires, and the mode in which
it is swallowed. In animals which subsist on
vegetable materials, requiring more complete
maceration than those which feed on flesh, the
salivary glands are of large size : they are parti-
cularly large in the Rodentia, which feed on the
hardest materials, requiring the most complete
trituration ; and in these animals we find that the
largest quantity of saliva is poured out opposite
to the incisor teeth, which are those principally
employed in this kind of mastication. In Birds
and Reptiles, which can hardly be said to mas-
ticate their food, the salivary glands are compa-
ratively of small size ; the exceptions to this rule
occurring chiefly in those tribes which feed on

176 THE VITAL FUNCTIONS.

vegetables ; for in these the glands are more con-
siderable.* In Fishes there is no structure of this
kind provided, there being no mastication per-
formed ; and the same observation applies to the
Cetacea. In the cephalopodous and gastero-
podous Mollusca, we find a salivary apparatus
of considerable size: Insects, and the Annelida,'^
also, generally present us with organs which
appear to perform a similar office.

The passage of the food along the throat is
facilitated by the mucous secretions, which are
poured out from a multitude of glands inter-
spersed over the whole surface of the membrane
lining that passage. The Camel, which is formed
for traversing dry and sandy deserts, where the
atmosphere as well as the soil is parched, is spe-
cially provided with a glandular cavity, placed
behind the palate, and which furnishes a fluid
for the express purpose of moistening and lubri-
cating the throat.

In the structure of the (Esophagus, which is
the name of the tube along which the food
passes from the mouth to the stomach, we may
trace a similar adaptation to the particular kind
of food taken in by the animal. When it is
swallowed entire, or but little changed, the

* The large salivary gland in the woodpecker, is seen at s.
Fig. 271, page 132.

t The bunch of filaments, seen at s, Fig. 260 (p. 103) are
the salivary organs of the leech.

DEGLUTITION. 177

oesophagus is a very wide canal, admitting of
great dilatation. This is the case with many-
carnivorous birds, especially those that feed on
fishes, where its great capacity enables it to
hold, for a considerable time, the large fish which
are swallowed entire, and which could not con-
veniently be admitted into the stomach. Bhi-
menbach relates that a sea-gull, which he kept
alive for many years, could swallow bones of
three or four inches in length ; so that only
their lower ends reached the stomach, and were
digested ; while their upper ends projected into
the oesophagus, and descended gradually in
proportion as the former were dissolved. Ser-
pents, which swallow animals larger than them-
selves, have, of course, the oesophagus, as well
as the throat, capable of great dilatation ; and
the food occupies a long time in passing through
it, before it reaches the digesting cavity. The
turtle has also a capacious oesophagus, the inner
coat of which is beset with numerous firm and
sharp processes, having their points directed
towards the stomach : these are evidently in-
tended to prevent the return of the food into the
mouth. Grazing quadrupeds, which, while they
eat, carry their heads close to the ground, have
a long oesophagus, with thick muscular coats,
capable of exerting considerable power in pro-
pelling the food in the direction of the stomach,
which is contrary to that of gravity.

VOL. II. N

178 THE VITAL FUNCTIONS.

§ 7. Receptacles for retaining Food.

Provision is often made for the retention of the
undigested food in reservoirs, situated in different
parts of the mouth, or the oesophagus ; instead of
its being immediately introduced into the sto-
mach. These reservoirs are generally employed
for laying in stores of provisions for future
consumption. Many quadrupeds have cheek
pouches for this purpose : this is the case with
several species of Monkeys and Baboons ; and
also with the Mus cricetus, or Hamster. The
Mus bursarius, or Canada rat, has enormous
cheek pouches, which, when distended with food,
even exceed the bulk of the head. Small cheek
pouches exist in that singular animal, the Orni-
thorhyncJms. The Sciurus palmarmn, or Palm
squirrel, is also provided with a pouch for laying
in a store of provisions. A remarkable dilatation
in the lower part of the mouth and throat,
answering a similar purpose, takes place in the
Pelican; a bird which displays great dexterity
in tossing about the fish with which it has
loaded this bag, till it has brought it into the
proper position for being swallowed. The Whale
has also a receptacle of enormous size, extending
from the mouth to a considerable distance under
the trunk of the body.

RECEPTACLES FOR RETAINING FOOD.

17.9

Analogous in design to these pouches are
the dilatations of the oesophagus of birds, deno-
minated crops. In most birds which feed on
grain, the crop is a capacious globular sac,
placed in front of the throat, and resting on the
furcular bone. The crop of the Parrot is repre-
sented at c. Fig. 299 ; where, also, s indicates

the cardiac portion of the
stomach, and g the giz-
zard, of that bird. The
inner coat of the crop is
furnished with numerous
glands, supplying consi-
derable quantities of fluid
for macerating and sof-
tening the dry and hard
texture of the grain,
which, for that purpose,
remains there for a considerable time. Many
birds feed their young from the contents of
the crop ; and, at those seasons, its glands are
much enlarged, and very active in preparing
their peculiar secretions : this is remarkably
the case in the Pigeon, which, instead of a
single sac, is provided with two, (seen at c, c.
Fig. 300), one on each side of the oesophagus (o).
The pouting pigeon has the faculty of filling
these cavities with air ; producing that dis-
tended appearance of the throat from which it
derives its name. Birds of prey have, in general.

180 THE VITAL FUNCTIONS.

very small crops, their food not requiring any
previous softening ; but the Vulture^ which
gorges large quantities of flesh at a single meal,
has a crop of considerable size, forming, when
filled, a visible projection in front of the chest.
Birds which feed on fish have no separate dila-
tation for this purpose, probably because the
great width of the oesophagus, and its having the
power of retaining a large mass of food, render
the further dilatation of any particular part of
the tube unnecessary. The lower portion of the
oesophagus appears often, indeed, in this class of
animals, to answer the purpose of a crop, and to
effect changes in the food which may properly
be considered as a preliminary stage of the
digestive process.

Chapter VII.

Digestion.

All the substances received as food into the
stomach, whatever be their nature, must neces-
sarily undergo many changes of chemical com-
position before they can gain admission into the
general mass of circulating fluids ; but the extent
of the change required for that purpose will, of
course, bie in proportion to the diflference be-

DIGESTION. 181

tween the qualities of the nutritive materials in
their original, and in their assimilated state.
The conversion of vegetable into animal matter
necessarily implies a considerable modification
of properties ; but even animal substances, how-
ever similar may be their composition to the
body which they are to nourish, must still pass
through certain processes of decomposition, and
subsequent recombination, before they can be
brought into the exact chemical state in which
they are adapted to the purposes of the living
system.

The preparatory changes we have lately been
occupied in considering, consist chiefly in the
reduction of the food to a soft consistence, which
is accomplished by destroying the cohesion of
its parts, and mixing them uniformly with the
fluid secretions of the mouth ; effects which may
be considered as wholly of a mechanical nature.
The first real changes in its chemical state are
produced in the stomach, where it is converted
into a substance termed Chyme; and the process
by which thi& first step in the assimilation of the
food is produced, constitutes what is properly
termed I>igestion.

Nothing has been discovered in the anato-
mical structure of the stomach tending to throw
any light on the means by which this remark-
able chemical change is induced on the materials
it contains. The stomach is in most animals

182 THE VITAL FUNCTIONS.

a simple sac, composed of several membranes,
enclosing thin layers of muscular fibres, abun-
dantly supplied with blood-vessels and with
nerves, and occasionally containing structures
which appear to be glandular. The human sto-
mach, which is delineated in Fig. 301, exhibits

one of the simplest forms of this organ ; c being
the cardiac portion, or part where the oesopliagus
opens into it ; and p the pyloric portion, or that
which is near its termination in the intestine.
At the pylorus itself, the diameter of the pas-
sage is much constricted, by a fold of the inner
membrane, which is surrounded by a circular
band of muscular fibres, performing the office of
a sphincter, and completely closing the lower
orifice of the stomach, during the digestion of
its contents.

The principal agent in digestion, as far as the
ordinary chemical means are concerned in that
operation, is a fluid secreted by the coats of the

DIGESTION. 183

stomach, and termed the Gastric juice. This
fluid has, in each animal, the remarkable pro-
perty of dissolving, or at least reducing to a
pulp, all the substances which constitute the na-
tural food of that particular species of animal ;
while it has comparatively but little solvent
ix>wer over other kinds of food. Such is the
conclusion which has been deduced from the
extensive researches on this subject made by
that indefatigable experimentalist, Spallanzani,
who found in numberless trials that the gastric
juice taken from the stomach, and put into glass
vessels, produced, if kept at the usual tempera-
ture of the animal, changes to all appearance
exactly similar to those which take place in
natural digestion.* In animals which feed on
flesh, the gastric juice was found to dissolve only
animal substances, and to exert no action on
vegetable matter ; while, on the contrary, that
taken from herbivorous animals, acted on grass
and other vegetable substances, without pro-

* The accuracy of this conclusion has been lately contested
by M. De Montegre, whose report of the effects of the gastric
juice of animals out of the body, does not accord with that of
Spallanzani; but the difference of circumstances in which his
experiments were made, is quite sufficient to account for the
discrepancy in the results; and those of M. De Montegre,
therefore, by no means invalidate the general facts slated in
the text, which have been established by the experiments, not
only of Spallanzani, but also of Reaumur, Stevens, Leuret, and
Lassaigne. See Alison's Outlines of Physiology and Pathology,
p. 170.

184 THE VITAL FUNCTIONS.

ducing any effect on flesh ; but in those animals,
which, like man, are omnivorous, that is, par-
take indiscriminately of both species of aliment,
it appeared to be fitted equally for the solution
of both. So accurate an adaptation of the che-
mical powers of a solvent to the variety of sub-
stances employed as food by different animals,
displays, in the most striking manner, the vast
provision of nature, and the refined chemistry
she has put in action for the accomplishment
of her different purposes.

In the stomachs of many animals, as also in
the human, it is impossible to distinguish with
any accuracy the organization by which the
secretion of the gastric juice is effected : but
where the structure is more complex, there may
be observed a number of glandular bodies inter-
spersed in various parts of the internal coats of
the stomach. These, which are termed the
Gastric glands, are distributed in various ways
in different instances : they are generally found
in greatest number, and often in clusters, about
the cardiac orifice of the stomach ; and they are
frequently intermixed with glands of another
kind, which prepare a mucilaginous fluid, serving
to protect the highly sensible coats of the sto-
mach from injurious impressions. These latter
are termed the mucous glands, and they are often
constructed so as to pour their contents into
intermediate cavities, or small sacs, which are

DIGESTION.

185

denominated /(>///cZe5, where the fluid is collected
before it is discharged into the cavity of the sto-
mach. The gastric glands of birds are larger
and more conspicuous than those of quadrupeds;
but, independently of those which are situated
in the stomach, there is likewise found, in
almost all birds, at the lower termination of the
oesophagus, a large glandular organ, which has
been termed the biilhulus glandulosus. In the
Ostrich, this organ is of so great a size as to give
it the appearance of a separate stomach. A
view of the internal surface of the stomach of
the African ostrich is given in Fig. 302 ; where

303

304

c is the cardiac cavity, the coats of which are
studded with numerous glands ; g, g, are the
two sides of the gizzard. Fig. 303 shows one of
the gastric glands of the African ostrich ; Fig.

18(5 THE VITAL FUNCTIONS.

304, a gland from the stomach of the American
ostrich ; and Fig. 305, a section of a gastric
gland in the beaver, showing the branching of
the ducts, which form three internal' openings.
In birds that live on vegetable food, the structure
of the gastric glands is evidently different from
that of the corresponding glands in predaceous
birds ; but as these anatomical details have not
as yet tended to elucidate in any degree the pur-
poses to which they are subservient in the pro-
cess of digestion, I pass them over as being
foreign to the object of our present inquiry.*

It is essential to the perfect performance of
digestion, that every part of the food received
into the stomach should be acted upon by the
gastric juice ; for which purpose provision is
made that each portion shall, in its turn, be
placed in contact with the inner surface of that
organ. Hence the coats of the stomach are
provided with muscular fibres, passing, some in
a longitudinal, and others in a transverse, or
circular direction ; while a third set have an
oblique, or even spiral course. t When the
greater number of these muscles act together,

* These structures have been examined with great care and
minuteness by Sir Everard Home, who has given the results of
his inquiries in a series of papers, read from time to time to the
Royal Society, and published in their Transactions.

t See Fig. 51, vol. i. p. 137, and its description, p. 138.

DIGESTION. 187

tliey exert a considerable pressure upon the
contents of the stomach ; a pressure which, no
doubt, tends to assist the solvent action of the
gastric juice. When different portions act in
succession, they propel the food from one part
to another, and thus promote the mixture of
every portion with the gastric juice. We often
find that the middle transverse bands contract
more strongly than the rest, and continue con-
tracted for a considerable time. The object
of this contraction, which divides the stomach
into two cavities, appears to be to separate its
contents into two portions, so that each may
be subjected to different processes; and, indeed,
the differences in structure, which are often
observable between these two portions of the
stomach, would lead to the belief that their func-
tions are in some respects different.

During digestion the exit of the food from the
stomach into the intestine is prevented by the
pylorus being closed by the action of its sphinc-
ter muscle. It is clear that the food is required
to remain for some time in the stomach in order
to be perfectly digested ; and this closing of the
pylorus appears to be one means employed for
attaining this end ; and another is derived from
the proper! 3^ which the gastric juice possesses of
coagulating, or rendering solid, every animal or
vegetable fluid susceptible of undergoing that

188 THE VITAL FUNCTIONS.

change. This is the case with fluid albumen :
the white of an egg, for instance, which is
nearly pure albumen, is very speedily coagu-
lated when taken into the stomach ; the same
change occurs in milk, which is immediately
curdled by the juices that are there secreted;
and these effects take place quite independently
of any acid that may be present. The object
of this change from fluid to solid appears to be
to detain the food for some time m the stomach,
and thus to allow of its being thoroughly acted
upon by the digestive powers of that organ.
Those fluids which pass quickly through the
stomach, and thereby escape its chemical action,
however much they may be in themselves nu-
tritious, are very imperfectly digested, and con-
sequently afford very little nourishment. This
is the case with oils, with jelly, and with all
food that is much diluted.* Hunter ascer-

* A diet consisting of too large a proportion of liquids,
although it may contain much nutritive matter, yet if it be
incapable of being coagulated by the stomach, will not be
sufficiently acted upon by that organ to be properly digested,
and will not only afford comparatively little nourishment, but be
very liable to produce disorder of the alimentary canal. Thus
soups will not prove so nutritive when taken alone, as when
they are united with a certain proportion of solid food, capable
of being detained in the stomach, during a time sufficiently long
to allow of the whole undergoing the process of digestion. I was
led to this conclusion, not only from theory, but from actual

DIGESTION. 189

tained that this coagulating power belongs to
the stomach of every animal, which he exa-
mined for that purpose, from the most perfect
down to reptiles.* Sir E. Home has prosecuted
the inquiry with the same result, and ascertained
that this property is possessed by the secretion
from the gastric glands, which communicates it
to the adjacent membranes.t

The gastric juice has also the remarkable
property of correcting putrefaction. This is par-
ticularly exemplified in animals that feed on
earrion, to whom this property is of great im-
portance, as it enables them to derive wholesome
nourishment from materials which would other-
wise taint the whole system with their poison,
and soon prove destructive to life.

observation of what took place among the prisoners in the Mil-
bank Penitentiary, in 1823, when, on the occasion of the extensive
prevalence of scorbutic dysentery in that prison, Dr. P. M. Latham
and myself were appointed to attend the sick, and inquire into
the origin of the disease. Among the causes which concurred
to produce this formidable malady, one of the most prominent
appeared to be an impoverished diet, consisting of a large
proportion of soups, on which the prisoners had subsisted for the
preceding eight months. A very full and perspicuous account
of that disease has been drawn up, with great ability, by my
friend Dr. P. M. Latham, and published under the title of "An
Account of the disease lately prevalent in the General Peniten-
tiary." London, 1825.

* Observations on the Animal Economy, p. 172.

t Phil. Trans, for 1813, p. 96.

190 THE VITAL FUNCTIONS.

It would appear that the first changes which
constitute digestion take place principally at
the cardiac end of the stomach ; and that the
mass of food is gradually transferred towards
the pylorus ; the process of digestion still con-
tinuing as it advances. In the Rabbit it has
been ascertained that food , newly taken into
the stomach is always kept distinct from that
which was before contained in it, and which
has begun to undergo a change : for this pur-
pose the new food is introduced into the centre
of the mass already in the stomach ; so that
it may come in due time to be applied to the
coats of that organ, and be in its turn digested,
after the same change has been completed in
the latter.*

As the flesh of animals has to undergo a less
considerable change than vegetable materials,
so we find the stomachs of all the purely carni-
vorous tribes consisting only of a membranous
bag, which is the simplest form assumed by
this organ. But in other cases, as we have
already seen, the stomach exhibits a division
into two compartments, by means of a slight
contraction ; a condition which, as Sir E. Home
has remarked, is sometimes found as a tem-

* See Dr. Philip's Experimental Enquiry into the Laws of
the Vital Functions, 3d edition, p. 122.

STOMACHS OF MAMMALIA.

191

porary state of the human stomach ; * while,
in other animals, it is its natural and per-
manent conformation- The Ilodentia furnish
many examples of this division of the cavity
into two distinct portions, which exhibit even
differences in their structure : this is seen in the
Dormouse, (Fig. 306) the Beaver, the Hare, the
Rabbit, and the Cape Hyrax, (Fig. 307). The
first, or cardiac portion, is often lined with

cuticle, while the lower portion is not so lined ;
as is seen very conspicuously in the stomachs
of the Solipeda. The stomach of the Horse, in
particular, is furnished at the cardia, with a

* The figure given of the human stomach, p. 182, shows it in
the state of partial contraction here described.

192

THE VITAL FUNCTIONS.

spiral fold of the inner, or cuticular membrane,
which forms a complete valve, offering no impe-
diment to the entrance
311 ><^WMM^!5s^ of food from the oeso-

phagus, but obstruct-
ing the return of any
part of the contents of
the stomach into that
passage.* This valve
is shown in Fig. .'511,
which represents an
inner view of the car-
diac portion of the sto-
mach of the horse ; o
being the termination of the oesophagus.

The stomach of the Water Rat is composed
of two distinct cavities, having a narrow passage
of communication : the first cavity is lined with
cuticle, and is evidently intended for the mace-
ration of the food before it is submitted to the
agents which are to effect its digestion ; a process
which is completed in the second cavity, pro-
vided, for that purpose, with a glandular surface.
In proportion as nature allows of greater lati-
tude in diet, we find her providing greater com-
plication in the digestive apparatus, and subdi-
viding the stomach into a greater number of

* The total inability of a horse to vomit is probably a conse-
quence of the impediment presented by this valve. See Mem.
du Museum d'Hist. Nat. viii. 111.

.STOMACHS OF MAMMALIA. 193

cavities, each having probably a separate office
assigned to it, though concurring in one general
effect. A gradation in this respect may be
traced through a long line of quadrupeds, such
as the Hog, the Peccariyilie Porcupiuey(Fig/SOii),
and the Hippopotamus, where we find the number
of separate pouches for digestion amounting to
four or five. Next to these we may rank the
very irregular stomach of the Kanguroo, (Fig.
309) composed of a multitude of cells, in which
the food probably goes through several prepa-
ratory processes ; and still greater complication
is exhibited by the stomachs of the Cetacea, as,
for example, in that of the Porpus (Fig. 310).
As the fishes upon which this animal feeds are
swallowed whole, and have large sharp bones,
which would injure any surface not defended by
cuticle, receptacles are provided, in which they
may be softened and dissolved, and even con-
verted into nourishment by themselves, and
without interfering with the digestion of the soft
parts. The narrow communications between
these several stomachs of the Cetacea are pro-
bably intended to ensure the thorough solution
of their contents, by preventing the exit of all
such portions as have not perfectly undergone
this process.

Supernumerary cavities of this kind, be-
longing to the stomach, are more especially
provided in those animals which swallow food

VOL. II. o

194

THE VITAL FUNCTIONS.

either in larger quantity than is immediately
wanted, or of a nature which requires much pre-
paration previous to digestion. The latter is more
particularly the case with the horned ruminant
tribes that feed on the leaves or stalks of vege-
tables ; a kind of food, which, in proportion to its
bulk, affords but little nutriment, and requires,
therefore, a long chemical process and a compli-
cated digestive apparatus, in order to extract from
it the scanty nutritious matter it contains, and
prepare it for being applied to the uses of the
system. This apparatus is usually considered
as consisting of four stomachs ; and in order to
convey a distinct idea of this kind of structure I
have selected for representation, (in Fig. 3 12), that

of the Sheep, of which the four stomachs are
marked by the numbers 1, 2, 3, 4, respectively,
in the order in which they occur when traced
from the oesophagus (c) to the intestine (p).

STOMACHS OF RUMINANTS. 195

The grass which is devoured in large quan-
tities by these animals, and which undergoes
but little mastication in the mouth, is hastily
swallowed, and is received into a capacious
reservoir, (marked 1 in the figure,) called the
paunch. This cavity is lined internally with a
thick membrane, beset with numerous flattened
papillae, and is often divided into pouches by
transverse contractions. While the food remains
in this bag, it continues in rather a dry state ;
but the moisture with which it is surrounded
contributes to soften it, and to prepare it for a
second mastication ; which is effected in the
following manner. Connected with the paunch
is another, but much smaller sac (2), which is
considered as the second stomach ; and, from its
internal membrane being thrown into numerous
irregular folds, forming the sides of polygonal
cells, it has been called the Jioneycomh stomach,
or reticule. Fig. 313 exhibits the reticulated
appearance of the inner surface of this cavity.
A singular connexion exists between this sto-
mach and the preceding ; for while the oesophagus
appears to open naturally into the paunch, there
is on each side of its termination, a muscular
ridge which projects from the orifice of the latter,
so that the two together form a channel leading
into the second stomach ; and thus the food can
readily pass from the oesophagus into either of
these cavities, according as the orifice of the one
or the other is open to receive it.

196 THE VITAL FUNCTIONS.

It would appear from the observations of Sir
E. Home, that liquids drunk by the animal pass
at once into the second stomach, the entrance
into the first being closed. The food contained
in the paunch is transferred, by small portions
at a time, into this second, or honey-comb
stomach, in which there is always a supply of
water for moistening the portion of food intro-
duced into it. It is in this latter stomach, then,
that the food is rolled into a ball, and thrown up,
through the oesophagus, into the mouth, where it
is again masticated at leisure, and while the ani-
mal is reposing ; a process which is well known
by the name of chewing the cud, or ruinination.

When the mass, after being thoroughly ground
down by the teeth, is again swallowed, it passes
along the oesophagus into the third stomach (3) ;
the orifice of which is brought forwards by
the muscular bands, forming the two ridges
already noticed, which are continued from the
second stomach, and which, when they con-
tract, effectually prevent any portion of the
food from dropping into either of the preceding
cavities. In the Ox, this third stomach is dcr
scribed by Sir E. Home as having the form
of a crescent, and as containing twenty-four
septa, or broad folds of its inner membrane.
These folds are placed parallel to one another,
like the leaves of a book ; excepting that they
are of unequal breadths, and that a narrower
fold is placed between each of the broader ones.

STOMACHS OF RUMINANTS. 197

Fig. 3 1 4 represents this plicated structure in the
interior of the third stomach of a bullock.
Whatever food is introduced into this cavity,
which is named, from its foliated structure, the
many-plies stomach, must pass between these
folds, and describe three-fourths of a circle,
before it can arrive at the orifice leading to the
fourth stomach, which is so near that of the
third, that the distance between them does not
exceed three inches. There is, however, a more
direct channel of communication between the
oesophagus and the fourth stomach (4), along
which milk taken by the Calf, and which does
not require to be either macerated or ruminated,
is conveyed directly from the cesophagus to this
fourth stomach ; for at that period the folds of
the many-plies stomach are not yet separated,
and adhere closely together ; and in these ani-
mals rumination does not take place, till they
begin to eat solid food. It is in this fourth
stomach, which is called the reed, that the proper
digestion of the food is performed, and it is here
that the coagulation of the milk takes place ; on
which account the coats of this stomach are
employed in dairies, under the name of rennet,
to obtain curd from milk.

A regular gradation in the structure of rumi-
nating stomachs may be traced in the different
genera of this family of quadrupeds. In rumi-
nants with horns, as the Bullock and the Sheep,
there are two preparatory stomachs for retaining

198 THE VITAL FUNCTIONS.

the food previous to rumination, a third for
receiving it after it has undergone this process,
and a fourth for effecting its digestion. Rumi-
nants without horns, as the Camel, Dromedary,
and Lama, have only one preparatory stomach
before rumination, answering the purpose of the
two first stomachs of the bullock ; a second,
which I shall presently notice, and which takes
no share in digestion, being employed merely as
a reservoir of water; a third, exceedingly small,
and of which the office has not been ascertained ;
and a fourth, which receives and digests the food
after rumination. Those herbivorous animals
which do not ruminate, as the Horse and Ass,
have only one stomach ; but the upper portion
of it is lined with cuticle, and appears to per-
form some preparatory office, which renders the
food more easily digestible by the lower portion
of the same cavity.*

The remarkable provision above alluded to
in the Camel, an animal which nature has
evidently intended as the inhabitant of the
sterile and arid regions of the East, is that of
reservoirs of water, which, when once filled,
retain their contents for a very long time, and
may minister not only to the wants of the animal
that possesses it, but also to those of man. The
second stomach of the Camel has a separate

* Home, Phil. Trans. 8vo. 1806, p. 370.

DIGESTION. 199

compartment, to which is attached a series of
cellular appendages, (exhibited on a small scale,
in Fig. 315) : in these the water is retained by
strong muscular bands, which close the orifices
of the cells, while the other portions of the
stomach are performing their usual functions.
By the relaxation of these muscles, the water is
gradually allowed to mix with the contents of
the stomach ; and thus the Camel is enabled to
support long marches across the desert without
receiving any fresh supply. The Arabs, who
traverse those extensive plains, accompanied by
these useful animals, are, it is said, sometimes
obliged, when faint, and in danger of perishing
from thirst, to kill one of their camels, for the
sake of the water contained in these reservoirs,
which they always find to be pure and wholesome.
It is stated by those who have travelled in Egypt,
that camels, when accustomed to go journeys,
during which they are for a long time deprived
of water, acquire the power of dilating the cells,
so as to make them contain a more than ordi-
nary quantity, as a supply for their journey.*

When the Elephant, while travelling in very
hot weather, is tormented by insects, it has been
observed to throw out from its proboscis, directly
upon the part on which the flies fix themselves,
a quantity of water, with such force as to dislodge

* Home, Lectures on Comparative Anatomy, vol. i. p. 171.

200 THE VITAL FUNCTIONS.

them. The quantity of water thrown out, is in
proportion to the distance of the part attacked,
and is commonly half a pint at a time: and
this, Mr. Pierard, who resided many years in
India, has known the elephant repeat, eight or
ten times within the hour. This water is not
only ejected immediately after drinking, but
six or eight hours afterwards. The quantity of
water at the animal's command for this pur-
pose, observes Sir E. Home, cannot be less
than six quarts ; and on examining the struc-
ture of the stomach of that animal, he found
in it a cavity, like that of the camel, per-
fectly well adapted to afford this occasional
supply of water, which may probably, at other
times, be employed in moistening dry food for
the purposes of digestion.*

In every series of animals belonging to other
classes, a correspondence may be traced, as has
been done in the Mammalia, between the nature
of the food and the conformation of the diges-
tive organs. The stomachs of birds, reptiles
and fishes, are, with certain modifications,
formed very much upon the models of those
already described ; according as the food con-
sists of animal or of vegetable materials, or
presents more or less resistance from the co-
hesion of its texture. As it would be impos-

» Supplement to Sir E. Home's Lectures on Comparative
Anatomy, vol. vi. p. 9.

DIGESTION IN BIRDS. 201

sible in this place to enter into all the details
necessary for fully illustrating this proposition,
I must content myself with indicating a few of
the most general results of the inquiry.*

As the food of Birds varies, in different spe-
cies, from the softest animal matter to the
hardest grain, so we observe every gradation in
their stomachs, from the membranous sac of the
carnivorous tribes, which is one extreme, to the
true gizzard of granivorous birds, which occu-
pies the other extremity of the series. This
gradation is established by the muscular fibres,
which surround the former, acquiring, in dif-
ferent tribes, greater extent, and forming stronger
muscles, adapted to the corresponding variations
in the food ; more especially in as far as it par-
takes of the animal or the vegetable character.

In all the cold-blooded vertebrata, where di-
gestion is not assisted by any internal heat, that
operation proceeds more slowly, though in the
end not less effectually, than in animals where
the contents of the stomach are constantly main-
tained at a high temperature. They almost all
rank as carnivorous animals ; and have accord-

* The comparative anatomy of the stomach has been investi-
gated with great diligence by the late Sir E, Home, and the
results recorded in the papers he communicated from time to
time to the Royal Society, and which have been republished in
his splendid work, entitled *' Lectures on Comparative Anatomy,"
to which it will be seen that I have been largely indebted for the
facts and observations relating to this subject, detailed in the text.

202 THE VITAL FUNCTIONS.

ingly stomachs, which, however they may vary
in their form, are alike simply membranous in
their structure, and act by means of the solvent
power of their secretions. Among Reptiles, only
a few exceptions occur to this rule. The com-
mon Sea-Turtle which is brought to our tables,
is one of these ; for it is found to feed exclu-
sively on vegetable diet, and chiefly on the sea-
weed called zostira mavitima; but though very
muscular, it has not the cuticular lining which
forms an essential character of a gizzard. Some
Tortoises, also, which eat grass, make an ap-
proach to the same structure.

In Fishes, indeed, although the membranous
structure of the stomach invariably accompanies
the habit of preying upon other fish, yet there is
one species of animal food, namely, shell-fish,
which requires to be broken down by powerful
means before it can be digested. In many fish,
which consume food of this kind, its trituration
is effected by the mouth, which is, for this pur-
pose, as I have already noticed in the Wolf-fish,
armed with strong grinding teeth. But in
others, an apparatus similar to that of birds is
employed ; the ofiice of mastication being trans-
ferred to the stomach. Thus the Mullet has a
stomach endowed with a degree of muscular
power, adapting it, like the gizzard of birds, to
the double office of mastication and digestion ;
and the stomach of the Gillaroo trout, a fish

DIGESTION IN FISHES 203

peculiar to Ireland, exhibits nearly the same
structure as that of the turtle. The common
Trout^ also, occasionally lives upon shell-fish, and
swallows stones to assist in breaking the shells.

Among the invertebrated classes we occa-
sionally meet with instances of structures ex-
ceedingly analogous to a gizzard, and probably
performing the same functions. Such is the
organ found in the Sepia : the Earth-worm has
both a crop and a gizzard ; and Insects offer
numerous instances, presently to be noticed, of
great complexity in the structure of the stomach,
which is often provided, not only with a me-
chanism analogous to a gizzard, but also with
rows of gastric teeth.

Chapter VIII.

Cliylification.

The formation of Chyle, or the fluid which is
the immediate and exclusive source of nutriment
to the system, takes place in the intestinal tube,
into which the chyme prepared by the stomach
is received, and where further chemical changes
are effected in its composition. The mode in
which the conversion of chyme into chyle is
accomplished, and indeed the exact nature of the
changes themselves, being, as yet, very imper-

204 THE VITAL FUNCTIONS.

fectly known, it is consequently impossible to
trace distinctly the correspondence which, in all
cases, undoubtedly exists between the objects
to be answered and the means employed for
their attainment. No doubt can be entertained
of the importance of the functions which are
performed by structures so large and so compli-
cated, as those composing the alimentary canal
and its various appendages. We plainly per-
ceive that provision is made, in the interior of
that canal, for subjecting its contents to the
action, first, of an extensive vascular and nervous
surface ; and secondly, of various fluid secretions,
derived from different sources, and exercising
powerful chemical agencies on the digested
aliment ; that a muscular power is supplied, by
means of the layers of circular and longitudinal
fibres, contained between the outer and inner
coats of the intestine,* for exerting a certain
pressure on their contents, and for propelling
them forwards by a succession of contractions,
which constitute what is termed their peristaltic
motion; and lastly, that contrivances are at the
same time resorted to for retarding the progress
of the aliment in its passage along the canal, so
that it may receive the full action of these several
agents, and yield the utmost quantity of nutri-
ment it is capable of affording.

* See vol. i. p. 137.

CHYLIFICATION. 205

The total length of the intestinal tube differs
much in different animals ; being, in general, as
already stated, smaller in the carnivorous tribes
than in those which feed on substances of diffi-
cult digestion, or affording but little nourishment.
In these latter animals, the intestine is always of
great length, exceeding that of the body many
times; hence it is obliged to be folded into a
spiral or serpentine course ; forming many con-
volutions in the abdominal cavity. Sometimes,
probably for greater convenience of package,
instead of these numerous convolutions, a similar
effect of increasing the surface of the inner
membrane is obtained by raising it into a great
number of folds, which project into the cavity.
These folds are often of considerable breadth ;
contributing not only to the extension of the
surface for secretion and absorption, but also to
the detention of the materials, with a view to
their more complete elaboration. Remarkable
examples of this kind of struc-
ture occur in most of the carti-
laginous fishes ; the inner coat
of the large intestine being ex-
panded into a broad fold, which,
as is seen in Fig. 316, repre-
senting this structure in the in-
terior of the intestine of the
Shark, takes a spiral course ; and
this is continued nearly the whole

206 THE VITAL FUNCTIONS.

length of the canal, so that the internal surface
is much augmented without any increase in the
length of the intestine.*

When the nature of the assimilatory process
is such as to require the complete detention of
the food, for a certain time, in particular situa-
tions, we find this object provided for by means
of caca, or separate pouches, opening laterally
from the cavity of the intestine, and having no
other outlet. Structures of this description have
already been noticed in the Infusoria f ; and they
are met with, indeed, in animals of every class,
occurring in various parts of the alimentary tube,
sometimes even as high as the pyloric portion of
the stomach, and frequently at the commence-
ment of the small intestine. Their most usual
situation, however, is lower down, and especially
at the part where the tube, after having remained
narrow in the first half of its course, is dilated
into a wider cavity ; which is distinguished from
the former by the appellation of the great intes-
tine, and which is frequently more capacious
than the stomach itself. It is exceedingly pro-
bable that these two portions of the canal per-
form different functions in reference to the

* Structures of this description have a particular claim to
attention from the light they throw on the nature of several
fossil remains, lately investigated with singular success by Dr.
Buckland.

t Page 96, of this volume.

CHYLIFICATION. 207

assimilation of the food : but hitherto no clue
has been discovered to guide us through the
intricacies of this difficult part of physiology ;
and we can discern little more than the ex-
istence, already mentioned, of a constant rela-
tion between the nature of the aliment and the
structure of the intestines, which are longer,
more tortuous, and more complicated, and are
furnished with more extensive folds of the inner
membrane, and with larger and more numerous
caeca, in animals that feed on vegetable sub-
stances, than in carnivorous animals of the same
class.

The class of Insects supplies numberless
exemplifications of the accurate adaptation of
the structure of the organs of assimilation to the
nature of the food which is to be converted into
nutriment ; and also of the general principle
that vegetable aliment requires, for this purpose,
longer processes, and a more complicated appa-
ratus than that which has been already ani-
malized. In the herbivorous tribes, we find the
oesophagus either extremely dilatable, so as to
serve as a crop, or receptacle for containing the
food previously to its digestion, or having a dis-
tinct pouch appended to it for the same object ;
to this there generally succeeds a gizzard, or ap-
paratus for trituration, furnished, not merely with
a hard cuticle, as in birds, but also with numerous
rows of teeth, of various forms, answering most

208 THE VITAL FUNCTIONS.

effectually the purpose of dividing, or grinding
into the minutest fragments, all the harder parts
of the food ; and thus supplying any deficiency
of power in the jaws for accomplishing the
same object. Thence the aliment, properly
prepared, passes into the cavity appropriated
for its digestion, which constitutes the true sto-
mach.* In the lower part of this organ a pecu-
liar fluid secretion is often intermixed with it,
which has been supposed to be analogous to the
hile of the higher animals. It is prepared by
the coats of slender tubes, termed hepatic
vessels, which are often of great length, and
sometimes branched or tufted, or beset, like the
fibres of a feather, with lateral rows of filaments ;
and which float loosely in the general cavity of
the body, attached only at their termination,
where they open into the alimentary canahf

* It is often difficult to distinguish the portions of the canal,
which correspond in their functions to the stomach, and to the
first division of the intestines, or duodenum ; so that different
naturalists, according to the views they have taken of the pecu-
liar office of these parts, have applied to the same cavity the
term of chyliferous stomach.^ or of duodenum. See the memoir of
Leon Dufour, in the Annales des Sciences Naturelles, ii. 473.

f The first trace of a secreting structure, corresponding to
hepatic vessels, is met with in the Asterias, where the double row
of minute lobes attached to the caecal stomachs of those animals,
and discharging their fluid into these cavities, are considered by
Carus, as performing a similar office. The flocculent tissue
which surrounds the intestine of the Holothuria, is probably
also an hepatic apparatus.

DIGESTIVE ORGANS OF INSECTS. 209

In some insects these tubes are of larger dia-
meter than in others ; and in many of the Or-
thoptera, as we shall presently see, they open
into large receptacles, sometimes more capacious
than the stomach itself, which have been sup-
posed to serve the purpose of reservoirs of the
biliary secretion ; pouring it into the stomach on
those occasions only when it is particularly
wanted for the completion of the digestive
process.*

The distinction into small and great intestine
is more or less marked, in different insects, in
proportion to the quantities of food consumed,
and to its vegetable nature ; and in herbivorous
tribes, more especially, the dilatations in the
lower part of the canal are most conspicuous,
as well as the duplicatures of the inner mem-
brane, which constitute imperfect valves for
retarding the progress of the aliment. It is
generally at the point where this dilatation of
the canal commences, that a second set of
hepatic vessels is inserted ; having a structure
essentially the same as those of the first set ; but
generally more slender, and uniting into a small
number of ducts before they terminate. The
number and complication of both these sets of
hepatic vessels, appear to have some relation to

* A doubt is suggested, by Leon Dufour, whether the liquid
found in those pouches is real bile, or merely aliment in the pro-
gress of assimilation. Ann. 3c. Nat. ii. 478.
VOL. II. P

210 THE VITAL FUNCTIONS.

the existence and developement of the gizzard,
and consequently also to the nature and bulk of
the food. Vessels of this description are, indeed,
constantly found in insects ; but it is only where
a gizzard exists, that two sets of these secreting
organs are provided ; and in some larvae, remark-
able for their excessive voracity, even three
orders of hepatic vessels are met with.*

A muscular power has also been jDrovided, not
only for the strong actions exerted by the gizzard,
but also for the necessary propulsion, in dif-
ferent directions, of the contents both of the
stomach and intestinal tubes. The muscular
fibres of the latter are distinctly seen to consist
of two sets ; the one passing in a transverse or
circular, and the other in a longitudinal direc-
tion. Glandular structures, analogous to the
mucous follicles of the higher animals, are also
plainly distinguishable in the internal coat of the
canal, more especially of herbivorous insects.!
The whole tract of the alimentary canal is at-
tached to the sides of the containing cavity by a
fine membrane, or peritoneum^ containing numer-
ous air-vessels, or trachece.\

* See the Memoirs of Marcel de Serres, in the Annales du
Museum, xx. 48.

f Lyonet.

X It has been stated by Malpighi and by Swammerdam, and
the statement has been repeated by every succeeding ana-
tomist, that almost all the insects belonging to the tribe of

DIGESTIVE ORGANS OF INSECTS.

211

To engage in a minute description of the end-
less variations in the structure of the digestive
organs, presented in the innumerable tribes
which compose this class of animals, would
be incompatible with the limits of this treatise.
I shall content myself, therefore, with giving a
few illustrations of their prin-
cipal varieties, selected from
those in which the leading
characters of structure are
most strongly marked. I shall,
with this view, exhibit first one
of the simplest forms of the
alimentary organs, as they oc-
cur in the Mantis religiosa,
(Linn.) which is a purely car-
nivorous insect, belonging to
the order of Orthoptera. Fig.
317 represents those of this
insect, freed from their attach-
ments, and separated from the
body. The whole canal, as is
seen, is perfectly straight: it
commences by an oesophagus
(o), of great length, which is succeeded by a

Grylli, possessed the faculty of ruminating their food; but this
error has been refuted by Marcel de Serres, who has offered satis-
factory evidence that in no insect is the food subjected to a true
rumination, or second mastication, by the organs of the mouth.
See Annales du Museum, xx. 51 and 364.

212 THE VITAL FUNCTIONS.

gizzard (g) ; at the lower extremity of this organ
the upper hepatic vessels (b,b), eight in number,
and of considerable diameter, are inserted : then
follows a portion of the canal (d), which may be
regarded either as a digesting stomach, or a
chyliferous duodenum : farther downwards, the
second set of hepatic vessels, (h h), which are
very numerous, but as slender as hairs, are
received ; and after a small contraction (n) there
is again a slight dilatation of the tube (c) before
it terminates.

The alimentary canal of the Ciciridela campes-
tris^ (Lin.) which preys on other insects, is re-
presented in Fig. 318; where we see that the
lower part of the oesophagus (o), is dilated into
a crop (p), succeeded by a small gizzard (g),
which is provided for the purpose of bruising
the elytra, and other hard parts of their victims :
but, this mechanical division being once effected,
we again find the true digesting stomach (s)
simply membranous, and the intestine (i) very
short, but dilated, before its termination, into a
large colon (c). The hepatic vessels (h), of
which, in this insect, there is only one set, ter-
minate in the cavity of the intestine by four
ducts, at the point where that canal commences.

A more complicated structure is exhibited in
the alimentary tube of the Melolontha vulgaris^
or common cockchafFer, which is a vegetable

DIGESTIVE ORGANS OF INSECTS. 213

feeder, devouring great quantities of leaves of
plants, and consequently requiring a long and
capacious canal for their assimilation ; as is
shown in Fig. 319, which represents them pre-

pared in a manner similar to the former. In
this herbivorous insect, the oesophagus (o) is, as
might be expected, very short, and is soon dilated
into a crop (p) ; this is followed by a very long,
wide, and muscular stomach (s), ringed like an

214

THE VITAL FUNCTIONS.

320

earth-worm, and continued into a long and tor-
tuous intestine (i, i), which presents in
its course several dilatations (c, c),
and receives very elongated, convo-
luted, and ramified hepatic vessels
(h. h) . Fig. 320 is a highly magnified
view of a small portion of one of these
vessels, showing its branched form.
In the alimentary canal (Fig. 321*) of the
Acrida aptera (Stephens),
which is a species of grass-
hopper, feeding chiefly on the
dewberry, we observe a long
oesophagus (o), which is very
dilatable, enlarging occasion-
ally into a crop (i), and suc-
ceeded by a rounded or heart-
shaped gizzard (g), of very
complicated structure, and
connected with two remark-
ably large biliary pouches (u
and b), which receive, at their
anterior extremity, the upper
set of hepatic vessels (v v). A
deep furrow in the pouch (b),
which, in the horizontal posi-

* The figures relating to this insect were engraved from the
drawings of Mr. Newport, who was also kind enough to supply
me with the description of the parts they represent. Fig. 321 is
twice the natural size.

DIGESTIVE ORGANS OF INSECTS. 215

tion of the body, lies underneath the gizzard,
divides it apparently into two sacs. The intes-
tinal canal is pretty uniform in its diameter, re-
ceives in its course a great number of hepatic
vessels (h h) by separate openings, and after
making one convolution, is slightly constricted at
N : it is then dilated into a colon (c), on the coats
of which the longitudinal muscular bands are
very distinctly seen. Fig. 322 is a magnified
view of the gizzard laid open, to show its internal
structure. It is furnished with six longitudinal
rows of large teeth, and six intermediate double
rows of smaller teeth ; the total number of teeth
being 270. One of the rows of large teeth is
seen, detached, and still more magnified, in Fig.
323 : it contains at the upper part, five small
hooked teeth (f), succeeded below by four broad
teeth (d), consisting of quadrangular plates, and
twelve tricuspid teeth (t) ; that is, teeth having
three cusps, or points at their edges. Fig. 324
shows the profile of one of these teeth ; a, being
the sharp point by which the anterior acute angle
of the base terminates. Fig. 325 exhibits the
base of the same tooth seen from below ; e, e, e,
being the three cusps, and m, the triangular
hollow space for the insertion of the muscles
which move them, and which compose part of
the muscular apparatus of the gizzard. The
smaller teeth, which are set in double lines
between each of the larger rows, consist of twelve

2ICi THE VITAL FUNCTIONS.

small triangular teeth in each row. All the
teeth contained in this organ are of a brown
colour and horny texture, resembling tortoise-
shell.

The same insect, as we have seen, often
exhibits, at different periods of its existence,
the greatest contrast, not only in external form,
but also in its habits, instincts, and modes of
subsistence. The larva is generally remarkable
for its voracity, requiring large supplies of food
to furnish the materials for its rapid growth, and
frequently consuming enormous quantities of
fibrous vegetable aliment : the perfect insect, on
the other hand, having attained its full dimen-
sions, is sufficiently supported by small quantities
of a more nutritious food, consisting either of
animal juices, or of the fluids prepared by
flowers, which are generally of a saccharine
quality, and contain nourishment in a concen-
trated form. It is evident that the same appa-
ratus, which is necessary for the digestion of the
bulky food taken in during the former period,
would not be suited to the assimilation of that
which is received during the latter ; and that in
order to accommodate it to this altered condition
of its function, considerable changes must be
made in its structure. Hence, it will be interest-
ing to trace the gradual transitions in the confor-
mation of the alimentary canal, during the pro:
gressive developement of the insect, and more

DIGESTIVE ORGANS OF INSECTS.

217

especially, while it is undergoing its different
metamorphoses.

These changes are most conspicuous in the
Lepidoptera, where we may observe the suc-
cessive contractions which take place in the im-
mensely voluminous stomach of the caterpillar,
while passing into the state of chrysalis, and
thence into that of the perfect insect, in which
its form is so changed that it can hardly be
recognised as the same organ. I have given re-

presentations of these three different states of
the entire alimentary canal of the Sphinx lignstri.

218 THE VITAL FUNCTIONS.

or Privet Hawk-moth, in Figures 326, 3*^7, and
328* ; the first of which is that of the cater-
pillar ; the second, that of the chrysalis ; and the
third, that of the moth. The whole canal and
its appendages have been separated from their
attachments, and spread out, so as to display all
their parts ; and they are delineated of the
natural size, in each case, so as to show their
comparative dimensions in these three states.
In all the figures, a is the oesophagus ; b, the
stomach ; c, the small intestine ; d, the caecal
portion of the canal ; and e, the colon, or large
intestine. The hepatic vessels are shown at f ;
and the gizzard, which is developed only in the
moth, at G, Fig. 328.

It will be seen that in the caterpillar, (Fig.
326), the stomach forms by far the most consi-
derable portion of the alimentary tube, and that
it bears some resemblance in its structure and
capacity to the stomachs of the Annelida, already
described.-f This is followed by a large, but short,
and perfectly straight intestine. These organs
in the pupa (Fig. 327) have undergone con-
siderable modifications ; the whole canal, but
more especially the stomach, being contracted

* These figures also have been engraved from the drawings of
Mr. Newport, which he was so obliging as to make for me, from
preparations of his own, the result of very careful dissections.

t See the figures and description of those of the Nais and the
Leech, p. 102 and 103.

DIGESTIVE ORGANS OF MOLLUSCA. 219

both in length and width* : the shortening of
the intestine not being in proportion to that of
the whole body, requires its being folded upon
itself for a certain extent. In the moth, (Fig.
328), the contraction of the stomach has pro-
ceeded much farther ; and an additional cavity,
which may be considered as a species of crop,
or gizzard (g), is developed : the small intestine
takes a great many turns during its course ;
and a large pouch, or caecum, has been formed
at the part where it joins the large intestine.

The hepatic vessels are exceedingly numer-
ous in the Crustacea, occupying a very large
space in the general cavity ; and they compose
by their union an organ of considerable size,
which may be regarded as analogous in its
functions to the Liver of the higher classes
of animals. This organ acquires still greater
size and importance in the Mollusca ; where it
frequently envelopes the stomach, pouring the
bile into its cavity by numerous ducts. t As the
structure and course of the intestinal canal
varies greatly in different tribes of Mollusca,
they do not admit of being comprised in any

* Carus states that he found the stomach of a pupa, twelve
days after it had assumed that state, scarcely half as long, and
only one-sixth as wide as it had been in the caterpillar.

t Transparent crystalline needles, the nature and uses of which
are quite unknown, are frequently found in the biliary ducts of
this class of animals.

220

THE VITAL FUNCTIONS.

329

general description. The only examples I
think it necessary to give, in this class, are those
of the Patella, or Limpet, and
of the Pleurobranchus. The
intestinal tube of the Patella is
delineated in Fig. 329 ; where
M is the month ; t, the tongue
folded back ; o, the oesophagus ;
and s, the stomach, from which
the tortuous intestinal tube is
seen to be continued. All the
convolutions of this tube, as
well as the stomach itself, are enclosed, or rather
imbedded in the substance of the liver, which
is the largest organ of the body.

The Pleurohranchus Peronii (Cuv.) is remark-
able for the number and compli-
cation of its organs of digestion.
They are seen laid open in Fig.
330 ; where c is the crop ; g, the
gizzard ; p, a plicated stomach, re-
sembling the third stomach of ru-
minant quadrupeds; and d, a fourth
cavity, being that in which digestion
is completed. A canal of commu-
nication is seen at t, leading from
the crop to this last cavity : b is the
point where the biliary duct enters.

In the Cephalopoda, the structure of these

330

DIGESTIVE ORGANS OF FISHES. 221

organs is very complicated ; for they are pro-
vided with a crop, a muscular gizzard, and a
caecum, which has a spiral form. In these ani-
mals we also discover the rudiment of another
auxiliary organ, namely, the Pancreas, which
secretes a fluid contributing to the assimilation
of the food. This organ becomes more and more
developed as we ascend in the scale of animals ;
assuming a glandular character, and secreting
a watery fluid, which resembles the saliva, both
in its sensible and chemical properties. It has
been conjectured that many of the vessels,
which are attached to the upper portion of the
alimentary canal of insects, and have been
termed hepatic, may, in fact, prepare a fluid
having more of the qualities of the pancreatic
than of the biliary secretion.

The alimentary canal of fishes is in general
characterised by being short ; and the con-
tinuity of the stomach with the intestines is often
such as to offer no well marked line of distinc-
tion between them. The caeca are generally
large and numerous ; and a number of tubular
organs, connected more especially with the
pylorus, and called therefore the pyloric appen-
dices, are frequently met with, resembling a
cluster of worms, and having some analogy, in
situation at least, to the hepatic or pancreatic
vessels of insects. Their appearance in the

'22*2 THE VITAL FUNCTIONS.

Salmon is represented at p, in Fig. 331 . The pan-
creas itself is only met with, in
this class of animals, in the order
of cartilaginous fishes, and more
especially in the Ray and the
Shark tribes. A distinct gall-
bladder, or reservoir, is also met
with in some kinds of fish, but is
by no means general in that class.
In the classes both of Fishes and of Reptiles,
which are cold-blooded animals, the processes
of digestion are conducted more slowly than in
the more energetic systems of Birds and of
Mammalia ; and the comparative length of the
canal is, on the whole, greater in the former than
in the latter : but the chief differences in this
respect depend on the kind of food which is
consumed ; the canal being always shortest in
those tribes that are most carnivorous.* As the
Frog, in the different stages of its growth, lives
upon totally different kinds of food, so we find
that the structure of its alimentary canal, like
that of the moth, undergoes a material change
during these metamorphoses. The intestinal
canal of the tadpole is of great length, and is
collected into a large rounded mass, composed
of a great number of coils, which may easily be
distinguished, by the aid of a magnifying glass,
through the transparent skin. During its gra-
* See Home, Lectures, &c. I. 401.

DIGESTIVE ORGANS OF MAMMALIA. 223

dual transformation into a frog, this canal be-
comes much reduced in its length ; so that when
the animal has attained its perfect form, it
makes but a single convolution in the abdominal
cavity.

A similar correspondence exists between the
length of the canal and the nature of the food,
in the class of Birds. At the termination of the
small intestine there are usually found two caeca,
which in the gallinaceous and the aquatic fowls,
are of great length : those of the ostrich contain
in their interior a spiral valve. Sir E. Home is
of opinion that, in these animals, the functions
of the pyloric portion of the stomach are per-
formed by the upper part of the intestine.

In the intestines of the Mammalia contrivances
are employed with the apparent intention of
preventing their contents from passing along too
hastily : these contrivances are most effectual in
animals whose food is vegetable, and contains
little nourishment ; so that the whole of what the
food is capable of yielding is extracted from
them. Sir E. Home observes that the colon, or
large intestine of animals which live upon the
same species of food, is of greater length in pro-
portion to the scantiness of the supply. Thus
the length of the colon of the Elephant, which
inhabits the fertile woods of Asia, is only 2Gh
feet ; while in the Dromedary, which dwells in
the arid deserts of Arabia, it is 42 feet. This

224 THE VITAL FUNCTIONS.

contrast is still more strongly marked in birds.
The Cassowary of Java, which lives amidst a
most luxuriant supply of food, has a colon of one
foot in length, and two caeca, each of which is
six inches long, and one quarter of an inch in
diameter. The African ostrich, on the other
hand, which inhabits a country where the supply
of food is very scanty, has a colon forty-five
feet long ; each of the caeca is two feet nine
inches in length, and, at the widest part, three
inches in diameter ; in addition to which there
are broad valves in the interior of both these
cavities.*

On comparing the structure of the digestive
organs of Man with those of other animals
belonging to the class Mammalia, we find them
holding a place in the series intermediate be-
tween those of the purely carnivorous, and ex-
clusively herbivorous tribes ; and in some mea-
sure uniting the characters of both. The powers
of the human stomach do not, indeed, extend to
the digestion either of the tough woody fibres of
vegetables on the one hand, or the compact
texture of bones on the other ; but still they are
competent to extract nourishment from a wider

* Lectures, &c. I. 470. In the account above given of the
digestive organs I have purposely omitted all mention of the
spleen; because, although this organ is probably in some way
related to digestion, the exact nature of its functions has not yet
been determined with any certainty.

DIGESTIVE ORGANS OF MAN. 225

range of alimentary substances, than the diges-
tive organs of almost any other animal. This
adaptation to a greater variety of food may also
be inferred from the form and disposition of
the teeth, which combine those of different kinds
more completely than in most Mammalia ; ex-
cepting, perhaps, the Quadrumana, in which,
however, the teeth do not form, as in man, an
uninterrupted series in both jaws. In addition
to these peculiarities, we may also here observe
that the sense of taste, in the human species,
appears to be affected by a greater variety of
objects than in the other races of animals. All
these are concurring indications that nature, in
thus rendering man omnivorous, intended to qua-
lify him for maintaining life wherever he could
procure the materials of subsistence, whatever
might be their nature ; whether animal or vege-
table, or a mixture of both, and in whatever soil
or climate they may be produced ; and for endow-
ing him with the power of spreading his race,
and extending his dominion over every acces-
sible region of the globe. Thus, then, from the
consideration of the peculiar structure of the
vital, as well as the mechanical organs of the
human frame, may be derived additional proofs
of their being constructed with reference to fa-
culties of a higher and more extensive range than
those of any, even the most favoured species of
the brute creation.

VOL. II. Q

226

Chapter IX.

LACTEAL ABSORPTION,

The Chyle, of which we have now traced the
formation, is a fluid of uniform consistence,
perfectly bland and unirritating in its properties ;
and the elements of which have been brought
into that precise state of chemical composition,
which renders them fit to be distributed to
every part of the system for the purposes of nou-
rishment. In all the lower orders of animals
it is transparent; but the chyle of Mammalia
often contains a multitude of globules, which
give it a white colour, like milk. Its chemical
composition appears to be very analogous to
that of the blood into which it is afterwards con-
verted. From some experiments made by my
late much valued friend Dr. Marcet, it appears
that the chyle of dogs, fed on animal food alone,
is always milky, whereas in the same animals,
when they are limited to a vegetable diet, it is
nearly transparent and colourless.*

The chyle is absorbed from the inner surface
of the intestines by the Lac teals, which commence

* Medico- Chirurgical Transactions ; vi. 630.

LACTEAL ABSORPTION. 227

by very minute orifices, in incalculable numbers,
and unite successively into larger and larger
vessels, till they form trunks of considerable size.
They pass between the folds of a very fine and
delicate membrane, called the mesentery, which
connects the intestines with the spine, and which
appears to be interposed in order to allow them
that degree pf freedom of motion, which is so
necessary to the proper performance of their
functions. In the mesentery, the lacteals pass
through several glandular bodies, termed the
mesenteric glands, where it is probable that the
chyle undergoes some modification, preparatory
to its conversion into blood.

The mesenteric glands of the Whale contain
large spherical cavities, into which the trunks
of the lacteals open, and where the chyle is
probably blended with secretions proper to those
cavities; but no similar structure can be de-
tected in terrestrial mammalia.

It is only among the Vertebrata that lacteal
vessels are met with. Those of Fishes are simple
tubes, either wholly without valves ; or if there
be any, they are in a rudimental state, and
not sufficiently extended to prevent the free
passage of their fluid contents in a retrograde
direction. The lacteals of the Turtle are larger
and more distinct than those of fishes ; but their
valves are still imperfect, though they present
some obstruction to descending fluids. In Birds

228 THE VITAL FUNCTIONS.

and in Mammalia these valves are perfectly
effectual, and are exceedingly numerous ; giving
to the lacteals, when distended with fluid, the
appearance of strings of beads. The effect of
these flood-gates, placed at such short intervals,
is that every external pressure made upon the
tube, assists in the propulsion of the fluid in the
direction in which it is intended to move. Hence
it is easy to understand how exercise must tend
to promote the transmission of the chyle. The
glands are more numerous and concentrated in
the Mammalia, than in any other class.

From the mesenteric glands, the chyle is con-
ducted, by the continuation of the lacteals, into
a reservoir, which is termed the receptacle of the
chyle: whence it ascends through the thoracic
duct,* which passes along the side of the spine,
in a situation affording the best possible protec-
tion from injury or compression, and opens into
the great veins leading directly into the heart.

In invertebrated animals having a circulatory
system of vessels, the absorption of the chyle is
performed by veins instead of lacteal vessels.

The sanguification of the chyle, or its conver-
sion into blood, takes place during the course
of the circulation, and is principally effected by
the action of atmospheric air in certain organs,
hereafter to be described, where that action, or

* This duct is occasionally double.

SANGUIFICATION 229

aeration as it may be termed, in common with
an analogous process in vegetables, takes place.
In all vertebrated animals the blood has a red
colour, and it is also red in most of the Anne-
lida; but in all other invertebrated animals, it
is either white or colourless.* We shall, for the
present, then, consider it as having undergone
this change, and proceed to notice the means
employed for its distribution and circulation
throughout the system.

Chapter X.

Circulation.

§ 1. Diffused Circulation.

Animal life, implying mutual actions and re-
actions between the solids and fluids of the body,
requires for its maintenance the perpetual trans-
fer of nutritive juices from one part to another,
corresponding in activity to the extent of the
changes which are continually taking place in
the organized system. For this purpose we

* Vauquelin has observed that chyle has often a red tinge in
animals.

230 THE VITAL FUNCTIONS.

almost constantly find that a circulatory motion
of the nutrient fluids is established ; and the
function which conducts and regulates their
movements is emphatically denominated the Civ'
culation. Several objects of great importance
are answered by this function ; for, in the first
place, it is through the circulation that every
organ is supplied with the nutritive particles
necessary for its developement, its growth, and
its maintenance in a healthy condition ; and
that the glands, in particular, as well as the other
secreting organs, are furnished with the materials
they require for the elaboration of the products,
which it is their peculiar office to prepare. A
second essential object of the circulation, is to
transmit the nutritive juices to certain organs,
where they are to be subjected to the salutary in-
fluence of the oxygen of the atmosphere ; a pro-
cess, which in all warm-blooded animals, com-
bined with the rapid and extensive distribution
of the blood, difliises and maintains throughout
the system the high temperature required by the
greater energy of their functions. Hence it
necessarily follows that the particular mode in
w hich the circulation is conducted in each re-
spective tribe of animals, must influence every
other function of their economy, and must, there-
fore, constitute an essential element in deter-
mining their physiological condition. We find,
accordingly, that among the characters on

DIFFUSED CIRCULATION. 231

which systematic zoologists have founded their
great divisions of the animal kingdom, the ut-
most importance is attached to those derived
from differences of structure in the organs of
circulation.

A comprehensive survey of the different classes
of animals with reference to this function, enables
us to discern the existence of a regular gradation
of organs, increasing in complexity as we ascend
from the lower to the higher orders ; and showing
that here, as in other departments of the economy
of nature, no change is made abruptly, but
always by slow and successive steps. In the
very lowest tribes of Zoophytes, the modes by
which nutrition is accomplished can scarcely be
perceived to differ from those adopted in the ve-
getable kingdom, where, as we have already
seen, the nutritive fluids, instead of being con-
fined in vessels, appear to permeate the cellular
tissue, and thus immediately supply the solids
with the materials they require; for, in the
simpler kinds of Polypi, of infusoria, of Medusae,
and of Entozoa, the nourishment which has been
prepared by the digestive cavities is apparently
imbibed by the solids, after having transuded
through the sides of these organs, and without its
being previously collected into other, and more
general cavities. This mode of nutrition, suited
only to the torpid and half vegetative nature of
zoophytes, has been denominated ttourishment hy

-32 THE VITAL FUNCTIONS.

imbibition, in contradistinction to that by circu-
lation; a term, which, as we have seen, implies,
not merely a system of canals, such as those ex-
isting in Medusae, where there is no evidence of
the fluids really circulating, but an arrangement
of ramified vessels, composed of membranous
coats, through which the nutrient fluid moves in
a continued circuit.

The distinction which has thus been drawn,
however, is one on which we should be careful
not to place undue reliance ; for it is founded,
perhaps, more on our imperfect means of investi-
gation, than on any real differences in the proce-
dures of nature relative to this function. When
the juices, either of plants or of animals, are trans-
parent, their motions are imperceptible to the eye,
and can be judged of only by other kinds of evi-
dence ; but when they contain globules, differing
in their density from that of the fluid, and there-
fore capable of reflecting light, as is the case
with the sap of the Chara and Caulinia, we have
ocular proof of the existence of currents, which,
as long as the plant is living and in health, pur-
sue a constant course, revolving in a regular and
defined circuit ; and all plants which have milky
juices exhibit this phenomenon. Although the
extent of each of these vegetable currents is very
limited, compared with the entire plant, it still
presents an example of the tendency which the
nutrient fluids of organized structures have to

DIFFUSED CIRCULATION. 233

move in a circuit, even when not confined within
vessels or narrow channels ; for this movement
of rotation, or cyclosis, as it has been termed,*
whatever may be its cause, appears always to
have a definite direction. The current returns
into itself, and continues without intermission,
in a manner much resembling the rotatory move-
ments occasionally produced in fluids by electro-
magnetism, t

Movements, very similar in their appearance
and character to those of vegetable cyclosis,
have been recently discovered in a great number
of polypiferous Zoophytes, by Mr. Lister, who
has communicated his observations in a paper
which was lately read to the Royal Society, and
of which the following are the principal results.
In a specimen of the Tubularia indivisa, when
magnified one hundred times, a current of
particles was seen within the tubular stem of the
polype, strikingly resembling, in the steadiness
and continuity of its stream, the vegetable cir-
culation in the Chara. Its general course was
parallel to the slightly spiral lines of irregular
spots on the surface of the tube, ascending on
the one side, and descending on the other ;

* See pages 49 and 50 of this volume.

f So great is this resemblance, that it has led several physi-
ologists to ascribe these movements to the agency of electricity ;
but there does not, as yet, appear to be any substantial founda-
tion for this hypothesis.

234 THE VITAL FUNCTIONS.

each of the opposite currents occupying one^
half of the circumference of the cyhndric cavity.
At the knots, or contracted parts of the tube,
slight eddies were noticed in the currents ; and
at each end of the tube the particles were seen
to turn round, and pass over to the other side.
In various species of Sertularice the stream
does not flow in the same constant direction ;
but, after a time, its velocity is retarded, and
it then either stops, or exhibits irregular eddies,
previous to its return in an opposite course ;
and so on alternately, like the ebb and flow of
the tide. If the currents be designedly ob-
structed in any part of the stem, those in the
branches go on without interruption, and inde-
pendently of the rest. The most remarkable
circumstance attending these streams of fluid
is that they appear to traverse the cavity of the
stomach itself; flowing from the axis of the
stem into that organ, and returning into the stem
without any visible cause determining these
movements. Similar phenomena were observed
by Mr. Lister in Campanularice and Plumularice.
In some of the minuter species of Crustacea,
the fluids have been seen, by the aid of the
microscope, moving within the cavities of the
body, as if by a spontaneous impulse, without
the aid of a propelling organ, and apparently
without being confined in membranous channels,

VASCULAR CIRCULATION. 235

or tubes of any sort. This kind of diffused cir-
culation is also seen in the embryos of various
animals, at the earliest periods of their develope-
ment, and before any vessels are formed. >

^ 2. Vascular Circulalion.

The next step in the gradation of structures con-
sists in the presence of vessels, within which the
fluids are confined, and by which their course
and their velocity are regulated ; and in general
these vessels form a complete circuit. The first
rudiments of a vascular organization are those
observed and described by Tiedemann, in the
Asteriee, which are situated higher in the animal
scale than Medusse ; but whether any actual
circulation takes place in the channels consti-
tuted by these vessels, which communicate both
with the cavity of the intestine, and with the
respiratory organs, is not yet determined with
any certainty. The HolotlmricB , which also
belong to the order of Echinodermata, are fur-
nished with a complex apparatus of vessels, of
which the exact functions are still unknown.
In those species of Entozoa which exhibit a
vascular structure, the canals appear rather to
be ramifications of the intestinal tube, than
proper vessels ; for no distinct circulation can be

236 THE VITAL FUNCTIONS.

traced in them : an organization of this kind has
already been noticed in Tceni(E.^

It was, till very lately, the prevailing opinion
among naturalists that all true insects are nou-
rished by imbibition, and that there exists in
their system no real vascular circulation of
juices. In all the animals belonging to this
class, and in every stage of their developement,
there is found a tubular organ, called the dorsal
vessel, extending the whole length of the back,
and nearly of uniform diameter, except where it
tapers at the two ends. It contains a fluid,
which appears to be undulated backwards and
forwards, by means of contractions and dilata-
tions, occurring in succession in different parts
of the tube ; and it is also connected with
transverse ligamentary bands, apparently con-
taining muscular fibres, capable by their action
of producing, or at least of influencing these pul-
satory movements. An enlarged representation
of the dorsal vessel of the Melolontha vulgarisy
or common cockchaffer, isolated from its attach-
ments, is given in Fig. 333, showing the series
of dilatations (v, v, v) which it usually presents
in its course ; and in Fig. 334 the same vessel is
exhibited in connexion with the ligamentary and

* Page 83, of this volume ; Fig. 247. The family of Pla-
naricB present exceptions to this general rule ; for many species
possess a system of circulating vessels. See Dug^s, Annales
des Sciences Naturelles; xv, 161.

CIRCULATION IN INSECTS.

237

muscular apparatus which surrounds it, seen
from the lower side. In the last of these figures,

333

A is the tapering prolongation of the tube, pro-
ceeding towards the head of the insect ; v, one of
the dilated portions, or ventricles, as they have
been called, of the dorsal part of the tube ; f, one
of the small tendinous folds, to which the liga-
mentary bands are attached ; and l is one of
these bands, having a triangular, or, if considered
as continuous with that on the other side of the
vessel, a rhomboidal shape, and attached at r,

238 THE VITAL FUNCTIONS.

to the superior segments of the abdomen. At i
is seen a layer of the same fibres, which are
partly ligamentous and partly muscular, passing
underneath the dorsal vessel, and forming, in
conjunction with the layer that passes above it,
a sheath, which embraces and fixes that vessel
in its place : these inferior layers have been
removed from the other parts of the vessel, to
allow the upper layers to be seen, as is the case
at L. Fig. 335 gives a side view of the anterior
extremity of the same vessel, showing the curve
(a) which it describes as it bends downwards in
its course towards the head.

The function performed by the dorsal vessel,
which, judging from the universal presence of
this organ in insects, must be one of great im-
portance in their economy, was long a profound
mystery. Its analogy in structure and position
to the dorsal vessels of the Arachnida and the
Annelida, where it evidently communicates with
channels of circulation, and exhibits movements
of pulsation resembling those of insects, was
a strong argument in favour of the opinion that
it is the prime mover of a similar kind of circu-
lation ; but then, again, this hypothesis ap-
peared to be overturned by the fact that no
vessels of any kind could be seen extending
from it in any direction ; nor could any channels
for the transmission of a circulating fluid be
detected in any part of the body. Those organs,

CIRCULATION IN INSECTS. 239

which, in animals apparently of an inferior rank,
are most vascular, such as the stomach, the
intestinal tube, the eye, and other apparatus
of the senses, seemed to be constructed, and
to be nourished, by means totally different from
those adopted in the former animals. Although
extremely minute ramifications of air tubes are
every where visible in the interior of insects,
yet, neither Cuvier, nor any other anatomist,
could succeed, by the closest scrutiny, in de-
tecting the least trace of blood vessels ; and the
presumption, therefore, was, that none existed.

But it still remained a question, if the dorsal
vessel be not subservient to circulation, what
is its real function ? Marcel de Serres, who
bestowed great pains in investigating this sub-
ject, came to the conclusion that its use is to
secrete the fatty matter, which is generally
found in great abundance in the abdominal
cavity, and which is accumulated particularly
around the dorsal vessel.* A more attentive
examination of the structure of the vessel itself
brought to light a valvular apparatus, of which
the only conceivable purpose is that of deter-
mining the motion of the contained fluid in one
constant course ; a purpose necessarily incom-
patible with its supposed alternate undulation

* See his various papers in the Memoires du Museum d' Hist.
Nat. ; torn. iv. and v.

240 THE VITAL FUNCTIONS.

in opposite directions, from one end of the
tube to the other. These valves are exhibited in
Fig. 336, in a still more magnified view of a
longitudinal section of the dorsal vessel, showing
the semicircular folds (s, s) of its inner mem-
brane, which perform the function of valves by
closing the passage against any retrograde mo-
tion of the fluid. This discovery of valves in
the dorsal vessel, again made the balance of
probability incline towards the opinion that it
is the agent of some kind of circulation.

All doubt as to the reality of a circulation in
insects is now dispelled by the brilliant dis-
coveries of Professor Cams, who, in the year
1824, first observed this phenomenon in the
larva of the Agrion puella. In the transparent
parts of this insect, as well as of many others,
numerous streams of fluid, rendered manifest
by the motions of the globules they contain,
are seen meandering in the spaces which inter-
vene between the layers of the integument,
but without appearing to be confined within
any regular vessels. The streams on the sides
of the body all pass in a direction backwards
from the head, till they reach the neighbourhood
of the posterior end of the dorsal vessel, towards
which they all converge ; they are then seen to
enter that vessel, and to be propelled by its pul-
sations towards its anterior extremity, where they
again issue from it, and are subsequently divided

CIRCULATION IN INSECTS. 241

into the scattered streams, which descend along
the sides of the body, and which, after having
thus completed their circuit, return into the pul^
sating dorsal vessel.

This mixed kind of circulation, partly diffused
and partly vascular, is beautifully seen in the
larva of the Ephemera marginatay^ where, be-
sides the main current, which, after being dis-
charged from the anterior extremity of the dorsal
vessel, descends in a wide spreading stream
on each side and beneath that vessel, another
portion of the blood is conveyed by two lateral
trunks, which pass down each side of the body,
in a serpentine course, and convey it into the
lower extremity of the dorsal vessel, with which
they are continuous. These are decidedly ves-
sels, and not portions of the great abdominal
cavity, for their boundaries are clearly defined ;
yet they allow the blood contained in them
to escape into that cavity, and mix with the
portion previously diffused. All these wandering
streams sooner or later find their way into the
dorsal vessel ; being absorbed by it at various
points of its course, where its membranous coat
is reflected inwards to form the v^ve*. In the

* This insect is figured and described in Dr. Goring and
Mr. Pritchard's " Microscopic Illustrations," and its circulation
is very fully detailed, and illustrated by an engraving on a large
scale, by Mr. Bowerbank, in the Entomological Magazme, i, 239 ;
plate ii.

VOL. II. U

242

THE VITAL FUNCTIONS.

legs, the tail, and the antennae, the circulation is
carried on by means of vessels, which are con-
tinuous with the lateral vessels of the body ;
branching off from them in the form of loops,
ascending on one side, and then turning back to
form the descending vessel, so that the currents
in each move in contrary directions. Fig. 337

represents the appearance of these parallel ves-
sels in one of the antennae of the Semhlis viridis,
magnified thirty times its natural size. The
whole system of circulating vessels in that in-
sect, of which the former is only a detached
part, is shown in Fig. 338, where the course
of the blood is indicated by arrows; a, repre-
senting the currents in the antennae ; w, those in
the rudimental wings ; and t, those in the tail ;
in all which parts the vessels form loops, derived

CIRCULATION IN INSECTS. 243

from the main vessels of the trunk. In some
larvse the vascular loops, conveying these colla-
teral streams, pass only for a certain distance
into the legs ; sometimes, indeed, they proceed
no farther than the haunches. The currents of
blood in these vessels have not a uniform velo-
city ; being accelerated by the impulsions they
receive from the contractions of the dorsal
vessel, which appears to be the prime agent in
their motion.

As the insect advances to maturity, and passes
through its metamorphoses, considerable changes
are observed to take place in the organization of
the circulating system, and in the energy of the
function it performs. The vessels in the extreme
parts, as in the tail, are gradually obliterated,
and the circulation in them, of course, ceases ; the
blood appearing to retire into the more internal
parts. In the wings, on the other hand, where
the developement proceeds rapidly, the circula-
tion becomes more active ; and even after they
have attained their full size, and are yet in a
soft state, the motion of the blood in the centre
of all the nervures is distinctly visible : * but
afterwards, as the wings become dry, it ceases
there also, and is then confined to the vessels

* These currents in the wing of the Semblis bilineata have
been described and delineated by Cams, in the Acta Acad. Cses.
Leop. Carol. Nat. Cur. vol. xv. part ii, p. 9.

244 THE VITAL FUNCTIONS.

of the trunk. In proportion as the insect ap-
proaches to the completion of its developement,
these latter vessels also, one after the other, shrink
and disappear ; till at length nothing which had
once appertained to this system remains visible,
except the dorsal vessel. But as we observe
this vessel still continuing its pulsatory move-
ments, we may fairly infer that they are designed
to maintain some degree of obscure and imperfect
circulation of the nutrient juices, through vessels,
which may, in their contracted state, correspond-
ing to the diminished demands of the system, have
generally escaped detection. In confirmation of
these views it may be stated, that several ob-
servers have, at length, succeeded in tracing
minute branches, proceeding in different direc-
tions from the dorsal vessel, and distributed
to various organs. The division of the anterior
part of the dorsal vessel into descending branches
was noticed by Comparetti. Dug^s has observed
a similar division of this vessel in the corselet of
several species of Phalence^ and further ramifica-
tions in that of the Gryllus lineola: and Audouin
has traced them in many of the Hymenoptera.*

* Annales des Sciences Naturelles, xv. 308.

The figures which follow (from 339 to 345) are represen-
tations, of the natural size, of the dorsal vessel of the Sphinx
ligustri, or Privet Hawk-moth, which has been dissected in its
three diflerent stages, with great care, by Mr. Newport, from

CIRCULATION IN INSECTS.

245

The discovery of the circulation in insects, and
of its varying energy at different periods of

whose drawings these figures have been engraved, and to whom
I am indebted also for the description which follows : —

The dorsal vessel of this insect is an elongated and gradually
tapering vessel, extending from the hinder part of the abdomen,
along the back, towards the head ; and furnished with valves,

339

which correspond very nearly in their situation to the incisions of
the body. During the changes of the insect from the larva to the
imago state, it undergoes a slight modification of form. In
every state it may be distinguished into two portions, a dorsal and
an aortal. The dorsal portion, which is the one in which a pulsa-
tion is chiefly observable, is furnished with distinct valves, is at-
tached along the dorsal part of the body by lateral muscles, and
has vessels which enter it laterally, pouring into it the circulating
fluid, which is returning from the sides and inferior portions of
the body. In the caterpillar, this portion of the dorsal vessel ex-
tends from the twelfth to the anterior part of the fifth segment.
It is furnished with eight double valves, which are formed as
Mr. Bowerbank has correctly described them in the Ephemera
marginata ; namely, the upper valve " by a reflecting inwards

246 THE VITAL FUNCTIONS.

growth, has elucidated many obscure points in
the physiology of this important class. It ex-

and upwards of the inner coat, or coats of the artery," (by which
he means the dorsal vessel) " and the under one by a contraction
or projection of the like parts of a portion of the artery beneath,
so as to come within the grasp of the lower part of the valve
above it." The whole vessel is made up of three coats, the two
innermost of which, the lining, or serous, and the muscular, or
principal portion of the vessel, constitute the reflected portions, or
valves ; while the third, or outermost coat, which is exceedingly
thin and delicate, is continued over the vessel nearly in a straight
line, and does not appear at all to follow the reflections of
the other two. In the caterpillar, this portion of the vessel has
eight pairs of small suspensory muscles, (seen along the upper side
of Fig. 339,) which arise from the middle of the upper surface of
each valve, and are continued back to be attached over the middle
of the next valve : they seem to have considerable influence over
the contractions of the valves. The Aortal, or anterior portion
of the vessel, extends from the hinder part of the fourth segment
to its termination and division into vessels, to be distributed to the
head ; which division takes place after it has passed the oesopha-
gus, and at a point immediately beneath the supra-oesophageal
ganglion, or brain of the insect. This portion of the vessel is
much narrower than the dorsal, has no distinct valves, or muscles ;
nor do any vessels enter it laterally ; but it is very delicate and
transparent, and gradually diminishes in size from its commence-
ment to its anterior termination. Its course, in the caterpillar,
is immediately beneath the integument, along the fourth and
third segments, till it arrives at the hinder parts of the second
segment ; when it gradually descends upon the oesophagus, and,
immediately behind the cerebral ganglion, gives off" a pair of ex-
ceedingly minute vessels. It then passes beneath the ganglion ;
and, in the front part of the head, is divided into several branches,
as noticed by Mr. Newport in the anatomical description he has
given of the nerves of this species of Sphinx : (Phil. Trans. 1832,
p. 385.) These branches are best observed in the chrysalis (Fig.

CIRCULATION IN INSECT8. 247

plains why insects, after they have attained their
imago state, and the circulation is nearly oblite-

340) : in all the stages they may be divided into three sets ; the
first is given off immediately after the vessel has passed beneath
the ganglion, and consists of two lateral trunks, the united capa-
city of which is equal to about one-third of that of the aorta ; they
descend, one on each side of the mouth, and are each divided
into three branches. The second set consists of two pairs of
branches ; one going apparently to the tongue ; the other to the
antennse. The third set is formed by two branches, which pass
upwards, and are the continuations of the aorta ; they divide into
branches, and are lost in the integuments, and structures of the
anterior part of the head.

The pulsatory action of the dorsal vessel is continued along its
whole course, and seems to terminate at the division of the vessel
into branches. During the metamorphoses of the insect, this
vessel becomes considerably shortened ; but is stronger, and more
consolidated in its structure. Its course is likewise altered ; from
having, in the caterpillar (Fig. 339) passed along, nearly in
a straight line, it begins, in the chrysalis (Fig. 340), to descend
in the fifth segment, and to pass under what is to become the di-
vision between the thorax and abdomen in the perfect insect. It
then ascends in the fourth segment, and descends again in the
second ; so that when the insect has attained its perfect form,
(Fig. 341) its course is very tortuous. The vessels which enter
it are situated in the abdomen, and pass in laterally among the
muscles, chiefly at the anterior part of each segment or valve.
Fig. 342 is a superior, or dorsal view of the same vessel, in the
perfect state of the insect, which shows still more distinctly the
vessels entering it laterally, intermixed with the lateral muscles.
Fig. 343 is a magnified lateral view of the anterior extremity of
the dorsal vessel, corresponding to Fig. 341 ; and Fig. 344, a
similarly magnified view of the same portion of the vessel seen
from above, corresponding to Fig. 342. Fig. 345 shows the
mode in which the valves are formed by a duplicature of the
inner membrane in the perfect insect.

248 THE VITAL FUNCTIONS.

rated, no longer increase in size, and require but
little nourishment for the maintenance of life.
This, however, is a state not calculated for so
long a duration as that in which the develope-
ment is advancing ; and accordingly, the period
during which the insect remains in the imago
condition is generally short, compared to that of
the larva, where a large supply of nutriment, and
a rapid circulation of the fluids concur in main-
taining the vital functions in full activity. Thus
the Ephemera, which lives for two or three years
in the larva state, generally perishes in the course
of a few hours after it has acquired wings, and
reached its perfect state of maturity.

In proportion as the changes of form which
the insect undergoes are less considerable, the
evidences of a circulation become more distinct.
Such is the case in many of the Apterous In-
sects, composing the family of Myriapoda: in
the Scolopendra morsitans (Linn.), for instance,
Dug^s observed the dorsal vessel dividing into
three large branches.

Most of the tribes belonging to the class of
Arachnida have likewise a dorsal vessel very
analogous in its structure and situation to that of
insects ; and as none of them undergo any meta-
morphosis, their vascular system admits of being
considerably developed, and becomes a per-
manent part of the organization. Fig. 346
shows the dorsal vessel of the Aranea domes-

CIRCULATION IN THE ARACHNIDA. 249

tica, or house spider, with some of the arterial
trunks arising from it, lying
imbedded in a thick mass of
substance, having a similar oily
character to that which is con-
tained in large quantities in
the principal cavities of insects.
It is, in general, difficult to ob-
tain a view of the circulation in
the living spider, on account of
the thick covering of hair which is spread over
the body and the limbs ; but if a species, which
has no hair, be selected for examination, we can
see very distinctly, through the microscope, the
motion of the blood in the vessels, by means of
the globules it contains, both in the legs and in
other parts, where it presents appearances very
similar to those already described in the limbs
of the larvae of insects.

A complete vascular circulation is established
in all the animals which compose the class of
Annelida; the vessels being continuous through-
out, and having sufficient power to propel the
blood through the whole of its circuit. Great
variety exists in the arrangement and distribu-
tion of these vessels, depending on the form of
the animal, the complication of its functions,
and the extent of its powers. The first rudi-
ment of a distinct system of circulating vessels,
independent of the ramified tubes proceeding

250 THE VITAL FUNCTIONS.

from the intestinal canal, occurs in the Planarice,
which are a tribe of fiat vermiform animals, in
many respects allied to the more developed
Entozoa, and appearing placed as an interme-
diate link between them and the Annelida. In
many species, such as the Planaria nigra^ fusca,
and tremellaris, (Muller), Duges observed two
longitudinal trunks (Fig. 346*) running along
^Q^ the sides of the under surface of

the animal, and joining together,
both at their fore and hind ex-
tremities, so as to form a con-
tinuous channel of an oval form.t
A great number of smaller vessels
branch off from these main trunks
in every direction, and ramify ex-
tensively ; often uniting with those
from the opposite side, and esta-
blishing the freest communications
between them.
In the Annelida which have a more length-
ened and cylindric form, the principal vessels
have a longitudinal course ; but are difterently
disposed in different species. There is in all a
vascular trunk, extending along a middle line,
the whole length of the back, and especially
designated as the dorsal vessel : in general there

f De Blainville has described a structure similar to this in a
Planaria iVom Brazil. Diet, des Sc. Nat. t. xli. 216.

CIRCULATION IN THE ANNELIDA. 251

is also a corresponding trunk, occupying the
middle line of the lower, or abdominal side of
the body, and termed the abdominal vessel. This
latter vessel is sometimes double ; one being su-
perficial, and another lying deeper; the principal
nervous cord, and chain of ganglia being situated
between them. Frequently there are found, in
addition to these, vessels which run along the
sides of the body, and are therefore called the
lateral vessels. In every case there are, as we
have seen in the Planaria, numerous branches,
and collateral communications between the la-
teral, the abdominal, and dorsal vessels; more
especially at the two extremities of the body,
where the great mass of blood, which has been
flowing in one direction in one set of vessels,
is transferred into others which convey it in the
contrary direction, and complete the circuit of
its course. The ramifications and lateral con-
nexions of the minuter branches are often so
numerous, as to compose a vascular net-work
covering a considerable extent of surface. This
general description of the circulatory system is
applicable to the tribes of Annelida possessing
the simplest structure, such as the Naisy the
Nereis, and the Leech; genera which include
a great variety of species of different shapes
and sizes.

Although the vessels themselves may be
plainly discerned, it is not so easy to determine

252 THE VITAL FUNCTIONS.

the real course which the blood takes while
circulating within them ; and we accordingly
find very great discordance in the reports of
different physiologists on this subject. De
Blainville asserts that in all the Annelida, the
blood in the dorsal vessel is carried backwards,
that is, from the head to the tail ; a motion,
which, of course, implies its return in the con-
trary direction in either the lateral or the abdo-
minal vessels. In the Nais, the Nereis, and
the Leech, these last vessels are two in number,
situated at the sides of the abdominal surface of
the body. Carus adds his testimony in favour
of this mode of considering the circulation in
the Annelida. On the other hand, Spix, Bon-
net, Sir Everard Home, and Duges describe the
course of the blood as quite the opposite of this,
and maintain that it moves backwards, or to-
wards the tail, in the abdominal vessels ; and
forwards, or towards the head, in the dorsal
vessel. Morren, who is the latest authority
on this subject, gives his testimony in favour
of the latter view of the subject, as far as relates
to the dorsal vessel of the Erpohdella vulgaris^*
an animal, allied to the Leech, and already
noticed in the account of the mechanical func-
tions of this tribe : t but he considers the ab-

* Hirudo vulgaris. (Linn.) Nephelis vulgaris. (Savigny.)
t Vol. i. p. 271, where a delineation of this animal was given,
Fig. 130.

CIRCULATION IN THE ANNELIDA.

253

dominal vessel as performing also the same
function of carrying the blood forwards towards
the head, and the two lateral vessels as convey-
ing it backwards, thus completing the circuit.
This is illustrated by the diagram (Fig. 347) ;

where a is the anterior and p the posterior
extremity of the animal, the dorsal vessel occu-
pying the middle straight line between the two
lateral vessels, and the direction of the stream
in each being indicated by the adjacent arrows.
The blood in the abdominal vessel following
the same course as that in the dorsal vessel,
the same diagram represents also these vessels
seen from below. Fig. 348 is a lower view
of the Erpobdella, showing the numerous rami-
fications of the abdominal vessel ; the lesser
branches encircling the nervous ganglia, and
accompanying the principal nervous filaments
which proceed from them ; while the lateral

254 THE VITAL FUNCTIONS.

vessels are seen pursuing a slightly serpentine
course.*

The tribe of JLumhrici, which includes the
earth-worm, is distinguished from the annelida
already noticed, by being more highly organized,
and possessing a more extensive circulation, and
a more complicated apparatus for the per-
formance of this function. The greater extent
of vascular ramifications appears to require in-
creased powers for carrying the blood through
the numerous and intricate passages it has to
traverse ; and these are obtained by means of
muscular receptacles, capable, by their succes-
sive contraction, of adding to the impulsive force
with which the blood is driven into the trunks
that distribute it so extensively. These muscu-

* Duges represents the blood of this animal as moving in
different directions in the right and in the left lateral vessels ;
generally backwards in the former, and forwards in the latter :
at the same time that it moves backwards in the dorsal, and
forwards in the abdominal vessel. In the communicating
branches which pass transversely from one lateral vessel to the
other, the blood flows from left to right in those situated in the
anterior half of the body, and from right to left in those of
the posterior half; so that the plane in which its circuit is
performed is horizontal, instead of vertical. It is curious to
find an example of a similar transverse circulation, in the
vegetable kingdom; this has recently been observed by Mr.
Solly and Mr. Varley, in a sprout of the Chara vulgaris, near
the end of which the enclosed fluid revolves continually on
its own axis, instead of following the ordinary course of ascent
and descent along the sides of the cylindric cavity. — See Trans,
of the Society of Arts, xlix. 180.

CIRCULATION IN THE ANNELIDA,

255

lar appendages are globular or oval dilatations
of some of the large vascular trunks, which bend
round the sides of the anterior part of the body,
and establish a free communication between the
dorsal and the abdominal vessels. They are
described by Dug^s as consisting, in the Lum-
hricus gigas, of seven vessels on each side ; form-
ing a series of rounded dilatations, about twelve
in number, resembling a string of beads.*

In the Lwnhricus tei^restris, or common earth-
worm, there are only five pair of these vessels :
they have been described and figured by Sir
E. Home y\ but the most full and accurate
account of their structure has been given by
Morren, in his splendid work on the anatomy of
that animaLJ Fig. 349, which is reduced from

349

* They are termed by Dug^s, Vaisseaux moniliformes, ou
dorso-abdominaux. — Annales des Sciences Naturelles, xv, 299.

t Philos. Transact, for 1817, p. 3 : and PI. iii. Fig. 4.

X " De Lumbrici terrestris Historia naturalis, necnon Ana-
tomia Tractatus." Qto. Bruxelles, 1829.

•^56 THE VITAL FUNCTIONS.

his plates, represents these singular appendages
to the vascular system of the earth-worm, sepa-
rated from their attachments, and viewed in con-
nexion only with the dorsal and abdominal trunks
in which they terminate. The abdominal vessel,
(a, a), on arriving near the oesophagus, is dilated,
at the point b, into a globular bulb (c), which
is followed, at equal intervals, by four others
(c, c). From each of these bulbs, or ventri-
cles, as they are termed by Morren, a vessel (d)
is sent off at right angles, on each side ; this
vessel also enlarges into several nearly globular
dilatations (e), followed by a still larger, and
more elongated oval receptacle (f), which com-
pletes the semicircular sweep taken by the vessel
in bending round the sides of the body, in
order to join the dorsal vessel (g, g), in which
all the other four communicating vessels, pre-
senting similar dilatations, terminate. Sir E.
Home is of opinion that these dilated portions of
the vessel are useful as reservoirs of blood, for
supplying it in greater quantity to the neigh-
bouring organs, as occasion may require; but
Morren ascribes to them the more important
office of accelerating, by their muscular action,
the current of circulating blood. If the latter of
these views be correct, which the strong pulsa-
tions constantly visible in these bulbs render
extremely probable, this structure would offer
the first rudiments of the organ which, in all the

•CIRCULATION IN THE CRUSTACEA. 257

superior classes of animals, performs so impor-
tant an office in the circulation of the blood,
namely, the heart: and this name, indeed, is
given by Cuvier, Morren, and others, to these
dilated portions of the vascular systems of the
higher orders of Annelida.

Here, also, the statements of different anato-
mists are at variance, with regard to the direc-
tion taken by the blood while circulating in the
vessels ; Home and Duges represent it as pro-
ceeding forwards in the dorsal, and backwards
in the abdominal vessels; a course which im-
plies its descent along the lateral communicating
vessels just described ; while De Blainville and
Morren ascribe to it a course precisely the
reverse. Amidst these conflicting testimonies,
it is extremely difficult to determine on which
side the truth lies ; and a suspicion will natu-
rally arise, that the course of the blood in the
vessels may not be at all times uniform, but may
be liable to partial oscillations, or be even com-
pletely reversed, by the operation of particular
disturbing causes.

The larger Crustacea possess a circulatory
apparatus still more extensive and complete,
accompanied by a corresponding increase in the
energy of the vital functions. As we follow this
system in the more highly organized tribes of
this class, we find the powers of the dorsal
vessel becoming more and more concentrated

VOL. II. s

258 THE VITAL FUNCTIONS.

in its anterior extremity ; till in the Decapoda,
a family which comprehends the Lobster and
the Crab, we find this part dilated into an oval
or globular organ, with very muscular coats,
capable of vigorous contractions, propelling its
contents with considerable force into the vessels,
and therefore clearly entitled to the appellation
of heart. The distinction between arteries and
veins, which can scarcely be made with any
precision in the systems of the inferior tribes, is
here perfectly determined by the existence of
this central organ of propulsion : for the vessels
into which the blood is sent by its contractions,
and which, ramifying extensively, distribute
it to distant parts, are indisputably arteries;
and conversely, the vessels, which collect the
blood from all these parts, and bring it back
to the heart, are as decidedly veins. The heart
of the lobster is situated immediately under the
carapace, or shell of the dorsal region of the
thorax, in a plane posterior to the stomach,
where it is not liable to be pressed against the
resisting shell, when the stomach is distended.
Its pulsations are very distinct, and are per-
formed with great regularity.

The importance of the heart, as the prime
agent in the circulation, increases as we advance
to the higher classes of animals, whose more
active and energetic functions require a con-
tinual and rapid renewal of nutrient fluid, and
render necessary the introduction of further re-

CIRCULATION IN THE VERTEBRATA.

259

finements into its structure. The supply of
blood to the heart, being in a constant stream,
produces a gradual dilatation of the cavity which
receives it ; and the muscular fibres of that cavity
are not excited to contraction, until they are
stretched beyond a certain point. But in order
effectually to drive the blood into every part of
the arterial system, where it has great resistances
to overcome, a considerable impulsive force is
required ; implying a sudden as well as powerful
muscular action. This object is attained, in all
vertebrated animals, by providing a second
muscular cavity, termed a ventricle, into which
the first cavity, or auricle^ throws the blood it has
received from the veins, with a sudden impulse ;
and thus the ventricle, being rapidly distended,
is excited to a much more quick and forcible

350

contraction than the auricle, and propels the
blood it contains into the artery, with an impetus

2fJ0 THE VITAL FUNCTIONS.

incomparably greater than could have resulted
from the action of the auricle alone. Fig. .350
represents the heart with its two cavities ; d being
the auricle, and e the ventricle ; together with
the main trunks of the veins (c, c,) which con-
vey the blood into the auricle ; and those of the
arteries (a), which receive it from the ventricle,
for distribution over the whole system.

The force of contraction in the principal
cavity of the heart being thus increased, it
becomes necessary to provide additional secu-
rities against the retrograde motion of its fluid
contents. Valves are accordingly interposed
between the auricle and ventricle ; and great
refinement of mechanism is displayed in their
construction. Fig. 351 represents their appear-

ance at (v) when the cavities, both of the auricle
(d), and the ventricle (e) are laid open : c, c, as
before, being the upper and lower venae cavse,
and A, the main trunk of the aorta. Tliese

CIRCULATION IN THE VERTEBRATA. 261

valves are composed of two loose membranes,
tlie fixed edges of which are attached circularly
to the aperture of communication between the
cavities, and their loose edges project into the
ventricle ; so that they perform the office of
flood-gates, allowing a free passage to the blood
when it is impelled into the ventricle, and being
pushed back the moment the ventricle contracts;
in which latter case they concur in accurately
closing the aperture, and preventing the return
of a single drop into the auricle. These valves
being attached to a wide circular aperture, it is
necessary that they should be restrained from
inverting themselves into the auricle, at each
contraction of the ventricle. For this purpose
there are provided slender ligaments (which are
seen in Fig. 351), fixed by one end to the edge
of the valve, and by the other to some part of
the inner surface of the ventricle ; so that the
valve is always kept within the cavity of the
latter. In the auricle, the same purpose is
answered by the oblique direction in which the
veins enter it.

The arteries themselves, especially the main
trunk of the aorta, as it issues from the heart, are
muscular, and when suddenly distended, contract
upon their contents. It was necessary, therefore,
to provide means for preventing any reflux of
blood into the ventricle during their contraction ;
and for this purpose another set of valves (r, Fig.

262

THE VITAL FUNCTIONS.

351) is placed at the beginning of these tubes,
where they arise from the ventricle. These valves
consist usually of three membranes, which have
the form of a crescent, and are capable of closing
the passage so accurately, that not a drop of
blood can pass between them.*

In order to convey a more clear idea of the
course of the blood in the circulatory system, I

have drawn the diagram,
Fig. 352, exhibiting the
general arrangement of
its component parts. The
main arterial trunk, or
Aorta (a), while proceed-
ing in its course, gives off
numerous branches (b),
which divide and subdi-
vide, till the ramifications
(p p) arrive at an extreme
degree of minuteness ;
and they are finally distributed to every organ,
and to the remotest extremities of the body. They
frequently, during their course, communicate with
one another, or atiastomose, as it is termed, by
collateral branches ; so as to provide against in-

* In the artery of the Shark, and other cartilaginous fishes,
where the action of the vessel is very powerful, these valves are
much more numerous, and arranged in rovers, occupying several
parts of the artery. Additional valves are also met with in other
fishes at the branching of large arteries.

CIRCULATION IN THE VERTEBRATA. 263

terruptions to the circulation, which might arise
from accidental obstructions in any particular
branches of this extended system of canals.
The minutest vessels (p p), which in incalculable
numbers, pervade every part of the frame, are
named, from their being finer than hairs, capil-
lary vessels.

After the blood, thus transmitted to the differ-
ent parts of the body by the arteries, has supplied
them with the nourishment they require, it is
conveyed back to the heart by the veins, which,
commencing from the extreme ramifications of
the arteries, bend back again in a course di-
rected towards the heart. The smaller branches
join in succession to form larger and larger
trunks, till they are at length all united into one
or two main pipes, called the Vence cavcB, (c),
which pour their accumulated torrent of blood
into the general reservoir, the heart; entering
first into the auricle (d), and thence being carried
forward into the ventricle (e), which again pro-
pels it through the Aorta. The veins are larger
and more numerous than the arteries, and may
be compared to rivers, which collecting all the
water that is not imbibed by the soil, and recon-
veying it into its general receptacle, the ocean,
perform an analogous office in the economy of
the earth.

The communications of the capillary arteries
with the veins are beautifully seen, under the

264 THE VITAL FUNCTIONS.

microscope, in the transparent membranes of
frogs or fishes. The splendid spectacle, thus
brought within the cognizance of our senses, of
unceasing activity in the minutest filaments of
the animal frame, and of the rapid transit of
streams of fluid, bearing along with them minute
particles, which appear to be pressing forwards,
like the passengers in the streets of a crowded
city, through multitudes of narrow and winding
passages, can never fail, when first beheld, to
fill the mind with astonishment * ; a feeling,
which must be exalted to the highest admiration
on reflecting that what we there behold is at all
times going on within us, during the whole
period of our lives, in every, even the minutest
portion of our frame. How inadequate, then,
must be any ideas we are capable of forming
of the incalculable number of movements and of
actions, which are conducted in the living sys-
tem ; and how infinite must be the prescience
and the wisdom, by which these multifarious and
complicated operations were so deeply planned,
and so harmoniously adjusted !

* Lewenhoeck, speaking of the delight he experienced on
viewing the circulation of the blood in tadpoles, uses the follow-
ing expressions. " This pleasure has oftentimes been so recrea-
ting to me, that I do not believe that all the pleasure of foun-
tains, or water-works, either natural or made by art, could have
pleased my sight so well, as the view of these creatures has
given jme." — Phil. Trans, xxii. 453.

265

§ 3. Respiratory Circulation.

The object of the circulation is not merely to
distribute the blood through the general system
of the body ; it has also another and a very im-
portant office to perform. The blood undergoes,
in the course of its circulation, considerable
changes, both in its colour and in its chemical
composition. The healthy blood transmitted by
the arteries is of a bright scarlet hue ; that
brought back by the veins is of a dark purple,
from its containing an excess of carbon, and
is consequently unfit to be again circulated.
Whenever, from some derangement in the func-
tions, this dark blood finds its way into the
arteries, it acts as a poison on every organ which
it reaches, and would soon, if it continued to
circulate, destroy life. Hence it is necessary
that the blood which returns by the veins should
undergo purification, by exposure either to the
air itself, or to a fluid containing air, for the
purpose of restoring and preserving its salutary
qualities. The heart and vascular system have
therefore the additional task assigned them of
conveying the vitiated venous blood to certain
organs, where it may have access to the air, and
receive its vivifying influence ; and to this office
a distinct set of arteries and veins is appro-

266

THE VITAL FUNCTIONS.

priated, constituting a distinct circulation. This
I have endeavoured to illus-
trate by the diagram, Fig.
353, where d represents the
auricle, and e the ventricle
of the heart; and a and c,
the main arterial and venous
trunks; and where the two
circulations are, for the sake
of distinctness, supposed to
be separated from one ano-
ther, so that the two systems
of vessels may occupy dif-
ferent parts of the diagram.
The vessels which pervade the body generally
(b), and are subservient to nutrition, belong to
what is termed the greater^ or systemic circula-
tion : those which circulate the blood through
the respiratory organs, (r), for the purpose of
aeration, compose the system of the lesser, or
respiratory circulation.

Few subjects in Physiology present a field
of greater interest than the comparison of the
modes in which these two great functions are,
in all the various classes of animals, exactly
adjusted to each other. So intimately are the
organs of circulation related to those which dis-
tribute the blood to the respiratory organs, that
we never can form a clear idea of the former,
without a close reference to the latter of these

RESPIRATORY CIRCULATION. 2G7

systems. While describing the several plans
of circulation presented to us by the different
classes, I shall be obliged, therefore, to assume
both the necessity of the function of respiration,
and of a provision of certain organs for the
reception of air, (either in its gaseous form, or
as it is contained in water,) where the blood
may be subjected to its action. It is necessary,
also, to state that the organs for receiving atmos-
pheric air in its gaseous state are either lungs,
or jmlmo7iary cavities, while those which are
constructed for aquatic respiration are termed
gills, or branchicB ; the arteries and the veins
which carry on this respiratory circulation, being
termed pulmonary, or branchial, according as
they relate to the one or the other description
of respiratory organs.

In many animals it is only a part of the cir-
culating blood which undergoes aeration ; the
pulmonary or branchial arteries and veins being
merely branches of the general system of blood
vessels; so that in this case, which is repre-
sented in the preceding figure (353), the lesser
circulation is included as a part of the ge-
neral circulation. But in all the higher classes
the whole of the blood is, in some part of its
circuit, subjected to the influence of the air;
the pulmonary, being then distinct from the
systemic circulation. In the Annelida, for in-
stance, the venae cavae, which bring back the

268 THE VITAL FUNCTIONS.

blood from the system, unite to form one or
more vessels, which then assume the function
of arteries, subdividing and ramifying upon the
branchial organs ; after this the blood is again
collected by the branchial veins, which unite
into one trunk to form the arteries of the sys-
temic circulation.

Most insects, especially when arrived at the
advanced stages of their developement, have too
imperfect a circulation to effect the thorough
aeration of the blood : and indeed the greater part
of that fluid is not contained within the vascular
system, but permeates the cavities and cellular
texture of the body. It will be seen, when I
come to treat of respiration, that the same object
is accomplished by means totally independent of
the circulatory apparatus ; namely, by a system
of air-tubes, distributed over every part of the
body. But an apparatus of this kind is not
required in those Arachnida where the circulation
is vigorous, and continues during the whole of
life: here, then, we again meet with a pulmonary
as well as a systemic circulation, in conjunction
with internal cavities for the reception of air.

In the Crustacea the circulation is conducted
on the same general plan as in the Annelida ; the
blood from every part of the body being collected
by the Venai Cavae, which are exceedingly capa-
cious, and extend, on each side, along the lower
surface of the abdomen. They send out branches,

RESPIRATORY CIRCULATION.

269

which distribute the blood to the gills ; but these
branches, at their origin, suddenly dilate, so as
to form large receptacles, which are called
sinuses, where the blood is allowed to accumu-
late, and where, by the muscularity of the ex-
panded coats of the vessels, it receives an addi-
tional force of propulsion. From the branchiae
the blood is returned by another set of veins
to the elongated heart formerly described, and

propelled by that or-
gan into the systemic
arteries. Fig. 354
shows the relative si-
tuation of these ves-
sels, when isolated
and viewed from be-
hind, in the Maia squinado. c, c, are the venae
cavae ; e, e, the venous sinuses above-mentioned ;
F, F, are the branchial arteries ; g, the gills, or
branchiae ; and i, i, the branchial veins termina-
ting in the heart l.*

In the Mollusca, the heart acquires greater
size, compared with the other organs, and exerts
a proportionally greater influence as the "^prime
mover in the circulation. A beautiful gradation
may be perceived in the developement of this

* A minute account of the organs of circulation in the Crus-
tacea is given by Audouin and Milne Edwards, in the Annales
des Sciences Naturelles, xi, 283 and 352, from which work the
above figure is taken.

270 THE VITAL FUNCTIONS.

organ in the several orders of this class ; the
JBrancMopoda having two hearts, one placed upon
each of the two lateral trunks of the branchial
veins ; the Gasteropoda having a single heart,
furnished with an auricle ; and the Acephala
being provided with a heart, which has a single
ventricle, but two auricles, corresponding to the
two trunks of the branchial veins.*

The most remarkable variety of structure is
that exhibited by the Cephalopoda. We have
already seen, in the Crustacea, dilatations of the
venae cavae, at the origin of the branchial arte-
ries ; but in the Nautilus the dilatations of the
branchial veins are of such a size, as to be almost
entitled to the appellation of auricles. The
Sepia, in whose highly organized system there is
required great additional power to propel the
blood with sufficient force through the gills, is
provided with a large and complicated branchial
apparatus; and the requisite power is supplied
by two additional hearts, situated on the venae
cavse, of which they appear as if they were
dilatations, immediately before the branchial
arteries are sent ofF.t They are shown at e, e.
Fig. 355, which represents this part of the vas-

* A great number of bivalve Mollusca exhibit the singular pe-
culiarity of the lower portion of the intestinal tube traversing
through the cavity of the heart.

t These veins are surrounded by a great number of blind
pouches, which have the appearance of a fringe ; the use of this
singular structure is unknown.

RESPIRATORY CIRCULATION IN FISHES. 271

cular system of the Loligo, detached from the
surrounding parts ; the course of the blood being

indicated by arrows, c is one of the three
trunks constituting the venae cavae, proceeding
from above, dividing into two branches as it de-
scends, and terminating, conjointly with the two
venous trunks (d), which are coming from below,
into the lateral or branchial hearts (e, e), already
mentioned. Thence the blood is conveyed by the
branchial arteries, (f, f), on each side, to the gills
(g), and returned, by the branchial veins, (i), to
the large central, or systemic heart (l), which
again distributes it, by means of the systemic ar-
teries, to every part of the body. The cuttle-fish
tribe is the only one thus furnished with three
distinct hearts for carrying on a double circula-
tion : none of these hearts are furnished with
auricles.

272

THE VITAL FUNCTIONS.

356.

The remarkable distribution of the muscular
powers, which give an impulse to the circulating
fluids, met with in the Sepia, constitutes a step
in the transition from MoUusca to Fishes. In
this latter class of animals, the two lateral hearts
have united into a single central heart ; while the
aortic heart has entirely disappeared ; and thus
the position of the heart with respect to the two
circulations is just the reverse of that which it

has in the invertebrated
classes. The plan in Fishes
is shown in the diagram,
Fig. 356 ; where the cen-
tral organs are seen to con-
sist of four cavities, (c,d,e,
f), opening successively the
one into the other. The
heart belongs exclusively
to the gills ; and there pro-
ceeds from it, not the aorta,
but the trunk of those
branchial arteries (f), which convey the whole of
the blood to the respiratory organs (g, h). This
blood, after being there aerated, is collected by
the branchial veins (i,) which unite into a single
trunk (a), passing down the back, and perform-
ing, without any intermediate heart, the office of
an aorta ; that is, it divides into innumerable
branches, and distributes the blood to every part

RESPIRATORY CIRCULATION IN INSECTS. 273

of the system.* The blood is then reconveyed
to the heart by the ordinary' veins, which form a
large vena cava (c). This vein is generally con-
siderably dilated at its termination, or j list before
it opens into the auricle ; constituting what has
been termed a venous sinus. This, then, is fol-
lowed by the auricle (d) and the ventricle (e) ;
but, besides these cavities, there is also a fourth
(f), formed by a dilatation of the beginning of
the branchial artery, and termed the bulbus arte-
riosus; contributing, doubtless, to augment the
impetus with which the blood is sent into the
branchial arteries.

The circulation in Reptiles is not double, like
that of fishes ; for only a part of the blood is
brought under the influence of the air in the
pulmonary organs. All the animals belonging
to this class are cold-blooded, sluggish, and
inert ; they subsist upon a scanty allowance of
food, and are astonishingly tenacious of life.
The simplest form in which we meet with this
mode of circulation is in the Batrachia; it is

* The caudal branch of the aorta is protected by the roots of
the inferior spinous processes, joining to form arches through
which it passes ; and frequently the artery is contained in a bony
channel, formed by the bodies of the vertebrae, which effectually
secures it from all external pressure. In the Sturgeon even the
abdominal aorta is thus protected; being entirely concealed
within this bony canal.

VOL. II. T

274

THE VITAL FUNCTIONS.

357 H

shown in the diagram, Fig. 357. The heart of
the Frog, for example, may be considered as
consisting of a single auricle
(d), and a single ventricle
(e).* From the latter there
proceeds one great arterial
trunk, which is properly the
aorta. This aorta soon di-
vides into two trunks, which,
after sending branches to the
head and neck, bend down-
wards (as is seen at o, p),
and unite to form a single
trunk (a), which is the de-
scending aorta. From this vessel proceed all
the arteries which are distributed to the trunk
and to the limbs, and which are represented
as situated at b : these arterial ramifications are
continued into the great venous trunks, which,

* Dr. Davy has observed that although the auricle appears
single, when viewed externally, its cavity is in reality divided
into two compartments by a transparent membranous partition,
in which some muscular fibres are apparent : these communicate
with the cavity of the ventricle by a common opening, provided
with three semilunar valves. Edin. Phil. Journal; xix, 161.

Mr. Owen informs me that his own observations confirm
those of Dr. Davy ; and that he has discovered that the Siren
has also a distinct pulmonic auricle ; whence he infers that
wherever lungs are suflfiiciently developed to effect a change in
the blood, that fluid is conveyed to the ventricle by a distinct
route, and the pulmonary veins thus defended from the pressure
of the blood accumulated in the right auricle.

RESPIRATORY CIRCULATION IN REPTILES. 275

as usual, constitute the venae cavse (c), and ter-
minate in the auricle (d).

From each of the trunks which arise from the
primary division of the aorta, there proceed the
small arteries (f), which are distributed to the
lungs (g, h), and convey to those organs a part
only of the mass of circulating blood. To these
pulmonary arteries there correspond a set of
veins, uniting in the trunks (i), which bring
back the aerated blood to the auricle of the
heart (d), where it is mixed with the blood
which has returned by the venae cavae (c), from
the general circulation. Thus the blood is only
partially aerated ; in consequence of the lesser
circulation being here only a branch of the
greater.

Nothing is more curious or beautiful than the
mode in which Nature conducts the gradual tran-
sition of the branchial circulation of the tadpole,
into the pulmonary circulation of the frog. In
the former, the respiratory organs are constructed
on the model of those of fishes, and respiration
is performed in the same manner as in that class
of animals : the heart is consequently essentially
branchial ; sending the whole of its blood to the
gills, the veins returning from which (describing
the course marked by the dotted lines m, n, in
the diagram), unite, as in fishes, to form the
descending aorta. As the lungs develope, small
arterial branches, arising from the aorta, are

276 THE VITAL FUNCTIONS.

distributed to those organs; and in proportion
as these arteries enlarge, the branchial arteries
diminish ; until, on their becoming entirely ob-
literated, the course of the blood is wholly
diverted from them, and flows through the
enlarged lateral trunks (o, p,) of which the
junction constitutes the descending aorta. This
latter vessel now receives the whole of its blood
directly from the heart ; which, from being
originally a branchial, has become a systemic
heart.

The heart of the Chelonian reptiles, such as the
ordinary species of Tortoises and Turtles, has
two distinct auricles ; the one, receiving the blood
from the pulmonary veins ; the other, from those
of the body generally ; so that the mixture of
aerated and vitiated blood takes place, not in the
auricle, but in the ventricle itself. When all the
cavities are distended with blood, the two auricles
being nearly of the same size as the ventricle,
the whole has the appearance of a union of
three hearts. The circulatory system of the
Ophidia is constructed on a plan very similar
to that of the Chelonia.

In the Saurian reptiles, the structure becomes
again more complicated. In the Chameleon each
auricle of the heart has a large venous sinus,
appearing like two supplementary auricles.*

* Houston ; Trans. Roy. Irish Acad, xv, 189.

WARM-BLOODED CIRCULATION. 277

The heart of the Crocodile has not only two
auricles, but its ventricle is divided, by two par-
titions, into three chambers : each of the par-
titions is perforated to allow of a free communi-
cation between the chambers ; and the passages
are so adjusted as to determine the current of
aerated blood, returning from the lungs, into
those arteries, more especially, which supply the
head and the muscles of the limbs; while the
vitiated blood is made again to circulate through
the arteries of the viscera.*

It is in warm-blooded animals that the two
offices of the circulation are most efficiently per-
formed ; for the whole of the blood passes
alternately through the greater and the lesser
circulations ; and a complete apparatus is pro-

* It would appear, from this arrangement of the vessels, that
the brain, or central organ of the nervous system, requires,
more than any other part, a supply of oxygenated blood for the
due performance of its functions. The curious provision which
is made for sending this partial supply of blood of a particular
quality in the larger kinds of reptiles, such as the Crocodile,
has been pointed out by many anatomists ; but has been lately
investigated more particularly by M. Martin St. Ange. (See
the Report of G. St. Hilaire, Revue Medicale, for April, 1833).
It is found that in these animals, as well as in the Chelonia, a
partial respiratory system is provided for by the admission,
through two canals opening externally, of aerated water into
the cavity of the abdomen, where it may act upon the blood
which is circulating in the vessels. Traces of canals of this
description are also met with in some of the higher classes of
vertebrated animals, as, for instance, among the Mammalia, in
the Monoiremata and the Marsupialia.

278

THE VITAL FUNCTIONS.

vided for each. There are, in fact, two hearts ;
the one on the left side impelling the blood
through the greater, or systemic circulation ;
the other, on the right side, appropriated to the
lesser, or pulmonary circulation. The annexed

diagram (Fig. 359), il-
lustrates the plan of the
circulation in warm-
blooded animals. From
the left ventricle (l) the
blood is propelled into
the aorta (a), to be dif-
fused through the arte-
ries of the system (b) to
every part, and pene-
trating into all the capil-
lary vessels ; thence it
is returned by the veins, through the vense cavae
(c), to the right auricle (d), which delivers it
into the right ventricle (e). This right ventricle
impels the blood, thus received, through the
pulmonary arteries (f), into the lungs (at h),
where it is aerated, and whence it is recon-
veyed by the pulmonary veins (i), into the left
auricle (k), which immediately pours it into the
left ventricle (l), the point from whence we
set out.

Both the right and the left heart have their
respective auricles and ventricles ; but they are
all united in one envelope, so as to compose

WARM-BLOODED CIRCULATION. 27,9

ill appearance but a single organ:* still, how-
ever, the right and left cavities are kept per-
fectly distinct from one another, and are sepa-
rated by thick partitions, allowing of no direct
transmission of fluid from the one side to the
other. These two hearts may therefore be com-
pared to two sets of chambers under the same
roof; having each their respective entrances
and exits, with a party-wall of separation be-
tween them. This junction of the two hearts
is conducive to their mutual strength ; for the
fibres of each intermix and even co-operate in
their actions, and both circulations are carried
on at the same time ; that is, both ventricles
contract or close at the same instant; and the
same applies to the auricles. The blood which
has just returned from the body, and that from
the lungs, the former by the venae cavae, the

* A remarkable exception to this general law of consolidation

occurs in the heart of the Du-
gong, represented in Fig. 360,
in which it may be seen that
the two ventricles, (e and l), are
almost entirely detached from
each other. In this figure, which
is taken from the Philosophical
Transactions for 1820, d is the
right or systemic auricle ; e the
right or pulmonary ventricle ; f
the pulmonary artery ; k the left
or pulmonary auricle ; l the left
or systemic ventricle ; and a the aorta.

280 THE VITAL FUNCTIONS.

latter by the pulmonary veins, fill their respec-
tive auricles at the same instant; and both
auricles, contracting at the same moment, dis-
charge their contents simultaneously into their
respective ventricles. In the like manner, at
the moment when the left ventricle is propelling
its aerated blood into the aorta, for the purposes
of general nutrition, the right ventricle is like-
wise driving the vitiated blood into the pul-
monary artery, in order that it may be purified
by the influence of the air. Thus the same
blood which, during the interval of one pulsation,
was circulating through the lungs, is, in the
next, circulating through the body ; and thus
do the contractions of the veins, auricles, ven-
tricles, and arteries all concur in the same
general end, and establish the most beautiful
and perfect harmony of action.*

* Evidence is afforded of the human conformation being
expressly adapted to the erect position of the body by the
position of the heart, as compared with quadrupeds ; for in the
latter, the heart is placed directly in the middle of the chest,
with the point towards the abdomen, and not occupying any
portion of the diaphragm ; but in man, the heart lies obliquely
on the diaphragm, with the apex turned towards the left side.

281

^ 4. Distrihution of Blood-vessels.

In the distribution of the arteries in the animal
system, we meet with numberless proofs of wise
and provident arrangement. The great trunks
of both arteries and veins, which carry on the
circulation in the limbs, are conducted always
on the interior sides, and along the interior
angles of the joints, and generally seek the
protection of the adjacent bones. Grooves are
formed in many of the bones, where arteries
are lodged, with the evident intention of afford-
ing them a more secure passage. Thus the
principal arteries which supply the muscles of
the chest, proceed along the lower edges of
the ribs, in deep furrows formed for their pro-
tection. Arteries are often still more effectually
guarded against injury or obstruction by pass-
ing through complete tubes of solid bone. An
instance occurs in the arteries supplying the
teeth, which pass along a channel in the lower
jaw, exca¥ated through the whole length of the
bone. The aorta in fishes, after having supplied
arteries to the viscera of the abdomen, is con-
tinued to the tail, and passes through a channel,
formed by bony processes from the vertebrae ;
and the same kind of protection is afforded
to the corresponding artery in the Cetacea. In

282 THE VITAL FUNCTIONS.

the fore leg of the Lion, which is employed
in actions of prodigious strength, the artery,
without some especial provision, would have
been in danger of being compressed by the
violent contractions of the muscles : to guard
against this inconvenience, it is made to pass
through a perforation in the bone itself, where
it is completely secure from pressure. In like
manner the coffin bone of the Horse is per-
forated for the safe conveyance of the arteries
going to the foot.

The energy of every function is regulated
in a great measure by the quantity of blood
which the organs exercising that function re-
ceive. The muscles employed in the most
vigorous actions are always found to receive
the largest share of blood. It is commonly
observed that the right fore leg of quadrupeds,
as well as the right arm in man, is stronger
than the left. Much of this superior strength
is, no doubt, the result of education ; the right
arm being habitually more used than the left.
But still the different mode in which the arteries
are distributed to the two arms constitutes a
natural source of inequality. The artery sup-
plying the right arm with blood is the first
which arises from the aorta ; and it proceeds
in a more direct course from the heart than
the artery of the left arm, which has its
origin in common with the artery of that side

DISTRIBUTION OF BLOOD-VESSELS. 283

of the head. Hence it has been inferred that
the right arm is originally better supplied with
nourishment than the left. It may be alleged,
in confirmation of this view, that in birds, where
any inequality in the actions of the two wings
would have disturbed the regularity of flight,
the aorta, when it has arrived at the centre of
the chest, divides with perfect equality into two
branches, so that both wings receive precisely
the same quantity of blood ; and the muscles,
being thus equally nourished, preserve that
equality of strength, which their function rigidly
demands.

When a large quantity of blood is wanted in
any particular organ, and yet the force with
which it would arrive, if sent immediately by
large arteries, might injure the texture of that
organ, contrivances are adopted for diminishing
its impetus, either by making the arteries pursue
very winding and circuitous paths, or by sub-
dividing them, before they reach their destination,
into a great number of smaller arteries. The
delicate texture of the brain, for instance, would
be greatly injured by the blood being impelled
with any considerable force against the sides of
the vessels which are distributed to it ; and yet
a very large supply of blood is required by that
organ for the due performance of its functions.
Accordingly we find that all the arteries which
^0 to the brain are very tortuous in their course ;

284 THE VITAL FUNCTIONS.

every flexure tending considerably to diminish
the force of the current of blood.

In animals that graze, and keep their heads
for a long time in a dependent position, the
danger from an excessive impetus in the blood
flowing towards the head is much greater than
in other animals ; and we find that an ex-
traordinary provision is made to obviate this
danger. The arteries which supply the brain,
on their entrance into the basis of the skull,
suddenly divide into a great number of mi-
nute branches, forming a complicated net-work
of vessels ; an arrangement which, on the well
known principles of hydraulics, must greatly
check the velocity of the blood conducted
through them. That such is the real purpose
of this structure is evident from the branches
afterwards uniting into larger trunks when they
have entered the brain, through the substance of
which they are then distributed exactly as in
other animals, where no such previous sub-
division takes place.

In the Brady pus tridactylus, or great Ame-
rican Sloth, an animal remarkable for the slow-
ness of its movements, a plan somewhat ana-
logous to the former is adopted in the structure
of the arteries of the limbs. These arteries, at
their entrance into both the upper and lower ex-
tremities, suddenly divide into a great number
of cylindric vessels of equal size, communicating

FORCE OF THE HEART. 285

in various places by collateral branches. These
curiously subdivided arteries are exclusively
distributed to the muscles of the limbs ; for all
the other arteries of the body branch off in the
usual manner. This structure, which was dis-
covered by Sir A. Carlisle,* is not confined to
the Sloth, but is met with in other animals, as
the Lemur tardigradus, and the Lemur lorisy
which resemble the sloth in the extreme slug-
gishness of their movements. It is extremely
probable, therefore, that this peculiarity in the
muscular power results from this remarkable
structure in the arteries ; or is at least in some
way connected with it. In the Lion, and some
other beasts of prey, a similar construction is
adopted in the arteries of the head ; probably
with a view to confer a power of more permanent
contraction in the muscles of the jaws for hold-
ing a strong animal, such as a buffalo, and car-
rying it to a distance.

That we may form an adequate conception
of the immense power of the ventricle, or prime
mover in the circulation of the blood, we have
but to reflect on the numerous obstacles im-
peding its passage through the arterial system.
There is, first, the natural elasticity of the
coats of the arteries, which must be overcome
before any blood can enter them. Secondly,

* Phil. Trans, for 1800, p. 98, and for 1804, p. 17.

2HG THE VITAL FUNCTIONS.

the arteries are, in most places, so connected
with many heavy parts of the body, that their
dilatation cannot be effected without, at the same
time, communicating motion to them. Thus,
when we sit cross-legged, the pulsation of the
artery in the ham, which is pressed upon the
knee of the other leg, is sufficiently strong to
raise the whole leg and foot, at each beat of the
pulse. If we consider the great weight of the
leg, and reflect upon the length of the lever by
which that weight acts, we shall be convinced of
the prodigious force which is actually exerted by
the current of blood in the artery in thus raising
the whole limb. Thirdly, the winding course,
which the blood is forced to take, in following
all the oblique and serpentine flexures of the
arteries, must greatly impede its motion. But
notwithstanding these numerous and powerful
impediments, the force of the heart is so great,
that, in defiance of all obstacles or causes of
retardation, it drives the blood with immense ve-
locity into the aorta. The ventricle of the human
heart does not contain more than an ounce of
blood, and it contracts at least seventy times in
a minute ; so that above three hundred pounds of
blood are passing through this organ during
every hour that we live. " Consider," says Paley,
" what an affair this is when we come to very
large animals. The aorta of a whale is larger in
the bore than the main pipe of the water-works

VALVES OF THE VEINS. 287

at London Bridge; and the water roaring in its
passage through that pipe is inferior in impe-
tus and velocity to the blood gushing through
the whale's heart. An anatomist who under-
stood the structure of the heart, might say before-
hand that it would play ; but he would expect,
from the complexity of its mechanism, and the
delicacy of many of its parts, that it should always
be liable to derangement, or that it would soon
work itself out. Yet shall this wonderful ma-
chine go on, night and day, for eighty years
together, at the rate of a hundred thousand
strokes every twenty-four hours, having at every
stroke a great resistance to overcome, and shall
continue this action, for this length of time,
without disorder and without weariness. To
those who venture their lives in a ship, it has
often been said that there is only a plank be-
tween them and destruction ; but in the body,
and especially in the arterial system, there is
in many parts only a membrane, a skin, a
thread." Yet how well has every part been
guarded from injury : how providentially have
accidents been anticipated : how skilfully has
danger been averted !

The impulse which the heart, by its powerful
contraction, gives to the blood, is nearly ex-
pended by the time it has reached the veins :
nature has accordingly furnished them with
numerous valves, all opening in a direction

288 THE VITAL FUNCTIONS.

towards the heart; so that, as long as the
blood proceeds in its natural course, it meets
with no impediment ; while a retrograde motion
is effectually prevented. Hence external pres-
sure, occasionally applied to the veins, assists in
promoting the flow of blood to-
wards the heart ; and hence the
effects of exercise in accelerating
the circulation. Valves are more
especially provided in the veins
which pass over the muscles of the
extremities, or which run imme-
diately beneath the skin ; while
they are not found in the more
internal veins belonging to the
viscera, which are less exposed to unequal
pressure. These valves are delineated in Fig.
365, which represents the interior of one of the
large veins.

The situation and structure of the valves be-
longing to the hydraulic apparatus of the circu-
lation furnish as unequivocal proofs of design as
any that can be adduced. It was the observa-
tion of these valves that first suggested to the
mind of Harvey the train of reflexions which led
him to the discovery of the real course of the
blood in the veins, the arteries and the heart.
This great discovery was one of the earliest
fruits of the active and rational spirit of inquiry,
which at the era of Bacon's writings, was be-

VALVES OF THE VEINS. 289

ginning to awaken the human mind from its long
night of slumber, and to dissipate the darkness
which had, for so many ages, overshadowed the
regions of philosophy and science. We cannot
but feel a pride, as Englishmen, in the recollec-
tion, that a discovery of such vast importance as
that of the circulation of the blood, which has
led to nearly all the modern improvements in the
medical art, was made by our own countryman,
whose name will for ever live in the annals of
our race as one of its most distinguished bene-
factors. The consideration, also, that it had its
source in the study of comparative anatomy and
physiology, affords us a convincing proof of the
great advantages that may result from the culti-
vation of these sciences ; to which Nature, in-
deed, seems, in this instance, expressly to have
invited us, by displaying to our view, in the
organs of the circulation, an endless diversity of
combinations, as if she had purposely designed
to elucidate their relations with the vital powers,
and to assist our investigations of the laws of
organized beings.

VOL. II.

290

Chapter XI.

RESPIRATION.

<§ 1 . Respif'atioii in General.

The action of atmospheric air is equally neces-
sary for the maintenance of animal and vegetable
life. As the ascending sap of plants cannot
be perfected unless exposed to the chemical
agency of air in the leaves ; in like manner the
blood of animals requires the perpetual reno-
vation of its vital properties by the purifying in-
fluence of respiration. The great importance of
this function is evinced by the constant provision
which has been made by Nature, in every class
of animals, for bringing each portion of their
nutritive juices, in its turn, into contact with air.
Even the circulation of these juices is an object
of inferior importance, compared with their
aeration ; for we find that insects, which have
but an imperfect and partial circulation of their
blood, still require the free introduction of air
into every part of their system. The necessity
for air is more urgent than the demand for food ;
many animals being capable of subsisting for a

RESPIRATION. 291

considerable time without nourishment, but all
speedily perishing when deprived of air. The
influence of this element is requisite as well for
the production and developement, as for the con-
tinuance of organized beings in a living state.
No vegetable seed will germinate, nor will any
egg, even of the smallest insect, give birth to a
larva, if kept in a perfect vacuum. Experiments
on this subject have been varied and multiplied
without end by Spallanzani, who found that
insects under an air pump, confined in rarefied
air, in general lived for shorter periods in pro-
portion to the degree to which the exhaustion of
air had been carried. Those species of infu-
soria, which are most tenacious of life, lived in
very rarefied air for above a month : others
perished in fourteen, eleven, or eight days; and
some in two days only. In this imperfect
vacuum, they were seen still to continue their
accustomed evolutions, wheeling in circles, dart-
ing to the surface, or diving to the bottom of the
fluid, and producing vortices by the rapid vibra-
tion of their cilia, to catch the floating particles
which serve as their food : in course of time,
however, they invariably gave indications of un-
easiness ; their movements became languid, a
general relaxation ensued, and they at length
expired. But when the vacuum was rendered
perfect, none of the infusions of animal or vege-
table substances, which, under ordinary circum-

2})2 THE VITAL FUNCTIONS.

Stances, soon swarm with millions of these micro-
scopic beings, ever exhibited a single animal-
cule ; although they soon made their appearance
in great numbers, if the smallest quantity of air
was admitted into the receiver.

Animals which inhabit the waters, and remain
constantly under its surface, such as fishes, and
the greater number of mollusca, are necessarily
precluded from receiving the direct action of
atmospheric air in its gaseous state. But as all
water exposed to the air soon absorbs it in large
quantities, it becomes the medium by which that
agent is applied to the respiratory organs of
aquatic animals ; and the oxygen it contains may
thus act upon the blood with considerable effect;
though not, perhaps, to the same extent as when
directly applied in a gaseous state. The air
which is present in water is, accordingly, as
necessary to these animals as the air of the
atmosphere is to those which live on land : hence
in our inquiries into the respiration of aquatic
animals, it will be sufficient to trace the means
by which the surrounding water is allowed to
have access to the organs appropriated to this
function ; and in speaking of the action of the
water upon them, it will always be understood
that I refer to the action of the atmospheric air
which that water contains.

Respiration, in its different modes, may be
distinguished, according to the nature of the

AQUATIC RESPIRATION. 29;)

iiiedium which is breathed, into aquatic or atmo-
spheric ; and in the former case, it is either cuta-
neous, or branchial, according as the respiratory
organs are external or internal. Atmospheric
respiration, again, is either tracheal, or pulmo-
nary, according as the air is received by a
system of air tubes, denominated tracheae, or into
pulmonary cavities, composing the lungs.

§ 2. Aquatic Respiration.

Zoophytes appear in general to be unprovided
Avith any distinct channels for conveying aerated
water into the interior of their bodies, so that it
may act in succession on the nutritive juices,
and after performing this office, may be expelled,
and exchanged for a fresh supply. It has ac-
cordingly been conjectured, on the presumption
that this function is equally necessary to them
as it is to all other animals, that the vivifying
influence of the surrounding element is exerted
through the medium of the surface of the body.
Thus it is very possible that in Polypi, while the
interior surface of the sac digests the food, its
external surface may perform the office of res-
piration ; and no other mode of accomplishing
this function has been distinctly traced in the
Acalephee. Medusae, indeed, appear to have a

294 THE VITAL FUNCTIONS.

further object than mere progression in the
alternate expansions and contractions of the
floating edges of their hemispherical bodies ; for
these movements are performed with great regu-
larity under all circumstances of rest or motion ;
and they continue even when the animal is taken
out of the water and laid on the ground, as long
as it retains its vitality. The specific name of
the Medusa pulmo* (the Puhnone Marino of the
Italians), is derived from the supposed resem-
blance of these movements to those of the lungs
of breathing animals. The large cavities ad-
jacent to the stomach, and which have been
already pointed out in Fig. 249 and 252,t have
been conjectured to be respiratory organs, chiefly,
I believe, because they are not known to serve
any other purpose.

The JEjitozoa, in like manner, present no ap-
pearance of internal respiratory organs ; so that
they probably receive the influence of oxygen
only through the medium of the juices of the
animals on which they subsist. PlanariiB, which
have a more independent existence, though en-
dowed with a system of circulating vessels, have
no internal respiratory organs ; and whatever
respiration they perform must be wholly cuta-

* See the delineation of this animal in Fig. 135, vol. i. p. 276.
t Pages 86 and 87 of this volume.

AQ^UATIC RESPIRATION. 2.95

neons. Such is also the condition of several of
the simpler kinds of Annelida; but in those
which are more highly organized, an apparatus
is provided for respiration, which is wholly ex-
ternal to the body, and appears as an appendage
to it ; consisting generally of tufts of projecting
fibres, branching like a plume of feathers, and
floating in the surrounding fluid. The Lum-
bricus marinus, or lob-worm,* for example, has
two rows of branchial organs of this description,
one on each side of the body ; each row being
composed of from fourteen to sixteen tufts. In
the more stationary Annelida, which inhabit
calcareous tubes, as the Serpula and the Tere-
helltty these arborescent tufts are protected by a
sheath, which envelopes their roots ; and they
are placed on the head, as being the only part
which comes in contact with the water.

Most of the smaller Crustacea have branchiae
in the form of feathery tufts, attached to the
paddles near the tail, and kept in incessant
vibratory motion, which gives an appearance of
great liveliness to the animal, and is more
especially striking in the microscopic species.
The variety of shapes which these organs assume
in different tribes is too great to allow of any

* Arenicola piscatorum (Lam.). See a delineation of this
marine worm in Fig. 135, voh i. p. 276.

296 THE VITAL FUNCTIONS.

specific description of them in this place : but
amidst these varieties it is sufficiently apparent
that their construction has been, in all cases, de-
signed to obtain a considerable extent of surface
over which the minute subdivisions of the blood-
vessels might be spread, in order to expose them
fully to the action of aerated water.

The Mollusca, also, present great diversity in
the forms of their respiratory organs, although
they are all, with but a few exceptions, adapted
to aquatic respiration. In many of the tribes
which have no shell, as the Thetis, the Doris, and
the IVitonia, there are arborescent gills projecting
from different parts of the body, and floating in
the water. In the Lepas, or barnacle, a curious
family, constituting a connecting link between
molluscous and articulated animals, these organs
are attached to the bases of the cirrhi, or jointed
tentacula, which are kept in constant motion,
in order to obtain the full action of the water on
the blood-vessels they contain.

We are next to consider the extensive series
of aquatic animals in which respiration is carried
on by organs situated in the interior of the body.
The first example of internal aquatic respiration
occurs in the Holothnria, where there is an
organ composed of ramified tubes, situated in a
jCavity having an external opening for the ad-
mission of the aerated water, which is brought to

AQUATIC RESPIRATION. 297

act on a vascular net-work, containing the nutri-
tive juices of the animal, and apparently per-
forming a partial circulation of those juices. A
still more complicated system of respiratory
channels occurs, both in the Echinus and Aste-
rias, where they open by separate, but very
minute orifices, distinct from the larger aper-
tures through which the feet protrude ; and the
water admitted through these tubes is allowed to
permeate the general cavity of the body, and is
thus brought into contact with all the organs.

The animals composing the family of Ascidice
have a large respiratory cavity, receiving the
water from without, and having its sides lined
with a membrane, which is thrown into a great
number of folds ; thus considerably extending
the surface on which the water is designed to
act. The entrance into the oesophagus, or true
mouth, is situated at the bottom of this cavity ;
that is, at the part most remote from the ex-
ternal orifice ; so that all the food has to pass
through the respiratory cavity, before it can be
swallowed, and received into the stomach.

In several of the Annelida, also, we find in-
ternal organs of respiration . The Liimhricus ter-
restris, or common earth-worm, has a single row
of apertures, about 120 in number, placed along
the back, and opening between the segments of
the body : they each lead into a respiratory

298 THE VITAL FUNCTIONS.

vesicle, situated between the integument and the
intestine.* The Leech has sixteen minute ori-
fices of this kind on each side of the body, open-
ing internally into the same number of oval cells,
which are respiratory cavities ; the water passing
both in and out by the same orifices. ^

The Aphrodita aculeata has thirty-two orifices
on each side, placed in rows, opening into the
abdominal cavity, and admitting the water, which
is afterwards received into numerous pouches,
containing csecal processes of the intestine ; so
that the nutriment is aerated almost as soon as
it is prepared by the digestive organs.^

In all the higher classes of aquatic animals,
where the circulation is carried on by means
of a muscular heart, and where the whole of
the blood is subjected, during its circuit, to the
action of the aerated water, the immediate organs
of respiration consist of long, narrow filaments,
in the form of a fringe ; and the blood-vessels

* A minute description of these organs is given by Morren, in
pages 53 and 148 of his work already quoted.

t The blood, after being aerated in these cells, is conveyed
into the large lateral vessels, by means of canals, which pass
transversely, forming loops, situated between the cseca of the
stomach. These loops are studded with an immense number of
small rounded bodies of a glandular appearance, resembling those
which are appended to the vense cava? of the cephalopoda.

X Home, Philos. Trans, for 1815, p. 259.

AQUATIC RESPIRATION. 2i)i)

belonging to the respiratory system are exten-
sively distributed over the whole surface of these
filaments. Organs of this description are deno-
minated Bra7ichi{B, or Gills ; and the arteries
which bring the blood to them are called the
branchial arteries; the veins, which convey it
back, being, of course, the branchial veins.

The larger Crustacea have their branchiae
situated on the under side of the body, not only
in order to obtain protection from the carapace,
which is folded over them, but also for the sake
of being attached to the haunches of the feet-
jaws, and thoracic feet ; and thus participating in
the movements of those organs. They may be
seen in the Lobster, or in the Crab, by raising
the lower edge of the carapace. The form of
each branchial lamina is shown at g, in Fig.
354:* they consist of assemblages of many
thousands of minute filaments, proceeding from
their respective stems, like the fibres of a feather ;
and each group having a triangular, or pyra-
midal figure. The number of these pyramidal
bodies varies in the different genera ; thus the
Lobster has twenty-two, disposed in rows on
each side of the body ; but in the Crab, there
are only seven on each side. To these organs
are attached short and flat paddles, which are

* Page 269 of this volume.

.300 THE VITAL FUNCTIONS.

moved by appropriate muscles, and are kept in
incessant motion, producing strong currents of
water, evidently for the purpose of obtaining the
full action of that element on every portion of the
surface of the branchiae.

In the greater number of Mollusca, these im-
portant organs, although external with respect
to the viscera, are within the shell, and are
generally situated near its outer margin. They
are composed of parallel filaments, arranged like
the teeth of a fine comb ; and an opening exists
in the mouth for admitting the water which is
to act upon them.* In the Gasteropoda, or
inhabitants of univalve shells, this opening is
usually wide. In the Acephala, or bivalve mol-
lusca, the gills are spread out, in the form of
laminae, round the margin of the shell ; as
exemplified in the Oyster, where it is commonly
known by the name of heard. The aerated
water is admitted through a fissure in the
mouth ; and when it has performed its office

* These filaments appear, in many instances, to have the
power of producing currents of water in their vicinity by the
action of minute cilia, similar to those belonging to the tentacula
of many polypi, where the same phenomenon is observable.
Thus if one of the branchial filaments of the fresh water muscle
be cut across, the detached portion will be seen to advance in
the fluid by a spontaneous motion, like the tentaculum of a
polype, under the same circumstances. Similar currents of
water, according to the recent observations of Mr. Lister, and
apparently determined by the same mechanism of vibratory cilia,
take place in the branchial sac of Ascidi'*.

RKSPIRATION IN FISHES. 301

in respiration, is usually expelled by a sepa-
rate opening. The part of the mouth through
which the water is admitted to the branchiaB is
sometimes prolonged ; forming a tube, open at
the extremity, and at all times allowing free
ingress and egress to the water, even when the
animal has withdrawn its body wholly within
its shell. Sometimes one, and sometimes two
tubes of this kind are met with ; and they are
often protected by a tubular portion of shell, as
is seen in the Murex, JBuccinum^ and Stromhus ;
in other instances, the situation of the tube is
only marked by a deep notch in the edge of the
shell. In those mollusca which burrow in the
sand, this tube can be extended to a considerable
length, so as to reach the water, which is alter-
nately sucked in and ejected by the muscular
action of the mouth. In those Acephala which
are unprovided with any tube of this kind, the
mechanism of respiration consists simply in the
opening and shutting of the shell. By watch-
ing them attentively we may perceive that the
surrounding water is moved in an eddy by these
actions, and that the current is kept up without
interruption. All the Sepiae have their gills en-
closed in two lateral cavities, which communicate
with a funnel-shaped opening in the middle of
the neck, and alternately receiving and expelling
the water by the muscular action of its sides.
The forms assumed by the respiratory organs in

.'J02

THE VITAL FUNCTIONS.

this class are almost infinitely diversified, while
the general design of their arrangement is still
the same.

As we rise in the scale of animals, the respira-
tory function assumes a higher importance. In
Fishes the gills form large organs, and the con-
tinuance of their action is more essential to life
than it appears to be in any of the inferior
classes : they are situated, as is well known, on
each side of the throat in the immediate vicinity
of the heart. Their usual form is shown at g g,

Fig. 366, where they are represented on one side
only, but in their relative situations with respect
to the auricle (d), and ventricle (e), of the heart ;
the bulbus arteriosus (b), and the branchial ar-
tery (f). They have the same fringed structure

RESPIRATION IN FISHES. 303

as in the mollusca, the fibres being set close to
each other, like the barbs of a feather, or the
teeth of a fine comb, and being attached, on each
side of the throat, in double rows, to the convex
margins of four cartilaginous or osseous arches,
which are themselves connected with the jaws
by the bone called the os hyoides. The mode of
their articulation is such as to allow each arch
to have a small motion forwards, by which they
are separated from one another ; and by moving
backwards they are again brought together,
or collapsed. Each filament contains a slender
plate of cartilage, giving it mechanical sup-
port, and enabling it to preserve its shape
while moved by the streams of water, which
are perpetually rushing past. When their sur-
faces are still more minutely examined, they
are found to be covered with innumerable mi-
nute processes, crowded together like the pile
of velvet ; and on these are distributed myriads
of blood-vessels, spread, like a delicate net-work,
over every part of the surface. The whole
extent of this surface exposed to the action of
the aerated water, by these thickly set filaments,
must be exceedingly great.*

A large flap, termed the Operculum^ extends
over the whole organ, defending it from injury,

* Dr. Monro computed that in the Skate, the surface of the
gills is, at the least, equal to the whole surface of the human
body.

.*J04 THE VITAL FUNCTIONS.

and leaving below a wide fissure for the escape
of the water, which has performed its office in res-
piration. For this purpose the water is taken in
by the mouth, and forced by the muscles of the
throat through the apertures which lead to the
branchial cavities : in this action the branchial
arches are brought forwards, and separated to a
certain distance from each other ; and the rush
of water through them unfolds and separates
each of the thousand minute filaments of the
branchiae, so that they all receive the full action
of that fluid as it passes by them. Such appears
to be the principal mechanical object of the act
of respiration in this class of animals ; and it is
an object that requires the co-operation of a
liquid, such as water, capable of acting by its
impulsive momentum in expanding every part
of the apparatus on which the blood vessels are
distributed. When a fish is taken out of the
water, this effect can no longer be produced ; in
vain the animal reiterates its utmost efforts to
raise the branchiae, and relieve the sense of
suffocation it experiences in consequence of the
general collapse of the filaments of those organs,
which adhere together in a mass, and can no
longer receive the vivifying influence of oxygen.*

* It has been generally stated by physiologists, even of the
highest authority, such as Cuvier, that the principal reason why
fishes cannot maintain life, when surrounded by air instead of
water, is that the branchiae become dry, and lose the power of

RESPIRATION IN FISHES. 305

Death is, in like manner, the consequence of a
ligature passed round the fish, and preventing
the expansion of the branchiae and the motion of
the opercula.

In all osseous fishes the opening under the
operculum for the exit of the respired water, is
a simple fissure; but in most of the cartilaginous
tribes, there is no operculum, and the water
escapes through a series of apertures in the
side of the throat. Sharks have five oblong
orifices of this description, as may be seen in
Fig. 367*.

As the Lamprey employs its mouth more con-
stantly than other fish in laying hold of its prey,
and adhering to other bodies, the organs of res-
piration are so constructed as to be independent
of the mouth in receiving the water. There are
seven external openings on each side (Fig. 368),
leading into the same number of separate oval
pouches, situated horizontally, and the inner
membrane of which has the same structure
as gills : these pouches are seen on a larger

acting -'ien thus deprived of their natural moisture ; for it might
otherwise naturally be expected that the oxygen of atmospheric
air would exert a more powerful action on the blood which cir-
culates in the branchiso, than that of merely aerated water.
The rectification of this error is due to Flourens, who pointed
out the true cause of suffocation, stated in the text, in a Memoir
entitled " Experiences sur le Mechanisme de la Respiration des
Poissons." — Annales des Sciences Naturelles, xx, 5.

* They are also visible in Fig. 293, (page 166), which is that
of the Squalus pristis, a species belonging to this tribe.
VOL. II. X

306 THE VITAL FUNCTIONS.

scale than in the preceding figure, in Fig. 369.
There is also an equal number of internal open-
ings, seen in the lower part of this last figure,
leading into a tube, the lower end of which is
closed, and the upper terminates by a fringed
edge in the oesophagus. The water which is
received by the seven lateral openings, enters at
one side, and after it has acted upon the gills,
passes round the projecting membranes. The
greater part makes its exit by the same orifices ;
but a portion escapes into the middle tube, and
thence passes, either into the other cavities, or
into the oesophagus*.

. In the Myxiiie, which feeds upon the internal
parts of its prey, and buries its head and part
of its body in the flesh, the openings of the
respiratory organs are removed sufficiently far
from the head to admit of respiration going on
while the animal is so employed ; and there are
only two external openings, and six lateral,
pouches on each side, with tubes similar to those
in the lamprey.

The Perca scandens (DaldorfF)t, which is a
fish inhabiting the seas of India, has a very
remarkable structure, adapting it to the main-

* It was commonly supposed that the respired water is ejected
through the nostril : but this is certainly a mistake, for the
nostrU has no communication with the mouth, as was pointed
out by Sir E. Home. Phil. Trans, for 1815, p. 259. These
organs have also been described by Bloch and Gaertner.

f Anthias testudineus (Bloch) : Anuhas (Cuv.)

RESPIRATION IN FISHES. 307

teiiance of respiration, and consequently to the
support of life for a considerable time when out
of the water ; and hence it is said occasionally
to travel on land to some distance from the
coast*. The pharyngeal bones of this fish have
a foliated and cellular structure, which gives
them a capacity for retaining a sufficient quan-
tity of water, not only to keep the gills moist,
but also to enable them to perform their proper
office ; while not a particle of water is suffered
to escape from them, by the opercula being
accurately closed.

The same faculty, resulting from a similar
structure, is possessed by the Ophicephalus^ which
is also met with in the lakes and rivers of India
and China. Eels are enabled to carry on respi-
ration when out of water, for a certain period,
in consequence of the narrowness of the aperture
for the exit of the water from the branchial
cavity, which enables it to be closed, and the
water to be retained in that cavity. f

I have already stated that, in all aquatic ani-
mals, the water which is breathed is merely the
vehicle by w^hich the air it contains is brought
into contact with the organs of respiration. This

* This peculiar faculty has been already alluded to in
volume i, p. 433.

t Dr. Hancock states that the Doras costatus, {Silurus cos-
tatus, Linn.) or Hassar, in very dry seasons, is sometimes seen,
in great numbers, making long marches over land, in search of
water. Edin. Phil. Journal, xx. 396.

308 THE VITAL FUNCTIONS.

air is constantly vitiated by the respiration of
these animals, and requires to be renewed by
the absorption of a fresh portion, which can
only take place when the water freely commu-
nicates with the atmosphere ; and if this renewal
be by any means prevented, the water is no
longer capable of sustaining life. Fishes are
killed in a very few hours, if confined in a
limited portion of water, which has no access
to fresh air. When many fishes are enclosed in
a narrow vessel, they all struggle for the upper-
most place, (where the atmospheric air is first
absorbed,) like the unfortunate men imprisoned
in the black-hole at Calcutta. When a small
fish-pond is frozen over, the fishes soon perish,
unless holes be broken in the ice, in order to
admit air : they may be seen flocking towards
these holes, in order to receive the benefit of
the fresh air as it is absorbed by the water ;
and so great is their eagerness on these occa-
sions, that they often allow themselves to be
caught by the hand. Water, from which all
air has been extracted, either by the air-pump,
or by boiling, is to fishes what a vacuum is to
a breathing terrestrial animal. Humboldt and
Provencal made a series of experiments on the
quantities of air which fishes require for their res-
piration. They found that river-water generally
contains about one 36th of its bulk of air ; of
which quantity, one- third consists of oxygen,

KESPllJATION IN FISHES. 309

being about one per cent, of the whole voKime.
A tench is able to breathe when the quantity of
oxygen is reduced to the 5000th part of the bulk
of the water, but soon becomes exceedingly
feeble by tlie privation of this necessary ele-
ment. The fact, however, shows the admirable
perfection of the organs of this fish, which can
extract so minute a quantity of air from water
to which that air adheres with great tenacity.*

* The swimming bladder of fishes is regarded by many of the
German naturalists as having some relations to the respiratory
function, and as being the rudiment of the pulmonary cavity
of land animals ; the passage of communication with the oeso-
phagus being conceived to represent the trachea. The air con-
tained in the swimming bladder of fishes has been examined by
many chemists, but although it is generally found to be a mixture
of oxygen and nitrogen, the proportion in which these gases exist
is observed to vary considerably. Biot concluded from his expe-
riments, that in the air-bladder of fishes inhabiting the greatest
dopths of the ocean, the quantity of oxygen is greater, while in
those of fishes which come often to the surface, the nitrogen is
more abundant ; and De la Roche came to the same conclusion
from his researches on the fishes of the Mediterranean. From the
experiments of Humboldt and Provenqal, on the other hand, we
may conclude, that the quality of the air contained in the air-
bladder is but remotely connected with respiration. (Memoires
de la Societe d'Arcueil, ii, 359.)

According to Ehrmann, the Cohitis, or Loche, occasionally
swallows air, which is decomposed in the alimentary canal, and
eflPects a change in the blood-vessels, with which it is brought
into contact, exactly similar to that which occurs in ordinary
respiration. It is also believed that in all fishes a partial aeration
of the blood is the result of a similar action, taking place at the
surface of the body under the scales of the integuments. Cuvier,
sur les Poissons, I, 383.

310 THE VITAL FUNCTIONS.

§ 3. Atmospheric Respiration.

The next series of structures which are to come
under our review, comprehends all those adapted
to the respiration of atmospheric air in its
gaseous form ; and their physiology is no less
diversified than that of the organs by which
water is respired.

Air may be respired by tracJiecBy or by pul-
monary cavities ; the first mode is exemplified in
insects ; the second is that adopted in the larger
terrestrial animals.

The greater part of the blood of insects being
diffused by transudation through every internal
organ of their bodies, and a small portion only
being enclosed in vessels, and circulating in them,
the salutary influence of the air could not have
been generally extended to that fluid by any of
the ordinary modes of respiration, where the
function is carried on in an organ of limited extent.
As the blood could not be brought to the air, it
became necessary, therefore, that the air should
be brought to the blood. For this purpose there
has been provided, in all insects, a system of
continuous and ramified vessels, called trachece,
distributing air to every part of the body. The
external orifices, from which these air tubes

ATMOSPHERIC RESPIRATION.

311

commence, are called spiracles, or stigmata, and
are usually situated in rows on each side of

the body, as is shown in Fig. 370, which repre-
sents the lower or abdominal surface of the Dy-
tiscus marginalis. They are seen very distinctly
in the caterpillar, which has generally ten on
each side, corresponding to the number of abdo-
minal segments. In many insects we find them
guarded by bristles, or tufts of hair, and some-
times by valves, placed at the orifice, to prevent
the entrance of extraneous bodies. The spira-
cles are opened and closed by muscles provided
for that purpose. Fig. 371 is a magnified view
of spiracles of this description, from the Ceram-
hyx heros. (Fab.) They are the beginnings of
short tubes, which open into large trunks (as
shown in Fig. 372), extending longitudinally

3l2 THE VITAL FUNCTIONS.

on each side, and sending off radiating branches
from the parts which are opposite to the spi-
racles ; and these branches are farther subdi-
vided, in the same manner as the arteries of the
larger animals, so that their minute ramifications
pervade every organ in the body. This ramified
distribution has frequently occasioned their
being mistaken for blood vessels. In the wings
of insects the nervures, which have the appear-
ance of veins, are only large air-tubes. Jurine
asserts that it is by forcing air into these tubes
that the insect is enabled suddenly to expand
the wings in preparing them for flight, giving
them by this means greater buoyancy, as well as
tension.

The tracheae are kept continually pervious by
a curious mechanism ; they are formed of three
coats, the external and internal of which are
membranous ; but the middle coat is constructed
of an elastic thread coiled into a helix, or cylin-
drical spiral (as seen in Fig. 372) ; and the
elasticity of this thread keeps the tube constantly
in a state of expansion, and therefore full of air.
When examined under water, the tracheae have a
shining silvery appearance, from the air they
contain. This structure has a remarkable ana-
logy to that of the air vessels of plants, which
also bear the name of tracheae ; and in both
similar variations are observed in the contexture

RESPIRATION IN INSECTS. 313

of the elastic membrane by which they are kept
pervious.*

The tracheae, in many parts of their course,
present remarkable dilatations, which apparently
serve as reservoirs of air ; they are very conspi-
cuous in the Dytiscus marginalis, which resides
principally in water ; but they also exist in
many insects, as the Melolontha and the Ceram-
hyx, which live wholly in the air.f Those of
the Scolia hortorum (Fab.) are delineated in
Fig. 373, considerably magnified.

If an insect be immersed in water, air will be
seen escaping in minute bubbles at each spi-
racle ; and in proportion as the water enters into
the tubes, sensibility is destroyed. If all the
spiracles be closed by oil, or any other unctuous
substance, the insect immediately dies of suf-
focation ; but if some of them be left open,
respiration is kept up to a considerable ex-
tent, from the numerous communications which
exist among the air vessels. Insects soon perish
when placed in the receiver of an air-pump,
and the air exhausted ; but they are generally

* According to the observation of Dr. Kidd these vessels are
often annular in insects, as is also the case with those of plants.
He considers the longitudinal tracheae as connecting channels,
by which the insect is enabled to direct the air to particular parts
for occasional purposes. Phil. Trans, for 1825, p. 234.

t Leon Dufour, Annales des Sciences Naturelles ; viii. 26.

314 THE VITAL FUNCTIONS.

more tenacious of life under these circum-
stances than the larger animals, and often, after
being apparently dead, revive on the readmis-
sion of air.

Aquatic insects have tracheae, like those living
in air, and are frequently provided with tubes,
which are of sufficient length to reach the sur-
face of the water, where they absorb air for res-
piration. In a few tribes a complicated mode
of respiration is practised ; aerated water is
taken into the body, and introduced into cavities,
where the air is extracted from it, and trans-
mitted by the ordinary tracheae to the different
parts of the system.*

Such, then, is the extensive apparatus for
aeration in animals, which have either no circu-
lation of their nutritious juices, or a very im-
perfect one ; but no sooner do we arrive at the
examination of animals possessing an enlarged
system of blood vessels, than we find nature
abandoning the system of tracheae, and employ-
ing more simple means of effecting the aeration

* Mr. Dutrochet conceives that the principle on which this
operation is conducted is the same with that by which gases are
reciprocally transmitted through moistened membranes ; as in
the experiments of Humboldt and Gay Lussac, who, on enclosing
mixtures of oxygen, nitrogen, and carbonic acid gases, in any
proportion, in a membranous bladder, which was then immersed
in aerated water, found that there is a reciprocal transit of
the gases ; until at length pure atmospheric air remains in the
cavity of the bladder.

RESPIRATION IN INSECTS.

315

of the blood. Advantage is taken of the facility
afforded by the blood-vessels of transmitting the
blood to particular organs, where it may con-
veniently receive the influence of the air. Thus
Scorpions are provided, on each side of the
thorax, with four pulmonary cavities, seen at l,
on the left side of Fig. 374, into each of which

air is admitted by a separate external opening.
A, B, is the dorsal vessel, which is connected with
the pulmonary cavities by means of two sets of

316 THE VITAL FUNCTIONS.

muscles, the one set (m, m) being longer than the
other (m, m, m). The branchial arteries (v) are
seen ramifying over the inner surface of the pul-
monary cavities (u) on the right side, whence
the blood is conveyed by a corresponding set of
branchial veins to the dorsal vessel ; and other
vessels, which are ordinary veins, are seen at o,
proceeding from the abdominal cavity to join the
dorsal vessel. The membrane which lines the
pulmonary cavities is curiously plaited ; present-
ing the appearance of the teeth of a comb, and
partaking of the structure of gills ; and on this
account these organs are termed by Latreille
pneumo-hmichicB . Organs of a similar descrip-
tion exist in Spiders ; some species having eight ;
others four ; and some only two : but there is
one entire order of Arachnida which respire by
means of tracheae, and in these the circulation
is as imperfect as it is in insects.

It may here be remarked that an essential dif-
ference exists in the structure of the respiratory
organs, according to the nature of the medium
which is to act upon them ; for in aquatic res-
piration the air contained in water is made to
act on the blood circulating in vessels which
ramify on the external surface of the filaments
of the gills ; while in atmospheric respiration
the air in its gaseous state is always received
into cavities, on the internal surface of which the
blood-vessels, intended to receive its influence,

RESPIRATION IN MOLLUSCA. 317

are distributed. It is not difficult to assign the
final cause of this change of plan ; for in each
case the structure is accommodated to the me-
chanical properties of the medium respired. A
liquid, being inelastic and ponderous, is adapted,
by its momentum alone, to separate and sur-
round the loose floating filaments composing the
branchiae ; but a light gaseous fluid, like air, is,
on the contrary, better fitted to expand dilatable
cavities into which it may be introduced.

Occasionally, however, it is found that organs
constructed like branchiae, and usually perform-
ing aquatic respiration, can be adapted to respire
air. This is the case with some species of Crus-
tacea, of the order Decapoda, such as Crabs,
which, by means of a peculiar apparatus, dis-
covered by Audouin and Milne Edwards, retain
a quantity of water in the branchial cavity so as
to enable them to live a very long time out of the
water. It is only in their mature state of de-
velopement, however, that they are qualified for
this amphibious existence, for at an early period
of growth they can live only in water.

There is an entire order of Gasteropodous
Mollusca which breathe atmospheric air by
means of pulmonary cavities. This is the case
with the Limax, or slug, and also with the
Helix, or snail, the Testacella, the Ctausilia,
and many others, which, though partial to moist
situations, are, from the conformation of their

318 THE VITAL FUNCTIONS.

respiratory organs, essentially land animals. The
air is received by a round aperture near the
head, guarded by a sphincter muscle, which is
seen to dilate or contract as occasion may re-
quire, but which is sometimes completely con-
cealed from view by the mouth folding over it.
The cavity, to which this opening leads, is lined
with a membrane delicately folded, and over-
spread with a beautiful net-work of pulmonary
vessels. Other mollusca of the same order,
which are more aquatic in their habits, have
yet a similar structure, and are obliged at in-
tervals to come to the surface of the water in
order to breathe atmospheric air : this is the
case with the Onchidium, the Planorhis, the
Lym7icea, &c.

The structure of the pulmonary organs be-
comes gradually more refined and complicated
as we ascend to the higher classes of animals.
In all vertebrated terrestrial animals they are
called lungs, and consist of an assemblage of
vesicles, into which the air is admitted by a
tube, called the trachea, or wind-pipe, extending
downwards from the back of the mouth, parallel
to the oesophagus. Great care is taken to guard
the beginning of this passage from the intrusion
of any solid or liquid that may be swallowed. A
cartilaginous valve, termed the epiglottis, is
generally provided for this purpose, which is
made to descend by the action of the same

RESPIRATION BY LUNGS. 319

muscles that perform, deglutition, and which
then closes accurately the entrance into the air->
tube. It is an exceedingly beautiful contriv-
ance, both as to the simplicity of the mechanism,
and the accuracy with which it accomplishes
the purpose of its formation. At the upper
part of the chest the trachea divides into two
branches, called the bronchia, passing to the
lungs on either side. Both the wind-pipe and
the bronchia are prevented from closing by the
interposition of a series of firm cartilaginous
ringlets, interposed between their inner and
outer coats, and placed at small and equal dis-
tances from one another. The natural elasticity
of these ringlets tends to keep the sides of the
tube stretched, and causes it to remain open :
it is a structure very analogous to that of the
trachea of insects, or of the vessels of the same
name in plants.

The lungs of Reptiles consist of large sacs,
into the cavity of which the bronchia, proceed-
ing from the bifurcation of the trachea, open at
once, and without further subdivision. Cells are
formed within the sides of this great cavity, by
fine membranous partitions, as thin and delicate
as soap bubbles. The lungs of serpents have
scarcely any of these partitions, but consist of
one simple pulmonary sac, situated on the right
side, having the slender elongated form of all
the other viscera, and extending nearly the

320 THE VITAL FUNCTIONS.

whole length of the body. The lung on the left
side is in general scarcely discernible, being-
very imperfectly developed. In the Chamelion
the lungs have numerous processes which pro-
ject from them like caeca. In the Sauria, the
lungs are more confined to the thoracic region,
and are more completely cellular.

The mechanism, by which, in these animals,
the air is forced into the lungs, is exceedingly
peculiar, and was for a long time a subject of
controversy. If we take a frog as an example,
and watch its respiration, we cannot readily dis-
cover that it breathes at all, for it never opens
its mouth to receive air, and there is no motion
of the sides to indicate that it respires; and
yet, on any sudden alarm, we see the animal
blowing itself up, as if by some internal power,
though its mouth all the while continues to be
closed. We may perceive, however, that its-
throat is in frequent motion, as if the frog were
economising its mouthful of air, and transferring
it backwards and forwards between its mouth
and lungs ; but if we direct our attention to the
nostrils, we may observe in them a twirling
motion, at each movement of the jaws ; for it is,
in fact, through the nostrils that the frog receives
all the air which it breathes. The jaws are
never opened but for eating ; and the sides of
the mouth form a sort of bellows, of which the
nostrils are the inlets ; and by their alternate

' RESPIRATION IN REPTILES. 321

contraction and relaxation the air is swallowed,
and forced into the trachea, so as to inflate the
lungs. If the mouth of a frog be forcibly kept
open, it can no longer breathe, because it is
deprived of the power of swallowing the air
required for that function ; and if its nostrils be
closed, it is, in like manner, suffocated. The
respiration of most of the Reptile tribes is per-
formed in a similar manner ; and they may be
said rather to swallow the air they breathe, than
to draw it in by any expansive action of the
parts which surround the cavity of the lungs ;
for even the ribs of serpents contribute but little,
by their motion, to this effect, being chiefly use-
ful as organs of progression.

The Chelonia have lungs of great extent,
passing backwards under the carapace, and
reaching to the posterior part of the abdomen.
Turtles, which are aquatic, derive great advan-
tages from this structure, which enables them
to give buoyancy to their body, (encumbered as
it is with a heavy shell,) by introducing into it a
large volume of air ; so that the lungs, in fact,
serve the purposes of a large swimming bladder.
That this use was contemplated in their struc-
ture is evident from the volume of air received
into the lungs being much greater than is re-
quired for the sole purpose of respiration. The
section of the lungs of the turtle (Fig. 375),

VOL. II. Y

322

THE VITAL FUNCTIONS.

shows their interior structure, composed of large
cells, into which the trachea (r) opens.

Few subjects in animal physiology are more
deserving the attention of those whose object is
to trace the operations of nature in the progres-
sive developement of the organs, than the changes
which occur in the evolution of the tadpole, from
the time it leaves the egg till it has attained the
form of the perfect frog. We have already had
occasion to notice several of these transforma-
tions in the organs of the mechanical functions,
and also in those of digestion and circulation :
but the most remarkable of all are the changes
occurring in the respiratory apparatus, corres-
ponding with the opposite nature of the elements
which the same animal is destined to inhabit in
the different stages of its existence. No less

RESPIRATION IN REPTILES. 323

than three sets of organs are provided for respi-
ration ; the first two being branchiae, adapted to
the fish-like condition of the tadpole ; and the
last being pulmonary cavities, for receiving air,
to be employed when the animal exchanges its
aquatic for its terrestrial life. It is exceedingly
interesting to observe that this animal at first
breathes by gills, which project in an arbo-
rescent form from the sides of the neck, and
float in the water ; but these structures are
merely temporary, being provided only to meet
the immediate exigency of the occasion, and
being raised at a period when none of the in-
ternal organs are as yet perfected. As soon as
another set of gills, situated internally, can be
constructed, and are ready to admit the circu-
lating blood, the external gills are superseded in
their office ; they now shrivel, and are removed,
and the tadpole performs its respiration by
means of branchiae, formed on the model of
those of fishes, and acting by a similar mecha-
nism. By the time that the system has under-
gone the changes necessary for its conversion
into the frog, a new and very different apparatus
has been evolved for the respiration of air.
These are the lungs, which gradually coming
into play, direct the current of blood from the
branchiae, and take upon themselves the whole
office of respiration. The branchiae, in their
turn, become useless, are soon obliterated, and

.324 THE VITAL FUNCTIONS.

leave no other trace of their former existence
than the original division of the arterial trunks,
which had supplied them with blood directly
from the heart, but which, now uniting in the
back, form the descending aorta.*

There is a small family, called the Perenni-
branchia, belonging to this class, which, instead
of undergoing all the changes I have been des-
cribing, present, during their whole lives, a great
similitude to the first stage of the tadpole. This
is the case with the Axolotl, the Proteus angui-
nus, the Siren lacertina, and the Menohranchus
lateralis^ which permanently retain their external
gills, while at the same time they possess imper-
fectly developed lungs. It would therefore seem
as if, in these animals, the progress of develope-
ment had been arrested by nature at an early
stage, so that their adult state corresponds to the
larva condition of the frog-t

In all warm blooded animals respiration be-
comes a function of much greater importance,

* See Fig. 357, p. 274.

t Geoffroy St. Hilaire thinks there is ground for believing that
Crocodiles and Turtles possess, in addition to the ordinary pul-
monary respiration, a partial aquatic abdominal respiration,
effected by means of the two channels of communication which
have been found to exist between the cavity of the abdomen and
the external surface of the body : and also that some analogy
may be traced between this aquatic respiration in reptiles, by
these peritoneal canals, and the supposed function of the swim-
ming bladder of fishes, in subserviency to a species of aerial res-
piration.

RESPIRATION IN MAMMALIA. 325

the continuance of life being essentially depen-
dent on its vigorous and unceasing exercise.
The whole class of Mammalia have lungs of an
exceedingly developed structure, composed of
an immense number of minute cells, crowded
together as closely as possible, and presenting a
vast extent of internal surface. The thorax, or
cavity in which the lungs, together with the
heart and its great blood-vessels, are inclosed,
has somewhat the shape of a cone ; and its sides
are defended from compression by the arches of
the rib3, which extend from the spine to the
sternum, or breast-bone, and produce mechani-
cal support on the same principle that a cask is
strengthened by being girt with hoops, which,
though composed of comparatively weak mate-
rials, are yet capable, from their circular shape,
of presenting great resistance to any compress-
ing force.

While Nature has thus guarded the chest, with
such peculiar solicitude, against the efforts of
any external force, tending to diminish its capa-
city, she has made ample provision for enlarging
or contracting its diameter in the act of respira-
tion. First, at the lower part, or that which
corresponds to the basis of the cone, the only
side, indeed, which is not defended by bone,
there is extended a thin expansion, partly mus-
cular, and partly tendinous, forming a complete
partition, and closing the cavity of the chest on

,S2G THE VITAL FUNCTIONS.

the side next to the abdomen. This muscle is
called the Diaphragm : it is perforated, close to
its origin from the spine, by four tubes, namely,
the oesophagus, the aorta, the vena cava, and the
thoracic duct. Its surface is not flat, but convex
above, or towards the chest ; and the direction of
its fibres is such that when they contract they
bring down the middle part, which is tendinous,
and render it more flat than before, (the passage
of the four tubes already mentioned, not inter-
fering with this action,) and thus the cavity of
the thorax may be considerably enlarged. It is
obvious that if, upon the descent of the dia-
phragm, the lungs were to remain in their ori-
ginal situation, an empty space would be left
between them and the diaphragm. But no
vacuum can take place in the body ; the air
cells of the lungs must always contain, even in
their most compressed state, a certain quantity
of air ; and this air will tend, by its elasticity, to
expand the cells : the lungs will consequently be
dilated, and will continue to fill the chest ; and
the external air will rush in through the trachea
in order to restore the equilibrium. This action
is termed inspiration. The air is again thrown
out when the diaphragm is relaxed, and pushed
upwards, by the action of the large muscles of
the trunk ; the elasticity of the sides of the
chest, concur in producing the same effect ; and
thus eixpiration is accomplished.

RESPIRATION IN MAMMALIA. 327

The muscles which move the ribs conspire
also to produce dilatations and contractions of
the cavity of the chest. Each rib is capable of
a small degree of motion on that extremity by
which it is attached to the spine ; and this mo-
tion, assuming the chest to be in the erect posi-
tion, as in man, is chiefly upwards and down-
wards. But, since the inclination of the ribs is
such that their lower edges form acute angles
with the spine, they bend downwards as they
proceed towards the breast ; and the uppermost
rib being a fixed point, the action of the inter-
costal muscles, which produces an approxi-
mation of the ribs, tends to raise them, and to
bring them more at right angles with the spine ;
the sternum also, to which the other extremities
of the ribs are articulated, is elevated by this
motion, and consequently removed to a greater
distance from the spine. The general result of
all these actions is to increase the capacity of
the chest.

Thus there are two ways in which the cavity
of the thorax may be dilated ; namely, by the
action of the diaphragm, and by the action of
the intercostal muscles. It is only in peculiar
exigencies that the whole power of this appa-
ratus is called into action ; for in ordinary res-
piration the diaphragm is the chief agent em-
ployed, and the principal effect of the action of
the intercostal muscles is simply to fix the ribs,

328

THE VITAL FUNCTIONS.

and thu8 give greater purchase to the diaphragm.
The muscles of the ribs are employed chiefly to
assist the diaphragm, when, from any cause, a
difficulty arises in dilating the chest.

In Birds the mechanism of respiration pro-
ceeds upon a different plan, of which an idea
may be derived from the view given of the lungs

of the Ostrich, at l, l. Fig. 377. The construc-
tion of the lungs of birds is such as not to admit
of any change in their dimensions ; for they are
very compact in their texture, and are so closely
braced to the ribs, and upper parts of the cheet,

RESPIRATION IN BIRDS. 329

by firm membranes, as to preclude all possi-
bility of. motion. They in part, indeed, project
behind the intervals between the ribs, so that their
whole mass is not altogether contained within
the thoracic cavity. There is no large muscular
diaphragm by which any change in the capacity
of the chest could be effected, but merely a few
narrow slips of muscles, arising from the inner
sides of the ribs, and inserted into the thin trans-
paren membrane which covers the lower surface
of the lungs. They have the effect of lessening
the concavity of the lungs towards the abdomen,
at the time of inspiration ; and they thereby assist
in dilating the air-cells*. The bronchia, or divi-
sions of the trachea (t), after opening, as usual,
into the pulmonary air-cells, do not terminate
there, but pass on to the surface of the lungs,
where they open by numerous apertures. The
air is admitted, through these apertures, into seve-
ral large air-cells (c c c), which occupy a consi-
derable portion of the body, and which enclose
most of the large viscera contained in the abdo-
men, such as the liver, the stomach, and the in-
testines! ; and there are, besides, many lateral
cells in immediate communication with the

* Hunter on the Animal Economy, p. 78.

t It was asserted by the Parisian Academicians, that the air
gets admission into the cavity of the pericardium, in which the
heart is lodged. This error was first pointed out by Dr. Ma-
cartney. (See Rees's Cyclopeedia. — Art. Bird.)

330 THE VITAL FUNCTIONS.

lungs, and extending all down the sides of the
body. Numerous air-cells also exist between
the muscles, and underneath the skin ; and the
air penetrates even into the interior of the bones
themselves ; filling the spaces usually occupied
by the marrow, and thus contributing materially
to the lightness of the fabric*. All these cells
are very large and numerous in birds which
perform the highest and most rapid flight, such
as the Eagle. The bill of the Toucan, which is
of a cellular structure, and also the cells between
the plates of the skull in the Owl, are, in like
manner, filled with air, derived from the lungs :
the barrels of the large quills of the tails and
wings are also supplied with air from the same
source.

In birds, then, the air is not merely received
into the lungs, but actually passes through them,
being drawn forwards by the muscles of the ribs
when they elevate the chest, and produce an
expansion of the subjacent air-cells. The chest
is depressed, for the purpose of expiration, by
another set of muscles, and the air driven back :
this air, consequently, passes a second time
through the lungs, and acts twice on the blood
which circulates in those organs. It is evident
that if the lungs of birds had been constructed

* Im birds, not formed for extensive flight, as the gallinaceous
tribes, the humerus is the only bone into which air is introduced.
— Hunter on the Anhnal Economy, p. 81.

RESPIRATION IN BIRDS. 331

on the plan of those of quadrupeds, they must
have been twice as large to obtain the same
amount of aeration in the blood ; and conse-
quently must have been twice as heavy, which
would have been a serious inconvenience in an
animal formed for flying*. The diffusion of so
large a quantity of air throughout the body of
animals of this class presents an analogy with
a similar purpose apparent in the conformation
of insects, w^here the same object is effected by
means of tracheae!.

Thus has the mechanism of respiration been
varied in the different classes of animals, and
adapted to the particular element, and mode of
life designed for each. Combined with the
peculiar mode of circulation, it affords a tole-
rably accurate criterion of the energy of the

* I must mention, however, that the correctness of this view
of the subject is contested by Dr. Macartney, who thinks it
probable that the ab, on its return from the large air-cells, passes
directly by the large air-holes into the bronchia, and is not
pirought a second time into contact with the blood.

f The peculiarities of structure in the respiratory system of
birds have probably a relation to the capability we see them
possess, of bearing with impunity, very quick and violent changes
of atmospheric pressure. Thus the Condor of the Andes is
often seen to descend rapidly from a height of above 20,000
feet, to the edge of the sea, where the air is more than twice the
density of that which the bird had been breathing. We are ag
yet unable to trace the connexion which probably exists between
the structure of the lungs, and this extraordinary power of accom-
modation to such great and sudden variations of atmospheric
pressure.

332 THE VITAL FUNCTIONS.

vital powers. In Birds, the muscular activity is
raised to the highest degree, in consequence of
the double effect of the air upon the whole cir-
culating blood in the pulmonary organs. The
Mammalia rank next below birds, in the scale
of vital energy ; but they still possess a double
circulation, and breathe atmospheric air. The
torpid and cold-blooded Reptiles are separated
from Mammalia by a very wide interval ; because,
although they respire air, that air only influences
a part of the blood ; the pulmonary, being only
a branch of the general circulation. In Fishes,
again, we have a similar result ; because, al-
though the whole blood is brought by a double
circulation to the respiratory organs, yet it is
acted upon only by that portion of air which is
contained in the water respired, and which is
less powerful in its action than the same element
in its gaseous state. We may, in like manner,
continue to trace the connexion between the
extent of these functions and the degrees of
vital energy throughout the successive classes of
invertebrate animals. The vigour and activity
of the functions of Insects, in particular, have
an evident relation to the effective manner in
which the complete aeration of the blood is
secured by an extensive distribution of tracheae
through every part of their system.

333

§ 4. Chemical Changes effected by Respiration.

We have next to direct our attention to the che-
mical offices which respiration performs in the
animal economy. It is only of late years that
we may be said to have obtained any accurate
knowledge as to the real nature of this important
function ; and there is perhaps no branch' of
physiology which exhibits in its history a more
humiliating picture of the wide sea of error in
which the human intellect is prone to lose itself,
when the path of philosophical induction is
abandoned, than the multitude of wild and
visionary hypotheses, devoid of all solid founda-
tion, and perplexed by the most inconsistent rea-
sonings, which formerly prevailed with regard to
the objects and the processes of respiration. To
give an account, or even a brief enumeration of
these theories, now sufficiently exploded, would
be incompatible wdth the purpose to which I
must confine myself in this treatise.* I shall

* For an account of the history of the various chemical
theories which have prevailed on this interesting department of
Physiology, I must refer to the " Essay on Respiration," by Dr.
Bostock, and also to the " Elementary System of Physiology,"
by the same author, which latter work comprises the most com-
prehensive and accurate compendium of the science that has
yet appeared.

/

334 THE VITAL FUNCTIONS.

content myself, therefore, with a concise state-
ment of such of the leading facts relating to this
function, as have now, by the labours of modern
physiologists, been satisfactorily established, and
which serve to elucidate the beneficent intentions
of nature in the economy of the animal system.

Atmospheric air acts without difficulty upon
the blood, while it is circulating through the
vessels which are ramified over the membranes
lining the air cells of the lungs ; for neither
these membranes, nor the thin coats of the vessels
themselves, present any obstacle to the trans-
mission of chemical elements from the one to the
other. The blood being a highly compound
fluid, it is exceedingly difficult to obtain an ac-
curate analysis of it, and still more to ascertain
with precision the different modifications which
occur in its chemical condition at different times :
on this account, it is scarcely possible to deter-
mine, by direct observation, what are the exact
chemical changes, which that fluid undergoes
during its passage through the lungs ; and we
have only collateral evidence to guide us in the
inquiry.*

* Some experiments very recently made by Messrs. Macaire
and Marcet, on the ultimate analysis of arterial and venous
blood, taken from a rabbit, and dried, have shown that the
former contains a larger proportion of oxygen than the latter ;
and that the latter contains a larger proportion of carbon than
the former : the proportions of nitrogen and hydrogen being the

CHEMICAL EFFECTS OF RESPIRATION. 335

The most obvious effect resulting from the ac-
tion of the air is a change of colour from the dark
purple hue, which the blood has when it is brought
to the lungs, to the bright vermillion colour,
which it is found to assume in those organs, and
which accompanies its restoration to the qualities
of arterial blood. In what the chemical differ-
ence between these two states consists may, in
some measure, be collected from the changes
which the air itself, by producing them, has
experienced.

The air of the atmosphere, which is taken
into the lungs, is known to consist of about
twenty per cent, of oxygen gas, seventy-nine of
nitrogen gas, and one of carbonic acid gas.
When it has acted upon the blood, and is re-
turned from the lungs, it is found that a certain
proportion of the oxygen, which it had contained,
has disappeared, and that the place of this
oxygen is almost wholly supplied by an addition
of carbonic acid gas, together with a quantity
of watery vapour. It appears also probable that
a small portion of the nitrogen gas is consumed
during respiration.

same in both. The following are the exact numbers expressive
of these proportions :

Carbon, Oxygen. Nitrogen. Hydrogen.

Arterial blood . . . 50.2 . . . 26.3 . . . 16.3 ... 6.6
Venous blood . . . 55.7 . . . 21.7 . . . 16.2 ... 6.4
Memoires de la Societe de Physique et d'Hist. Naturelle de
Geneve. T. v. p. 400.

336 THE VITAL FUNCTIONS.

For our knowledge of the fact of the dis-
appearance of oxygen we are indebted to the
labours of Dr. Priestley. It had, indeed, been
long before suspected by Mayow, that some
portion of the air inspired is absorbed by the
blood ; but the merit of the discovery that it is
the oxygenous part of the air which is thus con-
sumed is unquestionably due to Dr. Priestley.
The exact quantity of oxygen, which is lost in
natural respiration, varies in different animals, and
even in different conditions of the same animal.
Birds, for instance, consume larger quantities of
oxygen by their respiration ; and hence require,
for the maintenance of life, a purer air than
other vertebrated animals. Vauquelin, however,
found that many species of insects and worms
possess the power of abstracting oxygen from
the atmosphere in a much greater degree than
the larger animals. Even some of the terres-
trial mollusca, such as snails, are capable of
living for a long time in the vitiated air in which
a bird had perished. Some insects, which con-
ceal themselves in holes, or burrow under ground,
have been known to deprive the air of every
appreciable portion of its oxygen. It is ob-
served by Spallanzani, that those animals, whose
modes of life oblige them to remain for a great
length of time in these confined situations,
possess this power in a greater degree than
others, which enjoy more liberty of moving in the

CHEMICAL EFFECTS OF RESPIRATION. 337

open air : so admirably have the faculties of
animals been, in every instance, accommodated
to their respective wants.

Since carbonic acid consists of oxygen and
carbon, it is evident that the portion of that gas
which is exhaled from the lungs is the result of
the combination of either the whole, or a part,
of the oxygen gas, which disappears during the
act of respiration, with the carbon contained
in the dark venous blood, which is brought to
the lungs. The blood having thus parted with
its superabundant carbon, which escapes in the
form of carbonic acid gas, regains its natural Ver-
million colour, and is now qualified to be again
transmitted to the different parts of the body for
their nourishment and growth. As the blood
contains a greater proportion of carbon than the
animal solids and fluids which are formed from
it, this superabundant carbon gradually accu-
mulates in proportion as its other principles,
(namely, oxygen, hydrogen, and nitrogen) are
abstracted from it by the processes of secretion
and nutrition. By the time it has returned to
the heart, therefore, it is loaded with carbon,
a principle, which, when in excess, becomes
noxious, and requires to be removed from the
blood, by combining it with a fresh quantity of
oxygen obtained from the atmosphere. It is not
yet satisfactorily determined whether the whole

VOL. II. z

338 THE VITAL FUNCTIONS.

of the oxygen, which disappears during respi-
ration, is employed in the formation of carbonic
acid gas : it appears probable, however, from
the concurring testimony of many experimen-
talists, that a small quantity is permanently
absorbed by the blood, and enters into it as one
of its constituents.

A similar question arises with respect to
nitrogen, of which, as I have already mentioned,
it is probable that a small quantity disappears
from the air when it is respired ; although the
accounts of experimentalists are not uniform on
this point. The absorption of nitrogen during
respiration was one of the results which Dr.
Priestley had deduced from his experiments :
and this fact, though often doubted, appears, on
the whole, to be tolerably well ascertained by the
inquiries of Davy, PfafF, and Henderson. With
regard to the respiration of cold-blooded animals,
it has been satisfactorily established by the
researches of Spallanzani, and more especially
by those of Humboldt and Proven9al, on fishes,
that nitrogen is actually absorbed. A confirma-
tion of this result has recently been obtained by
Messrs. Macaire and Marcet, who have found
that the blood contains a larger proportion of
nitrogen than the chyle, from which it is formed.
We can discover no other source from which
chyle could acquire this additional quantity of
nitrogen, during its conversion into blood, than

CHEMICAL EFFECTS OF RESPIRATION. 339

the air of tlie atmosphere, to which it is exposed
in its passage through the piihnonary vessels.*

According to these views of the chemical
objects of respiration, the process itself is ana-
logous to those artificial operations which effect
the combustion of charcoal. The food supplies
the fuel, which is prepared for use by the di-
gestive organs, and conveyed by the pulmonary
arteries to the place where it is to undergo com-
bustion : the diaphragm is the bellows, which
feeds the furnace with air; and the trachea is
the chimney, through which the carbonic acid,
which is the product of the combustion, escapes.

It becomes an interesting problem to deter-
mine whether this analogy may not be farther
extended ; and whether the combustion of car-
bon, which takes place in respiration, be not the
exclusive source of the increased temperature,
which all animals, but more especially those
designated as warm-blooded^ usually maintain
above the surrounding medium. The uniform
and exact relation which may be observed to
take place between the temperature of animals
and the energy of the respiratory function, or
rather the amount of the chemical changes
induced by that function, affords very strong
evidence in favour of this hypothesis. The
coincidence, indeed, is so strong, that notwith-
standing the objections that have been raised
* See the note at page 334.

340 THE VITAL FUNCTIONS.

against the theory founded upon this hypothesis,
from some apparent anomalies which occasion-
ally present themselves, we must, I think, admit
that it affords the best explanation of the phe-
nomena of any theory yet proposed, and that,
therefore, it is probably the true one.

The maintenance of a very elevated tempe-
rature appears to require the concurrence of two
conditions ; namely, first, that the whole of the
blood should be subjected to the influence of the
air, and, secondly, that that air should be pre-
sented to it in a gaseous state. These, then, are
the circumstances which establish the great dis-
tinction between warm and cold-blooded animals ;
a distinction which at once stamps the character
of their whole constitution. It is the condition
of a high temperature in the blood which raises
the Quadruped and the Bird to a rank, in the
scale of vitality, so far above that of the Reptile :
it is this wbich places an insuperable boundary
between Mammalia and Fishes. However the
warm-blooded Cetacea, who spend their lives
in the ocean, may be found to approximate
in their outward form, and in their external
instruments of motion, to the other inhabitants
of the deep, they are still, from the conformation
of their respiratory organs, dependent on another
element. If a Seal, a Porpoise, or a Dolphin
were confined, but for a short time, under the
surface of the water, it would perish with the

CHEMICAL EFFECTS OF RESPIRATION. 341

same certainty as any other of the mammalia,
placed in the same situation. We observe them
continually rising to the surface in order to
breathe, under every circumstance of privation
or of danger ; and however eagerly they may
pursue their prey, however closely they may be
pressed by their enemies, a more urgent want
compels them, from time to time, to respire air
at the surface of the sea. Were it not for this
imperious necessity, the Whale, whose enormous
bulk is united with corresponding strength and
swiftness, would live in undisturbed possession
of the widely extended domains of the ocean,
might view without dismay whole fleets sent out
against him, and might defy all the efforts that
man could practise for his capture or destruction.
But the constitution of his blood, obliging him
to breathe at the surface of the water, brings
him within the reach of the fatal harpoon. In
vain, on feeling himself wounded, does he plunge
for refuge into the recesses of the deep ; the same
necessity recurs, and compelling him again to
present himself to his foes, exposes him to their
renewed attacks, till he falls in the unequal
struggle. His colossal form and gigantic strength
are of little avail against the power of man, feeble
though that power may seem, when physically
considered, but which derives resistless might
from its association with an immeasurably su-
perior intellect.

342

Chapter XII.

SECRETION.

The capability of effecting certain chemical
changes in the cnide materials introduced into
the body, is one of the powers which more espe-
cially characterize life ; but although this power
is exercised both by vegetable and by animal
organizations, we perceive a marked difference
in the results of its operation in these two orders
of beings. The food of plants consists, for the
most part, of the simpler combinations of ele-
mentary bodies, which are elaborated in cellular
or vascular textures, and converted into various
products. The oak, for example, forms, by the
powers of vegetation, out of these elements, not
only the green pulpy matter of its leaves, and
the light tissue of its pith, but also the densest of
its woody fibres. It is from similar materials,
again, that the olive prepares its oil, and the
cocoa-nut its milk ; and the very same elements,
in different states of combination, compose, in
other instances, at one time the luscious sugar
of the cane, at another the narcotic juice of the
poppy, or the acrid principle of the euphorbium ;
and the same plant which furnishes in one part

SECRETION. 343

the bland farina of the potatoe, will produce in
another the poisonous extract of the nightshade.
Yet all these, and thousands of other vegetable
products, differing widely in their sensible quali-
ties, agree very nearly in their ultimate chemical
analysis, and owe their peculiar properties chiefly
to the order in which their elements are arranged ;
an order dependent on the processes to which
they have been subjected in the system of each
particular vegetable.

In the animal kingdom we observe these pro-
cesses multiplied to a still greater extent ; and
the resulting substances are even farther removed
from the original condition of unorganized matter.
In the first place, the food of animals, instead
of being simple, like that of plants, has always
undergone previous preparation ; for it has
either constituted a portion of some other organ-
ized being, or it has been a product of organiza-
tion ; in each case, therefore, partaking of that
complexity of composition which characterises
organized bodies. Still, whatever may be its
qualities when received into the stomach, it is
soon converted by the powers of digestion into
^ milky, or transparent fluid, having nearly the
same uniform properties. We have seen that
there is scarcely any animal or vegetable sub-
stance, however dense its texture, or virulent its
qualities, but is capable of affording nourish-
ment to various species of animals. Let us take

344 THE VITAL FUNCTIONS.

as an example the elytra of cantharides, which
are such active stimulants when applied in
powder to the skin in the ordinary mode of
blistering : we find that, notwithstanding their
highly acrid qualities, they constitute the natural
food of several species of insects, which devour
them with great avidity ; and yet the fluids of these
insects, though derived from this pungent food,
are perfectly bland, and devoid of all acrimony.
Cantharides are also, according to Pallas, the
favourite food of the Hedge-hog ; although to
other mammalia they are highly poisonous. It
has also been found that even those animal
secretions, (such as the venom of the rattle-
snake,) which, when infused into a wound, even
in the minutest quantity, prove quickly fatal,
may be taken into the stomach without produc-
ing any deleterious effects. These, and a mul-
titude of other well-known facts, fully prove
how completely substances received as aliment
may be modified, and their properties changed,
or even reversed, by the powers of animal
digestion.

No less remarkable are the transmutations,
which the blood itself, the result of these pre-
vious processes, is subsequently made to undergo
in the course of circulation, and when subjected
to the action of the nutrient vessels and secret-
ing organs ; being ultimately converted into the

SECRETION. 345

various textures and substances which compose
all the parts of the animal frame. All the modifi-
cations of cellular substance, in its various states
of condensation ; the membranes, the ligaments,
the cartilages, the bones, the marrow ; the mus-
cles, with their tendons ; the lubricating fluid of
the joints ; the medullary pulp of the brain ; the
transparent jelly of the eye ; in a word, all the
diversified textures of the various organs, which
are calculated for such different offices, are
derived from the same nutrient fluid, and may
be considered as being merely modified arrange-
ments of the same ultimate chemical elements.

In what, then, we naturally ask, consists
this subtle chemistry of life, by which nature
effects these multifarious changes ; and in what
secret recesses of the living frame has she con-
structed the refined laboratory in which she
operates her marvellous transformations, far sur-
passing even those which the most visionary
alchemist of former times had ever dreamed of
achieving? Questions like these can be fairly
met only by the confession of profound igno-
rance ; for, although the subject of secretion has
long excited the most ardent curiosity of physi-
ologists, and has been prosecuted with extraor-
dinary zeal and perseverance, scarcely any
positive information has resulted from their
labours ; and the real nature of the process

346 THE VITAL FUNCTIONS.

remains involved in nearly the same degree of
obscurity as at first.* It was natural to expect
that in this inquiry material assistance would be
derived from an accurate anatomical examina-
tion of the organs by which the more remarkable
secretions are formed ; yet, notwithstanding the
most minute and careful scrutiny of these organs,
our knowledge of the mode in which they are
instrumental in effecting the operations which
are there conducted, has not in reality advanced
a single step. To add to our perplexity we often
see, on the one hand, parts, to all appearance
very differently organized, giving rise to secre-
tions of a similar nature ; and on the other hand,
substances of very different properties produced
by organs, which, even in their minutest details,
appear to be identical in their structure. Secre-
tions are often found to be poured out from
smooth and membranous surfaces, such as those

* It is not yet precisely determined to what extent the organs
of secretion are immediately instrumental in producing the sub-
stance secreted ; and it has been even suggested that possibly
their office is confined to the mere separation, or filtration from
the blood, of certain animal products, which are always sponta-
neously forming in that fluid in the course of its circulation.
This hypothesis, in which the glands, and other secreting appa-
ratus are regarded as only very fine strainers, is supported by a
few facts, which seem to indicate the presence of some of these
products in the blood, independently of the secreting processes
by which they are usually supposed to be formed ; but the evi-
dence is as yet too scanty and equivocal to warrant the deduc-
tion of any general theory on the subject.

SECRETION. 347

which line the cavities of the abdomen, the chest,
and the head, and which are also reflected in-
wards so as to invest the organs therein contained,
as the heart, the lungs, the stomach, the intestines,
the liver, and the brain.* In other instances,
the secreting membrane is thickly set with
minute processes, like the pile of velvet : these
processes are called villi^ and their more obvious
use, as far as we can perceive, is to increase the
surface from which the secretion is prepared.
At other times we see an opposite kind of struc-
ture employed ; the secreting surface being the
internal lining of sacs or cells, either opening at
once into some larger cavity, or prolonged into
a tube, or duct, for conveying the secreted fluid
to a more distant point. These cells, or follicles,
as they are termed, are generally employed for
the mucous secretions, and are often scattered

* Sometimes the secreting organ appears to be entirely com-
posed of a mass of vessels covered with a smooth membrane ;
in other cases, it appears to contain some additional material, or
parenchyma, as it is termed. Vertebrated animals present us
with numerous instances of glandular organs employed for special
purposes of secretion : thus, in the eyes of fishes there exists a
large vascular mass, which has been called the choroid gland,
and which is supposed to be placed there for the purpose of
replenishing some of the humours of the eye, in proportion as
they are wasted. Within the air-bladder of several species of
fishes there is found a vascular organ, apparently serving to secrete
the air with which the bladder is filled ; numerous ducts, filled
with air, having been observed proceeding from the organ, and
opening on the inner surface of the air-bladder.

348 THE VITAL FUNCTIONS.

throughout the surfaces of membranes :* at other
times the secreting cavities are collected in great
numbers into groups ; and they then frequently
consist of a series of lengthened tubes, like caeca,
examples of which we have already seen in the
hepatic and salivary glands of insects.

A secretory organ, in its simplest form, con-
sists of short, narrow and undivided tubes ; we
next find tubes which are elongated, tortuous or
convoluted, occasionally presenting dilated por-
tions, or even having altogether the appearance
of a collection of pouches, or sacs ; while in other
cases they are branched, and extend into minute
ramifications. Sometimes they are detached, or
isolated ; at other times they are collected into
tufts, or variously grouped into masses, where still
the separate tubes admit of being unravelled. The
secreting filaments of insects float in the general
cavity, containing the mass of nutrient fluid, and
thence imbibe the materials they require for the
performance of their functions. It is only when
they receive a firm investment of cellular mem-
brane, forming what is termed a capsule, and
assuming the appearance of a compact body,
that they properly constitute a gland; and this
form of a secreting organ is met with only among
the higher animals. t

* See p. 185 of this volume; and in particular Fig. 305.
Sebaceous follicles are also noticed in vol. i. p. 114.

t Dr. Kidd, however, describes bodies apparently of a glan-
dular character, disposed in rows on the inner surface of the

SECRETION. 349

Great variety is observable both in the form and
structure of different glands, and in the mode in
which their blood-vessels are distributed. In
animals which are furnished with an extensive
circulation, the vessels supplying the glands with
blood are distributed in various modes ; and it
is evident that each plan has been designedly
selected with reference to the nature of the par-
ticular secretion to be performed, although we
are here unable to follow the connexion between
the means and the end. In some glands, for
example, the minute arteries, on their arrival at
the organ, suddenly divide into a great number
of smaller branches, like the fibres of a camel-
hair pencil : this is called the penicillated struc-
ture. Sometimes the minute branches, instead
of proceeding parallel to each other after their
division, separate like rays from a centre, pre-
senting a stellated, or star-like arrangement. In
the greater number of instances, the smaller
arteries take a tortuous course, and are some-
times coiled into spirals, but generally the con-
volutions are too intricate to admit of being
unravelled. It is only by the aid of the micro-
scope that these minute and delicate structures
can be rendered visible ; but the fallacy, to
which all observations requiring the application
of high magnifying powers are liable, is a serious

intestinal canal of the Gryllotalpa, or mole-cricket. Phil.Tran.
for 1825, p. 227.

3o0 THE VITAL FUNCTIONS.

obstacle to the advancement of our knowledge
in this department of physiology. Almost the
only result, therefore, which can be collected
from these laborious researches in microscopic
anatomy, is that nature has employed a great
diversity of means for the accomplishment of
secretion ; but we still remain in ignorance as to
the kind of adaptation, which must assuredly
exist, of each stnicture to its respective object,
and as to the nice adjustment of chemical affinities
which has been provided in order to accomplish
the intended effects.* Electricity is, no doubt,
an important agent in all these processes ; but

* The only instance in which we can perceive a correspondence
between the chemical properties of the secretion, and the kind
of blood from which it is prepared, is in the liver, which, unlike
all the other glands, has venous, instead of arterial blood, sent
to it for that purpose. The veins, which return the blood that
has circulated through the stomach, and other abdominal viscera,
are collected into a large trunk, called the vena portce, which
enters the liver, and is there again subdivided and ramified, as if
it were an artery : its minuter branches here unite with those
of the hepatic artery, and ramify through the minute lobules
which compose the substance of the liver. After the bile is
secreted, and carried off by hepatic ducts, the remaining blood
is conducted, by means of minute hepatic veins, which occupy
the centres of each lobule, into larger and larger trunks, till they
all unite in the vena cava, going directly to the heart. (See
Kiernan's Paper on the Anatomy and Physiology of the Liver,
Phil. Trans, for 1833, p. 711.) A similar system of venous
ramifications, though on a much smaller scale, has been dis-
covered by Jacobson, in the kidneys of most fishes and reptiles,
and even in some birds.

SECRETION. 351

in the absence of all certain knowledge as to
the mode in which it is excited and brought into
play in the living body, the chasm can for the
present be supplied only by remote conjecture.

The process which constitutes the ultimate
stage of nutrition, or the actual incorporation of
the new material with the solid substance of the
body, of which it is to form a part, is involved
in equal obscurity with that of secretion.

Chapter XIII.

ABSORPTION.

Absorption is another function, related to nutri-
tion, which deserves special notice. The prin-
cipal objects of this function are the removal of
such materials as have been already deposited,
and have become either useless or injurious, and
their conveyance into the general mass of circu-
lating fluids ; purposes which are accomplished
by a peculiar set of vessels, called the Lym-
phatics. These vessels contain a fluid, which,
being transparent and colourless like water,
has been denominated the lymph. The lym-
phatics are perfectly similar in their structure,
and probably also in their mode of action, to
the lacteals, which absorb the chyle from the

;}52 THE VITAL FUNCTIONS.

intestinal cavity : they are found in all the
classes of vertebrated animals, and pervade
extensively every part of the body. Exceed-
ingly minute at their origin, they unite toge-
ther as they proceed, forming larger and
larger trunks, generally following the course of
the veins, till they finally discharge their con-
tents either into the thoracic duct, or into some
of the large vein& in the vicinity of the heart.
Throughout their whole course they are, like the
lacteals, provided with numerous
valves, which, when the vessel is dis-
tended with lymph, give it a resem-
blance to a string of beads. Fig. 378.*
In the lower animals it appears that
the veins are occasionally endowed
with a power of absorption, similar
to that possessed by the lymphatics. None of
the invertebrata, indeed, possess lymphatics, and
absorption must consequently be performed by
the veins, when these latter vessels exist. The
addition of the system of lymphatic vessels, as

* In warm-blooded animals, the lymphatics are made to
traverse, in some part of their course, certain bodies of a
compact structure, resembling glands, and termed accordingly,
the lymphatic glands. One of these is represented in Fig. 378.
They correspond in structure, and probably also in their func-
tions, to the mesenteric glands, through which, in the mammalia,
the lacteals pass, before reaching the thoracic duct. It is chiefly
in the mammalia, indeed, that these glands are met with, for they
are rare among birds, and still more so among fishes and
reptiles.

ABSORPTION. 353

auxiliaries to the veins, may therefore be re-
garded as a refinement in organization, peculiar
to the higher classes of animals.*

Professor Muller, of Bonn, has lately disco-
vered that the frog, and several other amphibious
animals, are provided with large receptacles for
the lymph, situated immediately under the skin,
and exhibiting distinct and regular pulsations,
like the heart. The use of these lymphatic
hearts, as they may be called, is evidently to
propel the lymph in its proper course along the
lymphatic vessels. In the frog four of these
organs have been found ; the two posterior hearts
being situated behind the joint of the hip, and the
two anterior ones on each side of the transverse
process of the third vertebra, and under the
posterior extremity of the scapula. The pulsa-
tions of these lymphatic hearts do not correspond
with those of the sanguiferous heart; nor do
those of the right and left sides take place at
the same times, but they often alternate in an
irregular manner. Professor Muller has disco-
vered similar organs in the Toad, the Salaman-
der, and the Green Lizard ; and thinks it pro-
bable that they exist in all the amphibia.t

* Fohmann, who has made extensive researches on the ab-
sorbent vessels throughout all the classes of vertebrated animals,
has found that they terminate extensively in the veins. See his
work, entitled " Anatomische Untersuchungen uber die Ver-
bindung der Saugadern mit den Venen."

t Phil. Trans, for 1833, p. 89. . '

VOL. II. A A

354

Chapter XIV.

NERVOUS POWER.

The organs which are appropriated to the per-
formance of the various functions conducive to
nutrition, are generally designated the vital
organs, in order to distinguish them from those
which are subservient to sensation, volun-
tary motion, and the other functions of animal
life. The slightest reflection on the variety and
complication of actions comprised under the
former class of functions in the higher animals,
will convince us that they must be the result
of the combined operation of several different
agents ; but the principal source of mechanical
force required by the vital organs, is still, as in
all other cases, the muscular power. The coats
of the stomach and of the intestinal tube contain
a large proportion of muscular fibres, the con-
tractions of which effect the intermixture and
propulsion of the contents of these cavities, in
tlie manner best calculated to favour the che-
mical operations to which they are to be sub-
jected, and to extract from them all the nourish-
ment they may contain. In like manner, all

NERVOUS POWER. ,*J55

the tubular vessels, vrhich transmit fluids, are
endowed with muscular powers adapted to the
performance of that office. The heart is a strong
hollow muscle, with power adequate to propel
the blood, with immense force, through the
arterial and venous systems. The blood-vessels,
also, especially the minute, or capillary arteries,
besides being elastic, are likewise endowed with
muscular power, which contributes its share in
forwarding the motion of the blood, and com-
pleting its circulation. The quantity of blood
circulating in each part, the velocity of its
motion, and the heat which it evolves, are
regulated in a great measure by the particular
mode of action of the blood-vessels of that part.
The quantity, and sometimes even the qua-
lity of the secretions, are dependent, in like
manner, on the conditions of the circulation ;
and the action of the ducts, which convey the
secreted fluids to their respective destinations, is
also resolvible into the effects of a muscular
power.

The immediate cause which, in these organs,
excites the muscular fibre to contraction, may
frequently be traced to the forcible stretching of
its parts. This is the case in all hollow and
tubular muscles, such as the stomach, the heart,
and the blood-vessels, when they are mechani-
cally distended, beyond a certain degree, by the
presence of contained fluids, or other substances.

356 THE VITAL FUNCTIONS.

At other times, the chemical quality of their
contents appears to be the immediate stimulus
inciting them to contraction. But numerous in-
stances occur, in the higher orders of animals,
in which these causes alone are inadequate to
explain the phenomena of the vital functions.
No mechanical hypothesis will suffice to account
for the infinite diversity in the modes of action
of the organs which perform these functions, or
afford any clue to the means by which they are
made to co-operate, with such nicety of adj iist-
ment, in the production of the ultimate effect.
Still less will any theory, comprising only the
agency of the muscular power, and the ordinary
chemical affinities, enable us to explain how an
irritating cause, applied at one part, shall pro-
duce its visible effects on a distant organ ; or in
what way remote and apparently unconnected
parts shall, as if by an invisible sympathy, be
brought at the same moment to act in concert,
in the production of a common effect. Yet such
co-operation must, in innumerable cases, be
absolutely indispensable to the perfect accom-
plishment of the vital functions of animals.

Nature has not neglected objects so important
to the success of her measures ; but has pro-
vided, for the accomplishment of these purposes,
a controlling faculty, residing in the nervous
system, and denominated the nervous power.

NERVOUS POWER. 357

Experiments have shown that the due perform-
ance of the vital functions of digestion, of circu-
lation, and of secretion, requires the presence of
an agency, derived from different parts of the
brain and spinal marrow, and regulating the
order and combinations of the actions of the
organs which are to perform those functions.
The same influence, for example, which in-
creases the power of secretion in any particular
gland, is found to increase, at the same time,
the action of those blood-vessels which supply
that gland with the materials for secretion ; and
conversely, the increased action of the blood-
vessels is accompanied by an increased activity
of the secreting organ. Experience also shows
that when the influence of the brain and spinal
marrow is intercepted, although the afflux of
blood may, for a time, continue, yet the secretion
ceases, and all the functions dependent upon
secretion, such as digestion, cease likewise.
Thus the nervous power combines together dif-
ferent operations, adjusts their respective de-
grees, and regulates their succession, so as to
ensure that perfect harmony which is essential
to the attainment of the objects of the vital func-
tions ; and thus, not only the muscular power
which resides in the vital organs, but also the
organic affinities which produce secretion, and
all those unknown causes which effect the nutri-

3o8 THE VITAL FUNCTIONS.

tion, developement, and growth of each part, are
placed under the control of the nervous power.*
Although we are entirely ignorant of the na-
ture of the nervous power, we know that, when
employed in the vital functions, it acts through
the medium of a particular set of fibres, which
form part of the nervous system, and are classed,
therefore, among the nerves. The principal
filaments of this class of nerves compose what
is called the sympathetic nerve, from its being
regarded as the medium of extensive sympathies
among the organs ; but the whole assemblage of
these nerves is more commonly known by the
name of the ganglionic system, from the circum-
stance of their being connected with small masses
of nervous substance, termed ganglia, which are
placed in different parts of their course. Fig.
379, represents a ganglion (g), through which
the nerve (n), consisting at its origin of a number
of separate filaments (f), is seen to pass, before
it subdivides into branches (b). The numerous
communications and interchanges of filaments,
which subsequently take place at various parts,
forming what is called a plexus, are shown in

* As the functions of plants are sufficiently simple to admit
of being conducted without the aid of muscular power, still less
do they require the assistance of the nervous energy ; both of
which properties are the peculiar attributes of animal vitality.
We accordingly find no traces either of nervous or of muscular
fibres in any of the vegetable structures.

NERVOUS POWER.

359

Fig. 380 : where four trunks (t, t) divide into
branches, which are again separated, and va-

riously reunited in their course, Uke a ravelled
skein of thread, before they proceed to their
respective destinations.

The ganglia are connected by nervous fila-
ments with every part of the brain and spinal
marrow, the great central organs of the nervous
system ; and they also send out innumerable
branches, to be distributed all over the body.
All the parts receiving blood-vessels, and more
especially the organs of digestion, are abun-
dantly supplied with ganglionic nerves ; so that,
by their intervention, all these parts have ex-
tensive connexions with the brain and spinal
marrow, and also with one another. The ganglia
are more particularly the points of union between
nervous fibres coming from many different parts :
they may be considered, therefore, as performing,
with regard to the vital functions, an office ana-

360 THE VITAL FUNCTIONS.

logous to that which the brain and spinal marrow
perform v/ith regard to the other nerves, or as
being secondary centres of nervous power. Thus
there are two important objects for which the
nerves belonging to the ganglionic system have
been provided ; first, to serve as the channels
through which the affections of one organ might
be enabled to influence a distant organ ; and
secondly, to be the medium through which the
powers of several parts might be combined and
concentrated for effecting particular purposes,
requiring such co-operation. Hence it is by
means of the ganglionic nerves that all the
organs and all the functions are rendered effi-
cient in the production of a common object, and
are brought into one comprehensive and har-
monious system of operation.

The nervous power, the effects of which we
are here considering, should be carefully dis-
tinguished from that power which is an attribute
of another portion of the nervous system, and
which, being connected with sensation, volition,
and other intellectual operations, has been deno-
minated sensorial power* The functions of di-
gestion, circulation, absorption, secretion, and
all those included under the class of nutrient or
vital functions, are carried on in secret, are not

• This distinction has been most clearly pointed out, and illusr
'trated by Dr. A. P. W. Philip. See his " Experimental Inquiry
into the Laws of the Vital Functions."

NERVOUS POWKR. 361

necessarily, or even usually attended with sen-
sation, and are wholly removed from the control
of volition. Nature has not permitted processes,
which are so important to the preservation of
life, to be in any way interfered with by the will
of the animal. We know that in ourselves they
go on as well during sleep as when we are
awake, and whether our attention be directed
to them or not ; and though occasionally in-
fluenced by strong emotions, and other affections
of mind, they are in general quite independent
of every intellectual process. In the natural and
healthy condition of the system, all its internal
operations proceed quietly, steadily, and con-
stantly, whether the mind be absorbed in thought
or wholly vacant. The kind of existence result-
ing from these functions alone, and to which our
attention has hitherto been confined, must be
regarded as the result of mere vegetative, rather
than of animal life. It is time that we turn our
views to the higher objects, and more curious
field of inquiry, belonging to the latter.

PART III.

THE SENSORIAL FUNCTIONS.

Chapter I.

SENSATION.

The system of mechanical and chemical func-
tions which we have been occupied in reviewing,
has been established only as a foundation for
the endowment of those higher faculties which
constitute the great objects of animal existence.
It is in the study of these final purposes that
the scheme of nature, in the formation of the
animal world, opens and displays itself in all ita
grandeur. The whole of the phenomena we
have hitherto considered concur in one essential
object, the maintenance of a simply vital exist-
ence. Endowed with these properties alone, the
organized system would possess all that is abso-
lutely necessary for the continuance and support
of mere vegetative life. The machinery pro-
vided for this purpose is perfect and complete in
all its parts. To raise it to this perfection, not

SENSATION, 363

only has the Divine Architect employed all the
properties and powers of matter, which science
has yet revealed to man, but has also brought
into play the higher and more mysterious ener-
gies of nature, and has made them to concur in
the great work that was to be performed. On
the organized fabric there has been conferred a
vital force ; with the powers of mechanism have
been conjoined those of chemistry ; and to these
have been superadded the still more subtle and
potent agencies of caloric and of electricity :
every resource has been employed, every refine-
ment practised, every combination exhausted
that could ensure the stability, and prolong the
duration of the system, amidst the multifarious
causes which continually menace it with destruc-
tion. It has been supplied with ample means of
repairing the accidents to which it ic ordinarily
exposed ; it has been protected from the injurious
influence of the surrounding elements, and fitted
to resist for a lengthened period the inroads of
disease, and the progress of decay.

But can this, which is mere physical exist-
ence, be the sole end of life ? Is there no fur-
ther purpose to be answered by structures so
exquisitely contrived, and so bountifully pro-
vided with the means of maintaining an active
existence, than the mere accumulation and co-
hesion of inert materials, differing from the
stones of the earth only in the more artificial

364 THE SENSORIAL FUNCTIONS.

arrangement of their particles, and the more
varied configuration of their texture ? Is the
growth of an animal to be ranked in the same
class of phenomena as the concretion of a pebble,
or the crystallization of a salt? Must we not
ever associate the power of feeling with the idea
of animal life ? Can we divest ourselves of the
persuasion that the movements of animals, di-
rected, like our own, to obvious ends, proceed
from voluntary acts, and imply the operation of
an intellect, not wholly dissimilar in its spiritual
essence from our own ? In vain may Descartes
and his followers labour to sustain their paradox,
that brutes are only automata, — mere pieces of
artificial mechanism, insensible either to plea-
sure or to pain, and incapable of internal affec-
tions, analogous to those of which we are con-
scious in ourselves. Their sophistry will avail
but little against the plain dictates of the under-
standing. To those who refuse to admit that
enjoyment, which implies the powers of sensa-
tion, and of voluntary motion, is the great end of
animal existence, the object of its creation must
for ever remain a dark and impenetrable mys-
tery ; by such minds must all further inquiry
into final causes be at once abandoned as utterly
vain and hopeless. But it surely requires no
laboured refutation to overturn a system that
violates every analogy by which our reasonings
on these subjects must necessarily be guided ;

NERVOUS SYSTEM. 36*5

and no artificial logic or scholastic jargon
will long prevail over the natural sentiment,
which must ever guide our conduct, that animals
possess powers of feeling, and of spontaneous
action, and faculties appertaining to those of
intellect.

The functions of sensation, perception, and
voluntary motion require the presence of an
animal substance, which we find to be organized
in a peculiar manner, and endowed with very
remarkable properties. It is called the medul-
lary substance ; and it composes the greater part
of the texture of the brain, spinal marrow, and
nerves ; organs, of which the assemblage is
known by the general name of the nervous system.
Certain aff(ections of particular portions of this
medullary substance, generally occupying some
central situation, are, in a way that is totally
inexplicable, connected with affections of the
sentient and intelligent principle ; a principle
which we cannot any otherwise conceive than as
being distinct from matter ; although we know
that it is capable of being affected by matter
operating through the medium of this nervous
substance, and that it is capable of reacting
upon matter through the same medium. Of the
truth of these propositions there exist abundant
proofs ; but as the evidence which establishes
them will more conveniently come under our
notice at a subsequent period of our inquiry, I

306 THE SENSORIAL FUNCTIONS.

sh^ll postpone their consideration ; and, proceed-
ing upon the assumption that this connexion
exists, shall next inquire into the nature of the
intervening steps in the process, of which sen-
sation and perception are the results.

Designating, then, by the name of brai7i this
primary and essential organ of sensation, or the
organ of which the physical affections are imme-
diately attended by that change in the percipient
being which we term sensation ; let us first inquire
what scheme has been devised for enabling the
brain to receive impressions from such external
objects, as it is intended that this sentient being
shall be capable of perceiving. As these objects
can, in the first instance, make impressions only
on the organs situated at the surface of the
body, it is evidently necessary that some medium
of communication should be provided between
the external organ and the brain. Such a me-
dium is found in the nerves, which are white
cords, consisting of bundles of threads or fila-
ments of medullary matter, enveloped in sheaths
of membrane, and extending continuously from
the external organ to the brain, where they all
terminate. It is also indispensably requisite
that these notices of the presence of objects
should be transmitted instantly to the brain ; for
the slightest delay would be attended with se-
rious evil, and might even lead to fatal conse-
quences. The nervous power, of which, in our

NERVOUS SYSTEM. 367

review of the vital functions, we noticed some of
the operations, is the agent employed by nature
for this important office of a rapid communica-
tion of impressions. The velocity with which
the nerves subservient to sensation transmit the
impressions they receive at one extremity, along
their whole course, to their termination in the
brain, exceeds all measurement, and can be
compared only to that of electricity passing
along a conducting wire.

It is evident, therefore, that the brain requires
to be furnished with a great number of these
nerves, which perform the office of conductors of
the subtle influence in question ; and that these
nerves must extend from all those parts of the
body which are to be rendered sensible, and
must unite at their other extremities in that
central organ. It is of especial importance that
the surface of the body, in particular, should
communicate all the impressions received from
the contact of external bodies ; and that these
impressions should produce the most distinct
perceptions of touch. Hence we find that the
skin, and all those parts of it more particularly
intended to be the organs of a delicate touch,
are most abundantly supplied with nerves ; each
nerve, however, communicating a sensation dis-
tinguishable from that of every other, so as to
enable the mind to discriminate between them,
and refer them to their respective origins in dif-

368 THE SENSORIAL FUNCTIONS.

ferent parts of the surface. It is also expedient
that the internal organs of the body should have
some sensibility ; but it is better that this should
be very limited in degree, since the occasions
are few in which its exercise would be useful,
and many in which it would be positively inju-
rious ; hence the nerves of sensation are distri-
buted in less abundance to these organs.

It is not sufficient that the nerves of touch
should communicate the perceptions of the simple
pressure or resistance of the bodies in contact
with the skin : they should also furnish indica-
tions of other qualities in those bodies, of which
it is important that the mind be apprized ; such,
for example, as warmth, or coldness. Whether
these different kinds of impressions are all con-
veyed by the same nervous fibres it is difficult,
and perhaps impossible to determine.

When these nerves are acted upon in a way
which threatens to be injurious to the part im-
pressed, or to the system at large, it is also their
province to give warning of the impending evil,
and to rouse the animal to such exertions as may
avert it ; and this is effected by the sensation of
pain, which the nerves are commissioned to
excite on all these occasions. They act the part
of sentinels, placed at the outposts, to give sig-
nals of alarm on the approach of danger.

Sensibility to pain must then enter as a ne-
cessary constituent among the animal functions ;

NERVOUS SYSTEM. 369

for had this property been omitted, the animal
system would have been but of short duration,
exposed, as it must necessarily be, to perpetual
casualties of every kind. Lest any imputation
should be attempted to be thrown on the bene-
volent intentions of the great Author and De-
signer of this beautiful and wondrous fabric, so
expressly formed for varied and prolonged en-
joyment, it should always be borne in mind that
the occasional suffering, to which an animal is
subjected from this law of its organization, is far
more than counterbalanced by the consequences
arising from the capacities for pleasure, with which
it has been beneficently ordained that the healthy
exercise of the functions should be accompanied.
Enjoyment appears universally to be the main
end, the rule, the ordinary and natural condi-
tion ; while pain is but the casualty, the excep-
tion, the necessary remedy, which is ever tending
to a remoter good, in subordination to a higher
law of creation.

It is a wise and bountiful provision of nature
that each of the internal parts of the body has
been endowed with a particular sensibility to
those impressions which, in the ordinary course,
have a tendency to injure its structure ; while it
has at the same time been rendered nearly, if
not completely, insensible to those which are not
injurious, or to which it is not likely to be ex-
posed. Tendons and ligaments, for example,

VOL. II. r. B

370 THE SENSORIAL FUNCTIONS.

are insensible to many causes of mechanical
irritation, such as cutting, pricking, and even
burning; but the moment they are violently
stretched, (that being the mode in which they
are most liable to be injured,) they instantly
communicate a feeling of acute pain. The
bones, in like manner, scarcely ever communi-
cate pain in the healthy state, except from the
application of a mechanical force which tends
to fracture them.

The system of nerves, comprising those which
are designed to convey the impressions of touch,
is universally present in all classes of animals ;
and among the lowest orders, they appear to con-
stitute the sole medium of communication with
the external world. As we rise in the scale of
animals we find the faculties of perception ex-
tending to a wider range ; and many qualities,
depending on the chemical action of bodies, are
rendered sensible, more especially those which
belong to the substances employed as food.
Hence arises the sense of taste, which may be
regarded as a new and more refined species of
touch. This difference in the nature of the im-
pressions to be conveyed, renders it necessary
that the structure of the nerves, or at least of
those parts of the nerves which are to receive
the impression, should be modified and adapted
to this particular mode of action.

SENSATION. 371

As the sphere of perception is enlarged, it is
made to comprehend, not merely those objects
which are actually in contact with the body, but
also those which are at a distance, and of the
existence and properties of which it is highly
important that the animal, of whose sensitive
faculties we are examining the successive en-
dowment, should be apprized. It is more espe-
cially necessary that he should acquire an accu-
rate knowledge of the distances, situations and
motions of surrounding objects. Nature has
accordingly provided suitable organizations for
vision, for hearing, and for the perception of
odours ; all of which senses establish extensive
relations between him and the external world,
and give him the command of various objects
which are necessary to supply his wants, or
procure him gratification ; and which also ap-
prize him of danger while it is yet remote,
and may be avoided. Endowed with the power
of combining all these perceptions, he com-
mences his career of sensitive and intellectual
existence ; and though he soon learns that he
is dependent for most of his sensations on the
changes which take place in the external
world, he is also conscious of an internal
power, which gives him some kind of con-
trol over many of those changes, and that he
moves his limbs by his own voluntary act ;

372 THE SENSORIAL FUNCTIONS.

movements which originally, and of themselves,
appear, in most animals, to be productive of
great enjoyment.

To a person unused to reflection, the pheno-
mena of sensation and perception may appear to
require no elaborate investigation. That he
may behold external objects, nothing more seems
necessary than directing his eyes towards them.
He feels as if the sight of those objects were a
necessary consequence of the motion of his eye-
balls, and he dreams not that there can be any
thing marvellous in the function of the eye, or
that any other organ is concerned in this simple
act of vision. If he wishes to ascertain the
solidity of an object within his reach, he knows
that he has but to stretch forth his hand, and to
feel in what degree it resists the pressure he
gives to it. No exertion even of this kind is
required for hearing the voices of his companions,
or being apprized, by the increasing loudness of
the sound of falling waters, as he advances in a
particular direction, that he is coming nearer
and nearer to the cataract. Yet how much is
really implied in all these apparently simple
phenomena ! Science has taught us that these
perceptions of external objects, far from being
direct or intuitive, are only the final results of a
long series of operations, produced by agents of
a most subtle nature, which act by curious and
complicated laws, upon a refined organization,

SENSATION. 373

disposed in particular situations in our bodies,
and adjusted with admirable art to receive their
impressions, to modify and combine them in a
certain order, and to convey them in regular
succession, and without confusion, to the imme-
diate seat of sensation.

Yet this process, complicated as it may ap-
pear, constitutes but the first stage of the entire
function of perception : for before the mind can
arrive at a distinct knowledge of the presence
and peculiar qualities of the external object
which gives rise to the sensation, a long series of
mental changes must intervene, and many intel-
lectual operations must be performed. All these
take place in such rapid succession, that even
when we include the movement of the limb,
which is consequent upon the perception, and
which we naturally consider as part of the same
continuous action, the whole appears to occupy
but a single instant. Upon a careful analysis of
the phenomena, however, as I shall afterwards
attempt to show, we find that no less than twelve
distinguishable kinds of changes, or rather pro-
cesses, some of which imply many changes, must
always intervene, in regular succession, between
the action of the external object on the organ of
sense, and the voluntary movement of the limb
which it excites.

The external agents, which are capable of
affecting the different parts of the nervous sys-

374 THE SENSORIAL FUNCTIONS.

tern, so as to produce sensation, are of different
kinds, and are governed by laws peculiar to
themselves. The structure of the organs must,
accordingly, be adapted, in each particular case,
to receive the impressions made by these agents,
and must be modified in exact conformity with
the physical laws they obey. Thus the struc-
ture of that portion of the nervous system
which receives visual impressions, and which is
termed the retina, must be adapted to the action
of light ; and the eye, through which the rays
are made to pass before reaching the retina,
must be constructed with strict reference to the
laws of optics. The ear must, in like manner,
be formed to receive delicate impressions from
those vibrations of the air which occasion sound.
The extremities of the nerves, in these and other
organs of the senses, are spread out into a deli-
cate expansion of surface, having a softer and
more uniform texture than the rest of the nerve ;
whereby they acquire a susceptibility of being
affected by their own appropriate agents, and
by no other. The function of each nerve of
sense is determinate, and can be executed by no
other part of the nervous system. These func-
tions are not interchangeable, as is the case with
many others in the animal system. No nerve,
but the optic nerve, and no part of that nerve,
except the retina, is capable, however impressed,
of giving rise to the sensation of light : no part

SENSATION. 375

of the nervous system, but the auditory nerve can
convey that of sound ; and so of the rest.*

In almost every case the impression made
upon the sentient extremity of the nerve which
is appropriated to sensation, is not the direct
eftect of the external body, but results from the
agency of some intervening medium. There is
always a portion of the organ of sense interposed
between the object and the nerve on which the
impression is to be made. The object is never
allowed to come into direct contact with the
nerves ; not even in the case of touch, where
the organ is defended by the cuticle, through
which the impression is made, and by which
that impression is modified so as to produce the
proper effect on the subjacent nerves. This ob-
servation applies with equal force to the organs
of taste and of smell, the nerves of which are
not only sheathed with cuticle, but defended
from too violent an action by a secretion ex-
pressly provided for that purpose. In the
senses of hearing and of vision, the changes

* The credulity of the public has sometimes been imposed
upon by persons who pretended to see by means of their fingers :
thus, at Liverpool, the celebrated Miss M'Avoy contrived for a
long time to persuade a great number of persons that she really
possessed this miraculous power. Equally unworthy of credit
are all the stories of persons, under the influence of animal
magnetism, hearing sounds addressed to the pit of the stomach,
and reading the pages of a book a])plied to the skin over that
organ.

376 THE SENSORIAL FUNCTIONS.

which take place in the organs interposed be-
tween the external impressions and the nerves,
are still more remarkable and important, and
will be respectively the subjects of separate
inquiries. The objects of these senses, as well
as those of smell, being situated at a distance,
produce their first impressions by the aid of
some medium, exterior to our bodies, through
which their influence extends ; thus, the air is
the usual medium through which both light
and sound are conveyed to our organs. Hence,
in order to understand the whole series of phe-
nomena belonging to sensation, regard must be
had to the physical laws which regulate the
transmission of these agents. We are now to
consider these intermediate processes in the
case of each of the senses.

377

Chapter II.

TOUCH. >*?f«!j.'^^^

I HAVE already had occasion to point out the
structure of the integuments, considered in their
mechanical office of protecting the general frame
of the body ;* but we are now to view them in
their relation to the sense of touch, of which
they are the immediate organ. It will be recol-
lected that the corium forms the principal portion
of the skin ; that the cuticle composes the outer-
most layer ; and that between these there occurs
a thin layer of a substance, termed the rete mu-
cosum. The corium is constructed of an inter-
texture of dense and tough fibres, through which
a multitude of blood vessels and nerves are
interspersed ; but its external surface is more
vascular than any other part, exhibiting a fine
and delicate net-work of vessels ; and it is this
portion of the skin, termed by anatomists the
vascular plexus^ which is the most acutely sen-
sible in every point : hence, we may infer that
it contains the terminations of all the nervous
filaments distributed to this organ, and which

* Volume i, p. 112.

378 THE SENSORIAL FUNCTIONS.

are here found to be divided to an extreme
degree of minuteness.

When examined with the microscope, this ex-
ternal surface presents a great number of minute
projecting filaments. Malpighi first discovered
this structure in the foot of a pig ; and he gave
these prominences the name of papillce. It is
probable that each papilla contains a separate
branch of the nerves of touch, the ultimate
ramifications of which are spread over the sur-
face ; so that we may consider these papillae,
of which the assemblage has been termed the
corpus papular e, as the principal and immediate
organ of touch. This structure is particularly-
conspicuous on those parts of the skin which are
more especially appropriated to this sense, such
as the tips of the fingers, the tongue, and the lips :
in other parts of the surface, which are endowed
with less sensibility, the papillae are scarcely
visible, even with the aid of the microscope.

The surface of the corium is exquisitely sen-
sible to all irritations, whether proceeding from
the contact of foreign bodies, or from the im-
pression of atmospheric air. This extreme sen-
sibility of the corium would be a source of con-
stant torment, were it not defended by the cuticle,
which is unprovided with either blood-vessels or
nerves, and is, therefore, wholly insensible. For
the same reason, also, it is little liable to change,
and is thus, in both respects, admirably calcu-

TOUCH. 379

lated to afford protection to the finely organized
corium.

Although the cuticle exhibits no traces of vas-
cularity, it is by no means to be regarded as a
dead or inorganic substance, like the shells of
the mollusca. That it is still part of the living
system is proved by the changes it frequently
undergoes, both in the natural and the diseased
conditions of the body. It is perpetually, though
slowly, undergoing decay and renovation ; its
external surface drying off in minute scales,
and in some animals peeling off in large por-
tions. When any part of the human skin is
scraped with a knife, a grey dust is detached
from it, which is found to consist of minute
scales.

By repeated friction, or pressure of any part
of the skin, the cuticle soon acquires an increase
of thickness and of hardness : this is observable
in the soles of the feet, and palms of the hands,
and in the fingers of those who make much use
of them in laborious work. But this greater
thickness in the parts designed by nature to
suffer considerable pressure, is not entirely the
effect of education ; for the cuticle, which exists
before birth, is found, even then, to be much
thicker on the soles of the feet, and palms of the
hands, than on other parts. This example of
provident care in originally adjusting the struc-
tures of parts to the circumstances in which

.380 THE SENSORIAL FUNCTIONS.

they are to be placed at an after period, would
of itself, were it a solitary instance, be well fitted
to call forth our admiration. But as we study
each department of the animal economy in
detail, the proofs of design in the adaptation of
organs to their respective purposes multiply
upon us in such profusion, that we are apt to
overlook individual instances, unless they are
particularly brought before our notice. How
often have we witnessed and profited by the
rapid renewal of the cuticle, when by any acci-
dent it has been destroyed, without adverting
to the nature of the process which it implies ; or
reflected that the vessels of the skin must, on
all these occasions, supply the materials, out of
which the new cuticle is to be formed, must
effect their combination in the requisite pro-
portions, and must deposit them in the precise
situations in which they are wanted !

Different animals present remarkable differ-
ences in the thickness and texture of the cuticle,
according to the element they are destined to
inhabit, and the situations in which they are
most frequently placed. Provision is in many
cases made for preserving the cuticle from the
injury it would receive from the long continued
action of the air or water ; for it is apt to become
rigid, and to peel off", from exposure to a very
dry atmosphere ; and the constant action of
water, on the contrary, renders it too soft and
spongy. In order to guard against both these

TOUCH. 381

eftects, the skin has been furnished, in various
parts of its surface, with a secreting apparatus,
which pours out unctuous or mucilaginous fluids;
the oily secretions being more particularly em-
ployed as a defence against the action of the
air, and the mucilaginous fluids as a protection
against that of water.

The conditions on which the perfection of the
sense of touch depends are, first, an abundant
provision of soft papillae supplied with numerous
nerves ; secondly, a certain degree of fineness
in the cuticle ; thirdly, a soft cushion of cellular
substance beneath the skin ; fourthly, a hard
resisting basis, such as that which is provided in
the nails of the human fingers ; and lastly, it is
requisite that the organ be so constructed as to
be capable of being readily applied, in a variety
of directions, to the unequal surfaces of bodies ;
for the closer the contact, the more accurate will
be the perceptions conveyed. In forming an
estimate of the degree of perfection in which
this sense is exercised in any particular animal,
we must, accordingly, take into account the
mobility, the capability of flexion, and the figure
of the parts employed as organs of touch.

As touch is the most important of all the
senses, inasmuch as it is the foundation of all
our knowledge of the material world, so its rela-
tive degrees of perfection establish marked dif-
ferences in the intellectual sagacity of the several
tribes, and have a considerable influence on the

382 THE SENSORIAL FUNCTIONS.

assignment of their proper station in the scale of
animals.

Although the power of receiving obscure im-
pressions from the contact of external bodies,
and of perceiving variations of temperature, is
probably possessed by all animals, a small num-
ber only are provided with organs specially
appropriated for conveying the more delicate
sensations of touch. The greater part of the
surface of the body in the testaceous Mollusca is
protected by a hard and insensible covering of
shell. The integuments of Insects, especially
those of the Coleoptera, are in general too rigid
to receive any fine impressions from the bodies
which may come in contact with them ; and the
same observation applies, with even greater force,
to the Crustacea. The scales of Fishes, and of
Reptiles, the solid encasements of the Chelonia,
the plumage of Birds, the dense coating of the
Armadillo, the thick hides of the Rhinoceros,
and other Pachydermata, are evidently incom-
patible with any delicacy of touch. This nicer
faculty of discrimination can be enjoyed only
by animals having a soft and flexible integu-
ment, such as all the naked Zoophytes, Worms,
and Mollusca, among the lower orders, and Ser-
pents, among the higher. The flexibility of the
body or limbs is another condition which is ex-
tremely necessary towards procuring extensive
and correct notions of the relative positions of
external objects. It is essential therefore that

TOUCH. 383

those instruments which are more particularly
intended as organs of touch, should possess this
property.

It will not be necessary to enter into a minute
description of these organs, because they have,
for the most part, been already noticed as in-
struments of motion or prehension ; for the sense
of touch is in general exercised more particularly
by the same parts which perform this latter
function. Thus the tentacula of the various
tribes of Polypi, of Actiniae, and of Annelida,
are organs both of prehension and of touch.
The tubular feet of the Asterias and Echinus
are subservient both to the sense of touch, and
to the faculty of progressive motion. The feet
of Insects and of Crustacea are well calculated,
indeed, by their jointed structure, for being
applied to the surfaces, and ditferent sides of
bodies ; but they are scarcely ever employed
in this capacity ; being superseded by the palpi,
which are situated near the mouth. When
insects are walking, the palpi are incessantly
applied to the surface on which they advance,
as if these organs were especially employed to
feel their way. There can be little doubt, how-
ever, that, in most insects, the principal organs
of touch are the A7Uenn<s\ also denominated, from
their supposed office, the feelers.'^

Some idea of the great variety in the forms of

* The German name for them, fiihlhorner, or the feeling
horns, is founded on the same notion.

n84

THE SENSORIAL FUNCTIONS.

the antennae of insects may be obtained from
the specimens delineated in Fig. 381, which
shows a few of the most remarkable.*

* In this figure, A represents the form of antennae, technically
denominated Antenna capiiulo uncinato, as exemplified in the
Pausus. ^

B. is the A. piloso-verticillata, as in the Psychoda ocellaris.

C. .A. biclavata, (Claviger longicornis).
D. . A. triangularis, (Lopliosia).
E..A. clavata, (Masaris).

F. .A. capit. lamellato, ( Melolontha mas).

G. .A. capit. fissile, ( Aphodius fossor).
H..A. fusiformis, ( Zygcena).

I. .A. capitata, ( Ascalaphus ) .

K. . A. furcata, (Schizocera).

L. , A. bipectinata, (Ctenophora).

M. . A. irregularis, (Agaon paradoxum).

N..A. cordata, (Diaperis holeti).

0..A. bipectinata, (Bombyx).

P. .A. palmata, (Nepa cinerea).

Q. . A. ensiformis, (Truxalis).

R. .A. setacea, (Cerambyx).

TOUCH. 385

The universality of these organs among every
species of this extensive class of animals, their
great flexibility, arising from their jointed struc-
ture,* their incessant motion when the insect
is walking, and their constant employment in
examining the surfaces of all the bodies with
which they come in contact, sufficiently point
them out as instruments of a very delicate sense
of touch. Organs of this kind were particularly
necessary to insects, since the horny nature of
the integuments of the greater number pre-
cludes them from imparting any accurate per-
ceptions of touch.

It has been conjectured that the antennae of
insects are the organs of other senses besides
that of touch. If an insect be deprived of its
antennae, it either remains motionless, or if it
attempt to walk or fly, appears bewildered, and
moves without any apparent object. Huber
found that Bees are enabled, by feeling with
their antennae, to execute their various works in
the interior of the hive, where, of course, they
can have no assistance from light. They employ

* The number of segments into which these organs are divided
is often very great. In the Gryllotalpa, or mole cricket, it
amounts to above 100. (Kidd, Phil. Trans, for 1825, p. 211.)
This insect has, besides the antennae on the head, two posterior
or caudal antennae, which are not jointed, excepting at their
very commencement. These are extremely sensible, and serve
probably to give the animal notice of the approach of any
annoyance from behind, lb. p. 216.

VOL. II. C C

380 THE SENSORIAL FUNCTIONS.

these organs perpetually while building the
combs, pouring honey into the magazines, ascer-
taining the presence of the queen, and feeding
and tending the larvae. The same naturalist
observes, also, that it is principally by means of
the antennae that these social insects communi-
cate to one another their impressions and their
wants.

The different modes in which Ants, when they
happen to meet during their excursions, mu-
tually touch one another with their antennae,
appears to constitute a kind of natural lan-
guage understood by the whole tribe. This
contact of the antennae evidently admits of a
great variety of modifications, and seems capable
of supplying all the kinds of information which
these insects have occasion to impart. It would
seem impossible, indeed, for all the individuals
composing these extensive societies to co-operate
effectually in the execution of many works,
calculated for the general benefit of the com-
munity, unless some such means of commu-
nication existed. There is no evidence that
sound is the medium of this intercourse ; for
none, audible to us at least, was ever known
to be emited by these insects. Their mode
of communication appears to be simply by
touching one another in different ways with
the antennae. Huber's observations on this

TOUCH. 387

subject are exceedingly curious.* He remarks
that the signal denoting the apprehension of
danger, is made by the ant striking its head
against the corselet of every ant which it chances
to meet. Each ant, on receiving this intima-
tion, immediately sets about repeating the
same signal to the next ant which comes in its
way; and the alarm is thus disseminated with
astonishing rapidity throughout the whole so-
ciety. Sentinels are at all times stationed on
the outside of the nests, for the purpose of
apprizing the inhabitants of any danger that
may be at hand. On the attack of an enemy,
these guardians quickly enter into the nest, and
spread the intelligence on every side : the whole
swarm is soon in motion, and while the greater
number of ants rush forwards with desperate
fury to repel the attack, others who are entrusted
with the office of guarding the eggs and the
larvae, hasten to remove their charge to places of
greater security.

When the queen bee is forcibly taken away
from the hive, the bees which are near her at
the time, do not soon appear sensible of her ab-
sence, and the labours of the hive are carried on
as usual. It is seldom before the lapse of an
hour, that the working bees begin to manifest
any symptoms of uneasiness : they are then

* See his " Recherches sur les mceurs des fourmis indigenes,''

.388 THE SENSORIAL FUNCTIONS.

observed to quit the larvae which they had been
feeding, and to run about in great agitation, to
and fro, near the cell which the queen had oc-
cupied before her abduction. They then move
over a wider circle, and on meeting with such of
their companions as are not aware of the disaster,
communicate the intelligence by crossing their
antennae and striking lightly with them. The
bees which receive the news become in their
turn agitated, and conveying this feeling where-
ever they go, the alarm is soon participated by
all the inhabitants of the hive. All rush forwards
with tumultuous precipitation, eagerly seeking
their lost queen ; but after continuing the search
for some hours, and finding it to be fruitless,
they appear resigned to their misfortune ; the
noisy tumult subsides, and the bees quietly
resume their labours.

A bee, deprived of its antennae, immediately
becomes dull and listless : it desists from its
usual labours, remains at the bottom of the hive,
seems attracted only by th« light, and takes the
first opportunity of quitting the hive, never more
to return. A queen bee, thus mutilated, ran
about, without apparent object, as if in a state
of delirium, and was incapable of directing her
trunk with precision to the food which was
offered to her. Latreille relates that, having de-
prived some labouring ants of their antennae, he
replaced them near the nest ; but they wandered

TOUCH. 389

in all directions, as if bewildered, and uncon-
scious of what they were doing. Some of their
companions were seen to notice their distress,
and approaching them with apparent compas-
sion, applied their tongues to the wounds of the
sufferers, and anointed them with their saliva.
This trait of sensibility was repeatedly witnessed
by Latreille, while watching their movements
with a magnifying glass.

The Arachnida, from the mobility of their
limbs, and the thinness of their cutaneous invest-
ment, have a very delicate sense of touch.
Among the Mollusca, it is only the higher orders
of Cephalopoda that enjoy this sense in any con-
siderable degree ; and they are enabled to exer-
cise it by means of their long and flexible ten-
tacula. Many bivalve mollusca have, indeed,
a set of tentacula placed near the mouth, but
they are short, and of little power. It is pro-
bable that the foot may also be employed by
these animals as an organ of touch.

Fishes are, in general, very ill-constructed for
the exercise of this sense ; and their fins are used
for no other purposes than those of progressive
motion. That part of the surface which pos-
sesses the most acute feeling is the under-side,
where the integuments are the thinnest. The
chief seat of the sense of touch, however, is the
lip, or end of the snout, which is largely sup-
plied with nerves ; and perhaps the cirrhi, or

390 THE SENSORIAL FUNCTIONS.

little vermiform processes called barbels, which
in some species are appended to the mouth,
may be subservient to this sense.* These pro-
cesses in the Silurus glanis are moved by par-
ticular muscles.

Serpents, from the great flexibility of their
spine, are capable of grasping and twining round
objects of almost any shape, and of taking, as
it were, their exact measure. This conformation
must be exceedingly favourable to the acquisi-
tion of correct perceptions of touch. As it is
these perceptions, which, as we shall afterwards
find, lay the foundation of the most perfect ac-
quaintance with the tangible properties of sur-
rounding bodies, we may presume that this
power contributes much to the sagacity possessed
by these animals. It has been said of Serpents,
that their whole body is a hand, conferring some
of the advantages of that instrument. Hellman
has shown that the slender bifurcated tongue
of these animals is used for the purposes of

touch. t

In those species of Lizards which are ena-
bled by the structure of their feet to clasp the
branches of trees, as the Gecko and the C/ia-

* These kind of tentacula are remarkable for their length and
mobility in the Lophius piscatorius, or Angler ; and it is said
that they are employed by the fish, while lurking in ambush, as
a decoy to other fishes, which they entice by their resemblance to
worms.

t Quoted by Blumenbach.

TOUCH. 391

tnelion, and whose tails also are prehensile,
we must, for the same reason, presume that the
sense of touch exists in a more considerable
degree than in other Saurian Reptiles, which do
not possess this advantage. The toes of Birds
are also well calculated to perform the office of
organs of touch, from the number of their arti-
culations and their divergent position, and from
the papillae with which their skin abounds ; ac-
companied as they are with a large supply of
nerves. Those birds, which, like the Parrot,
employ the feet as organs of prehension, probably
enjoy a greater developement of this sense. The
skin which covers the bills of aquatic birds is
supplied by very large nerves, and consequently
possesses great sensibility. This structure enables
them to find their food, which is concealed in the
mud, by the exercise of the sense of touch
residing in that organ. A similar structure,
probably serving a similar purpose, is found in
the Ornithorhynchus.

Among Mammalia, we find the seat of this
sense frequently transferred to the lips, and ex-
tremity of the nostrils ; and many have the nose
prolonged and flexible, apparently with this
view. This is the case with the Shrew and the
Mole, which are burrowing animals, and still
more remarkably w ith the Pachydermata, where
this greater sensibility of the parts about the
face seems to have been bestowed as some com-

392 THE SENSORIAL FUNCTIONS.

pensation for the general obtuseness of feeling
resulting from the thickness of the hide which
covers the rest of the body. Thus the Rhi-
7ioceros has a soft, hook-shaped extension of
the upper lip, which is always kept moist,
in order to preserve its sensibility as an organ
of touch. The Hog has the end of the nose
also constructed for feeling ; though it is not so
well calculated for distinguishing the form of
objects, as where the organ is prolonged in the
form of a snout, which it is in the Tapir, and in
a still higher degree in the admirably constructed
proboscis of the Elephant, which as an organ,
both of prehension and of touch, forms the
nearest approach to the perfect structure of the
human hand.

The Lion, Tiger, Cat, and other animals of the
genus Felis, have whiskers, endowed at their
roots with a particular sensibility, from being
largely supplied with nerves. The same is the
case with the whiskers of the Seal.

The prehensile tails of the American Monkeys
are doubtless fitted to convey accurate percep-^
tions of touch, as well as the feet and hands ;
as may be inferred from the great size of the
nervous papillae, and the thinness of the cuticle
of those parts.

The sense of touch attains its greatest degree
of excellence in the human hand, in which it is
associated with the most perfect of all instru-

TASTE. 393

merits of prehension. But as the structure and
functions of this organ are the exclusive subjects
of another of these treatises, I shall refrain from
any farther remarks respecting it.

Chapter 111.

TASTE.

The senses of Taste and Smell are intended to
convey impressions resulting from the chemical
qualities of bodies, the one in the fluid, the other
in the gaseous state.* There is a considerable
analogy between the sensations derived from
these two senses. The organ of taste is the
surface of the tongue, the skin of which is fur-
nished with a large proportion of blood-vessels
and nerves. The vascular plexus immediately
covering the corium is here very visible, and
forms a distinct layer, through which a great
number of papillae pass, and project from the
surface, covered with a thin cuticle, like the pile

* Bellini contended that the different flavors of saline bodies
are owing to the peculiar figures of their crystalline particles.
It is strange that Dumas should have thought it worth while
seriously to combat this extravagant hypothesis, by a laboured
refutation.

394 THE SENSORIAL FUNCTIONS.

of velvet. In the fore part of the human tongue
these papillae are visible even to the naked eye ;
and especially in certain morbid conditions of
the organ.* They are of different kinds ; but
it is only those which are of a conical shape
that are the seat of taste. If these papillae be
touched with a fluid, which has a strong taste,
such as vinegar, applied by means of a camel-
hair pencil, they will be seen to become elon-
gated by the action of the stimulus ; an effect
which probably always accompanies the percep-
tion of taste.

The primary use of this sense, the organ of
which is placed at the entrance of the alimen-
tary canal, is evidently to guide animals in the
choice of their food, and to warn them of the
introduction of a noxious substance into the
stomach. With respect to the human species,
this use has been, in the present state of society,
superseded by many acquired tastes, whicli have
supplanted those originally given to us by na-
ture ; but in the inferior animals it still retains
its primitive office, and is a sense of great im-
portance to the safety and welfare of the indivi-

* This is particularly the case in scarlatina, in the early stage
of which disease they are elongated, and become of a bright red
colour, from their minute blood-vessels being distended with
blood. As the fever subsides the points of the papillae collapse,
and acquire a brown hue; giving rise to the appearance known
by the name of the strawberry tongue.

TASTE. 395

dual, from its operation being coincident with
those of natural instincts. If, as it is said, these
instincts are still met with among men in a
savage state, they are soon weakened or effaced
by civilization.

The tongue, in all the inferior classes of ver-r
tebrated animals, namely Fishes, Reptiles, and
Birds, is scarcely ever constructed with a view
to the reception of delicate impressions of taste ;
being generally covered with a thick, and often
horny cuticle ; and being, besides, scarcely ever
employed in mastication. This is the case,
also, with those quadrupeds which swallow their
food entire, and which cannot, therefore, be
supposed to have the sense of taste much de-
veloped.

Insects which are provided with a tongue or
a proboscis may be conceived to exercise the
sense of taste by means of these organs. But
many insects possess, besides these, a pair of
short feelers, placed behind the true antennae ;
and it has been observed that, while the insect
is taking food, these organs are in incessant mo-
tion, and are continually employed in touching
and examining the food, before it is introduced
into the mouth : hence, some entomologists have
concluded that they are organs of taste. But it
nuist be obvious that in this, as in every other
instance in which our researches extend to
beings of such minute dimensions, and which

390 THE SENSORIAL FUNCTIONS.

occupy a station, in the order of sensitive exist-
ence, so remote from ourselves, we are wander-
ing into regions where the only light that is
afforded us must be borrowed from vague and
fanciful analogies, or created by the force of a
vivid and deceptive imagination.

Chapter IV.

SMELL.

Animal life being equally dependent upon the
salubrious qualities of the air respired, as of
the food received, a sense has been provided
for discriminating the nature of the former,
as well as of the latter. As the organs of taste
are placed at the entrance of the alimentary
canal, so those of smell usually occupy the be-
ginning of the passages for respiration, where
a distinct nerve, named the olfactory^ appro-
priated to this office, is distributed.

The sense of smell is generally of greater
importance to the lower animals than that of
taste ; and the sphere of its perceptions is in
them vastly more extended than in man. The
agents, which give rise to the sensations of

SMELL. 397

smell, are certain effluvia, or particles of ex-
treme tenuity, which are disseminated very
quickly through a great extent of atmospheric
air. It is exceedingly difficult to conceive how
matter so extremely rare and subtile as that
which composes these odorous effluvia can re-
tain the power of producing any sensible im-
pression on the animal organs ; for its tenuity is
so extraordinary as to exceed all human com-
prehension. The most copious exhalations from
a variety of odoriferous substances, such as musk,
valerian, or assafoetida, will be continually ema-
nating for years, without any perceptible loss
of weight in the body which supplies them. It
is well known that if a small quantity of musk
be enclosed for a few hours in a gold box, and
then taken out, and the box cleaned as carefully
as possible with soap and water, that box will
retain the odour of musk for many years ; and
yet the nicest balance will not show the smallest
increase of its weight from this impregnation.
No facts in natural philosophy afford more
striking illustrations of the astonishing, and
indeed inconceivable divisibility of matter, than
those relating to odorous effluvia.

It would appear that most animal and vege-
table bodies are continually emitting these subtle
effluvia, of which our own organs are not suffi-
ciently delicate to apprize us, unless when they

398 THE SENSORIAL FUNCTIONS.

are much concentrated, but which are readily
perceived and distinguished by the lower ani-
mals ; as may be inferred from their actions. A
dog is known to follow its master by the scent
alone, through the avenues and turnings of a
crowded city, accurately distinguishing his track
amidst thousands of others.

The utility of the sense of smell is not con-
fined to that of being a check upon the respira-
tion of noxious gases ; for it is also a powerful
auxiliary to the sense of taste, which of itself,
and without the aid of smell, would be very
vague in its indications and limited in its range.
What may have been its extent and delicacy in
man, while he existed in a savage state, we have
scarcely any means of determining ; but in the
present artificial condition of the race, resulting
from civilization and the habitual cultivation of
other sources of knowledge, there is less neces-
sity for attending to its perceptions, and our sen-
sibility to odours may perhaps have diminished
in the same proportion. It is asserted both by
Soemmerring and Blumenbach that the organ
of smell is smaller in Europeans, and other civi-
lized races of mankind, than in those nations of
Africa or America, which are but little removed
from a savage state : it is certainly much less
developed in man than in most quadrupeds. To
the carnivorous tribes, especially, it is highly

SMELL. 399

useful in enabling them to discover their natural
food at great distances.

The cavity of the nostrils, in all terrestrial
vertebrated animals, is divided into two by a
vertical partition ; and the whole of its internal
surface is lined by a soft membrane, called the
Schneiderian memhrane* which is constantly
kept moist, is supplied with numerous blood-
vessels, and upon which are spread the ultimate
ramifications of the olfactory nerves. The rela-
tive magnitude of these nerves is much greater
in carnivorous quadrupeds than in those which
subsist on vegetable food. In quadrupeds, as
well as in man, these nerves are not collected
into a single trunk, in their course towards the
brain, but compose a great number of filaments,
which pass separately through minute perfora-
tions in a plate of bone, (called the ethmoid hone)^
before they enter into the cavity of the skull,
and join that part of the cerebral substance with
which they are ultimately connected.

The surface of the membrane which receives
the impressions from odorous effluvia, is con-
siderably increased by several thin plates of
bone, which project into the cavity of the nos-
trils, and are called the turbinated hones. These
are delineated at t, t, in Fig. 382, as they appear

* It has been so named in honour of Schneider, the first ana-
tomist who gave an accurate description of this membrane.

400

THE SENSORIAL FUNCTIONS.

in a vertical and longitudinal section of the
cavity of the human nostril, where they are seen

covered by the Schneiderian membrane.* A
transverse and vertical section of these parts is
given in Fig. 383. t The turbinated bones are
curiously folded, and often convoluted in a spiral
form, with the evident design of obtaining as

* This figure shows the branches of the olfactory nerve (o),
passing through the thin cribriform plate of the ethmoid bone,
and distributed over that membrane. Several of the cells, which
open into the cavity, are also seen ; such as the large sphenoidal
sinus (s), the frontal sinus (f), and one of the ethmoidal cells
(c). N, is the nasal bone ; p, the palate ; and e, the month of
the Eustachian tube, which leads to the ear.

t In this figure, s, is the septum, or partition of the nostrils,
on each side of which are seen the sections of the turbinated
bones projecting into the cavity ; the ethmoid cells (c), situated
between the orbits (o) ; and the Antrum maxillare (a), which
is another large cavity communicating with the nostrils.

SMELL.

401

great an extent of surface as possible within
the confined space of the nasal cavity. This

turbinated, or spiral shape, chiefly characterises
these bones among herbivorous quadrupeds :
in the Horse, for example, the turbinated
bones are of a large diameter, and extend the
whole length of the prolonged nostrils. Their
structure is exceedingly intricate ; for while
they retain externally the general shape of an
oblong spiral shell, they are pierced on all
their internal sides with numerous perforations,
through which the membrane, together with the
fine branches of the nerves, passes freely from
one side to the other. The cavities resulting
from the convolutions are intersected by un-
perforated partitions of extraordinary tenuity ;
serving both to support the arches of bone, and

VOL. II. D D

402

THE SENSORIAL FUNCTIONS.

to furnish a still greater surface for the extension
of the olfactory membrane. In the Sheep, the
Goat, and the Deer, the structure is very similar
to that just described ; but the convolutions are
double, with an intermediate partition, so as to
resemble in its transverse section the capital of
an Ionic column.* They are shown at t, Fig.
384, which exhibits the transverse section of the
nostrils of a sheep.

In carnivorous quadrupeds the structure of
these bones is still more intricate, and is cal-
culated to afford a far more extensive surface

* In a species of Antelope described by Mr. Hodgson, cavities
exist, situated immediately behind the ordinary nostrils, and
communicating with them. These accessory nostrils are conjec-
tured to be useful to this exceedingly fleet animal by facilitating
its breathing, while it is exerting its utmost speed ; for the
expansion of the nostrils opens also these posterior cavities, the
sides of which, being elastic, remain dilated. (Journal of the
Asiatic Society, Feb. 1832, p. 59.)

SMELL. 403

for the (listribiition of the olfactory nerve. In
the Seal this conformation is most fully de-
veloped ; and the bony plates are here not tur-
binated, but ramified, as shown at t in Fig. 385.
Eight or more principal branches arise from the
main trunk ; and each of these is afterwards
divided and subdivided to an extreme degree of
minuteness, so as to form in all many hundred
plates. The olfactory membrane, with all its
nerves, is closely applied to every plate in this
vast assemblage, as well as to the main trunk,
and to the internal surface of the surrounding
cavity ; so that its extent cannot be less than
120 square inches in each nostril. An organ of
such exquisite sensibility requires an extraor-
dinary provision for securing it against injury,
by the power of voluntarily excluding noxious
vapours ; and nature has supplied a mechanism
for this express purpose, enabling the animal to
close at pleasure the orifice of the nostril. The
Hog^ which, in its natural state, subsists wholly
on vegetable food, resembles herbivorous tribes
in the external form and relative magnitude
of the turbinated bones ; but they are more
simple in their structure, being formed of single,
and slightly convoluted plates, without partitions
or perforations. In this respect they approach
to the human structure, which is even less com-
plicated, and indicates a greater affinity to
vegetable than to animal feeders. Man, in-

404 THE SENSORIAL FUNCTIONS.

deed, distinguishes more accurately vegetable
odours than those proceeding from animal sub-
stances ; while the reverse is observed with re-
gard to quadrupeds whose habits are decidedly
carnivorous. A dog, for instance, is regardless of
the odour of a rose or violet ; and probably, as
he derives from them no pleasure, is unable to
discriminate the one from the other. Preda-
ceous animals, as Sir B. Harwood observes,
require both larger olfactory nerves, and a more
extensive surface for their distribution, than the
vegetable eaters. The food of the latter is ge-
nerally near at hand ; and as they have occasion
only to select the wholesome from the noxious
plants, their olfactory organs are constructed for
the purpose of arresting the effluvia of odorous
substances immediately as they arise. The former
are often under the necessity of discovering the
lurking places of their prey at a considerable
distance, and are therefore more sensible to the
weak impressions of particles widely diffused
through the surrounding medium, or slightly ad-
hering to those bodies, with which the object of
their pursuit may have come into contact.

The olfactory bones of Birds are constructed
very much on the model of the spiral bones of
herbivorous quadrupeds, and vary but little in
the different species. Fig. 386 exhibits their
appearance in the Turkey : but the size of the
olfactory nerves of birds of prey greatly exceeds

SMELL. 405

that of the same nerves in granivorous birds.
In the latter, indeed, they are exceedingly small ;
and as the natural food of that tribe has but little
odour, we find that they are easily deceived by

any thing which bears a resemblance to it. Sir
Busick Harwood relates that some poultry, which
were usually fed with a mixture of barley meal
and water, were found to have swallowed, by
mistake, nearly the whole contents of a pot of
white paint. Two of the fowls died, and two
others became paralytic. The crops of the
latter were opened, and considerably more than
a pound of the poisonous composition taken from
each ; and the crops, either naturally, or from
the sedative effects of the paint, appeared to
have so little sensibility that, after the wounds
were sewed up, both the fowls eventually reco-
vered.

The olfactory nerves are conspicuous in the
Duck, both from their size and mode of distribu-

406 THE SENSORIAL FUNCTIONS.

tion: They are seen in Fig. 387, passing out
through the orbit of the eye (o) in two large

branches, an upper one (u), and a lower one (l),
the ramifications of which are spread over the
mandibles, both within and without. For the
protection of the highly sensible extremity of
the beak against the injurious impressions of
hard bodies, a horny process (p), similar, both in
form and office, to the human nail, is attached to
it, and its edges guarded by a narrow border of
the same horny material ; these receive a first,
and fainter impression, and admonish the animal
of approaching danger ; if none occur, the mat-
ter is then submitted to the immediate scrutiny
of the nerves themselves, and is swallowed or
rejected according to their indication.*

It has been generally asserted that Vultures,
and other birds of prey, are gifted with a highly
acute sense of smell ; and that they can discover
by means of it the carcass of a dead animal at
great distances : but it appears to be now suffi-

* Such is the account given by Sir Busick Harwood, in his
" System of Comparative Anatomy and Physiology," p. 26.

SMELL. 407

ciently established by the observations and ex-
periments of Mr. Audubon, that these birds in
reality possess the sense of smell in a degree
very inferior to carnivorous quadrupeds ; and
that so far from guiding them to their prey from
a distance, it affords them no indication of its
presence, even when close at hand. The follow-
ing experiments appear to be perfectly con-
clusive on this subject. Having procured the
skin of a deer, Mr. Audubon stuffed it full of
hay ; and after the whole had become perfectly
dry and hard, he placed it in the middle of an
open field, laying it down on its back, in the
attitude of a dead animal. In the course of a
few minutes afterwards, he observed a vulture
flying towards it, and alighting near it. Quite un-
suspicious of the deception, the bird immediately
proceeded to attack it, as usual, in the most vul-
nerable points. Failing in his object, he next,
with much exertion, tore open the seams of the
skin, where it had been stitched together, and
appeared earnestly intent on getting at the flesh,
which he expected to find within, and of the
absence of which, not one of his senses was able
to inform him. Finding that his efforts, which
were long reiterated, led to no other result than
the pulling out large quantities of hay, he at
length, though with evident reluctance, gave up
the attempt, and took flight in pursuit of other
game to which he was led by the sight alone,

408 THE SENSORIAL FUNCTIONS.

and which he was not long in discovering and
securing.

Another experiment, the converse of the first,
was next tried. A large dead hog was concealed
in a narrow and winding ravine, about twenty
feet deeper than the surface of the earth around
it, and filled with briers and high cane. This
was done in the month of July in a tropical
climate, where putrefaction takes place with
great rapidity. Yet, although many vultures
were seen, from time to time, sailing in all di-
rections over the' spot where the putrid carcass
was lying, covered only with twigs of cane, none
ever discovered it; but in the mean while,
several dogs had found their way to it, and had
devoured large quantities of the flesh. In an-
other set of experiments it was found that young
vultures, enclosed in a cage, never exhibited any
tokens of their perceiving food, when it could
not be seen by them, however near to them it
was brought.*

It has been doubted whether fishes, and other
aquatic animals, possess the sense of smell ; in
some of the Whale tribe, indeed, neither the or-
gan of smell nor the olfactory nerves are found-t
Some physiologists have gone the length of de-

* Edinburgh New Journal of Science, ii. 172. The accuracy
of these results, which had been contested by Mr. Waterton, is
fully established by the recent observations and experiments of
Mr. Bach man, which are detailed in Loudon's Magazine of Na-
tural History, vii. 167.

t Home; Lectures on Comparative Anatomy, i. 17.

SMELL. 409

nying the capability of water to serve as the ve-
hicle of odorous effluvia. But as water is known
to contain a large quantity of air, which acts upon
the organs of respiration, it is easy to conceive
that it may also convey to the nostrils the pecu-
liar agents which are calculated to excite percep-
tions of smell. Fishes are, in fact, observed to
be attracted from great distances by the effluvia
of substances thrown into the water ; and they
are well known to have a strong predilection for
all highly odoriferous substances. Baits used
by anglers are rendered more attractive by being
impregnated with volatile oils, or other sub-
stances having a powerful scent, such as assa-
fcetida, camphor, and musk. Mr. T. Bell* has
discovered in the Crocodile and Alligator, a
gland, which secretes an unctuous matter, of a
strong musky odour, situated beneath the lower
jaw, on each side. The external orifice of this
gland is a small slit, a little within the lower
edge of the jaw ; and the sac, or cavity contain-
ing the odoriferous substance, is surrounded by
two delicate bands of muscular fibres, apparently
provided for the purpose of first bringing the
gland into a proper position, and then, by com-
pressing it, discharging its contents. Mr. Bell
conceives that the use of this secretion is to act
as a bait for attracting fish towards the sides of
the mouth, where they can be readily seized in

* Phil. Trans, lor 1827, p. 132.

410 THE SENSORIAL FUNCTIONS.

the mode usual to the alligator, which is that of
snapping sideways at the objects he aims at de-
vouring.

The organs of smell in Fishes are situated in
cavities, placed one on each side, in front of the
head : they are merely blind sacs, having no
communication with the mouth or throat, and
indeed no other outlet but the external openings,
which are generally two to each sac. The prin-
cipal entrance is furnished with a valve, formed
by a moveable membrane, appearing like a par-
tition dividing each nostril into two cavities, and
serving the purpose of preventing the introduc-
tion of any foreign body. The organ itself is
situated behind this valve, and consists either of
a membrane, curiously plaited into numerous
semicircular folds, or of tufted or arborescent
filaments. Fig. 388 shows this cavity (s), with

its plaited membrane in the Perch; and Fig. 389,
in the Skate; the laminae in the former being
radiated, and in the latter, foliated, or parallel to
each other. On the surface of these organs,
whatever be their shape, the olfactory nerves

SMELL. 411

(n), arising from the anterior lobes (o) of the
brain, are distributed ; and the great size of these
nerves would lead us to infer considerable acute-
ness in the sense which they supply. When
the fish is swimming, their situation in front of
the snout exposes them to the forcible impulse of
the water, which strikes against them. Accord-
ing, to Geoffroy St. Hilaire, the water enters the
cavity by the upper orifice, and escapes by the
lower. Scarpa alleges that fishes exercise this
sense by compressing the water against the
membrane. On the other hand, it is contended
by Dumeril, that the perceptions communicated
by this organ, being the result of the action of a
liquid instead of a gas, should be classed under
the head of taste rather than of smell. This
seems, however, to be a mere verbal criticism, in
making which it appears to have been forgotten
that the impressions of odorous effluvia, even
in animals breathing atmospheric air, always
act upon the nerve through the intermedium of
the fluid which lubricates the membrane of the
nostril.

That the nasal cavities of fishes are rudimental
forms of those of the mammalia, although they
do not, as in the latter class, open into the respi-
ratory organs, is shown by the curious transform-
ation of the one into the other during the de-
velopement of the tadpole, both of the Frog and
of the Salamander. We have already seen that
during the first periods of their existence, these

412 THE SENSORIAL FUNCTIONS.

animals are perfectly aquatic ; breathing water by
means of gills, and having all their organs formed
on the model of the fish. Their nasal cavities are
not employed for respiration at this early period;
nor even for some time after they have begun to
take in air, which they do by the mouth, swal-
lowing it in small portions at a time, and after-
wards throwing it out in bubbles by the same
orifice. But when they quit the water, and
become land animals with pulmonary respiration,
the nostrils are the channels through which the
air is received and expelled ; and it is here also
that the sense of smell continues to be exercised.
We know very little respecting the seat of
the sense of smell in any of the invertebrated
animals, though it is very evident that insects,
in particular, enjoy this faculty in a very high
degree. Analogy would suggest the spiracles as
the most probable seat of this sense, being the
entrances to the respiratory passages. This
office has, however, been assigned by many to
the antennae ; while other entomologists have
supposed that the palpi are the real organs of
smell.* Experiments on this subject are at-
tended with great difficulty, and their results
must generally be vague and inconclusive.
Those which Mr. P. Huber made on bees seem,
however, to establish, with tolerable certainty,
that the spiracles are insensible to strong odours,

* On the subject of this sense in insects, See Kirby and
Spence's Introduction to Entomology, vol. iv. p. 249.

SMELL. 413

such as that of oil of turpentine, which is ex-
ceedingly offensive to all insects. It was only
when a fine camel-hair pencil containing this
pungent fluid was presented near the cavity of
the mouth, above the insection of the proboscis,
that any visible effect was produced upon the
insect, which then gave decisive indications of
strong aversion. Mr. Kirby has discovered in
the anterior part of the nose of the Necrophorus
vespillo, or burying-beetle, which is an insect
remarkable for the acuteness of its smell, a pair
of circular pulpy cushions, covered with a mem-
brane, beautifully marked with fine transverse
furrows. These he considers as the organs of
smell ; and he has found similar structures in
several other insects.*

No distinct organs of smell have been disco-
vered in any of the Mollusca ;t but as there is
evidence that some of the animals belonging to
that class possess this sense, it has been con-
jectured that it resides either in the whole
mucous surface of the mantle, or in the respi-
ratory organs. Swammerdam observed, long-
ago, that snails are evidently affected by odours ;
and the cuttle-fish is said to show a decided
aversion to strongly scented plants.

* Ibid. vol. iii. 481 ; and iv, 254.

t A group of laminse, closely resembling the olfactory organs
of Fishes, has been lately observed by Mr. Owen.

414

Chapter V.

HEARING.

§ 1. Acoustic Principles.

The knowledge acquired by animals of the pre-
sence and movements of distant objects is de-
rived almost wholly from the senses of hearing
and of sight ; and the apparatus, necessary for
the exercise of these senses, being more elaborate
and refined than any of the organs we have yet
examined, exhibits still more irrefragable evi-
dence of those profound designs, and that infinite
intelligence, which have guided the construction
of every part of the animal frame.

Sound results from certain tremulous or vi-
bratory motions of the particles of an elastic
medium, such as air or water, excited by any
sudden impulse or concussion given to those
particles by the movements of the sounding
body. These sonorous vibrations are trans-
mitted with great velocity through those fluids,
till they strike upon the external ear ; and, then,
after being concentrated in the internal passages
of the organ, they are made to act on the fila-

HEARING. 415

ments of a particular nerve called the acoustic^
or auditory nerve, of which the structure is
adapted to receive these peculiar impressions,
and to communicate them to the brain, where
they produce changes, which are immediately
followed by the sensation of sound. Sound
cannot traverse a void space, as light does ; but
always requires a ponderable material vehicle
for its transmission ; and, accordingly, a bell
suspended in the vacuum of an air-pump, gives,
when struck, no audible sound, although its
parts are visibly thrown into the usual vibratory
motions. In proportion as air is admitted into
the receiver, the sound becomes more and more
distinct ; and if, on the other hand, the air be
condensed, the sound is louder than when the
bell is surrounded by air of the ordinary den-
sity.*

The impulses given by the sounding body to
the contiguous particles of the elastic medium^
are propagated in every direction, from particle
to particle ; each in its turn striking against the
next, and communicating to it the whole of
its own motion, which is destroyed by the re-
action of the particle against which it strikes.
Hence, after moving a certain definite distance,
(a distance, indeed, which is incalculably small,)

* These facts were first ascertained by Dr. Hauksbee. See
Philosophical Transactions for 1705, vol. xxiv. p. 1902,
1904.

/

416 THE SENSORIAL FUNCTIONS.

each particle returns back to its former situation,
and is again ready to receive a second impulse.
Each particle, being elastic within a certain
range,* suffers a momentary compression, and
immediately afterwards resumes its former
shape : the next particle is, in the mean time,
impelled, and undergoes the same succession of
changes ; and so on, throughout the whole series
of particles. Thus the sonorous undulations
have an analogy to waves, which spread in
circles on the surface of water, around any body,
which by its motion ruffles that surface ; only
that instead of merely extending in a horizontal
plane, as waves do, the sonorous undulations
spread out in all directions, forming, not circles
in one plane, but spherical shells ; and, what-
ever be the intensity of the sounds, the velocity
with which the undulations advance is uniform,
as long as they continue in a medium of uniform
density. This velocity in air is, on an average,
about 1100 feet in a second, or twelve and a half
miles in a minute : it is greater in dense, and
smaller in rarefied air ; being, in the same
medium, exactly proportional to the elasticity
of that medium.

* The particles of water are as elastic, within a limited dis-
tance, as those of the most solid body ; although, in consequence
of their imperfect cohesion, or rather their perfect mobility in all
directions, this property cannot be so easily recognised in masses
of fluids, as in solids.

HEARING. 417

Water is the medium of sound to aquatic
animals, as the air is to terrestrial animals.
Sounds are, indeed, conveyed more quickly, and
to greater distances, in water than in air, on ac-
count of the greater elasticity of the constituent
particles of water, within the minute distance
required for their action in propagating sound.
Stones, struck together under water, are heard
at great distances by a person whose head is
under water. Franklin found by experiment
that sound, after travelling above a mile through
water, loses but little of its intensity. According
to Chladni, the velocity of sound in water is
about 4900 feet in a second, or between four and
five times greater than it is in air.

Solid bodies, especially such as are hard and
elastic, and of uniform substance, are also ex-
cellent conductors of sound. Of this we may
easily convince ourselves by applying the ear
to the end of a log of wood, or a long iron rod,
in which situation we shall hear very distinctly
the smallest scratch made with a pin at the
other end ; a sound, which, had it passed
through the air only, would not have been heard
at all. In like manner, a poker suspended by
two strings, the ends of which are applied to the
two ears, communicates to the organ, when struck,
vibrations which would never have been heard
under ordinary circumstances. It is said that
the hunters in North America, when desirous of

VOL. II. E E

418 THE SENSORIAL FUNCTIONS.

hearing the sounds of distant footsteps, which
would be quite inaudible in any other way, apply
their ears close to the earth, and then readily
distinguish them. Ice is known to convey
sounds, even better than water; for if cannon
be fired from a distant fort, where a frozen river
intervenes, each flash of light is followed by two
distinct reports ; the first being conveyed by the
ice, and the second by the air. In like manner,
if the upper part of the wall of a high building
be struck with a hammer, a person standing close
to it on the ground, will hear two sounds after
each blow, the first descending through the wall,
and the second through the air.

As sounds are weakened by diffusion over a
larger sphere of particles, so they are capable of
having their intensity increased by concentra-
tion into a smaller space ; an effect which may
be produced by their being reflected from the
solid walls of cavities, shaped so as to bring the
undulations to unite into a focus : it is on this
principle that the ear-trumpet, for assisting per-
sons dull of hearing, is constructed ; and the
same effect sometimes takes place in echoes,
which occasionally reflect a, sound of greater
loudness than the original sound which was
directed towards them.

If the impulses given to the nerves of the ear
be repeated at equal intervals of time, provided
these intervals be very small, tlie impressions

HEARING. 419

become so blended together as not to be dis-
tinguishable from one another, and the sensation
of a uniform continued sound, or musical note,
is excited in the mind. If the intervals between
the vibrations be long, the note is grave ; if short,
that is, if the number of vibrations in a given
time be great, the note is, in the same proportion,
acute. The former is called a low, the latter a
high note ; designations which were perhaps
originally derived from the visible motions of the
throat of a person who is singing these different
notes ; for, independently of this circumstance,
the terms of high and low are quite arbitrary ;
and it is wxll known that they were applied by
the ancients in a sense exactly the reverse of
that in which we now use them.

The different degrees of tension given to the
chord or wire of a stringed musical instrument,
as well as its different lengths, determine the
frequency of its vibrations ; a greater tension, or
a shorter length, rendering them more frequent,
and consequently producing a higher note ; and
on the contrary, the note is rendered more grave
by either lessening the tension, or lengthening
the chord or wire. In a wind instrument, the
tone depends chiefly upon the length of the tube
producing the sound.

There are, therefore, two qualities in sound
recognisable by the ear, namely, loudness, or
intensity, and quality, or tone ; the former de-

420 THE SENSORIAL FUNCTIONS.

pending on the force of the vibrations ; the
latter, on their frequency. These acoustic prin-
ciples are to be borne in mind in studying the
comparative physiology of hearing ; and since
the functions of the different parts of the organ
of this sense are, as yet, but imperfectly under-
stood, I shall, in treating of this subject, deviate
from the plan I have hitherto followed, and pre-
mise an account of the structure of the ear in its
most perfectly developed state, as it appears to
be in Man.

§ 2. Physiology of Hearing in Man.

That part of the organ of hearing, which, above
all others, is essential to the performance of this
function, is the acoustic nerve, of which the
fibres are expanded, and spread over the surface
of a fine membrane, placed in a situation
adapted to receive the full impression of the
sonorous undulations which are conveyed to
them. This membrane, then, witli its nervous
filaments, may be regarded as the immediate
organ of the sense ; all the other parts consti-
tuting merely an accessory apparatus, designed
to collect and to condense the vibrations of the
surrounding medium, and to direct their concen-
trated action on the auditory membrane.

HEARING.

421

1 have endeavoured, in Fig. 390, to exhibit,
in one view, the principal parts of this compli-
cated organ, as they exist in man, in their rela-
tive situations, and of their natural size ; thereby
affording a scale by which the real dimensions
of those portions, which I shall afterwards have
occasion to explain by magnified representations,
may be properly appreciated.*

The Concha, or external ear (c), is formed of
an elastic plate of cartilage, covered by inte-
gument, and presenting various elevations and
depressions, which form a series of parabolic
curves ; apparently for the purpose of collecting
the sonorous undulations of the air, and of di-

* In this and all the following figures, the parts of the right
ear are shown, and, similar parts are always indicated by th(;
same letters.

422 THE SENSORIAL FUNCTIONS.

reeling them into a funnel-shaped canal (m),
termed the meatus auditorius, which leads to the
internal ear. This canal is composed partly of
cartilage and partly of bone ; and the integu-
ment lining it is furnished with numerous small
glands, which supply a thick oily fluid, of an
acrid quality, apparently designed to prevent the
intrusion of insects : the passage is also guarded
by hairs, which appear intended for a similar
purpose.

The meatus is closed at the bottom by a
membrane (d), which is stretched across it like
the skin of a drum, and has been termed, from
this resemblance, the membrane of the tympanum,
or the ear-drum.^ It performs, indeed, an office
corresponding to its name ; for the sonorous un-
dulations of the air, which have been collected,
and directed inwards by the grooves of the
concha, strike upon the ear-drum, and throw it
into a similar state of vibration. The ear-drum
is composed of an external membrane, derived
from the cuticle which lines the meatus ; an in-
ternal layer, which is continuous with that of
the cavity beyond it ; and a middle layer, which
consists of radiating muscular fibres, proceeding
from the circumference towards the centre, where
they are inserted into the extremity of a minute

* The inner surface of the ear-drum is shown in this figure ;
the cavity of the tympanum, which is behind it, being laid
open.

HEARING. 423

bony process (h), presently to be described.*
This muscular structure appears designed to
vary the degree of tension of the ear-drum, and
thus adapt the rate of its vibrations to those
communicated to it by the air. There is also a
slender muscle, situated internally, which by
acting on this delicate process of bone, as on a
lever, puts the whole membrane on the stretch,
and enables its radiating fibres to effect the
nicer adjustments required for tuning, as it may
be called, this part of the organ. t

Immediately behind the membrane of the ear-
drum, there is a hollow space (r), called the
cavity of the tympanum^ of an irregular shape,
scooped out of the most solid part of the tem-
poral bone, which is here of great density and
hardness. This cavity is always tilled with air ;
but it would obviously defeat the purpose of the
organ if the air were confined in this space;
because unless it were allowed occasionally to
expand or contract, it could not long remain in
equilibrium with the pressure exerted by the
atmosphere on the external surface of the ear-
drum ; a pressure which, as is well known, is
subject to great variations, indicated by the rise
and fall of the barometer. These variations

* In many quadrupeds their insertion into this process is at
some distance from the centre of the membrane. These mus-
cular fibres are delineated in Fig. 45, vol. i, p. 136.

t Home, Lectures, &c., iii, 268.

424 THE SENSORIAL FUNCTIONS.

would expose the membrane of the ear-drum to
great inequalities of pressure at its outer and
inner surfaces, and endanger its being forced,
according to the state of the weather, either out-
wards or inwards, which would completely inter-
fere with the delicacy of its vibrations. Nature
has guarded against these evils by establishing
a passage of communication between the tym-
panum and the external air, by means of a tube
(e), termed the Eustachian tube, which begins by
a small orifice from the inner side of the cavity
of the tympanum, and opens by a wide mouth at
the back of the nostrils.* This tube performs
the same office in the ear, as the hole which it
is found necessary to make in the side of a drum,
for the purpose of opening a communication
with the external air ; a communication which
is as necessary for the functions of the ear, as it
is for the proper sounding of the drum. We
find, accordingly, that a degree of deafness is
induced whenever the Eustachian tube is ob-
structed ; which may happen either from the
swelling of the membrane lining it, during a
cold, or from the accumulation of secretion in
the jDassage. It is also occasionally useful as a
channel through which sounds may gain admit-
tance to the internal ear ; and it is perhaps for

* This opening is seen at e, in Fi^. 382, p. 400, representing-
a vertical and longitudinal section of the right nostril.

HEARING.

42o

this reason tliat we instinctively open the mouth
when we are intent on hearing a very faint or
distant sound.

On the side of the cavity of the tympanum,
which is opposite to the opening of the Eu-
stachian tube, is situated the beginning of
another passage, leading into numerous cells,
contained in the mastoid process of the temporal
bone, and therefore termed the mastoid cells:
these cells are likewise filled with air. The
innermost side of the same cavity, that is the
side opposite to the ear-drum, and which is
shown in Fig. 391, is occupied by a rounded
eminence (p), of a triangular shape, termed the
promontory ; on each side of m hich there is an

opening in the bone, closed, however, by the
membrane lining the whole internal surface of
the cavity. The opening (o), which is situated
at the upper edge of the promontory, is called
the fenestra ovalis, or oval window ; and that
near the under edge (u), is the fenestra rotunda,
or round window.

Connected with the membrane of the ear-

426 TPIE SENSORIAL FUNCTIONS.

drum, at one end, and with the fenestra ovahs
at the other, there extends a chain of very
minute moveable bones, seen at b, in Fig. 390 ;
but more distinctly at m, i, s, in Fig. 393, which
is drawn on a somewhat larger scale, and in
which, as before, p is the promontory ; and r
the fenestra rotunda. These bones, which may
be called the tympanic ossicula, are four in
number, and are represented, enlarged to twice
the natural size, in Fig. 392. The names they
have received are more descriptive of their
shape than of their office. The first is the
malleus, or hammer (m) ; and its long handle
(h) is affixed to the centre of the ear-drum :
the second is the incus, or anvil (i) ; the third,
which is the smallest in the body, being about
the size of a millet seed, is the orbicular bone
(o)* ; and the last is the stapes, or stirrup (s),
the base of which is applied to the membrane
of the fenestra ovalis. These bones are regu-
larly articulated together, with all the ordinary
apparatus of joints, and are moved by small
muscles provided for that purpose. Their office
is apparently to transmit the vibrations of the
ear-drum to the membrane of the fenestra ovalis,
and probably, at the same time, to increase
their force.

* Blumenbach, and other anatomists, consider this as not
being a separate bone, but only a process of the incus ; a view
of the subject which is supported by the observations of Mr.
Shrapnell, detailed in the Medical Gazette, xii, 172.

HEARING.

427

The more internal parts of the ear compose
what is designated, from the intricacy of its wind-
ing passages, the labyrinth. It is seen at s v k

in Fig. 390, in connex-
x,^^^^ ^^s^^y iqh ^ith the tympanum ;

but in Fig. 394, it is repre-
sented, on a very large
scale, detached from every
other part, and separated
from the solid bone in
which it lies embedded.
It consists of a middle por-
tion, termed the vestibule
(v), from which, on its
upper and posterior side, proceed the three
tubes (x, Y, z), called, from their shape, the
semicircular canals; while to the lower anterior
side of the vestibule there is attached a spiral
canal, resembling in appearance the shell of a
snail, and on that account denominated the
Cochlea (k). All these bony cavities are lined
with a very delicate membrane, or periosteum,
and are filled with a transparent watery, or thin
gelatinous fluid, which is termed by Breschet
the perilymph *

Within the cavity of the osseous labyrinth now
described, are contained membranes having
nearly the shape of the vestibule and semicir-

* Annales des Sciences Naturelles, xxix, 97. It has also
been called the Aqua labyrinthi, and the fluid of Cotunnius,
from the name of the Anatomist who first distinctly described it»

428

THE SENSORIAL FUNCTIONS.

cular canals, but not extending into the cochlea.
These membranes, which compose what has
been termed, for the sake of distinction, the
memhranous labyrinth, form one continuous, but
closed sac, containing a fluid*, perfectly similar

in appearance to the perilymph, which sur-
rounds it on the outer side, and intervenes be-
tween it and the sides of the osseous labyrinth,
preventing any contact with those sides. In

* De Blainville has termed this fluid " la vitrine auditive,"
from its supposed analogy to the vitreous humour of the eye.

HEARING. 429

Fig. 395, which is on a still larger scale than
the preceding figure, the osseous labyrinth is
laid open, so as to show the parts it encloses,
and more especially the membranous labyrinth,
floating in the perilymph (p). The form of
this latter part is still more distinctly seen, in
Fig. 396, where it is represented in a position
exactly corresponding to the former figure, but
wholly detached from the bony labyrinth, and
connected only with the nervous filaments which
are proceeding to be distributed to its different
parts.

A simple inspection of these figures, in both
of which the corresponding parts are marked by
the same letters, will show at once the form and
the connexions of the three semicircular canals,
(x, Y, z), each of which present, at their origin
from the vestibule, a considerable dilatation,
termed an mnpulla (a, a, a), while, at their other
extremities, where they terminate in the vesti-
bule, there is no enlargement of their diameter ;
and it will also be seen that two of these canals
(x and v) unite into one before their termination.
The same description applies in all respects
both to the osseous and to the membranous
canals contained within them ; the space (p)
which intervenes between the two, being filled
with the perilymph. But the form of the
membranous vestibule demands more particular
notice, as it is not so exact an imitation of that

430 THE SENSORIAL FUNCTIONS.

of the osseous cavity ; being composed of two
distinct sacs, opening into each other : one of
these (u) is termed the utricle;* and the other
(s), the sacculns. Each sac contains in its in-
terior a small mass of white calcareous matter,
(o, o), resembling powdered chalk, which seems
to be suspended in the fluid contained in the
sacs by the intermedium of a number of nervous
filaments, proceeding from the acoustic nerves (g
and n), as seen in Fig. 396. From the universal
presence of these cretaceous substances in the
labyrinth of all the mammalia, and from their
much greater size and hardness in aquatic
animals, there can be little doubt that they per-
form some office of great importance in the phy-
siology of hearing.^ Their size and appearance
in the Dog are shown in Fig. 397 ; and in the
Hare, in Fig. 398.

The Cochlea, again, is an exceedingly curious
structure, being formed of the spiral convolu-
tions of a double tube ; or rather of one tube,
separated into two compartments by a partition
(l), called the lamina spiralis, which extends its
whole length, except at the very apex of the

* Scarpa and Weber term it the sinus, or alveus utriculosus ;
it is called by others the sacculus vestibuli. Breschet gives it
the name of le sinus median. See the Memoir already quoted,
p. 98.

t These cretaceous bodies are termed by Breschet otolithes,
and otoconies, according as they are of a hard or soft consistence.
Ibid. p. 99.

HEARING. 431

cone, where it suddenly terminates in a curved
point, or hook (h), leaving an aperture by which
the two portions of the tube communicate to-
gether. In Fig. 395, a bristle (b, b) is passed
through this aperture. The central pillar, round
which these tubes take two and a half circular
turns, is termed the modiolus. Its apex is seen
at M. One of these passages is distinguished
by the name of the vestibular tube* in conse-
quence of its arising from the cavity of the ves-
tibule ; and the other by that of the tympanic
tube,-\ because it begins from the inner side of
the membrane which closes the fenestra rotunda,
and forms the only separation between the
interior of that tube, and the cavity of the tym-
panum. The trunk of the auditory nerve occu-
pies a hollow space immediately behind the
ventricle ; and its branches pass through minute
holes in the bony plate which forms the wall of
that cavity ; being finally expanded on the dif-
ferent parts of the membranous labyrinth. |

* Scala vestibuli. f Scam tympani.

X In Fig. 396, the anterior trunk of the auditory nerve is seen
(at g) distributing branches of the ampullae (a, a), the utricle
(u), and the calcareous body it contains; while the posterior
trunk (n) divides into a branch, which supplies the sacculus (s)
and its calcareous body (o), and a second branch (k) which is
distributed over the cochlea, (d) is the nerve called the portio
dura, which merely accompanies the auditory nerve, but has no
relation to the sense of hearing. In Fig. 390, the auditory
nerve (n) is seen entering at the back of the vestibule.

4'i2 THE SENSORIAL FUNCTIONS.

Great uncertainty prevails with regard to the
real functions performed by the several parts of
this very complex apparatus. It is most pro-
bable, however, that the sonorous vibrations of
the air which reach the external ear, are directed
down the meatus, and striking against the ear-
drum which closes the passage, throw that mem-
brane into vibrations of the same frequency ; to
which the action of its muscles, which appear in-
tended to regulate its tension, may also contribute.
The vibrations of the ear-drum, no doubt, excite
corresponding motions in the air contained in
the cavity of the tympanum ; which, again, com-
municates them to the membrane of the fenestra
rotunda; while, on the other hand, the mem-
brane closing the fenestra ovalis receives similar
impressions from the stapes, conveyed through
the chain of tympanic ossicula, which appear to
serve as solid conductors of the same vibrations.
Thus the perilymph, or fluid contained in the
labyrinth, is affected by each external sound, both
through the medium of the air in the tympanum,
and by means of the ossicula : the undulations
thus excited produce impressions on the extre-
mities of the nervous filaments, which are spread
over the membranous labyrinth ; and these im-
pressions being conveyed to the brain, are imme-
diately followed by the sensation of sound.

With regard to the purposes which are an-
swered by the winding passages of the semi-

HEARING. 433

circular canals, and cochlea, hardly any plaus-
ible conjecture has been offered ; yet no doubt
can be entertained that the uses of all these parts
are of considerable importance, both as to deli-
cacy and correctness of hearing. There is an
obvious correspondence between the positions of
the three semicircular canals, (two of which are
vertical, and one horizontal, and of which the
planes are reciprocally perpendicular to one ano-
ther,) and the three dimensions by which the geo-
metrical relations of space are estimated ; and it
might hence be conjectured that the object of
this arrangement is to allow of the transmission
of vibrations of every kind, in whatever direction
they may arrive. It is not an improbable sup-
position that the return into the vestibule, of
undulations which have passed through these
canals, has the effect of at once putting a
stop to all further motion of the fluid, and pre-
venting the continuance of the impression which
has been already made on the nerves. The
same use may be assigned to the double spiral
convolutions of the tubes of the cochlea ; for the
undulations of the fluid in the tympanic tube,
received from the membrane of the fenestra
rotunda, will meet those proceeding along the
vestibular tube, derived from the membrane of
the fenestra ovalis, and like two opposing waves,
will tend to destroy one another. Thus each
external sound will produce but a single mo-

VOL. II, F F

434 THE SENSORIAL FUNCTIONS.

mentary impression ; the prolongation of the
undulations of the fluid of the labyrinth being
prevented by their mutual collision and neutral-
ization.*

§ 3. Comparative Physiology of Hearing,

The structure of the organs of hearing in the
lower animals presents a regular gradation from
the simple vestibule, with its membranous sac,
supplied with nervous filaments, which may be
regarded as the only essential part of this organ,
through the successive additions of semicircular
canals, fenestra ovalis, tympanic cavity, ossicula,
ear-drum, meatus auditorius, cochlea, and con-
cha, till we arrive at the combination of all
these parts in the higher orders of the Mam-
malia. The simpler forms are generally met

* The preliminary steps in the process above described are not
absolutely essential to hearing, for many instances have occurred
in which the power of hearing has been perfectly retained after
the membrane of the ear-drum, and also the ossicula had been
destroyed by disease. A small aperture in the membrane does
not interfere with its power of vibration ; but if the whole ear-
drum be destroyed, and the ossicula lost, an almost total deafness
generally ensues. After a time, however, the hearing may be in
a great measure recovered, with an undiminished power of dis-
tinguishing musical tones. See two papers by Sir Astley Cooper,
in the Phil. Trans, for 1800, p. 151; and for 1801, p. 437.

HEARING. 435

with in aquatic animals ; probably because the
sonorous undulations of water are communicated
more readily, and with greater force, than those
of air, and require no accessory apparatus for
their concentration. The Lobster ^ for instance,
has a vestibular cavity (seen at v, in Fig. 399),
containing a membranous sac, with a striated
groove (g),* and receiving the filaments of the
auditory nerve. This vestibule is protected by
the shell on all sides, except at one part, where
it is closed only by a membrane (e), which may
therefore be considered as corresponding to the
fenestra ovalis. The outer-side of this mem-

401

brane in the Astacus Jluviatilis, or cray-fish, is
seen at f in Fig. 401 ; while Fig. 402, shows
an interior view of the same membrane (f), with
the vestibule (v) laid open, and the auditory
nerve (n) passing through the shell to be dis-
tributed on the sacculus.

It appears from a variety of observations that
Insects, both in their larva and their perfect

* This groove is represented magnified in Fig. 400.

43G THE SENSORIAL FUNCTIONS.

state, possess the faculty of hearing ; but no
certain knowledge has been obtained of the
parts which exercise this sense. The prevailing
opinion among entomologists is that it resides in
some part of the antennae ; organs, which are
supposed to have a peculiar sensibility to aerial
undulations. This hypothesis is founded princi-
pally on the analogy of the Crustacea, whose
antennae contain the vestibular cavity already
described ; but on the other hand it is opposed
by the fact that Spiders, which hear very acutely,
have no antennae ; and it is also reported that
insects, when deprived of their antennae, still
retain the power of hearing.*

None of the MoUusca appear to possess, even
in the smallest degree, the sense of hearing, if
we except the highly organized Cephalopoda ;
for in them we find, at the lower part of the car-
tilaginous ring, which has been supposed to ex-
hibit the first rudiment of a cranium, a tubercle,
containing in its interior two membranous vesi-
cles, contiguous to each other, and surrounded
by a fluid. They evidently correspond to the
vestibular sacs, and contain each a small cal-
careous body, suspended from the vesicles by

* Comparetti has described structures in a great number of
insects, which he imagined were organs of hearing ; but his
observations have not been confirmed by subsequent inquifers,
and their accuracy is therefore doubtful. See De Blainville
" De rOrganisation des Animaux," i, 565.

HEARING. 437

slender nervous filaments, like the clapper of a
bell, and probably performing an office ana-
logous to that instrument ; for, being thrown into
a tremulous motion by every undulation of the
surrounding fluid, they will strike against the
membrane, and communicate similar and still
stronger impulses to the nerves by which they are
suspended, thus increasing the impression made
on those nerves. The mechanical effect of an
apparatus of this kind is shown by the simple
experiment, mentioned by Camper, of enclosing
a marble in a bladder full of water, and held in
the hand ; when the slightest shaking of the
bladder will be found instantly to communicate
motion to the marble, the reaction of which on
the bladder gives an unexpected concussion to
the hand.

The ear of Fishes contains, in addition to the
vestibule, the three semicircular canals, which
are in general greatly developed.* An enlarged
view of the membranous labyrinth of the Lophius
piscatorius is given in Fig. 403, showing the form
and complication of its parts, which are repre-
sented of twice the natural size, x, v, z, are
the semicircular canals, with their respective
ampullae (a, a, a), m is the Sinus medianus, or
principal vestibular sac, with its anterior ex-

* In the Lamprey, these canals exist only in a rudimental
state, appearing as folds of the membrane of the vestibule ; and
there are also no cretaceous bodies in the vestibular sac.

438 THE SENSORIAL FUNCTIONS.

pansion, termed the Utricle (u). The Sacculus
(s) has, in like manner, a posterior appendage
(c) termed the Cysticule. The hard calcareous
bodies (o, o, o) are three in number ; and the
branches of nerves (i, i, i), by which they are
suspended in the fluid contained in the mem-
branes, are seen passing into them ; while the
ampullae are supplied by other branches (n,n,n).

In all the osseous fishes the labyrinth is not en-
closed in the bones of the cranium, but projects
into its cavity ; but in the larger cartilaginous
fishes, as the Ray and Shark tribes, it is sur-
rounded by solid bone, and is not visible within
the cranium. In these latter fishes, we first
meet with a rudiment of the meatus, in a passage
extending from the inner side of the vestibule
to the upper and back part of the skull ; where
it is closed by a membrane, which is covered by
the skin.

HEARING. 439

Aquatic reptiles have ears constructed nearly
on the same plan as those of fishes; thus the
Triton or Newt, has a vestibule containing only
one cretaceous body, and three semicircular
canals, unprotected by any surrounding bone.
In the Frog, however, we first perceive the ad-
dition of a distinct cavity, closed by a mem-
brane, which is on a level with the integuments,
on each side of the head. From this cavity,
which corresponds to that of the tympanum,
there proceeds an Eustachian tube ; and within
it, extending from the external membrane,
which must here be regarded as an ear-drum,
to the membrane of the vestibule, or fenestra
ovalis, is found a bone, shaped like a trumpet,
and termed the Columella. This bone is seen
at c in Fig. 404, attached by its base (b) to the
fenestra ovalis of the vestibule (v), which con-

tains the cretaceous body (o). There is also a
small bone (i) attached in front to the columella.
In the Chelonia, the structure of the ear is
essentially the same as in the Frog, but tlie

440 THE SENSORIAL FUNCTIONS.

tympanum and columella are of greater length.
In the saurian reptiles the cavity of the tym-
panum is still more capacious, and the ear-drum
very distinctly marked ; and these animals pos-
sess great delicacy of hearing. The labyrinth
of the Crocodile is enclosed in bone, and con-
tains three calcareous bodies : it presents also
an appendage which has been regarded as the
earliest rudiment of a cochlea ; and there are
two folds of the skin, resembling eye-lids, at the
external orifice of the organ, which appear like
the first step towards the developement of an
external ear.

The structure of the ear in the Crocodile is
but an approximation to that which we find pre-
vailing in Birds, where the organ is of large size
compared with that of the head. The rudi-
mental cochlea, as seen at k in Fig. 405, which
represents these organs in the Turkey, is of
large size, and slightly curved. In the cavity
of the tympanum (r) is seen the columella, which
extends to the fenestra ovalis ; and beyond it, the
semicircular canals (s), the bony cells (b) which
communicate with the tympanum, the os quad-
ratum (q), the zygomatic process (z), and the
lower jaw (j). The ear-drum is now no longer
met with at the surface, but lies concealed at the
bottom of a short meatus, the orifice of which is
surrounded with feathers, arranged so as to serve
as a kind of imperfect concha, or external ear»

HEARING. 441

In Owls these feathers are a prominent and cha-
racteristic feature ; and in these birds there is,
besides, a membranous flap, acting as a valve to
guard the passage.

The chief peculiarity observable in the in-
ternal ears of Mammalia is the great develope-
ment of the cochlea, the tubes of which are con^^"
voluted, turning in a spiral, and assuming the
figure of a turbinated shell. From an extensive
comparison of the relative size of the cochlea in
difterent tribes of quadrupeds, it has been in-
ferred that it bears a tolerably constant propor-
tion to the degree of acuteness of hearing, and
that, consequently, it contributes essentially to
the perfection of that faculty : Bats, for instance,
which are known to possess exquisite delicacy
of hearing, have a cochlea of extraordinary size,
compared with the other parts of the ear. The
tympanic ossicula are completely developed
only in the Mammalia.* It is also in this class
alone that we meet with a concha, or external
ear, distinctly marked ; and the utility of this
part, in catching and collecting the sonorous
undulations of the air, may be inferred from the
circumstance, that a large and very moveable
concha is generally attended with great acute-

* These tympanic ossicula are regarded by GeofFroy St. Hilaire
as corresponding to the opercular bones of fishes, where, accord-
ing to his theory, they have attained their highest degree of
developement.

442 THE SENSORIAL FUNCTIONS.

ness of hearing. This is more particularly the
case with feeble and timid quadrupeds, as the
Hare and Rahbit^ which are ever on the watch
to catch the most distant sounds of danger, and
whose ears are turned backwards, or in the
direction of their pursuers ; while, on the con-
trary, the ears of predaceous animals are directed
forwards, that is, towards the objects of their
pursuit. This difference in direction is not
confined to the external ear, but is observable
also in the bony passage leading to the tym-
panum.

The Cetacea, being strictly inhabitants of the
water, have no external ear; and the passage
leading to the tympanum is a narrow and wind-
ing tube, formed of cartilage instead of bone,
and having a very small external aperture. In
the Dolphin tribe the orifice will barely admit
the entrance of a pin ; it is also exceedingly
small in the Dugong; these structures being
evidently intended for preventing the entrance
of any quantity of water.* It is apparently
with the same design that in the Seal the pas-
sage makes a circular turn ; and that, in the
Ornithorhynchus paradoxus, it winds round the
temporal bone, and has its external orifice at a
great distance from the vestibule. The internal

* It is probable that in these animals the principal channel
by which sounds reach the internal organ is the Eustachian
tube.

HEARING. 443

parts of the organ of hearing in the Whale and
other Cetacea, are inclosed in a bone of extra-
ordinary hardness, which, instead of forming a
continuous portion of the skull, is connected to
it only by ligaments, and suspended in a kind of
osseous cavity, formed by the adjacent bones.
The cochlea is less developed than in quad-
rupeds; for it only takes one turn and a half,
instead of two and a half. The existence of the
semicircular canals in the Cetacea was denied
by Camper ; but they have since been discovered
by Cuvier.

Several quadrupeds, which are in the habit of
burrowing, or of diving, as the Sorex fodiens, or
water-shrew, are furnished with a valve, com-
posed of a double membrane, capable of accu-
rately closing the external opening of the meatus,
and protecting it from the introduction of water,
earth, or other extraneous bodies.* In like
manner the external ear of the Hippopotamus,
which feeds at the bottom of rivers, is guarded
by an apparatus which has the effect of a valve.

We find, indeed, the same provident care dis-
played in this, as in every other department of
the animal economy : every part, however mi-
nute, of the organ of this important sense, being
expressly adapted, in every species, to the par-
ticular circumstances of their situation, and to

* Geoffioy St. Hilaire; Memoires du Museum, i. 305.

444 THE SENSORIAL FUNCTIONS.

that degree of aciiteness of perception, M^hich is
best suited to their respective wants and powers
of gratification .*

Chapter VI.

VISION.

§ 1 . Object of the Sense of Vision.

To those who study nature with a view to the
discovery of final causes, no subject can be more
interesting or instructive than the physiology of
Vision, the most refined and most admirable of
all our senses. However well we may be ac-
quainted with the construction of any particular
part of the animal frame, it is evident that we
can never form a correct estimate of the excel-
lence of its mechanism, unless we have also a
knowledge of the purposes to be answered by it,
and of the means by which those purposes can
be accomplished. Innumerable are the works of
creation, the art and contrivance of which we

* The Comparative Physiology of the Voice, a function of
which the object, in animals as well as in man, is to produce
sounds, addressed to the ear, and expressive of their ideas, feel-
ings, desires and passions, forms a natural sequel to that of
Hearing ; but Sir Charles Bell having announced his intention
of introducing it in his Treatise on the Hand, I have abstained
from entering into tliis extensive subject.

VISION. 445

are incompetent to understand, because we per-
ceive oiily the ultimate effects, and remain igno-
rant of the operations by which those effects are
produced. In attempting to investigate these
obscure functions of the animal or vegetable
economy, we might fancy ourselves engaged in
the perusal of a volume, written in some un-
known language, where we have penetrated the
meaning of a few words and sentences, sufficient
to show us that the whole is pregnant with the
deepest thought, and conveys a tale of surpassing
interest and wonder, but where we are left to
gather the sense of connecting passages by the
guidance of remote analogies or vague conjecture.
Wherever we fortunately succeed in decyphering
any continued portion of the discourse, we find
it characterized by that perfection of style, and
grandeur of conception, which at once reveal
a master's hand, and which kindle in us the
most ardent desire of supplying the wide chasms
perpetually intervening in the mysterious and
inspiring narrative. But in the subject which
now claims our attention we have been permitted
to trace, for a considerable extent, the continuity
of the design, and the lengthened series of means
employed for carrying that design into execu-
tion ; and the view which is thus unfolded of
the magnificent scheme of creation is calculated
to give us the most sublime ideas of the wisdom,

THE POWER, AND THE BENEVOLENCE OF GoD.

On none of the works of the Creator, which

446 THE SENSORIAL FUNCTIONS.

we are permitted to behold, have the characters
of intention been more deeply and legibly en-
graved than on the organ of vision, where the
relation of every part to the effect intended to
be produced is too evident to be mistaken, and
the mode in which they operate is at once
placed within the range of our comprehension.
Of all the animal structures, this is, perhaps, the
one which most admits of being brought into
close comparison with the works of human art ;
for the eye is, in truth, a refined optical instru-
ment, the perfection of which can never be fully
appreciated until we have instituted such a com-
parison ; and the most profound scientific inves-
tigations of the anatomy and physiology of the
eye concur in showing that the whole of its
structure is most accurately and skilfully adapted
to the physical laws of light, and that all its parts
are finished with that mathematical exactness
which the precision of the effect requires, and
which no human effort can ever hope to ap-
proach,— far less to attain.

To the prosecution of this inquiry we are
farther invited by the consciousness of the in-
calculable advantages we derive from the sense
of sight, the choicest and most enchanting of our
corporeal endowments. The value of this sense
must, indeed, appear inestimable, when we con-
sider of how large a portion of our sensitive
and intellectual existence it is the intermediate

VISION. 447

source. Not only has it given us extensive com-
mand over the objects which surround us, and
enabled us to traverse and explore the most dis-
tant regions of the globe, but it has introduced
us to the knowledge of the bodies which compose
the solar system, and of the countless hosts of
stars which are scattered through the firmament ;
thus expanding our views to the remotest con-
fines of creation. As the perceptions supplied
by this sense are at once the quickest, the most
extensive, and the most varied, so they become
the fittest vehicles for the introduction of other
ideas. Visual impressions are those which, in
infancy, furnish the principal means of deve-
loping the powers of the understanding : it is to
this class of perceptions that the philosopher
resorts for the most apt and perspicuous illustra-
tions of his reasonings ; and it is also from the
same inexhaustible fountain that the poet draws
his most pleasing and graceful, as well as his
sublimest imagery.

The sense of Vision is intended to convey to
its possessor a knowledge of the presence, situa-
tion, and colour of external and distant objects,
by means of the light which those objects are
continually sending off, either spontaneously, or
by reflection from other bodies. It would ap-
pear that there is only one part of the nervous
system so peculiarly organized as to be capable

448 THE SENSORIAL FUNCTIONS.

of being affected by luminous rays, and convey-
ing to the mind the sensation of light ; and this
part is the Retina, so named from the thin and
delicate membranous net-work, on which the
pulpy extremities of the optic nerves, establish-
ing an immediate communication between that
part and the brain, are expanded.

If the eye were so constructed as to allow the
rays of light, which reach it from surrounding
objects, simply to impinge on the retina as they
are received, the only perception which they
could excite in the mind, would be a general
sensation of light, proportionate to the total
quantity which is sent to the organ from the
whole of the opposite hemisphere. This, how-
ever, does not properly constitute Vision ; for in
order that the presence of a particular object in
its real direction and position with respect to us,
may be recognised, it is necessary that the light,
which comes from it, and that light alone, should
produce its impression exclusively on some parti-
cular part of the retina ; it being evident that if
the light, coming from any other object, were
allowed to act, together with the former, on the
same part, the two actions would interfere with
one another, and only a confused impression
would result. The objects in a room, for ex-
ample, are all throwing light on a sheet of paper
laid on the floor; but this light, being spread
equally over every part of the surface of the

VISION. 449

paper, furnishes no means of distinguishing the
sources from which each portion of the light has
proceeded ; or, in other words, of recognising
the respective figures, situations, and colours of
the objects themselves. We shall now^ proceed
to consider the modifications to be introduced
into the structure of the organ, in order to attain
these objects.

§ 2. Modes of accomplishing the Objects of Vision.

Let us suppose that it were proposed to us as
a problem to invent an apparatus, by which,
availing ourselves of the known properties of
light, we might procure the concentration of
all the rays, proceeding from the respective
points of the object to be viewed, on separate
points of the retina, and obtain likewise the ex-
clusion of all other rays ; and also to contrive
that the points of the retina, so illuminated,
should have the same relative situations among
one another, which the corresponding points of
the surrounding objects have in nature. In
other words, let us suppose ourselves called upon
to devise a method of forming on the retina a
faithful delineation, in miniature, of the external
scene.

As it is a fundamental law in optics that the

VOL. II. G G

450

THE SENSORIAL FUNCTIONS.

rays of light, while they are transmitted through
the same medium, proceed in straight lines, the
simplest mode of accomplishing the proposed end
would be to admit into the eye, and convey to
each particular point of the retina, only a single
ray proceeding directly from that part of the
object which is to be depicted on it, and to ex*
elude all other rays. For carrying this desiga
into effect we have the choice of two methods,
both of which we find resorted to by nature
under different circumstances.

The first method consists in providing for
each of these single rays a separate tube, with
darkened sides, allowing the ray which traverses
it, and no other, to fall on its respective point of
the retina, which is to be applied at the opposite
end of the tube. The most convenient form to
be given to the surface of the retina, which is to

be spread out to receive the
rays from all these tubes,
appears to be that of a con-
vex hemisphere ; and the
most eligible distribution of
the tubes is the placing them
so as to constitute diverging
radii, perpendicular, in
every part, to the surface of
the retina. This arrange-
ment will be understood by
reference to Fig. 406, which represents a section

406

VISION.

451

of the whole organ ; t, t, being the tubes dis-
posed in radii every where perpendicular to the
convex hemispherical surface of the retina (r).
Thus will an image be formed, composed of the
direct rays from each respective point of the
objects, to which the tubes are directed ; and
these points of the image will have, among them-
selves, the same relative situation as the external
objects, from which they originally proceeded,
and which they will accordingly faithfully re-
present.

The second method, which is nearly the in-
verse of the first, consists in admitting the rays
through a small aperture into a cavity, on the
opposite and internal side of which the retina is
expanded, forming a concave, instead of a convex

m

Ml WIIIIII^^^

^^*-..^

1

1

'1

1 1

jBlll^^^^fe

^^

hemispherical surface. The mode in which this
arrangement is calculated to answer the intended
purpose will be easily understood by conceiv-
ing a chamber (as represented in Fig. 407), into

452 THE SENSORIAL FUNCTIONS.

which no light is allowed to enter, except what
is admitted through a small hole in a shutter,
so as to fall on the opposite side of the room.
It is evident that each ray will, in that case,
illuminate a different part of the wall ; and that
the whole external scene will be there faith-
fully represented ; for the several illuminated
points, which constitute these images, preserve
among themselves the same relative situation
which the objects they represent do in nature;
although with reference to the actual objects
they have an inverted position. This inversion
of the image is a necessary consequence of the
crossing of all the rays at the same point ;
namely, the small aperture in the shutter,
through which they are admitted.

One inconvenience attending the limiting of
the illumination of each point of the wall to that
of a single ray, in the mode last pointed out, is
that the image produced must necessarily be
very faint. If, with a view of remedying this
defect, the aperture were enlarged, the image
would, indeed, become brighter, but would at
the same time, be rendered more indistinct,
from the intermixture and mutual interference
of adjacent rays; for all the lines would be
spread out, the outlines shaded off, and the whole
picture confused.

The only mode by w^hich distinctness of image
can be obtained with increased illumination, is

VISION. 453

to collect into one point a great number of rays
proceeding from the corresponding point of the
object to be represented. Such a collection of
rays proceeding from any point, is termed, in
the language of optics, a pencil of rays ; and the
point into which they are collected is called a
focus. For the purpose of collecting a pencil of
rays into a focus, it is evident that all of them,
except the one which proceeds in a straight line
from the object to that focus, must be deflected,
or bent from their rectilineal course. This effect
may be produced by lefraction, which takes
place according to another optical law ; a law
of wl ich the following is the exposition.

It is only when the medium which the rays
are traversing is of uniform density that their
course is constantly rectilineal. If the density
change, or if the rays pass obliquely from one
medium into another of a different density, they
are refracted ; each ray being deflected towards
a line situated in the medium of greatest density,
and drawn from the point where the ray meets
the new medium, perpendicular to the refracting
surface. Thus the ray r. Fig. 408, striking ob-
liquely on the surface of a denser medium, at the
point s, instead of pursuing its original course
along the line s o, is refracted, or turned in the
direction s t, which is a line situated between s o,
and s p ; this latter line being drawn perpen-
dicularly to the surface of the medium, at the

454 THE SENSORIAL FUNCTIONS.

point s, and within that medium. When the
ray arrives at t, and meets the posterior sur-

408

face of the dense medium, passing thence into
one that is less dense, it is again refracted
according to the same law ; that is, it inclines
towards the perpendicular line t i, drawn from
T, within the denser medium, and describes the
new course t u instead of t v. The amount
of the deflection corresponds to the degree of
obliquity of the ray to the surface which re-
fracts it ; and is mathematically expressed, by
the law, that the sines of the two angles formed
with the perpendicular by the incident and the
refracted rays retain, amidst all the variations of
those angles, the same constant proportion to one
another. We may hence derive a simple rule
for placing the plane of the refracting surface so
as to produce the particular refraction we wish
to obtain. When a ray is to be deflected from
its original course to a particular side, we have
only to turn the surface of the medium in such
a manner as that the perpendicular line to that

VISION. 455

surface, contained within the denser medium,
shall lie still farther on the same side. Thus, in
Fig. 408, if we wish to turn the ray r s, from
s o to s T, we must place the dense medium so
that the perpendicular s p, which is within it,
shall be still farther from s o, than s t is ; that
is, shall lie on the other side of s t. The same
rule applies to the contrary refraction of the ray
s T from T V to T u, when it passes out of a dense,
into a rare medium ; for the perpendicular t i
must still be placed on the same side of t v as
T u is situated.

Let us now apply these principles to the case
before us ; that is, to the determination of the
form to be given to a dense medium, in order to
collect a pencil of rays, proceeding from a distant

409

object, accurately to a focus. We shall suppose
the object in question to be very remote, so that
the rays composing the pencil may be consi-
dered as being parallel to each other; for at
great distances their actual deviation from
etrict parallelism is wholly insensible ; and let

456 THE SENSORIAL, FUNCTIONS.

A, B, c, D, E, (Fig. 409), represent these rays.
There must evidently be one of these rays (c),
and only one, which, by continuing its rectilineal
course, would arrive at the point (r) intended to
be the focus of the rays. This ray, then, may
be suffered to pass on, without being subjected
to any refraction ; the surface of the medium
should, therefore, be presented to the ray (at i)
perpendicularly to its course, so that it may pass
through at right angles to that surface. Those rays
(b and d) which are situated very near to this
direct, or central ray (c), will require but a small
degree of refraction in order to reach the focus (r) :
this small refraction will be effected by a slight
degree of obliquity in the medium at the points
(h and k) where those rays meet it. In propor-
tion as the rays (such as those at a and e) are
more distant from the central ray, a greater
amount of refraction, and consequently a greater
obliquity of the surfaces (g and l) will be re-
quired, in order to bring them to the same focus.
The convergence of these rays, after they have
passed this first surface, which would have
brought them to the point r, may be farther in-
creased by interposing new surfaces of other
media at the proper angles. If the new medium
be still denser than the last, the inclination of its
surface must be similar to that already described ;
if rarer, they must be in an opposite direction.
Thi« last case, also, is illustrated in the figure,

VISION.

457

where m, n, o, p, q, show the inclinations of the
surfaces of a rarer medium, calculated to in-
crease the convergence of the rays ; that is, to
bring them to a nearer focus (f). The result
of the continued change of direction in the
refracting surface, is a regular curvilineal sur-
face, which, in the present case, approaches very
nearly to that of a sphere. Hence by giving
thete refractive media spherical surfaces, we
adapt them, with tolerable exactness, to produce
the convergence of parallel rays to a focus, and
by making the denser medium convex on both
sides (as shown in Fig. 410), both surfaces will

conspire in producing the desired effects. Such
an instrument is termed a double convex lens;
and it has the property of collecting into a focus
rays proceeding from distant points.*

Having obtained this instrument, we may now

* The refraction by spherical surfaces does not, strictly speak-
in^-, unite a pencil of parallel or divergent rays into a mathe-
matical point, or focus; for in reality the rays which are near
the central line are made to converge to a point a little more

458

THE SENSORIAL FUNCTIONS.

venture to enlarge the aperture through which
the light was admitted into our dark chamber,
and fit into the aperture a double convex lens.
We have thus constructed the well-known op-
tical instrument called the Camera Ohscura, in
which the images of external objects are formed
upon a white surface of paper, or a semi-trans-
parent plate of glass ; and these images must
evidently be in an inverted position with re-
spect to the actual objects which they re-
present.

Such is precisely the construction of the eye,
which is, to all intents, a camera obscura : for
in both these instruments, the objects, the prin-

distant than that to which the remoter rays converge : an effect
which I have endeavoured to illustrate by the diagram Fig. 411 ;
where, in order to render it obvious to the eye, the disparity
is exaggerated : for on ordinary occasions, where great nicety is

411

not required, this difference in the degree of convergence be-
tween the central rays and those near the circumference of the
lens, giving rise to what is termed the Aberration of Sphericity ,
is too small to attract notice.

VISION.

459

ciples of construction, and the mode of operation
are exactly the same ; and the only difference
is, that the former is an infinitely more perfect
instrument than the latter can ever be rendered
by the utmost efforts of human art.

With a view of simplifying the subject, I have assumed, in
the account given in the text, that the rays which arrive at the
eye are parallel, which in mathematical strictness they never
are. The focus of the rays refracted by a convex lens is more

412

413

414

remote in proportion as the rays are more divergent ; or, in other
words, proceed from nearer objects. This is illustrated by
Figures 412, 413, and 414; to which I shall again have occa-
sion to refer in the sequel.

460 THE SENSORIAL FUNCTIONS.

^ 3. Structure of the Eye.

One of the many points of superiority which the
eye possesses over the ordinary camera obscura
is derived from its spherical shape, adapting the
retina to receive every portion of the images
produced by refraction, which are themselves
curved ; whereas had they been received on a
plane surface, as they usually are in a camera
obscura, a considerable portion of the image
would have been indistinct. This spherical form
is preserved by means of the firm membranes
which protect the eye, and which are termed
its Coats; and the transparent media which
they enclose, and which effect the convergence
of the rays, are termed the Humours of the Eye.
There are in this organ three principal coats,
and three humours ; composing altogether what
is called the Globe of the Eye. Fig. 415, which
gives an enlarged view of a horizontal section
of the right eye, exhibits distinctly all these
parts.

The outermost coat (s), which is termed the
Sclerotica, is exceedingly firm and dense, and
gives to the globe of the eye the mechanical sup-

VISION.

401

port it requires for the performance of its deli-
cate functions. It is perforated behind by the
optic nerve (o), which passes onwards to be ex-
panded into the retina (r). The sclerotica does
not extend farther than about four-fifths of the
globe of the eye; its place in front being sup-
plied by a transparent convex membrane (c).

called the Cornea, which is more prominent than
the rest of the eye-ball. A line passing through
the centre of the cornea and the centre of the
globe of the eye is called the axis of the eye.
The Sclerotica is lined internally by the Choroid
coat (x), which is chiefly made up of a tissue of

462 THE SENSORIAL FUNCTIONS.

blood vessels, for supplying nourishment to the
eye. It has on its inner surface a layer of a dark
coloured viscid secretion, known by the name of
the Pigmentum nigrum, or black pigment. Its
use is to absorb all the light which may happen
to be irregularly scattered through the eye, in
consequence of reflection from different quarters;
and it serves, therefore, the same purpose as the
black paint, with which the inside of optical in-
struments, such as telescopes, microscopes, and
camerae obscurae, is darkened. Within the pig-
mentum nigrum, and almost in immediate con-
tact with it*, the Retina (r) is expanded ; form-
ing an exceedingly thin and delicate layer of
nervous matter, supported by a fine membrane.
More than three-fourths of the globe of the
eye are filled with the vitreous humour (v), which
has the appearance of a pellucid and elastic
jelly, contained in an exceedingly delicate tex-
ture of cellular substance. The Crystalline
humour (l), which has the shape of a double
convex lens, is formed of a denser material than
any of the other humours, and occupies the fore-
part of the globe of the eye, immediately in front
of the vitreous humour, which is there hollowed
to receive it. The space which intervenes be-

* Between the pigmentum and the retina there is found a very
fine membrane, discovered by Dr. Jacobson : its use has not
been ascertained.

VISION. 463

tween the lens and the cornea is filled with a
watery secretion (a), called the Aqueous humour.
This space is divided into an anterior and a pos-
terior chamber by a flat circular partition (i),
termed the Iris.

The iris has a central perforation (p), called
the Pupil^ and it is fixed to the edge of the cho-
roid coat, by a white elastic ring (q), called the
Ciliary Ligament. The posterior surface of the
iris is called the Uvea, and is lined with a dark
brown pigment. The structure of the iris is very
peculiar, being composed of two layers of con-
tractile fibres ; the one, forming concentric cir-
cles ; the other, disposed like radii between the
outer and inner margin.* When the former act,
the pupil is contracted ; when the latter act, the
breadth of the iris is diminished, and the pupil
is, of course, dilated. By varying the size of the
pupil the quantity of light admitted into the
interior of the eye is regulated, and accommo-
dated to the sensibility of the retina. When the
intensity of the light would be injurious to that
highly delicate organ, the pupil is instantly con-
tracted, so as to exclude the greater portion ;
and, on the contrary, when the light is too
feeble, it is dilated, in order to admit as large a
quantity as possible. The iris also serves to in-

* See Fig. 47. vol. i, p. 136.

464 THE SENSORIAL FUNCTIONS.

tercept such rays as would have fallen on parts
of the crystalline lens less fitted to produce their
regular refraction, the object of which will be
better understood when we have examined the
functions of this latter part. But, before engag-
ing in this inquiry, it will be proper to complete
this sketch of the anatomy of the eye by
describing the principal parts of the apparatus
belonging to that organ, which are exterior to
the eye-ball, and may be considered as its ap-
pendages.

The purposes answered by the parts exterior
to the eye-ball are chiefly those of motion, of
lubrication, and of protection.

As it is the central part of the retina which is
endowed with the greatest share of sensibility,
it is necessary that the images of the objects to
be viewed should be made to fall on this part ;
and consequently that the eye should be capa-
ble of having its axis instantly directed to those
objects, wherever they maybe situated. Hence
muscles are provided within the orbits, for effect-
ing the motions of the
eye-ball. A view of these
muscles, with their attach-
ments to the ball of the
eye, but separated from
the other parts, is given
in Fig. 416. Four of these
proceed in a straight course from the bottom of

VISION. 463

the orbit, arising from the margin of the aperture
through which the optic nerve passes, and being
inserted by a broad tendinous expansion into the
fore-part of the sclerotic coat. Three of these
are marked a, b, and c in the figure ; and the
edge of the fourth is seen behind and above b.
These, straight muscles, as they are called, sur-
round the optic nerve and the eye-ball, forming
four longitudinal bands ; one (a) being situated
above, for the purpose of turning the eye up^
wards ; a second (c), situated below, for turning
it downwards ; and the two others, on either side,
for performing its lateral motions to the right or
left. The cavity of the orbits being considerably
larger than the eye-ball, the intervening space,
especially at the back part, is filled up by
fat, which serves as a soft cushion for its pro-
tection, and for enabling it to roll freely in all
directions.

Besides these straight muscles, there are also
two others (s and i), termed the oblique muscles,
which give the eye-ball a certain degree of rota-
tion on its axis. When these act in conjunction,
they draw the eye forwards, and serve as anta-
gonists to the combined power of the straight
muscles. The upper oblique muscle (s) is re-
markable for the artificial manner in which its
tendon passes through a cartilaginous pulley (p)
in the margin of the orbit, and then turns back
again to be inserted into the eye-ball ; so that the

VOL. II. H H

466 THE SENSORIAL FUNCTIONS.

effect produced by the action of the muscle is a
motion in a direction exactly the reverse of that
in which its fibres contract. This mechanism,
simple as it is, affords one of the most palpable
instances that can be adduced of express contri^
vance ; for in no other situation could the muscle
have been so conveniently lodged as within the
eye-ball ; and in no other way could its tendon
have been made to pull in a direction contrary to
that of the muscle, than by the interposition of a
pulley, turning the tendon completely round.

The fore-part of the globe of the eye, which is
of a white colour, is connected with the sur-
rounding integuments by a membrane, termed
the Conjunctiva* This membrane, on arriving
at the base of the eye-lids, is folded forwards so
as to line their inner surfaces, and to be con-
tinuous with the skin which covers their outer
sides. The surfaces of the conjunctiva and of
the cornea are kept constantly moist by the
tears, which are as constantly secreted by the,
JLacrymal glands. Each gland, (as shown at l,
Fig. 417,) is situated above the eye, in a hol-

* An abundant supply of nerves has been bestowed on this
membrane for the purpose of conferring upon it that exquisite
degree of sensibility which was necessary to give immediate warn-
ing of the slightest danger to so important an organ as the eye
from the intrusion of foreign bodies. That this is the intentior^
is apparent from the fact that the internal parts of the eye possess
but little sensibility compared with the external surface.

VISION. 467

low of the orbit ; and the ducts (d) proceeding
from it open upon the inner side of the upper
eye-lid (e). This fluid, the uses of which are
obviously to wash away dust, or other irritating
substances which may happen to get introduced,
is distributed over the outer surface of the eye
by means of the eye-lids. Each lid is sup-

ported by an elastic plate of cartilage, shaped
like a crescent, and covered by integuments.
An orbicular muscle, the fibres of which run in
a circular direction, immediately underneath the
skin, all round the eye,* is provided for closing
them. The upper eye-lid is raised by a separate
muscle, contained within the orbit, immediately
above the upper straight muscle of the eye-ball.

* See Fig. 46, vol. i. p. 136.

468 THE SENSORIAL FUNCTIONS.

The eye-lashes are curved in opposite directions,
so as not to interfere with each other when the
eye-lids are closed. Their utility in guarding
the eye against the entrance of various sub-
stances, such as hairs, dust, or perspiration, and
also in shading the eye from too strong im-
pressions of light, is sufficiently apparent. The
eye-lids, in closing, meet first at the outer
corner of the eye ; and their junction proceeds
along the line of their edges, towards the inner
angles, till the contact is complete: by this
means the tears are carried onwards in that
direction, and accumulated at the inner corner
of the eye ; an effect which is promoted by the
bevelling of the margins of the eye-lids, which,
when they meet, form a channel for the fluid to
pass in that manner. When they arrive at the
inner corner of the eye, the tears are conveyed
away by two slender ducts, the orifices of which,
called the puncta lacrymalia (p, p), are seen at
the inner corner of each eye-lid, and are sepa-
rated by a round projecting body (c), connected
with a fold of the conjunctiva, and termed the
lacrymal caruncle. The two ducts soon unite to
form one passage, which . opens into a sac (s),
situated at the upper part of the sides of the
nose, and terminating below (at n) in the cavity
of the nostrils, into which the tears are ulti-
mately conducted. When the secretion of the
tears is too abundant to be carried off by this

VISION. 469

channel, they overflow upon the cheeks; but
when the quantity is not excessive, the ten-
dency to flow over the eye-lid is checked by an
oily secretion proceeding from a row of minute
glands, situated at the edge of the eye-lids, and
termed the Meibomian glands.

The eye-brows are a further protection to the
eyes ; the direction of the hairs being such as
to turn away from them any drops of rain or
of perspiration, which may chance to fall from
above.

Excepting in front, where the eyes are covered
and protected by the eye-lids, these important
organs are on all sides effectually guarded from
injury by being contained in a hollow bony
socket, termed the orbit, and composed of seven
portions of bone. These seven elements may be
recognised in the skulls of all the mammalia,
and perhaps also in those of all other verte-
brated animals ; affording a remarkable illustra-
tion of the unity of the plans of nature in the
construction of the animal fabric.

§ 4. Physiology of perfect Vision.

The rays of light, proceeding from a distant
object, strike upon the convex surface of the
cornea, which being of greater density than the

470 THE SENSORIAL FUNCTIONS.

air, refracts them, and makes them converge to-
wards a distant focus. This effect, however, is
in part counteracted on their emergence from
the concave posterior surface of the cornea,
when the rays enter into the aqueous humour.
On the whole, however, they are refracted, and
made to converge to a degree equal to that
which they would have undergone if they had
at once impinged against the convex surface of
the aqueous humour, supposing the cornea not
to have been interposed.

A considerable portion of the light which has
thus entered the aqueous humour is arrested in
its course by the iris ; so that it is only those
rays which are admitted through the pupil that
are subservient to vision. These next arrive at
the crystalline lens, where they undergo two re-
fractions ; the one at the anterior, the other at
the posterior surface of that body. Both these
surfaces being convex outwardly, and the lens
being a denser substance than either the aque-
ous or the vitreous humours, the effect of both
these refractions is to increase the convergence
of the rays, and to bring them to unite in a focus
on the retina at the bottom of the eye. The
most considerable of these refractions is the
first ; because the difference of density between
the air and the cornea, or rather the aqueous
humour, is greater than that of any of the hu-
mours of the eye compared with one another.

VISION. 471

The accurate convergence of all the rays of
light, which enter through the pupil, to their
respective foci on the retina, is necessary for the
perfection of the images there formed ; but for
the complete attainment of this end various nice
adjustments are still requisite.

In the first place, the Aberration of Sphericity *
which is a consequence of the geometrical law
of refraction, introduces a degree of confusion in
the image ; which is scarcely perceptible, indeed,
on a small scale, but which becomes sensible in
instruments of much power ; being one of the
greatest difficulties which the optician has to
overcome in the construction of the telescope and
the microscope. Nature, in framing the human
eye, has solved this difficulty by the simplest,
yet most effectual means, and in a manner quite
inimitable by human art. She has in the first
place given to the surfaces of the crystalline
lens, instead of the spherical form, curvatures
more or less hyperbolical or elliptical ; and has,
in the next place, constructed the lens of an
infinite number of concentric layers, which in-
crease in their density, as they succeed one ano-
ther from the surface to the centre. The refract-
ing power, being proportional to the density, is
thus greatest at the centre, and diminishes as it
recedes from that centre. This admirable ad-

* See Fig. 411, and the note referring to it, p. 457.

472 THE SENSORIAL FUNCTIONS.

justment exactly corrects the deficiency of re-
fraction, which always takes place in the central
portions of a lens composed of a material of
uniform density, as compared with the refraction
of the parts more remote from the centre.*

The second adjustment for perfect vision has
reference to the variations in the distance of
the focus which take place according as the
rays arrive at the eye from objects at different
distances, and which may be called the Aberra-
tions of Parallax. When the distance of the
object is very great, the rays proceeding from
each point arrive at the eye with so little
divergence, that each pencil may be considered
as composed of rays which are parallel to each
other ; the actual deviation from parallelism
being quite insensible. But if the same object
be brought nearer to the eye, the divergence of
the rays becomes more perceptible ; and the
effect of the same degree of refraction is to
collect them into a focus more remote than
before. t For every distance of the object there

* Sir David Brewster has ascertained that the variations of
density producing the doubly refracting structure, in the crys-
talline lens of fishes, are related, not to the centre of the lens,
but to the diameter which forms the axis of vision ; an arrange-
ment peculiarly adapted for correcting the spherical aberrations.
Philos. Trans, for 1816, p. 317.

t This is illustrated by Fig. 412, 413, and 414; the first of
which shows the rapid convergence of rays proceeding from a
very distant object, and which may be considered as parallel.

VISION. 473

is a corresponding focal distance ; and when the
eye is in a state adapted for distinct vision at
one distance, it will have confused images of
objects at another distance ; because the exact
foci of the rays will be situated either before or
behind the retina. It is evident that if the
retina be not placed exactly at the point where
the focus is situated, it will either intercept the
pencil of rays before they are united into a point,
or receive them after they have crossed one
another in passing through the focus ; in either
of which cases, each pencil will throw upon the
retina a small circle of light, brighter at the
middle and fainter at the edges, which will mix
itself with the adjacent pencils, and create con-
fusion in the image.

It is found, however, that the eye has a power
of accommodating itself to the distinct vision of
objects at a great variety of distances, according
as the attention of the mind is directed to the
particular object to be viewed. The mode in
which this change in the state of the eye is
effected has been the subject of much contro-
versy. The increase of the refracting power of
the eye necessary to adapt it to the vision of near
objects is evidently the result of a muscular
effort, of which we are distinctly conscious when

The second shows that divergent rays unite at a more distant
focus ; and the third, that the focus is more distant the greater
the divergence.

474 THE SENSORIAL FUNCTIONS.

we accurately attend to the accompanying sen-
sations. The researches of Dr. Young have ren-
dered it probable that some change takes place
in the figure of the lens, whereby its convexity,
and perhaps also its distance from the retina, are
increased. He has shown by a very decisive
experiment, that any change which may take
place in the convexity of the cornea has but
little share in the production of the effect ; for
the eye retains its power of adaptation when im-
mersed in water, in which the form of the cornea
can in no respect influence the refraction.

But the rays of light are of different kinds ;
some exciting the sensation of red, others of
yellow, and others again of blue ; and these
different species of light are refracted, under
similar circumstances, in different degrees.
Hence the more refrangible rays, that is the
violet and the blue, are brought to a nearer
focus, than those which are less refrangible, that
is the orange and the red rays ; and this want
of coincidence in the points of convergence of
these different rays, (all of which enter into the
composition of white light), necessarily impairs
the distinctness of all the images produced by
refraction ; shading off* their outlines with various
colours, even when the object itself is colourless.
This defect, which is incident to the power of a
simple lens, and which is termed the Chromatic
Aberration, is remedied almost perfectly in the

VISION. 475

eye, by the nice adjustment of the powers of the
different refracting media, which the rays of
Hght have to traverse before they arrive at the
retina, producing what is called an achromatic
combination ; * and it is found that the eye,
though not an absolutely achromatic instrument,
as was asserted by Euler,t is yet sufficiently so
for all the ordinary practical purposes of life.

The object, then, of the whole apparatus ap-
pended to the optic nerve, is to form inverted
images of external objects on the retina, which,
as we have seen, is the expanded extremity of
that nerve. That this effect is actually pro-
duced, may be easily shown by direct obser-
vation ; for if the sclerotic and choroid coats be
carefully dissected off from the posterior part of
the eye of an ox, or any other large quadruped,
leaving only the retina, and the eye so prepared
be placed in a hole in a window-shutter, in a
darkened room, with the cornea on the outside,
all the illuminated objects of the external scene
will be beautifully depicted, in an inverted posi-
tion, on the retina.

Few spectacles are more calculated to raise
our admiration than this delicate picture, which

* For the exposition of the principles on which these achro-
matic combinations of lenses correct this source of aberration, I
must refer to works which treat professedly on Optics,

t For the rectification of this error we are indebted to Dr.
Young.

476 THE SENSORIAL FUNCTIONS.

nature has, with such exquisite art, and with the
finest touches of her pencil, spread over the
smooth canvass of this subtle nerve ; a picture,
which, though scarcely occupying a space of
half an inch in diameter, contains the deline-
ation of a boundless scene of earth and sky, full
of all kinds of objects, some at rest, and others
in motion, yet all accurately represented as to
their forms, colours and positions, and followed
in all their changes, without the least inter-
ference, irregularity, or confusion. Every one
of those countless and stupendous orbs of fire,
whose light, after traversing immeasurable re-
gions of space, at length reaches our eye, is col-
lected on its narrow curtain into a luminous focus
of inconceivable minuteness ; and yet this al-
most infinitesimal point shall be sufficient to
convey to the mind, through the medium of the
optic nerve and brain, a knowledge of the exist-
ence and position of the far distant luminary,
from which that light has emanated. How infi-
nitely surpassing all the limits of our conception
must be the intelligence, and the power of that
Being, who planned and executed an instrument
comprising, within such limited dimensions, such
vast powers as the eye, of which the perceptions
comprehend alike the nearest and most distant
objects, and take cognizance at once of the most
minute portions of matter, and of bodies of the
largest magnitude !

VISION. 477

§ 5. Comparative Physiology of Vision.

In the formation of every part of the animal
machinery we may generally discern the predo-
minance of the law of gradation ; but this law
is more especially observed in those organs
which exhibit, in their most perfect state, the
greatest complication and refinement of struc-
ture ; for on following all their varieties in the
ascending series, we always find them advancing
by slow gradations of improvement, before they
attain their highest degree of excellence. Thus
the organ of vision presents, amidst an infinite
variety of constructions, successive degrees of
refinement, accompanied by corresponding ex-
tensions of power. So gradual is the progress of
this developement, that it is not easy to determine
the point where the faculty of vision, properly so
called, begins to be exercised, or where the first
rudiment of its organ begins to appear.

Indications of a certain degree of sensibility to
light are afforded by many of the lower tribes of
Zoophytes, while no visible organ appropriated
to receive its impressions can be traced. This is
the case with many microscopic animalcules;
and still more remarkably with the Hydra, and
the Actinia, which show by their movements that

478 THE SENSORIAL FUNCTIONS.

they feel the influence of this agent ; for, when
confined in a vessel, they always place them-
selves, by preference, on the side where there is
the strongest light.* The Veretillum cynomorium,
on the other hand, seeks the darkest places, and
contracts itself the moment it is exposed to
light. t In a perfectly calm sea, the Medus(B
which are rising towards the surface, are seen to
change their course, and to descend again, as
soon as they reach those parts of the water which
receive the full influence of the sun's rays, and
before any part of their bodies has come into
contact with the atmosphere. J But, in all these
instances a doubt may arise whether the ob-
served actions may not be prompted by the mere
sensation of warmth excited by the calorific rays
which accompany those of light ; in which case
they would be evidence only of the operation of
a finer kind of touch.

The first unequivocal appearance of visual
organs is met with in the class of Annelida;
although the researches of Ehrenberg would
induce us to believe that they may be traced
among animals yet lower in the scale ; for
he has noticed them in several of the more
highly organized Infusoria, belonging to the

* Such is the uniform report of Trembley, Baker, Bonnet,
Goeze, Hanow, Roesel, and SchsefFer.

t Rapp ; Nov. Act. Acad. Nat. Cur. of Bonn, xiv, 645.
X Grant ; Edin. Journal of Science : No. 20.

VISION. 479

order Rotifera, and particularly in the Hydatina
senta, where he has found the small black points,
observable in other species, united into a single
spot of larger size. Nitsch, also, states that the
Cercaria viridis possesses three organs of this
kind. Pla?iaricB present two or three spots,
which have been regarded as visual organs ; and
these have been found by Baer to be composed,
in the Planaria torvtty of clusters of black grains,
situated underneath the white or transparent in-
tegument.* The eyes of the Nais prohoscidea are
composed, according to Gruithuisen, simply of a
small mass of black pigment, attached to the
extremity of the optic nervet ; and organs ap-
parently similar to these are met with in many
of the inferior tribes of Annelida. In all these
cases it is a matter of considerable doubt whe-
ther the visual organs are constructed with any
other intention than merely to convey general
sensations of light, without exciting distinct per-
ceptions of the objects themselves from which
the light proceeds ; this latter purpose requiring,
as we have seen, a special optical apparatus of
some degree of complexity. An approach to
the formation of a crystalline lens takes place in

^ * Nov. Act. Acad. Nat. Cur. of Bonn. xiii. 712. See also
the Memoir of Dug^s, entitled " Recherches sur I'Organisation
et les Moeurs des Planaires," in the Annales des Sc. Nat. xv.
148.

t Nov. Act. Acad. Nat. Cur. of Bonn, xi, 242. '

480 THE SENSORIAL FUNCTIONS.

the genus Eunice of Cuvier,(iycom,Sav.)which^
from the account given by Professor Muller*,
has four eyes, situated on the hinder part of the
head, and covered with the epidermis ; but con-
taining in their interior a spherule, composed of
an opaque white substance, surrounded by a
black pigment, and penetrated by an optic
nerve, which is continued to the brain. On the
other hand, Professor Weber found in the Hirudo
medicinalis, or common leech, no less than ten
minute eyes, arranged in a semicircle, in front
of the head, and projecting a little from the sur-
face of the integument : they present externally
a convex, and perfectly transparent cornea;
while internally, they are prolonged into cylin-
drical tubes, containing a black pigment t ;
structures, apparently subservient to a species
of vision of a higher order than that which con-
sists in the simple recognition of the presence of
light.

No organs having the most distant relation to
the sense of vision have ever been observed in
any of the Acephalous, or bivalve Mollusca ; but
various species of Gasteropoda have organs
which appear to exercise this sense, situated
sometimes at the base, sometimes at the middle,
and frequently at the extremity of the tenta-

* Annales des Sciences Naturelles, xxii, 23.
t Meckel, Archiv fiir Anatomie und Physiologic; 1824,
p. 301.

VISION. 481

cula. Of the latter we have examples in the
common Slug and Snail, vi'here these tentacula,
or horns, are four in number, and are capable of
being protruded and again retracted, by folding
inwards like the finger of a glove, at the pleasure
of the animal. According to Muller,*the eye of
the Helix pomatia, represented at e, (Fig. 4 1 8),
is situated a little to one side of the rounded
extremity, or papilla (p), of the tentaculum, and

418

is attached to an oval bulb of a black colour.
It receives only a slender branch (o) from a
large nerve (n n) which is distributed to the
papilla of the tentaculum, and appears to be
appropriated exclusively to the sense of touch.
The bulb, with the eye attached to it, is repre-
sented, in this figure, as half retracted within the
tubular sheath of the tentaculum (s s) ; but it
can exercise its proper function only when fully
exposed, by the complete unfolding and protru-
sion of the tentaculum. This eye contains,
within its choroid coat, a semi-fluid and per-
fectly transparent substance, filling the whole of

* Annales des Sciences Naturelles; xxii. 12.
VOL. II. I I

482 THE SENSORIAL FUNCTIONS.

the globe; and Muller also discovered at the
anterior part, another transparent body, having
the shape of a lens.* A structure very similar
to this was found to exist in the eye of the 3Iurex
tritonisy with the addition of a distinct iris, per-
forated so as to form a pupil ; a part which ha4
also been observed, together with a crystalline
lens of very large size, in the Voluta cymbium^
by De Blainville-t Thus the visual organs of
these Gasteropoda appear to possess every re-
quisite for distinct vision, properly so called.
Experiments are said to have been recently made,
both by Leuchs, and by Steifensand,| in which
a snail was repeatedly observed to avoid a small
object presented near the tentaculum ; thus
affording evidence of its possessing this sense.

The accurate investigation of the anatomy of
the eyes of insects presents considerable diffi-
culty, both from the minuteness of their parts
and from the complication of their structure ; so|
that notwithstanding the light which has recently
been thrown on this interesting subject by the
patient and laborious researches of entomologists,
great obscurity still prevails with regard to the^

* Muller thus confirms the accuracy of Swammerdam's account
of the anatomy of the eye of the snail, which had been contested
by Sir E. Home (Phil. Trans. 1824, p. 4) and other writers.

t Principes d' Anatomic Comparee, i, 445.

X Quoted by Muller ; ibid, p. 16. These results also corro-
borate the testimony of Swammerdam, who states that he had
obtained proofs that the snail could see by means of these
organs.

VISION. 483

mode in which these diminutive beings exercise
the sense of vision. Four descriptions of visual
organs are met with in the class of Articulated
animals ; the first are the simple eyes, or stem-
mata, as they are termed, which appear as lucid
spots, resembling those we have noticed in the
higher orders of Annelida ; the second, are the
conglomerate eyes, which consist of clusters or
aggregations of simple eyes ; the third, are the
compound eyes, which are formed of a vast
assemblage of small tubes, each having its re-^
spective apparatus of humours and of retina, and
terminating externally in a separate cornea,
slightly elevated above the general surface of
the organ : the fourth kind of eyes, which have
not yet been distinguished by any particular
appellation, are constituted by a number of
separate lenses, and subjacent retinae, but the
whole covered by a single cornea common to
them all.

Few insects are wholly destitute of visual
organs, either in their larva or perfect states.*
The larvae of those insects which undergo a com-
plete metamorphosis have only stemmata ; but
those which are subjected only to a partial
change of form, as the Orthoptera, the Hemip-

• This is the case, however, with the genus Claviger, among
the Coleoptera ; Braula (Nitzch) among Diptera, and also some
of the species of Pupipara, Nycterihia^ and Melophagus, which
are all parasitic insects : there are also five species of ants, whose
neuters have no eyes. (Muller, Annales des Sc. Nat. xvii. 366.)'

484 THE SENSORIAL FUNCTIONS.

tera, and the aquatic Neuroptera, have com-
pound as well as simple eyes. Perfect insects,
with the few exceptions above noticed, have
always compound eyes, generally two in num-
ber, placed on the sides of the head ; and they
are often accompanied by stemmata situated
between, or behind them, on the upper part of
the head. These stemmata, when met with, are
generally three in number, and are either placed
in a row, or form a triangle. Their structure
has been minutely examined by Professor Muller,
who found them to contain a hard and spherical
crystalline lens, a vitreous humour, and a choroid
coat, with its accompanying black pigment ; the
whole being covered externally by a convex
cornea. The stemmata of a caterpillar, which
has eight of these eyes, are shown in Fig. 419,

connected together by a circular choroid mem-
brane (x x) common to the whole ; together with
the separate branches (o o) of the optic nerve
(n) belonging to each.

All the Arachnida possess eyes of this latter
description ; and from their greater size afford

> • VISION. 485

facilities for dissection, which are not met with
among proper insects. Their number in Spiders
is generally eight; and they are disposed with
great symmetry on the upper side of the head.
Fig. 420 represents, on a magnified scale, one of
the large stemmata, on the head of the Scorpio
tuiiensis, dissected so as to display its internal
parts; in which are seen the cornea (c), derived
from an extension of the integument (i) ; the
dense spherical crystalline lens (l) ; the choroid
coat, with its pigment (x),* forming a wide open-
ing, or pupil ; the vitreous humour (v), covered
behind by the retina (r), which is closely ap-
plied to it ; and the optic nerve (o), with which
the retina is continuous.

Examples of the conglomerate eye occur in
the Myriapoda : in the Scolopendra, for instance,
they consist of about twenty contiguous circular
pellucid lenses, arranged in five lines, with one
larger eye behind the rest, which Kirby com-
pares to a sentinel, or scout, placed at some little
distance from the main body. In the Julus
terrestriSf or common Millepede, these eyes,
amounting to 28, form a triangle, being disposed
in seven rows ; the number in each regularly
diminishing from the base to the apex ; an
arrangement which is shown in Fig. 421. -f

* Marcel de Serres states, that some of the stemmata of the
insects which he examined contain a thin choroid, having a sil-
very lustre, as if intended as a reflector of the light which falls
on it.

t Kirby and Spence's Introduction, &c., iii. 494.

486 THE SENSORIAL FUNCTIONS.

The corapound eyes of insects are formed of a
vast number of separate cylinders or elongated
cones,* closely packed together on the surface
of a central bulb, which may be considered as
a part of the optic nerve ; while their united
bases or outer extremities constitute the surface
of a hemispherical convexity, which often occu-
pies a considerable space on each side of the
head. The usual shape of each of these bases is
that of a hexagon, a form which admits of their
uniform arrangement with the greatest economy
of space, like the cells of a honey-comb ; and
the hexagonal divisions of the surface are very
plainly discernible on viewing the surface of
these eyes with a microscope ; especially as there
is a thin layer of black pigment intervening
between each, like mortar between the layers of
brick. The appearance they present in the
Melolonthay when highly magnified, is shown in
Fig. 422.1 The internal structure of these eyes
will be best understood from the section of that

* The number of these cones or cylinders which compose the
entire organ differs much in different species. In the ant, there
are only 50; in a Scarabceus, 3180; in the Bombyx mori, 6236;
in the house-fly (Musca domestica), 8000 ; in the Melolontha
vulgaris, 8820 ; in the Phalena cossus, 1 1 ,300 ; in the Libellula,
12,544; in the Papilio, 17,325; and in the Mordella, 25,088.

t In the PhalencB, and other tribes, they are arranged in
squares (as shown in Fig. 423), instead of hexagons, and fre-
quently much less regularly ; as must necessarily happen, in
many parts, from the curvature of the spherical surface.

VISION.

487

of the Libelhila vulgata, or grey Dragon-fly,
shown in Fig. 424, aided by the highly mag-
nified views of smaller portions given in the
succeeding figures, in all of which the same
letters of reference are used to indicate the same
objects.* The whole outer layer (c c) of the

422

424

compound eye may be considered as corres-
ponding to the cornea ; each separate division of
which has been termed a Corneule, being com-
posed of a horny and perfectly transparent
material. Each corneule (c) has the form of a
truncated pyramid, the length of which (l) is
between two and three times the diameter of the
base (b). The outer surface (b) is very convex ;
but the internal, or truncated end (d) is con-
cave ; and the concavity of the latter being

* These figures, as well as the account of the anatomy of the
eye of the Libellula, are taken from the memoir of Duges, in the
Annales des Sciences Naturelles, xx. 341.

48B

THE SENSORIAL FUNCTIONS.

smaller than the convexity of the former, its
optical effect is that of a meniscus, or concavo-
convex lens, with power of converging to a dis-
tant focus the rays of light which traverse iti

425

428

rh^

426

X

-iL

' "

»iHi

( J M

427

_4

m ""

■r^

■N

m

EG^

Within these corneules there is extended a layer
of an opaque black pigment (x), probably con-
nected with a choroid coat, which, from the deli-
cacy of its texture, has hitherto escaped obser-
vation. There exists opposite to the centre, or
axis of each corneule, a circular perforation (p),
which performs the functions of a pupil.* Dug^s
states, indeed, that he has witnessed in this part

* This pupillary aperture was discovered by MuUer, after it had
eluded all the efforts of former observers to detect it ; and it was
accordingly the prevailing notion that the black pigment lined

VISION 489

movements of contraction and dilatation, like
those of the iris in vertebrated animals. He has
likewise found that there is a small space (a)
intervening between the extremity of each cor-
neule and the iris, and filled with an aqueous
humour. The compartments formed by the sub-
stance of the choroid (x) are continued inwards
towards the centre of the general hemisphere ;
tlie cylindrical spaces which they enclose being
occupied each by a transparent cylinder (v),
consisting of an outer membrane, filled with a
viscid substance analogous to the vitreous hu-
mour. Their general form and situation, as
they lie embedded in the pigment, may be seen
from the magnified sections; each cylinder
commencing by a rounded convex base, imme-
diately behind its respective pupil, and slightly
tapering to its extremities, where it is met by
a filament (n) of the optic nerve ; and all these
filaments, after passing for a certain distance
through a thick mass of pigment, are united to
the large central nervous bulb (g. Fig. 427),
which is termed the optic ganglioti.*

the whole surface of the cornea, and interposed an insuperable
barrier to the passage of light beyond the cornea. It was evi-
dently impossible, while such an opinion was entertained, that
any intelligible theory of vision, with eyes so constructed, could
be formed.

* Numberless modifications of the forms of each of these con-
stituent parts occur in different species of insects. Very fre-

490 THE SENSORIAL FUNCTIONS.

It thus appears that each of the constituent
eyes, which compose this vast aggregate, con-
sists of a simple tube, furnished with all the ele-
ments requisite for distinct vision, and capable
of receiving impressions from objects situated in
the direction of the axis of the tube. The rays
traversing adjacent corneules are prevented from
mixing themselves with those which are proper
to each tube by the interposition of the black
pigment, which completely surrounds the trans-
parent cylinders, and intercepts all lateral or
scattered light. Thus has nature supplied the
want of mobility in the eyes of insects, by the

quently the vitreous humour (v), instead of forming an elon-
gated cylinder, has the shape of a short cone, terminating in a
fine point, as shown in Fig. 426. Straus Durckheim appears to
have mistaken this part for an enlarged termination of the optic
nerve ; believing it to be opaque, and to form a retina applied to
the back of the corneule, vphich latter part he considered as pro-
perly the crystalline lens. In his elaborate vpork on the ana-
tomy of the Melolontha, he describes the filaments (r) of the op^
tic nerve, in their progress inwards, as passing through a second
membrane (k, Fig. 428), which he denominates the common
choroid, and afterwards uniting to form an expanded layer, or
more general retina (r), whence proceed a small number of
short but thick nervous columns (n), still converging towards the
large central ganglion (g), in which they terminate. The use he
ascribes to this second choroid is to intercept the light, which,
in so diminutive an organ, might otherwise penetrate to the gene-
ral retina and produce confusion, or injurious irritation. The
colour of the pigment is not always black, but often has a bluish
tint : in the common fly, it is of a bright scarlet hue, resembling
blood. In nocturnal insects, the transverse layer of pigment
between the corneule and the vitreous humour is absent.

VISION. 491

vast multiplication of their number, and by pro-
viding, as it were, a separate eye for each sepa-
rate point which was to be viewed ; and thus
has she realized the hypothetical arrangement,
which suggested itself in the outset of our in-
quiries, while examining all the possible modes
of effecting this object.

This mode of vision is probably assisted by
the converging powers of each corneule ; although
in parts which are so minute it is hardly pos-
sible to form an accurate estimate of these
powers by direct experiment. In corroboration
of this view I am fortunately enabled to cite a
valuable observation of the late Dr. WoUaston,
relative to the eye of the Astacus fluviatilisy or
cray-fish, where the length of each component
tube is short, compared with that of the Li-
bellula. On measuring accurately the focal
distance of one of the corneules, Dr. Wollaston
ascertained that it corresponds with great exact-
ness to the length of the tube attached to it ;
so that an image of an external object is formed
precisely at the point where the retina is placed
to receive it.*

Little is known of the respective functions of
these two kinds of eyes, the simple and the com

* This interesting fact was communicated to me by Captain
Kater, who, together with Mr. Children, assisted Dr. Wollaston
in this examination.

492 THE SENSORIAL FUNCTIONS.

pound ; both of which are generally possessed by
the higher orders of winged insects. From the
circumstance that the compound eyes are not
developed before the insect acquires the power
of flight, it has been inferred that they are more
particularly adapted to the vision of distant ob-
jects ; but it must be confessed that the expe-
riments made on this subject have not, hitherto,
led to any conclusive results. Dug^s found, in
his trials, that after the stemmata had been
covered, vision remained apparently as perfect
as before ; whiJe, on the other hand, when in-
sects were deprived of the use of the compound
eyes, and saw only with the stemmata, they
seemed to be capable of distinguishing nothing
but the mere presence or absence of light.
Others have reported, that if the stemmata be
covered with an opaque varnish, the insect loses
the power of guiding its flight, and strikes
against walls or other obstacles ; whereas if the:
compound eyes be covered while the stemmata
remain free, the insect generally flies away,
rising perpendicularly in the air, and continuing
its vertical ascent as long as it can be followed
by the observer. If all the eyes of an insect
be covered, it will seldom make any attempt
whatsoever to fly.

The eyes of insects, whether simple or com-
pound, are immoveably fixed in their situations ;
but the compound eyes of the higher orders of

VISION. 493

the class Crustacea, are placed at the ends
of moveable pedicles ; so as to admit of being
turned at pleasure towards the objects to be
viewed.* This, however, is not the case with
the Entomostraca, comprising the various species
of Monoculi, in which the two eyes are brought
so close to one another as apparently to consti-
tute a single organ, corresponding in its struc-
ture to the fourth class of eyes already enume-
rated ; that is, the separate lenses it contains
have a general envelope of a transparent mem-
brane, or cornea. Muscles are provided for
moving the eye in its socket; so that we have
here indications of an approach to the structure
of the eye which prevails in the higher classes
of animals. There is, however, a still nearer
approximation to the latter in the eye of the
Cephalopoda ; for SepicB differ from all the
tribes belonging to the inferior orders of mollusca
in having large and efficient eyes, containing a
hemispherical vitreous humour, placed imme-
diately before a concave retina, and receiving in
front a large and highly convex crystalline lens,
which is soft at its exterior, but rapidly increases
in density, and contains a nucleus of great hard-
ness ; there is also a pigmentum nigrum, and a

* Latreille describes a species of Crab, found on the shores of
the Mediterranean, having its eyes supported on a long jointed
tube, consisting of two articulations, which enables the animal
to move them in various directions, like the arms of a telegraph.

494 THE SENSORIAL FUNCTIONS.

distinct iris, with a kidney-shaped pupil. This
eye is remarkable for the total absence of a
cornea ; the integuments of the head being
continued over the iris, and reflected over the
edges of the pupil, giving a covering to the ex-
ternal surface of the lens ; there is, of course,
no chamber for containing an aqueous humour.
The globe of the eye is nearly spherical ; but the
sclerotica is double, leaving, at the posterior part,
between its two portions, a considerable space,
occupied by the large ganglion of the optic
nerve, with its numerous filaments, which are
embedded in a soft glandular substance.*

The eyes of Fishes differ from those of Sepise
principally in the addition of a distinct cornea,
exterior to the lens and iris, but having only a
slight degree of convexity. This, indeed, is the
case with all aquatic animals ; for, since the
difference of density between the cornea and
the external medium is but small, the refractive
power of any cornea, however convex, would be
inconsiderable ; ,and the chief agent for per-
forming the requisite refraction of the rays is
the crystalline lens. We accordingly in general
find the cornea nearly flat, and the globe of the
eye approaching in shape to a hemisphere ;
while the lens itself is nearly spherical, and of

* See Cuvier, sur les Mollusques ; Memoir sur le Poiilpe,
p, 37. In the Octopus there are folds of the skin, which appear
to be rudiments of eye-lids.

VISION. 495

great density. These circumstances are shown
in the section of the eye of the Perch, Fig. 430.*
The flatness of the cornea leaves scarcely any
space for aqueous humour, and but little for the
motions of the iris.

The surface of the eye in fishes, being con-
tinually washed by the water in which it is
immersed, requires no provision
430 ^ of a secreted fluid for that pur-

pose ; and there are consequent-
ly neither lacrymal apparatus,
nor proper eye-lids ; the integu-
ments supplying only a thin
transparent membrane, which
passes over and protects the cornea, serving the
office of a conjunctiva. The eye retains its form
by the support it receives from the sclerotic coat,
which is of extraordinary thickness and density.
In the Shark and the Skate the eye is supported
from the bottom of the orbit, by a cartilaginous
pedicle, which enables it to turn as on a pivot,
or lever.

Sir David Brewster has recently made an in-
teresting analysis of the structure of the crystal-

* In this figure, as in the others, c is the cornea ; l, the lens ;
V, the vitreous humour ; r, the retina : o, the optic nerve ; and
s, the sclerotica. There is also found in the eyes of most fishes
an organ, lodged in the space k, termed the Choroid gland,
which envelopes the optic nerve, is shaped like a horse-shoe, is
of a deep red colour, and highly vascular ; its use is quite un-
known.

496

THE SENSORIAL FUNCTIONS.

line lens of the Cod, to which he was led by
noticing some remarkable optical appearances
presented by thin layers of this substance when
transmitting polarised light. He found that the
hard central portion is composed of a succession
of concentric, and perfectly transparent, sphe-
roidal laminae, the surfaces of which, though
apparently smooth, have the same kind of iri-
descence as mother-of-pearl, and arising from
the same cause ; namely, the occurrence of re-
gularly arranged lines, or strice* These lines,
which mark the edges of the separate fibres
composing each lamina, converge like meridians
from the equator to the two poles of the sphe-

roid, as is shown in Fig. 431. The fibres them-
selves are not cylindrical, but flat ; and they
taper at each end, as they approach the points of
convergence. The breadth of the fibres in the
most external layer, at the equator, is about the
5,500th of an inch. The observation of another
optical phenomenon, of a still more delicate kind,
* See vol. i. p. 232.

VISION. 4P9T

led Sir David Brewster to the further discovery
of the curious mode in which, (as is represented
in Fig. 432,) the fibres are locked together at
their edges by a series of teeth, resembling
those of rack-work. He found the number of
teeth in each fibre to be 12,500 ; and as the
whole lens contains about 5,000,000 fibres, the
total number of these minute teeth amounts to
62,500,000,000.*

Some fishes, which frequent the depths of the
ocean, being found at between three and four
hundred fathoms below the surface, to which it
is impossible that any sensible quantity of the
light of day can penetrate, have, like nocturnal
quadrupeds, very large eyes-t In a few spe-
cies, which dwell in the muddy banks of rivers,
as the C(Ecilia^ and Miircend cceca, or blind eel,
the eyes are quite rudimental, and often nearly
imperceptible ; and in the Gastrohranchus, De
Blainville states that it is impossible, even by
the most careful dissection, to discover the least
trace of eyes.

Reptiles, being destined to reside in air as

* As far as his observations have extended, this denticulated
structure exists in the lenses of all kinds of fishes, and likewise in
those of birds. He has also met with it in two species of Lizards,
and in the Ornithorhynchus ; but he has not been able to find it
in any of the Mammalia, not even in the Cetacea. (Phil. Trans,
for 1833, p. 323.)

t See " Observations sur les Poissons recueillis dans un Voy-
age aux lies Baleares et Pythiuses. Par M. Delaroche."
VOL. II. K K

498 THE SENSORIAL FUNCTIONS.

well as in water, have eyes accommodated
to these variable circumstances. By the pro-
trusion of the cornea, and the addition of an
aqueous humour, they approach nearer to the
spherical form than the eyes of fishes ; and the
lens has a smaller refractive power, because
the principal refraction is now perfonned by
the cornea. Rudiments of eye-lids are met
with in the Salamander, but they are not of suf-
ficient extent to cover the whole surface of the
eyes. In some serpents, the integuments pass
over the globe of the eye, forming a transparent
conjunctiva, or external cornea, behind which
the eye-ball has free motion. This membrane
is shed, along with the cuticle, every time that
the serpent is moulting ; and at these epochs,
while the cornea is preparing to detach itself,
air insinuates itself underneath the external
membrane and renders it opaque ; so that until
this operation is completed, and an entire sepa-
ration effected, the serpent is rendered blind.
Serpents have no proper eyelids ; but the cor-
nea is covered by a transparent integument,
which does not adhere to it.* Lizards have

* It was the general opinion, until very lately, that serpents
are unprovided with any lacrymal apparatus; but a small la-
crymal passage has been recently discovered by Cloquet, leading
from the space in the inner corner of the eye, between the trans-
parent integument and the cornea. This lacrymal canal opens
into the nasal cavity in venomous snakes, and into the mouth in
those that are not venomous.

VISION. 499

usually a single perforated eye-lid, which, when
closed by its orbicular muscle, exhibits merely a
horizontal slit. There is also a small internal
fold, forming the rudiment of a third eye-lid.
The Chameleon has remarkably projecting eyes,
to which the light is admitted through a very
minute perforation in the skin constituting the
outer eye-lid. This animal has the power of
turning each eye, independently of the other, in
a great variety of directions.

The eyes of Tortoises exhibit an approach
to those of birds : they are furnished with large
lacrymal glands, and with a very moveable
memhrana nictitans, or third eye-lid.

Birds present a still further developement of
all these parts : their eyes are of great size com-
pared with the head ; as may be seen from the
large portion of the skull which is occupied on
each side by the orbits. The chief peculiarities
of the internal structure of these organs are ap-
parently designed to accommodate them to vision
through a very rare medium, and to procure their
ready adjustment to objects situated at very dif-
ferent distances. The form of the eye appears
calculated to serve both these purposes ; for the
great prominence of its anterior portion, which
has often the shape of a short cone, or cylinder,
prefixed to the front of a hemispherical globe,
and which is terminated by a very convex cornea,
affords space for a larger quantity of aqueous

500 THE SENSORIAL FUNCTIONS.

humour, and also for the removal of the lens
to a greater distance from the retina ; whereby
the vision of near objects is facilitated, while at
the same time the refracting powers are suscep-
tible of great variation.

For the purpose of preserving the hemisphe-
rical form of the sclerotica, this membrane in
birds is strengthened by a circle of bony plates,
which occupy the fore-part, and are lodged
between the two layers of which it consists.
These plates vary in number from fifteen
to twenty ; and they lie close together, their
edges successively overlapping each other.
There is manifest design in this arrangement ;
for it is clear that a ring formed of a number of
separate plates is better fitted to resist fracture,
than an entire bony circle of the same thick-
ness.

There is a dark-coloured membrane, called the
Marsupium, situated in the vitreous humour, the
use of which is unknown, though it appears to
be of some importance, as it is found in almost
every bird having extensive powers of vision.*
The comparative anatomy of the eye offers,
indeed, a great number of special stinictures of

* It is shown at m, Fig. 433, which is a magnified section of
the eye of a Goose, c is the cornea ; i, the iris ; p, the ciliary
processes, s, the sclerotic coat, and o, the optic nerve.

VISION.

501

which we do not understand the design, and
which I have therefore purposely omitted to
notice, as being foreign to the object of this
treatise.

In most birds the memhrana nictitans, or third
eye-lid, is of considerable size, and consists of a
semi-transparent fold of the conjunctiva, lying,
when not used, in the inner corner of the eye,
with its loose edge nearly vertical : it is repre-
sented at N, Fig. 434, covering half the surface
of the eye : its motion, like that of a curtain, is
horizontal, and is effected by two muscles : the
first of which, seen at q, in Fig. 435, is called
from its shape the quadratus, and arises from the
upper and back part of the sclerotica : its fibres
descending in a parallel course towards the optic

nerve, where they terminate, by a semi-circular
edge, in a tubular tendon. This tendon has no

502 THE SENSORIAL FUNCTIONS.

particular attachment, but is employed for the
purpose of serving as a loop for the passage of
the long tendon of the second muscle (p), which
is called the pyramidalis, and which arises from
the lower and back part of the sclerotica. Its
tendon (t), after passing through the channel
above described, which has the effect of a pulley,
is conducted through a circular sheath, furnished
by the sclerotica to the under part of the eye,
and is inserted into the lower portion of the
loose edge of the nictitating membrane. By
the united action of these two muscles, the
former of which serves merely to guide the
tendon of the latter, and increase the velocity of
its action, the membrane is rapidly drawn over
the front of the globe. Its return to its former
position is effected simply by its own elasticity,
which is sufficient to bring it back to the inner
corner of the eye. If the membrane itself had
been furnished with muscular fibres for effecting
this motion, they would have interfered with its
use by obstructing the transmission of light.

The eyes of quadrupeds agree in their general
structure with those of man. In almost all the
inferior tribes they are placed laterally in the
head; each having independent fields of vision,
and the two together commanding an extensive
portion of the whole sphere. This is the case
very generally among Fishes, Reptiles, and
Birds. Some exceptions, indeed, occur in par"

VISION. 503

ticular tribes of the first of these classes, as in the
Uranoscopus, where the eyes are directed imme-
diately upwards ; in the Ray and the Callio-
nymus^ where their direction is oblique ; and in
the Pleuronectes, where there is a. remarkable
want of symmetry between the right and left
sides of the body, and where both eyes, as well
as the mouth, are apparently situated on one
side. Among birds, it is only in the tribe of Owls,
which a:re nocturnal and predaceous, that we find
both eyes placed in front of the head. In the
lower quadrupeds, the eyes are situated laterally,
so that the optic axes form a very obtuse angle
with each other. As we ascend towards the
Quadrumana we find this angle becoming
smaller ; till at length the approximation of the
fields of view of the two eyes is such as to
admit of their being both directed to the same
object at the same time. In the human species
the axes of the two orbits approach nearer to
parallelism than in any of the other mammalia ;
and the fields of vision of both eyes coincide
nearly in their whole extent. This is probably
a circumstance of considerable importance with
regard to our acquisition of correct perceptions
by this sense.

In the magnitude of the organ compared with
that of the body, we may occasionally observe
some relation to the character of the animal and
the nature of its pursuits. Herbivorous animals,

504 THE SENSORIAL FUNCTIONS.

and especially those whose bulk is great, as
the Elephant, the Ithinoceros, and the Hippo-
potamus, have comparatively small eyes ; for
that of the elephant does not exceed two inches
in diameter. The eye of the Whale is not much
more than the 200th part of the length of the
body. In the purely carnivorous tribes, which
are actively engaged in the chase of living prey,
the organ of vision is large, and occupies a con-
siderable portion of the head ; the orbit is much
developed, and encroaches on the bones of the
face ; while, at the same time, the bony par-
tition separating the globe of the eye from the
temporal muscle is supplied by ligament alone :
so that when that muscle is in strong action, the
eye is pressed outwards, giving a peculiar ferocity
of expression to the countenance.

While nature has thus bestowed great acute-
ness of sight on pursuing animals, she has,
on the other hand, been no less careful to arm
those which are the objects of pursuit, with
powers of vision, enabling them to perceive
their enemies from afar, and avoid the impend-
ing danger. Thus, large eyes are bestowed
on the Rodentia and the Ruminant ia. Those
tribes which pursue their prey by night, or
in the dusk of the evening, as for example
the Lemur and the Cat, are furnished with
large eyes. Bats, however, form an exception

VISION. 505

to this rule, their eyes being comparatively
small ; but a compensation has been afforded
them in the superior acuteness of their other
senses. In many quadrupeds a portion of the
choroid coat is highly glistening, and reflects
a great quantity of coloured light : the object
of this structure, which is termed the Tapetuniy
is not very apparent.

Among the lesser quadrupeds which burrow
in the ground, we find many whose eyes are
extremely minute ; so much so, indeed, as to be
scarcely serviceable as visual organs. The eye
of the Sorex, or shrew mouse, is very small, and
surrounded by thick hair, which completely
obstructs vision, and requires to be removed
by the action of the subcutaneous muscles,
in order to enable the animal to derive any
advantage from its eyes. These organs in the
Mole are still more remarkably deficient in
their developement, not being larger than the
head of a pin, and consequently not easily
discovered.* It is therefore probable that this
animal trusts chiefly to its sense of hearing,
which is remarkably acute, for intimations of
the approach of danger, especially as, in its
subterranean retreats, the vibrations of the solid

* Magendie asserts that the mole has no optic nerve ; but G.
St. Hilaire and Carus recognise the existence of a very slender
nervous filament, arising from the brain, and distributed to the
eye of that animal.

506 THE SENSORIAL FUNCTIONS.

earth are readily transmitted to its ears. The
Mus typhlus^ or blind rat of Linnaeus, (the
Zemni of Pallas,) which is an inhabitant of
the western parts of Asia, cannot be supposed
to possess even the small degree of vision of
the mole ; for no external organ of this sense
has been detected in any part of that animal.
The whole side of the head is covered with
a continuous integument of uniform thickness,
and equally overspread with a thick velvety
hair. It is only after removing the skin that
a black spot is discovered on each side, of ex-
ceeding small size, apparently the mere imper-
fect rudiment of an eye, and, as far as we can
perceive, incapable of exercising any of the
functions of vision.

Those mammalia whose habits are aquatic,
having the eye frequently immersed in a dense
medium, require a special provision for accom-
modating the refractive power of that organ to
this variation of circumstances. Accordingly it
is found that in the Seal, and other amphibious
tribes, the structure of the eye approaches to
that of fishes ; the lens being denser and more
convex than usual, the cornea thin and yield-
ing, and both the anterior and posterior seg-
ments of the sclerotic thick and firm ; but
the middle circle is very thin and flexible,
admitting of the ready separation or approxi-
mating of the other portions, so as to elongate

VISION. 507

or contract the axis of the eye ; just as a tele-
scope can be drawn out or shortened, in order
to adapt it to the distance of the object to be
viewed. The whole eye-ball is surrounded by
strong muscles which are capable of effecting
these requisite changes of distance between the
cornea and the retina. The Dolphin, which lives
more constantly in the water, has an eye still
more nearly approaching in its structure to that
of fishes ; the crystalline lens being nearly
spherical, and the globe of the eye furnished
with strong and numerous muscles. In birds
which frequently plunge their heads under
water the crystalline lens is more convex than
in other tribes; and the same is true also of
aquatic reptiles.

508

Chapter VII.

PERCEPTION.

The object of nature in establishing the organ-
izations we have been reviewing is to produce
certain modified impressions on the extremities
of particular nervous filaments provided to
receive them ; but these impressions constitute
only the commencement of the series of cor-
poreal changes which terminate in sensation ;
for they have to be conveyed along the course
of the nerves to the brain, or central organ of
the nervous system ;* where, again, some phy-
sical change must take place, before the re-
sulting affection of the mind can be produced.
The particular part of the brain where this last
physical change, immediately preceding the
mental change, takes place, is termed the Sen-
sorium. Abundant proofs exist that all the
physical changes here referred to really occur,
and also that they occur in this order of suc-

* It is usual to designate the end of the nerve which is next
to the sensoriura, as the origin of that nerve ; whereas it should
more properly be regarded as its termination ; for the series of
changes which end in sensation commence at the organ of sense,
and are thence propagated along the nerve to the sensorium.

PERCEPTION. 509

cession ; for they are invariably found to be
dependent on the healthy state, not only of the
nerve, but also of the brain : thus, the destruc-
tion, or even compression of the nerve, in any
part of its course between the external organ
and the sensorium, totally prevents sensation ;
and the like result ensues from even the slight-
est pressure made on the sensorium itself.

Although the corporeal or physical change
taking place in the sensorium, and the mental
affection we term sensation, are linked together
by some inscrutable bond of connexion, they
are, in their nature, as perfectly distinct as the
subjects in which they occur; that is, as mind
is distinct from matter ; and they cannot, there-
fore, be conceived by us as having the slightest
resemblance the one to the other. Yet sen-
sations invariably suggest to the mind ideas,
not only of the existence of an external agent
as producing them, but also of various qualities ,
and attributes belonging to these agents ; and
the term Perception expresses the belief, or
rather the irresistible conviction, thus forced
upon us, of the real existence of these external
agents, which we conceive as constituting the
material world.

Various questions here present themselves
concerning the origin, the formation, and the
laws of our perceptions. This vast field of
curious but difficult inquiry, situated on the

510 THE SENSORIAL FUNCTIONS.

confines of the two great departments of human
knowledge, (of which the one relates to the
phenomena of matter, and the other to those
of mind,) requires for its successful cultivation
the combined efforts of the physiologist and
the metaphysician. For although our sensa-
tions are purely mental affections, yet inasmuch
as they are immediately dependent on physical
causes, they are regulated by the physical laws
of the living frame ; whereas the perceptions
derived from these sensations, being the results
of intellectual processes, are subject rather to
the laws which regulate mental than physical
phenomena. It is certain, from innumerable
facts, that in the present state of our existence,
the operations of the mind are conducted by the
instrumentality of our bodily organs ; and that
unless the brain be in a healthy condition, these
operations become disordered, or altogether
cease. As the eye and the ear are the instru-
ments by which we see and hear, so the brain
is the material instrument by which we retrace
and combine ideas, and by which we remember,
we reason, we invent. Sudden pressure on this
organ, as in a stroke of apoplexy, puts a total
stop to all these operations of the mind. If the
pressure be of a nature to admit of remedy,
and has not injured the texture of the brain,
recovery may take place; and immediately on
the return of consciousness, the person awakes

PERCEPTION. 511

as from a dream, having no sense of the time
which has elapsed since the moment of the
attack. All causes which disturb the healthy
condition of the brain, such as alcohol, opium,
and other narcotic drugs, or which disorder
more especially the circulation in that organ,
such as those inducing fever, or inflammation,
produce corresponding derangements of the in-
tellectual powers; modifying the laws of the
association of ideas, introducing confusion in
the perceptions, irregularity in the trains of
thought, or incapacity of reasoning, and lead-
ing to the infinitely diversified forms of mental
hallucination, delirium, or insanity. Even the
strongest minds are subject to vicissitudes
arising from slighter causes, which affect the
general tone of the nervous system. Vain,
indeed, was the boast of the ancient Stoics
that the human mind is independent of the
body, and impenetrable to external influences.
No mortal man, whatever may be the vigour
of his intellect, or the energy of his application,
can withstand the influence of impressions on
his external senses ; for, if sufficiently reiterated
or intense, they will always have power, if not
to engross his whole attention, at least to in-
terrupt the current of his thoughts, and direct
them into other channels. Nor is it necessary
for producing this effect that cannon should
thunder in his ears ; the mere rattling of a

512 THE SENSORIAL FUNCTIONS.

window, or the creaking of a hinge will often
be sufficient to disturb his philosophical medi-
tations, and dissever the whole chain of his
ideas. " Marvel not," says Pascal, " that this
profoynd statesman is just now incapable of
reasoning justly; for behold, a fly is buzzing
round his head. If you wish to restore to
him the power of correct thinking, and of dis-
tinguishing truth from falsehood, you must
first chase away the insect, holding in thraldom
that exalted reason, and that gigantic intellect,
which govern empires and decide the destinies
of rnankind."

Although we must necessarily infer, from the
evidence furnished by experience, that some
physical changes in the brain accompany the
mental processes of thought, we are in utter ig-
norance of the nature of those actions ; and all
our knowledge on this subject is limited to the
changes which we are conscious are going on in
the mind. It is to these mental changes, there-
fore, that our attention is now to be directed.

In experiencing mere sensations, whatever be
their assemblage or order of succession, the, mind
is wholly passive : on the other hand, the mind
is active on all occasions when we combine into
one idea sensations of different kinds, (such as
those which are derived from each separate
sense) ; when we compare sensations or ideas with
one another ; when we analyze a compound idea.

PERCEPTION. 513

and unite its elements in an order or mode of
combination different from that in which they
were originally presented. Many of these active
operations of mind are implied in the process of
perception ; for although it might be supposed
that the diversity in the nature of our sensations
would sufficiently indicate to us a corresponding
variety in the qualities of the material agents,
which produce their impressions on our senses,
yet these very qualities, nay, even the existence
of the objects themselves, are merely inferences
deduced by our reasoning powers, and not the
immediate effects of those impressions on the
mind. We talk, for instance, of seeing a distant
body ; yet the immediate object of our perception
can only be the light, which has produced that
particular impression on our retina ; whence we
infer, by a mental process, the existence, the
position, and the magnitude of that body. When
we hear a distant sound, the immediate object of
our perception is neither the sounding body
whence it emanates, nor the successive undula-
tions of the medium conveying the effect to our
ear ; but it is the peculiar impression made by
the vibrating particles of the fluid, which are in
direct contact with the auditory nerve. It is
not difficult to prove that the objects of percep-
tion are mere creations of the mind, suggested,
probably instinctively, by the accompanying-
sensations, but having no real resemblance or

VOL. II. L L

514 THE SENSORIAL FUNCTIONS.

correspondence either with the impressions them-
selves, or with the agencies which produce them ;
for many are the instances in which our actual
perceptions are widely different from the truth,
and have no external prototype in nature. In
the absence of light, any mechanical pressure,
suddenly applied to the eye, excites, by its effect
on the retina, the sensation of vivid light. That
this sensation is present in the mind we are cer-
tain, because we are conscious of its existence :
here there can be no fallacy. But the percep-
tion of light, as a cause of this sensation, being
inseparably associated with such sensation, and
wholly dependent on it, and corresponding in all
respects, both as to its duration and intensity,
with the same circumstances in the sensation,
we cannot avoid having the perception as well as
the sensation of light : yet it is certain that no
light has acted. The error, then, attaches to the
perception ; and its source is to be traced to the
mental process by which perception is derived
from sensation.

Many other examples might be given of falla-
cious perceptions, arising from impressions made
in an unusual manner on the nerves of the
senses. One of the most remarkable is the ap-
pearance of a flash of light from the transmission
of the galvanic influence through the facial
nerves. If a piece of silver, or of gold, be
passed as high as possible between the upper

PERCEPTION. ' 515

lip and the gums, while at the same time a
plate of zinc is laid on the tongue, or applied to
the inside of the cheeks ; and if a communica-^
tion be then made between the two metals,
either by bringing them into direct contact, or
by means of a wire touching both of them at
the same time, a flash of light is seen by the
person who is the subject of the experiment.
This appearance is the effect of an impression
made either on the retina, or on the optic nerve,
and is analogous to that occasioned by a mecha-
nical impulse, such as a blow directed to the
same part of the nervous system, both being
phenomena totally independent of the presence
of light. A similar fallacy occurs in the per-
ception of taste, which arises in the well known
experiment of placing a piece of zinc and another
of silver, the one on the upper and the other
on the under surface of the tongue, and making
them communicate, when a pungent and dis-
agreeable metallic taste is instantly perceived :■
this happens because the nerves of the tongue,
being acted upon by the galvanism thus excited,
communicate the same sensation as that which
would be occasioned by the actual application
of sapid bodies to that organ. Thus it appears
that causes which are very different in their
nature, may, by acting on the same nerves,^
produce the very same sensation ; and it follows,
therefore, that our sensations cannot be depended

/

€510 THE SENSORIAL FUNCTIONS.

upon as being always exactly correspondent with
the qualities of the external agent which excites
them.

Evidence to the same effect may also be
gathered from the consideration of the narrow-
ness of those limits within which all our senses
are restricted. It requires a certain intensity in
the agent, whether it be light, or sound, or che-
mical substances applied to the senses of smell
or taste, in order to produce the very lowest
degree of sensation. On the other hand, when
their intensity exceeds a certain limit, the
nature of the sensation changes, and becomes
one of pain. Of the sensations commonly re-
ferred to the sense of touch, there are many
which convey no perception of the cause pro-
ducing them. Thus a slighter impression than
that which gives the feeling of resistance pro-
duces the sensation of itching, which is totally
different in its kind. The sensation of cold is
equally positive with that of warmth, and differs
from it, not in degree merely, but in species ;
although we know that it is only in its degree
that the external cause of each of these sensa-
tions differs.

The only distinct notions we are capable of
forming respecting Matter, are that it consists of
certain powers of attraction and repulsion, occu-
pying certain portions of space, and capable of
moving in space ; and that its parts thereby

PERCEPTION. 517

assume different relative positions or configura-
tions. But of 7nindy our knowledge is more ex-
tensive and more precise; because we are con-
scious of its existence, and of many of its opera-
tions, which are comprised in the general term
thought. To assert that thought can be a pro-
perty of matter, is to extend the meaning of the
term matter to that with which we cannot per-
ceive it has any relation. All that we know of
matter has regard to space : nothing that we
know of the properties and affections of mind
has any relation whatsoever to space.

A similar incongruity is contained in the pro-
position that thought is a Junction of the brain.
It is not the brain which thinks, any more than
it is the eye which sees ; though each of these
material organs is necessary for the production
of their respective effects. That which sees and
which thinks is exclusively the mind ; although
it is by the instrumentality of its bodily organs
that these changes take place. Attention to this
fundamental distinction, which, although obvious
when explicitly pointed out, is often lost sight
of in ordinary discourse, will furnish a key to
the solution of many questions relating to per-
ception, which have been considered as difficult
and embarrassing.

The sensations derived from the different
senses have no resemblance to one another, and
have, indeed, no property in common, except

518 THE SENSORIAL FUNCTIONS.

that they are felt by the same percipient being.
A colour has no sort of resemblance to a sound ;
nor have either of these any similarity to an
odour, or a taste, or to the sensations of heat, or
cold. But the mind, which receives these in-
congruous elements, has the power of giving
them, as it were, cohesion, of comparing them
with one another, of uniting them into combina-
tions, and of forming them into ideas of external
objects. All that nature presents is an infinite
number of particles, scattered in different parts
of space ; but out of these the mind forms indi-
vidual groups, to which she gives a unity of her
own creation.

All our notions of material bodies involve that
of space ; and we derive this fundamental idea
from the peculiar sensations which attend the
actions of our voluntary muscles. These actions
first give us the idea of our own body ; of its
various parts, and of their figure and movements ;
and next teach us the position, distances, magni-
tudes, and figures of adjacent objects. Com-
bined with these ideas are the more immediate
perceptions of touch, arising from contact with
the skin, and especially with the fingers. All
these perceptions, variously modified, make us
acquainted with those mechanical properties of
bodies, which have been regarded by many
as primary or essential qualities. The per-
ceptions derived from the other senses can only

PERCEPTION. 519

add to the former the ideas of partial, or secon-
dary qualities, such as temperature, the peculiar
actions which produce taste and smell, the sounds
conveyed from certain bodies, and lastly their
visible appearances.

The picture formed on the retina by the re-
fracting power of the humours of the eye, is the
source of all the perceptions which belong to the
sense of vision ; but the visible appearances
which these pictures immediately suggest, when
taken by themselves, could have given us no
notion of the situation, distances, or magnitudes
of the objects they represent ; and it is altogether
from the experience acquired by the exercise of
other senses that we learn the relation which
these appearances have with those objects. In
process of time the former become the signs and
symbols of the latter ; while abstractedly, and
without such reference, they have no meaning.
The knowledge of these relations is acquired by
a process exactly analogous to that by which we
learn a new language. On hearing a certain
sound in constant conjunction with a certain idea,
the two become inseparably associated together
in our minds ; so that on hearing the name, the
corresponding idea immediately presents itself.
In like manner, the visible appearance of an
object is the sign, which instantly impresses us
with ideas of the presence, distance, situation,
form, and dimensions of the body that gave rise

520 THE SENSORIAL FUNCTIONS.

to it. This association is, in man at least, not
original, but acquired. The objects of sight and
touch, as Bishop Berkeley has justly observed,
constitute two worlds, which although they have
a very important correspondence and connexion,
yet bear no sort of resemblance to one another.
The tangible world has three dimensions,^
namely, length, breadth, and thickness ; the
visible world only two, namely, length and
breadth. The objects of sight constitute a kind
of language, which Nature addresses to our eyes,
and by which she conveys information most im-
portant to our welfare. As, in any language, the
words or sounds bear no resemblance to the
things they denote, so in this particular language
tlie visible objects bear no sort of resemblance to
the tangible objects they represent.

The theory of Berkeley received complete
confirmation by the circumstances attending the
well-known case, described by Cheselden, of a
boy, who, from being blind from birth, suddenly
acquired, at the age of twelve, the power of see-
ing, by the removal of a cataract. He at first
imagined that all the objects he saw touched his
eyes, as what he felt did his skin ; and he was
unable either to estimate distances by the sight
alone, or even to distinguish one object from
another, until he had compared the visual with
what has been called the tactual impression.
This theory also affords a satisfactory solution

VISUAL PERCEPTIONS. 521

of a question which has frequently been sup-
posed to involve considerable difficulty ; namely,
how it happens that we see objects in their true
situation, when their images on the retina, by
which we see them, are inverted. To expect
that the impression from an inverted image on
the retina should produce the perception of a
similar position in the object viewed, is to com-
mit the error of mistaking these images for the
real objects of perception ; whereas they are only
the means which suggest the true perceptions.
It is not the eye which sees; it is the mind. The
analogy which the optical part of the eye bears
to a camera obscura has perhaps contributed to
the fallacy in question ; for, in using that instru-
ment, we really contemplate the image which is
received on the paper, and reflected from it to our
eyes. But in our own vision nothing of this kind
takes place. Far from there being any contem-
plation by the mind of the image on the retina,
we are utterly unconscious that such an image
exists ; and still less can we be sensible of the
position of the image with respect to the object.
All that we can distinguish as to the locality of
the visual appearance which an object produces,
is that this appearance occupies a certain place
in the field of vision ; and we are taught, by the
experience of our other senses, that this is a sign
of the existence of the external object in a parti-
cular direction with reference to our own body.

522 THE SENSORIAL FUNCTIONS.

It is not until long after this association has
been established that we learn, by deduction
from scientific principles, that the part of the
retina, on which the impression causing this
appearance is made, is on the side opposite to
that of the object itself; and also that the image
of a straight object is curved, as well as inverted.
But this subsequent information can never in-
terfere with our habitual, and perhaps instinc-
tive reference of the appearance, resulting from
an impression made upon the upper part of
the retina, to an object situated below us ; and
vice versa. Hence we at once refer impressions
made on any particular part of the retina to a
cause proceeding from the opposite side. Thus
if we press the eye-ball with the finger applied
at the outer corner of the orbit, the luminous
appearance excited by the pressure is imme-
diately referred to the opposite or inner side of
the eye.

If we place a card perpendicularly between
the two eyes, and close to the face, the card will
appear double, because, although each surface is
seen by the eye which is adjacent to it, in the
direction in which it really is with regard to that
eye, yet, being out of the limits of distinct vision,
it is referred to a much greater distance than
its real situation ; and consequently, the two sides
of the object appear separated by a wide interval,
and as if they belonged to two different objects.

VISUAL PERCEPTIONS. 523

Many other examples might be given of similar
fallacies in our visual perceptions.

All impressions made on the nerves of sensa-
tion have a definite duration, and continue for a
certain interval of time after the action of the
external agent has ceased. The operation of this
law is most conspicuous in those cases where the
presence or absence of the agent can readily be
determined. Thus we retain the sensation of a
sound for some time after the vibrations of the
external medium have ceased; as is shown by
the sensation of a musical note being the result
of the regular succession of aerial undulations,
when the impression made by each continues
during the whole interval between two consecu-
tive vibrations. Whether light be caused by the
emission of material particles, or the undulations
of an etherial fluid, its impulses on the retina
are unquestionably consecutive, like those of
sound ; but being repeated at still shorter inter-
vals, they give rise to a continuous impression.
A familiar instance of the same principle occurs
in the appearance of an entire luminous circle,
from the rapid whirling round of a piece of lighted
charcoal ; for the part of the retina which re-
ceives the brilliant image of the burning char-
coal, retains the impression with nearly the same
intensity during the entire revolution of the
light, when the same impression is renewed.
For the same reason a rocket, or a fiery meteor,

524 THE SENSORIAL FUNCTIONS.

shooting across the sky in the night, appears to
leave behind it a long luminous train. The
exact time during which these impressions con-
tinue, after the exciting cause has been with-
drawn, has been variously estimated by different
experimentalists, and is very much influenced
indeed, by the intensity of the impression.*

When the impressions are very vivid, another
phenomenon often takes place; namely, their

* Many curious visual illusions may be traced to the ope-
ration of this principle. One of the most remarkable is the
curved appearance of the spokes of a carriage wheel rolling on
the ground, when viewed through the intervals between vertical
parallel bars, such as those of a palisade, or Venetian window-
blind. On studying the circumstances of this phenomenon, I
found that it was the necessary result of the traces left on
the retina by the parts of each spoke which became in succession
visible through the apertures, and assumed the curved appear-
ances in question. A paper, in which I gave an account of
the details of these observations, and of the theory by which I ex-
plained them, was presented to the Royal Society, and published
in the Philosophical Transactions, for 1825, p. 131. About
three years ago, Mr. Faraday prosecuted the subject with the
usual success which attends all his philosophical researches,
and devised a great number of interesting experiments on the
appearances resulting from combinations of revolving wheels ;
the details of which are given in a paper contained in the first
volume of the Journal of the Royal Institution of Great Britain,
p. 205. This again directed my attention to the subject, and led
me to the invention of the instrument which has since been intro-
duced into notice under the name of the Phantasm ascope or
Phenakisticope. I constructed several of these at that period,
(in the spring of 1831) which I showed to many of my friends ;
but in consequence of occupations and cares of a more serious
kind, I did not publish any account of this invention, which was
last year reproduced on the continent.

VISUAL PERCEPTIONS. 52«!>

subsequent recurrence, after a certain interval,
during which they are not felt, and quite in-
dependently of any renewed application of the
cause which had originally excited them. If,
for example, we look steadfastly at the sun for
a second or two, and then immediately close our
eyes, the image, or spectrum of the sun remains
for a long time present to the mind, as if its
light were still acting on the retina. It then
gradually fades and disappears ; but if we con-
tinue to keep the eyes shut, the same impression
will, after a certain time, recur, and again vanish ;
and this phenomenon will be repeated at inter-
vals, the sensation becoming fainter at each re-
newal. It is probable that these reappearances
of the image, after the light which produced the
original impression has been withdrawn, are oc-
casioned by spontaneous affections of the retina
itself, which are conveyed to the sensorium.
In other cases, where the impressions are less
strong, the physical changes producing these
spectra are perhaps confined to the sensorium.
These spectral appearances generally undergo
various changes of colour ; assuming first a yel-
low tint ; passing then to a green ; and lastly
becoming blue, before they finally disappear.

Another general law of sensation is, that all
impressions made on the nerves of sense tend to
exhaust their sensibility ; so that the continued
or renewed action of the same external cause
produces a less effect than at first ; while, on the

526 THE SENSORIAL FUNCTIONS.

Other hand, the absence or diminution of the
usual excitement leads to a gradual increase of
sensibility, so that the subsequent application of
an exciting cause produces more than the usual
effect. One of the most obvious exemplifica-
tions of this law presents itself in the case of the
sensations of temperature. The very same body
may appear warm to the touch at one time, and
cold at another, (although its real temperature has
not varied,) according to the state of the organ
induced by previous impressions : and a very
different judgment will be formed of its tempe-
rature, when felt by each hand in succession,
if the one has immediately before been exposed
to cold, while the other has retained its natural
warmth. Similar phenomena may be observed
with regard to all the other senses ; thus the
flavour of odorous, as well as sapid bodies, de-
pends much on the previous state of the organ
by which they are perceived ; any strong im-
pression of taste made on the nerves of the
tongue, rendering them, for some time, nearly
insensible to weaker tastes. Sounds, which
make a powerful impression on the auditory
nerves, will, in like manner, occasion temporary
deafness with regard to faint sounds. The con-
verse of this is observed when hearing has been
suddenly restored in deaf persons, by the opera-
tion of perforating the ear-drum.* The sensi-

* See the note in p. 434 of this volume.

VARIATIONS OF SENSIBILITY. 527

bility of the auditory nerves, which had not
been accessible to impressions of sound, is
found to be increased to a morbid degree. This
was remarkably exemplified in the case of a
gentleman, who for several years had been very
deaf, in consequence of the obliteration of the
Eustachian tube, so that he could scarcely hear
a person speaking in a loud voice close to his
ear. As soon as the instrument which had
made the perforation was withdrawn, the by-
standers began to address him in a very low
tone of voice, and were surprised at receiving no
answer, and at his remaining immoveable in his
chair, as if stunned by a violent blow. At
length he burst out into the exclamation, '* For
God's sake, gentlemen, refrain from crying out
so terribly loud ! you are giving me excessive pain
by speaking to me." The surgeon,* upon this,
retired across the room ; unfortunately, however,
the creaking of his boots caused the gentleman to
start up in an agony from his chair, at the same
time applying his hand instinctively to cover his
ear ; but in doing this, the sound of his fingers
coming in contact with his head was a fresh
source of pain, producing an effect similar to
that of a pistol suddenly fired close to him. For
a long time after, when spoken to, even in the
lowest whisper, he complained of the distressing

* M. Maunoir, of Geneva, on whose authority I have given
this account.

528 THE SENSORIAL FUNCTIONS.

loudness of the sounds ; and it was several weeks
before this excessive sensibility of the auditory
nerves wore off: by degrees, however, they ac-
commodated themselves to their proper function,
and became adapted to the ordinary impressions
of sound. Some time afterwards, this gentleman
had a similar operation performed on the other
ear, and with precisely the same results : the
same degree of excessive sensibility to sounds
was manifested on the restoration of hearing in
this ear as had occurred in the first ; and an
equal time elapsed before it was brought into
its natural state.

The most striking illustrations of the extent
of this law are furnished by the sense of vision.
On entering a dark chamber, after having been
for some time exposed to the glare of a bright
sunshine, we feel as if we were blind ; for the
retina, having been exhausted by the action of a
strong light, is insensible to the weaker impres-
sions which it then receives. It might be sup-
posed that the contraction of the pupil, which
takes place on exposure to a strong light, and, of
course, greatly reduces the quantity admitted to
the retina, is a cause adequate to account for
this phenomenon ; but careful observation will
show that the pupil very rapidly enlarges to its
full expansion when not acted upon by light ;
while the insensibility of the retina continues
for a much longer time. It regains its usual

VARIATIONS OF SENSIBILITY. 529

sensibility, indeed, only by slow degrees. By
remaining in the dark its sensibility is still
farther increased ; and a faint light will excite
impressions equal to those produced in the
ordinary state of the eye by a much stronger
light ; and while it is in this state, the sudden
exposure to the light of day produces a dazzling
and painful sensation.

This law of vision was usefully applied by Sir
William Herschel in training his eye to the
acquisition of extraordinary sensibility, for the
purpose of observing very faint celestial objects.
It often happened to hiin, when, in a fine winter's
night, and in the absence of the moon, he was
occupied during four, five, or six hours in taking
sweeps of the heavens with his telescope, that,
by excluding from the eye the light of surround-
ing objects, by means of a black hood, the sen-
sibility of the retina was so much increased, that
when a star of the third magnitude approached
the field of view, he found it necessary imme-
diately to withdraw his eye, in order to preserve
its powers. He relates that on one occasion the
appearance of Sirius announced itself in the
field of the telescope like the dawn of the morn-
ing ; increasing by degrees in brightness, till the
star at last presented itself with all the splendour
of the rising sun ; obliging him quickly to re-
treat from the beautiful but overpowering spec-
tacle.

VOL. II. M M

530 THE SENSORIAL FUNCTIONS.

The peculiar construction of the organ of
vision allows of our distinguishing the effects of
impressions made on particular parts of the
retina from those made on the rest, and from
their general effect on the whole surface. These
partial variations of sensibility in the retina give
rise to the phenomena of ocular spectra, as they
are called, which were first noticed by BufFon,
and afterwards more fully investigated by Dr.
Robert Darwin. A white object on a dark
ground, after being viewed steadfastly till the
eye has become fatigued, produces, when the eye
is immediately directed to another field of view,
a spectrum of a darker colour than the surround-
ing space, in consequence of the exhaustion
of that portion of the retina on which its image
had been impressed. The converse takes place,
when the eye, after having been steadfastly
directed to a black object on a light ground,
is transferred to another part of the same field ;
and in this case a bright spectrum of the object
is seen.

It is a still more curious fact that the sensi-
bility of the retina to any particular kind of
light, may, in like manner, be increased or
diminished, without any change taking place
in its sensibility to other kinds of light. Hence
the spectrum of a red object appears green ;
because the sensibility of that portion of the
retina, on which the red image has been im-
pressed, is impaired with regard to the red rays,

OCULAR SPECTRA. 531

while the yellow and the blue rays still continue
to produce their usual effect ; and these, by com-
bining their influence, produce the impression of
green. For a similar reason, the spectrum of a
green object is red ; the rays of that colour being
those which alone retain their power of fully
impressing the retina, previously rendered less
sensible to the yellow and the blue rays com-
posing the green light it had received from the
object viewed.

The judgments we form of the colours of
bodies are influenced, in a considerable degree,
by the vicinity of other coloured objects, which
modify the general sensibility of the retina.
When a white or grey object of small dimen-
sions, for instance, is viewed on a coloured
ground, it generally appears to assume a tint of
the colour which is complementary to that of
the ground itself.* It is the etiquette among the
Chinese, in all their epistles of ceremony, to
employ paper of a bright scarlet hue ; and I am
informed by Sir George Staunton, that for a long
time after his arrival in China, the characters
written on this kind of paper appeared to him to
be green ; and that he was afterwards much sur-
prised at discovering that the ink employed was
a pure black, without any tinge of colour ; and on
closer examination he found that the marks were

* Any two colours which, when combined together, produce
white light, are said to be complementary to one another.

532 THE SENSORIAL FUNCTIONS.

also black. The green appearance of the letters,
in this case, was an optical illusion, arising from
the tendency of the retina, which had been
strongly impressed with red light, to receive im-
pressions corresponding to the complementary
colour, which is green.

A philosophical history of the illusions of the
senses would afford ample evidence that limits
have been intentionally assigned to our powers
of perception ; but the subject is much too ex-
tensive to be treated at length in the present
work.* I must content myself with remarking,
that these illusions are the direct consequences
of the very same laws, which, in ordinary cir-
cumstances, direct our judgment correctly, but
are then acting under unusual or irregular com-
binations of circumstances. These illusions may
be arranged under three classes, according as
they are dependent on causes of a physical,
physiological, or mental kind.

The first class includes those illusions in
which an impression is really made on the
organ of sense by an external cause ; but in a
way to which we have not been accustomed.
To this class belong the acoustic deceptions
arising from echoes, and from the art of ven-

* In the Gulstonian Lectures, which I was appointed to read
to the Royal College of Physicians, in May, 1832, I took occa-
sion to enlarge on this subject. A summary of these lectures
was given in the London Medical Gazette, vol. x. p. 273.

ILLUSIONS OF THE SENSES. 533

triloquism ; the deceptive appearances of the
mirage of the desert, the looming of the hori-
zon at sea, the Fata Morgana of the coast of^
Calabria, the gigantic spectre of the Brocken in
the Hartz, the suspended images of concave
mirrors, the visions of the phantasmagoria, the
symmetrical reduplications of objects in the
field of the kaleidoscope, and a multitude of
other results of the simple combinations of the
laws of optics.

The second class comprehends those in which
the cause of deception is more internal, and
consists in the peculiar condition of the nervous
surface receiving the impressions. Ocular spec-
tra of various kinds, impressions on the tongue
and the eye from galvanism, and those which
occasion singing in the ears, arising generally
from an excited circulation, are among the
many perceptions which rank under this head.

The third class of fallacies comprehends those
which are essentially mental in their origin, and
are the consequences of errors in our reasoning
powers. Some of these have already been
pointed out with regard to the perceptions of
vision and of hearing, the formation of which is
regulated by the laws of the association of ideas.
But even the sense of touch, which has been
generally regarded as the least liable to fallacy,
is not exempt from this source of error, as is
proved by the well known experiment of feeling

534 THE SENSORIAL FUNCTIONS.

a single ball, of about the size of a pea, between
two fingers which are crossed ; for there is then
a distinct perception of the presence of two
balls instead of one.

But limited as our senses are in their range
of perception, and liable to occasional error, we
cannot but perceive, that, both in ourselves, and
also in every class of animals, they have been
studiously adjusted, not only to the properties
and the constitution of the material world, but
also to the respective wants and necessities of
each species, in the situations and circumstances
where it has been placed by the gracious and
beneficent Author of its being.

If the sensorial functions had been limited to
mere sensation and perception, conjoined with
the capacity of passive enjoyment and of suf-
fering, the purposes of animal existence would
have been but imperfectly accomplished ; for in
order that the sentient being may secure the pos-
session of those objects which are agreeable and
salutary, and avoid or reject those which are
painful or injurious, it is necessary that he should
possess the power of spontaneous action. Hence
the faculty of Voluritary Motion is superadded
to the other sensorial functions. The muscles
which move the limbs, the trunk, the head, and
organs of sense, — all those parts, in a word,
which establish relations wdth the external
world, are, through the intermedium of a sepa-
rate set of nervous filaments, totally distinct from

VOLUNTARY MOTION. 535

those which are subservient to sensation,* made
to communicate directly with the sensorium, and
are thereby placed under the direct control and
guidance of the will. The mental act of volition
is doubtless accompanied by some corresponding
physical change in that part of the sensorium,
whence the motor nerves, or those distributed to
the muscles of voluntary motion, arise. Here,
then, we pass from mental phenomena to such
as are purely physical ; and the impression,
whatever may be its nature, originating in the
sensorium, is propagated along the course of the
nerve to those muscles, whose contraction is re-
quired for the production of the intended action.
Of the function of voluntary motion, as far as
concerns the moving powers and the mechanism
of the instruments employed,! I have already

* On this subject I must refer the reader to the researches of
Sir Charles Bell, and Magendie, who have completely established
the distinction between these two classes of nerves.

t A voluntary action, occurring as the immediate consequence
of the application of an external agent to an organ of the senses,
though apparently a simple phenomenon, implies the occurrence
of no less that twelve successive processes, as may be seen
by the following enumeration. First, there is the modifying
action of the organ of the sense, the refractions of the rays, for
instance, in the case of the eye : secondly, the impression made
on the extremity of the nerve : thirdly, the propagation of this
impression along the nerve : fourthly, the impression or physical
change in the sensorium. Next follow four kinds of mental
processes, namely, sensation, perception, association, and volition.
Then, again, there is another physical change taking place in the
sensorium, immediately consequent on the mental act of volition :
this is followed by the propagation of the impression downwards

536 THE SENSORIAL FUNCTIONS.

treated at sufficient length in the first part of
this work.

Every excitement of the sensorial powers is,
sooner or later, followed by a proportional de-
gree of exhaustion ; and when this has reached
a certain point, a suspension of the exercise of
these faculties takes place, constituting the
state of sleep, during which, by the continued
renovating action of the vital functions, these
powers are recruited, and rendered again adequate
to the purposes for which they were bestowed.
In the ordinary state of sleep, however, the ex-
haustion of the sensorium is seldom so complete
as to preclude its being excited by internal
causes of irritation, which would be scarcely
sensible during our waking hours ; and hence
arise dreams, which are trains of ideas, sug-
gested by internal irritations, and which the
mind is bereft of the power to control, in con-
sequence of the absence of all impressions froni
the external senses.* In many animals, a much
more general suspension of the actions of life,
extending even to the vital functions of respi-
ration and circulation, takes place during the
winter months, constituting what is termed
Hyhemation.

along the motor nerve ; then an impression is made on the
muscle ; and lastly we obtain the contraction of the muscle,
which is the object of the whole series of operations.

* The only indications of dreaming given by the lower animals
occur in those possessed of the greatest intellectual powers, such
as the Dog, among quadrupeds, and the Parrot, among birds.

537

Chapter VIII.

COMPARATIVE PHYSIOLOGY OF THE NERVOUS
SYSTEM.

§ 1 . Nervous Systems of Iiivertehrated Animals.

Our knowledge of the exact uses and functions
of the various parts which compose the nervous
system, and especially of its central masses, is
unfortunately too scanty to enable us to discern
the correspondence, which undoubtedly exists,
between the variations in the functions and the
diversities in the organization. The rapid re-
view which I propose to take of the different
plans, according to which the nervous system is
constructed in the several classes of animals,
will show that these central masses are multi-
plied and developed in proportion as the facul-
ties of the animal embrace a wider range of
objects, and are carried to higher degrees of
excellence.

In none of the lowest tribes of Zoophytes,
such as Sponges^ Polypi, and Medusce, have any
traces of organs, bearing the least analogy to a
nervous system, been discovered ; not even in

538 THE SENSORIAL FUNCTIONS.

the largest specimens of the last named tribe,
some of which are nearly two feet in diameter.
All these animals give but very obscure indica-
tions of sensibility; for the contractions they
exhibit, when stimulated, appear to be rather
the effect of a vital property of irritability than
the result of any sensorial faculty. Analogy,
however, would lead us to the belief that many
of their actions are really prompted by sensa-
tions and volitions, though in a degree very
inferior to those of animals higher in the scale
of being ; but whatever may be their extent,
it is probable that the sensorial operations in
these animals take place without the inter-
vention of any common centre of action. It
is at the same time remarkable that their
movements are not effected by means of mus-
cular fibres, as they are in all other animals ;
the granular flesh, of which their whole body
is composed, appearing to have a generally
diffused irritability, and perhaps also some de-
gree of sensibility ; so that each isolated granule
may be supposed to be endowed with these com-
bined properties, performing, independently of
the other granules, the functions both of nerve
and muscle. Such a mode of existence exhibits
apparently the lowest and most rudimental con-
dition of the animal functions. Yet the actions
of the Hydra, of which I have given an account,
are indicative of distinct volitions ; as are also, in

NERVOUS SYSTEM OF INVERTEBRATA. 539

a still more decided manner, those of the Infu-
soria. In the way in which the latter avoid
obstacles while swimming in the fluid, and
turn aside when they encounter one another,
and in the eagerness with which they pursue
their prey, we can hardly fail to recognise
the evidence of voluntary action.

To seek for an elucidation of these mysteries
in the structure of animals whose minuteness
precludes all accurate examination, would be a
hopeless inquiry. Yet the indefatigable Ehren-
berg has recently discovered, in some of the
larger species of animalcules belonging to the
order Rotifera, an organization, which he be-
lieves to be a nervous system. He observed, in
the Hydatina senta^ a series of six or seven grey
bodies, enveloping the upper or dorsal part of
the oesophagus, closely connected together, and
perfectly distinguishable, by their peculiar tint,
from the viscera and the surrounding parts.
The uppermost of these bodies, which he con-
siders as a ganglion, is much larger than the
others, and gives off slender nerves, which, by
joining another ganglion, situated under the in-
teguments at the back of the neck, form a circle
of nerves, analogous to that which surrounds the
oesophagus in the mollusca : from this circle two
slender nervous filaments are sent off to the
head, and a larger branch to the abdominal sur-
face of the body. The discovery of a regular

540 THE SENSORIAL FUNCTIONS.

structure of muscular bands of fibres, in these
animalcules, is a further evidence of the con-
nexion which exists between nerves and muscles.
We again meet with traces of nervous fila-
ments, accompanied also with muscular bands of
fibres, in some of the more highly organized
Entozoa. In the Ascaris, or long round worm,
a slender and apparently single filament is seen
passing forwards, along the lower side of the
abdomen, till it reaches the oesophagus, where it
splits into two branches, one passing on each
side of that tube, but without exhibiting any
ganglionic enlargement. This may be consi-
dered as the first step towards the particular
form of the nervous system of the higher classes
of articulated animals, where the principal ner-
vous cord is obviously double throughout its
whole length ; or, if partially united at different
points, it is always readily divisible into two, by
careful manipulation. In addition to this cha-
racteristic feature, these cords present in their
course a series of enlargements, appearing like
knots ; one pair of these generally corresponding
to each of the segments of the body, and sending
off, as from a centre, branches in various direc-
tions. It is probable that these knots, or ganglia,
perform, in each segment of the worm, an office
analogous to that of the brain and special mar-
row of vertebrated animals, serving as centres of
nervous, and perhaps also of sensorial powers.

NERVOUS SYSTEM OF ARTICULATA. 541

Many facts, indeed, tend to show that each
segment of the body of articulated animals, of
an annular structure and cylindric form, such as
the long worms and the myriapoda, has in many
respects an independent sensitive existence, so
that when the body is divided into two or more
parts, each portion retains both the faculty of
sensation, and the power of voluntary motion.
As far as we can judge, however, the only ex-
ternal sense capable of being exercised by this
simple form of nervous system, is that of touch ;
all the higher senses evidently requiring a much
more developed and concentrated organization
of nervous ganglia.

In this division of the animal kingdom, the
primary nervous cords always pass along the
middle of the lower surface of the body, this
being the situation which, in the absence of a
vertebral bony column, affords them the best
protection. They may be considered as ana-
logous to the spinal marrow, and as serving to
unite the series of ganglia, through which they
pass, into one connected system. On arriving
at the oesophagus, they form round it a circle, or
collar, studded with ganglia, of which the up-
permost, or that nearest the head, is generally of
greater size than the rest, and is termed the
oesophageal, cephalic, or cerebral ganglion ; being
usually regarded as analogous to the brain of
larger animals. Perhaps a more correct view of

542 THE SENSORIAL FUNCTIONS.

its functions would be conveyed by calling it the
principal brain, and considering the other ganglia
as subordinate brains. This large ganglion, which
supplies an abundance of nervous filaments to
every part of the head, seems to be the chief
organ of the higher senses of vision, of hearing,
of taste, and of smell, and to be instrumental in
combining their impressions, so as to constitute
an individual percipient animal, endowed with
those active powers which are suited to its rank
in the scale of being.

Such is the general form of the nervous system
in all the Annelida ; but in the higher orders of
Articulata we find it exhibiting various degrees
of concentration. The progress of this concen-
tration is most distinctly traced in the Crustacea*
One of the simplest forms of these organs occurs
in a little animal of this class, which is often
found in immense numbers, spread over tracts of
sand on the sea shore, and which is called the
"^^^^^^^arimsKsasiiite. Talitrus locusta, or Sand-hopper,
(Fig. 438). The central parts of
its nervous system are seen in
Fig. 439, which represents the abdominal side
of this animal laid open, and magnified to twice
the natural size. The two primary nervous
cords, which run in a longitudinal direction, are

* See the account of the researches of Victor Audouin, and
H. M. Edwards, on this subject, given in the Annales des
Sciences Naturelles; xix. 181.

NERVOUS SYSTEM OF CRUSTACEA.

543

here perfectly distinct from one another, and
even separated by a small interval : they present
a series of ganglia, which are nearly of equal
size, and equidistant from one another ; one pair
corresponding to each segment of the body,*
and vmited by transverse threads ; and other
filaments, diverging laterally, proceed from each
ganglion. During the progress of growth, the
longitudinal cords approach somewhat nearer to
each other, but still remain perfectly distinct.

439

440

The first pair of ganglia, or the cephalic, have
been considered, though improperly, as the brain
of the animal.

The next step in the gradation occurs in the

* These segments are numbered in this and the following
figure in their proper order, beginning with that near the head.
A is the external antenna ; a, the internal antenna ; and e, the
eye.

544 THE SENSORIAL FUNCTIONS.

Phyllosoma (Leach), where the ganglia composing
each pair in the abdomen and in the head, are
united into single masses, while those in the
thoracic region are still double. In the Cymo-
thoa (Fab,), which belongs to the family of
Oniscus, there is the appearance of a single chain
of ganglia, those on the one side having coa-
lesced with those on the other ; each pair com-
posing a single ganglion, situated in the middle
line ; while the longitudinal cords which connect
them still remain double, as is shown in Fig. 440,
which represents the interior of this crustaceous
animal, nearly of the natural size. But in the
higher orders of Crustacea, as in the Lobster,
these longitudinal cords are themselves united in
the abdominal region, though still distinct in the
thorax.

In following the ascending series of crustace-
ous animals, we observe also an approximation
of the remoter ganglia towards those near the
centre of the body : this tendency already shows
itself in the shortening of the hinder part of the
nervous system of the CymotJioa, as compared
with the Talitrus; and the concentration pro-
ceeds farther in other tribes. In the Palemon,
for example, most of the thoracic ganglia, and in
the Palinurus (Fab.), all of them, have coalesced
into one large oval mass, perforated in the
middle, and occupying the centre of the thorax ;

NERVOUS SYSTEM OF CRUSTACEA. 545

and lastly, in the Maia squinado, or Spider
Crab (Fig. 441),* this mass acquires still greater
compactness, assumes a more globular form, and
has no central perforation.

These different forms of structure are also
exemplified in the progress of the developement

* In this figure are seen the great thoracic ganglion (b), from
which proceed the superior thoracic nerves (x), those to the
fore feet (f), to the hinder feet (f), and the abdominal nervous
trunk (n) ; the cephalic ganglion (c), communicating by means
of two nervous cords (o), which surround the oesophagus and
entrance into the stomach (s), with the thoracic ganglion (b) ;
and sending off the optic nerve (e) to the eyes (e), and the motor
nerves (m), to the muscles of those organs ; and also the nerves
(a) to the internal antennae, and the nerves (x) to the external
antennae (a).

VOL. II. N N

546 THE SENSORIAL FUNCTIONS.

of the higher Crustacea : thus, in the Lobster, the
early condition of the nervous system is that of
two separate parallel cords, each having a dis-
tinct chain of ganglia, as is the case in the Tali-
trus : then the cords are observed gradually to
approximate, and the ganglia on each side to
coalesce, as represented in the Cymothoa; and
at the period when the limbs begin to be deve-
loped, the thoracic ganglia approach one ano-
ther, unite in clusters, and acquire a rapid en-
largement, preparatory to the growth of the
extremities from that division of the body ; the
abdominal ganglia remaining of the same size as
before. The cephalic ganglion, which was ori-
ginally double, and has coalesced into one, is
also greatly developed, in correspondence with
the growth of the organs of sense. The next
remarkable change is that taking place in the
hinder portions of the nervous cords, which are
shortened ; at the same time that their ganglia
are collected into larger masses, preparatory to
the growth of the tail and hinder feet ; so that
throughout the whole extent of the system the
number of ganglia diminishes in the progress of
developement, while their size is augmented.

All Insects have the nervous system con-
structed on the same general model as in the
last mentioned classes ; and it assumes, as in the
Crustacea, various degrees of concentration in
the different stages of developement. As an

NERVOUS SYSTEM OF INSECTS.

547

example we may take the nervous system of the
Sphinx ligustri, of which representations are
given in the larva, pupa, and imago states,
wholly detached from the body, and of their
natural size, in Figures 442, 443, and 444.*

444

4i3

442

* These figures were drawn by Mr. Newport, from original
preparations made by himself. The same numbers in each refer
to the same parts ; so that by comparing the figures with one
another, a judgment may be formed of the changes of size and
situation which occur in the progress of the principal transfor-
mations of the insect. Numbers 1 to 11 indicate the series of
ganglia which are situated along the under side of the body, and
beneath the alimentary canal. Of these the first five are the
thoracic, and the last six the abdominal ganglia ; while the ce-

548 THE SENSORIAL FUNCTIONS.

This system in the larva (Fig. 442) has the
same simple form as in the Annelida, or in the

phalic, or cerebral ganglion (17) is situated above the oesophagus
and dorsal vessel, and communicates by two nervous cords with
the first of the series, or sub-oesophageal ganglion (1), which is, in
every stage of the insect, contained within the head, and distri-
butes nerves to the parts about the mouth. The next ganglion
(2) becomes obliterated at a late period of the change from the
pupa to the imago state : the third (3) remains, but the two
next (4, 5) coalesce to form, in the imago, the large thoracic
ganglion ; while the two which follow (6 and 7), become wholly
obliterated before the insect attains the imago state, the interven-
ing cords becoming shorter, and being, with the nerves they send
out, carried forwards. The last four (8, 9, 10, 11) of the abdo-
minal ganglia remain, with but little alteration, in all the stages
of metamorphosis : in the larva, they supply nerves to the false
feet. The nerves (12, 13) which supply the wings of the imago,
are very small in the larva ; and they arise by two roots, one de-
rived from the cord, and one from the ganglion. The nerves sent
to the three pair of anterior, or true legs, are marked 14, 15, 16.

The nervous system of the larva is exhibited in Fig. 442, that
of the pupa in Fig. 443, and that of the imago in Fig. 444, It
will be seen that in the pupa the abdominal ganglia are but little
changed ; but those situated more forward (6, 7) are brought
closer together by the shortening of the intervening cord, prepa-
ratory to their final obliteration in the imago ; a change which
those in front of them (4, 5) have already undergone. The pro-
gressive developement of the optic (18) and antennal (19) nerves
may also be traced. Mr. Newport has also traced a set of nerves
(20) which arise from distinct roots, and which he found to be
constantly distributed to the organs of respiration.

A detailed account of the anatomy of the nervous system of the
Sphinx lir/ustri, and of the changes it undergoes up to a certain
period, is given by Mr. Newport in a paper in the Phil. Trans, for
1832, p. 383. He has since completed the inquiry to the last
transformation of this and other insects, and has lately presented
to the Royal Society an account of his researches.

NERVOUS SYSTEM OF INSECTS. 549

Talitrus, for it consists of a longitudinal series of
ganglia, usually twelve or thirteen in number,
connected in their whole length by a double
filament. By degrees the different parts of
which it consists approach each other ; the tho-
racic ganglia, in particular, coalescing into
larger masses, and becoming less numerous ;
some being apparently obliterated : the whole
cord becomes in consequence shorter, and the
abdominal ganglia are carried forwards. The
optic nerves are greatly enlarged during the
latter stages of transformation ; and each of them
is often of greater magnitude than the brain
itself. A set of nerves has also been discovered,
the course of which is peculiar, and appears to
correspond with the sympathetic or ganglionic
system of nerves in vertebrated animals ; while
another nerve resembles in its mode of distri-
bution, the pneumo- gastric nerve, or par vagum.
Very recently Mr. Newport has distinctly traced
a separate nervous tract, which he conceives
gives origin to the motor nerves, while the
subjacent column sends out the nerves of sen-
sation.

In the next great division of the animal king-
dom, which includes all molluscous animals,
the nervous ganglia have a circular, instead of
a longitudinal arrangement. The first example
of this type occurs in the Asterias, where the
nervous system (Fig. 445) is composed of small

550

THE SENSORIAL FUNCTIONS.

ganglia, equal in number to the rays of the
animal, and disposed in a circle round the cen-
tral aperture or mouth, but occupying situations
intermediate between each of the rays. A nerve
is sent off from both sides of each ganglion, and
passes along the side of the rays, each ray
receiving a pair of these nerves. In the Holo-
thuria there is a similar chain of ganglia,

446

445

448

encircling the oesophagus ; and the same mode
of arrangement prevails in all the bivalve Mol-
lusca, except that, besides the oesophageal ganglia,
others are met with, in different parts of the
body, distributing branches to the viscera, and
connected with one another and with the oeso-
phageal ganglia by filaments, so as to form with
them one continuous nervous system. In the

NERVOUS SYSTEM OF MOLLUSCA. 551

Gasteropoda, which are furnished with a distinct
head, and organs of the higher senses, (such as
the Aplysia, of which the nervous system is exhi-
bited in Fig. 446), there is generally a special
cephalic ganglion (c), which may be supposed to
serve the office of brain.* In others, again, as
in the Patella (Fig. 447), the cephalic ganglion
is scarcely discernible, and its place is supplied
by two lateral ganglia (l, l) ; and there is be-
sides a transverse ganglion (t), below the oeso-
phagus. The cephalic ganglion, on the other
hand, attains a considerable size in the Cepha-
lopoda (c. Fig. 448), where it has extensive con-
nexions with all the parts of the head : the
optic ganglia (o, o), in particular, are of very
great size, each of them, singly, being larger
than the brain itself. t

* This figure also shows a ganglion (a), which is placed higher,
and communicates by lateral filaments with the cephalic ganglion
(c) ; two lateral ganglia (l, l), of great size ; and a large abdo-
minal ganglion (g).

f Some peculiarities in the structure of the cephalic ganglion
of the Sepia have been supposed to indicate an approach to the
vertebrated structure ; for this ganglion, together with the laby-
rinth of the ear, is enclosed in a cartilaginous ring, perforated at
the centre to allow of the passage of the oesophagus, and imagined
to be analogous to a cranium.

552

THE SENSORIAL FUNCTIONS.

4-19

D <

-M

::: w

Q <S

Z ;?

458

NERVOUS SYSTEM OP VEHTEBRATA. 553

§ 2. Nervous System of Vertehrated Animals.

The characteristic type of the nervous system of
vertehrated animals is that of an elongated cy-
linder of nervous matter (m z, Fig. 449), ex-
tending down the back, and lodged in the canal
formed by the grooves and arches of the verte-
brae. It has received the name of spinal marrow,
or more properly spinal cord; and, (as is seen in
the transverse section, Fig. 450), is composed of
six parallel columns, two posterior, two middle,
and two anterior, closely joined together, but
leaving frequently a central canal, which is filled
with fluid. On each side of the spinal cord, and
between all the adjacent vertebrae, there proceed
two sets of nervous filaments ; those which are
continuous with the posterior columns (p), being
appropriated to the function of sensation ; and
those arising from the anterior columns (a), being
subservient to voluntary motion. The former,
soon after their exit from the spine, pass through
a small ganglion (g), and then unite with the
nerves from the anterior column ; composing, by
the intermixture of their fibres, a single nerv-
ous trunk (n), which is afterwards divided and
subdivided in the course of its further distribu-
tion, both to the muscular and the sentient
organs of the body. Each of these spinal nerves
also sends branches to the ganglia of the sympa-

554 THE SENSORIAL FUNCTIONS.

thetic nerve, which, as was formerly described,
passes down on each side, parallel and near
to the spine.

Enlargements of the spinal marrow are ob-
served in those parts (w and l, Fig. 449), which
supply the nerves of the extremities ; the increase
of diameter being proportional to the size of the
limbs requiring these nerves. In Serpents,
which are wholly destitute of limbs, the spinal
marrow is not enlarged in any part, but is a
cylindrical column of uniform diameter. In
Fishes, these enlargements appear to have a re-
lation to the size of the organs of motion or sen-
sation, and correspond to them in their situation.
Thus in the Trigla lyra (the Red or Piper Gur-
nard), and the Trigla gurnardus (the Grey Gur-
nard), there are, at the commencement of the
spinal marrow, numerous enlargements, present-
ing a double row of tubercles, (as seen in the
space between m and s, Fig. 451). The nerves
from these tubercles supply the detached rays,
or feelers, anterior to the pectoral fin. Fishes
which possess electrical organs have a consider-
able dilatation of the spinal marrow, answering
to the large nerves which are distributed to those
organs. Birds which fly but imperfectly, as the
Gallinaceous tribe and the Scansores, have the
posterior enlargement much greater than the an-
terior ; a disproportion which is particularly re-
markable in the Ostrich. On the contrary, the
anterior enlargement is much more considerable

NERVOUS SYSTEM OF VERTEBRATA. 55t)

than the posterior in birds which have great
power of flight. In the Dove, of which the brain
and whole extent of the spinal marrow are shown
in Fig. 449, the enlargements (w and l) corres-
ponding to the wings and legs respectively, are
nearly of equal size. In Quadrupeds, we like-
wise find the relative size of these enlargements
corresponding to that of fore and hind extremi-
ties. When the latter are absent, as in the Ce-
tacea, the posterior dilatation does not exist.

The brain (b) may be regarded as an expan-
sion of the anterior or upper end of the spinal
marrow ; and its magnitude, as well as the
relative size of its several parts, vary much
in the different classes and families of ver-
tebrated animals. This will appear from the
inspection of the figures I have given of this
organ in various species, selected as specimens
from each class, viewed from above ; and in all
of which I have indicated corresponding parts
by the same letters of reference.

The portion (m) of the brain, which appears
as the immediate continuation of the spinal
marrow (s), is termed the medulla oblongata.
The single tubercle (c), arising from the ex-
pansion of the posterior columns of the spinal
marrow, is termed the cerebellum^ or little brain.
Next follow the pair (t) which are termed the
optic tubercles, or lobes* and appear to be pro-

* In the Mammalia, and in Man, they have been often desig-
nated by the very inappropriate name of Corpora quadrigemina.

556 THE SENSORIAL FUNCTIONS.

ductions from the middle columns of the spinal
marrow. These are succeeded by another pair
of tubercles (h), which are called the cerebral
hemispheres, and the origin of which may be
traced to the anterior columns of the spinal
marrow. There is also generally found, in front
of the hemispheres, another pair of tubercles (o),
which, being connected with the nerves of smell-
ing, have been called the olfactory lohes, or
tubercles* These are the principal parts of the
cerebral mass to be here noticed ; for I pur-
posely omit the mention of the minuter divisions,
which, though they have been objects of much
attention to anatomists, unfortunately furnish no
assistance in understanding the physiology of
this complicated and wonderful organ.

On comparing the relative proportions of the
brain and of the spinal marrow in the four
classes of vertebrated animals, a progressive in-
crease in the size of the former will be observed,
as we ascend from Fishes to Reptiles, Birds,
and Mammalia. This increase in the magnitude
of the brain arises chiefly from the enlargement
of the cerebral hemispheres (h), which in the
inferior orders of fishes, as in the Trigla lyra, or
Piper Gurnard (Fig. 451), and in the Murcena
conger, or Conger Eel (Fig. 452), are scarcely

* Several cavities, termed Ventricles, are occasionally found
in the interior of the principal tubercles of the brain ; but their
use is unknown.

NERVOUS SYSTEM OF VERTEBRATA. 557

discernible. They are very small in the Perca
fluviatilis, or common Perch (Fig. 45.3) ; but
more developed in Reptiles, as in the Testudo
mydas, or Green Turtle (Fig. 454), and in the
Crocodile (Fig. 455) ; and still more so in Birds,
as is seen in the brain of the Dove (Fig. 449) ;
but most of all in Mammalia, as is exemplified
in the brain of the Lion (Fig. 456). On the other
hand, the optic tubercles (t) are largest, com-
pared with the rest of the brain, in Fishes ; and
their relative size diminishes as we ascend to
Mammalia ; and the same observation applies
also to the olfactory lobes (o).

The relative positions of the parts of the brain

are much influenced by their proportional deve-

lopement. This will be rendered manifest by

the lateral views of the brains of the Perch, the

Turtle, the Dove, and the Lion, presented in

Figures 457, 458, 459, and 460, respectively,

where the same letters are employed to designate

the same parts as in the preceding figures. In

Fishes, all the tubercles which compose this

organ, are disposed nearly in a straight line,

continuous with the spinal marrow, of which, as

they scarcely exceed it in diameter, they appear

to be mere enlargements. As the skull expands

more considerably than the brain, this organ

does not fill its cavity, but leaves a large space,

filled with fluid. Some degree of shortening,

however, may be perceived in the brain of the

558 THE SENSORIAL FUNCTIONS.

Perch (Fig. 457) ; for the medulla oblongata (m)
is doubled underneath the cerebellum (c), push-
ing it upwards, and rendering it more prominent
than the other tubercles. This folding inwards,
and shortening of the whole mass, proceeds to a
greater extent as we trace the structure upwards,
as may be seen in the brain of the Green Turtle
(Fig. 458). In that of Birds, of which Fig. 459
presents a vertical section, the optic tubercles
have descended from their former place, and
assumed a lateral position, near the lower sur-
face of the brain, lying on each side of the
medulla oblongata, at the part indicated by the
letter t. In Mammalia, as in the Lion (Fig.
460), they are lodged quite in the interior of the
organ, and concealed by the expanded hemi-
spheres (h) ; their position only being marked
by the same letter (t). These changes are con-
sequences of the increasing developement of the
brain, compared with that of the cavity in which
it is contained, requiring every part to be more
closely packed ; thus the layers of the hemi-
spheres in Mammalia are obliged, from their
great extent, to be plaited and folded on
one another, presenting at the surface curious
windings, or convolutions, as they are called
(seen in Fig. 456), which do not take place
in the hemispheres of the inferior classes. The
foldings of the substance of the cerebellum pro-
duce, likewise, even in birds, transverse furrows

NERVOUS SYSTEM OF VERTEBRATA. 559

on the surface ; and from the interposition of a
substance of a grey colour between the laminae
of the white medullary matter, a section of the
cerebellum presents the curious appearance
(seen in Fig. 459), denominated, from its fancied
resemblance to a tree, the Arbor VitcB.

Thus far we have followed an obvious gradation
in the developement and concentration of the dif-
ferent parts of the brain ; but on arriving at Man,
the continuity of the series is suddenly disturbed
by the great expansion of the hemispheres,
(Fig. 461), which, compared with those of quad-
rupeds, bear no sort of proportion to the rest of
the nervous system. Both Aristotle and Pliny
have asserted that the absolute, as well as the
comparative size of the human brain is greater
than in any other known animal ; exceptions,
however, occur in the case of the Elephant, and
also in that of the Whale^ whose brains are cer-
tainly of greater absolute bulk than that of man.
But all the large animals, with which we are
familiarly acquainted, have brains considerably
smaller ; as will readily appear from an exami-
nation of their skulls, which are narrow and com-
pressed at the part occupied by the brain ; the
greater part of the head being taken up by the
developement of the face and jaws. In Man, on
the other hand, the bones of the skull rise per-
pendicularly from the forehead, and are extended
on each side, so as to form a capacious globular

5(10

THE SENSORIAL FUNCTIONS.

cavity for the reception and defence of this most
important organ. It is chiefly from the expan-
sion of the hemispheres, and the developement
of its convolutions, that the human brain derives
this great augmentation of size.*

* This will be apparent from the vertical section of the human
brain, Fig. 461 ; where, as before, s is the spinal marrow ; m,
the medulla oblongata; c, the cerebellum, with the arbor vitm ;
T, the optic tubercles, or corpora quadrigemina, dwindled to a
very small size, compared with their bulk in fishes ; p, the
pineal gland, supposed by Des Cartes to be the seat of the soul ;
V, one of the lateral ventricles; q, the corpus callosum ; and
ir, H, H, the hemispheres.

Several expedients have been proposed for estimating the
relative size of the brain in different tribes of animals, with a
view of deducing conclusions as to the constancy of the relation
which is presumed to exist between its greater magnitude and
the possession of higher intellectual faculties. The most cele-
brated is that devised by Camper, and which he termed the
facial angle, composed of two lines, one drawn in the direction

FUNCTIONS OF THE BRAIN. 561

§ 3. Functions of the Brain.

Physiologists have in all ages sought for an
elucidation of the functions of the brain by the
accurate examination of its structure, which
evidently consists of a congeries of medullary
fibres, arranged in the most intricate manner.
Great pains have been bestowed in unravelling
the tissue of these fibres, in the hope of dis-
covering some clue to the perplexing labyrinth of
its organization ; but nearly all that has been
learned from the laborious inquiry, is that the
fibres of the brain are continuous with those
which compose the columns of the spinal
marrow ; that they pass, in their course, through
masses of nervous matter, which appear to be
analogous to ganglia ; and that their remote
extremities extend to the surface of the convo-
lutions ,of the brain and cerebellum, which are
composed of a softer and more transparent grey
matter, termed the cortical or cineritious sub-
stance of the brain.

of the basis of the skull, from the ear to the roots of the upper
incisor teeth, and the other from the latter point, touching
the most projecting part of the forehead. Camper conceived
that the magnitude of this angle would correctly indicate the
size of the brain, as compared with the organs of the principal
senses which compose the face ; but the fallacy of this criterion
of animal sagacity has been shown in a great many cases.

VOL. II. O O

562 THE SENSORIAL FUNCTIONS.

It is a remarkable fact, that in vertebrated
animals all the organs which are subservient to
the sensorial functions are double ; those on one
side being exactly similar to those on the other.
We see this in the eyes, the ears, the limbs, and
all the other instruments of voluntary motion;
and in like manner the parts of the nervous
system which are connected with these functions
are all double, and arranged symmetrically on
the two sides of the body. The same law of
symmetry extends to the brain : every part of
that organ, which is found on one side, is repeated
on the other ; so that, strictly speaking, we have
two brains, as well as two optic nerves and two
eyes. But in order that the two sets of fibres
may co-operate, and constitute a single organ of
sensation, corresponding with our consciousness
of individuality, it was necessary that a free
communication should be established between
the parts on both sides. For this purpose there
is provided a set of medullary fibres, passing
directly across from one side of the brain to the
other ; these constitute what are called the Com-
missures of the Brain.*

* The principal commissure of the human brain, called the
corpus callosum, is seen at q, Fig. 461. Dr. Macartney, in a
paper which he read at the late meeting at Cambridge of the
British Association for the Advancement of Science, described
the structure of the human brain, as discovered by his peculiar
mode of dissection, to be much more complicated than is

FUNCTIONS OF THE BRAIN. 563

The question, however, still recurs; — What
relation does all this artificial intertexture and
accumulation of fibres bear to the mental opera-
tions of which we are conscious, such as memory,
abstraction, thought, judgment, imagination, vo-
lition ? Are there localities set apart for our
different ideas in the store-house of the cerebral
hemispheres ; and are they associated by the
material channels of communicating fibres ?
Are the mental phenomena the effects, as was
formerly supposed, of a subtle fluid, or animal
spirits, circulating with great velocity along
invisible canals in the nervous substance; or
shall we, with Hartley, suppose them to be the
results of vibrations and vibratiuncles, agitating
in succession the finer threads of which this
mystic web has been constructed ? A little
reflection will suffice to convince us that these,
and all other mechanical hypotheses, which the
most fanciful imagination can devise, make not
the smallest approach to a solution of the diffi-
culty ; for they, in fact, do not touch the real
subject to be explained, namely, how the affec-
tions of a material substance can influence and
be influenced by an immaterial agent. All that

generally supposed. He observed that its fibres are interlaced
in the most intricate manner ; resembling the plexuses met with
among the nerves, and establishing the most extensive and
general communications between every part of the cerebral
mass.

564 THE SENSORIAL FUNCTIONS.

we have been able to accomplish has been to
trace the impressions from the organ of sense
along the communicating nerve to the sen-
sorium : beyond this the clue is lost, and we can
follow the process no farther.

The exact locality of the sensorium has been
eagerly sought for by physiologists in every age.
It would appear, from the results of the most
recent inquiries, that it certainly does not extend
to the whole mass of the brain, but has its seat
more especially in the lower part, or basis of
that organ. It differs, however, in its locality,
in different classes of animals. In man, and
the mammalia which approach the nearest to
him in their structure, it occupies some part of
the region of the medulla oblongata ; probably
the spot where most of the nerves of sense are
observed to terminate. In the lower animals it
is not confined to this region, but extends to the
upper part of the spinal marrow. As we de-
scend to the inferior orders of the animal king-
dom, we find it more and more extensively dif-
fused over the spinal marrow ; and in the In-
vertebrata the several ganglia appear to be
endowed with this sensorial property ; but,
becoming less and less concentrated in single
masses, the character of individuality ceases to
attach to the sensorial phenomena ; until, in
Zoophytes, we lose all traces of ganglia and of
nervous filaments, and every part appears to

FUNCTIONS OF THE BRAIN. 565

possess an inherent power of exciting sensation,
as well as performing muscular contractions.

Beyond this point we can derive no further
aid from Anatomy ; since the intellectual ope-
rations of which we are conscious bear no con-
ceivable analogy to any of the configurations
or actions of a material substance. Although
the brain is constructed with evident design, and
composed of a number of curiously wrought
parts, we are utterly unable to penetrate the
intention with which they are formed, or to
perceive the slightest correspondence which
their configuration can have with the functions
they respectively perform. The map of regions
which modern Phrenologists have traced on the
surface of the head, and which they suppose to
have a relation to different faculties and pro-
pensities, does not agree either with the natural
divisions of the brain, or with the metaphysical
classification of mental phenomena.* Experi-
ments and pathological observations, however,
seem to show that the hemispheres of the brain
are the chief instruments by which the intel-
lectual operations are carried on ; that the
central parts, such as the optic lobes and the

* For a summary of the doctrines of Drs. Gall and Spurzheim,
I beg leave to refer the reader to an account which I drew up,
many years ago, for the Encyclopaedia Britannica, and which
composed the article " Cranioscopy" in the last supplement
to that work, edited by Mr. Napier.

560 THE SENSORIAL FUNCTIONS.

medulla oblongata, are those principally con-
cerned in sensation ; and that the cerebellum is
the chief sensorial agent in voluntary motion.

§ 4. Comparative Physiology of Perception.

Of the perceptions of the lower animals, and of
the laws which they obey, our knowledge must, of
necessity, be extremely imperfect, since it must
be derived from a comparison with the results of
our own sensitive powers, which may differ very
essentially from those of the subjects of our
observation. The same kind of organ which, in
ourselves, conveys certain definite feelings, may,
when modified in other animals, be the source
of very different kinds of sensations and per-
ceptions, of which our minds have not the power
to form any adequate conception. Many of the
qualities of surrounding bodies, which escape
our more obtuse senses, may be distinctly per-
ceived, in all their gradations, by particular
tribes of animals, furnished with more delicate
organs. Many quadrupeds and birds possess
powers of vision incomparably more extensive
than our own ; in acuteness of hearing, we are
excelled by a great number of animals ; and in
delicacy of taste and smell, there are few quad-
rupeds that do not far surpass us. The organ

PERCEPTIONS OF ANIMALS. 567

of smell, in particular, is often spread over a
vast extent of surface, in a cavity occupying the
greatest part of the head ; so that the per-
ceptions of this sense must be infinitely diver-
sified.

Bats have been supposed to possess a peculiar,
or sixth sense, enabling them to perceive the
situations of external objects without the aid
either of vision or of touch. The principal facts
upon which this opinion has been founded were
discovered by Spallanzani, who observed that
these animals would fly about rapidly in the
darkest chambers, although various obstacles
were purposely placed in their way, without
striking against or even touching them. They
continued their flight with the same precision as
before, threading their way through the most
intricate passages, when their eyes were com-
pletely covered, or even destroyed! Mr. Jurine,
who made many experiments on these animals,
concludes that neither the sense of touch, of
hearing, or of smell, was the medium through
which bats obtain perceptions of the presence
and situation of surrounding bodies ; but he
ascribes this extraordinary faculty to the great
sensibility of the skin of the upper jaw, mouth,
and external ear, which are furnished with very
large nerves.*

* Sir Anthony Carlisle attributes this power to the extreme
delicacy of hearing in this animal.

568 THE SENSORIAL FUNCTIONS.

The wonderful acuteness and power of dis-
crimination which many animals exercise in the
discovery and selection of their food, has often
suggested the existence of new senses, different
from those which we possess, and conveying
peculiar and unknown powers of perception.
An organ, which appears to perform some sen-
sitive function of this kind, has been discovered
in a great number of quadrupeds by Jacobson.*
In the human skeleton there exists a small per-
foration in the roof of the mouth, just behind the
sockets of the incisor teeth, forming a communi-
cation with the under and fore part of the nos-
trils. This canal is perceptible only in the dried
bones ; for, in the living body, it is completely
closed by the membrane lining the mouth, which
sends a prolongation into it ; but in quadrupeds,
this passage is pervious, even during life, and
is sometimes of considerable width. Jacobson
found, on examining this structure with atten-
tion, that the canal led to two glandular organs
of an oblong shape, and enclosed in carti-
laginous tubes : each gland has in its centre a
cavity, which communicates above with the
general cavity of the nostrils. These organs lie
concealed in a hollow groove within the bone,
where they are carefully protected from injury :
and they receive a great number of nerves and

* See Annales du Musee; xviii. 412.

PERCEPTIONS OF ANIMALS. 5()9

blood-vessels, resembling in this respect the
organs of the senses. Their structure is the
same in all quadrupeds in which they have been
examined ; but they are largest in the family of
the Rodent ia, and next in that of the Ruminantia:
in the Horse, they are still very large, but the
duct is not pervious ; while in carnivorous quad-
rupeds, they are on a smaller scale. In Mon-
keys, they may still be traced, although ex-
tremely small ; appearing to form a link in the
chain of gradation connecting this tribe with the
human race, in whom every vestige of these
organs has disappeared, excepting the aperture
in the bones already noticed. Any use that can
be attributed to these singularly constructed
organs must evidently be quite conjectural. The
ample supply of nerves which they receive
would indicate their performing some sensitive
function ; and their situation would point them
out as fitting them for the appreciation of objects
presented to the mouth to be used as food :
hence it is probable that the perceptions they
convey have a close affinity with those of smell
and taste.

The larger cartilaginous fishes, as Sharks and
Rays, have been supposed by Treviranus to be
endowed with a peculiar sense, from their having
an organ of a tubular structure on the top of the
head, and immediately under the skin. Roux
considers it as conveying sensations intermediate

570 THE SENSORIAL FUNCTIONS.

between those of touch and hearing ; while De
Blainville and Jacobson regard it merely as the
organ of a finer touch.

The perceptive powers of Insects must em-
brace a very different, and, in many respects,
more extended sphere than our own. These
animals manifest by their actions that they per-
ceive and anticipate atmospheric changes, of
which our senses give us no information. It is
evident, indeed, that the impressions made by
external objects on their sentient organs must be
of a nature widely different from those which the
same objects communicate to ourselves. While
with regard to distance and magnitude our per-
ceptions take a wider range, and appear infi-
nitely extended when compared with those of
insects, yet they may, in other respects, be
greatly inferior. The delicate discrimination of
the more subtle affections of matter is perhaps
compatible only with a minute scale of organi-
zation. Thus the varying degrees of moisture
or dryness of the atmosphere, the continual
changes in its pressure, the fluctuations in its
electrical state, and various other physical con-
ditions, may be objects of distinct perception to
these minute animals. Organs may exist in
them, appropriated to receive impressions, of
which we can have no idea ; and opening
avenues to various kinds of knowledge, to which

PERCEPTIONS OF ANIMALS. 571

we must ever remain utter strangers. Art, it is
true, has supplied us with instruments for dis-
covering and measuring many of the properties
of matter, which our unassisted senses are in-
adequate to observe. But neither our ther-
mometers, nor our electroscopes, our hygro-
meters, nor our galvanometers, however skilfully
devised or elaborately constructed, can vie in
delicacy and perfection with that refined appa-
ratus of the senses, which nature has bestowed
on even the minutest insect. There is reason to
believe, as Dr. Wollaston has shown, that the
hearing of insects comprehends a range of per-
ceptions very different from that of the same
sense in the larger animals ; and that a class of
vibrations too rapid to excite our auditory nerves,
is perfectly audible to them. Sir John Herschel
has also very clearly proved that, if we admit
the truth of the undulatory theory of light, it is
easy to conceive how the limits of visible colour
may be established ; for if there be no nervous
fibres in unison with vibrations more or less
frequent than certain limits, such vibrations,
though they reach the retina, will produce no
sensation. Thus it is perfectly possible that
insects, and other animals, may be incapable of
being affected by any of the colours which we
perceive ; while they may be susceptible of re-
ceiving distinct luminous impressions from a

572 THE SENSORIAL FUNCTIONS.

class of vibrations which, applied to our visual
organs, excite no sensation.* The functions of
the antennae, which, though of various forms,
are organs universally met with in this class of
animals, must be of great importance, though
obscurely known ; for insects when deprived of
them appear to be quite lost and bewildered.

The Torpedo, the Gymnotus, and several other
fishes, are furnished with an electrical appa-
ratus, resembling the Voltaic battery, which they
have the power of charging and discharging at
pleasure. An immense profusion of nerves is
distributed upon this organ ; and we can hardly
doubt that they communicate perceptions, with
regard to electricity, very different from any that
we can feel. In general, indeed, it may be re-
marked, that the more an organ of sense differs
in its structure from those which we ourselves
possess, the more uncertain must be our know-
ledge of its functions. We may, without any
great stretch of fancy, conceive ourselves placed
in the situation of the beasts of the forest, and
comprehend what are the feelings and motives
which animate the quadruped and the bird.
But how can we transport ourselves, even in
imagination, into the dark recesses of the ocean,
which we know are tenanted by multitudinous
tribes of fishes, zoophytes, and mollusca ? How

* Encyclopaedia Metropolitana, Article " Light."

PERCEPTIONS OF ANIMALS. 573

can we figure to ourselves the sensitive exist-
ence of the worm or the insect, organized in so
different a manner from ourselves, and occupy-
ing so remote a region in the expanse of creation?
How can we venture to speculate on the percep-
tions of the animalcule, whose world is a drop of
fluid, and whose fleeting existence, chequered
perhaps by various transformations, is destined
to run its course in a few hours ?

Confining our inquiries, then, to the more
intelligible intellectual phenomena displayed by
the higher animals, we readily trace a gradation
which corresponds with the dev elopement of the
central nervous organ, or brain. That the com-
parison may be fairly made, however, it is neces-
sary to distinguish those actions which are the
result of the exercise of the intellectual facul-
ties, from those which are called instinctive, and
are referable to other sources. The actions of
animals appear on various occasions to be
guided by a degree of sagacity not derivable
from experience, and apparently implying a
fore-knowledge of events, which neither expe-
rience nor reflection could have led them to
anticipate. We cannot sufficiently admire the
provident care displayed by nature in the pre-
servation both of the individual and of the spe-
cies, which she has entrusted, not to the slow
and uncertain calculations of prudence, but to
innate faculties, prompting, by an unerring im-

574 THE SENSORIAL FUNCTIONS.

pulse, to the performance of the actions required
for those ends. We see animals providing
against the approach of winter, the effects of
which they have never experienced, and em-
ploying various means of defence against ene-
mies they have never seen. The parent consults
the welfare of the offspring she is destined never
to behold ; and the young discovers and pursues
without a guide that species of food which is
best adapted to its nature. All these unex-
plained, and perhaps inexplicable facts, we must
content ourselves with classing under the head
of instinct; a name which is, in fact, but the
expression of our ignorance of the nature of that
agency, of which we cannot but admire the
ultimate effects, while we search in vain for the
efficient cause.

In all the inferior orders of the animal crea-
tion where instincts are multiplied, while the
indications of intellect are feeble, the organ
which performs the office of the brain is compa-
ratively small. The sensitive existence of these
animals appears to be circumscribed within the
perceptions of the moment, and their voluntary
actions have reference chiefly to objects which
are present to the sense. In proportion as the
intellectual faculties of animals are multiplied,
and embrace a wider sphere, additional magni-
tude and complication of structure are given to
the nervous substance which is the organ of

PERCEPTIONS OF ANIMALS. 57-5

those faculties. The greater the power of com-
bming ideas, and of retaining them in the me-
mory, the greater do we find the developement
of the cerebral hemispheres. These parts of the
brain are comparatively small, as we have seen,
in fishes, reptiles, and the greater number of
birds ; but in the mammalia they are expanded
in a degree nearly proportional to the extent
of memory, sagacity, and docility. In man, in
whom all the faculties of sense and intellect are
so harmoniously combined, the brain is not only
the largest in its size, but beyond all comparison
the most complicated in its structure.*

A large brain has been bestowed on man, evi-
dently with the design that he should exercise
superior powers of intellect; the great distin-
guishing features of which are the capacity for
retaining an immense variety of impressions,
and the strength, the extent, and vast range of
the associating principle, which combines them
into groups, and forms them into abstract ideas.
Yet the lower animals also possess their share of
memory, and of reason : they are capable of
acquiring knowledge from experience ; and, on

* All the parts met with in the brain of animals exist also in
the brain of man ; while several of those found in man are either
extremely small, or altogether absent in the brains of the lower
animals. Soemmerring has enumerated no less than fifteen ma-
terial anatomical differences between the human brain and that
of the ape.

57G THE SENSORIAL FUNCTIONS.

some rare occasions, of devising expedients for
accomplishing particular ends. But still this
knowledge and these efforts of intellect are con-
fined within very narrow limits ; for nature has
assigned boundaries to the advancement of the
lower animals, which they can never pass. If
one favoured individual be selected for a special
education, some additional share of intelligence
may, perhaps, with infinite pains, be infused ;
but the improvement perishes with that indivi-
dual, and is wholly lost to the race. By far the
greater portion of that knowledge which it im-
ports them to possess is the gift of nature, who
has wisely implanted such instinctive impulses
as are necessary for their preservation. Man
also is born with instincts, but they are few in
number compared with those of the lower ani-
mals, and unless cultivated and improved by
reason and education, would, of themselves, pro-
duce but inconsiderable results. That of which
the effects are most conspicuous, and which is
the foundation of all that is noble and exalted in
our nature, is the instinct of Sympathy. The
affections of the lower animals, even between
individuals of the same species, are observable
only in a few instances ; for in general they are
indifferent to each other's joys or sufferings, and
regardless of the treatment experienced by their
companions. The attachment, indeed, of the
mother to her offspring, as long as its wants and

INTELLECTUAL FACULTIES OF MAN. 577

feebleness require her aid and protection, is as
powerful in the lower animals, as in the human
species ; but its duration, in the former case, is
confined, even in the most social tribes, to the
period of helplessness ; and the animal instinct is
not succeeded, as in man, by the continued in-
tercourse of affection and kind offices, and those
endearing relations of kindred, which are the
sources of the purest happiness of human life.

While Nature has apparently frowned on the
birth of man, and brought him into the world
weak, naked, and defenceless, unprovided with
the means of subsistence, and exposed on every
side to destruction, she has in reality implanted
in him the germ of future greatness. The help-
lessness of the infant calls forth the fostering
care and tenderest affections of the mother, and
lays the deep foundations of the social union.
The latent energies of his mind and body are
successively, though slowly developed. While
the vital organs are actively engaged in the exe-
cution of their different offices, while the diges-
tive apparatus is exercising its powerful chemis-
try, while myriads of minute arteries, veins, and
absorbents are indefatigably at work in building
and modelling this complex frame, the sentient
principle is no less assiduously and no less inces-
santly employed. From the earliest dawn of sen-
sation it is ever busy in arranging, in combining,
and in strengthening the impressions it receives.

VOL. II. p p

578 THE SENSORIAL FUNCTIONS.

Wonderful as is the formation of the bodily fabric,
and difficult as it is to collect its history, still
more marvellous is the progressive construction
of the human mind, and still more arduous the
task of tracing the finer threads which connect
the delicate web of its ideas, which fix its fleet-
ing perceptions, and which establish the vast
system of its associations ; and of following the
long series of gradations, by which its affections
are expanded, purified, and exalted, and the
soul prepared for its higher destination in a
future stage of existence.

Here, indeed, we perceive a remarkable inter-
ruption to that regular gradation, which we have
traced in all other parts of the animal series ;
for between man and the most sagacious of the
brutes there intervenes an immense chasm, of
which we can hardly estimate the magnitude.
The functions which are purely vital, and are
necessary for even the lowest degree of sensitive
existence, are possessed equally by all animals :
in the distribution of the faculties of mere sen-
sation a greater inequality may be perceived :
the intellectual faculties, again, are of a more
refined and nobler character, and being less
essential to animal life, are dealt out by nature
with a more sparing and partial hand. Between
the two extremities of the scale we find an infi-
nite number of intermediate degrees. The more
exalted faculties are possessed exclusively by

INTELLECTUAL FACULTIES OF MAN. fi79

man, and constitute the source of the immense
superiority he enjoys over the brute creation,
which so frequently excels him in the perfection
of subordinate powers. In strength and swift-
ness he is surpassed by many quadrupeds. In
vain may he wish for the power of flight pos-
sessed by the numerous inhabitants of air. He
may envy that range of sight which enables the
bird to discern, from a height at which it is itself
invisible to our eyes, the minutest objects on the
surface of the earth. He may regret the dull-
ness of his own senses, when he adverts to the
exquisite scent of the hound, or the acute hear-
ing of the bat. While the delicate perceptions
of the lower animals teach them to seek the food
which is salutary, and avoid that which is inju-
rious, man alone seems stinted in his powers of
discrimination, and is compelled to gather in-
struction from a painful and hazardous expe-
rience. But if nature has created him thus
apparently helpless, and denied him those in-
stincts with which she has so liberally furnished
the rest of her offspring, it was only to confer
upon him gifts of infinitely higher value. While
in acuteness of sense he is surpassed by inferior
animals, in the powers of intellect he stands
unrivalled. In the fidelity and tenacity with
which impressions are retained in his memory,
in the facility and strength with which they are
associated, in grasp of comprehension, in extent

580 THE SENSORIAL FUNCTIONS.

of reasoning, in capacity of progressive improve-
ment, he leaves all other animals at an immea-
surable distance behind. He alone enjoys in
perfection the gift of utterance ; he alone is able
to clothe his thoughts in words ; in him alone
do we find implanted the desire of examining
every department of nature, and the power of
extending his views beyond the confines of this
globe. On him alone have the high privileges
been bestowed of recognising and of adoring the
Power, the Wisdom, and the Goodness of the
Author of the Universe, from whom his being
has emanated, to whom he owes all the blessings
which attend it, and by whom he has been
taught to look forward to brighter skies and to
purer and more exalted conditions of existence.
Heir to this high destination, Man discards all
alliance with the beasts that perish : confiding in
the assurance that the dissolution of his earthly
frame destroys not the germ of immortality
which has been implanted within him, and by
the developement of which the great scheme of
Providence here commenced, will be carried on,
in a future state of being, to its final and perfect
consummation.

PART IV.

THE REPRODUCTIVE FUNCTIONS.

Chapter I.

REPRODUCTION.

Limits have been assigned to the duration of all
living beings. The same power to whom they
owe their creation, their organization, and their
endowments, has also subjected them to the in-
exorable Law of Mortality; and has ordained
that the series of actions which characterise the
state of life, shall continue for a definite period
only, and shall then terminate. The very same
causes which, at the earlier stages of their exist-
ence, promoted their developement and growth,
and which, at a maturer age, sustained the
vigour and energies of the system, produce, by
their continued and silent operation, gradual
changes in the balance of the functions, and, at
a later period, effect the slow demolition of the
fabric they had raised, and the successive de-
struction of the faculties they had originally

582 THE REPRODUCTIVE FUNCTIONS.

nurtured and upheld.* With the germs of life,
in all organized structures, are conjoined the
seeds of decay and of death ; and however
great may be the powers of their vitality, we
know that those powers are finite, and that a
time must come when they will be expended,
and when their renewal, in that individual, is no
longer possible.

But although the individual perishes. Nature
has taken special care that the race shall be
constantly preserved, by providing for the pro-
duction of new individuals, each springing from
its predecessor in endless perpetuity. The pro-
cess by which this formation, or rather this ap-
parent creation, of a living being is effected,
surpasses the utmost powers of the human com-
prehension. No conceivable combinations of
mechanical, or of chemical powers, bear the
slightest resemblance, or the most remote ana-
logy, to organic reproduction, or can afford the
least clue to the solution of this dark and hope-
less enigma. We must be content to observe
and generalize the phenomena, in silent wonder
at the marvellous manifestation of express con-
trivance and design, exhibited in this depart-
ment of the economy of created beings.

Throughout the whole, both of the vegetable

* See the article "Age," in the Cyclopedia of Practical
Medicine, where I have enlarged on this subject.

REPRODUCTION. 583

and animal world, Nature has shown the utmost
solicitude to secure not only the mutiplication
of the species, but also the dissemination of their
numbers over every habitable and accessible
region of the globe; and has pursued various
plans for the accomplishment of these important
objects.

The simplest of all the modes of multiplica-
tion consists in the spontaneous division of the
body of the parent into two or more parts ; each
part, when separated, becoming a distinct indi-
vidual, and soon acquiring the size and shape of
the parent. We meet with frequent examples of
this process of Jissiparous generation, as it is
termed, among the infusory animalcules. Many
species of Monads, for instance, which are natu-
rally of a globular shape, exhibit at a certain
period of their developement a slight circular
groove round the middle of their bodies, which
by degrees becoming deeper, changes their form
to that of an hour-glass ; and the middle part
becoming still more contracted, they present the
appearance of two balls, united by a mere point.
The monads in this state are seen swimming
irregularly in the fluid, as if animated by two
different volitions ; and, apparently for the pur-
pose of tearing asunder the last connecting
fibres, darting through the thickest of the crowd
of surrounding animalcules ; and the moment
this slender ligament is broken, each is seen

584 THE REPRODUCTIVE FUNCTIONS.

moving away from the other, and beginning its
independent existence. This mode of separation
is illustrated by Fig. 462, representing the suc-
cessive changes of form during its progress.

462

463

In this animalcule the division is transverse, but
in others, for example in the Voi'ticella, (as
shown in Fig. 463), and in most of the larger
species, the line of separation is longitudinal.
Each animalcule, thus formed by the subdivision
of its predecessor, soon grows to the size which
again determines a further spontaneous subdivi-
sion into two other animalcules ; these, in course
of time, themselves undergo the same process,
and so on, to an indefinite extent. The most
singular circumstance attending this mode of
multiplication is that it is impossible to pro-
nounce which of the new individuals thus
formed out of a single one should be regarded as
the parent, and which as the offspring ; for they
are both of equal size. Unless, therefore, we
consider the separation of the parts of the parent
animal to constitute the close of its individual
existence, we must recognise an unbroken conti-

REPRODUCTION. 585

nuity in the vitality of the animal, thus trans-'
mitted in perpetuity from the original stem,
throughout all succeeding generations. This,
however, is one of those metaphysical subtleties
for which the subject of reproduction affords
abundant scope, but which it would be foreign
to the object of this work to discuss:

It is in the animal kingdom only that we
meet with instances of this spontaneous division
of an organic being into parts, where each re-
produces an individual of the same species. All
plants, however, are capable of being multiplied
by artificial divisions of this kind : thus a tree
may be divided longitudinally into a great num-
ber of portions, or slips, as they are called, any
one of which, if planted separately and supplied
with nourishment, may continue to grow, and
may, in time, reproduce a tree similar in all
respects to the one from which it originated.
This inherent power of reproduction exists even
in smaller fragments of a plant ; for, when all
circumstances are favourable, a stem will shoot
from the upper end of the fragment, and roots
will be sent forth from its lower end ; and ulti-
mately a complete plant will be formed.* These

* Among the conditions necessary for these evolutions of
organs are, first, the previous accumulation of a store of nourish-
ment in the detached fragment, adequate to supply the growth
of the new parts ; and secondly, the presence of a sufficient
quantity of circulating sap, as a vehicle for the transmission of

586 THE REPRODUCTIVE FUNCTIONS.

facts, which are well known to horticulturalists,
exhibit only the capabilities of vegetative power
under circumstances which do not occur in the
natural course of things, but have been the effect
of human interference.

Reproductive powers of a similar kind are
exhibited very extensively in the lower depart-
ments of the animal kingdom. The Hydra, or
fresh water polype, is capable of indefinite mul-
tiplication by simple division : thus, if it be cut
asunder transversely, the part containing the
head soon supplies itself with a tail ; and the
detached tail soon shoots forth a new head, with
a new set of tentacula. If any of the tentacula,
or any portion of one of them, be cut off, the
mutilation is soon repaired ; and if the whole
animal be divided into a great number of pieces,
each fragment acquires, in a short time, all the
parts which are wanting to render it a complete
individual. The same phenomena are observed,
and nearly to the same extent, in the Planaria.
The Asterias, the Actinia, and some of the lower
species of Annelida, as the Nais, are also capable

that nourishment. It has been found that when these conditions
are present, even the leaf of an orange tree, when planted in a
favourable soil, sends down roots, and is capable of giving origin
to an entire tree. According to the observations of Mirandola,
the leaf of the Bryophyllum, when simply laid on moist ground,
strikes out roots, which quickly penetrate into the soil. (De
CandoUe, Pysiologie Vegetale, ii. 677.) The leaves of the mo-
nocotyledonous plants often present the same phenomenon.

REPRODUCTION. 587

of being multiplied by artificial divisions ; each
segment having the power of supplying others,
and containing within itself a kind of separate
and individual vitality.

A power of more partial regeneration of mu-
tilated parts by new growths, which is very
analogous to that of complete reproduction,
exists in the higher orders of animals, though it
does not extend to the entire formation of two
individuals out of one. The claws, the feet, and
the antennae of the Crustacea, and the limbs of
the Arac/inida, are restored, when lost, by a
fresh growth of these organs. If the head of a
Snail be amputated, the whole of that part of
the animal, including the telescopic eyes, and
other organs of sense, will be reproduced. Even
among the Vertebrata we find instances of these
renovations of mutilated parts ; as happens with
respect to the fins of fishes : for Broussonet
found that in whatever direction they are cut,
the edges easily unite ; and the rays themselves
are reproduced, provided the smallest part of
their base has been left. The tails of Newts,
and of some species of Lizards, will grow again,
if lost ; and what is more remarkable, the eyes
themselves, with all their complex apparatus of
coats and humours, will, if removed, be replaced
by the growth of new eyes as perfect as the
former. We have seen that the teeth of Sharks,
and other fishes, are renewed with the utmost

588 THE REPRODUCTIVE FUNCTIONS.

facility, when by accident they have been lost-
Among Mammalia, similar powers exist, although
they are restricted within much narrower limits;
as is exemplified in the formation of new bones,
replacing those which have perished. When
we advert to the numberless instances of the
reparation of injuries happening to various parts
of our own frame, we have abundant reason to
admire and be grateful for the wise and bountiful
provisions which Nature has made for meeting
these contingencies.

The multiplication of the species by buds, or
Gemmiparous reproduction, is exemplified on the
largest scale in the vegetable creation. Almost
every point of the surface of a plant appears to
be capable of giving rise to a new shoot, which,
when fully developed, exactly resembles the
parent stock, and may, therefore, be regarded as
a separate organic being. The origin of buds is
wholly beyond the sphere of our observation ;
for they arise from portions of matter too minute
to be cognizable to our organs, with every
assistance which the most powerful microscopes
can supply. These imperceptible atoms, from
which organic beings take their rise, are called
germs.

Vegetable germs are of two kinds ; those
which produce stems, and those which produce
roots; and although both may be evolved from
every part of the plant, the former are usually

REPRODUCTION. 589

developed at the axillcB of the leaves ; that is, at
the angles of their junction with the stem ; and
also at the extremities of the fibres of the stems ;
their developement being determined by the
accumulation of nourishment around them.
They first produce huds, which expanding, and
putting forth roots, assume the form of shoots ;
and the successive accumulation of shoots, which
remain attached to the parent plant,* and to
each other, is what constitutes a tree. What
are called knots in wood are the result of germs,
which, in consequence of the accumulation of
nourishment around them, are developed to a
certain extent, and then cease to grow. The
Lemna, or common Duckweed, which consists
of a small circular leaf, floating on the surface
of stagnant pools, presents a singular instance
of the developement of germs from the edges of
the leaves, and the subsequent separation of the
new plant thus formed. In this respect the

* In some rare instances the shoots are removed to a distance
from the parent plant, by a natural process : this occurs in some
creeping plants, which propagate themselves by the horizontal
extension of their branches on the ground, where they dip, and
strike out new roots, giving rise to stems independent of the
original plant. This also sometimes happens in the case of
tuberous roots, as the potatoe, which contain a number of germs,
surrounded by nutritive matter, ready to be developed when cir-
cumstances are favourable. These portions are called eyes ;
and each of them, when planted separately, are readily evolved,
and give rise to an individual plant.

590 THE REPRODUCTIVE FUNCTIONS.

process is analogous to the natural mode of mul-
tiplication met with in the lower orders of Zoo-
phytes, such as the Hydra. At the earliest
period at which the young of this animal is
visible, it appears like a small tubercle, or bud,
rising from the surface of the parent hydra : it
grows in this situation, and remains attached for
a considerable period; at first deriving its nou-
rishment, as well as its mechanical support,
from the parent ; then occasionally stretching
forth its tentacula, and learning the art of catch-
ing and of swallowing its natural prey. The
tube, which constitutes its stomach, at first com-
municates by a distinct opening with that of its
parent : but this opening afterwards closes ; and
the filaments by which it is connected with the
parent becoming more and more slender, at
length break, and the detached hydra imme-
diately moves away, and commences its career
of independent existence. This mode of multi-
plication, in its first period, corresponds exactly
with the production of a vegetable by buds ;
and may therefore be classed among the in-
stances of gemmiparous reproduction ; although
at a later stage, it differs from it in the complete
detachment of the offspring from the parent.

Another plan of reproduction is that in which
the germs are developed in the interior of the
animal, assuming, at the earliest period when
they become animated, the form of the parent.

REPRODUCTION. 591

In this case they are termed gemmules instead
of buds. This mode of reproduction is exem-
plified in the Volvox, which, as we have already
seen, is an infusorial animalcule of a spherical
form, exhibiting incessant revolving move-
ments.* The germs of this animal are deve-
loped, in great numbers, in its interior, having
a globular shape, and visible, by the aid of the
microscope, through the transparent covering ;
and while yet retained within the body of the
parent other still minuter globules are developed
within these, constituting a third generation of
these animals. After a certain period, the young,
which have thus been formed, escape by the
bursting of the parent volvox, which in conse-
quence perishes. Similar phenomena are pre-
sented by many of the Infusoria. In some of
the Entozoa, likewise, as in the Hydatid, the
young are developed within the parent ; and this
proceeds successively for an indefinite number
of generations.^ In most cases of the spon-

* Vol. i. p. 188. This animal is delineated in Fig. 79.

t The mode in which infusory animalcules are produced and
multiplied is involved in much obscurity. Many distinguished
naturalists, adopting the views of BufFon, have regarded them as
the product of an inherent power belonging to a certain class of
material particles, which, in circumstances favourable to its ope-
ration, tends to form these minute organizations ; and in this
manner they explain how the same organic matter which had
composed former living aggregates, on the dissolution of their
union, reappears under new forms of life, and gives rise to the
phenomenon of innumerable animalcules, starting into being,

592 THE REPRODUCTIVE FUNCTIONS.

taneous evolution of gemmules within the parent,
channels are provided for their exit ; but the
gemmules of the Actinia force their way through
the sides of the body, which readily open to give

and commencing a new, but fleeting career of existence. Yet
the analogy of every other department of the animal and vege-
table kingdoms is directly opposed to the supposition that any
living being can arise without its having been originally derived
from an individual of the same species as itself, and of which it
once formed a part. The difficulty which the hypothesis of the
spontaneous production of infusory animalcules professes to
remove, consists in our inability to trace the pre-existence of the
germs in the fluid, where these animalcules are found to arise ;
and to follow the operations of nature in these regions of infinite
minuteness. The discoveries of Ehrenberg relative to the orga-
nization of the Rotifera go far towards placing these diminutive
beings more on a level, both in structure and in functions, with
the larger animals, of whose history and economy we have a
more familiar and certain knowledge ; and in superseding the
hypothesis above referred to, by showing that the bold assump-
tion on which it rests, is not required for the explanation of the
observed phenomena. In many of these animalcules, he has
seen the ova excluded in the form of extremely minute globules,
the 12,000th of an inch in diameter. When these had grown
to the size of the 1700th of an inch, or seven times their original
diameter, they were distinctly seen to excite currents, and to
swallow food. The same diligent observer detected the young
of the Rotifer vulgaris, perfectly formed, moving in the interior
of the parent animalcule, and excluded in a living state ; thus
constituting them viviparous animals, as the former were ovi-
parous. Other species, again, imitate the hydra, in being what
is termed gemmiparous, that is, producing gemmules (like the
budding of a plant), which shoot forth from the side of the
parent, and are soon provided with cilia, enabling them, when
separated, to provide for their own subsistence; although they
are of a very diminutive size when thus cast off.

REPRODUCTION. 593

them passage ; after which, the lacerated part
soon heals.

In the instances which have now passed under
our review, the progeny is, at first, in direct
communication with its parent, and does not
receive the special protection of membranous
envelopes, containing a store of nourishment for
its subsequent growth. But in all the more
perfect structures, both of animals and vege-
tables, the germ is provided with auxiliary
coverings of this kind ; the whole together com-
posing what is called a seed, or an ovum; the
former term being usually applied to vegetable,
and the latter to animal productions ; and in
both cases the organ which originally contained
them is termed the ovary.

The formation and evolution of vegetable
seeds takes place, not indiscriminately at every
point, as we have seen is the case with simple
germs, but only in particular parts of the plant.
The Filices, or fern tribes, may be taken as
examples of this mode of reproduction ; the seeds
being formed at the under surface of the leaves,
apparently by a simple process of evolution ;
and when detached and scattered on the ground,
being further developed into a plant similar to
the parent. The Linnean class of Cryptogamia
includes all the plants coming under this de-
scription. In Animals, likewise, it is only in
the particular organs termed ovaries, that ova

VOL. II. « Q

594 THE REPRODUCTIVE FUNCTIONS.

are formed ; and they are generally divided into
compartments; the whole being enclosed in a
membranous covering, bearing a great resem-
blance to the seed-capsules of plants.

The propagation of living beings by means of
ova or seeds, is a process of a totally different
class from their multiplication by mere slips or
buds ; and the products of the former retain
less of the peculiar characters of the individual
from which they spring, than those of the latter.
This is remarkably exemplified in the case of
orchard trees, such as apples and pears ; for all
the trees which derive their origin from shoots, or
grafts from the same individual, partake of the
same properties, and produce a fruit of nearly
the same flavour and qualities ; whereas trees of
the same species, which grow from seed, have
the characters of distinct individuals, and losing
all the peculiarities that may have distinguished
the parent, revert to the original type of the
species to which they belong. Thus from the
seeds of the golden pippin, or nonpareil, arise
trees bearing the common crab apple, which is
the natural fruit of the species. By continued
graftings, after a long period, the vitality of the
particular variety is gradually exhausted, and
the grafts no longer bear the same fruit. This
has already happened with regard to the two
varieties of apples just mentioned. For these
curious facts, and the theory which explains

REPRODUCTION. 595

them, we are indebted to the observation and
sagacity of Mr. Andrew Knight.*

The plans hitherto noticed are suited only to
the simplest of vegetable or animal beings ; but
for the continuance of the higher races in both
kingdoms of nature there is required a more
complex procedure. The latent germ, contained
in the seed or ovum, is never developed beyond
a certain point, unless it be vivified by the action
of a peculiar fluid, which is the product of other
organs. Thus there are established two distinct
classes of structures ; the office of the one being
the formation of the seed or ovum, and that of
the other the production of the vivifying fluid.
The effect of this vivifying fluid upon the dor-
mant germ is termed FecuHdatioti ; and the
germ, when fecundated, receives the name of
Embryo.

The modes in which the fecundation of the
germ is accomplished are exceedingly various in
different classes of organized beings. In all
Phanerogamous plants, (so named in contra-
distinction to those which are Cryptogamous),
the whole of the double apparatus required for
reproduction is contained in the flower. One
set of organs contains the rudiment of the seed,
enclosed in various envelopes, of which the as-
semblage constitutes an ovary, and to which is

* See his various papers in the Philosophical Transactions.

596 THE REPRODUCTIVE FUNCTIONS.

appended a tube, (the pistil), terminated by a
kind of spongiole, (the stigma). The fecunda-
ting organs are the stamens, which are columns,
{ox filament s), placed generally near and parallel
to the pistil, and terminated by a glandular
organ, (the anther). This organ, when mature,
contains, enclosed in a double envelope, a fine
powder, (the pollen), consisting of very minute
vesicles, filled with a viscous liquor, {ihefovilla),
in which are seen extremely small granules.
Fecundation takes place by a portion of the
pollen being received by the stigma, and con-
veyed through the tubular pistil to the seed,
which it impregnates by imparting to it the fluid
it contains.

By far the greater number of plants com-
posing the vegetable kingdom have these two
sets of organs contained in the same flower ; or
at least in flowers belonging to the same indivi-
dual plant. In the animal kingdom this ar-
rangement is also adopted ; but only in a com-
paratively small number of tribes. In these the
ova, in their passage from the ovary, along a
canal termed the oviduct, are fecundated by
receiving a secretion from another set of organs
in the same system, which is conveyed by a
duct, opening into the oviduct in some part of
its course. In a limited number of plants, com-
posing the class Dioecia, the individuals of the
same species are distinguished by their bearing

REPRODUCTION. 597

flowers which contain only one of the kinds of
reproductive apparatus ; so that the stamens and
the pistils are situated on separate plants ; and
the impregnation of the ovaries in the latter, can
be effected only by the transference of the pollen
from the former. A similar separation of offices
is established among all the higher classes of the
animal kingdom. In most Fishes, and in all
Batrachian reptiles, the ova are impregnated
after their expulsion from the body : in all other
cases their impregnation is internal ; and their
subsequent developement takes place in one or
other of the four following ways.

1. The ovum, when defended by a firm enve-
lope, which contains a store of nutriment, is
termed an egg, and is deposited in situations
most favourable for the developement of the
embryo ; and also for its future support when it
emerges from the egg. Birds, as is well known,
produce eggs which are encased in a calcareous
shell, and hatch them by the warmth they com-
municate by sitting on them with unwearied
constancy. All animals which thus lay eggs
are termed oviparous.

2. There are a few tribes, such as the Viper
and the Salamander, whose eggs are never laid,
but are hatched in the interior of the parent ; so
that they bring forth living offspring, although
originally contained in eggs. Such animals are
said to be Ovo-viviparous. There are other

598 THE REPRODUCTIVE FUNCTIONS.

tribes, again, which, according to circumstances,
are either oviparous, or ovo-viviparous : this is
the case with the Shark.

3. Viviparous animals are those in which no
egg, properly so called, is completed ; but the
ovum, after proceeding through the oviduct,
sends out vessels, which form an attachment to
the interior of a cavity in the body of the parent,
whence it draws nourishment, and therefore has
attained a considerable size at the time of its
birth.

4. Marsupial animals are those, which, like
the KanguroOy and the Opossum, are provided
with abdominal pouches, into which the young,
born at a very early stage of developement, are
received, and nourished with milk, secreted from
glands contained within these pouches. As the
young, both in this and in the last case, are nou-
rished with milk prepared by similar glands, or
MammcB, the whole class of viviparous and mar-
supial animals has received, from this charac-
teristic circumstance, the name of Mammalia.

599

Chapter II.

ORGANIC DEVELOPEMENT.

Although the study of organic structures in
their finished state must tend to inspire the
most sublime conceptions of the Great Creator
of this vast series of beings, extending from the
obscurest plant to the towering tenant of the
forest, and from the lowest animalcule to the
stately elephant and gigantic whale, there yet
exists another department of the science of
Nature, removed, indeed, from the gaze of ordi-
nary observers, but presenting to the philosophic
inquirer subjects not less replete with interest,
and not less calculated to exalt our ideas of the
transcendent attributes of the Almighty. To a
mind nurtured to reflection, these divine attri-
butes, whether of power, of wisdom, or of bene-
ficence, are no where manifested with greater
distinctness, or arrayed in greater glory, than in
the formation of these various beings, and in
the progressive architecture of their wondrous
fabric.

Our attention has already been directed, in a
former part of these inquiries, to the successive

600 THE REPRODUCTIVE FUNCTIONS.

changes which constitute the metamorphoses of
winged insects,* and of Batrachian reptiles,
phenomena which are too striking to have
escaped the notice of the earliest naturalists :
but the patient investigations of modern inquirers
have led to discoveries still more curious, and
have shown that all vertebrated animals, even
those belonging to the higher classes, such as
birds, and mammalia, not excepting man him-
self, undergo, in the early stages of their deve-
lopement, a series of changes fully as great and
as remarkable as those which constitute the
transformations of inferior animals. They have
also rendered it extremely probable that the
organs of the system, instead of existing simul-
taneously in the germ, arise in regulated succes-
sion, and are the results not of the mere expan-
sion of pre-existing rudiments, but of a real
formation by the union of certain elements ;
which elements are themselves successively
formed by the gradual coalescence or juxta-
position of their constituent materials. On con-
templating the infinitely lengthened chain of
means and ends, and of causes and effects,
which, during the construction and assemblage
of the numerous parts composing the animal

* The researches of Nordmann, on different species of Lemcea,
have brought to light the most singular succession of forms
during the progress of developement of the same individual
animal.

ORGANIC DEVELOPEMENT. 601

machine, are in constant operation, adapting
them to their various purposes, and combining
them into one efficient and liarmonious system,
it is impossible not to be deeply impressed with
the extent and the profoundness of the views of
Providence, which far exceed the utmost boun-
daries of our vision, and surpass even the powers
of the human imagination.*

The clearest evidence of enlarged and provi-
dent designs may be collected from observing the
order in which the nascent organs are succes-
sively brought forwards, and added to the grow-
ing fabric ; such order appearing, in all cases, to
be that best calculated to secure the due per-
formance of their appointed functions, and to
promote the general objects of the system. The
apparatus first perfected is that which is imme-
diately necessary for the exercise of the vital
functions, and which is therefore required for the
completion of all the other structures ; but pro-

* ** Si Ton applique," says Cuvier, when speaking of the ana-
tomy of insects, " k chacune de ces especes, par la pensee, ce
qu'il seroit bien impossible qu'un homme entreprit de verifier en
effet pour toutes, une organisation a-peu-pres egale en complica-
tion k celle qui a ete decrite dans la chenille par Lyonet^ et
tout recemment dans le hanneton par M. Straus, et cependant
plus ou moins difFerente dans chaque insecte, I'imagination
commencera a concevoir quelque chose de cette richesse ef-
frayante, et de ces millions de millions de parties, et de parties
de parties, toujours correlatives, toujours en harmonie, qui con-
stituent le grand ouvrage de la nature." (Histoire des Progres
des Sciences Naturclles, iv. 145.)

002 THE REPRODUCTIVE FUNCTIONS.

vision is likewise made for the establishment of
those parts which are to give mechanical sup-
port to each organic system in proportion as it
is formed ; while the foundations are also pre-
paring ft ' endowments of a higher kind, by the
early developement of the organs of the external
senses, the functions of which so essentially
minister to the future expansion of the intellec-
tual faculties, embracing a wide range of per-
ceptions and of active powers. Thus in the
early, as well as in all the subsequent periods of
life, the objects of nature vary as the respective
necessities of the occasion change. At first, all
the energies of vitality are directed to the raising
of the fabric, and to the extension of those
organs which are of greatest immediate utility ;
but still having a prospective view to further and
more important ends. For the accomplishment
of this primary object unremitting exertions are
made, commensurate with the magnitude of the
design, and giving rise to a quick succession of
varied forms, both with regard to the shape of
each individual organ, and to the general aspect
of the whole assemblage.

In the phenomena of their early evolution.
Plants and Animals present a striking contrast,
corresponding to essential differences in the
respective destinations of these two orders of
beings. The primary object of vegetable struc-
tures appears to be the establishment of the

ORGANIC DEVELOPEMENT. 603

functions of nutrition ; and we accordingly find
that whenever the seed begins to germinate, the
first indication of developement is the appear-
ance of the part called the plumula, which is a
collection of feathery fibres, bursting from the
enveloping capsule of the germ, and which,
whatever may have been its original position,
proceeds immediately to extend itself vertically ;
while, at the same time, slender filaments,
or radicles, shoot out below to form the roots.
Thus early are means provided for the absorp-
tion and the aeration of the nutrient matter,
which is to constitute the materials for the
subsequent growth of the plant, and for the
support and protection of the organs by which
these processes are to be carried on. But animal
vitality, being designed to minister to a higher
order of endowments, is placed in subordination
to a class of functions, of which there exists no
trace in vegetables, namely, those of the nervous
system. By attentively watching the earliest
dawn of organic formation, in the transparent
gelatinous molecule, for example, which, with its
three investing pellicles, constitutes the embryo of
a bird, (for the eggs of this class of animals best
admit of our following this interesting series of
changes,) the first opaque object discoverable by
the eye is a small dark line, called the primitive
trace, formed on the surface of the outermost
pellicle. Two ridges then arise, one on each

004 THE REPRODUCTIVE FUNCTIONS.

side of this dark line* ; and by the union of
their edges, they soon form a canal, containing a
deposit of semi-fluid matter, which, on acquiring
greater consistence and opacity, discloses two
slender and delicate threads, placed side by
side, and parallel to one another, but separated
by a certain space. These are the rudiments of
the spinal cord, or the central organ of nervous
power, on the endowments of which the whole
character of the being to be formed depends.
We may next discern a number of parallel equi-
distant dots, arranged in two rows, one on the
outer side of each of the filaments already no-
ticed : these are the rudiments of the vertebrae,
parts which will afterwards be wanted for giving
protection to the spinal marrow, and which soon
form, for this purpose, a series of rings embracing
that organ. I

The appearance of the elementary filaments
of the spinal cord is soon followed by the deve-
lopement of its upper or anterior extremity, from
w^hich there arise three vesicles, each forming
w^hite tubercles ; these are the foundations of the
future brain. The tubercles are first arranged

* The pliccB primitivcB of Pander ; the laminee dorsales of
Baer. See a paper on embryology by Dr. Allen Thomson, in
the Edin. New Phil. Journal for 1830 and 1831.

f These rings have, by speculative physiologists, been sup-
posed to be analogous to those which form the skeleton of the
Annelida.

ORGANIC DEVELOPEMENT. 005

in pairs and in a longitudinal series, like those
we have seen constituting the permanent form
of the brain in the inferior fishes : but, in birds,
they are soon folded together into a rounded
mass ; while, in the mean time, the two filaments
of the spinal cord have approached each other,
and united into a single column, the form which
they ever after retain. Even at this early period
the rudiments of the organs of the higher senses,
(first of the eye, and next of the labyrinth of the
ear,) make their appearance ; but, on the other
hand, those of the legs and wings do not show
themselves until the brain has acquired greater
solidity and developement. The nerves which
are to connect these organs of sensation and of
motion with the spinal cord and brain are formed
afterwards, and are successively united to the
nervous centres.

Although the plan of the future edifice has
thus been sketched, and its foundations laid in
the homogeneous jelly, by the simpler efforts of
the vital powers, the elevation of the vast super-
structure demands the aid of other machinery,
fitted to collect and distribute the requisite
materials. Here, then, we might, perhaps,
expect to meet with a repetition of those vege-
tative processes, having similar objects in view,
and the adoption of analogous means for their
accomplishment ; but so widely different in cha-
racter is the whole organic economy of these two

606 THE REPRODUCTIVE FUNCTIONS.

orders of beings, that we perceive no resem-
blance in the mechanism employed for their
formation. For the purposes of animal life the
nutrient juices must be brought into active circu-
lation by means of vessels extensively pervading
the system. Nature, then, hastens to prepare
this important hydraulic apparatus, without
which the work of construction could not pro-
ceed. What may be the movements of the
transparent nutrient juices at the very earliest
period must, of course, remain unknown to us,
since we can only follow them by the eye after
the nutritive substance they contain has become
consolidated in the form of opaque globules.
These globules are at first seen to meander
through the mass, unconfined by investing ves-
sels ; presently, however, a circular vessel is dis-
covered, formed by the foldings of the membrane
of the embryo, along which the fluids undulate
backwards and forwards, without any con-
stancy.* A delicate net- work of vessels is next
formed in various parts of the area of the circle,
which are seen successively to join by the for-
mation of communicating branches, and ulti-
mately to compose larger trunks, so as to
establish a more general system of vascular orga-
nization. But increased power for carrying on

* These phenomena are similar to those which were noticed
as presented by the larvse of some insects and other inferior
animals.

ORGANIC DEVELOPEMENT. 607

this extended circulation will soon be wanted ;
and for this purpose there must be provided a
central organ of propulsion, or heart, the con-
struction of which is now commenced, at a
central point, by the folding inwards of a lamina
of the middle membrane, forming first a simple
groove, but, after a time, converted, by the
union of its outer edges, into a kind of sac,
which is soon extended into a longitudinal tube.*
The next object is to bring this tube, or rudi-
mental heart, into communication with the
neighbouring vascular trunks ; and this is effected
by their gradual elongation, till their cavities
meet, and are joined ; one set of trunks (the
future veins,) first uniting with the anterior end
of the tube ; and then another set (the future
arteries,) joining its other end. The addition of
this central tube to the vessels previously formed
completes the continuity of their course ; so that
the uniform circulation of the blood is esta-
blished in the direction in which it is ever after
to flow ; and we may now recognise this central
organ as the heart, which, under the name of the
piinctum saliens, testifies by its quick and regular
pulsations that it has already begun to exercise its
appropriate function. It is long, however, before
it acquires the form which it is permanently to

* The discovery of this fact is due to Pander. See also the
works of Rolando, WoIfF, Prevost and Dumas, and Serres.

60S THE REPRODUCTIVE FUNCTIONS.

retain ; for from being at first a mere lengthened
tube, presenting three dilatations, which are the
cavities of the future auricle, ventricle, and bulb
of the aorta, it assumes in process of time a
rounded shape, by the folding of its parts, tlie
whole of which are coiled, as it were, into a
knot ; by which means the different cavities
acquire relative situations more nearly corre-
sponding to their positions in the developed and
finished organ.

The blood-vessels, in like manner, undergo a
series of changes quite as considerable as those
of the heart, and totally altering their arrange-
ment and distribution. Serres maintains that
the primitive condition of all the organs, even
those which are generally considered as single,
is that of being double, or being formed in pairs ;
one on the right, and another exactly similar to
it on the left of the middle, or mesial plane, as
if each were the reflected image of the other.*

* A remarkable exemplification of this tendency to symmetric
duplication of organs occurs in a very extraordinary parasitic
animal, which usually attaches itself to the gills of the Cyprinus
brama, and which has been lately examined by Nordmunn, and
named by him the Diplozoon paradoxum, from its having the
semblance of two distinct animals of a lengthened shape, each
bent at an obtuse angle, and joined together in the form of the
letter X. The right and left halves of this cross are perfectly
similar in their organization, having each a complete and inde-
pendent system of vital organs ; excepting that the two alimentary
canals join at the centre of the cross to form a single cavity, or
stomach. (Annales des Sciences Naturelles, xxx, 373.)

ORGANIC DEVELOPEMENT. ()0i)

Such is obviously the permanent condition of all
the organs of sensation, and also of the appa-
ratus for locomotion ; and it has just been shown
that those portions of the nervous system which
are situated in the mesial plane, such as the
spinal cord and the brain, consisted originally
of two separate sets of parts, which are brought
together and conjoined into single organs. In
like manner we have seen that the constituent
laminae of the heart are at first double, and
afterwards form by their union a single cavity.
The operation of the same law has been traced
in the formation of those vascular trunks, situated
in the mesial plane, which are usually observed
to be single, such as the aorta and the vena
cava ; for each were originally formed by the
coalescence of double vascular trunks, running
parallel to each other, and at first separated by
a considerable interval ; then approaching each
other, adhering together, and quickly converted,
by the obliteration of the parts which are in
contact, into single tubes, throughout a consider-
able portion of their length.*

Nature, ever vigilant in her anticipations of

* These facts were first observed by Serres (Annales des Sc.
Nat. xxi. 8.), and their accuracy has been confirmed by the ob-
servations of Dr. Allen Thomson. In Reptiles this union of the
two constituent trunks of the aorta is effected only at the pos-
terior part, while the anteiior portion remains permanently
double. (See Fig. 357, vol. ii. p. 274.)

VOL. II. R R

GIO THE UEPRODUCTIVE FUNCTIONS.

the wants of the system, has accumulated round
the embryo ample stores of nutritive matter, suf-
ficient for maintaining the life of the chick, and
for the building of its frame, while it continues
in the egg, and is consequently unable to obtain
supplies from without ; yet, with the same fore-
sight of future circumstances, she delays not,
longer than is necessary for the complete esta-
blishment of the circulation, to construct the
apparatus for digestion, on which the animal is
to rely for the means of support in after life.
The alimentary canal, of which no trace exists
at an earlier period, is constructed by the for-
mation of two laminae, arising from folds of the
innermost of the pellicles which invest the
embryo ; that is, on the surface opposite to the
one which has produced the spinal marrow.
These laminae, which are originally separate,
and apart from one another, are brought toge-
ther, and by the junction or soldering of their
opposite edges, formed into a tube,* which, from
being at first uniform in diameter, afterwards
expands into several dilated portions, corre-
sponding with the cavities of the stomach, crop,
gizzard, &c. into which they are to be converted,
when the time shall come for their active em-
ployment. These new organs are, however, even
in this their rudimental state, trained to the per-

* Wolff' is the author of this discovery.

ORGANIC DEVELOrEMENT. 6*11

forniance of their proper offices ; receiving into
their cavities, through a tube temporarily pro-
vided for that purpose, the fluid of the yelk, and
preparing nourishment from it.

In the mean time, early provision is made for
the aeration of the fluids by an extensive but
temporary system of vessels, spread over the
membrane of the egg, and receiving the influ-
ence of atmospheric oxygen through the sub-
stance of the shell, which is sufficiently porous
to transmit it ; and these vessels, being brought
into communication with the circulatory system
of the chick, convey to its blood this vivifying
agent. As the lungs cannot come into use till
after the bird is emancipated from its prison,
and as it was sufficient that they should be in
readiness at that epoch, these organs are among
the last that are constructed ; and as the mecha-
nism of respiration in this class of animals does
not require the play of the diaphragm, this mus-
cular partition is only begun, but not completed,
and there is no separation between the cavities
of the thorax and the abdomen.

The succession of organic metamorphoses is
equally remarkable in the formation of the
diversified apparatus for aeration, which is re-
quired to be greatly modified, at diflerent periods,
in order to adapt it to different elements ; of this
we have already seen examples in those insects
which, after being aquatic in their larva state,

612 THE REPRODUCTIVE FUNCTIONS.

emerge from the water when they have acquired
wings ; and also in the steps of transition from
the tadpole to the frog. But similar, though less
conspicuous changes occur in the higher verte-
brated animals, during the early periods of their
formation, corresponding to the differences in
the modes of aeration employed at different
stages of developement. In the primeval con-
ditions this function is always analogous to that
of aquatic animals, and requires for its perform-
ance only the simpler form of heart already
described, consisting of a single set of cavities ;
but the system being ultimately designed to
exercise atmospheric respiration, requires to be
gradually adapted to this altered condition ; and
the heart of the Bird and the Quadruped must
be separated into two compartments, corre-
sponding to the double function it will have
to perform. For this purpose a partition wall
is built in its cavity ; and this wall is begun
around the interior circumference of the ven-
tricle, and is gradually carried on towards the
centre; there being, for a time, an aperture of
communication between the right and left cavi-
ties; but this aperture is soon closed, and the
ventricle is now effectually divided into two.
Next the auricle, which at first was single,
becomes double ; not, however, by the growth of
a partition, but by the folding in of its sides,
along a middle line, as if it were encompassed

ORGANIC DEVELOPEMENT. 613

by a cord, which was gradually tightened. In
the mean while the partition, which had divided
the ventricle, extends itself into the trunk of the
main artery, which it divides into two channels ;
and these afterwards become two separate ves-
sels; that which issues from the left ventricle
being the aorta ; and the other, which proceeds
from the right ventricle, being the pulmonary
artery ; and each of these vessels is now pre-
pared to exercise its appropriate function in the
double circulation which is soon to be estab-
lished.*

A mode of subdivision of blood vessels, very
similar to that just described, takes place in
those which are sent to the first set of organs
provided for aeration, and which resemble
branchiae. These changes may be very dis-
tinctly followed in the Batrachia ;'\ for we see,
in those animals, the trunk of the aorta under-
going successive subdivisions, by branches sent
off from it and forming loops, which extend in
length and are again subdivided, in a manner not
unlike the unravelling of the strands of a rope ;
each subdivision, however, being preceded by
the formation of a double partition in the cavity
of the tube ; so that at length the whole forms

* The principal authorities for the facts here stated are Baer
and Rolando. See the paper of Dr. Thomson already quoted.

+ See the investigations of Rusconi, and of Baer, on this
subject.

614 THE REPRODUCTIVE FUNCTIONS.

an extensive ramified system of branchial arte-
ries and veins. Still all these are merely tem-
porary structures ; for when the period of change
approaches, and the branchiae are to be super-
seded in their office, every vessel, one after
another, becomes obliterated ; and there remain
only the two original aortae, which unite into a
single trunk lower down, and from which pro-
ceed the pulmonary arteries, conveying either the
whole, or a portion of the blood, to the newly
developed respiratory organs, the lungs.

By a similar process of continued bifurcation,
or the detachment of branches in the form of
loops, new vessels are developed in other parts
of the body ; as has been particularly observed
in the finny tail, and the external gills of the
frog, and the newt, parts which easily admit of
microscopical examination.*

Progress is in the mean while making in the
building of the skeleton ; the forms of the prin-
cipal bones being modelled in a gelatinous sub-
stance, which is converted into cartilage ; begin-
ning at the surface, and gradually advancing
towards the centre of each portion or element of
the future bone ; and thus a temporary solid
and elastic scaffolding is raised, suited to the

* Such is the result of the concurring observations of Spallan-
zani, Fontana, and Dcillinger.

ORGANIC DEVELOPEMENT. ( j ! 5

yielding texture of the nascent organs : lastly,
the whole fabric is surrounded by an outer wall,
the building of which is begun from the dorsal
region, and conducted round the sides of the
body, till the two portions come to meet in the
middle abdominal line, where they are finally
united into one general and continuous integu-
ment. The eyes, which were hitherto unpro-
tected, receive special means of defence, by the
addition of eyelids, which are formed by a
further extension and folding of these inte-
guments; and the greater part of the surface
of the body gives rise to a growth of temporary
down, which, as we have seen, is provided as a
covering to the bird at the time it is ready to
quit the shell. But this hard shell, which had
hitherto afforded it protection, is now opposed to
its emancipation ; and the chick, in order to
obtain its freedom, must, by main force, break
through the walls of its prison ; its beak is, how-
ever, as yet too tender to apply the force requi-
site for that purpose. Here, again, we find
Nature expressly interposing her assistance ; for
she has caused a pointed horny projection to
grow at the end of the beak, for the special
object of giving the chick the power of batter-
ing its shell, and making a practicable breach,
through which it shall be able to creep out, and
begin its new career of life. That this horn is

616 THE REPRODUCTIVE FUNCTIONS.

provided only for this temporary use appears
from the circumstance of its falling off spon-
taneously in the course of three or four days
after it has been so employed.

But though the bird has now gained its
liberty, it is still unable to provide for its own
maintenance, and requires to be fed by its pa-
rent till it can use its wings, and has learned
the art of obtaining food. The pigeon is fur-
nished by nature with a secretion from the crop,
with which it feeds its young. In the Mammalia
the same object is provided for still more ex-
pressly, by means of glands, whose office it is to
prepare ^mVyfc ,- a fluid which, from its chemical
qualities, is admirably adapted to the powers of
the digestive organs, when they first exercise
their functions. The Cetacea have also mam-
mary glands; but as the stnicture of the mouth
and throat of the young in that class does not
appear adapted to the act of sucking, there has
always been great difficulty in understanding
how they obtain the nourishment so provided.
A recent discovery of Geoffi:'oy St. Hilaire ap-
pears to have resolved the mystery with respect
to the Delphinus globiceps, for he found that the
mammary glands of that animal contain each
a large reservoir, in which milk is accumulated,
and which the dolphin is capable, by the action
of the surrounding muscles, of emptying at once

ORGANIC DEVELOPEMENT. 017

into the mouth of its young, without requiring
from the latter any effort of suction.*

The rapid sketch which I have attempted to
draw of the more remarkable steps of the early
stages of organic developement in the higher
animals, taken in conjunction with the facts al-
ready adverted to in various parts of this Trea-
tise, and particularly those relating to ossifica-
tion, dentition, the formation of hair, of the quills
of the porcupine, of the antlers of the stag, and
of the feathers of birds, will suffice to show that
they are regulated by laws which are definite,
and preordained according to the most enlarged
and profound views of the future circumstances
and wants of the animal. The double origin of all
the parts of the frame, even those which appear
as single organs, and the order of their forma-
tion, which, in each system, commences with the
parts most remote from the centre, and proceeds
inwards, or towards the mesial plane, are among
the most singular and unexpected results of this
train of inquiries. t We cannot but be forcibly

* The account of this discovery is contained in a memoir which
was read at the " Institut," March 24, 1834.

t The first of these two laws is termed by Serres, who has
zealously prosecuted these investigations, " la loi de syrnmetrie ;"
and the second, " la loi de conjugaison." He maintains that
they are strictly applicable to all the parts of the body having a
tubular form, such as the trachea, the Eustachian tube, the canals
and perforations of bones, &c. See the preliminary discourse to

018 THE REPRODUCTIVE FUNCTIONS.

struck with the numerous forms of transition
through which every organ has to pass before
arriving at its ultimate and comparatively per-
manent condition : we cannot but wonder at the
vast apparatus which is provided and put in
action for effecting all these changes ; nor can
we overlook the instances of express contrivance
in the formation of so many temporary struc-
tures, which are set up, like the scaffold of an
edifice, in order to afford the means of trans-
porting the materials of the building in propor-
tion as they are wanted ; nor refuse to recognise
the evidence of provident design in the regular
order in which the work proceeds, every organ
growing at its appointed time, by the addition
of fresh particles brought to it by the arteries,
while others are carried away by the absorbents,
and are gradually aquiring the form which is
to qualify it for the performance of its proper
office in this vast and complicated system of
animal life.

his " Anatomie comparee du cerveau," p. 25 ; and also his se-
veral memoirs in the " Annales des Sciences Naturelles," vols. xi.
xii. xvi. and xxi.

An excellent summary of the principal facts relating to the
developement of the embryo is given by Mr. Herbert Mayo, in
the third edition of his " Outlines of Human Physiology."

(il9

Chapter III.

DECLINE OF THE SYSTEM.

To follow minutely the various steps by which
Nature conducts the individual to its state of
maturity, would engage us in details incom-
patible with the limits of the present work.
I shall only remark, in general, that during the
period when the body is intended to increase in
size, the powers of assimilation are exerted to
prepare a greater abundance of nourishment, so
that the average supply of materials rather ex-
ceeds the consumption ; but when the fabric has
attained its prescribed dimensions, the total
quantities furnished and expended being nearly
balanced, the vital powers are no longer exerted
in extending the fabric, but are employed in
consolidating and perfecting it, and in qualifying
the organs for the continued exercise of their
respective functions, during a long succession of
years.

Yet, while every function is thus maintained
in a state of healthy equilibrium, certain changes
are in progress which, at the appointed season,

620 DECLINE OF THE SYSTEM.

will inevitably bring on the decline, and ulti-
mate destruction of the system.* The process
of consolidation, begun from the earliest period
of developement, is still advancing, and is pro-
ducing in the fluids greater thickness, and a
reduction of their total quantity ; and in the
solids, a diminution in the proportion of gelatin,
and the conversion of this element into albumen.
Hence, all the textures acquire increasing so-
lidity, the cellular substance becomes firmer and
more condensed, and the solid structures more
rigid and inelastic : hence the tendons and liga-
mentous fibres growing less flexible, the joints
lose their suppleness, and the contractile power

* It would appear from the researches of De Candolle, that
the vegetable system is not, like the animal, subject to the
destructive operation of internal causes ; for the agents which
destroy vegetable life are always extraneous to its economy.
Each individual tree is composed of an accumulation of the shoots
of every successive year since the commencement of its growth ;
and although, from the continued deposition of lignin, and the
consequent obliteration of many of its cells and vessels, the vi-
tality of the interior wood may be destroyed, and it then becomes
liable to decay by the action of foreign agents, yet the exterior
layers of the liber still vegetate with undiminished vigour; and,
imless injured by causes extraneous to its own system, the life of
the tree will continue to be sustained for an indefinite period.
If, on the other hand, we were to regard each separate shoot as an
individual organic body, and every layer as constituting a dis-
tinct generation of shoots, the older being covered and enclosed
in succession by the younger, the great longevity of a tree would,
on this hypothesis, indicate only the permanence of the species,
not the indefinitely protracted duration of the individual plant.

DECLINE OF THE SYSTEM. 0*21

being also impaired, the muscles act more tardily
as well as more feebly, and the limbs no longer
retain the elastic spring of youth. The bones
themselves grow harder and more brittle ; and
the cartilages, the tendons, the serous mem-
branes, and the coats of the blood-vessels, acquire
incrustations of ossific matter, which interfere
with their uses. Thus are all the progressive
modifications of structure tending, slowly but
inevitably, to disqualify the organs for the due
performance of their functions.

Among the most important of the internal
changes consequent on the progress of age are
those which take place in the vascular system.
A large proportion of the numerous arteries,
which were in full activity during the building
of the fabric, being now no longer wanted, are
thrown, as it were, out of employment ; they, in
consequence, contract, and becoming impervious,
gradually disappear. The parts of the body, no
longer yielding to the power applied to extend
them, oppose a gradually increasing resistance
to the propelling force of the heart ; while, at
the same time, this force, in common with all the
others, is slowly diminishing. Thus do the vital
powers become less equal to the demands made
upon them ; the waste of the body exceeds the
supply, and a diminution of energy becomes
apparent in every function.

Such are the insensible gradations by which.

62'2 DECLINE Ol' THE SYSTEM.

while gliding down the stream of time, we lapse
into old age, which insidiously steals on us
before we are aware of its approach. But the
same provident power which presided at our
birth, which superintended the growth of all the
organs, which infused animation into each as
they arose, and which conducted the system
unimpaired to its maturity, is still exerted in
adjusting the conditions under which it is
placed in its season of decline. New arrange-
ments are made, new energies are called forth,
and new resources are employed, to accommo-
date it to its altered circumstances, to prop the
tottering fabric, and retard the progress of its
decay. In proportion as the supply of nutritive
materials has become less abundant, a more
strict economy is practised with regard to their
disposal ; the substance of the body is hus-
banded with greater care ; the absorbent vessels
are employed to remove such parts as are no
longer useful; and when all these adjustments
have been made, the functions still go on for
a considerable length of time without material
alteration.

The period prescribed for its duration being
at length completed, and the ends of its exist-
ence accomplished, the fabric can no longer
be sustained, and preparation must be made
for its inevitable fall. In order to form a cor-
rect judgment of the real intentions of nature,

DECLINE OF THE SYSTEM. 623

with regard to this last stage of life, its pheno-
mena must be observed in cases where the sys-
tem has been wholly entrusted to the operation
of her laws. When death is the simple conse-
quence of age, we find that the extinction of the
powers of life observes an order the reverse of
that which was followed in their evolution. The
sensorial functions, which were the last perfected,
are the first which decay ; and their decline is
found to commence with those mental faculties
more immediately dependent on the physical
conditions of the sensorium, and more especially
with the memory, which is often much impaired,
while the judgment remains in full vigour. The
next faculties which usually suffer from the effects
of age are the external senses ; and the failure of
sight and of hearing still farther contributes to
the decline of the intellectual powers, by with-
drawing many of the occasions for their exercise.
The actual demolition of the fabric commences
whenever there is a considerable failure in the
functions of assimilation ; but the more imme-
diate cause of the rapid extinction of life is
usually the impediment which the loss of the
sensorial power, necessary for maintaining the
movements of the chest, creates to respiration.
The heart, whose pulsations gave the first indi-
cations of life in the embryo, generally retains
its vitality longer than any other organ ; but its
powers being dependent on the constant oxida-

624 DECLINE OF THE SYSTEM.

tion of the blood in the lungs, cannot survive
the interruption of this function ; and on the
heart ceasing to throb, death may then be con-
sidered as complete in every part of the system.
It is an important consideration, with reference
to final causes, that generally long before the
commencement of this

" Last scene of all,
That ends this strange eventful history,"

the power of feeling has wholly ceased, and the
physical struggle is carried on by the vital powers
alone, in the absence of all consciousness of the
sentient being, whose death may be said to pre-
cede, for some time, that of the body. In this,
as well as in the gradual decline of the sensorial
faculties, and the consequent diminution both of
mental and of physical sensibility in advanced
age, we cannot fail to recognise the wise ordi-
nances of a superintending and beneficent pro-
vidence, kindly smoothing the path along which
we descend the vale of life, spreading a narcotic
mantle over the bed of death, and giving to the
last moments of departing sensation the tran-
quillity of approaching sleep.

625

Chapter IV.

UNITY OF DESIGN.

The inquiries on Animal and Vegetable Physi-
ology in which we have been engaged, lead to
the general conclusion that unity of design and
identity of operation pervade the whole of nature ;
and they clearly point to one Great and only
Cause of all things, arrayed in the attributes of
infinite power, wisdom, and benevolence, whose
mighty works extend throughout the boundless
regions of space, and whose comprehensive plans
embrace eternity.

In examining the manifold structures and
diversified phenomena of living beings, we can-
not but perceive that they are extensively, and
perhaps universally connected by certain laws of
Analogy ; a principle, the recognition of which
has given us enlarged views of a multitude
of important facts, which would otherwise have
remained isolated and unintelligible. Hence
naturalists, in arranging the objects of their
study, according to their similarities and ana-
logies, into classes, orders and genera, have but

VOL. II. s s

626 UNITY OF DESIGN.

followed the footsteps of Nature herself, who in
all her operations combines the apparently op-
posite principles of general resemblance, and of
specific variety ; so that the races which she
has united in the same group, though possessed
of features individually different, may easily be
recognised by their family likeness, as the off-
spring of a common parent.

" Facies non omnibus una ;
Nee diversa tamen ; qualem deeet esse sororum."

We have seen that in each of the two great
divisions, or kingdoms of organic nature, the
same general objects are aimed at, and the same
general plans are devised for their accomplish-
ment ; and also that in the execution of those
plans similar means and agencies are employed.
In each division there prevails a remarkable
uniformity in the composition and properties of
their elementary textures, in the nature of their
vital powers, in the arrangement of their organs,
and in the laws of their production and develope-
ment. The same principle of analogy may be
traced, amidst endless modifications of detail, in
all the subordinate groups into which each
kingdom admits of being subdivided, both in
respect to the organization and functions of
the objects comprehended in each assemblage;
whether we examine the wonders of their me-
chanical fabric, or study the series of processes by
which nutrition, sensation, voluntary motipn, and

UNITY OF DESIGN. 627

reproduction are effected. To specify all the
examples which might be adduced in confirma-
tion of this obvious truth is here unnecessary ;
for it would be only to repeat the numerous facts
already noticed in every chapter of this treatise,
relative to each natural group of living beings ;
and it was, indeed, chiefly by the aid of such
analogies, that we were enabled to connect and
generalize those facts. We have seen that, in
constructing each of the divisions so established,
Nature appears to have kept in view a certain
definite type, or ideal standard, to which, amidst
innumerable modifications, rendered necessary
by the varying circumstances and diff*erent des-
tinations of each species, she always shows a
decided tendency to conform. It would almost
seem as if, in laying the foundations of each or-
ganized fabric, she had commenced by taking
an exact copy of this primitive model ; and, in
building the superstructure, had allowed herself
to depart from the original plan only for the pur-
pose of accommodation to certain specific and
ulterior objects, conformably with the destina-
tion of that particular race of created beings.
Such, indeed, is the hypothetical principle,
which, under the title of unity of composition,
has been adopted, and zealously pursued in all
its consequences, by many naturalists of the
highest eminence on the continent. As the
facts on which this hypothesis is supported, and
the views which it unfolds, are highly deserving

028 UNITY OF DESIGN.

of attention, I shall here briefly state them ; but
in so doing I shall beg to premise the caution
that these views should for the present be re-
garded as hypothetical, and as by no means pos-
sessing the certainty of philosophical generali-
zations.

The hypothesis in question is countenanced,
in the first place, by the supposed constancy
with which, in all the animals belonging to the
same natural group, we meet with the same con-
stituent elements of structure, in each respective
system of organs ; notwithstanding the utmost
diversity which may exist in the forms of the
organs, and in the uses to which they are ap-
plied. This principle has been most strikingly
exemplified in the osteology of vertebrated ani-
mals : but its truth is also inferred from the
examination of the mechanical fabric of Insects,
Crustacea, and Arachnida ; and it appears to
extend also to the structures subservient to other
functions, and particularly those of the nervous
system. Thus Nature has provided foj* the
locomotion of the serpent, not by the creation
of new structures, foreign to the type of the
vertebrata, but by employing the ribs in this
new office ; and in giving wings to a lizard,
she has extended these same bones to serve as
supports to the superadded parts. In arming
the elephant with tusks, she has merely caused
two of the teeth in the upper jaw to be developed

UNITY OF DESIGN. 629

into these formidable weapons ; and in providing
it with an instrument of prehension has only
resorted to a greater elongation of the snout.

The law of GradatioHy in conformity to which
all the living, together with the extinct races, of
organic nature, arrange themselves, more or less,
into certain regular series, is one of the conse-
quences which have been deduced from the
hypothesis we are considering. Every fresh
copy taken of the original type is supposed to
receive some additional extension of its faculties
and endowments by the graduated developement
of elements, which existed in a latent form in
the primeval germ, and which are evolved, in
succession, as nature advances in her course.
Thus we find that each new form which arises,
in following the ascending scale of creation,
retains a strong affinity to that which had
preceded it, and also tends to impress its own
features on those which immediately succeed ;
and thus their specific differences result merely
from the different extent and direction given to
these organic developements ; those of inferior
races proceeding to a certain point only, and
there stopping ; while in beings of a higher
rank they advance farther, and lead to all the
observed diversities of conformation and endow-
ments.

It is remarked, in further corroboration of
these views, that the animals which occupy the

630 UNITY OF DESIGN.

highest stations in each series possess, at the
commencement of their existence, forms exhibit-
ing a marked resemblance to those presented in
the permanent condition of the lowest animals
in the same series ; and that, during the pro-
gress of their developement, they assume, in
succession, the characters of each tribe, corre-
sponding to their consecutive order in the
ascending chain ; so that the peculiarities which
distinguish the higher animal, on its attaining
its ultimate and permanent form, are those
which it had received in its last stage of embry-
onic evolution. Another consequence of this
hypothesis is that we may expect occasionally
to meet, in inferior animals, with rudimental or-
gans, which from their imperfect developement
may be of little or no use to the individual, but
which become available to some superior species,
in which they are sufficiently perfected. The
following are among the most remarkable facts
in illustration of these propositions.

In the series of Articulated Animals, of which
the Annelida constitute the lowest, and winged
Insects the highest terms, we find that the larvae
of the latter are often scarcely distinguishable,
either in outward form, or in internal organiza-
tion, from Vermes of the lower orders ; both
being equally destitute of, or but imperfectly
provided with external instruments of locomo-
tion ; both having a distinct vascular circulation.

UNITY OF DESIGN. 631

and multiple organs of digestion ; and the
central filaments of the nervous system in both
being studded with numerous pairs of equidis-
tant ganglia. In the worm all these features
remain as permanent characters of the order : in
the insect they are subsequently modified and
altered during its progressive metamorphoses.
The embryo of a crab resembles in appearance
the permanent forms of the Myriapoda^ and of
the lower animals of its own class, but acquires,
in the progress of its growth, new parts ; while
those already evolved become more and more
concentrated ; passing, in their progress, through
all the forms of transition which characterise
the intermediate tribes of Crustacea; till the
animal attains its last state, and then exhibits
the most developed condition of that particular
type.*

However different the conformations of the
Fish, the Reptile, the Bird, and the Warm
blooded Quadruped, may be at the period of
their maturity, they are scarcely distinguishable
from one another in their embryonic state; and
their early developement proceeds for some time

* This curious analogy is particularly observable in the suc-
cessive forms assumed by the nervous system, which exhibits a
gradual passage from that of the Talitrus, to its ultimate great-
est concentration in the Maia. (See Figures 439 and 441,
p. 543 and 545.) Milne Edwards has lately traced a similar
progression of developement in the organs of locomotion of the
Crustacea. (Annales des Sciences Naturelles; xxx, 354.)

632 UNITY OF DESIGN.

in the same manner. They all possess at first
the characters of aquatic animals ; and the
Frog even retains this form for a considerable
period after it has left the egg. The young
tadpole is in truth a fish, whether we regard
the form and actions of its instruments of pro-
gressive motion, the arrangement of its organs
of circulation and of respiration, or the condition
of the central organs of its nervous system. We
have seen by what gradual and curious transi-
tions all these aquatic characters are changed
for those of a terrestrial quadruped, furnished
with limbs for moving on the ground, and with
lungs for breathing atmospheric air; and how
the plan of circulation is altered from branchial
to pulmonary, in proportion as the gills wither
and the lungs are developed. If, while this
change is going on, and while both sets of
organs are together executing the function of
aeration, all further dev elopement were pre-
vented, we should have an amphibious animal,
fitted for maintaining life both in air and in
water. It is curious that this precise condition
is the permanent state of the Siren and the
Proteus; animals which thus exemplify one of
the forms of transition in the metamorphoses of
the Frog.

In the rudimental form of the feet of serpents,
which are so imperfectly developed as to be
concealed underneath the skin, and to be use-

UNITY OF DESIGN. 633

less as organs of progressive motion, we have an
example of the first stage of that process, which,
when carried farther in the higher animals,
gives rise to the limbs of quadrupeds, and which
it would almost seem as if nature had instituted
with a prospective view to these more improved
constructions. Another, and a still more re-
markable instance of the same kind occurs in
the rudimental teeth of the young of the Whale,
which are concealed within the lower jaw, and
which are afterwards removed, to give place to
the curious filtering apparatus, which occupies
the roof of the mouth, and which nature has
substituted for that of teeth ; as if new objects,
superseding those at first pursued, had arisen
in the progress of developement.

Birds, though destined to a very different
sphere of action from either fishes or reptiles, are
yet observed to pass, in the embryonic stage of
their existence, through forms of transition, which
successively resemble these inferior classes.
The brain presents, in its earliest formation, a
series of tubercles, placed longitudinally, like
those of fishes, and only assuming its proper
character at a later period. The respiratory
organs are at first branchiae, placed, like those of
fishes, in the neck, where there are also found
branchial apertures similar to those of the lam-
prey and the shark ; and the heart and great
vessels are constructed like those of the tadpole.

634 UNITY OF DESIGN.

with reference to a branchial circulation. In
their conversion to the purposes of aerial respi-
ration, they undergo a series of changes pre-
cisely analogous to those of the tadpole.

Mammalia, during the early periods of their
dev elopement, are subjected to all the transform-
ations which have been now described ; com-
mencing with an organization corresponding to
that of the aquatic tribes ; exhibiting not only
branchiae, supported on branchial arches, but
also branchial apertures in the neck ; and thence
passing quickly to the conditions of structure
adapted to a terrestrial existence. The deve-
lopement of various parts of the system, more
especially of the brain, the ear, the mouth, and
the extremities, is carried still farther than in
birds. Nor is the human embryo exempt from
the same metamorphoses ; possessing at one
period branchiae and branchial apertures similar
to those of the cartilaginous fishes,* a heart with
a single set of cavities, and a brain consisting of
a longitudinal series of tubercles ; next losing
its branchiae, and acquiring lungs, while the
circulation is yet single, and thus imitating the
condition of the reptile ; then acquiring a double
circulation, but an incomplete diaphragm, like
birds ; afterwards, appearing like a quadruped,

* These facts are given on the authorities of Rathke, Baer,
Huschke, Breschet, &c. Ann. des Sc. Naturelles, xv. 266. See
also the paper of Dr. A. Thomson, already quoted.

UNITY OF DESIGN. (iS5

with a caudal prolongation of the sacrum, and
an intermaxillary bone ; and lastly, changing its
structure to one adapted to the erect position,
accompanied by a great expansion of the cerebral
hemispheres, which extend backwards so as
completely to cover the cerebellum. Thus does
the whole fabric arrive, by a gradual process of
mutation, at an extent of elaboration and refine-
ment, which has been justly regarded as con-
stituting a climax of organic developement,
unattainable by any other race of terrestrial
beings.*

It must, I think, be admitted that the analo-
gies, on which the hypothesis in question is
founded, are numerous and striking ; but great
care should be taken not to carry it farther than
the just interpretation of the facts themselves

* A popular opinion has long prevailed, even among the
well informed, that mis-shapen or monstrous productions, or
lusus naturce, as they were termed, exhibit but the freaks of
nature, who was believed, on these occasions, capriciously to
abandon her usual course, and to amuse herself in the production
of grotesque beings, without any special object. But it is now
found that all defective formations of this kind are occasioned
by the imperfect developement of some parts of the embryo,
while the natural process is carried on in the rest of the system ;
and thus it happens that a resemblance may often be traced, in
these malformations, with the type or the permanent condition
of some inferior animal. Hence all these apparent anomalies
are, in reality, in perfect harmony with the established laws of
organic developement, and afford, indeed, striking confirmations
of the truth of the theory here explained.

636 UNITY OF DESIGN.

may warrant. It should be borne in mind
that these facts are few, compared with the
entire history of animal developement ; and that
the resemblances which have been so ingeniously
traced, are partial only, and fall very short of
that universality, which alone constitutes the
solid basis of a strictly philosophical theory.
Whatever may be the apparent similarity be-
tween one animal and another, during different
periods of their respective developements, there
still exist specific differences, establishing be-
tween them an impassable barrier of separation,
and effectually preventing any conversion of one
species into another, however nearly the two
may be mutually allied. The essential charac-
ters of each species, amidst occasional varieties,
remain ever constant and immutable. Although
gradations, to a greater or less*extent, may be
traced among the races both of plants and
animals, yet in no case is the series strictly
continuous ; each step, however short, being in
reality an abrupt transition from one type of
conformation to another. In many instances the
interval is considerable ; as for example in the
passage from the invertebrate to the vertebrated
classes; and indeed in every instance where
great changes in the nature and arrangement of
the functions take place.* It is in vain to allege

* See a paper on this subject, by Cuvier, in the Ann. des
Sciences Naturelles, xx. 241.

UNITY OF DESIGN. 037

that the original continuity of the series is indi-
cated by a few species presenting, in some res-
pects, intermediate characters, such as the Orni-
thorliynchus^ between birds and mammaUa, and
the Cetacea^ between fishes and warm blooded
quadrupeds ; for these are but detached links of
a broken chain, tending, indeed, to prove the
unity of the designs of Nature, but showing also
the specific character of each of her creative
efforts. The pursuit of remote and often fanciful
analogies has, by many of the continental physi-
ologists, been carried to an unwarrantable and
extravagant length ; for the scope which is given
to the imagination in these seductive specu-
lations, by leading us far away from the path of
philosophical induction, tends rather to obstruct
than to advance the progress of real knowledge.
By confining our inquiries to more legitimate
objects, we shall avoid the delusion into which
one of the disciples of this transcendental school
appears to have fallen, when he announces, with
exultation, that the simple laws he has discovered
have now explained the universe ; * nor shall we
be disposed to lend a patient ear to the more
presumptuous reveries of another system-builder,
who, by assuming that there exists in organized
matter an inherent tendency to perfectibility,

* " L'univers est explique, et nous le voyons ; c'est un petit
nombre de principes generaux et feconds qui nous en ont donne
la clef." Serres, Ann. des Sc. Nat. xi. 50.

038 UNITY OF DESIGN.

fancies that he can supersede the operations of
Divine agency.*

Very different was the humble spirit of the
great Newton, who, struck with the immensity
of nature, compared our knowledge of her ope-
rations, into which he had himself penetrated so
deeply, to that of a child gathering pebbles on
the sea shore. Compared, indeed, with the
magnitude of the universe, how narrow is the
field of our perceptions, and how far distant
from any approximation to a knowledge of the
essence of matter, of the source of its powers, or
even of the ultimate configurations of its parts !
How remote from all human cognizance are the
intimate properties of those imponderable agents.
Light, Heat, and Electricity, which pervade
space, and exercise so potent ^ control over all
the bodies in nature! Doubtless there exist

* Allusion is here made to the celebrated theory of Lamarck,
as exposed in his " Philosophic Zoologique." He conceives
that there was originally no distinction of species, but that each
race has, in the course of ages, been derived from some other,
less perfect than itself, by a spontaneous effort at improvement ;
and he supposes that infusorial animalcules, spontaneously
formed out of organic molecules, gave birth, by successive trans-
formations, to all other animals now existing on the globe. He
believes that tribes, originally aquatic, acquired by their own
efforts, prompted by their desire to walk, both feet and legs,
fitting them for progression on the ground; and that these
members, by the long continued operation of the wish to fly,
were transformed into wings, adapted to gratify that desire. If
this be philosophy, it is such as might have emanated from the
college of L?»puta.

UNITY OF DESIGN. 639

around us, on every side, influences of a still
more subtle kind, which " eye hath not seen, nor
ear heard," neither can it enter into the heart or
imagination of man to conceive. How scanty is
our knowledge of the mind ; how incompre-
hensible is its connexion with the body ; how
mysterious are its secret springs, and inmost
workings ! What inefi'able wonders would burst
upon us, were we admitted to the perception
of the spiritual world, now encompassed by
clouds impervious to mortal vision !

The Great Author of our being, who, while he
has been pleased to confer on us the gift of
reason, has prescribed certain limits to its
powers, permits us to acquire, by its exercise, a
knowledge of some of the wondrous works of his
creation, to interpret the characters of wisdom
and of goodness with which they are impressed,
and to join our voice to the general chorus
which proclaims "His Might, Majesty, and Do-
minion." From the same gracious hand we also
derive that unquenchable thirst for knowledge,
which this fleeting life must ever leave unsatis-
fied ; those endowments of the moral sense,
with which the present constitution of the world
so ill accords ; and that innate desire of per-
fection which our present frail condition is so
inadequate to fulfil. But it is not given to
man to penetrate into the counsels, or fathom
the designs of Omnipotence ; for in directing his

640 UNITY OF DESIGN.

views into futurity, the feeble light of his reason
is scattered and lost in the vast abyss. Although
we plainly discern intention in every part of the
creation, the grand object of the whole is placed
far above the scope of our comprehension. It is
impossible, however, to conceive that this enor-
mous expenditure of power, this vast accumula-
tion of contrivances and of machinery, and this
profusion of existence resulting from them, can
thus, from age to age, oe prodigally lavished,
without some ulterior end. Is Man, the favoured
creature of nature's bounty, " the paragon of
animals," whose spirit holds communion with
celestial powers, formed but to perish with the
wreck of his bodily frame? Are generations
after generations of his race doomed to follow in
endless succession, rolling darkly down the
stream of time, and leaving no track in its path-
less ocean ? Are the operations of Almighty
power to end with the present scene ? May we
not discern, in the spiritual constitution of man
the traces of higher powers, to which those he
now possesses are but preparatory ; some embryo
faculties which raise us above this earthly habi-
tation ? Have we not in the imagination, a power
but little in harmony with the fetters of our
bodily organs ; and bringing within our view
purer conditions of being, exempt from the illu-
sions of our senses and the infirmities of our
nature, our elevation to which will eventually

UNITY OF DESIGN. 641

prove that all these unsated desires of know-
ledge, and all these ardent aspirations after
moral good, were not implanted in us in vain ?

Happily there has been vouchsafed to us,
from a higher source, a pure and heavenly light
to guide our faltering steps, and animate our
fainting spirit, in this dark and dreary search ;
revealing those truths which it imports us
most of all to know ; giving to morality higher
sanctions ; elevating our hopes and our affections
to nobler objects than belong to earth, and
inspiring more exalted themes of thanksgiving
and of praise.

VOL. II. T T

INDEX.

Abdomen of insects, i. 324.
Aberration, chromatic, ii. 474.
Aberration of parallax, ii. 459,

472.
Aberration, spherical, ii. 458,

471.
Absorption, vegetable, ii. 19,22.
Absorption, animal, ii. 12,351.
Absorption, lacteal, ii. 226.
Absorption of shell, i. 239.
Acalepha, i. 192 ; ii. 293.
Acarus, i. 297.
Achatina zebra, i. 242.
Achromatic power, ii. 475.
Acephala, i. 217 ; ii. 1 16, 300.
Acetabulum, i. 405.
Acid secretions, ii. 46.
Acrida, ii. 214.
Acridium, i. 333.
Acoustic principles, ii. 414.
Actinia, i. 182, 197; ii. 99,

383, 477, 586, 592.
Adanson, i. 253.
Adipose substance, i. 123.
Adductor muscle, i. 218.
Aeration of sap, ii. 29.
Aeration, animal, ii. 34, 611.
jEschna, i. 351.
Affinities, organic, ii. 7.
Agaon, ii. 384.
Agastric medusae, ii. 92.
Age of trees, i. 85.
Age, effects of, ii. 620.
Agouti, i. 498.
Agrion, ii. 240.
Air-bladder, i. 429.

Air cells of plants, i. 76.
Air cells of birds, ii. 329.
Air, rarefaction of, in birds, i.

557.
Air tubes in plants, i. 73.
Albumen, i. 105.
Alburnum, i. 85; ii. 41.
Algse, ii. 19.

Alimentary canal, ii. 107.
Alimentary canal, formation of,

ii. 610.
Alitrunk, i. 345.
Alligator, i. 458, 460 ; ii. 409.
Amble, i. 495.
Ambulacra, i. 201.
Amici, i. 77 ; ii. 50.
Amphibia, i. 436, 487.
Amphisbsena, i. 447, 448.
Amphitrite, i. 281.
Anabas, ii. 306.
Analogy, law of, i. 49 ; ii. 625.
Anarrhichas, ii. 128.
Anchylosis, i. 382.
Ancillaria, i. 241.
Anemone, sea, i. 198.
Angler, i. 422 ; ii. 390.
Anguis, i. 447, 454.
Animal functions, i. 39.
Animal organization, i. 96.
Animalcules. See Infusoria.
Annelida, i. 269 ; ii. 249, 297,

383, 479.
Annular vessels, i. 74.
Anodon, i. 231.
Ant, ii. 386, 483,486.
Ant-eater, i. 524 ; ii. 134,

644

INDEX.

Antelope, ii. 147, 402.
Antelope, horn of, i. 515.
Antennae, i. 288 ; ii. 383.
Antennulse, ii. 124.
Anther, ii. 596.
Anthias, ii. 306.
Anthophora, i. 352.
Antipathes, i. 166.
Antler of deer, i. 509.
Antrum maxillare, ii. 400.
Aorta, ii. 108, 609.
Aphodius, ii, 384.
Aphrodite, ii. 102, 125, 298.
Aplysia, ii. 126, 168, 551.
Apodes, i. 423.
Apterous insects, i. 296.
Aquatic animals, i. 146.
Aquatic plants, ii. 48.
Aquatic larvse, i. 309.
Aquatic insects, i. 335.
Aquatic birds, i. 592.
Aquatic respiration, ii. 293.
Aqueous humour, ii. 463.
Arachnida, i. 282; ii. 121,

316, 389, 485, 587.
Aranea. See Spider.
Arbor vitee, ii. 559.
Arenicola, i. 277 ; ii. 295.
Argonauta, i. 265.
Aristotle, i. 149 ; ii. 559.
Aristotle, lantern of, ii. 119.
Arm, human, i. 544.
Armadillo, ii. 382.
Arteries, i. 41 ; ii. 108.
Articulata, i. 268.
Ascalaphus, ii. 384.
Ascaris, ii. 114, 540.
Ascidia, i. 137; ii. 297.
Ass, i. 516.

Assimilation, i. 41 ; ii. 11.
Astacus, ii. 435, 491.
Asterias, i. 200; ii. 100, 208,

235, 297, 383, 549, 586.
Ateles, i. 399, 534.
Atlas of Lion, i. 529.
Atmosphere, purification of, ii.

35.
Atmospheric respiration, ii. 310.

Atriplex, ii. 48.

Audouin, I 290, 323, 324;

ii. 244, 317, 542.
Audubon, ii. 407.
Auricle, ii. 108, 259.
Auricula, i. 251.
Avicula, i. 235.
Axillge of plants, i. 90 ; ii.

589.
Axelotl, ii. 324.

Babiroussa, ii. 141.
Bacculite, i. 267.
Baer, ii. 479,613,634.
Baker, ii. 478.
Balsena. See Whale.
Balance of affinities, ii. 7.
Balistes, i. 432.
Banks, i. 453.
Barbels of fish, ii. 390.
Bark, formation of, i. 86.
Barnacle, i. 257 ; ii. 296.
Bat, i. 551 ; ii. 136, 567.
Batrachia, i. 436 ; ii. 597.
Batrachospermum, ii. 48.
Bauer, i. 63.
Bear, ii. 146.
Beard of oyster, ii. 300.
Beaver, i. 524; ii. 149, 186,

191.
Bee, i. 351 ; ii. 387.
Belchier, i. 384.
Bell (Sir C), ii. 535.
BeZZ (Thomas), i. 482; ii. 409.
Bellini, ii. 393.
Berberis, i. 127.
Berkeley, ii. 520.
Beroe, i. 194, 203.
Berzelius, ii. 18.
Bicuspid teeth, ii. 144.
Bipes canaliculatus, i. 457.
Birds, i. 554; ii. 130, 328,

404, et passim.
Blind-worm, i. 454, 457.
Blood, ii. 334.
Blood-vessels, ii. 281.
Blumenbach, ii. 426.
Boa, i. 447, 448.

INDEX.

64.5

Boar, i. 56; ii. 141, 161.
Bombyx, i. 300, 304, 312 ;

ii. 384, 486.
Bone, i. Ill, 365, 375.
Bon7iet,u 53; ii. 17, 79, 92,

252, 478.
Borelli, i. 588.
Bostock, ii. 333.
Bound of deer, i. 495.
Bowerbank, ii. 241.
Boyle, ii. 16.
Bractese, i. 94.
Bradypus, i. 481 ; ii. 284.
Brain, i. 35; ii. 366, 555, 515.
Brain, formation of, ii. 605.
Branchiae, ii. 267, 293, 299.
Brassica, ii. 48, 53.
Braula, ii. 483.
Breschet, ii. 427.
Brewster, i. 232 ; ii. 472, 495.
Brocken, spectre of, ii. 533.
Broussonnet, ii. 587.
Bruguiere, i. 248.
Bryophyllum, ii. 586.
Buccinum, i. 215, 229, 242;

ii. 126, 301.
Buckland, ii. 206.
Buds, i. 86 ; ii. 588.
Buffon, i. 185; ii. 530, 591.
Bulb of hair, i. 117.
Bulb of feather, i. 577.
Bulbus arteriosus, ii. 273.
Bulbulus glandulosus, ii. 185.
Bulimus, i. 249.
Bulla, ii. 168.
Burrowing of the mole, i. 525.

Cabbage, ii. 48, 53.
Cachalot, i. 484 ; ii. 142.
Cseca, ii. 101,206.
Csecilia, ii. 497.
Calamary, i. 261.
Callionymus, ii. 503.
Calosoma, i. 320.
Cambium, ii. 40.
Camel, i. 108; ii. 176, 198.
Camelopard, i. 481, 498; ii.
135.

Camera obscura, ii. 458.
Camerated shells, i. 265.
Campanularia, ii, 234.
Camper, ii. 437, 443, 561.
Canada rat, ii. 178.
Cancelli, i. 374.
Cannon bone, i. 505.
Capibara, ii. 160.
Capillaries, ii. 263.
Capsular ligaments, i. 106.
Caput Medusae, i. 212.
Carapace, i. 290, 463.
Carbon, non-absorption of, ii.

17.
Carbonic acid, ii. 30, 337.
Cardia, ii. 182.
Cardium, i. 131, 221, 222,

224.
Carduus, i. 127.
Carinated sternum, i. 5QQ.
Carlisle, i. 426, 434 ; ii. 285,

567.
Carnivora, i. 528 ; ii. 66, 145.
Carp, i. 411,429.
Carpus, i. 405.
Cartilage, i. 109.
Caruncle, lacrymal, ii. 468.
Cams, i. 366; ii. 208, 219,

240, 252, 505.
Cassowary, i. 586 ; ii. 224.
Cat, ii. 392, 505.
Caterpillar, i. 305, 315; ii.484.
Caudal vertebrae, i. 404.
Cavolini, i. 159.
Celandine, ii. 48.
Cells of plants, i, 66, 69.
Cellular texture, animal, i. 99.
Centaurea, i. 127.
Cephalic ganglion, ii. 541.
Cephalo-thorax, i. 282.
Cephalopoda, i. 258 ; ii. 220,

551.
Cerambyx, i. 313, 328 ; ii.

311, 313, 384.
Cercaria, i. 186 ; ii. 479.
Cerebellum, ii. 555.
Cerebral ganglion, ii. 541.
Cerebral hemispheres, ii. 556.

646

INDEX.

Cerithium, i. 249.
Ceroxylon, ii. 48.
Cetacea, i. 481, 482; ii. 142,

176, 193, 442, 555, 616.
Chabrier, i. 108, 346.
Chain of being, i. 53 ; ii. 629.
Chalcides, i. 448, 457.
Chameleon, i. 462; ii. 129,

390, 499.
Chara, ii. 50, 254.
Chelidonium, ii. 48.
Chelonia, i. 463 ; ii. 130,276,

321, 439.
Chemistry, organic, ii. 5, 333.
Cheselden, ii. 520.
Chevreuil, i. 123.
Children, \. 318; ii. 491.
Chitine, i. 318.
Chladni, ii. 417.
Chondrilla, ii. 52.
Choroid coat, ii. 462.
Choroid gland, ii. 495.
Chromatic aberration, ii. 474.
Chromule, i. 70.
Chrysalis, i. 307.
Chyle, ii. 107, 203.
Chyme, ii. 181.
Cicada, i. 340.
Cicindela, ii. 212.
Cilia, i. 126, 154, 157, 173,

195, 203, 215.
Ciliary ligament, ii. 463.
Cimbex, i. 333.
Cimex, ii. 124.
Cineritious, ii. 561.
Circulation, ii. 11, 229.
Cirrhi, ii. 296, 389.
Cirrhopoda, i. 257.
Classification, i. 51 ; ii. 625.
Clausilia, ii. 317.
Clausium, i. 253.
Clavicle, i. 404, 523, 566.
Claviger, ii. 384, 483.
Claw in lion's tail, i. 531.
Clio, i. 258 ; ii. 138.
Cloquet, ii. 498.
Clypeaster, i. 21 1.

Cobitis, ii. 309.

Cobra de capello, i. 549 ; ii, 1 64.

Coccygeal bone, i. 404.

Cochlea, ii. 427.

Cockchaffer. See Melolontha.

Cockle, i. 221. See Cardium.

Cod, lens of, i. 59. ; ii. 496.

Coenurus, ii. 84.

Co-existence of forms, i. 50.

Coffin-bone, i. 517.

Coleoptera, i. 348 ; ii. 382.

Collar-bone, i. 404.

Colours of insects, i. 318.

Colours, perceptions of, ii. 531.

Coluber, i. 448, 450 ; ii. 164.

Columella, i. 243 ; ii. 439.

Commissures of brain, ii. 562.

Comparetti, ii. 244, 436.

Complementary colours, ii. 531.

Compound eyes, ii. 483.

Concha of the ear, ii. 421.

Condor, ii. 331.

Conger eel, ii. 556.

Conglomerate eyes, ii. 483.

Conjunctiva, ii. 466.

Consumption of animal mat-
ter, ii. 60.

Contractility, muscular, i. 125.

Conus, i. 250.

Convolutions of the brain, ii.
558.

Convolvulus, ii. 48.

Cooper, ii. 434.

Coracoid bone, i. 404, 566.

Coral, i. 166.

Coral islands, i. 15.

Corium, i. 112.

Cornea, ii. 461.

Corneule, ii. 487.

Cornu Ammonis, i. 267.

Coronet bone, i. 517.

Corpora quadrigemina, ii. 555.

Corpus callosum, ii. 562.

Corpus papillare, ii. 378.

Cortical substance, ii. 561.

Cossus, i. 300, 312,355.

Cotunnius, ii. 427.

INDEX.

647

Cowrie, i. 247.

Crab,i. 290; ii. 258, 299,317,
493.

Cranium, i. 399, 400, 443,
470.

Cranium of insects, i. 322.

Craw, ii. 169.

Cray-fish, ii. 435,491.

Cribriform plate, ii. 400.

Crinoidea, i. 212.

Crocodile, i. 458, 460, 462;
ii. 142, 163, 276, 409, 440,
557.

Crop, ii. 179.
Cross-bill, ii. 131.
Crotalus, i. 450.
Crust, i. 111,292.
Crusta petrosa, ii. 152.
Crustacea, i. 286; ii. 269,

295, 299, 542, 587.
Cryptogamia, i. 71 ; ii. 593.
Crystalline iens, i. 59; ii.

462.
Crystalline needles in biliary

ducts, ii. 219,
Ctenophora, ii. 384,
Curculio, i, 328.
Cushions of insects, i. 331.
Cuticle, vegetable, i. 77.
Cuticle, animal, i. 112; ii.

377.
Cuttle-fish. See Sepia.
Cuvier, passim.
Cuvier (F.), i. 120, 574.
Cyclidium, i. 186.
Cyclocsela, ii. 98.
Cyclosis, ii. 49, 233.
Cyclostomata, ii. 116.
Cymbia, i. 241.
Cymothoa, ii. 544.
Cyprsea, i. 247.
Cyprinus, i. 116, 411.
Cysticule, ii. 438.

Baldxyrff, i. 433 ; ii. 306.

Darwin, i. 89.

Darwin (Dr. R.), ii. 530.

Davy, ii. 17,338.
Davy (Dr.), ii. 274.
Death, ii. 624.

De Blainville, i. 63, 248, 366;

ii. 252, 428, 482, 497, 570.

De Candolle, i. 93 ; ii. 19,25,

28, 30, 38, 51, 620.
De Candolle (junior), ii. 47.
Decapoda, ii. 258.
Decline of the system, ii. 619.
Decollated shells, i. 249.
Deer, i. 507; ii.402.
Def ranee, i. 256.
De Geer, i. 341.
Deglutition, ii. 174.
Delaroche, ii. 309, 497.
De Montegre, ii. 183.
Dermo-skeleton, i. 366.
De Saussure (Th.), ii. 30.
Des Cartes, ii. 364, 560.
De Serres, ii. 211, 239, 485.
Design, evidence of, i. 28.
Design, unity of, ii. 625.
Developement, vegetable, i. 82.
Developement, animal, ii. 599.
Diaperis, ii. 384.
Diaphragm, ii. 326, 611.
DiflPusion of animals, ii. 64.
Digestion, i. 41 ; ii. 180.
Digitigrada, i. 533.
Diodon, i. 433.
Dioecia, ii. 596.
Dionsea, i. 128.
Diplozoon, ii. 608.
Diptera, i. 323, 353; ii. 115.
Diquemare, i. 220.
Distoma, ii. 113.
Divisibility of matter, ii. 397.
Dollinger, ii. 614.
Dolphin, ii. 142, 442,507,616.
Doras costatus, ii. 307.
D'Orbigny, i. 1Q5.
Doris, ii. 126, 296.
Dormouse, ii. 191,
Dorsal vessel, ii, 236.
Dory, i. 421.
Dove, ii. 554, 557.

648

INDEX.

Down of plants, i. 94.
Down of birds, i. 572.
Draco volans, i. 56, 547.
Dragon-fly, i. 310, 351 ; ii.

487.
Dreaming, ii. 536.
Dromedary, ii. 223.
Duck, ii. 130,405.
Duckweed, ii. 589.
Dufour (Leon), ii. 209, 313.
Dugh, ii. 244, 250, 479, 487,

491.
Dugong, ii. 142, 279, 442.
Duhamel, ii. 16, 20.
Dumas, ii. 393.
Dumeril, i. 400, 590 ; ii. 411.
Dumortier, i. 366.
Duodenum, ii. 208.
Dutrochet, i. 75, 190 ; ii. 314.
Dytiscus, i. 29, 311, 333, 336 ;

ii. 311, 313.

Eagle, ii. 130.

Ear, ii. 421.

Ear-drum, ii. 422.

Earle, i. 560.

Earths in plants, ii. 43.

Earth-worm, (see Lumbricus).

Echinodermata, i. 199.

Echinus, i. 203,210; ii. 101,

119,297, 383.
Edwards, ii. 317, 542, 631.
Eel, i. 424 ; ii. 307.
Egg, ii. 597.
Ehrenberg, i. 13, 186, 189;

ii. 93, 478, 592.
Ehrmann, ii. 309.
Elaboration, successive, ii. 13.
Elastic ligaments, i. 107.
Elater, i. 341.
Elearine, i. 123.
Electric organs, i. 31.
Electricity, ii. 350.
Elements, organic, ii. 6.
Elephant, i. 56, 108, 491, 518 ;

ii. 141, 154,162, 199, 223,

392, 504, 559.

Ellis, i. 150.

Elytra, analysis of, i. 318, 349.

Embryo, ii. 595.

Emu, i. 586.

Emys, i. 474.

Enamel of teeth, ii. 150.

Endogenous plants, i. 83.

Entomoline, i. 115, 318.

Entomostraca, ii. 493.

Entozoa, i. 282; ii. 83, 113,

235,294, 540, 591.
Ephemera, i. 311 ; ii. 241.
Epidermis, vegetable, i. 88.
Epidermis, animal, i. 112, 113,

231.
Epiphragma, i. 253.
Equivocal generation, ii. 591.
Equorea, ii. 85.
Erato, i. 247.
Erect vision, ii. 521.
Erpobdella, i. 272 ; ii. 252.
Eryx, i. 447.
Esox, i. 427.
Ethmoid bone, ii. 400
Eudora, ii. 91.
Euler, ii. 475.
Eunice, ii. 480.
Euphorbium, ii. 59.
Euryale, i. 212.
Eustachian tube, ii. 424.
Evil from animal warfare, i. 46 ;

ii. 67.
Excretion, ii. 12.
Excretion, vegetable, ii. 46, 51 .
Exhalation by leaves, ii. 27.
Exocetus, i. 547.
Exogenous plants, i. 83.
Eye, i. 31 ; ii. 460, 587, 589.
Eye, formation of, ii. 605.
Eye-lids, formation of, ii. 615.

Fabricius, i. 195.

Facial angle, ii. 561.

Fairy rings, ii. 55.

Fallacies of perception, ii. 514.

Fangs of serpents, ii. 163.

Faraday, ii. 524,

INDEX.

649

Fasciola, ii. 113.

Fasciolaiia, i. 249.

Fat, i. 123.

Fata Morgana, ii. 533.

Feathers, i. 568, 591.

Fecula, i. 70.

Fecundation, ii. 595.

Feelers, i. 288 ; ii. 383.

Feet-jaws, i. 289.

Feet of birds, i. 584.

Femur, i. 287, 328, 405.

Fenestrse of ear, ii. 425.

Ferns, i. 83 ; ii. 593.

Fibre, animal, i. 98, 105.

Fibula, i. 405.

Fig-tree, ii. 48.

Fig Marygold, ii. 48.

Filaments of feathers, i. 569.

Filaria, i. 63.

Filices, i. 83 ; ii. 593.

Final causes, i. 1,22, et passim.

Fins of fishes, i. 421.

Fins of cetacea, i. 486.
Fishes, i. 109, 408; ii. 127,
272, 389, 410, 494, et pas-
sim.
Fissiparous reproduction, ii.
583.

Flea, i. 297.

Flight, i. 344, 545.

Flourens, ii. 305.

Flower, ii. 595.

Fluidity, organic, i. 61.

Flustra, i. 165, 169, 172.

Flying fish, i. 547.

Flying lizard, i. 547.

Flying squirrel, i. 550.

Focus, ii. 453.

Fohmann, ii. 353.

Follicles, i. 114; ii. 185.

Fontana, ii. 614.

Food of plants, ii. 15.

Food of animals, ii. 57.

Foot of moUusca, i. 221.

Forces, physical, i. 6.

Fordyce, ii. 172.

Foviila, ii. 596.

French bean, ii. 52.

Frog, i. 437;ii. 128,222,274,

439.
Fucus vesiculosus, i. QQ.
Functions, i. 34, 38 ; ii. 69.
Fungi, ii. 55.
Furcular bone, i. 566.
Furcularia, i. 62.
Fusiform fibres, i. 75.
Fusiform roots, ii. 21,
Future existence, ii. 580, 640.

Gaede, ii. 86.
Gaillon, i. 156.
Gaimard, i. 97.
Galeopithecus, i. 550.
Galileo, i. 81.
Gallinse, ii. 554.
Gallop, i. 495.
Galvanism, ii. 514.
Ganglion, ii. 358.
Gasteropoda, i. 227; ii. 176,

300, 480.
Gastric juice, ii. 183.
Gastric teeth, ii. 167, 214.
Gastric glands, ii. 184.
Gastrobranchus, i. 407, 416 ;

ii. 116, 497.
Gay Lussac, ii. 314.
Gecko, i. 460 ; ii. 390.
Gelatin, i. 105.
Gemmiparous reproduction, ii.

588.
Gemmule, i. 156 ; ii. 591.
Geometer caterpillars, i. 315.
Germs, vegetable, i. 86 ; ii.

588.
Geronia, ii. 91.
Gibson, i. 385.
Gillaroo trout, ii. 202.
Gills, i. 439 ; ii. 267, 299.
Gimbals, i. 330.
Gizzard, ii. 169,214.
Glands, vegetable, i. 77 ; ii.

45.
Glands, animal, ii. 348.
Glands in crocodile, ii. 409.

650

INDEX.

Glands, gastric, ii. 184.

Gleichen, ii. 94.

Globules, i. 64, 98.

Glossa, ii. 124.

Glossopora, ii. 104.

Gnat, ii. 115.

Goat, ii. 402.

Goeze, ii 478.

Gonium, i. 187.

Goose, ii. 173, 500.

Gordius, i. 63, 276.

Gorgonia, i. 166.

Gradation of being, i. 53 ; ii.

629.
Grampus, ii. 142.
Grallae, i. 585, 592.
Grant, i. 147, 151, 169, 172,

175, 195,203,215,587; ii.

478.
Gray, i. 219, 239, 254.
Growth, vegetable, i. 84 ; ii.

21,599.
Gruithuisen, ii. 479.
Gryllotalpa, i. 342 ; ii. 385.
Gryllus, ii. 244.
Guinea-pig, i. 498.
Gulstonian lectures, ii. 532.
Gum, ii. 37.
Gurnard, ii. 554.
Gymnotus, i. 424 ; ii. 572,

Hsematopus, ii. 131.
Haidinger, i. 205.
Hair, vegetable, i. 94.
Hair, animal, i. 117, 319.
Hair-worm, i. 276.
Hales, ii. 26.
Haliotis, i. 231.
Haller, i. 98.
Halteres, i. 353.
Hamster, ii. 178.
Hancock, ii. 307.
Hand, i. 544 ; ii. 392.
Hanow, ii. 478.
Hare, i. 497; ii. 149,191.
Hartley, ii. 563.
Harwood, ii. 404, 405.

Hatchett, ii. 43.

Hauksbee, ii. 415.

Haunch in insects, i. 287, 328.

Hawk, ii. 130.

Head of insects, i. 322.

Hearing, ii. 414, 571.

Heart, i. 41,138; ii. 258,607.

Hedge-hog, i. 524, 527.

Hedysarum gyrans, i. 127.

Hedwig, i. 74.

Helix, i. 242, 253; ii. 126,

317,481.
Hellman, ii. 390.
Hemiptera, i. 309, 350; ii.

115.
Hemispheres, cerebral, ii. 556.
Henbane, ii. 59.
Henderson, ii. 338.
Hepatic vessels, ii. 208, 214.
Herring, i. 421.
Herschel (Sir W.), ii. 529.
Herschel (Sir John), i. 232 ;

ii. 44, 571.
Hervey, ii, 288.
Hesperia, i. 356.
Hexastoma, ii. 113.
Hippopotamus, ii. 141, 151,

152, 162, 193, 443, 504.
Hirudo, i. 138, 281 ; ii. 102,

125,252,298,480.
Hodgkin, i. 99, 127.
Hodgson, ii. 403.
Hog, i. 402, 521 ; ii. 193, 392.
Holothuria, ii. 208, 235, 296,

550.
Home (Sir Everard), passim.
Honey-comb stomach, ii. 195.
Hooded snake, i. 549.
Hooks on feetof insects, i. 331.
Hop, i, 91.
Horn, i. 115,514.
Horn on beak of chick, ii. 6 1 5,
Horse, i. 516; ii, 191, 401,

569,
Horse-fly, ii, 115,
Hostilities of animals, i. 46; ii.
67.

INDEX.

651

Houston, ii. 129.

Huber, ii. 386, 413.

Human fabric, i. 536; ii. 559.

Humboldt, ii. 308, 314, 338.

Humerus, i. 405.

Humours of the eye, ii. 460.

Hunter, i. 108; ii. 171, 188,

330.
Hysena, i. 499 ; ii. 61, 149.
Hybernation, ii. 536.
Hydatid, ii. 84, 113, 591.
Hydatina, ii. 97, 98, 479, 539.
Hydra, i. 162, 176; ii. 74,

477, 538, 586, 590.
Hydrogen, ii. 45.
Hydrophilus, i. 311.
Hydrostatic acalepha, i. 196.
Hyla, i. 445.
Hymenoptera, i. 323, 351 ; ii.

116,244.
Hyoid bone, ii. 132, 303.
Hyrax, ii. 191.

Ichthyosaurus, i. 469.
Ilium, i. 405.
Imago, i. 307, 317.
Incisions of insects, i. 327.
Incisor teeth, ii. 143.
Incus, ii. 426.
Indian walrus, ii. 142.
Individuality of polypes, i. 173.
Infusoria, i. 183; ii. 539, 583.
Injuries, reparation of, ii. 3,

587.
Inorganic world, i. 7.
Insects, i. 11, 108, 296; ii.

207, 236, 395, 436, 546,

570.
Insectivora, i. 525.
Instinct, ii. 574.
Integuments, i. Ill; ii. 377.
Intercellular spaces, i. 70.
Intermaxillary bone, ii. 143,

634.
Interspinous bones, i. 396.
Intestine, ii. 101.
Iriartea, ii. 48.

Iridescence, i. 232.
Iris, i. 136 ; ii. 463.
Ischium, i. 405.
Isis, i. 168.
Ivy, i. 92.

Jacobson, ii. 568, 570.
Jerboa, i. 497, 538.
Johnson, ii. 104.
Julus, i. 298 ; ii. 485.
Jurine, ii. 567.

Kaleidoscope, ii. 533.
Kanguroo, i. 399, 497, 538;

ii. 193, 598.
Kater, ii. 491.
Kerona, i. 186.
Kidd, i. 342; ii. 313, 348,

385.
Kiernan, ii. 350.
Kieser, i. 66, 74.
Kirby, i. 327; ii. 413, 485.
Knight, ii. 595.
Knots in wood, ii. 589.
Koala, i. 527.
Kolpoda, i. 187.

Labium of insects, ii. 123.
Labrum of insects, ii, 123.
Labyrinth, ii. 427.
Lacerta, i. 457, 458.
Lacrymal organs, ii. 466.
Lacteals, ii. 107, 226.
Lamarck, ii. 93, 637.
Lamina spiralis, ii. 430.
Lamprey, i. 416 ; ii. 1 16, 305,

437.
Lancets of diptera, ii. 115.
Language of insects, ii. 386.
Lark, i. 582.
Larva, i. 304, 306.
Lassaigne, i. 318 ; ii. 183.
Latham, ii. 189.
Latreille, I 290 ; ii. 316, 389,

493.
Laws of nature, i. 6.
Law of mortality, i. 42.

652

INDEX.

Law of co-existence of forms,

i. 50.
Law of gradation, ii. 629.
Law of analogy, i. 49 ; ii. 625.
Leach, i. 219.
Leaves, ii. 29, 44.
Leech, (see Hirudo).
Lemur, i. 533, 550; ii. 285,

505.
Lens, crystalline, i. 59 ; ii.

462, 496.
LenticellsD, i. 93.
Lepas, i. 257 ; ii. 296.
Lepidoptera, i. 304, 354 ; ii.

114,217.
Lepisma, i. 297, 298, 356.
Lerneea, i. 302 ; ii. 600, 608.
Leuchs, ii. 482.
Leucophra, ii. 96.
Leuret, ii. 183.
Lewenhoeck, i. 356 ; ii. 264.
Libellula, i. 310, 351; ii.486,

487.
Liber, i. 88; ii. 41.
Lichen, ii- 19.
Life, i. 34, 42.
Ligaments, i. 106.
Ligamentum nuchse, i. 108,

501.
Light on plants, i. 91 ; ii. 28.
Lignin, i. 70; ii. 41.
Lilium, i. 78.
Limax, ii. 126, 317.
Limpet, (see Patella).
Link, i. 75.
Lion, i. 108,496,529; ii. 136,

392, 557.
Lister, ii. 233, 300.
Liver, ii. 107, 219, 350.
Lizard, ii. 129, 390, 497, 587.
Lobster, i. 292; ii. 167, 258,

299, 435, 544.
Lobularia, i. 161.
Loche, ii. 309.
Locomotion, i. 143.
Locusta, ii. 122.
Loligo,i. 261,407; ii. 271.

Longevity of trees, ii. 620.

Lophius, i.422; ii. 390,437.

Lophosia, ii. 384.

Loxia, ii. 131.

Lucanus, i. 359.

Lumbricus marinus, i. 277 , 295.

Lumbricus terrestris, ii. 102,

114,254,297.
Lungs, ii. 267,611.
Lycopodium, i. 78.
Lycoris, ii. 480.
Lymphatics, ii. 352.
Lymphatic hearts, ii. 353.
Lyonet,\. 300, 312, 355.

Macaire, ii. 51, 54, 58, 334.
Macartney, i. 590; ii. 329,

331, 562.
Macavoy, ii. 375.
Mackerel, i. 425.
Macleay, i. 54.
Madder, i. 384.
Madrepore, i. 166.
Magendie, ii. 505, 535.
Magilus, i. 249.
Maia, ii. 269, 545.
Malleus, ii. 426.
Malpighi, ii. 378.
Mammae, ii. 598, 616.
Mammalia, i. 477 ; ii. 325,

441,598.
Man, i. 536 ; ii. 559.
Man of war, Portuguese, i. 196.
Manatus, ii. 142.
Mandible, i. 289.
Mantis, ii. 211.
Mantle, i. 113, 237.
Many-plies stomach, ii. 197.
Marcet, ii. 58, 226, 334, 338.
Marginella, i. 247.
Marmot, ii. 149.
Marsigli, i. 150.
Marsupialia, ii. 277, 598.
Marsupium, ii. 500.
Masaris, ii. 384.
Mastication, ii. 140.
Mastoid cells, ii. 425.

INDEX.

653

Matrix of feather, i. 576.
Matter, ii. 516.
Maunoir, ii. 527.
Maxillae, ii. 123.
Mayer, i. 447.
Mayo, ii. 618.
Meatus auditorius, ii. 422.
Mechanical functions, i. 38.
Meckel, i. 482 ; ii. 480.
Medulla oblongata, ii. 555.
Medullary substance, ii. 365.
Medullary rays, i. 86.
Medusa, i. 96, 192 ; ii. 63, 72,

85, 294, 478.
Meibomian glands, ii. 469.
Melolontha, i. 300 ; ii. 122,

212,236,313,384,486,490.
Melophagus, ii. 483.
Membrana nictitans, ii. 499,

501.
Membrane, i. 101.
Menobranchus, ii. 324.
Mercurialis, ii. 53.
Mergus, ii. 130.
Merrythought of fowl, i. 566.
Mesembryanthemum, ii. 48.
Mesenteric glands, ii. 227.
Mesentery, ii. 108.
Mesial plane, i. 270.
Mesothorax, i. 323.
Metacarpus, i. 405.
Metals in plants, ii. 43.
Metamorphoses, i. 302, 437 ;

ii. 632, 634.
Metatarsus, i. 405.
Metathorax, i. 323.
Milk, ii. 616.
Millepedes, ii. 485.
Millepora, i. 167.
Mimosa, i. 127.
Mint, ii. 17, 30.
Mirandola, ii. 586.
Mirbel, i. 69, 72.
Mite, i. 297.
Mitra, i. 248.
Modiolus, ii. 431.
Molar teeth, ii. 144.

Moldenhawer, i. 74.
Mole, i. 524, 525 ; ii. 391 , 505.
Mole cricket, i. 342.
MoUusca, i. 213 ; ii. 269, 389,

550.
Monas, i. 13,184; ii. 96, 583.
Monkey, i. 533 ; ii. 149, 392,

569.
Monoculus, ii. 493.
Monothalamous shell, i. 265.
Monotremata, ii. 277.
Monro, i. 123, 132; ii. 303.
Mordella, ii. 486.
Morpho, i. 354.
Morren, ii. 252, 255.
Mortality, i. 42; ii. 581.
Mother of pearl, i. 232.
Motion, voluntary, i. 37 ; ii.

534.
Motion, vegetable, i. 127.
Motor nerves, ii. 535.
Mucous membrane, i. 112.
Mucous glands, ii. 184.
Mulberry, ii. 59.
Muller, i. 183; ii. 92, 353,

480.
Mullet, ii. 202.
Multilocular shells, i. 265.
Multivalves, i. 257.
Mursena, ii. 497, 55Q.
Murex, i. 245, 252; ii. 126,

301,482.
Mus, ii. 178, 506.
Musca, i. 332.

Muscle (shell fish), i. 222, 224.
Muscle, i. 124, 127, 300.
Muscles of eye, ii. 464.
Muscular power in plants, ii.

358.
Muscular power in birds, i.

593.
Mushroom, ii. 19.
Musk shrew, ii. 135.
Musical tone, ii. 419.
Mya, i. 223.

Myriapoda, i. 297, ii. 248.
Myrmecophaga, ii. 134.

654

INDEX.

Mysis Fabricii, i. 289.
Mytilus, i. 222.
Myxine, i. 407,416; ii. 116,
497.

Nacreous structure, i. 231.

Nais, ii. 102, 251,479,586.

Narwhal, i. 56; ii. 14L

Nature, i. 6, 13.

Nautilus, i. 242, 266 ; ii. 270.

Necrophorus, ii. 413.

Needles in biliary ducts, ii. 219.

Nemazoaria, i. 156.

Nepa, ii. 384.

Nereis, i. 271, 274, 280; ii.

251.
Nerve, i. 36 ; ii. 366.
Nervous system, ii. 365, 537,

553.
Nervous power, ii. 354.
Nettle, ii. 47.
Neuro-skeleton, i. 366.
Neuroptera, i. 351.
Newport, i. 352; ii. 102, 214,

218, 244, 547, 548.
Newt, ii. 439, 587.
Nightshade, ii. 59.
Nitrogen, ii. 14, 338.
Nordmann, ii. 600, 608.
Notonecta, i. 29, 337.
Nursling sap, ii. 24.
Nutrition, ii. 1, 10, 13, 57.
Nutrition in lower orders, ii. 74.
Nutrition in higher orders, ii.

104.
Nutritive functions, i. 38.
Nycteribia, ii. 483.

Octopus, i. 261 ; ii. 494.
Ocular spectra, ii. 530.
Odier,'u 318.

(Esophagus, ii. 101, 107, 176.
Oken, i. 349, 400.
Olfactory nerve, ii. 396.
Olfactory lobes, ii. 556.
Olivee, i. 241, 250.
Oniscus, ii. 544.

Onocrotalus, i. 556.
Operculum of Mollusca, i. 252.
Operculum of fishes, ii. 303.
Ophicephalus, ii. 307,
Ophidia, i. 447.
Ophiosaurus, i. 454, 457.
Ophiura, i. 212.
Opossum, ii. 136, 598.
Optic axis, ii. 503.
Optic ganglion, ii. 489.
Optic lobes, ii. 555.
Opuntia, i. 127.
Orache, ii. 48.
Orbicular bone, ii. 426.
Orbicular muscle, i. 136.
Orchidese, i. 69.
Organic Mechanism, i. 59, 96.
Organic developement, ii. 599.
Ornithorhynchus, i. 395 ; ii.

136, 178, 391,442,497.
Orobanche, ii. 54.
Orthoceratite, i. 267.
Orthoptera, i. 309, 349.
Os hyoides, ii. 132, 303.
Osier, i. 206, 220, 223, 277,

280. "
Osseous fabric, i. 365.
Ossicula, tympanic, ii. 426.
Ossification, i. 375, 556.
Ostracion, i. 432.
Ostrich, i. 563, 587, 590; ii.

185,224,328,554.
Otter, sea, ii. 149.
Ovary, ii. 593, 594.
Oviduct, ii. 596.
Oviparous animals, ii. 597.
Ovo-viviparous animals, ii. 597.
Ovula, i. 247.
Ovum, ii. 593.
Owen, i. 563', ii. 274, 413.
Owl, ii. 330,441,503.
Ox, horn of, i. 515.
Oxygen, ii. 29.
Oyster, i. 131, 220,221.
Oyster-catcher, ii. 131.

Paces of quadrupeds, i. 492.

INDEX.

655

Pachydermata, i. 518 ; ii. 382,

391.
Package of organs, i. 102.
Pain, ii. 368.
Paleraon, ii. 544.
Paley,\. 102,571; ii. 286.
Palinurus, ii 544.
Pallas, i. 150; ii. 344.
Palms, i. 83.
Palm squirrel, ii. 178.
Palmer, ii. 30.
Palpi, i. 289 ; ii. 124.
Pancreas, ii. 107, 221.
Pander, ii. 607.
Panniculus carnosus, i. 527.
Panorpa, i. 326.
Paper nautilus, i. 265.
Papilio, i. 357 ; ii. 486.
Papillee, ii. 378, 394.
Par vagum, ii. 549.
Parakeet, ii. 131.
Parallax, aberration of, ii. 472.
Parrot, ii. 179, 391.
Pastern, i. 517.
Patella, i. 228; ii. 220, 551.
Patella of knee, i. 406.
Patellaria, ii. 46.
Paunch, ii. 195.
Pausus, ii. 384.
Pearl, i. 232.
Peccari, ii. 193.
Pediculus, i. 297.
Pelican, i. 556; ii. 178.
Pelvis, i. 404.
Pencil of rays, ii. 453.
Penguin, i. 592.
Penitentiary, ii. 189.
Pennatula, i. 174; ii. 82.
Penniform muscle, i. 133.
Pentacrinus, i. 212.
Perca, i. 116, 433; ii. 306,

410,495,557.
Perception, i. 36 ; ii. 372, 508.
Perch (See Perca).
Perennibranchia, ii. 324.
Perilymph, ii. 427.
Periostracum, i. 237.

Peristaltic motion, ii. 204.
Peron, i. 97 ; ii. 72.
Pfaff, ii. 338.
Phalanges, i. 405.
Phalena, ii. 244, 486.
Phanerogamous plants, ii. 595.
Phantasmagoria, ii. 533.
Phantasmascope, ii. 524.
Phaseolus, ii. 52.
Phenakisticope, ii. 524.
Philip, ii. 190, 360.
Phoca, i. 487.
Pholas, i. 220, 256.
Phosphorescence of the sea,

i. 194; ii. 63.
Phrenology, ii. 565.
Phyllosoma, ii. 544.
Physalia, i. 196.
Physiology, i. 21.
Physsophora, i. 197.
Phytozoa, i. 146.
Pierard, ii. 200.
Pigeon, ii. 179, 616.
Pigmentum of skin, i. 112.
Pigmentum of the eye, ii. 462.
Pike, i. 427.
Pileopsis, i. 252.
Pineal gland, ii. 560.
Pinna, i. 224, 235.
Pistil, ii. 596.
Pith of plants, i. 85.
Pith of quill, i. 580.
Placuna, i. 233.
Planaria, ii. 114, 236, 250,

294, 479, 586.
Planorbis, i. 227, 242.
Plantigrada, i. 533.
Plastron, i. 463.
Pleurobranchus, ii. 220.
Pleuronectes, i. 431 ; ii. 503.
Plexus, nervous, ii. 359.
Pliny, ii. 559.
Plumula, ii. 603.
Plumularia, ii. 234.
Pneumo-branchise, ii. 316.
Pneumo-gastric nerve, ii. 549.
Podura, i. 297.

656

INDEX.

Poisers, i. 353.

Poison of nettle, ii. 47.

Poll, i. 227, 235.

Pollen, ii. 596.

Polygastriea, ii. 97.

Polypi, i. 161; ii. 74, 81, 293,

383.
Polystoma, ii. 113.
Polythalamous shell, i. 265.
Pontia brassica, i. 354.
Pontobdella, i. 271.
Poppy, ii. 48.
Porcupine quills, i. 120.
Porcupine, i. 527; ii. 149, 193.
Porifera, i. 147.
Porpita, i. 195.
Porpus, ii. 142, 193.
Porterfield, i. 374.
Potatoe, ii. 589.
Prehension of food, ii. 113,117.
Priestley, ii. 29, 336, 338.
Pristis, i, 56; ii. 166.
Pritchard, ii. 241.
Privet Hawk moth, ii. 218.
Proboscis of insects, ii. 114.
Proboscis of mollusca, ii. 126.
Proboscis of Elephant, i. 520.
Progressive motion, i. 144.
Prolegs, i. 313.
Promontory of ear, ii. 425.
Proteus, i. 187; ii. 324,632.
Prothorax, i. 323.
Prout, ii. 37, 41.
Provencal, ii. 308.
Proximate principles, ii. 6.
Psychoda, ii. 384.
Pterocera, i. 246.
Pteropoda, i. 257.
Pteropus, ii. 136.
Pubic bone, i. 405.
Pulmonary organs, ii. 267.
Puncta lacrymalia, ii. 468.
Punctum saliens, ii. 607.
Pupa, i. 304, 307.
Pupil, ii. 463.
Pupipara, ii. 483.
Pyloric appendices, ii. 221.

Pylorus, ii. 107, 182.
Pyramidalis muscle, ii. 501 .
Python, i. 447.

Quadratus muscle, ii. 500.
Quadrumana, i. 533; ii. 149.
Quadrupeds, i. 487.
Quagga, i. 516.
Quail, i. 582.

Quills of porcupine, i. 120.
Quills of feathers, i. 568.
Quoy, i. 97.

Rabbit, i. 497; ii. 149, 190.

Racoon, i. 112.

Radiata, i. 164.

Radicles, ii. 603.

Radius, i. 405.

Ranunculus, i. 79.

Rapp, ii. 478.

Rat, ii. 148, 149, 192.

Rathke, ii. 634.

Rattle-snake, i. 450.

Ray, i. 11.

Ray, i. 420, 422, 423 ; ii. 503,

569.
Rays of fins, i. 424.
Razor-shell-fish, i. 222.
Reaumur, i. 199, 202, 227,

237,292; ii. 115, 170, 183.
Receptacles of food, ii. 178.
Receptaculumchyli, ii. 108,228.
Reed of ruminants, ii. 197.
Refraction, law of, ii. 453.
Regeneration of claw, i. 295.
Rennet, ii. 197.
Reparation, ii. 3, 9, 587.
Repetition of organs, i. 57.
Reproduction, i. 43; ii. 581.
Reptiles, i. 435 ; ii. 273.
Resinous secretions, ii. 47,
Respiration, i. 41 ; ii. 11, 265,

290.
Rete muscosum, i. 112.
Reticulated cells, i. 69.
Reticule of Ruminants, ii. 195.
Retina, ii. 374, 448, 462.

INDEX.

657

Returning sap, ii. 36.
Revelation, ii. 641.
Reviviscence, i. 62 ; ii. 255.
Rhea, i. 586.
Rhinoceros, i. 515; ii. 135,

151,382, 392,504.
Rhipiptera, i, 350.
Rhizostoma, ii. 87.
Rhyncops, ii. 132.
Ribs, i. 401; ii. 327.
Ricinus, i. 297.
Rings of annelida, i. 272.
Rodentia, i. 523; ii. 148, 151,

162, 175, 191, 504.
Rcesel, ii. 478.
Roget, ii. 9, 524, 532, 582.
Rolando, ii. 613.
Roosting, i. 588.
Roots, i. 93 ; ii. 20.
Ross, i. 16.
Rostrum, ii. 124.
Rotifer, i. 62,189; ii.92,479,

539, 591.
Roux, ii. 569.
Rudimental org-ans, i. 55; ii.

632.
Rudolphi, i. 75.
Rumfurd, i. 76.
Ruminantia, i. 499; ii. 196,

504,
Rusconi, ii. 613.

Sabella, i. 277.
Sacculus of ear, ii. 430.
Sacrum, 404.
St. Ange, ii. 277.
St. Hilaire, passim.
Salamander, i. 446 ; ii. 128,

498, 597.
Salicaria, ii. 54.
Saline substances in plants, ii.

43.
Saliva, ii. 175,
Salmon, ii. 222.
Sand-hopper, ii. 542.
Sap, ii. 24.
Sauria, i. 457 ; ii. 276. "

VOL. II.

^am^rny, i. 274, 290 ; ii. 119,

124.
Saw-fish, ii. 166.
Scala tympani et vestibuli, ii.

431.
Scales of lepidoptera, i, 354.
Scales of fishes, i. 116.
Scansores, i. 586 ; ii. 554,
Scapula, i. 404.
Scarabseus, ii. 486.
Scarf skin, i. 112.
Scarpa, i. 101 ; ii. 411, 430.
Schoeffer, ii. 478.
Schizocera, ii. 384.
Schneiderian membrane, ii.

399.
Schultz, ii. 49.
Sciurus, i. 550; ii. 178.
Sclerotica, ii. 460.
Scolopendra, i. 298; ii. 248,

485.
Scoreshy, \. 194.
Scorpion, ii. 315, 485,
Scuta, abdominal, i, 453.
Scutella, i. 211,
Scyllsea, i. 229.
Sea, phosphorescence of, i. 194 ;

ii. 63.
Sea-hare, ii. 126, 168, 551.
Sea-mouse, ii. 102, 125, 298.
Sea-otter, ii. 149.
Seal, i. 487; ii. 403, 442, 506.
Sebaceous follicles, i. 114.
Secretion, ii. 12, 45, 342.
Seed, ii. 593.

Segments of insects, i. 320.
Semblis, ii. 242.
Semicircular canals, ii. 427.
Senecio, ii. 53.
Sennebier, ii. 20, 29.
Sensation, ii. 362.
Sensibility, variationsof, ii. 526.
Sensitive plant, i. 127.
Sensorial power, ii. 360.
Sensorium, ii. 508.
Sepia, i. 261 ; ii. 126, 203,

413,493.

u u

658

INDEX.

Seps, i. 458.

Series of organic beings, i. 53.
Serous membranes, i. 102.
Serpents, i. 447; ii. 129, 163,

390.
Serpula, i. 277 ; ii. 295.
Serves, ii. 609, 617.
Sertularia, i. 165; ii. 234.
Serum, i. 102.
Sesamoid bones, i. 406.
Setee, i. 274.
Shark, ii. 162, 205, 262, 495,

569, 587, 598.
Sheep, ii. 153, 194, 402,
Shell, i. 111,230.
Sheltopusic, i. 457.
Shrapnell, ii. 426.
Shrew, ii. 149, 391.
Shuttle bone, i. 517.
Silica, ii. 18, 44.
Silk worm, i. 305; ii. 59.
Silurus, ii. 307, 390.
Sinistral shells, i. 243.
Siphonaria, i. 252.
Siren, i. 457; ii. 274, 324.
Skate, ii. 303, 410, 495.
Skeleton, i. 365, 386.
Skeleton, vegetable, i. 95 ; ii. 42.
Skimmer, ii. 132.
Skin, ii. 377.
Skull (see Cranium).
Slack, i. 66.
Sleep, ii. 536.

Slips, propagation by, ii. 585.
Sloth, i. 481, 498, 524; ii. 284.
Slug, ii. 126, 317.
Smell, ii. 396.
Smith, ii. 166.
Snail, ii. 317, 413,587.
Snake-lizard, i. 448.
Snout, i. 521.
Snow, red, i. 16.
Soemmerring, ii. 575.
Soils, fertility of, ii. 18.
Solar light, ii. 31.
Solen, i. 222, 224.
Solipeda, i. 516.
Solly, ii, 354.

Sorex, ii. 135, 443, 505.
Sound in fishes, i. 429.
Sound, ii. 414.
Spallanzani,i. 62; ii. 79, 170,

183, 338, 567, 614.
Spatangus, i. 205, 211.
Spectra, ocular, ii. 525, 530.
Spectre of the Brocken, ii. 533.
Speed of quadrupeds, i. 496.
Spermaceti, i. 484.
Spherical aberration, ii. 471.
Sphincter muscle, i. 136.
Sphinx, ii. 217, 244,547.
Spicula, in sponge, i. 154.
Spider, i. 282, 284 ; ii. 248.
Spider-crab, ii. 545.
Spider-monkey, i. 399, 534.
Spine, i. 387, 392.
Spinal cord, or Spinal marrow,

ii. 553, 604.
Spiracles, ii. 311.
Spiral threads in plants, i. 68.
Spiral vessels, i. 73.
Spiral growth of plants, i. 90.
Spiral valve in fishes, ii. 205.
Spirits, animal, ii. 563.
Spirula, i. 242, 265.
Spix, ii. 252.
Spleen, ii. 224.
Splint bone, i. 517.
Spokes, curved spectra of, ii.

524.
Sponge, i. 147 ; ii. 84.
Spongiole, i. 79; ii. 20, 21.
Spotted cells of plants, i. 69.
Spring-tail, i. 297.
Spur of cock, i. 586.
Squalus (see Shark).
Squalus pristis, ii. 166.
Squirrel, i. 524, 550; ii. 178.
Stability of trees, i. 81.
Stability of human frame, i. 541.
Stag, skeleton of, i. 507.
Stamen, ii. 596.
Stapes, ii. 426.

Star-fish, i. 200 (see Asterias).
Starch, i. 70; ii. 41.
Staunton, ii. 531.

INDEX.

659

Stearine, i. 123.
Steifensand, ii. 482.
Steins, vegetable, i. 81.
Stemmata, ii. 483.
Stentor, ii. 98.
Sternum, i. 402.
Stevens, ii. 183.
Stigma, vegetable, ii. 596.
Stigmata of insects, ii. 311.
Sting of bee, i. 352.
Stipulse, i. 94.
Stomach, ii. 72, &c.
Stomata, i. 77 ; ii. 19.
Stones, swallowing of, ii. 171.
Stone-wort, ii. 50.
Stork, i. 590.
Stratiomys, i. 310, 348.
Straus Durckheim, i. 300,

323 ; ii. 490.
Strepsiptera, i. 350.
Striated structures, i. 232.
Strombus, i. 246 ; ii. 301.
Styloid bone, i. 517.
Subbrachieni, i, 423.
Suckers, i. 136, 260, 332.
Sugar, ii. 5.

Sun, action of, on plants, i. 91.
Surveyor caterpillars, i. 315.
Sus iEthiopicus, ii. 161.
Suture, i, 381.
Swammerdam, i. 352 ; ii. 413,

482.
Swan, i. 559,593; ii. 169.
Swimming of fishes, i. 412.
Swimming bladder, i. 429.
Symmetry, lateral, i. 57 ; ii.

609.
Sympathy, ii. 576.
Sympathy of ants, ii. 389.
Sympathetic nerve, ii. 358.
Synovia, i. 102.
Syphon of shells, i. 267.
Systemic circulation, ii. 266.

Tabanus, i. 333; ii. 115.
Tadpole, i. 437 ; ii. 222, 322,
632.

Taenia, ii. 83, 114, 236.
Tail, i. 398, 524, 531, 583; ii.

392, 634.
Talitrus, ii. 542.
Tapetum, ii. 505.
Tapeworm, ii. 83, 114,236
Tapir, i. 521 ; ii. 392.
Tarsus, i. 288, 328, 330, 405.
Taste, ii. 393.
Teeth, ii. 140.

Tegmina of orthoptera, i. 349.
Telegraphic eyes, ii. 493.
Tellina, i. 224.
Temperature, animal, ii. 340.
Tendons, i. 106,134.
Tendrils, i. 94.

Tentacula, i. 161,171; ii.383.
Terebella, i. 277,278; ii. 95.
Terebra, i. 249.
Teredo, i. 235.
Testacella, ii. 317.
Testudo, i. 470; ii. 557.
Tetrodon, i. 420, 433.
Textures, vegetable, i. 66.
Textures, animal, ii. 97.
Thetis, ii. 296.
Thoracic duct, ii. 108, 228.
Thorax, i. 323 ; ii. 325.
Thorns, i. 94.
Thought, ii. 517.
Threads, elastic, in plants, i. 68.
Tibia, i. 328, 330, 405.
Tick, i. 297.
Tiedemann, ii. 235.
Tiger, i. 496; ii. 136, 145,

146, 392.
Tipula, i. 331.
Tone, musical, ii. 419.
Tongue of insects, ii. 124.
Tongue, strawberry, ii. 394.
Torpedo, i. 31 ; ii. 572.
Tortoise, i. 463 ; ii. 499.
Tortryx, i. 447, 448.
Toucan, ii. 131, 330.
Touch, ii. 377, 534.
Trachese of animals, ii. 293,

310.

660

INDEX.

Tracheae of plants, i. 73.
Tradescantia, ii. 51.
Trapezius muscle, i. 135.
Trembley,i. Ill ; ii, 79, 478.
Treviranus, i. 73, 75; ii. 569.
Trichechus, i. 487.
Trichoda, ii. 97.
Trigla, ii. 554.
Trionyx, i. 475.
Tristoma, ii. 113.
Triton, i. 252, 446.
Tritonia, ii. 296.
Trituration of food, internal, ii.

167.
Trochanter, i. 328.
Trochilus, ii. 117.
Trophi, ii. 121.
Trot, actions in, i. 494.
Trunk-fish, i. 432.
Trunk of elephant, i. 520.
Truxalis, ii. 384.
Tuberose roots, ii. 589.
Tubicolac, i. 277.
Tubipora, i. 165.
Tubularia, ii. 233.
Turbinated shells, i. 216.
Turbinated bones, ii. 400.
Turbo, i. 253.
Turkey, ii. 173, 440.
Turritella, i. 249.
Turtle, i. 463; ii. 202, 557.
Tusks, ii. 141.
Tympanum, ii. 422.
Type, i. 48 ; ii. 627.
Typhlops, i. 447.

Ulna, i. 405.
Ungual bone, i. 405,
Unio batava, i. 217.
Unity of design, ii. 625.
Uranoscopus, ii. 503.
Urceolaria, i. 187.
Urchin, sea. See Echinus.
Utricle of labyrinth, ii. 430.
Uvea, ii. 463.

Valves, i. 31, 104: ii. 260,288.

Vampire bat, ii. 117.
Van Helmont, ii. 16.
Vane of feather, i. 568.
Variety, law of, i. 11, 48; ii.

626.
Varley, ii. 254.
Vascular circulation, ii. 235.
Vascular plexus, ii. 377
Vauquelin, ii. 229, 336.
Vegetable kingdom, i. 14, 40.
Vegetable organization, i. Q5.
Vegetable nutrition, ii. 15.
Veins, i. 41 ; ii. 108.
Velella, i. 195.
Velocity of fishes, i. 434.
Velvet coat of antler, i. 510.
Vena cava, ii. 263,
Ventricle of heart, ii. 108, 259.
Ventricles of brain, ii. 55Q.
Veretillum, ii. 82, 478.
Vertebra, i. 387 ; ii. 604.
Vertebrata, i. 361.
Verticillated arrangement, i. 90.
Vesicles of plants, i. 66.
Vespertilio, i. 551 ; ii. 136,567.
Vessels of plants, i. 71.
Vessels of animals, i. 103; ii.

606, 613, 621.
Vestibule of ear, ii. 427.
Vibrations, ii. 563.
Vibrio, i. 63, 186.
Vicq d'Azyr, i. 190.
Villi, ii. 347.
Viper, i. 447; ii. 597.
Vision, ii. 444.
Vision, erect, ii. 521.
Visual perceptions, ii. 520.
Vital functions, i. 38 ; ii. 1, 69.
Vital organs, ii. 354.
Vitality, i. 20.
Vitreous humour, ii. 462.
Vitreous shells, i. 236.
Viviparous reproduction, ii.

598.
Voice, ii. 444.
Voltaic battery of torpedo, ii.

572.

INDEX.

661

Voluntary motion, i. 37; ii.

534.
Volute, i. 248 ; ii. 126,482.
Volvox, i. 186, 188; ii. 591.
Voracity of hydra, ii. 77.
Vorticella, i. 62, 182; ii. 97,

584.
Vulture, ii. 180, 406.

Wading birds, i. 585, 592.
Walking, i. 492, 542.
Waller, ii. 134.
Walrus, i. 487; ii. 141.
Warfare, animal, i. 46 ; ii. 67.
Warm-blooded circulation, ii.

278.
Water not the food of plants,

ii. 16.
Water-beetle (see Dytiscus).
Water-boatman, i. 29, 337.
Wax, vegetable, ii. 48.
Web-footed birds, i. 592.
Weber, ii. 430, 480.
Whale, i. 55; ii. 178, 443,

504, 559.
Whalebone, ii. 136.
Wheel animalcule, i. 189.
Wheel spokes, spectre of, ii.

524.

Whelk (see Buccinum).
Whiskers, ii. 392.
Whorls of plants, i. 90.
Whorls of shells, i. 243.
Willow, i. 79.
Wings, i. 344, 567.
Winged insects, i. 299.
Withers, i. 518.
Wolf-fish, ii. 128.
Wollaston, i. 92; ii. 55,491,

571.
Wombat, i. 527.
Woodhouse, ii. 30.
Woodpecker, ii. 132.
Woody fibres, i. 71, 75.
Worms (see Annelida and En-

tozoa).

Yarrell, ii. 131.
Young, ii. 474, 475.

Zebra, i. 516.
Zemni, ii. 506.
Zoanthus, i. 162, 182.
Zoocarpia, i. 156.
Zoophytes, i. 146; ii. 477,537.
Zostira, ii. 202.
Zygsena, ii. 384.
Zygodactyli, i. 586.

FINIS.

C. WHITTINGHAM, TOOKS COURT, GHANCEKV LANK

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