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2000

I..

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.

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

ANIMAL

AND

VEGETABLE

PHYSIOLOOY,

CONSIDERED WITH REFERENCE

TO

NATrRAli THEOIiOGY.

BY

PETER MARK ROGET, M. D.

SECRETARY TO THE ROYAL SOCIETY, FULLERIAN PROFESSOR OF PHYSIOLOGY IN THE ROYAL

INSTITUTION OF GREAT BRITAIN, VICE PRESIDENT OF THE SOCIETY OF ARTS,
FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS, CONS0LTIMQ PHYSICIAN TO THE QOEEN
CHARLOTTE'S LYING-IN HOSPITAL, AND TO THE NORTHERN
DISPENSARY, ETC, ETC.

VOL. II.

PHILADELPHIA:
CAREY, LEA & BLANCHARD.

1836.

GRIGGS & CO., PRINTERS.

COIVTEIVTS

OF THE SECOND VOLUME.

PART II.— THE VITAL FUNCTIONS.

Page
Chapter I. — Objects of Nutbition ----- 9

Chapter II. — Nutrition ix Vegetables - - . . 19

§ 1. Food of Plants - - - . - - 19

2. Absorption of Nutriment by Plants - - - 21

3. Exhalation - - - - - - 27

4. Aeration of the Sap ----- 28

5. Return of the Sap - - - - - 32

6. Secretion in Vegetables - - - - 38

7. Excretion in Vegetables - - - - - 43

Chapter III. — Animal Nutrition in general . . . ^

§1. Food of Animals - - - - - - 47

2. Series of Vital Functions - - - - SS

Chapter IV. — ^Nutrition in the lower orders of Animals - 58

Chapter V. — Nutrition in the higher orders of Animals - 80

Chapter VI. — Preparation of Food - - - - 86

§ 1. Prehension of Liquid Food - - - - 86

2. Prehension of Solid Food - - - - 89

3. Mastication by means of Teeth - _ - 104

4. Formation and Development of the Teeth - - 114

5. Trituration of Food in Internal Cavities - - 122

6. Deglutition - - - - - - 127

7. Receptacles for retaining Food . - - 130

Chapter VII. — Digestion - * - - - - - 132

Chapter VIIL — Chyhfication - - - - - 148

21991

VI

CONTENTS.

Chapter IX.— L.UTKAL Absoiiptio.v -

ClIAPTEIl X. CincCLATIOX

§ 1. Diffused Circulation

2. Vascular Circulation -

3. Respiratory Circulation -

4. Distributionof Blood Vessels

CHAPTEnXI.-— Rf.si'iiiatiojt . . , _

§ 1. Respiration in g-eneral
2. Aquatic Respiration - . .

o. Atmospheric Respiration
4. Chemical Chang^cs effected by Respiration

Chapter XII.— Seciietiox

Chapteii XIII. — AnsoiiPTiojr

Chapter XIV.— Nervous Power

rage
164

167

167
170
191
201

208

208

210

221

236

243

250

252

PART III.— THE SENSORIAL FUNCTIONS.

Chapter I. — Sensatiout - . . ,

Chapter II. — Touch

Chapter III.— Taste

Chapter IV.— Sxell

Chapter V — Hearing

§ 1. Acoustic Principles -

2. Physiology of Hearing- in Man

3. Comparative Physiology of Hearing -

Chapter Vf.— Vision

§ 1. Object of the Sense of Vision

2. Modes of accomplishing the objects of Vision

3. Structureof the Eye -

4. Physiology of Perfect Vision

5. Compai-ativc Physiology of Vision .

Chapter VII.— Pekceptiox -

- 258

268

- 279
281

- 294
294

- 298
308

- 315
315

- 318
325

- 332 '

)7

oot

558

CONTENTS. Vii

Page

Chapter VIII. — Comparative Phtsiolooy of the Nervous System 378

§1. Nervous System of Invcrteb rated Animals - - 378

2. Nervous System of Vertebrated Animals - - - 388

3. Functions of the Brain .... 395

4. Comparative Physiolog-y of Perception - - - 398

PART IV.— THE REPRODUCTIVE FUNCTIONS.

Chapter I. — Reproduction ..... 408

Chapter II. — Organic Development . - • . 420

Chapter HI. — Decline of the System . - . . 433

Chapter IV. — Unity of Design - - . . . 437

Index - - - - - - - * - 449

$

fMfEKTr UBMART

SS. C State Colkse

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 attention in the preceding
volume, are necessary for the maintenance of life, and tlie
exercise of the vital powers. But, however artificially that
fabric may have been constructed, and however admirable
the skill and the foresight that have been displayed in en-
suring the safety of its elaborate mechanism, audi in pre-
serving 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 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 miany causes inevita-
bly leading to their ultimate destruction. Continued friction
must necessarily occasion a loss of substance in the harder

Vol. II. 2

10 THE VITAL FUNCTIONS.

pnrts of the frame, and evaporation is constantly tending to
exhaust the fluids. The repeated actions of the muscles in-
duce certain changes in these organs, both in their mecha-
nical properties and chemical composition, which impair
their powers of contraction, and which, if suffered to con-
tinue, would, in no long time, render them incapable of ex-
ercising 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 remedy-
ing 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 development of the organs, and ge-
neral 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 exist-
ed, and become identified with the livinjr 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 materials is also
wanted for the reparation of occasional injuries, to which,
in the course of its long career, the body is unavoidably ex-
posed. Like a ship fitted out for a long voyage, and forti-
fied against the various dangers of tempests, of icebergs, 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 enerjiies,
which may be called into action when demanded by the ex-
igencies of the occasion.

Any one of the circumstances above enumerated would
of itself be sufficient to establish the necessity of supplies of
nourishment for the maintenance of life. But there are

OBJECTS OF NUTRITION. 11

other considerations, equally important in a physiological
point of view, and derived from the essential nature of or-
ganization, 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 vitality must therefore be selected and
arranged with a view to their continual modification, cor-
responding to these ever varying changes of condition. The
artificer, whose aim is to construct a machine for perma-
nent use, and to secure it as much as possible from the de-
terioration arising from friction or other causes of injury,
would, 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 inces-
sant, 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 or-
ganized structures. In order that these may be qualified
for exercising the functions of life, they must be capable of
continual alterations, displacements, and adjustments, vary-
ing perpetuall}'^, both in kind and in degree, according to
the progressive stages of their internal development, and to
the different circumstances which may arise in their exter-
nal condition. The materials which nature has employed
in their construction, are, therefore, neither the elementary
bodies, nor even their simpler and more permanent combi-
nations; but such of their compounds as are of a more plastic
nature, and which allow of a variable proportion of ingre-
dients, and of great diversity in the modes of their combi-
nation. So great is the complexity of these arrangements,
that although chemistry is fully competent to the analysis
of organized substances into their ultimate elements, no hu-

12 THE VITAL FUNCTIONS.

man 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 'refmcd operations of vitality, the only power
that could produce this adjustment, that they have been
brought into that condition.

We may take as an example one of the simplest of orga-
nic products, namely, Sugar; a substance which has been
analyzed with the greatest accuracy by modern chemists:
yet to reproduce this sugar, by the artificial combination of
its simple elements, is a problem that has hitherto baffled all
the efforts of philosophy. Chemistry, notwithstanding the
proud rank it justly holds among the physical sciences, and
the noble discoveries with which it has enriched the arts;
notwithstanding it has unveiled to us many of the secret ope-
rations of nature, and placed in our hands some of her most
powerful instruments for acting upon matter; and notwith-
standing it is armed with full powers to destroy, cannot, in
any one organic product, rejoin that w^hich has been once
dissevered. Through the medium of chemistry we are ena-
bled, perhaps, to form some estimate of the value of what
we find executed by other agencies; but the imitation of
the model, even in the smallest part, is far beyond our power.
No means w^hich the laboratory can supply, no process,
which the most inventive chemist can devise, have ever yet
approached those delicate and refined operations which na-
ture silently conducts in the organized texture of living
plants and animals.

The elements of organic substances are not very nume-
rous; the principal of them being oxygen, carbon, hydrogen,
nitrogen, sulphur, and phosphbrus, together with a few of
the alkaline, earthy, and metallic bases. These substances
are variously united, so as to form certain specific com-
pounds, wdiich, although they are susceptible, in different
instances, of endless modifications, yetf possess such a gene-
ral character of uniformity, as to allow of their being ar-
ranged in certain classes; the most characteristic substance
in each class constituting what is called a proximate orga-

ORGANIC CHEMISTRY. ' 13

7iic principle. Thus, in the vegetable kingdom we have
Lignin, Tannin, Mucilage, Vil, Sitgar, Fecula, &c. The
animal kingdom, in like manner, furnishes Gelatin, Mhu-
men, Fihnn, Mucus, Entomoline, Elearin, Stearin, and
many others.

The chemical constitution of these organic products,
formed, as they are, of but few primary elements, is strik-
ingly contrasted with that of the bodies belonging to the mi-
neral 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 vegetable 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 combination in each sepa-
rate compound is infinitely more complex, and presents in-
calculable diversity. Simple binary compounds are rarely
ever met with; but, in place of these, w^e find three, four,
five, or even a greater number of constituent elements ex-
isting in very complicated states of union.

This peculiar mode of combination gives rise to a remark-
able condition, which attaches to the chemical properties of
organic compounds. The attractive forces, by which their
several ingredients are held together, being very numerous,
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 pro-
duce rapid changes of form, or even complete decomposi-
tion. The principles, thus retained in a kind of forced
union, have a constant tendency to react upon one another,
and to produce, from slight variations of circumstances, a to-
tally new order of combinations. Thus, a degree of heat,
which would occasion no change in most mineral substances,
will at once effect the complete disunion of the elements of
an animal or vegetable body. Organic substances are, in

14 ' THE VITAL FUNCTIONS.

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 consti-
tuting fermentation and putrefaction, which occur when their
vitality is extinct, and when they are consequently aban-
doned to the uncontrolled operation of their natural chemi-
cal 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 suffered to
remain, would accelerate the destruction of the adjoining
par^are immediately removed, and their place supplied by
parWles tliat have been modified for that purpose, and
which, when they afterwards lose these salutary properties,
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, especial-
ly in the higher classes of animals. In the fabric of those
animals which possess an extensive system of circulating
and absorbing vessels, the changes that 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 por-
tion of the body after the lapse of a certain time. The pe7
riod assigned by the ancients for this entire change of the
substance of the body, was seven or eight years: but modern
inquiries, which show us the rapid reparation that takes
place in injured parts, and the quick renewal of the bones
themselves, tend to prove that even a shorter time than this

ORGANIC CHEMISTRY. 15

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 series of the most refined
and artificial operations is required, in order to bring about
the complicated 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 Phy-
siology, 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 mechanism 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 processes, consisting of chemical combinations and de-
compositions which are continually going on in the organic
laboratory of living beings, would reveal still greater won-
ders, and would fill us with a more fervent admiration of
the infinite art and prescience which are even now manifest-
ed to us in every department both of the vegetable and ani-
mal economy.

The processes by which all these important purposes are
fulfilled, comprise a distinct class of functions, the final ob-
ject of which may be termed Nutrition, that is, the repara-
tion of the waste of the substance of the organs, their main-
tenance in the state fitting them for the exercise of their
respective offices, and the application of properly prepared
materials to their development and growth.

The functions subservient to nutrition may be distinguished
according as the processes they comprise relate to seven
principal periods in the natural orders of their succession.
The first series of processes has for its objects the reception
of the materials from without, and their preparation and
gradual conversion into proper nutriment, that is, into mat-
ter having the same chemical properties with the substance

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

16 THE VITAL FUNCTIONS.

of the organs with which it is to he incorporated; and their
purpose being to assimilate the food as much as possible
to the nature of the organic body it is to noiTrish, all these
functions have been included under the term Assimila-
tion.

The second scries of vital functions comprise 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 ac-
complished by means of canals, called vessels, through which
the nutritive fluids move in a kind of circuit: in this case the
function is denominated the Circulation.

It is not enough that the nutritive juices are assimilated:
another chemical process is still required to perfect their ani-
malization, and to retain them in their proper chemical con-
dition for the purposes of the system. This third object is
accomplished by the function oi Respiration.

Fourthly, several chemical products, which are wanted in
different parts of the economy, are required to be formed
by a peculiar set of organs, of which the intimate structure
eludes observation; although we may perceive that in many
instances among the higher orders of beings, a special appa-
ratus 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 Se-
cretion.

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 excretories, 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 must be taken for

POWERS OF ASSIMILATION. 17

the removal of these useless or noxious materials, by trans-
ferring them to the general mass of circulating IdIoocI, so as
either to be again usefully employed, or altogether discard-
ed by excretion from the system. This object is accom-
plished by a peculiar set of vessels; and the function they
perform is termed Msorjjtion.

Lastly, the conversion of the fluid nutriment into tlic
solids of the body, and its immediate application to the
purposes of the development of the organs, of their preser-
vation 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, m.ore 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 ne-
vertheless perceive that in following them successively from
the simplest vegetables to the higher orders of the animal
Idngdpm, 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 nourish-
ment from the crude and simple materials which they ab-
sorb from the earth, the waters,, and the air that surround
them; materials which consist almost wholly of water, with
a small proportion of carbonic acid, and a few saline ingre-
dients, of which that water is the vehicle. But these, after
having been converted by the powers of vegetable assimila-
tion, into the substance of the plant, acquire the character-
istic 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 nutn-

VoL. 11. 3

18 THE VITAL FUNCTIONS.

tive to other plants, which absorb them, and with more fa-
cility adapt them to the purposes of their own systems.
Here they receive a still higher degree of elaboration; and
thus the same materials may pass through several successive
series of modifications, till they become the food of animals,
and are tlien made to under2;o still farther chansces. New
elements, and in j^articulnr nitrogen, is added to the oxy-
gen, 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
appertaining 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 carnivorous animals, which generally
hold a higher rank in the scale of organization, than those
that subsist only on vegetables.

Thus has each created beino; been formed in reference, not
merely to its own welfare, but also to that of multitudes of
others which arc dependent on it for their support, their pre-
servation,— nay, even for their existence. In contemplating
this mutual relationship, this successive 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 pro-
gressive attainment of higher objects ; imtil, in the pqrfcct sys-
tem, and exalted endowments of man, we behold the last re-
sult that has been manifested to us of creative power.

• Nitrog-cn, however, frequently enters into the composition of veg-eta-
blcs: though, in general, in a much smaller proportion than into the sub-
stance of animals, of which lust it always appears to be an essential constitu-
ent.

( la )

CHAPTER II.

NUTRITION IN VEGETABLES.

§1. 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 modern chemistry, it was very generally be-
lieved that plants could subsist on w^ater alone ; and Boyle,
and Van Helmont, in particular, endeavoured 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 ascertained 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 scj^cely di-
minished in weight, while the w^illow had grown into a tree,
and had acquired an additional weight of one hundred and
fifty pounds : whence he concluded that the w^ater had been
the only source of its nourishment. But it does not seem to
have been, at that time, known, that rain-water always con-
tains 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 w^holly 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-chesnut 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, in-

JO THE VITAL FUNCTIONS.

<lcctl, but tlicy were exceedingly stinted in their growth, and
evidently derived little or no sustenance from the water with
which thev were supplied. Experiments of a similar nature
were made by Bonnet, and with the like result. When plants
«'ire contained in closed vessels, and regularly supplied with
water, but denied all access to carbonic acid gas, they are
developed oidy to a very limited extent, determined by the
.store of nutritious matter which had been already collected in
each plant when the experiment commenced, and which, by
combining with the water, may have allbrded a temporary
supplv of nourishment.

13ut the water which nature furnishes to the vegetable or-
gans is never perfectly pure; for, besides 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 mate-
rials 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^arried along with it into the
vegetable system. It does not appear, however, that purq
carbon is ever admitted; for Sir II. Davy, on mixing char-
coal, ground to an impalpable powder, with the water into
which the roots of mint were imm.ersed, could not discover
that the smallest quantity of that substance had been, in any
case, absorbed.* But in the form of carbonic acid, this ele-
ment 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, de-
rived 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 nourish-
picnt.

* Elements of AgricuIturLil Cliemistiy, Lcct. \l. p. 234.

rOOD OF PLANTS. 21

The soil is also the source whence plants derive their sa-
line; 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 Ber-
zilius, 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 surface,
or collected in the interior of many plants, such as the bam-
boo, and various species of grasses. The small degree of
solubility of many substances thus required for the con-
struction of the solid vegetable fabric, is, probably, one of
the reasons why plants require so large a supply of water
for their subsistence.

§ 2. Msorption 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 tdlgseAor instance, which are aquatic plants.
In LichenSy on the other hand, absorption takes place more
partially; but the particular parts of the surface where it oc-
curs are not constantly the same, and appear to be deter-
mined more by mechanical causes than by any peculiarity
of structure: some, however, are found to be provided in
certain parts of the surface with stomata, which De Can-
dolle supposes may act as sucking orifices. Many mush-
rooms appear to be capable of absorbing 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 in
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 circumstances,
the leaves, or the stems of plants are found to absorb mois-

22 THE VITAL FUNCTIONS.

ture, which they have hcen supposed to do by the stomata
interspersed on their surface. This, however, is not their
natural action; and they assume it only in forced 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 water-
ing, even before any moisture can be supposed to have pe-
netrated to the roots.

It is by the extremities of the roots alone, or rather by
the spongioles which are there situated, that absorption takes
place: for the surface of the root, being covered in every
other part by a layer of epidermis, is incapable of perform-
ing this office. It was long ago remarked by Duhamel, that
trees exhaust the soil only in those parts which surround the
extremities of the roots: but the fact that absorption is ef-
fected only at those points has been placed beyond a doubt by
the direct experiments of Senncbier, who, taking two car-
rots 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 ano-
ther carrot in the fluid, with the exception of the extremity
of the root, which was raised so as to be above the surface,
no absorption whatever took place. Plants having' a yim-
form, or spindle-shaped root, such as the carrot and the
radish, are the best for these experiments.

In the natural progress of growth, the roots are constant-
ly shooting forwards in the direction they have first taken,
whether horizontally, or vertically, or at any other inclina-
tion. 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 preserved between their lateral
extension and the horizontal spreading of the branches, the

VEGETABLE ABSORPTION. 23

greater part of the rain which falls upon the tree, is made to
drop from the leaves at the exact distance from. the trunk,
where, after it has soaked through the earth, it will be re-
ceived by the extremities 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 different departments of na-
ture, and which are made to concur in the production of re-
mote effects, that could never have been accomplished with-
out 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 partly, also, by what has been termed
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 tlie stem of the plant.
The most probable explanation of this phenomenon is, that
the progressive movement of the fluid is produced by alter-
nate contractions and dilatations of the cells themselves,
which compose the texture of the plant: these actions being
themselves referrible 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 vis-
cid or glutinous a consistence 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 exhi-
bited when the saline solutions of a certain strength arc pre-
sented to the roots: the water of the solution, with only a

24 THE VITAL FUNCTIONS.

small proportion of the salts, being taken up, and the re-
maining part of the fluid being found to be more sti'ongly
impregnated with the salts than before this absorption liad
taken place. It would appear, however, that all this is
merely the result of a mechanical operation, and that it fur-
nishes no evidence of any discriminating faculty in the
.spongiole: for it is found that, provided the material pre-
sented be in a state of perfect sohition and limpidity, it is
sucked in with equal avidity, whether its qualities be dele-
terious or salubrious. Solutions of sulphate 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 exceedingly in-
jurious when admitted into the system of the plant, from
the excess of nutriment it contains. 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 re-
ceive 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 in-
gredients necessary for the nourishment of the plant, is in
a perfectly crude state. It rises in the stem of the plant, un-
dergoing scarcely any perceptible cliange in its ascent; and
is in this state conducted to the leaves, where it is to expe-
rience 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 tlie 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 de-

VEGETABLE ABSORPTION. 25

velopment, which usually takes place when the genial
warmth of spring has penetrated heyond the surface, and
expanded the fibres and vessels of the plant, there arises an
urgent demand for nourishment, which the roots arc active-
ly employed in supplying. As the leaves are not yet com-
pleted, tl\p sap is at first applied to purposes somewhat dif-
ferent from those it is destined to fulfil at a more advanced
period, when it has to nourish the fully expanded orgp.ns:
this fluid has, accordingly, received a distinct appellation,
being termed the nii7\sling sap. Instead of rising through
the alburnum, the nursling sap ascends through the inner-
most circle of wood, or that which is immediately contigu-
ous to the pith, and is thence transmitted, by unknown
channels, through the several layers of wood, till it reaches
the buds, which it is to supply with nourishment. During
this circuitous passage, it probably undergoes a certain de-
gree of elaboration, fitting it for the ofilce which it has to
perform: it apparently 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 nursling sap, provided for the nourish-
ment 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 through 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 circum-
stances, it is not transmitted through any of the distinguisha-
ble 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 inter-
cellular spaces. That the latter is the course it takes, is the
opinion of De Candollc, who adduces a variety of arguments
in its support. The sap, he observes, is found to rise equally

Vol. II. 4

2b THE VITAL FUNCTIONS.

well in plants whose structure is wholly cellular ; a fact which
proves the vessels arc not, in all cases, necessary for its con-
veyance. In many instances, the sap is known to deviate
from its usual rectilinear path, and to pursue a circuitous
course, very difUjrent from that of any of the known vessels
of the plant. The dillusion of the sap in diiferent ^directions,
and its suhsidcnce in llie lowest parts, on certain occasions,
are facts irreconcilahlc with the supposition that it is confined
in these vessels.

Numerous experiments have heen made to discover the ve-
locity with which the sap rises in plants, and the force it ex-
erts in its ascent. Those of Hales are well known : hy lopping
off the top of a young vine, and applying to the truncated ex-
tremity a glass tuhe, which closed round it, he found that the
fluid in the tuhe rose to a height, which, taking into account
the specific gravity of the fluid, was equivalent to a perpen-
dicular column of water of more than forty-three feet; and,
consequently, exerted a force of propulsion considerably great-
er than the pressure of an additional atmosphere. The velo-
city, as well as the force of ascent, must, however, be hable
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 eflccted ; but, although it seems
likely to be resolved into the vital movements of the cellular
structure already mentioned, 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 ve-
getable tissue, and exerted, under certain circumstances, and
in conformity to certain laws, which we have not yet succeed-
ed in determining.

VEGETABLE EXHALATION. 27

• • § 3. Exhalation,

The nutrient sap, which, as we have seen, rises in the
stem, and is transmitted to the leaves witliout any cliange in
its qualities or composition, 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 propor-
tion 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, docs not
contain more than a 10,000,000th part of the foreign mat-
ter with which it was impregnated when first absorbed by
the roots. The water thus exhaled, being dissolved by the
air the moment it escapes, passes ofT 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 daily loss of the plant by exha-
lation was twenty ounces ; and this, he computes, is a quanti-
ty seventeen times greater than that lost by insensible per-
spiration from an equal portion of the surface of the human

bodv.

The comparative quantities of fluid exhaled by the same
plant, at different times, are regulated, not so much by tem-
perature, 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 arti-
ficial light of lamps produces on the leaves an effect similar
to that of the solar rays, and iji a degree pro})ortionate to its
intensity.* As it is only through the stomata that exhalation
proceeds, the number of these pores in a given surlace must
considerably influence the quantity of fluid exhaled.

By the loss of so large a portion of the water which, in the
rising sap, had held in sdution various foreign materials, these

* Physiologic Vegetale, i. 112.

23 THE VITAL FUNCTIONS.

substances are rendered more disposed to separate from the
fluid, and to become consolidated on the sides of the cells of
vessels, to which they are conducted from the leaves. This,
tlien, is the lirst modification in the qualities of the sap which
it undergoes hi those organs.

§ 4. Aeration of the Sap.

A CHEMICAL chang;c much more considerable and im-
portant than the prccedini;- is next effected on the sap by the
leaves, when they are subjected 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 di-
rectly from the surrounding atmosphere. In either case its
oxygen is separated, and is disengaged in the form of gas;
while its carbon is retained, and composes an essential in-
gredient of the altered sap, which, as it now possesses one of
the principal elements of vegetable structures, may be con-
sidered as having made a near approach to its complete assi-
mi/ation, 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
vSennebier we are indebted for the first observation that the
presence of carbonic acid is required for the disengagement
of oxygen in this process, and that the oxygen is derived
from the decomposition of the carbonic acid, and these lat-
ter facts have since been fully established by the researches
of Mr. Woodhouse, of Pennsylvania, ]Mr. Theodore de Saus-
sure, and Mr. Palmer. They are proved in a very satisfac-
tory 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 otlicr 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 communication between

AERATION OF THE SAP. 29

the water and the atmosphere. The sprig of mint was ex-
posed to the light of the sun for twelve days consecutively:
at the end of each day the carbonic acid was seen to di-
minish in quantity, the water rising in the jar to supply the
place of what was lost, and at the same time the plant ex-
haled a quantity of oxgyen, exactly equal to that of the carbo-
nic acid which had disappeared. A similar sprig of mint,
placed in ajar of the same size,full of distilled water,but with-
out 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, ox-
ygen gas was substituted in the first jar instead of carbonic
acid gas, no gas was disengaged 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 experiment
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 deter-
mining this decomposition of carbonic acid: but the experi-
ments are of so delicate a nature, that this result requires to
be confirmed by a more rigid investigation, before it can be
admitted as satisfactorily established.

That the carbon resulting from this decomposition of car-
bonic 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 previously con-
tained.

It is in the screen substance of the leaves alone that this
process is conducted: a process, which, from the strong ana-
logy that it bears to a similar function in animals, may be
considered as the respiration of vegetables. The cfiect ap-

30 THE VITAL FUNCTIONS.

pears to 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 cOcct which is in some respects
the opposite of this; for they have a tendency to absorb oxy-
o-en, 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, absorb a portion of that element. The oxygen
thus absorbed enters immediately into combination 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.

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 expect 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 or-
der to produce the result intended.

The water which is absorbed by the roots generally car-
ries with it a certain quantity of soluble animal or vegetable
materials, which contain carbon. This carbon is transmit-
ted to the leaves, where, during the night, it is made to
combine with the oxygen they have absorbed. It is thus

AERATION OF THE SAP. 31

converted into carbonic acid, which, when daylight pre-
vails, 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 disinte-
gration, to which it is reduced at the moment of its separa-
tion 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 available 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 neces-
sity of its having to undergo this double operation of first
combining 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 sys-
tem, 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 reduce 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 ^Seration.

Thus the great object to be answered by this vegetable
aeration is exactly the converse of that which we shall af-
terwards 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 dis-
charging the superfluous quantity of carbon from the animal
system. The absorption of oxygen, and the partial disen-
gagement of carbonic acid, which constitute the nocturnal
changes effected by plants, must have a tendency to deteri-
orate the atmosphere with respect to its capability of sup-
porting animal life; but this effect is much more than com-
pensated 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

32 THE VITAL FUNCTIONS.

of which efTects tend to its purification and to Its remaining
adapted to the respiration of animals. Nearly the whole of
the carhon accumulated by vegetables 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 arc longer than the nights;
so that the diurnal process of purification goes on for a great-
er number of hours than the nocturnal process by which the
air is vitiated.

The oxygen given out by plants, and the carbonic acid
resulting from animal respiration, and from the various pro-
cesses 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 diffusion, but
also from the action of the winds, which are continually agi-
tating the whole mass, and promoting the thorough mingling
of its different portions, so as to render it perfectly homo-
geneous in every region of the globe, and at every elevation
above the surface.

Thus are the two great organized kingdoms of the crea-
tion made to co-operate in the execution of the same design:
each ministering to the other, and preserving that due ba-
lance 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 suspended. It is impossible to contem-
plate so special an adjustment of opposite effects without ad-
miring this beautiful dispensation of Providence, 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 particles, diluted with a

RETURN OF THE SAP. 33

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 nutri-
ment; and that nutriment has been reduced to those particu-
lar forms and states of composition which render it applica-
ble to the growth of the organs, and the other purposes of
vegetable life. This fluid, therefore, corresponds to tlic
blood of animals, which, like the elaborated sap, may be re-
garded as fluid nutriment, perfectly assimilated to that par-
ticular kind of organization, with which it is to be afterwards
incorporated. 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 re-
turning sap, that it might be distinguished from the crude
fluid which arrives at the leaves, and which is termed tlie
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 exhaled by
the leaves, has been actually decomposed, and that its sepa-
rated elements, the oxygen and the hydrogen, have been
combined with certain proportions of carbon, hydrogen, ni-
trogen and various earths, metals and salts, so as to form the
proximate vegetable products, which are found in the re-
turning 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

* According- to the investig-ations 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 wliich they would have united to
form 586 grains of water; together witli 414 of carbon, or the base of carbo-
nic acid. This, according to the doctrine of chemical equivalents, corre-
sponds to one molecule of water, and one molecule of carbon. I'hil. Trans.
1827, 584.

Vol. 11. 5

34 THE VITAL FUNCTIONS.

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 arc convcrtai)le into tliis product, Gum may be
fairly assumed to be the principal basis of vegetable nutri-
ment; and its simjilo and definite composition points it out
as being the immediate result of the chemical changes
which the sap experiences in the leaves. During the de-
scent of the sap, however, this iluid undergoes, in various
parts of the plant, a farther elaboration, which gives rise
to other ])roducts. We arc 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 alburnum,
or outermost layer of the wood. With regard to the ex-
act channels through which it passes, the same degree of un-
certainty prevails as with regard to those which transmit
the ascending sap. I)e Candollc maintains that, in either
case the fluids find their way through the intercellular
spaces: other physiogists, however, are of opinion, that par-
ticular vessels are appropriated to the ofTice of transmitting
the descending sap. The extreme minuteness of the organs
of veo-etables has hitherto presented insuperable obstacles to
the investigation of this important question: and consequent-
ly our reasonings respecting it can be founded only on in-
direct evidence. The processes of the animal economy,
where the channels of distribution, and the organs of pro-
pulsion are plainly observable, afford but imperfect analogies
to guide us in this intricate inquiry: for although it is true
that in the higher classes of animals the circulation of the
nutrient fluid, or blood, through distinct vessels, is sufficient-
ly obvious, yet in the lower departments of the animal king-
dom and in the embryo condition even of the more perfect
species, the nutritious juices are distributed without being
confined within any visible vessels; and they either perme-

RETURN OF THE SAP. 35

ate extensive cavities in the interior of the body, or pene-
trate through the interstices of a celkilar tissue. That this
latter is the mode of transmission adopted in tlie vegetable
system has been considered probable, from the circumstance
that the nutritious juices arc diffused throughout those plants
which contain no vessels whatsoever with the same facility
as throughout 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 distribution to dilferent parts, is involved in equal
obscurity 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
been latest formed; that is, through the innermost layers of
their structure.

The returning sap, w^hile traversing these several parts of
the plant, deposites in each the particular 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
contributes: 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 consequence, produced,
which, from the intricacies 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 results, namely, the
gradual deposition of the materials of the future alburnum
and liber. These materials are first deposited in the form
of a layer of glutinous substance, termed the Cambium; a
substance which appears to consist of the solid portion of the

36 THE VITAL FUNCTIONS.

sap, precipitated from it by the separation of the o-fcater part
of the water that held it in solution. Tlie cambium becomes,
in process of time, more and more consolidated, and acquires
the organization proju^' to the ])lant of which it now form.s
an inte2;rant part: it constitutes two layers, the one, belona;-
ins to the wood, l^einir tlie all)urnuni; tlie other, belonirinj^
to the bark, beinii the liber.

The all)urnum and the liber, which have been thus con-
structed, perform an important part in inducing ulterior
changes on tlie nutrient materials which the returning sap
continues to supj)ly. Tlieir cells absorb the gummy sub-
stance from the surrounding fluid, and by their vital powers
effect a still farther elaboration in its composition; convert-
ing it either into starch, or sugar, or lignin, according to the
mode in which its constituent elements are arranged. Al-
though 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 ef-
fect these conversions, are comparatively slight, and may
readily take place in the sim])le cellular tissue.*

In tiie series of decompositions which are artificially ef-
fected in the laboratory of the chemist, it has been found
that gum and sugar are intermediate products, or states of
transition between various others; and they appear to be pe-
culiarly calculated, from their great solubility, for being easi-
ly conveyed from one organ to another. Starch and lignin,
on the other hand, are compounds of a more permanent
character, and especially adapted for being retained in the
organs. Starch, which, though solid, still possesses consi-

• According to the aniilyscs of Dr. I'rout, the following- is the composition
of these siibstunces: 1000 parts of

Pure Gum Arabic consist of 586 of oxygen and liydrogcn, united in tlie

proportions in which they exist in water, and 414 of carbon.
Dried Starch, or Fecula, of 560 water, and 440 - - -
Pure ciystallized Sugar, - - 572 - - - 428 - - -
Lignin from Boxwood, - - - 500 - - - 500 - - -

RETURN OP THE SAP. 37

derable solubility, is peculiarly fitted for being applied to
the purposes of nourishment: it is accordingly hoarded in
magazines, with a view to future employment, being to ve-
getables, what the fat is to animals, a resource for the exi-
gencies that may subsequently arise. With this intention,
it is carefully stored in small cells, the coats of which pro-
tect it from the immediate dissolving action of the surround-
ing watery sap, but allow of the penetration of this fluid, and
of its solution, when the dem.ands of the system require it.
The tuberous root of the potato, that invaluable gift of Pro-
vidence 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 inten-
tion of forming a permanent part of the vegetable structure,
constituting the basis of the woody fibre, and giving mecha-
nical support and strength to the whole fabric of the plant.
These latter structures may be compared to the bones of ani-
mals, composing, by their union, the solid frame work, or
skeleton of the organized system. The woody fibres do not
seem to be capable of farther alteration in the living vegeta-
ble, and are never, under any circumstances, taken up and
removed to other parts of the system, as is the case with nu-
tritive matter of a more convertible kind.

The sap holds in solution, besides carbonaceous matter,
some saline compounds and a few earthy and metallic bases:
bodies which, in however minute a quantity they may be
present, have unquestionably a powerful influence in deter-
mining certain chemical changes among the elements of or-
ganic products, and in imparting to them peculiar proper-
ties; for it is now a w^ell ascertained fact that a scarcely
sensible portion of any one ingredient is capable of pro-
ducing important dificrences 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 neighbourhood of melt-

38 THE VITAL FUNCTIONS.

ed gold, have the power of destroying its ductility.* In the
experiments made by Sir John Herschel on some remarka-
ble 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 compound, are suflicient to communicate
sensible mechanical motions, and delinite properties to the
bodies witli which they are mixed. t

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 exhalation
of the water which held it dissolved; and it is found, ac-
cordingly, 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 accu-
mulation of silicious particles takes place, and by clogging
up their cells and vessels, 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 ulti-
mate destruction of the leaves: their foot-stalks, more espe-
cially suffering from tliis obstruction, perish, 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. Seo^elion in Vegetables.

While the powers of the simpler kinds of cells are ade-
quate to produce in the returning sap the modifications
above described, by which it is converted into gummy, sac-
charine, amylaceous, or ligneous products; there are other
cellular organs, endowed with more extensive powers of

• Hatchclt. f Philosophical Transactions for 1824, p. 162,

VEGETABLE SECRETION. 39

chemical action, which effect still greater changes. The na-
ture of the agents by which these changes are produced arc
unknown, and are therefore referred generally to the vital
energies of vegetation; but the process itself has been termed
Secretion, and the organs in which it is conducted, and
which are frequently very distinguishable as separate and
peculiar structures, are called Glands. When the products
of secretion are chemically analyzed, the greater number
are found to contain a large quantity of hydrogen, in addi-
tion to that which is retained in combination with oxygen
as the representative of water: this is the case with all the
oily secretions, whether they be fixed or volatile, and also
with those secretions which are of a resinous quality. Some,
on the contrary, are found to have an excess of oxygen; and
this is the condition of most of the acid secretions; while
others, again, appear to have acquired an addition of nitro-
gen.

All these substances have their respective uses, although
it may frequently be difficult to assign them correctly. Some
are intended to remain permanently enclosed in the vesicles
where they were produced; others are retained for the pur-
pose of being employed at some other time; while those be-
longing to a third class are destined to be thrown off from
the system as being superfluous or noxious: these latter sub-
stances, which are presently to be noticed, are specially de-
signated as excretions. Many of these fluids find their way
from one part of the plant to another, without appearing to
be conducted along any definite channels, and others are
conveyed by vessels, which appear to be specially appro-
priated to this office.

The following are examples of the uses to which the pe-
culiar secretions of plants are applied. Many lichens, which
fix themselves on calcareous rocks, such as the Fatcllaria
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 substance, analogous to
that which many marine worms arc known to possess. The

40 THE VITAL FUNCTIONS.

agent appears in both instances to be an acid, which here is
probably the oxalic, acting uj)on the carbonate of lime, and
producing the gradual excavation of the rock. This view is
confirmed by the observation that the same species of lichen,
when attached to the rocks which are not calcareous, re-
mains always at the surface, and does not penetrate 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. Tliis is the case with the Nettle.
The slightest pressure made by the hand on the hairs grow-
ing on the leaves of this plant, causes the fluid in their vesi-
cles to pass out from their points, so as to be instilled into
the skin, and occasion the well known irritation which en-
sues. INI. De CandoUe, junior, has ascertained, by chemical
tests, that the stinging fluid of the nettle is of an alkaline na-
ture. In some species of this genus of plants, the hairs are
so large that the whole mechanism above described is visi-
ble to the naked eye. This apparatus bears a striking re-
semblance 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 effec-
tually defending the young buds from the injurious effects
of moisture; and 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 Ceroxylojiy
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 re-
pelling water: the leaves of the Mesembryanthemum, or
Fig-marigold, of the JJtripIex, or Orache, and of the Bras-
sica, or Cabbage, may be given as examples of this curious
provision. Such plants, if completely immersed in water,
may be taken out without being wetted in the slightest de-
gree; thus presenting us with an analogy to the plumage of
the cygnet, and other aquatic birds, which are rendered com-
pletely water-proof by an oily secretion spread over their

CIRCULATION IN PLANTS. 41

surface. Many aquatic plants, as the BatrachospcTjnum,
are, in like manner, protected by a viscid layer, which ren-
ders the leaves slippery to the feel, and which is impermea-
ble to water.

Several tribes of plants contain liquids that 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 Chelidonium, or Celandine. All
these juices are of a resinous nature, and usually highly
acrid, and even poisonous in their qualities; and their opaci-
ty is occasioned 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 peculiar kind, and exhibit
ramifications and junctions, 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 remarkable 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 1S20; and he designated it by the term
Cyclosis, in order to distinguish it from a real circulation,
if, on farther 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 evi-
dence of the existence of a general circulation of nutrient
juices in the vegetable systems of those plants which ex-
hibit them; for it would appear that in reality the ob-
served motions of the fluid, are, in every case, partial, and

* "Die Nutur der Icbcndig-en Pflaiize." See, also, Annulcs dcs Sciences
Naturellcs, xxiii. 75.

Vol. II. a

42

THE VITAL FUNCTIONS.

the extent of the circuit very limited. The cause of these
motions is not yet known; but probably they are ultimately
referrible to a vital contraction of the vessels; for they cease
the moment that the plant has received an injury, and are
more active in proportion as the temperature of the atmo-
sphere 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 Caulinafragilis, &:c.,* where the double
currents are beautifully seen under the microscope, perform-
ing a complete circulation
within the spaces of the stem
that lie between two adja-
cent knots or joints; and
where, by the proper adjust-
ment of the object, it is easy
to see at one view both the
ascending and descending
streams passing on opposite
sides of the stem. Fig. 239
shows this circulation in the
cells of the Caulinia fragalis
very highly magnified, 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 Tradescantia
virginica,\ in each cell of which the same circulatory motion
of the fluids is perceptible.

• Amici, Annalcs des Sciences Naturelles, ii. p. 41.
f Fig. 239 is taken from Amici, and Fig. 240 from that given by Mr.
Slack, Trans. Soc. Arts, vol. xlix.

VEGETABLE EXCRETIONS. 43

§ 7. Excretion in Vegetables.

It had long been conjectured by De Candollc, 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 originally; and also why
plants of a different species are frequently found to flourish
remarkably well in the same situation where this apparent
deterioration of the soil has taken place. The truth of this
sagacious conjecture has been established in a very satisfac-
tory manner by the recent experiments of M. Macaire.*
The roots of the Chondrilla muralifi 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 sm.ell
and taste, the presence of a bitter narcotic substance, analo-
gous to that of opium; a result which was farther confirmed
by the application of chemical tests, and by the reddish
brown residuum obtained from the water by evaporation.
M. Macaire ascertained that neither the roots nor the stems
of the same plants, when completely detached, and im-
mersed in water, could produce this effect, which he there-
fore concludes is the result of an exudation from the roots,
continually going on while the plant is in a state of healthy
vegetation. By comparative experiments on the quantity
of matter thus excreted by the roots of the French bean
{Phaseotus 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 cor-

• An account of these experiments was first published in the fifth volume
of the "Memoires de la Society de Physique ct d'Histoire Naturelle dc Ge-
neve," and repeated in the " Annales des Sciences Naturclles," xxviii. 402.

44 THE VITAL FUNCTIONS.

responding; absorption by the roots is also suspended. This
was confirmed 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 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 imbibed through the roots, was also proved
by another set of experiments on the Mcrciirialls annua,
the Senecio vulgaris, and Brassica campcstris, 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 separate ves-
sel. 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 va-
ried by first allowing the plant to remain with its roots
immersed in a similar solution, and then removing it,
after carefully 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 ace-
tate of lead, and were attended with the like results. De
CandoUe has ascertained, that certain maritime plants which
yield soda, and which flourish in situations very distant
from the coast, provided they occasionally receive breezes
from the sea, communicate a saline impregnation to the
soil in their immediate vicinity, derived from the salt which
they doubtless 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

VEGETABLE EXCRETIONS. 45

species, it does not therefore follow that they are incapa-
ble of supplying salutary nourishment to other kinds of
plants: thus, it has been observed that the Salicaria flou-
rishes particularly in the vicinity of the willow, and the Oro-
banche, or broom-rape, in that of hemp. This fact has also
been established experimentally by M. Macaire, who found
that the vvater 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 different kind.

This fact is of great importance in the theory of agricul-
ture, since it perfectly explains the advantage derived from
a continued rotation of different crops in the same field, in in-
creasing the productiveness of the soil. It also gives a satis-
factory explanation of the curious phenomenon oi fairy rings,
as they are called, that is, of circles of dark green grass, oc-
curring in old pastures: these Dr. Wollaston has traced to the
growth of successive generations of certain /^/7^^^^, or mush-
rooms spreading from a central point.* The soil, which has
once contributed to the support of these fungi, becomes ex-
hausted or deteriorated with respect to the future crops of
the same species, and the plants, therefore, cease to be pro-
duced on those spots: the second year's crop consequently
appears in the space of a small ring, surrounding the origi-
nal 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 direc-
tion, and occasions the circle of fungi continually to proceed
by annual enlargement from the centre outwards. An ap-
pearance of luxuriance of the grass follows as a natural con-
sequence; 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 herbage is for a time totally desti'byed,

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

46 THE VITAL FUNCTIONS.

giving rise to the appearance of a ring bare of grass, sur-
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 otlier's
progress, they not only do not cross each other, but both
circles are invariably obliterated between the points of con-
tact: for the exhaustion occasioned by each obstructs the pro-
gress of the other, and both are starved. It would appear
that ditferent species of fungi often require the same kind of
nutriment; for, in cases of the interference 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 previous vegetation of its
own species.

The only final cause we can assign for the series of phe-
nomena constituting the nutritive functions of vegetables is
the formation of certain organic products calculated to sup-
ply sustenance 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 ope-
rations of vital chemistry, a power too subtle for human
science to detect, or for human art to imitate, and by which
these materials are combined into a variety of nutritive sub-
stances. Of these substances, so prepared, one portion is
consumed by the plants themselves in maintaining their
own structures, and in developing the embryos 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
subsequent and more extended vegetation, or else, buried in
the bosom of the earth, it forms part of that vast magazine
of combustible matter, destined to benefit future communi-
ties of mankind, when the arts of civilization shall have de-
veloped the mighty energies of human power.

( 47 )

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 enjoying a wider range of ac-
tion, animals, compared with plants, require a considerably
larger supply of nutritive materials for their sustenance, and
for the exercise of their various and higher faculties. The
materials of animal nutrition must, in all cases, have previ-
ously been combined in a peculiar mode; which the powers
of organization alone can effect. In the conversion of vege-
table into animal matter, the principal changes in chemical
composition which the former undergoes, are, first, the ab-
straction of a certain proportion of carbon; and secondly, the
addition of nitrogen.* Other changes, however, less easily
appreciable, though perhaps as important as the former,
take place in greater quantity, with regard to the propor-
tions of saline earthy, and metallic ingredients; all of which,
and more especially iron, exist in greater quantity in ani-
mal than in vegetable bodies. The former also contain a
larger proportion of sulphur and phosphorus than the latter.

* 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 ultimate analysis. They found, in particular,
the same proportion of nitrogen in the chyle, whatever kind of food the ani-
mal habitually 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. {Mcmoires dc la Socicle dc Physique d d'llistoire
Naturelle dc Geneve^ v. 389.)

48 THE VITAL FUNCTIONS.

The equitable mode in which nature dispenses to her in-
numerable offspring the food she has provided for their sub-
sistence, apportioning to each the quantity and the kind
most consonant to enlarged views of prospective benefi-
cence, is calculated to excite our highest wonder and admi-
ration. 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, become the food
of some species of insect, or contribute 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 assigned
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 attached each to a
particular plant which they prefer to all others. There are
at least fifty difierent 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
Eitphorbium, Henbane, and Nightshade, afford a whole-
some and delicious food to others. Innumerable tribes of
animals subsist upon fruits and seeds, while others feast
upon the juices which they extract from flowers, or other
parts of plants; others, again, derive their principal nourish-
ment 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, whe-
ther 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

ECONOMY OP NUTRITIVE MATTER. 49

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 fo-
rest, displaying an equally ferocious and insatiable rapacity.
Other families, again, generally of still smaller size, are de-
signed 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
carnivorous quadrupeds, it never fails to be soon attacked by
swarms of insects, which speedily consume its softer tex-
tures, leaving only the bones.* These, again, are the fa-
vourite food of the Hyena, whose powerful jaws are pecu-
liarly formed for grinding them into powder, and whose
stomach can extract from them an abundant portion of nu-
triment. No less speedy is the work of demolition among
the inhabitants of the waters, where innumerable fishes,
Crustacea, annelida, and mollusca, are on the. watch, to de-
vour 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 conspi-
cuous, 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

* So strongly was Linnzeus impressed with the immensity of the scale on
which these works of demohtion by insects are carried 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 by three flesh flies {Musca voynitoria)
and their immediate progeny: for it is known that one female fly will g-ive
birth to at least 20,000 young larvae, 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 tin|ps its weight: and a few days are sufiicicnt to the produc-
tion of a third generation.

Vol. II. 7

50 THE VITAL FUNCTIONS.

they appear to require for the purposes of nutrition. We
may hence infer that, in tlicir formation, otlier ends were
contemplated, besides 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 atmo-
sphere, or the waters, with infection, and spread a wide ex-
tent 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 admiring how carefully she has pro-
vided the means for turning to the best account every parti-
cle 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 ultimate 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 fras:-
ments of decomposed organizations, which we might con-
ceive 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 favoura-
ble circumstances, again partaking of animation, and invested
with various forms of infusory animalcules, which sport, in
countless myriads, their ephemeral existence, within the
ample regions of every drop. Yet, even these are still qua-
lified 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 useless-
ly in the vast abyss, but contribute to maintain in existence
incalculable hosts of animal beings, which people every por-
tion of the wide expanse of ocean, and which^rise, in regu-
lar gradation, from the microscopic monad, and scarcely vi-

ECONOMY OF NUTRITIVE MATTER. 51

sible medusa,* through endless tribes of mollusca, and of
fishes, up to the huge Leviathan of the deep.

Not even are these portions of organic matter, which, in
the course of decomposition, escape in the form of gases,
and are widely diffused through the atmosphere, wholly
lost for the uses of living nature: for, in course of time,
they, also, as we have seen, re-enter into the vegetable sys-
tem, resuming the solid form, and reappearing as organic
products, destined again to run through the same never end-
ing 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 regu-
lated b}' the seasons, 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 con-
tinual interchanges produced, contributing to colonize the
earth, and extend its animal population over every habitable
district. But in all these changes we may discern the ulti-
mate relation they ever bear to the condition of the vegeta-
ble world, which is placed as an intermediate and necessary
link between the mineral and the animal kingdoms. All
those regions which are incapable of supporting an exten-
sive vegetation, are, on that account, unfitted for the habita-
tion 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 summits of the
highest mountain chains; and such is the wide expanse of
sand, which covers the largest portions both of Africa and

* The immensity of the numbers of these microscopic medusae, which peo-
ple every region of the ocean, may be judged of from the phenomenon of
the phosphorescent light which is so frequently exhibited by the sea, when
agitated, and which, as 1 have already observed, is found to arise from the
presence of an incalculable multitude of these minute animals.

52 THE VITAL FUNCTIONS.

of Asia: and often have we heard of the sunken spirits of the
traveller through the weary desert, from the appalling si-
lence that reigns over those regions of eternal desolation;
hut no sooner is his eye refreshed hy the reappearance of
vegetation, than he again traces the footsteps and haunts of
animals, and welcomes the cheering sound of sensitive be-
ings.

The kind of food which nature has assigned to each par-
ticular race of animals has an important influence, not mere-
ly on its internal organization, but, also, on its active powers
and disposition; for the faculties of animals, as well as their
structure, have a close relation to the circumstances con-
nected 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 de-
velopment: such, for instance, is the condition of herbivo-
rous quadrupeds, 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 aflbrds them the means of support. Predaceous
animals, on the contrary, being prompted by the calls of ap-
petite to wage war with living beings, are formed for a more
active and martial career; their muscles are more vigorous,
their bones are stronger, their limbs more robust, their senses
more delicate 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 jackal? All the perceptions
of carnivorous animals are more accurate, their sagacity em-
braces a greater variety of objects, and, in feats of strength
and agility, they far surpass the herbivorous tribes. A tiger
will take a spring of fifteen 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 ele-
phant. \

INFLUENCE OF THE DEMAND FOR FOOD. 53

While herbivorous animals are almost constantly 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 ex-
haustion consequent upon fasting for two or three days,
whereas, the wolf and the 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. Herbivo-
rous 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 ne-
ver so great as when he is nearly famished, and he rushes
to the attack, reckless of consequences, 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 oflfence, which it
wants not the courage to employ, when necessary, is capa-
ble of being tamed with the greatest ease, is readily brought
to submit to the authority of man, and requites with afifec
tion the benefits he receives.

On first contemplating this extensive destruction of ani-
mal life, by modes the most cruel and revolting to all our
feelings, we naturally recoil with horror from the sanguina-
ry scene; and cannot refrain from asking how all this is con-
sistent with the wisdom and benevolence so conspicuously
manifested in all other parts of the creation. The best theo-
logians have been obliged to confess that a difficulty does
here exist,* and that the only plausible solution which it ad-
mits of, is to consider the pain and suffering thus created, as
one of the necessary consequences of those general laws

* See, in particular, Palcy's Natural Thcolog-y, chap. xxvi.

54 THE VITAL FUNCTIONS.

which secure, on the whole, the greatest and most permanent
good. There can he no douht that the sclieme, hy which
one animal is made directly conducive to the subsistence of
another, leads to the extension of the benefits of existence
to an infinitely greater number of beings than could other-
wise have enjoyed them. This system, besides, is the spring
of motion and activity in every part of nature. While the
pursuit of its prey forms the occupation, 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, success-
ful; for, wherever nature has inspired sagacity in the percep-
tion 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 destruc-
tion; others, among the myriapoda, fold themselves into the
smallest possible compass, so as to escape detection. 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 glo-
bular body for the same purpose, and floats on the sea, armed
at all the points of its surface; the cuttle-fish screens itself
from pursuit by efiusing 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 reiterated discharges from
its electric battery; the butterfly avoids capture by its irre-
gular movements in the air, and the hare puts the hounds at
fault hy her mazy doublings. Thus does the animated crea-
tion present a busy scene of activity and employment: thus
arc 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 de-
grees, as well as of a larger amount of enjoyment, than ap-

SERIES OF VITAL FUNCTIONS. 55

pears 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, Respiration, Secretion, Excretion,
Absorption, and Nutrition; some of which even admit of
farther subdivision. This is the case more particularly
with the processes of assimilation, which are generally nu-
merous, and require a very complicated apparatus for acting
on the food in all the stages of its conversion into blood, a
fluid which, like the returning sap of plants, consists of nu-
triment in its completely assimilated state. It will be ne-
cessary, therefore, to enter into a more particular examina-
tion of the objects of these different processes.

In the more perfect structures belonging to tlie 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 chemical agents to which it
is afterwards to be subjected. From the mouth, after it has
been sufficiently masticated, and softened by fluid secretions
prepared by neighbouring glands, the food must be con-
veyed into an interior cavity, called the Stomach,, where,
as in a chemical laboratory, it is made to undergo the par-
ticular change which results from the operation termed Di-
gestion. 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 ap-
propriate channels of circulation, not only to the respiratory

56 THE VITAL FUNCTIONS.

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 de-
scend in the scale, we find them more and more simplified,
by the concentration of organs, and the union of many of-
fices in a single organ, till we arrive, in the very lowest or-
ders, at little more than a simple digestive cavity, perform-
ing 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, whether internal or
external, the muscular and nervous systems, the glands, ves-
sels, 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 peculiar cha-
racteristic of the animal system as contrasted 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, howe-
ver 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

• See the first chapter of this vohime, p. 23.

•(• In some species of animals belonging to the tribe of mediisx, as the
Eiidora, Berenice, Orylhia, Favonia, Lymnoriay and Geryonia, no central
cavity corresponding" to a stomach has been discovered: they appear, there-
fore, to constitute an exception to the general rule. See Peron, Annales de
Museum, xiv. 227 and 326.

INFLUENCE OF THE DEMAND FOR FOOD. 57

nearly constant supply, and have no receptacle for collcctlni:;
it at its immediate entry; the sap, as we have seen, passnig
at once into the cellular tissue of tlie plant, where llie pro-
cess of its gradual elaboration is commenced. x\nimals, on
the other hand, are capable of receiving at once large sup-
plies of food, in consecjucncc of having an internal cavity,
adapted for the immediate reception of a considerable quan-
tity. 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 organs of assimilation within itself, and
having receptacles in which it can lay in a store of provi-
sions, it may be said to be nourished from within; for it is
from these interior receptacles that the lacteals, or absorb-
ing vessels corresponding in their office to the roots of ve-
getables, imbibe nourishment. Imj^ortant consequences
ilow 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, absolutely neces-
sary to beings which have their subsistence to seek. It is
this necessity, again, which calls for the continued exercise
of their senses, intelligence, and more active energies; and
that lead, in a word, to the possession of all those higher
powers which raise them so far above the level of the ve-
getable creation.

Vol. II.

( 58 )

CHAPTER IV.

241

Nutrition in the lower Orders of Jlnimals.

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 nothing more.
A simple sac, or tube, adapted to receive and digest food, is
the only visible organ of the body. It exhibits not a trace
of either brain, nerves, or organs of sense, nor any part

corresponding to lungs, heart, or even
^■J^ 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 thickness of the walls of this
cavity. The structure of these w^alls 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 through its
substance, probably by means of invisible pores, of llic nu-
tritious particles thus extracted from the food, for the pur-
pose of its being incorporated and identified with the ge-
latinous 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 microsco})e. Trembley watched them with unwearied
perseverance for days together, and has given the following

NUTRITION IN POLYTI. 59

account of his observations. The liydra, though it does not
pursue the animals on which it feeds, yet devours with avi-
dity all kinds of living prey that come witliin the reach
of its tentacula, and which it can overcome and introduce
into its mouth. The larvai of insects, naides, and other
aquatic worms, minute Crustacea, and even small fishes, arc
indiscriminately laid hold of, if they happen ])ut to touch
any part of the long filaments which the animal spreads out,
in different directions, like a net in search of food. The
struggles of the captive which finds itself entangled in the
folds of these tentacula, are generally ineffectual, and the hy-
dra, like the boa constrictor, contrives, by enormously ex-
panding its mouth, slowly to draw into its cavity animals
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 de-
vouring the vermiform larva of a Tijjula, which it has en-

circled with its tentacula, to which it has applied its expand-
ed mouth, and of which it is absorbing the juice, before
swallowing it. Fig. 243 shows the same animal, after it has
succeeded, though not without a severe contest, in swallow-
ing a minnow, or other small fish, the form of which is still
visible through the transparent 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

60 THE VITAL FUNCTrONS.

its dificrcnt ends, that a struggle between them ensues, anil
that llic strongest, having obtained tlie victory, swallows at
a single gulp, not only the object of contention, but its an-
tagonist also. The scene is represented in Fig. 244, where
the tail of the hydra, of which the body has been swallowed
by the victor, is seen protruding from the mouth of the lat-
ter. It soon, however, extricates itself fi'om this situation,
apparently without having suflcred the smallest injur3\ The
voracity of tlie hydra is very great, especially after long fast-
ing; and it will then devour a great number of insects, one af-
ter another at one meal, gorging itself till it can hold no more,
and its body becoming dilated to an extraordinary size: and
yet the same animal can continue to live for more than four
months without any visible supply of food. .

On attentively observing the changes induced upon the
food by the action of the stomach of these animals, they ap-
pear to consist of a gradual melting down of the softer parts,
which are resolved into a kind of jelly, leaving unaltered
only a few fragments of the harder and less digestible parts.
These changes arc 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-
di 'tested materials being collected together and rejected by
the mouth, the remaining fluid is seen to contain opaque
••lobules of vai'ious sizes, some of which are observed to pe-
netratc through the sides of the stomach, and enter into the
granular structure which composes the flesh of the animal.
Some portion of this opaque fluid is distributed to the tenta-
cula, into the tubular cavities of which it may be seen en-
tering by passages of communication with the stomach. By
watching attentively the motions of the globules, it will be
perceived that they pass backwards and forwards through
these passages, like ebbing and flowing tides.

All these phenomena may be observed with greater dis-
tinctness when the food of the animal contains colouring
matter, capable of giving a tinge to the nutritious fluid, and

NUTRITION IN POLYri. 61

allowing of its progress being traced into the granules which
are dispersed throughout the substance of the body. Trem-
bley 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 nourishment. 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 successively, in an order determined by their
distance from the surface of the stomach. Trembley ascer-
tained 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 tenta-
cula, which had encircled it: but these tentacula always ul-
timately came out of the stomach, sometimes after having
remained there twenty-four hours, without the least detri-
ment.

The researches of Trembley have brought to light the
extraordinary fact that not only the internal surface of the
stomach of the polypus is endow^ed with the power of di-
gesting food, but that th^ same property belongs also to the
external surface, or wiiat we might call the skin of the ani-
mal. He found that b}^ a dexterous manipulation, the hy-
dra 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 accommo-
dates itself in the course of a day or two to the transforma-
tion, and resumes all its natural habits, eagerly seizing ani-
malcules with its tentacula, and introducing them into its
newly formed stomach, which has for its interior surface
what before was the exterior skin, aaul which digests them
with perfect ease. When the discovery of this curious phe-
nomenon was first made known to the world, it excited
great astonishment, and many naturalists were incredulous

62 THE VITAL FUNCTIONS.

as to the correctness of the observations. But the researches
of l^oiinet and of Spallanzani, who repeated the experiments
of Trembley, have borne aniple testimony to their accuracy,
which those of every subsequent observer have farther con-
tributed to confirm.

The experiments of Trembley have also proved that eve-
ry portion of the hydra possesses a wonderful power of re-
pairing 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 asunder; 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 sepa-
rate 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, another will be
formed v/ith double that number. If any of the parts of this
compound polypus be cut off, as many new ones will spring
up to replace them; the mutilated heads at the same time
acquiring fresh bodies, and becoming as many entire poly-
pi. Fig. 245 represents a hydra with seven heads, the re-
:sult of several operations of this kind. The hydra will
sometimes of its own accord split into two; each division
becoming independent of the other, and growing to the
same size as the original hydra. Trembley found that dif-
ferent portions of one polype might be ingrafted on ano-
ther, by cutting their surfaces, and pressing tlicm together;
for by this 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 individual. A number of heads and bodies may
thus be joined together artificially, so as to compose living

NUTRITION IN POLYPI. 63

monsters more complicated tlian the wildest fancy has con-
ceived.

Still more complicated arc the forms and economy 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, witli widely ex-
tended flowering branches. Myriads of mouths open upon
the surface of the animated mass; each mouth being sur-
rounded with one or more circular rows of tentacula, which
are extended to catch their prey: but as the stationary con-
dition of these polypes prevents them from moving in search
of food, their tentacula are generally furnished with a mul-
titude of cilia, which, by their incessant vibrations, deter-
mine currents of water to flow towards the mouth, carrying
with them the floating animalcules on which the entire po-
lypus subsists.

Each mouth leads into a separate stomach: whence the
food, after its digestion, passes into several channels, gene-
rally five in number, which proceed in different directions
from the cavity of each stomach, dividing it into many
branches, and being distributed over all the surrounding por-
tions of the flesh. These branches communicate with simi-
lar channels proceeding from the neighbouring 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 structure in
the Veretilla, which belongs to this order of polypi: he also
found it in the Pennotula, 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 poly-
pus is an individual animal, associated with the rest in a sort
of republic, where the labours of all are exerted for the com-
mon benefit of the whole society. But it is, perhaps, more
consonant with our ideas of the nature of vitality, to consider
the extent of the distribution of nutritive fluid in any organic

64

THE VITAL FUNCTIONS.

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 indivi-
dual animal; for there is no more inconsistency in sup])Osing
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.

Some of the Entozoa^ or parasitic worms, exhibit a gene-
ral diiTusion,or circulation of nourishment through numerous
channels of communication, into which certain absorbing

vessels convey it from a great number of external orifices, or
mouths, as they may be called. This is the case with the
Txnia, 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 pro-
vided with a tube (o,) having external openings for imbibing
nourishment from the surrounding fluids. Although each
segment is thus provided with a nutritive apparatus, com-
plete within itself, and so far, therefore, independent of the
rest, the individuality of the whole animal is sufiiciently de-
termined by its having a distinct head at one extremity, pro-
vided with instruments for its attachment to the surfaces it
inhabits.

NUTRITION IN MEDUSiE. 65

The Hydatid^ (I'^^g- 248,) is another parasitic worm, of
the simplest possible construction. It has a head (o,) of
which H is a magnified representation, furnished with four
suckers, and a tubular neck, which terminates in a globular
sac. When this sac, which is the stomach, is fully distend-
ed with fluid, its sides are stretched, so as to be reduced to
a very thin transparent membrane, having a perfectly sphe-
rical shape; after this globe has become swollen to a very
large size, the neck yields to the distention, and disappears;
and the head can then be distinguished only as a small point
on the surface of the globular sac. It is impossible to con-
ceive a more simple organic structure than this, which may,
in fact, be considered as an isolated living stomach. The
Cceniu'us, which is found in the brain of sheep, has a struc-
ture a little more complicated; for, instead of a single head,
there is a great number spread over the surface, opening into
the same general cavity, and when the sac is distended, ap-
pearing only as opaque spots on its surface.

The structure of the Sponge has been ah'eady fully de-
scribed; and the course of the minute channels pointed out,
in which a kind of circulation of sea water is carried on for
the nourishment of the animal. The mode by which nutri-
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 b}' animals belong-
ing to the tribe of Medusse is of a peculiar kind. I have
already described the more ordinary form of tliese singular
animals, which resembles a mushroom, from the hemispheri-
cal form of their bodies, and their central foot-stalk, or pedi-
cle. In the greater number of species there exists at the
extremity of this pedicle, a single aperture, which is tiie be-
ginning of a tube leading into a large central cavity in the
interior of the body, and which may, therefore, be regarded
as the mouth of the animal: but in those species which have
no pedicle, as the Equorca, the mouth is situated at the
centre of the under surface. The aperture is of sufiicienl

Vol. II. 9

66 THE VITAL FUNCTIONS.

width to admit of the entrance of prey of considerable size,
as appears from the circumstance that fishes of some inches
in length are occasionally found entire in the stomachs of
those medusae which have a single mouth. The central ca-
vity, 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 al-
most be considered as constituting four stomachs, joined at
a common centre. Such, indeed, is the actual structure in
the Medusa auinla, in which Gaede found the stomach to
consist of four spherical sacs, completely separated by par-
titions. 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, general-
ly, 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 com-
plicated net-work of vessels. These ramifications 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 farther communication is established between this lat-
ter 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 elongations 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 com-
mencement of a vascular system, taking its rise from that
cavity, and calculated to distribute the nutritious juices to
' every part of the organization.

There are other species of Medusae, composing the ge-
nus Rhizostoma of Cuvier, which, instead of having only

* Journal dc Physique, Ixxxix. 146.

NUTRITION IN MEDUSiE.

67

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 num-
ber of fringed processes, which, on examination, are found
to contain ramified tubes, with orifices opening at the ex-
tremity of each process. In this singular tribe of animals
there is properly no mouth or central orifice, the only ave-
nues 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,
which terminates in the general cavity of the stomach. The
Medusa pulmo, of which a figure was given in Vol. i., page
142, belongs to this modern genus, and is now termed the
Rhizostovia Cuvieri,

The course of these absorbent vessels is most conveni-
ently traced after they have been filled with a dark coloured
liquid. The appearances they present in the Rhizosioma

* It is from this circumstance tliat the g-eniis has received the name it now
bears, and wiiich is derived from two Greek words, signifying root-like
mouths.

G8

THE VITAL FUNCTIONS.

Ciivieriy after beinj:; 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 re-
moved by a horizontal section, at its origin from the hemi-
spherical body, or cupola, as it may be termed, where it has
a square prismatic form, so that its section presents the
square surface, 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, ii, 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 hemi-
sphere presented in Fig. 249. On the other hand, the dot-
ted line V, V, in Fig. 249, is that along which the vertical
section of the same animal, represented in Fig. 252, was

made, four of the arms (a, a, a, a,) descending from the pe-
dicle being left attached to it. In these arms, or tentacula,

NUTRITION IN MEDUSiE. 69

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 towainls the centre of the
pedicle, and terminate by a common tube, which may be
considered as the oesophagus (o,) in one large central cavity,
or stomach (s,) situated in the upper part of the cupola. The
section of this esophagus is visible at the centre of Fig. 249,
where its cavity has the form of a cross. The stomach has
a quadrangular shape, as in the ordinary medusae, and from
each of its four corners there proceed vessels, which are con-
tinuous with its cavity, and are distributed by endless rami-
fications over the substance of the cupola, extending even to
the fringed margin, all round its circumference. The mode
of their distribution, and their numerous 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 membranous partitions: these cavities have, by some,
been supposed to perform an office in the system of the JNIe-
dusa, corresponding to respiration; an opinion, however,
which is founded rather on analogy than on any direct ex-
perimental evidence. 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
Flz. 253, showino; 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
tube, the cavities of some of the smaller branches (b, b,)
which are proceeding to join it, are also visible.

The reo-ular gradation which nature has observed in the
complexity of the digestive cavities and other organs, of the
various species of this extensive tribe, is exceedingly re-
markable: for while some, as the Eudora, have, to all ap-
pearance, no internal cavity corresponding to a stomach, and

70 THE VITAL FUNCTIONS.

are totally unprovided with either pedicle, arms, or tenta-
cula; others, furnished with these latter appendages, are
equally destitute of such a cavity; and those helonging to a
third family possea* a kind of pouch, or false stomach, at
the upper part of the pedicle, apparently formed by the
mere folding in of the integument. This is the case with
the Gcronia, depicted in Fig. 250, whose structure, in this
respect, approaches that of the Hydra, already dcscri])cd,
where the stomach consists of an open sac apparently formed
by the integuments alone. Thence may a regular progres-
sion be followed, through various species, in which the aper-
ture of this pouch is more and more completely closed, and
where the tube which enters it branches out into ramifica-
tions more or less numerous, as we have seen in the Rhizos-
toma.* It is difficult to conceive in what mode nutrition
is performed in the agastric tribes, or those destitute of any
visible stomach; unless we suppose that their nourishment is
imbibed by direct absorption from the surface.

Ever since the discovery of the animalcula of infusions,
naturalists have been extremely desirous of ascertaining the
nature of the organization of these curious beings: but as no
mode presented itself of dissecting objects of such extreme
minuteness, it was only from the external appearances they
present under the microscope that any inferences could be
drawn with regard to the existence and form of their inter-
nal organs. In most of the larger species, the opaque glo-
bules, 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,t 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. JNlullcr, and several other observers,
have witnessed the larger animalcules devouring the smaller;
and the inference was obvious, that, in common with all

* See Peron, Annates du Museum, xiv. 330.
fVol. i. p.58, Fit,^ 1.

NUTRITION IN THE INFUSORIA. 71

other animals, they also must possess a stomach. But, as no
such structure had been rendered visible in the smallest spe-
cies of infusoria, such as monads, it was too hastily concluded
that these species were formed upon a different and a sim-
pler model. Lamark characterized them as bcinc;, tbrough-
out, of a homogeneous substaifce, destitute of mouth and di-
gestive cavity, and nourished simply by means of the ab-
sorption 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
upon the subject by Professor Ehrenberg, whose researches
have recently disclosed fresh scenes of interest and of won-
der 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 fortunate expedient of
supplying them with coloured food, which might communi-
cate its tinge to the cavities into which it passed, and exhi-
bit their situation and course. Obvious as this method may
appear, it was not till after a labour often years that Ehren-
berg succeeded in discovering the fittest substances, and in
applying them in the manner best suited to exhibit the phe-
nomena satisfactorily. W.e have already seen that Trembley
had adopted the same plan for the elucidation of the struc-
ture of the hydra. Gleichen also had made similar attempts
with regard to the infusoria; but, in consequence of his
having employed metallic or earthy colouring materials,

* The results of Ehrenberg's labours were first communicated to the Ber-
lin Academy; they have since been published in two works in German: the
first of wliich appeared at Berlin in 1830, under the title of " Orga/iisatiany
Systematik und Geographischcs J'erhalhiiss der Infusionsthierchen." The
second work appeared in 1832, and is entitled " Zur Erkenntniss der Organi-
sation in dtr Richtung des kkinsten Baumes." Both are in folio, with plates.
An able analysis of the contents of the former of these works, by Dr. Gaird-
ner, is given in The Edinburgii New Philosophical Journal for 1831, p. 201,
of which I have availed myself largely in the account which follows.

72 THE VITAL FUNCTIONS.

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 cer-
tain quantity of white lead, a substance highly deleterious to
all animals; but, at length, by employing an indigo which
was fpiite pure, he succeeded perfectly.* The moment a
minute particle of a highly attenuated solution of this sub-
stance is applied to a drop of water in which are some pe-
dunculated vorticellai, occupying the field of the microscope,
the most 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 distinguished 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 num-
ber of distinctly circular spots, of a dark blue colour, evi-
dently produced by particles of indigo accumulated in those
situations. In some species, particularly those which have
a contracted part, or neck, between the head and the body,
as the Rotifer vulgains, these particles can be traced in a
continuous line in their progress from the mouth to these
internal cavities.

In this way, by the employment of colouring matters, Eh-
renberg 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

• 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 verj- minute division. Indigo, sap, green, and carmine, answer these
conditions, and being also easily recognised under the microscope, are well
ad;ij)tcd for these obscnations. 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.

73

among this order of beings. Whole generations of these
transparent gelatinous animalcules may remain immersed
for weeks in an indigo solution, without presenting any co-
loured points in their tissue, except the circumscribed cavi-
ties above described.

Great variety is found to exist in the forms, situations,
and arrangement of the organs of digestion in the Infusoria.
They differ also in their degree of complication, but with-
out any obvious relation to the magnitude of the animalcule.
The Monas aiotnus, the minutest of the whole tribe, exhi-
bits a number of sacs, opening by as many separate orifices,
from a circumscribed part of the surface. In others, as in
the Leucophra paiida^ of which Fig. 22S represents the ap-
pearance under the microscope, there is a long alimentary

25^

canal, traversing the greater part of the body, taking several
spiral turns, and furnished with a great number of blind
pouches, or cxca, as sacs of this description, proceeding la-
terally from any internal canal, and having no other outlet,
are technically termed. These cavities become filled with
coloured particles immediately after their entrance into the
alimentary canal; and must, therefore, be considered as so
many stomachs provided for the digestion of the food which
they receive.* But they are not all filled at the same time,

* Ehrenbcrg terms these Pohji^asiric infusuriuy from the Greek, signify-
ing with many stomachs.

Vol. II. 10

74 THE VITAL FUNCTIONS.

for some continue long in a contracted state, so as not to be
visible; while, at another time, they readily admit the co-
loured fooil. It is, therefore, only by dint of patient watch-
ing that the wliole extent of the alimentary tube, and its
apparatus of stomachs, can be fully made out. Fig. 255,
above referred to, exhibits the Leiicophra palulaoi^ Ehrcn-
berg,* witli a few of its stomachs filled with the opaque par-
ticles: but Fig. 256 shows the whole series of organs as it
would appear if it could be taken out of the body, and placed
in the same relative situation with the eye of the observer,
as they are seen 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
Ihjdalina 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 termination. In some Vorticellse, the intestine, from
whicb proceed numerous caeca, makes a complete circular
turn, ending close to its commencement: Ehrcnberg forms
of these the tribe of Cyclocsela, of which the Vorticella cil-
rinctj and the Slentor polymorplms, are examples. Thus
do we discover the same diversity in the structure of the
digestive organs of the several races of these diminutive be-
ings, as is found in the other classes of animals.

The Ihjdatina senta, one of the largest of the infusoria,
was found by Ehrcnberg to j)0ssess a highly developed
structure with respect to many systems of organs, which we
should never have expected to meet with so low in the
scale of animals. As connected with the nutritive func-
tions, it may here be mentioned that the head of this ani-
malcule is provided with a regular apparatus for mastica-
tion, consisting of serrated jaws, each having from two to
six teeth. These jaws are seen actively opening and shut-
ling when the animal is taking its food, which consists of

• Trichoda pulula. Mailer.

NUTRITION IN THE ACTINIA.

15

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 as-
similation 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
nourishment to different parts; but all these cavities and ca-
nals being simply hollowed out of the solid substance of the
body. As we ascend a step higher in the scale, we find
that the stomach and intestinal tube, together with tlieir
appendages, 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 re-
ceives and collects the nutritious juices after their elabora-
tion in these organs. The t^ctinia, or Sea Jlnemone, 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 sto-
mach constitute also those of
the body, the whole being one
simple sac; in the actinia, spaces
intervene 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 sto-
mach: they are divided by numerous membranous parti-
tions passing vertically between the skin, and the membrane
of the stomach, and giving support to that organ. Fig. 257,
representing a vertical section of the Jictinia coriaccd, dis-
plays this internal structure, b is tlie base, or disk, by
which the animal adheres to rocks: i is the section of the
coriaceous integument, showing its thickness: m is the cen-
tral aperture of the upper surface, which performs the oflice

76

THE VITAL FUNCTIONS.

of a moiitli, leading to s, the stomach, of which tlic lower
orifice is open, and which is suspended in the general cavi-
ty, hv means of vertical partitions, of which the cut edges
are seen helow, uniting at a central point, c, and passing be-
tween the stomach and the integument. These muscular
partitions arc connected above with three rows of tentacula,
of which tlie points are seen at t. The ovaries (o) are seen
attached to the partition; and also the apertures in the lower
part of the stomach, by whicli they communicate with its

cavity.

If we considered the medusa as having four stomachs, we
mi'^^ht in like manner rcsrard the Jlsterias, or star-fish, as
having ten, or even a greater number. The mouth of this

.^, ^'!!"l'":!'' frT.

^.

'1,1

\

\

radiated animal is at the centre of the under surHice; it leads
into a capacious bag, situated immediately above it, and
which is properly the stomach. From this central sac there
proceed ten prolongations, t)r canals, which occupy in pairs
the centre of each ray, or division of the body, and subdi-
vide into numerous minute ramifications. These canals,
with their branches, are exhibited at c, c. Fig. 25S, which
represents one of tlie rays of the Asterias, laid open from
the upper side. The canals are supported in their positions
by membranes, connecting them with the sides of the ca-
vity in which they are suspended.

In the various species of Echini, we find that the ali-
mentary tube has attained a more perfect development; for
instead of constituting merely a blind pouch, it passes en-
tirely through tlic body of the animal. We here find an
oesophagus, or narrow tube, leading from the mouth to the

NUTRITION IN THE ASTERIAS. 77

stomach; and the stomach 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 frequently ex-
hibits 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 por-
tions are more properly denominated the great intestine,
by way of distinction from the middle portions of the tube,
which are generally narrower, and are termed the smalt in-
testine. We often see blind pouches, or cseca, projecting
from different parts of the canal; this is the case with the
intestine of the Jiphrodita aculeata, or sea-mouse. The in-
testine 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 Lumbricus
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
num.erous stomachs, backwards and forwards many times,
before its digestion is accomplished.

The stomach of the Leech is very peculiar in its struc-
ture: its form, when dissected off, and removed from the
body, is shown in Fig. 2G0. It is of great capacity, occu-
pying the larger part of the interior of the body; and its ca-
vity is expanded by folds of its internal membrane into se-
veral pouches (c, c, c.) Mr. Newport, who has lately

78

THE VITAL FUNCTIONS.

S.^

examined its structure with great care, finds that each of the
262 ten portions into which it is divided,
sends out, on tlic part most remote
jC^ from tlic ocsopliagus (o,) two lateral
C-/>«?^ pouches, or caica; which, as they are

traced along the canal, hecome both
wider and longer, so that the tenth pair
of caeca (a) extends to the hinder extre-
mity of the animal; tlie intestine (i,)
which is very short, lying between
them.* It has long been known, that
if, after the leech has fastened on the
skin, a portion of the tail be cut off, the
animal will continue to suck blood for
an indefinite time: this arises from the
circumstance, 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.
i^'V'^ A structure very similar to that of

the leech is met with in the digestive organs of the Glosso-

264 po7^a tubercxilata, (Ilirudo com-

planata, 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,*thc cavity
of the stomach is distinctly
seen, prolonged into several
cells, divided by partitions, and
directed towards the tail. The two last of these cells (c c)

• This figure was engraved from a drawing made, at my request, by Mr.
Newport, from a specimen which he dissected, and which he was so oblig-ing
as to sliow me. Fig. 261 represents the mouth, within wlilch 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

NUTRITION IN THE ANNELIDA. 79

are much longer than the rest, and terminate in two blind
sacs, between which is situated a tortuous intestinal tube.*

at a meeting- of the Royal Society. See the abstracts of the proceedings in
the Society, for June, 1833.

* In both these figures, t is the tubular tongue, projected from tlie mouth.
In Fig, 263, e are the six eyes, situated on tlie extremity whicli coiresponds
to the head; and a double longitudinal 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,

( so )

CHAPTER V.

Nutrition in the highe)' oi^dcrs 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 complicated in their structure. The long series
of processes requisite for the perfect elaboration of nutri-
ment, 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 func-
tions of the stomach and of the heart. The manufacture of
nutriment, if we may so express it, is, in these lower zoo-
phytes, conducted upon a small scale, by less refined me-
thods, and with the strictest economy of means; the appara-
tus 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 development, we find a farther division of
labour introduced. Of this we have already seen the com-
mencement 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 is carried to a much great-
er extent than in the inferior departments of the animal
creation. Besides the stomach, or receptacle for the unas-
similated food, another organ, the heart, is provided for the

COMPLEX APPARATUS FOR NUTRITION.

SI

uniform distribution 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 ves-
sels, for transmitting the fluids from the organs which pre-
pare 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 in-
dividual species. For the purpose of conveying clearer no-
tions of the arrangement of this extensive system of vital
organs, I have drawn the annexed plan (Fig. 265,) which

^ •

exhibits them in their natural order of connexion, and as
they might be supposed to appear in a side view of the in-
terior of a quadruped. To this diagram I shall make fre-
quent reference in the following description of this sys-
tem.

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 secre-
tions of the mouth. It is then collected into a mass, by the
action of the muscles of the cheek and tongue, and swal-

VOL. II.

II

S2 THE VITAL rUXCTIOXS.

lowed by the regulated contractions of the different parts of
the throat. It now passes along a muscular tube, called
the Oesophagus, (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 che-
mical changes; after which it is conducted through the aper-
ture termed the pi/lninis (r,) 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 neiglil)ourhood, 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 Lacleals, which transmit it to the heart (ii.) These
vessels are exceedingly numerous, and arise by open orifices
from the inner surface of the intestines, whence they ab-
sorb, or drink up the chyle. They may be compared to
internal roots, which unite as they ascend along the mesen-
tery (m,) or membrane connecting the intestines with the
back; forming larger and larger trunks, till they terminate
in an intermediate reservoir (r,) which has been named the
Ixeccptacic of the Chyle. From this receptacle there pro-
ceeds a tube, which, from its passing through the thorax, is
called the Thoracic 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 pouring their contents
into tlie auricle, or first cavity of the heart (u.) whence it
immediately passes into the ventricle, or second cavity of
that organ (ii.) Suc|), in the more perfect animals, is the
circuitous and guarded route, which every particle of nou-
rishment must take before it can be added to the general
mass of circuhiting fluid.

By its admixture with the blood already contained in
these vessels, and its purification by the action of the air in
the respiratory organs (i?,) the chyle becomes assimilated,
and is distributed by the heart through appropriate'channels
of circulation called arteries (of which the common trunk,

COMPLEX APPARATUS FOR NUTRITION. 83

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 which these functions are conducted in tlie seve-
ral tribes of animals will be described hereafter. It will be
sufficient for our present purpose to state, by way of com-
pleting the outline of this class of functions, that, like the
returning sap of plants, the blood is made to undergo far-
ther 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 pro-
ducts are here formed, and new properties acquired, adapted
to the respec^e purposes which they are to serve in the
animal economy. The whole is one vast Laboratory, where
mechanism is subservient to Chemistry, where Chemistry
is the agent of the higher powers of Vitality, and where
these powers themselves minister to the more exalted fiicul-
ties 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 respective
ends, than the mechanical functions. This arises from the
circumstance that the processes they efTect 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 producing each of the chemi-
cal changes which the food must necessarily undergo during
its assimilation. We only know that all these changes are
slowly and gradually effected; the materials having to pass
through a great number of intermediate stages before they
can attain their final state of elaboration.

Hence we are furnished with a kind of scale, whereby,
whenever we can ascertain the degrees of difference exist-
ing between the chemical condition of the substance taken
into the body, and that of the product derived from it, we
may estimate the length of the process required, and the

S4 THE VITAL FUNCTIONS.

amount of power necessary for its conversion into that pro-
duct. It is obvious, for example, that the chemical changes
which vciictable food must be made to under2;o, in order to
assimilate it to blood, must be considerably greater than
tiiuse recjuircd to convert animal food into the same fluid,
because the latter is itself derived, with only slight modifi-
cation, immediately from the blood. We accordingly find
it to be an established rule, that the digestive organs of ani-
mals which feed on vegetable materials are remarkable for
their size, their length, and their complication, when com-
pared witli those of carnivorous 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, 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 ima-
gine contained any sensible portion. 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 materi-
als that happen to be intermixed with the soil; and this slen-
der nutriment is sufllcient for the subsistence of that ani-
mal. Many marine worms, in like manner, feed apparently
upon sand alone; but that sand is generally intermixed with
fragments of shells, which have been pulverized by the con-
tinual rolling of the tide and the surge; and the animal mat-
ter contained in these fragments, aflbrds them a supply of
nutriment adequate to their wants. It is evident, 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 mechani-
cal texture, as well as the chemical qualities of the food;
and this we lind to be the case in a variety of instances,
which will hereafter be specified.

COMPLEX APPARATUS FOR NUTRITION. 85

The activity of the digestive functions and the structure
of the organs, will also be regulated by a great variety of
other circumstances in the condition of the animal, inde-
pendently of the mechanical or chemical nature of the food.
The greater the energy with which the more peculiarly
animal functions of sensation and muscular action are ex-
ercised, the greater must be the demand for nourishment, in
order to supply the expenditure of vital force created by
these exertions. 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 chameleon,
will, indeed, live for months without taking any food.
Fishes, which, like reptiles, are cold-blooded animals, al-
though at all times exceedingly voracious when supplied
with food, yet can endure long fasts with impunity.

The rapidity of development has also great influence
on the quantity of food which an animal requires. Thus,
the caterpillar, which grows very quickly, and must re-
peatedly throw off its integuments, during its continuance
in the larva state, consumes a vast quantity of food com-
pared with the size of its body; and hence we find it pro-
vided with a digestive apparatus of considerable size.

( 86 )

CHAPTER VI.

1

PREPARATION OF FOOD.

§ 1. Pi'chcnsion of Liquid Food.

In studying the series of processes which constitute assi-
mihition, our attention is first to be directed to the mode in
which the food is introduced into the body, and to the me-
chanical changes it is made to undergo before it is subjected
to the chemical action of the digestive organs. The nature
of these preliminary processes will, of course, vary accord-
ins to the texture and mechanical condition of the food.
Where it is already in a fluid state, mastication is unneces-
sary, and the receiving organs consist simply of an appara-
tus for suction. This is the case very generally with the
Entozoa, which subsist upon the juices of other animals,
and which arc all provided with one or more sucking ori-
fices, often extended in tlic form of a tube or proboscis.*
The Hydatid, for instance, has four sucking apertures dis-
posed round the head of the animal: the Txnia has orifices
of this kind in each of its jointed segments: the Jiscains and
the Earih-ivorm 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 Flancuna, there is, besides the

• Some species of Fasclolx, or flukes, are furnished with two, three, six,
or more sucking- disks, by which they adhere to surfaces: to these animals
the names Dlstonia, Trisioma, Hexastoma, and Pohjstnma have been g-iven;
but these denominations, implyinjj a plurality of mouths, are evidently in-
correct, since tlic suckintj 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. Sec Edinburgh Philos. Journal, xx. 101.

PREHENSION OF LIQUID FOOD.

87

ordinary mouth, a tube also provided for suction, in a dif-
ferent part of the body, and leading into the same stomach.*
When the instrument for suction extends for some length
from the mouth, it is generally termed a 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 Lejjidoptera, (Fig. 266,) is a double tube, con-
structed by the two edges being rolled
longitudinally till they meet in the
middle of the lower surface, thus form-
ing a tube on each side, but leaving
also another tube, intermediate to the
two lateral ones. This middle tube is
formed by the junction of two grooves,
which, by the aid of a curious appara-
tus of hooks, resembling those of the
laminae 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 is that which 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.!

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 suc-
tion, consisting of a tube, of which the sides are Mrong and
fleshy, and moveable in every direction, like the trunk of
an elephant: it has at its extremity a double fold, resemb-
ling 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 ofiice,

* Phil. Trans, for 1822, 442.

•f Volume i. p. 393.

\ Kirby and Spcnce's Entomology, vol. ii, p. 390.

SS THE VITAL FUNCTIONS.

lancets.* In the gnat, they are five or six in number, finer
than a hair, exceedingly sharp, and generally barbed on one
side. In the Tabamts, or horse-fly, they are flat like the
blade of a knife. These instruments are sometimes con-
structed 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 -^ymc7zo/?/er«, 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 projected forwards by
being carried on a pedicle, which can be folded back when
the tube is not in use. The mouths of the Jicephaloits Mol-
lusca are merely sucking apertures, with folds like lips, and
without either jaws, tongue, or teeth, but having often ten-
tacula arising from their margins.

Among fishes, we meet with the family of Cydosiomata,
so called from tJicir having a circular 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 attachment 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 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

♦ Kirby and Spcncc's Entomology, vol. ill. p. 467'.

PREHENSION OP SOLID FOOD. 89

is said to be folded into a tubular sbapc for that purpose.
But suction among the mammalia is almost always performed
by the muscles of the lips and cheeks, aided by the move-
ments 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 remarka-
ble. 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 prehension and introduction into
the mouth; secondly, for their detention when so introduced;
and thirdly, for their mechanical division into smaller frag-
ments.

Of those instruments of prehension which are not portions
of the mouth itself, and which form a series of variously
constructed organs extending from the tentacula of the po-
lypus to the proboscis of the elephant, and to the human arm
and hand, some account has already been given in the his-
tory of the mechanical functions: but, in a great number of
instances, prehension is performed by the mouth, or the
parts which are extended from it, and maybe considered as
its appendices. The prehensile power of the mouth is de-
rived principally from the mechanical 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. Ano-
ther principal purpose effected by the jaws is that of giving
mechanical jiower to the muscles, which, by acting upon
the sides of the cavity of the mouth, tend to compress and

Vol. II. 12

90 THE VITAL FUNCTIONS.

propel tlifi contained food. We find, accordingly, that all
animals of a highly developed structure arc provided with

jaws.

Among the animals which are ranked in the class of Zoo-
phytes the highest degrees of development arc exhibited
by the Kchinodermata, and in them we fmd a remarka-
l)le jM'rfection in tlu; organs of mastication. The mouth of
the Echinus is surroumled hy a frame-work of shell, con-
sisting of five converging pieces, each armed with a long
tooth; and for the movement of each part there are provided
sei)arate muscles, of whicli the anatomy has been minutely
described by Cuvier. In the shells of the echini that arc
cast on the shore, this calcareous frame is usually found en-
tire in the inside of the outer case; and Aristotle having
noticed its resemblance to a lantern, it has often gone by the
whimsical name of the lantern of Jiristotlc.

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 ad-
mits of great diversity in the dillcrent tril)es; 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 description. In most insects, also, their minuteness
is an additional obstacle to the accurate observation of their
anatomy, and of the mechanism of their action. The re-
searches, however, of Savigny,* and other modern entomo-
logists, have gone far to prove, that, amidst the infinite va-
riations observable in the form and arrangement of the se-
veral parts of these organs, there is still ])reserved, in the
general plan of their construction, 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 ap-

• Sec l)is '♦Thcoric cks Organcs de la bouche des Aniniaux invcrtcbrcs et
articiiles," which forms the first part of the "Mcnioircs sur Ics Animaux
sans vcrtt'brcs." Paris, 181G.

JAWS OF THE ECHINUS. 91

parently most remote, and organs destined for widely dif-
ferent uses: so that even when there lias l)een a complete
change of purpose, we still perceive the same design fol-
lowed, the same model copied, and the same uniformity of
plan preserved in the construction of the organs of every
kind of mastication; and there prevails in tiicm 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 fd^rous tex-
tures; they are fashioned, 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 tu-
bular proboscis, capable of suction, and adapted to the drink-
ing; of fluid aliment. Pursuino; the examination of these or-
gans in another series of articulated animals, we And 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 metamorphoses of or-
gans 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 some-
times provided also with supernumerary organs of each de-
scription. In the Arachnida, in particular, we frequently
meet with supernumerary jaws, together with various ap-
pendices, which present remarkable analogies of form with
the attennse, and the legs and feet of the Crustacea.

The principal elementary parts which enter into the com-
position of the mouth of an insect, when in its most perfect
state of development, are the seven following: a pair of up-
per jaws, a pair of lower jaws, an upper and a lower lip, and
a tongue.t These parts in the Locusta vhHdisswia, or com-

♦ Vol. i. p. 206.

f All these parts, taken together, were termed by Fabriclus ijistrumcnla

92

THE VITAL FUNCTIONS.

mon grasshopper, arc delineated in llieir relative situations^
but detached from one another, in lig. 267. The upper
jaws, {m,) which are termed the mandibles, are those prin-

cipally employed for the mastication of hard substances;
they are, accordingly, of greater strength than the lower
jaws, and thcii- 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 arc designed to be applied. Thus, the mandi-
bles of some Melolonthx have a projection, rendered rough
by numerous deep transverse furrows, converting it into a
file for wearing down the dry leaves, w'hich are their natural
food."* In most cases, indeed, we are, in like manner, ena-
bled, from a simple inspection of the shape of the teeth, to

clbarla; and upon their varieties of structure he founded his cclcbnitcd sys-
tem of entomological classification. Kirby and Spence have denominated
them iropfiL Sec their introduction to Entomology, vol. iii. p. 417. To
the seven elements above enumerated, Savigny adds, in the Hcmlptcra, an
eighth, whicli he terms the Epv^lotasu.
* Knoch, quoted by Kirby.

TAWS OF INSECTS. 93

forin tolerably accurate ideas of the kind of food on which
the insect naturally subsists.*

Above, or rather in front of the mandibles, is situated the
labrum, 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 maxillx, 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 arti-
culations with the head, to motions in a horizontal plane
only, so that they open and close by a lateral movement, and
not vertically upwards and downwards, as is the case with
the jaws of vertebrated animals. The maxillcC are, in most
cases, employed principally for holding the substances on
which the dividing or grinding apparatus of the mandibles
is exerted. A similar use may be assigned also to the or-
gans denominated Palpi, or Aiitennulx (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 maxillay^y , 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 generally found.

For an account of the various modifications which these
parts receive in different tribes and species, I must refer to
works which treat professedly of this branch of comparative
anatomy. I shall content myself with giving a single exam-
ple of the conversion of structure here alluded to, in that of
the I'ostrum, or proboscis of the Cimex nigricoriiis. This
insect belongs l(\ the order Hemiptera, which has been
usually characterized as being destitute of both mandibles

* See a memoir by Marcel tics Scrrcs, in the Annalcs du MusC'um d'llist.
Nat. xiv. 56.

94

THE VITAL FUNCTIONS.

and jaws, and as having, instead of these parts, an apparatus
of very different construction, designed to pierce the skin of
animals, and suck their juices, l^ut Savigny, on applying
the j)rinciples of his theory, has recognised, in the proboscis
of the Cimex, the existence of all the constituent elements
that are found in the mouth of insects formed for the masti-
cation of solid food. This proboscis consists of four clon-
sated filaments, contained in a kind of sheath: these fila-

ments are represented in Fig. 2GS, se-
])arated 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 man-
dibles (j,) and the other set (p) being,
in like manner, prolongations of the
maxillae (m.) Between these filaments,
and near their commencement, is seen
a pointed cartilaginous body (g,) which
is the glossa, or tongue; and the ajier-
ture seen at its root is the passage into
the oesophagus. The sheath is merely
the elongated labium, of which the
base is seen at l, in Fig. 2GS; but is
represented, in its whole length, in
Fis;. 2G9, where the £!;roove for con-
P \q W taining 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 Jlphro-
dile, or sea-mouse, is furnished with four teeth of this de-
scription. The Leech has, immediately within its lips,
three semi-circular teeth, with round and sharp cutting
edges: they are delineated in P^ig. 262, in their relative po-
sitions; and Fig. 2G3 represents one of the teeth detached
from the rest. It is with these teeth that the leech pierces
tJie skin of tiie animals whose blood it sucks; and, as soon as

JAWS OF INSECTS. 95

the wound is inflicted, tlic teeth, heing moveahle at their
base, fall back, leavinp; the openini^ 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
Bucciniim, the Miirex, and the Valuta, as also in the
Doris, which, though it has no shell, is likewise a gastero-
pode. In those mollusca of this order which have not a
proboscis, as the Limax, or slug, the Helix, or snail, and
the ^plysia, or sea-hare, the mouth is furnished with broad
270 lips, and is supported by an internal cartilage,
iSfl^ having several tooth-like projections, which assist
in laying hold of the substances taken as food.
That of the snail is represented in Fig. 270.

All the Sepiss, or cuttle fish tribe, are furnished, at the
entrance of the mouth, with two horny jaws, having a re-
markable resemblance to the bill of a parrot; excepting that
the lower piece is the larger of the two, and covers the up-
per one, which is the reverse of what takes place in the
parrot. These constitute a powerful instrument for break-
ing the shells of the molhisca 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 principally as organs of
prehension and detention; and the upper jaw, as well as the
lower one, being moveable upon the cranium, they are ca-
pable of opening to a great width. The bony pieces which
compose the jaws are more numerous than the correspond-
ing bones in the higher classes of vertebrata, and tlicy ap-
pear, therefore, as if their development had not proceeded

96 THE VITAL FUNCTIONS.

t

sufTicicntly i-^x to effect tlieir consolidation into more com-
pact 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 di-
viding it, so as to adapt it to being swallowed. These teeth
are of various shapes, though usually sharp at the points, and
cither conical or hooked at the extremity, with the points al-
wa\'s directed backwards, in order to prevent the escape of the
animal which has been seized. Those fishes which subsist on
testaceous mollusca have teeth with grinding surfaces, and
their jaws arc 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 tlie 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, appear to be formidable instru-
ments; 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 des-
titute of teeth. Frogs, indeed, exhibit two rows of very fine
points; the one in the upper jaw, and the other passing trans-
versely across the palate: they may be considered as teeth
existing in a rudimental state; for they are not sufficiently
developed to be useful in mastication. There arc about
forty of these minute teeth on each side in the frog. In the
Salamander, there are sixty above and below; and also thirty
on each side of the palate.

The tongue of tlic frog is of great length; its root is at-
tached close to the fore part of the lower jaw, while its point,

• AUcmpts liavc been made to trace analogies between the difTerent seg-
ments of the jaws of fishes and covrcsponding parts of the mouths of cms-
tacea and of insects: but the justness of these analogies is yet far from being
satisfactorily proved.

JAWS OP BIRDS. 97

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 approached witliin 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 froo; bavin"; 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 mo-
tion. The Chameleon 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 from a considera-
ble distance, by a similar manoeuvre to that practised by the
frog.*

Serpents and Lizards have generally curved or conical
teeth, calculated rather for tearing and holding the food, than
for masticating it: like those of fishes, they are affixed partly
to the jaws, and partly to the palate. The Chelonian rep-
tiles have no teeth, their office 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 exceedingly strong hooked beak,
for tearing and dividing 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 fffittened 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 fre-
quently serrated, to allow the fluid to filter through, while

• Mr. Houston has given a description of the structure of this organ, and
of the muscles by which it is moved, in a paper contained in the Transactions
of the Royal Irish Academy, vol. xv. p. 177.

Vol. II. 13

98 THE VITAL FX^NCTIOXS.

the solid portions arc rclalncd in the inoiitli. Tlic duck af-
fords an instance of tliis structure; wliich is, however, still
more stron«j;ly marked in the genus Mergiis^ or Merganser,
where tiie whole Icngtli of tlie inarL!;in of the hill is beset
with small sliarp pointed teeth, directed backwards: thc}^ are
jiarticularlv conspicuous in the Merf^its sej'ratnr, or red-
l)rcastc(l Mcri^nnsor. 'J'he oI)ject of the ])arbs and fringed
processes which arc apj^ended to the tongue in many birds,
such as that of the Tuvcan and the Pan^akcet^ appears in
like manner, to be the detention of substances introduced
into the mouth.

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

In the Loxia curvirosira, or Cross-])ill, 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 tlie seeds, by insinuating the
bill between the scales. It can split cherry stones with the
utmost ease, and in a very short lime, by means of this pe-
culiarly shajK'd 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 changes its position till it is adapted
for swallowinc;.

The li/iijnchnps, or black Skimmer, has a very singular-
ly 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
plotigh-share, catching the prey which is on the surface of
the sea. •

The Woodpecker is furnished with a singular apparatus
for enabling it to dart out with great velocity its long and
jminted tongue, and transfix the insects on which it princi-
pally feeds; and these motions arc performed so quickly

• Ser .1 papor on the nicclianism of the bill of this bird, by Mr. Yan-cll,
in the Zoolojric.il .lonmal, iv. 4.'>9.

TONGUE OF THE WOODPECKER.

99

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 tlie
parts dissected. The tongue itself (t) is a slender sharj)-
pointed horny cylinder, having its extremity (b) beset with
barbs, of which the points are directed backwards: it is sup-
ported on a slender 0.9 Hyoides, or lingual bone, to the
])Osterior 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, 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 di-
verging as they proceed backwards; until, bending down-
wards (at c,) they pass obliquely round the sides of the
neck, connected by a membrane (m;) then, being again in-
flected 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, whicli ex-
tends forwards, along the middle line of the cranium (e,)
till it arrives between the eyes. From this point, the groove

* Tl\cse cartilages correspond in situation, at the part, at least, where they
are jouied to the os hyoklcs, to what arc called the curnuUy or horns of that
bone, ui other annuals.

100 THE VITAL FUNCTIONS.

and the two cartilages it contains, which arc now more
closely conjoined, are deflected towards the right side, and
terminate at the edge of the aperture of the right nostril (r,)
into which the united cartilages arc 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 croove ])rovided 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 cartilages, so that their
curvature is diminished, and the tongue protruded to a con-
siderable distance, 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 ex-
tremity. This muscular effort is, however, 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 m.uscular power.t 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 picking up the minutest insects, witli the ut-
most celerity and precision. On meeting w^th 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 Mamm.alia w-hich have no teeth, the Myrme-
cophuf^a, or Ant-eater, practises a remarkable manoeuvre
for catching its prey. The tongue of this animal is very
long and slender, and has a great resemblance to an earth-
worm: tliat of the two-toed ant-eater is very nearly one-
third of the length of the whole body; and at its base is

• S Is the hrge salivary gland on the right side.

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

TONGUE OF THE ANT-EATER. 101

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 elonga-
tion and other movements are effected by circular fibres,
which are exterior to the former. When laid on the g-round
in the usual track of ants, it is soon covered with these in-
sects, and being suddenly retracted, transfers them into the
mouth; and as, from their minuteness, they require no mas-
tication, they are swallowed undivided, 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 Rhi-
noceros^ whose upper lip is so extensible as to be capable of
performing the office of a small proboscis. The Sorcx mos-
chatus, 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 nmd. More fre-
quently, however, this office of prehension is performed by
the tongue, which for that purpose is very flexible and much
elongated, as we see in the Camelcojjard, where it acts like
a hand in grasping and bringing down the branches of a
tree.*

In the animals belonging to the genus Fells, each of the
papillae of the tongue is armed with a horny sheath termi-
nating 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 produc-
tions have been regarded as analogous to the lingual teeth
of fishes, already noticed.

The mouth of the Ornithorhyncus has a form of con-
struction intermediate between that of quadrupeds and

• Home, Lectures, &c. vi. Plate 33.

102

THE VITAL FUNCTIONS.

birds; being furnished, like the former, with grinding teeth
at the posterior part of both the ui)per and lower jaws, but
they are of a horny substance; and the mouth is terminated
in front by a horny bill, greatly resembling that of the duck,
or the spoon-bill.

The JVfialc is furnished with a singular apparatus de-
signed for filtration on a large scale. The palate has the
form of a concave dome, and from its sides there descends
vertically into the mouth, a multitude of thin plates set pa-
rallel to each other, with one of their edges directed towards
the circumference, and the other towards the middle of the

j)alate. These plates are known by the
name of ivhalebone, and their general
form and appearance, as they hang from
the roof of the palate, are shown in
Fig. 272, which represents only six of
these plates."" They are connected to
the bone by means of a white ligamen-
tous substance, to which they arc im-
mediately attached, and from which
they appear to grow: at their inner
margins, the fd^rcs, 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 so-
lid or even gelatinous substances that
may have been admitted into the cavity
of the mouth, which is exceedingly ca-
pacious; for as the plates of vv'halebonc
grow only from the margins of the up-
per jaw, they leave a large space with-
in, which, though narrow anteriorly, is wider as it extends
backwards, and is capable of holding a large quantity of

• In ihc Pihed Ulialc the plates of whalebone are placed very near to-
gether, not being a (|uartcr of an inch asunder; and there are above three
hundred plates in the outer rows on each bide of the mouth.

!

h'

MOUTH OF THE WHALE.

103

water. Thus, llic whale is cnal)lccl to collect a whole shoal
of mollusca, and other small prey, hy taking into its mouth
the sea water which contains these animals, and allowing it
to drain off through the sides, after passing through the in-
terstices of the net work formed hy the fdaments of the
whalebone. Some contrivance of this kind was even neces-
sary to this animal, because the entrance into its oesophagus
is too narrow to admit of the passage of any prey of consi-
derable size; and it is not furnished with teeth to reduce the
food .into smaller parts. The principal food of the Bahina
Mysticetiis, or great whalebone whale of the Arctic Seas, is
the small Clio Borealis, which swarms in immense num-
bers 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 teelh; and, accordingly, no traces of
teeth are to be discovered either in the upper or lower jaw.
Yet a tendency to conform to the type of the mammalia is
manifested in the early conformation of the whale; for rudi-
ments of teeth exist in the interior of the lower jaw before
birth, lodged in deep sockets, and forming a row on each
side. The development of these imperfect teeth proceeds
no farther; they even disappear at a very early period, and
the groove which contained them closes over, and, after a
short time, can no longer be seen. For the discovery of
this curious fact we arc indebted to Geoffroy St. Hilaire.t
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 external appearance
exactly resembling the rudiments 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 deposition from a vascular pulp,
connected with the jaw. These tubercles are afterwards

• Vol. i. p. 186.

f Cuvicv, Ossemens Fossiles, 3me edition, torn. v. p. 360.

104 THE VITAL FUNCTIONS.

consolidated into one piece In each jaw, forming, by their
union, the beak of the parrot, in a manner perfectly analo-
gous to that which leads to the construction of the compound
tooth of the elephant, and which I shall presently describe.
The original indentations are obliterated as the beak ad-
vances in growth; but they are permanent in the bill of the
duck, where the structure is very similar to that above de-
scribed in the embryo of the parrot.

§ 3. Mastication hy means of Teeth.

The teeth, being essential instruments for seizing and
holding the food, and effecting that degree of mechanical di-
vision 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 requires considera-
ble preparation previously to its digestion. There exist, ac-
cordingly, the most intimate relations between the kind of
food upon which each animal of this class is intended by na-
ture to subsist, and the form, structure, and position of the
teeth; and these relations may, indeed, be also traced in the
shape of the jaw, in the mode of its articulation with the
head, in the proportional size and distribution of the mus-
cles 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 indicated
by the structure and arrangement of the tcetii, it is evident
that these latter organs, in particular, will afford to the na-
turalist most important characters for establishing a systema-
tic classification of animals, and more especially of quadru-
peds, where the differences among the teeth are very consi-
derable; and these differences have, accordino-lv, been the
objects of much careful study. To the physiologist they
present views of still higher interest, by exhibiting most

OFFICES OF THE TEETH. 105

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 principally 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. Occasionally, some
particular teeth are much enlarged, in order to serve as
weapons of attack or defence; for which purpose, they ex-
tend beyond the mouth, and are then generally denominated
tiisks; this we see exemplified in the Elephant, {\\e Narwhal^
the Walrus^ the Hlppopotainus, the Boar, and the Babi-
roussa.

Four principal forms have been given to teeth, which ac-
cordingly may be distinguished into the conical, the sharp-
edged, the flat and the tuberculated teeth; though we occa-
sionally 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 especially adapted. The conical teeth,
which are generally also sharp-pointed, are principally em-
ployed in seizing, piercing, and holding objects: such arc
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 Porpiis, 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 Por-
pus, shows the form of these simply prehensile teeth.

The Cachalot has a similar row of teeth, which are, how-
ever, confined to the lower jaw. All these animals subsist
upon fish, and their teeth are therefore constructed very
much on the model of those offish; while those Cetacea, on
the other hand, which are herbivorous, as the Manatus and

Vol. II. • 14

106 THE VITAL FUNCTIONS.

the Diigong, or Indian Walrus, liave teeth very differently
formed. Tiic tusks of animals must necessarily, as respects
their shape, be classed among the conical teeth.

273

The sharp-edged teeth perform the office of cutting and
dividing the yielding textures presented to them; they act
inilividually 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 tuberculated
teeth, of which the surfaces present a number of rounded
eminences, corresponding to depressions in the teeth op-
posed to them in the other jaw, act more by their direct
pressure in breaking down hard substances, and pounding
them, as they would be in a mortar.

The position of the teeth in the jaws has been another
ground of distinction. In those INIammalia 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
iiicisors. The incisors of the upper jaw are always im-
planted in a bone, intermediate between the two upper jaw
bones, and called the intermaxillary bones.* The conical

• Tliose teeth of the lower juw which coiTespond with the incisors of the
upper j:iw, are also considered as incisors. In Man, and in the species of
qiiadrumana that most nearly resemble him, the sutures which divide the in-
tennaxillary from the maxillary bones are obliterated before birth, and leave
in the adult no trace of their former existence.

TEETH OP CETACEA. 107

teeth, immediately following the incisors, are called cuspi-
date, or canine teeth, from their being particularly conspi-
cuous in dogs; as they arc, indeed, in all the purely carnivo-
rous tribes. In the larger beasts of prey, as the lion and the
tiger, they become most powerful weapons of destruction:
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 of inoUn^
teeth, or 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 eminences; these are called the false molar teeth,
and, also, from their having generally two points, or cusps,
the bicuspidate teeth. The posterior molar teeth are diffe-
rently shaped in carnivorous animals, for they are raised into
sharp and often serrated ridges, having many of the proper-
ties of cutting teeth. In insectivorous and frugivorous ani-
mals their surface presents prominent tubercles, either point-
ed or rounded, for pounding the food; while in quadrupeds
that feed on grass or grain they are flat and rough, for the
purpose simply of grinding.

The apparatus for giving motion to the jaws is likewise
varied according to the particular movements required to act
upon the food in the diflferent tribes. The articulation of the
lower jaw with the temporal bone of the skull, approaciics
to a hinge joint; but considerable latitude is allowed to its
motions by the interposition of a moveable cartilage between
the two surfaces of articulation, a contrivance admirably an-
swering the intended purpose. Hence, in addition to the
principal movements of opening and shutting, which are
made in a vertical direction, the lower jaw has also some de-
gree of mobility in a horizontal or lateral direction, and is
likewise capable of being moved backwards or forwards, to
a certain extent. The muscles which effect the closing of
the jaw are principally the temporal and the masseter mus-
cles; the former occupying the hollow of the temples, the
latter connecting the lower angle of the jaw with the zygo-

108

THE VITAL FUNCTIONS.

matic arch. The lateral motions of the jaw are effected by
muscles placed internally 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 carnivorous and herbivorous
animals. In the former, of which the Tiger, Fig. 274, may

be taken as an example, the whole apparatus for mastication
is calculated for the destruction of life, and for tearing and
dividing the fleshy fibres. The molar teeth aj-e armed with
pointed eminences, which correspond in the opposite jaws
so as exactly to lock into one another, like wheel-work, 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 arc 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 promi-
nent; and the arch bone (z,) from which the masseter arises,
takes a wide span outwards, so as to give great strength to

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

JAWS AND TEETH OF HERBIVORA.

109.

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 Jintelope^ Fig. 275, the greatest force is

bestowed, not so much on the motions of opening and shut-
ting, as on those which are necessary for grinding, and
which act in a lateral direction. The temporal muscles, oc-
cupying the space t, are comparatively sm.all and feeble;
the condyles of the jaw are broad and rounded, and more
loosely connected with the skull by ligafnents; 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 of the molar teeth are flattened and of great
extent, and they are at the same time kept rough, like those
of mill-stones, their office being in fact very similar to that
performed by these implements for grinding. AH 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 al-
ready had occasion to notice, compose a well-marked fiimily
of Mammalia. These animals are formed for subsisting on

110 THE VITAL FUNCTIONS.

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 diflicult of digestion. They are
all animals of diminutive stature, whose teeth are expressly

formed for gnawing, nibbling,
and wearing away by conti-
nued attrition, the harder tex-
tures of organized bodies.
The 7i«/, whose skull is de-
lineated in Fig. 216, belongs
to this tribe. They are all
furnished with two incisor teeth in 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 instruments of tritura-
tion. The zygomatic arch is exceedingly slender and fee-
ble; 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 adapted, and
by which the grinding operation is performed. The Beaver,
the Baty the Marmot, and the Porcup'uie, present exam-
ples of this structure, among the omnivorous rodentia: and
the Hare, the Babbit, the Shrew, among those that are
principally herbivorous.

The Quadrumajia, or Monkey tribes, approach nearest
to the human structure in the conformation 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 de-
scribed, corresponding to greater varieties in the nature of
their food. Thus, the teeih and jaws of the Hyena are
formed more csj)ccially 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.

%'

STRUCTURE OF TEETH.

Ill

The teeth, though composed of the same chemical ingre-
dients as the ordinary hones, differ from them by having a
greater density and compactness of texture, whence they
derive that extraordinary degree of hardness which they re-
quire for the performance of their peculiar office. The sub-
stances of which they are composed 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 ge-
nerally given to it. Its earthy ingredient oonsists almost
entirely of phosphate of lime, the proportion of the carbo-
nate of that earth entering into its composition being very
small; and the animal portion is albumen, with a small quan-
tity of gelatin.

A layer of a still harder substance, termed the enamel,
usually covers the ivory, and, in teeth of the simplest struc-
ture, forms the whole of their outer surface: this is the

case with the teeth of man and of carnivorous quadrupeds.
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 atrace of gelatin;

112 THE VITAL FUNCTIONS.

it is the hardest of all animal substances, and is capable of
striking fire with steel. It exhibits a fibrous structure, ap-
proaching to a crystalline arrangement, and the direction of
its fibres, as shown by the form of its fragments when bro-
ken, is every where ])crpendicular to the surface of the ivory
to whicli it is applied. Tlie ends of the fibres arc 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 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 en-
amel only on their anterior convex surfaces, 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. 2S0.) This mode of growth is admi-
rably calculated to presefve these chisel teeth fit for use
during the whole lifetime 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 structure 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 received various names, but it is most commonly known
by that of the Criistd pctrosa: it resembles ivory both in
its composition and its extreme hardness; but is generally
more opaque and yellow than that substance.

STRUCTURE OF TEETH. 113

Other herbivorous quadrupeds, as the horse, and animals
belongnig to the ruminant tribe, have also complex teeth com-
posed of these three substances; and their grinding surfaces
present ridges of enamel intermixed in a more irregular
manner with the ivory and crusta pctrosa; but still giving
the advantasfe of a very rougrh surface to the 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 crusta petrosa (c,) marked by waving lines, is seen.
On the outside of all there is a plate of bone, which has been
left w^hite. In ruminants, the plates of enamel form cres-
cents, which are convex outwardly in the lower, and in-
wardly in the upper jaw; thus providing for the crossing of
the ridgeS'Of the two surfaces, an arrangement similar to
that which is practised in constructing those of mill-stones.
The teeth of the lower javv 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. 279 shows the grinding surface of the tooth oi d. 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 of the grinding tooth
of the elephant, in which they are more completely and
equally intermixed than in that of any other animal. Fig.
281 presents a vertical section of the grinding tooth of the
Asiatic elepiiant, 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 pe-
trosa, c, is less regularly deposited, p is the cavity which
had been occupied by the pulp. In this tooth, which is still

Vol. II. 15

114

THE VITAL FUNCTIONS.

rn a growing state, the fangs are not )-et 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
horizontal section of its interior structure, in which the plates
of white enamel are seen forming waved ridges. These con-
stitute, in the Asiatic Elephant, a series of narrow transverse
bands, (Fig. 2S3,) and in tlie African Elephant, a series of
loy.enge-shapcd lines, (Fig. 2S2,) having the ivory on their
interior, and the yellow crusta petrosa on their outer sides;
which latter substance also composes the whole circumfe-
rence of the section.

§ 4. Formation and Development of the Teeth.

Few processes in animal development are more remarka-
ble than those which arc employed to form the teeth; for
they are, by no means, the same as those by which ordina-
ry bone is constructed; and being commenced at a very ear-
ly period, they aftbrd a signal instance of Nature's provident
anticipation of the futiu'e necessities of the animal. Tiic
teeth, being the hardest parts of the body, require a peculiar

DENTITION. 115

system of operations for giving them this extraordinary-
density, which no gradual consolidation could have impart-
ed. The formation of the teeth is, in some respects, analo-
gous to that of shell; inasmuch as all their parts, when once
deposited, remain as permanent structures, hardly ever ad-
mitting of removal or of renewal by the vital powers. Un-
like the bones, which contain within their solid substance
vessels of different kinds, by which they are nourished, mo-
dified, and occasionally removed, the closeness of the texture
of the teeth is such as to exclude all vessels whatsoever.
This circumstance renders it necessary that they should ori-
ginally be formed of the exact size and shape which 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
deposite the calcareous substance, which is to compose the
ivory, at the most prominent points of that part of the vesi-
cle, which corresponds in situation to the outer la3'er of the
crown of the tooth. The thin scales of ivory thus formed
increase by farther depositions made on 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, se-
cretes the substance that is to compose the enamel, and tie-
posites it in layers on the surface of the ivory. This double
operation proceeds step by step, fresh layers of ivory being

116 THE VITAL FUNCTIONS.

deposited, and building up the body of the tooth, and in the
same projiortion cncroachinii; upon tlie cavity occupied by
the pulp, which retires before it, until it is shrunk into a
small compass, and fdls only the small cavity which remains
in the centre of the tooth. The ivory has by this time re-
ceived from the capsule a complete coating 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 disaj^pears. But the formation of
ivory still continuing at the part most remote from the
crown, the fangs are gi'adually formed by a similar process
from the pulp; and a pressure being thereby directed against
the bono of the socket at the part where it is the thinnest,
that portion of the jaw is absorbed, and the ])rogress of the
tooth is onl}^ 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 by feeding a young animal at dif-
ferent 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 quadruj:)eds,
which have three kinds of substance, is conducted in a still
more artificial and complicated manner. Thus, in the ele-
phant, the pulp which deposites 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 du-
plicatures of membrane in the intervals between the plates.
Hence the ivory constructed by the pulp, and the enamel
deposited over it, are variously intermixed; but besides this,
the crusta petrosa is deposited on the outside of the enamel.
Cuvier asserts that this deposition is made by the same cap-
sule which has formed the enamel, and which, previously
to this change of function, has become more spongy and
vascular than before. Hut his brother, M. Frederic Cuvier,

• Cuvier. Dictionnnirc dcs Sciences Medicalcs, t. viii. p. 320.

DENTITION. 117

represents the depositc of crusta pctrosa, as pcrforined 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 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 described, excepting that there is no outer cap-
sule, and therefore no outer crust of enamel. The whole of
the substance of the tusk is constructed by successive depo-
sites of layers, having a conical shape, from the pulp which
occupies the axis of the growing tusk; just as happens in the
formation of a univalve shell which is not turbinated, as, for
instance, the patella. Hence, any foreign substance, a bul-
let, 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, re-
tires in proportion as its place is occupied by the fresh de-
posites of ivory.

The young animal requires teeth long before it has attained
its full stature; and these teeth must be formed of dimen-
sions adapted to that of the jaw, wdiile it is yet of small size.
But, as the jaw enlarges, and the teeth it contains admit not
of any corresponding increase, it becomes necessary that
they should be shed to make room for others of larger di-
mensions, formed in a more capacious mould. Provision is
made for this necessary change at a very early period of the
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 en-
larges, the second set of teeth gradually acquire their full
dimensions, and then, by their outward pressure, occasion

118 THE VITAL FUNCTIONS.

the absorption of the fangs of the temporary teeth, and, push-
ing them out, occupy tlieir 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 la-
teral 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, ratlier smaller
than the others, and are called the wisdom-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 molars.

The teeth of carnivorous animals are, from the nature of
their food, less liable to be worn, than those of animals
living on grain, or on the harder kinds of vegetable sub-
stances; so that the simple plating of enamel is sufficient to
preserve them, even during a long life. But in many herbi-
vorous 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 imme-
diately behind them are pushed forwards by the growth Of
a new tooth at the back of the jaw; and this process goes on
continually, giving rise to a succession of teeth, each of
which is larger than that which has preceded it, during the
whole period that the animal lives. A similar succession of
teeth takes place in the wild boar, and, also, though to a less
extent, in the Sus Q^thiopicusA This mode of dentition

• It Is stated by Rousseau that the shedding- of the first molar tooth both
of the Guinea-pig, and the Capihara, and its replacement by the permanent
tooth, take place a few days before birth. Anatomic Compai-ce du systeme
dentaire, p. 164-.

t Home, Phil. Ti-ans. for 1799, p. 237; and 1801, p. 319.

DENTITION. 119

appears to be peculiar to animals of great longevity, and
which subsist on vegetable substances containing a large pro-
portion 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 contained, at the
same time, in the alveolar portion of the jaw.

An expedient of a different kind has been resorted to in
the Rodentictf 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 ani-
mal would perish in consequence, 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 princi-
ples, 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 em-
bedded in the jaw, in a large bony canal provided for that
purpose; and their cavity is always filled with the vascular
pulp, from which the 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 arc
lodged in each jaw, but one only of these rows projects and
is in use at the same time; the rest lying flat, but ready to
rise in order to replace those that 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 great-
est variety occurs in the mode of the succession of the teeth,.

The teeth of the Crocodile, which arc sharp-pointed hoi-

120

THE VITAL FUNCTIONS.

low cones, composed of ivory and enamel, are renewed by
the new tooth (as is shown at a, in Fig. 2S4,) being formed
234 in the cavity of the one (b) wliich it is

_____ to replace, and not being enclosed in any
C ,/ sejiarate cavity of the jaw bone (c.) As
this new tooth increases in size, it press-
es against the base of the old one, and
entering its cavity, acquires the same
conical form; so that when the latter is
shed, it is already in its place, and fit
for immediate use. This succession of
teetli 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 li-
quor, which is squeezed out by 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 interven-
tion 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 arc ready to supply their places
when wanted: for which purpose there are commonly pro-
vided two or three half-grown fangs, which arc 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 l^ase and extre-
mity of the fang, by which means there remain apertures at
both ends for the passage of the fluid poison. This struc-
ture was discovered by jNlr. T. Smith in the Coluber naia,

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

FANGS OF SERPENTS.

121

or, Cobra de Capcllo;'^- and is shown in Fig. 285, which
represents the full grown tooth, where the slight furrow, in-
dicating the junction of the two sides of the original groove,
maybe plainly seen; as also the two apertures (a and b)
above mentioned. This mode of formation of the tube is
farther illustrated by Fig. 286, which shows a transverse

291

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, resembling 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 complete-
ly filled up, even in this early stage of growth. Fig. 292
is a section of a full-growni fang of the same species of ser-
pent, at the same part as the preceding; and here the cavity

* Philosophical Transactions, 1818, p. 471.
Vol. II. 16

122

THE VITAL FUNCTIONS.

of the pulp is seen much contracted from tlie more advanced
staf];e of growth.

It is a remarka])le circumstance, noticed by Mr. Smith,
that a similar lonii;itudinal furrow is perceptible on every-
one of the teeth of the same serpent; and that this appear-
ance is most marked on those which are nearest to the poi-
sonous fangs: these furrows, however, in the teeth that are
not venomous, are confmed entirely 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 arc converted
to uses widely different from mastication, may be noticed
that of the Squalus pristis, or Saw-fish, where the teeth are
set horizontally on the two lateral edges of the upper jaw,
which is prolonged in the form of a snout (seen in a, Fig.
293,) constituting a most formidable weapon of offence, b

is a more enlarged view of a portion of this instrument, seen
from the under side.

§. 5. Tr 'duration of Food in Internal Cavities.

The mechanical apparatus, provided for triturating the
harder kinds of food, does not belong exclusively to the

GASTRIC TEETH.

123

mouth, or entrance into the alimentary canal, for in many-
animals we find this office performed by interior organs.

294 Among the inferior classes, we

find examples of this conforma-
tion in the Crustacea, the Mol-
lusca, and above all in Insects.
Thus, there is found in the sto-
mach of the Lobster, a cartilagi-
nous frame-work, in which are
implanted hard calcareous 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 stomach 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,) 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 mollusca introduced into the
stomach. These teeth are the result of a secretion of calca-
reous matter from the inner coat of that organ, just as the
outer shell of the animal is a production of the integu-
ment: and at each casting of the shell, these teeth, together
wnth 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 Mollusca, se-
veral species of Bullx have stomachs armed
with calcareous plates, which act as cutting
or grinding teeth. The Bulla aperta has
three instruments of this description, as
may be seen in Fig. 295, which shows the
interior of the stomach of that species.
Similar organs are found in the Bulla lig-

124

THE VITAL PUNCTIOXS.

nciria. The ^^p^ysia lias a considerable number of these gas-
tric teeth. An apparatus of a still more complicated kind
is provided in most of the insects belonging to the order of
Orthoptera; but I shall not enter at present in their de-
scription, as it will be more convenient to include them
in the sreneral account of the alimentary canal of insects,
which will be the subject of future consideration.

The internal machinery for grinding is exemplified on
the largest scale in granivorous birds; where it forms part

of the stomach itself, and is termed
a Gizzard. It is shown in Fig.
298, representing the interior of
the stomach of a Swan. Both the
structure and the mode of operation
of this organ bear a striking analo-
gy to a mill for grinding corn, for
it consists of two powerful mus-
\ cles (g,) of a hemispherical shape,
'] with their flat sides applied to each
other, and their edges united by a
strong tendon, which leaves a va-
cant 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 ac-
tion, performs an office similar to that of mill-stones. In
most birds, there is likewise a sac, or receptacle, termed
the Craw, (represented laid open at c) in which the food
is collected for the purpose of its being dropped, in small
quantities at a time, into the gizzard, in proportion as the
latter gradually becomes emptied.* Thus, the analogy be-
tween 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

• The gastric glands, wliich are spread over the greater part of the inter-
nal surface of the craw, and which prcjjare a secretion for macerating the
grain, are also seen in this part of the fig^ire.

ACTION OF THE GIZZARD. 125

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 wliich have been made,
particularly by Reaumur and Spallanzani, with a view to
ascertain the force of compression exerted by the gizzard
on its contents. 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 ad-
miration to physiologists; and being echoed from mouth to
mouth, were received with a sort of passive astonishment,
till Hunter directed the powers of his mind to the inquiry,
and gave the first rational explanation of the mechanism by
which they are produced. He found that the motion of the,
sides of the gizzard, w^hen 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 surfaces, and never perpendicularly to them;
and thus the edges and points of sharp instruments are either
bent or broken off by the lateral pressure, without their
having an opportunity of acting directly upon tlfose sur-
faces. Still, however, it is evident that the effects we ob-
serve produced upon sharp metallic points and edges, could
not be accom.plished by the gizzard without some assistance
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 constantly found
to contain a certain quantity of small pebbles, which must
have been swallowed by the animal. The most natural rea-
son that can be assigned for the presence of these stones, is,
that they aid the gizzard in triturating the contained food,
and that they, in fact, supply the office of teeth in that ope-

126 THE VITAL FUNCTIONS.

ration. Spallanzani, however, has called in question the
souiuhicss of this explanation, and has contended that the
pebbles found in the gizzard are swallowed merely by acci-
dent, 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 giz-
zard, when the bird has been able to meet with the requi-
site supply, and they are never swallowed but along with
the food. Several hundred w^ere 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 ac-
cident. If the alleged mistake could ever occur, w^e 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 be-
• ino- 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
instinct in the choice of the proper materials for food, and
for assistinsi its digestion: and if a mixture of a large quan-
tity of stones with a small proportion of grain be set before
them, they will at once pick out the grain, and swallow
alono- with it only the proper proportion of stones. The
best proof of the utility of these substances may be derived
from the experiments of Spallanzani himself, who ascer-
tained that grain is not digested in the stomachs of birds,
when it is protected from the effects of trituration.

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

Considerable differences are met with in the structure of
the gizzards of various kinds of birds, corresponding to dif-

SALIVARY APPARATUS. 127

ferences In the texture of their natural food. In the Tin^keyy
the two muscles which compose the gizzard are of unequal
strength, that on the left side being considerably larger than
that on the right; so that while the principal cifort is made
by the former, a smaller force is used by the latter to restore
the parts to their situation. These muscles produce, by
their alternate action, two effects; the one a constant tritura-
tion, 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 flat-
tened, and its lateral edges are very thin. The surfaces ap-
plied 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
intervening 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
between them, and fitting it, by this peculiar kind of tritu-
ration, for being digested.'^

^ 6. Beglulition,

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 subjected to
mechanical trituration, but it must also be mixed with a cer-

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

128 THE VITAL FUNCTIONS.

tain proportion of fluid. Hence, all animals that masticate
their food are provided with organs wliich secrete a fluid,
called the Saliva, and which pour this fluid into the mouth
as near as possible to the <;;rinding surfaces of the teeth.
These organs are glands, ])laced in such a situation as to he
compressed hy the action of the muscles which move the
jaw, and to pour out the fluid tliey secrete in greatest quan-
tity, just at the time when the food is undergoing masti-
cation. 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 luhricate the passage
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 ve-
getable materials, requiring more complete maceration than
those which feed on flesh, the salivary glands are of large
size: they are particularly large in the Rodent ia, which feed
on the hardest materials, requiring the most complete tritu-
ration: 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 masti-
cate their food, the salivary glands are comparatively of
small size: the exceptions to this rule occurring chiefly in
those trlhes which feed on vegetables, for in these the glands
are more considerable.* In Fishes there is no structure of
this kind provided, there being no mastication performed:
and the same observation applies to the Cetacea. In the
cephalopodous and gasteropodous Mollusca, we find a sali-
vary apparatus of considerable size: Insects, and the t^nne-
lida,\ also, generally present us with organs which appear
to perform a similar oflice.

• The l.'irge salivary gland in the woodpecker, is seen at s, Fig-. 271, page
99.

t The bunch of filaments, seen at s, Fig. 260 (p. 78) are the salivary or-
gans of tliclccch.

DEGLUTITION. 129

•

Tho passage of, the food along the lliroat is facilitated by
the mucous secretions, which are poured out from a multi-
tude of glands interspersed over the whole surface of the
membrane lining that passage. The Cmiid, which is formed
for traversing dry and sandy deserts, where the atmosphere
as well as the soil is parched, is specially provided with a
glandular cavity placed behind the palate, and which fur-
nishes a fluid for the express purpose of moistening and lu-
bricating the throat.

In the structure of the (Esopha^iis^y^X^xoXx 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 cosophagus 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 considera-
ble time, the large fish which arc swallowed entire, and
which could not conveniently be admitted into the stomach.
Blumenbach 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
CESophagus, and descended gradually, in proportion as the
former were dissolved. Serpents, which swallow animals
larger than themselves, 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 oeso-
phagus, the inner coat of which is beset with numerous firm
and sharp processes, having their points directed towards
the stomach; these are evidently intended to prevent the re-
turn of the food into the mouth. Grazing quadrupeds, who,
while they eat, carry their heads close to the ground, have
a long oesophagus, with thick muscular coats, capable of ex-
erting considerable power in ])ropelling the food in the di-
rection of the stomach, which is contrary to that of gravity.
Vol. II. 17

130 THE VITAL FUNCTIONS.

§ 7. Bccepfaclcsfor retaining Food,

Provision is often made for the retention of the undigest-
ed food in reservoirs, situated in difTercnt parts of the mouth,
or the ccsopliagus, instead of its being immediately intro-
duced into the stomach. These reservoirs are generally em-
ployed for laying in stores of provisions for future consump-
tion, ^lany quadrupeds have cheek pouches for this purpose:
this is the case with several species of Monkeys and Ba-
boons; and, also, willi the Miis cricclus, or Hamster. The
Mus buscu'ius, or Canada rat, has enormous check pouches,
■which, when distended with food, even exceed the bulk of
the head. Small cheek pouches exist in that singular ani-
mal, the Ornil/ior/iT/ncns. The Scitirus palmariim, 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 dex-
terity in tossing about the fish with which it has loaded this
bag, till it is brought into the proper position for being swal-
lowed. The JV/iale has also a receptacle of enormous size,
extending from the mouth to a considerable distance under
the trunk of the body.

Analogous in design to these pouches are the dilatations
of the oesophagus of birds, denominated c?'ops. 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 Pam^ot is represented at c. Fig. 299; where
also, s indicates the cardiac portion of the stomach, and g the
gizzard, of that l)ird. The inner coat of the crop is furnished
with numerous glands, which pour out considerable quantities
of fluid for macerating and softening 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 arc

RECEPTACLES FOR RETAINING FOOD.

131

much enlarged, and very active in preparing tlieir peculiar
secretions: this is remarkably the case in the Pigeon (Fig.

300,) which, instead of a single
sac, is provided with two (seen
at c, c, Fig. 300,) one on each
side of the ccsophagus (o.) The
pouting pigeon has the facul-
ty of filling these cavities with
air, which produces that distend-
ed appearance of the throat from
which it derives its name. Birds
of prey have, in general, very
small crops, their food not re-
quiring any previous softening;
but the Vulture, which gorges large quantities of flesh
at a single meal, has a crop of considerable size, form-
ing, when filled, a visible projection in front of the chest.
Birds which feed on fish have no separate dilatation 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 farther dilatation of any particular part of the
tube unnecessary. The lower portion of the oesophagus ap-
pears 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.

( 132 )

CHAPTER VII.

Digestion.

All the substances received as food into the stomach,
whatever be their nature, must necessarily undergo many
changes of chemical composition before they can gain ad-
mission into the general mass of circulating fluids; but the
extent of tiie change required for that purpose will, of course,
be in proportion to the difference between the (qualities of
the nutritive materials in their original, and in their assimi-
lated state. The conversion of vesretable into animal mat-
ter necessarily implies a considerable modification of proper-
ties; but even animal substances, however similar may be
their composition to the body which they are to nourish,
must still pass through certain j)rocesses of decomposition,
and subsequent recombination, before they can be brought
into the exact chemical state in which they are adapted to
the pur])oses 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 co-
hesion of its parts, and mixing them uniformly with the fluid
secretions of the mouth; eficcts wdiich 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 con-
verted into a sul)stance termed Chyme; and the process by
which this first step in the assimilation of the food is pro-
duced, constitutes what is proj^erly termed Digestion.

Nothing has been discovered in the anatomical structure
of the stomach, tending to throw any light on the means by
which this remarkable chemical change is induced on the

DIGESTION.

133

materials it contains. The stomacli is, in most animals, a
simple sac, composed of several membranes, enclosing thin
layers of muscular fibres, abundantly supplied with blood-
vessels and with nerves, and occasionally containing struc-
tures which appear to be glandular. The human stomach,
which is delineated in Fig. 301, exhibits one of the simplest

form^of this organ; c being the cardiac portion, or part
where the oesophagus opens into it; and p the pyloric por-
tion, or that which is near its termination in the intestine.
At the pylorus itself, the diameter of the passage is much
constricted, by a fold of the inner membrane, which is sur-
rounded 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 arc concerned in that operation, is a fluid
secreted by the coats of the stomach, termed the Gastric
juice. This fiuid has, in each animal, the remarkable pro-
perty of dissolving, or, at least, reducing. to a pulp, all the
substances which constitute the natural food of that particu-
lar species of animal; w'hile it has comparatively but little
solvent power over other kinds of food. Such is the con-
clusion which has been deduced from the extensive re-
searches on this subject, made by that indefatigable experi-
mentalist, Spallanzani, who found, in numberless trials, that
the gastric juice taken from the stomach, and put into glass

134 THE VITAL FUNCTIONS.

vessels, produced, if kept at the usual temperature 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 mat-
ter; while, on the contrary, that taken from herbivorous ani-
mals, acted on grass and other vegetable substances, without
producing any eflect on flesh; but in those animals, which,
like man, are omnivorous, that is, partake indiscriminately
of both species of aliment, it appeared to be fitted equally
for the solution of both. So accurate an adaptation of the
chemical powers of a solvent to the variety of substances
employed as food by diflcrent animals, displays, in the most
striking manner, the vast resources of nature, and the re-
fined chemistry she has put in action for the accomplishment
of her difierent purposes.

In the stomachs of many animals, as also in the human, it
is impossible to distinguish with any accuracy the organi-
zation by which the secretion of the gastric juice is effected:
but where the structure is more complex, there may be ob-
served a number of glandular bodies interspersed in various
parts of tlic 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 ori-
fice 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 stomach
from injurious impressions. These latter are termed the

* The accuracy of this conclusion has been lately contested by M. Ue
Montcgrc, whose report of the effects of the gastric juice of animals out of
the body, docs not accord with that of Spallanzani; but the difference of cir-
cumstances in which liis experiments were made, is quite sufficient to ac-
count for the discrepancy in the results; and those of M. De Montcgre, there-
fore, by no means, invalidate the general facts stated in the text, which have
been estabhslicd by the experiments, not only of Spallanzani, but also of
Reaumur, Stevens, Leuret, and Lassaigne. See Alison's Outlines of Phy-
siology and Pathology, p. 170.

DIGESTION.

135

mucous glands, and they are often constructed so as to
pour their contents into intermediate cavities, or small sacs,
which are denominated follicles, where the fluid is collected
before it is discharged into the cavit}^ of the stomach. The
o-astric dands of birds arc 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 ocsophngus, a large
glandular organ, which has been termed the bulbulus glan-
dulosus. In the Oslrich, 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 c is the cardiac cavity, the coats

305

303

304

of which are studded with numerous glands; G, G, are the
two sides of the gizzard. Fig, 303 shows one of the gas-
tric glands of the African ostrich; Fig. 304, a gland from
the stomach of the American ostrich, and Fig. 305, a sec-
tion 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 prcdaceous birds; but as these anatomical details
have not as yet tended to elucidate in any degree the pur-

. 136 THE VITAL FUNCTIOB^S.

poses to which they are subservient in the process of diges-
tion, 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 tlie food received into the stomach sliould be
acted upon by tlic gastric juice; for which purpose provi-
sion is made that each portion shall, in its turn, be placed
in contact with tlic inner surface of that organ. This is the
more necessar}', as many facts render it ])robable, as will be
noticed more particularly hereafter, that, besides the chemi-
cal action of the gastric juice, an influence, derived from the
nerves, essentially contributes to the accomplishment of the
chemical changes which the food undergoes in the stomach.
For this reason it is that the coats of the stomach are pro-
vided with muscular fibres, passing, some in a longitudinal,
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, they 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 succes-
sion, they propel the food from one part to another, and
thus promote the mixture of every portion with the gastric
juice. We often fnid that the middle transverse bands con-
tract more strongly tlian the rest, and continue contracted
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 differ-
ences in structure, which are often observable between these
two portions of the stomach, would lead to the belief that
their functions are in some respects different.

• These structures have been examined M'ith great care and minuteness
by Sir Everard Home, who lias given the results of his inquiries in a scries
of papers, read from time to time to the lioyal Society, and published in
their Tmnsactions.

t Sec rig. 51, vol. i. p. lOG, and its description, p. 107.

DIGESTION. 137

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 sphincter 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 j)ylo-
rus appears to be one means employed for attaining this
end; and another is derived from the property which the
gastric juice possesses of coagulating, or rendering solid,
every animal or vegetable fluid susceptible of undergoing
that change. This is the case with fluid albumen; the white
of an egg, for instance, which is nearly pure albumen, is
very speedily coagulated when taken into the stomach; the
same change occurs in milk, which is immediately curdled
by the juices that are there secreted, and these efi^ects 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 in 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 ac-
tion, however much they may be in themselves nutritious,
are very imperfectly digested, and consequently afibrd very
little nourishment. This is the case with oils, with jelly,
and with all food that is much diluted.* Hunter ascertained

* 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 proper-
ly 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 pro-
cess of digestion. I was led to this conclusion, not only from theory, but
from actual observation of what took place among the prisoners in the Mil-
bank Penitentiary, in 1823, when on the occasion of the extensive preva-
lence 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
Vol. II. 18

13S THE VITAL FUNCTIONS.

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

The gastric juice has also the remarkable property of cor-
recting putrefaction. This is particularly exemplified in
animals that feed on carrion, to whom this property is of
great importance, as it enables them to derive wholesome
nourishment from materials which would otherwise taint
the whole system with their poison, and soon prove de-
structive to life.

It would appear that the first changes wdiich 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 continuing
as it advances. In the Babbit 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
bco-un to undergo a change: for this purpose the new food
is introduced into the centre of the mass already in the sto-
mach; 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. :j:

As the flesh of animals has to undergo a less considera-
ble change than vegetable materials, so we find the stomachs
of all the purely carnivorous tribes consisting only of a mem-
branous bag, which is the simplest form assumed by this or-

largc proportion of soups, on which the prisoners had subsisted for the pre-
ceding; cii^ht months. A very full and perspicuous account of that disease
has been drawn up, with great abihty, by my friend Ur. P. M. Latliam, and
published under the title of "An account of the disease lately prevalent in
the General Penitentiary." London, 1825.

• Observations on the Animal Economy, p. 172.

I Phil. Trans, for 1813, p. 9G.

\ Sec Dr. Philip's ExperimcnUd Inquiry into the Laws of the Vital Func-
tions, 3d edition, p. 122.

STOMACHS OF MAMMALIA.

13a

gan. But in other cases, as we have already seen, the sto-
mach exhibits a division into two compartments by means
of a slight contraction; a condition which, as Sir E. liome
has remarked, is sometimes found as a temporary slate of
the human stomach;* while, in other animals, it is the na-
tural and permanent conformation. The Rodcntia furnish
many examples of this division of the cavity into two dis-
tinct portions, which exhibit even differences in their struc-
•ture: 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 spiral fold of
the inner, or cuticular membrane, which forms a complete
valve, offering no impediment to the entrance of food
from the oesophagus, but obstructing the return of any part
of the contents of the stomach into that passage.! This

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

f The total inabihty of a horse to vomit is probably a consequence of tlie
impediment presented by this valve. Sec Mem. du Museum d'llist. Nat. viii.
111.

140

THE VITAL FUNCTIONS.

valve is shown in Fig. 311, wliich represents an inner view
of the cardiac portion of the stomach of the horse; o being
the termination of the oesophagus.

The stomach of the Water
Rat is composed of two dis-
tinct cavities, having a narrow
passage of communication:
the first cavity is lined with
cuticle, and is evidently in-*
tended for the maceration of
the food before it is submit-
ted to the agents which are to
efl'ect its digestion; a process
which is completed in the se-
cond cavity, provided, for that
purpose, with a glandular sur-
face.
In proportion as nature allows of greater latitude In diet, we
find her providing great complication in the digestive appara-
tus, and subdividing the stomach into a greater numijer of ca-
vities, each having probably a separate office assigned to it,
thougli concurring in one general effect. A gradation in this
respect mf»y be traced through a long line of quadrupeds,
such as the Hog, the Peccari, the Porcupine, (Fig. 308,) and
the Hippopotamus, where we find the number of separate
pouches for digestion amounting to four or five. Next to
these we may rank tlie very irregular stomach of the Kan-
gnroo, (Fig. 309) composed of a multitude of cells, in which
the food probably goes through several preparatory processes:
and still greater complication is exhibited by the stomachs
of the Cctacea, as, for example, in that of the Porpus (Fig.
310.) As the fishes upon which this animal feeds are swal-
lowed wliole, 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 inter-
fering with the di;rrstion of the soft parts. The narrow com-

STOMACHS OF MAMMALIA.

141

munications between these several ^stomachs of the cetacea
are probably intended to ensure the thorough solution of their
contents, by preventing the exit of all such portions as have
not perfectly undergone that process.

Supernumerary cavities of this kind, belonging to the
stomach, are more especially provided in those animals
which swallow food either in larger quantity than is imme-
diately wanted, or of a nature which requires much prepa-
ration previous to digestion. The latter is more particularly
the case with the horned ruminant tribes that feed on the
leaves or stalks of vegetables, a kind of food, which, in pro-
portion to its bulk, affords but little nutriment, and requires,
therefore, a long chemical process, and a complicated diges-
tive apparatus, in order to extract from it the scanty nutri-
tious matter it contains, and prepare it for being applied t^
the uses pf 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 re-
presentation, in Fig. 312, that of the Sheep, of which the

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

142 THE VITAL FUNCTIONS.

The orass, which is devoured in large quantities 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 tlu; figure, called ihc paunch. This
cavity is lined internally with a thick membrane, beset with
numerous flattened ])a])illa^, and is often divided into pouches
by transverse contractions. While the food remains in this
bail, it continues in rather a diy state; but the moisture with
which it is surrounded contributes to soften it, and to pre-
pare it for a second mastication; which is effected in the fol-
lowing manner. Connected with the paunch is another, but
much smaller sac (2,) which is considered as the second sto-
mach; and, from its internal membrane being thrown into
numerous irregular folds, forming the sides of polygonal
tells, it has been called the honcy-comh stomach, or reticule.
Fig. 313 exhibits this reticulated appearance of the inner
surface of this cavity. A singular connexion exists between
this stomach and the preceding; for, while the asophagus ap-
pears to open naturally into the paunch, there is, on each
side of its termination, a muscular ridge, wdiich projects from
the orifice of the latter, so that the two together form a chan-
nel leading into the second stomach; and thus the food can
readily pass from the oesophagus into either of these cavi-
ties, according as the orifice of the one or the other is open
to receive it.

It would appear, from the observations of Sir E. Home,
that liquids drank 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, iil wiiich
there is always a supply of water for moistening the portion
of food introduced 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 wliile the animal is reposing; a process which
is well known by the name of chewing the cud, or rumi-
7iation.

STOMACHS OF RUMINANTS. 143.

When the mass, after being thoroughly ground down by
the teeth, is again swallowed, it passes along the esophagus
into the third stomach (3,) the orifice of which is brought
forward by the muscular bands, forming the two ridges al-
ready noticed, which are continued from the second sto-
mach, and which, wheti they contract, effectually prevent
any portion of the food from dropping into either of the pre-
ceding cavities. In the ox, this third stomach is described
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. Fig. 314 represents this plicated struc-
ture in the interior of the third stomach of a bullock. What-
ever food is introduced into this cavity, w^hich is named,
from its foliated structure, the maiiy-plies stomach, must
pass between these folds, and describe three-fourths of a cir-
cle, 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, how-
ever, 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 oeso-
phagus 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 animals, rumination does not
take place, till they begin to eat solid food. It is in tliis
fourth stomach, which is called the reed, that the proper di-
gestion of the food is performed, and it is here that the coa-
gulation 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 rciiular jiradation in the structure of ruminatino; sto-
machs may be traced in the different genera of this family
of quadrupeds. In ruminants with horns, as the bullock

144 THE VITAL FUNCTIONS.

and the sheep, tlicre are two preparatory stomachs for re-
taining the food previous to rumination, a third for receiving
it after it lias undergone this process, and a fourth for effect-
ing its digestion. Ruminants without horns, as tiic Camel,
Dromedar}", and Lama, have only one preparatory stomach
before rumination, answering the purpose of the two first sto-
machs of the bullock; a second, which I shall presently no-
tice, 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 both receives and dij2;csts 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 perform some pre-
paratory office, which renders the food more easily digesti-
ble 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 inha-
bitant of the steril 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
compartment, to which is attached a series of cellular ap-
pendages; (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 sto-
mach are performing their usual functions. By the relaxa-
tion of these muscles, the water is gradually allowed to mix
with the contents of the stomach, and thus the Camel is ena-
bled to sujDport long marches across the desert, without re-
ceiving 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

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

DIGESTION. 145

of the water contained in these reservoirs, which they al-
ways 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 de-
prived of water, acquire the power of dilating tlie cells, so
as to make them contain a more than ordinary 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 them. The quantity of water thrown out is in pro-
portion to the distance of the part attacked, and is common-
ly half a pint at a time: and this, Mr. Pierard, who resided
many years in India, has known the elephant to repeat eight
or ten times within the hour. The quantity of water at the
animaPs command for this purpose, observes Sir E. Home,
cannot, therefore, be less than six quarts. This water is not
only ejected immediately after drinking, but six or eight
hours afterwards. Upon receiving this information, Sir E.
Home examined the structure of the stomach of that animal,
and found in it a cavity, like that of the camel, perfectly
well adapted to afford this occasional supply of water, which
may, 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 jNIam-
malia, between the nature of their food and the conformation
of their digestive 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 consists of animal or of vegetable materials, or presents
more or less resistance from the cohesion of its texture. As
it would be impossible, in this place, to enter into all the de-

* Home, Lectures on Comparative Anatomy, vol. i. p. 171.
t Supplement to Sir E. Home's Lectures on Comparative Anatomy, vol.
vi. p. 9.

Vol. II. 19

146 THE VITAL FUNCTIONS.

tails necessary for fully illustrating this proposition, I must
content myself with indicatino; a few of the most general re-
suits of the inquiry.*

As tiie food of birds varies, in difierent species, 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 occupies the other
extremity of the series. This gradation is established by
the muscular fibres, which surround the former, acquiring,
in difTcrent tribes, greater extent, and forming stronger mus-
cles, adapted to the corresponding variations in the food,
more especially as it partakes of the animal or vegetable
character.

In all the cold-blooded vcrtebrata, where digestion is not
assisted by any internal heat, that operation proceeds more
slowly, though in the end not less effectually, than in ani-
mals where the contents of the stomach are constantly main-
tained at a high temperature. They almost all rank as car-
nivorous animals, and have accordingly stomachs, which,
however they may vary in their form, are alike simply
membranous in their structure, and act by means of the sol-
vent power of their secretions. Among reptiles, only a few
exceptions occur to this rule. The common sea-turtle that
is brought to our tables, is one of these; for it is found to
feed exclusively on vegetable diet, and chiefly on the sea-
weed called zostira maritima, and the structure of its sto-
mach corresponds exactly to the gizzard of birds. Some
tortoises, also, which eat grass, make an approach to the
same structure.

In fishes, indeed, although the membranous structure of

* The comparative anatomy of the stomach has been investig-atcd with
great diligence by the late Sir E. Home, and the results recorded in the pa-
pers he communicated, from time to time, to the I{oyul Society, and which
have been republished in his splendid work, entitled *' Lectures on Compara-
tive Anatomy," to which it will be seen that I have been largely indebted for
the ficts and observations relating to this subject, detailed in the text.

DIGESTION IN PISHES. 147

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 purpose, 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 em-
ployed; the office of mastication being transferred to the
stomach. Thus, the Mullet has a stomach endowed witli 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 peculiar to Ireland,
exhibits, though in a less degree, the same structure. The
common trout, also, occasionally lives upon shell-fish, and
swallows stones to assist in breaking the shells. |

Among the invertcbrated classes we occasionally meet
with instances of structures exceedingly analogous to a giz-
zard, 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, wdiich is often provided, not only with a
mechanism analagous to a gizzard, but also with rows of
gastric teeth.

( 14S )

CIIx\PTEll VIII.

Chrjl'ificalion.

The formation of Chyle, or the Huid which is the imme-
diate and cxchisivc source of nutriment to the system, takes
])lace in the intestinal tube, into which the chyme ])repared
by the stomach is receiv'ed, and where farther 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 imperfectly known, it is consequently impos-
sible 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 that
are performed by structures so large and so complicated as
are those composing the alimentary canal, and its various
appendages. We plainly perceive that provision is made
in the interior of that canal, for subjecting its contents to
the action, first, of an extensive vascular and nervous sur-
face; and secondly, of various fluid secretions, derived from
different sources, and exercising powerful chemical agencies
on the digested aliment; that a muscular power is su])plied,
by means of the layers of circular and longitudinal fibres,
contained between the outer and inner coats of the intes-
tine,* for exerting a certain pressure on their contents, and
for propelling them forwards by a succession of contractions,
which constitutes what is termed their pcrisiallic motion;
and lastly, that contrivances arc at the same time resorted
to for retarding the progress of the aliment in its passage

• See vol. i. p. 106.

CHYLIFICATION.

149

i

along the canal, so that it may receive tlie full action of
these several agents, and yield the utmost (quantity of nutri-
ment it is capable of affording.

The total length of the intestinal tube differs much in dif-
ferent animals, being in general, as already stated, smaller in
the carnivorous tribes, than those which iced on substances
of difficult dige stion, 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 oi* serpentine course, forming many
convolutions in the abdominal cavity. Sometimes, probably
for greater convenience of package, instead of these nume-
rous convolutions, a similai' effect of increasing the suri\ice
of the inner membrane is obtained by raising it into a great
number of folds, which project into tl>c cavity. These folds
are often of consiflerable breadth, contributing not only to the
extension of the surface foi' 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

structure occur in most of the cartilaginous
fishes, when the inner coat of the large in-
testine is expanded into a broad fold, which,
as is seen in fig. 316, representing this struc-
ture in the interior of the intestine of the
sliark, takes a spiral course; and this is con-
tinued nearly the whole length of tlie canal,
so that tlie internal surface is much augment-
ed without any increase in the length of the

intestine.*

When the nature of the assimilatory pro-
cess is such as to require the complete detention of the food,
for a certain time, in particular situations, we find this ob-
ject provided for by means of casca, or separate pouches

• Structures of ttiis description have a particular claim to attention, from
the light they tlirow on the nature of several fossil remains, lately investi-
gated with singular success by Dr. Duckland.

150 THE VITAL FUNCTIONS.

opening laterally from the cavity of the intestine, and having
no otlicr outlet. Structures of this description have already
hccn noticed in the infusoria,! 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 por-
tion of the stomach, and frequently at the commencement
of the small intestine. Their most usual situation, however,
is lower down, and especially at the part where the tube, af-
ter 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 intestine, and which
is frequently more capacious than the stomach itself. It is
exceedingly probable that these two portions of the canal
perform different functions in reference to the assimilation
of the food: but hitherto no clew has been discovered to guide
us through the intricacies of this difficult part of physiology;
and we can discern little more than the existence already
mentioned, of a constant relation 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 ca:ca, in animals that feed on ve-
getable substances, than in carnivorous animals of the same

class.

The class of insects supplies numberless exemplifications
of tlie accurate adaptation of the structure of the organs of as-
similation to the nature of the food which is to be convert-
ed into nutriment, and of the general principle that vegeta-
ble aliment requires longer processes and a more compli-
cated apparatus for this purpose, than that which has been al-
ready animalized. In the herbivorous tribes, we find the oeso-
phagus cither extremely dilatable, so as to serve as a crop, or
receptacle for containing the food previous to its digestion, or
having a distinct pouch appended to it for the same object:
to this there generally succeeds a gizzard, or apparatus for

• Page 73, of this volume.

CHYLIFICATION. 151

trituration, furnished, not merely with a hard cuticle, as in
birds, but also with numerous rows of teeth, of various forms,
answering most effectually the purjDOse of dividing, or grind-
ing 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 dio-estion, which constitutes the true stomach.* In the
lower part of this organ a peculiar fluid secretion is often in-
termixed with it, which has been supposed to be analogous
to the bile 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 fila-
ments, and which float loosely in the general cavity of the
body, attached only at their termination, where tliey open
into the alimentary canal. t In some insects, these tubes are
of larger diameter than in others: and in many of the or-
thoptera, as we shall presently see, they open into large re-
ceptacles, sometimes more capacious than the stomach itself,
which have been supposed 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 com-
pletion of the digestive process. J

* It is often diflficult to distinguish the portions of the canal, which cor-
respond in their functions to the stomach, and to the first division of the in-
testines, or duodenum; so that different naturalists, according to the views
they take of the peculiar office of tliese parts, have applied to the same ca-
vity the term of chjllferous stomach, or of du'odmum. See the memoir of
Leon IJufour, 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 Asierlas, where the double row of minute lobes attached
to the cxcal stomachs of those animals, and discharging their fluid into these
cavities, are considered by Cams, as performing a similar office. The floc-
culent tissue which surrounds the intestine of the Ilolothuria, is probably,
also, an hepatic apparatus.

\ A doubt is suggested, by Lcon Dufour, whether the liquid found in
these pouches is real bile, or merely aUmcnt in the progress of assimilation,
Ann. Sc. Nat. ii. 478.

152 THE VITAL FUNCTIONS.

Tlic distinction into small and great intestine is more or
less marked, in different insects, in proportion to the quan-
tities 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 llie inner membrane, which constitute im-
perfect valves for rctardinij; the progress of the alim.ent. It
is generally at the j)oint 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, iKit 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
liave some relation to the existence and development 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, remarkable for their excessive voracity, even three
orders of hepatic vessels are met with.*

A muscular power has also been provided, not only for
the strong actions exerted by the gizzard, but, also, for the
necessaiy propulsion, in different 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 direction. Glandular structures, analogous to
the mucous follicles of the higlicr animals, are also plainly
distinguishable in tlie internal coat of the canal, more espe-
cially of herl)ivorous insects.! The whole tract of the ali-
mentary canal is attached to the sides of the containing ca-
vity by a fine membrane, ov ])eritoncuin, containing nume-
rous air-vessels, or truchece.X

• Sec the IMemoirs of Marcel dcs Serres, in the Annalcs du Museum,
XX. 48.
■j- Lyonet.
4^ It has been stated by Malpighi and by Swammcrdam, and the statement

DIGESTIVE ORGANS OF INSECTS.

153

To engage in a minute description of the endless varia-
tions 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. T

shall content myself, therefore, with
giving a few illustrations of their prin-
cipal varieties, selected from those in
which the leading characters of struc-
ture are most strongly marked. I shall,
with this view, exhibit first one of the
simplest forms of the alimentary or-
gans, as they occur in the Mantis reli-
giosa, (Linn.) which is a purely carni-
vorous 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 perfect-
ly straight: it commences by an oeso-
phagus (o,) of great length, which is
succeeded by a gizzard (g;) at the low-
er extremity of this organ the upper
hepatic vessels (b, b,) eight in number,
and of considerable diameter, are in-
serted: 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 termi-
nates.

has been repeated by every succeeding- anatomist; that almost all tlie insects
belong-ing" to the tribe of Grylll, possessed the faculty of ruminating- tiieir
food; but this error has been refuted by Marcel des Serres, wlio luis offered
satisfactory evidence that in no insect is the food subjected to a true rumina-
tion, or second mastication, by the organs of the mouth. See Annales du Mu-
seum, XX. 51 and 364.

Vol. II. 20

154

THE VITAL FUNCTIONS.

The alimentary canal of the Cicinchla campcstris, (Lin.)
which preys on other insects, is represented in Fi^^. 31S;
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, their mechanical
division being once efiected, 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 (ii,) of which, in this insect, there
is only one set, terminate in the cavity of the intestine by
four ducts, at the point where that canal commences.

A more complicated structure is exhibited in the alimen-
tary tube of the Meloloniha vidf^aris^ or common cock-
chafier, which is a vegetable feeder, devouring great quanti-
/lies of leaves of jjlants, and consequently requiring a long

DIGESTIVE ORGANS OF INSECTS.

1^5

and capacious canal for their assimilation; as is shown in
Fig. 319, which represents them prepared in a similar man-
ner to the former. In this herbivorous insect, the oesopha-
gus (o) is, as might be expected, very short, and is soon di-
320 lated into a crop (*p;) this is followed by a very

long, wide, and muscular stomach (s,) ringed
like an earth-worm, and continued into a long
and tortuous intestine (i, i,) which presents
in its course several dilatations (c, c,) and re-
ceives very elongated, convoluted, and rami-
fied 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"^) o^ihe ^crida 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 occasionally
into a crop (i,) and succeeded by a round-
ed or heart;shaped gizzard (g,) of very
o 325 complicated structure, and connected with
two remarkably large biliary pouches (u
and B,) which receive, at their anterior
324 extremity, the upper set of hepatic ves-
sels (v V.) A deep furrow in the pouch
(b,) which, in the horizontal position of
the body, lies underneath the gizzard,
divides it apparently into two sacs. The
intestinal canal is pretty uniform in its
diameter, receives in its course a great
K number of hepatic vessels (h ir,) by se-
parate openings, and after making one
convolution, is slightly constricted at n,
and is dilated into a colon (c,) on the

* 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.

156 THE VITAL FUNCTIONS.

coats of which the longitudinal muscular bands are very dis-
tinctly 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 intermedi-
ate 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 up-
per 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 ante-
rior 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 com-
pose 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 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 dif-
ferent 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 m,a-
terials for its rapid growth, and frequently consuming enor-
mous quantities of fibrous vegetable aliment: the perfect in-
sect, on the other hand, having attained its full dimensions,
is sufliciently supported by small quantities of a more nu-
tritious food, consisting cither of animal juices, ^r of the
fluids prepared by flowers, which are generally of a saccha-
rme quality, and contain nourishment in a concentrated
form. It is evident that the same apparatus, which is ne-
cessary 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

DIGESTIVE ORGANS OP INSECTS.

157

to accommodate it to this altered condition of its function,
considerable changes must be made in its structure. Hence,
it will be interesting to trace the gradual transitions in the
conformation of the alimentary canal, during the progressive
development of the insect, and more especially while it is
undergoing its different metamorphoses.

These changes are most conspicuous in the Lepidoptera,
where we may observe the successive contractions which
take place in the immensely voluminous stomach of the ca-
terpillar, 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

328

327

326

I

given representations of these three different states of the en-
tire alimentary canal of the Sjjhinx ligiislri, or Privet Ilawk-
moth, in Figures 326, 327, and 32S;* the first of which

* Tliesc figures also have been engraved from the drawings of Mr. New-
port, which he was so obliging as to make for mc, from preparations of his
own, the result of very careful dissections.

158 THE VITAL FUNCTIONS.

is that of tlic caterpillar; 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 de-
lineated of the natural size, and 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 ccecal 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 sto-
mach forms by far the most considerable portion of the ali-
mentary tube, and that it bears some resemblance in its struc-
ture and capacity to the stomachs of the Annelida, already
described.* This is followed by a large, but short, and per-
fectly straight intestine. These organs in the pupa (Fig.
327) have undcgone considerable modifications, the whole
canal, but more especially the stomach, being contracted both
in length and width if the shortening of the intestine not
being in proportion to that of the whole body, obliges it to
be folded upon itself for a certain extent. In the moth,
Fig. 328,) the contraction of the stomach has proceeded
much farther; and an additional cavity, which may be consi-
dered 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 cxcum, has been formed at the part
where it joins the large intestine.

The hepatic vessels are exceedingly numerous in the Crus-
tacea, 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

• Sec the figures and description of those of the Nais and the Leech, p.

102 and 103.

f Cams states tliat he found tlie 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.

DIGESTIVE ORGANS OF MOLLUSCA.

159

the Liver of the higher classes of animals. This organ ac-
quires still greater size and imporUnce in the Mollusca,
where it frequently envelops the stomach, pouring the bile
into its cavity by numerous ducts.* As the structure and
course of the intestinal canal varies greatly in diflcrcnt tribes
of Mollusca, they do not admit of being comprised in any
general description. The only examples I think it necessa-
ry to give, in this class, are those of the
Patella, or Limpet, and of the Pleiiro-
branchiis. The intestinal tube of the
Patella is delineated in Fig. 329; where
M is the mouth; t, the tongue folded
back; o, the oesophagus; and s, the sto-
mach, 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 Pleurobranchiis Peronii (Cuv.) is remarkable for
the number and complication 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,resemblingthc third stomach
of ruminant quadrupeds; and d, a fourth ca-
vity, being that in which digestion is com-
pleted. A canal of communication 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
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
animals we also discover the rudiment of another auxiliary

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

160

THE VITAL FUNCTIONS.

/

organ, namclvj the Panaceas, which secretes a fluid contri-
buting to tlie assimihitfon of the food. This organ becomes
more and more developed as we ascend in the scale of ani-
mals, 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 characterized
by being short; and the continuity of the stomach with the
intestines is often such as to. oflcr no well marked line of
distinction between them. The ca3ca are generally large and
numerous; and a number of tubular organs, connected more
especially with the pyloric appendices, arc 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 Sabyion is re-
presented at p, in Fig. 331. The pancreas
itself is only met with, in this class of ani-
mals, in the order of cartilaginous fishes, and
ire 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 class both of Fishes and of Reptiles,
which are cold-blooded animals, the processes of digestion
are conducted more slowly than in the more energetic sys-
tems 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 diflcrenccs in this respect
depend on the kind of food which is consumed, the canal
being always shortest in those tribes that are most carnivo-
rous.^ As the Frog, in the different stages of its growth.

• See Home, Lectuies, &c. I. 401.

DIGESTIVE ORGANS OF MAMMALIA. 161

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 tad-pole 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 gradual transformation into a frog, tliis canal be-
comes much reduced in its length; so that when the animal
has attained its perfect form, it makes but a single convolu-
tion 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 performed by the upper part of the intes-
tine.

In the intestines of the Mammalia contrivances are em-
ployed with the apparent intention of preventing their con-
tents from passing along too hastily: tliese contrivances are
most effectual in animals whose food is vegetable, and con-
tains 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
proportion to the scantiness of the supply. Tluis, the length
of the colon of the Elephant, which inhabits the fertile woods
of Asia, is only 26^ feet; while, in the Dromedary, which
dwells in the arid deserts of Arabia, it is 42. This con-
trast is still more strongly rwarked in birds. The Cassowa-
ry of Java, which lives amidst a most luxuriant suppl}' of
food, has a colon of one foot in length, and two caica, each
of which is six inches long, and one quarter of an inch in
Vol. II. 21

162 THE VITAL FUNCTIONS.

diameter. The African ostrich, on the other hand, which
inhabits a country where the supply of food is very scanty,
has the colon forty-five feet long; each of the cseca is two
feet nine inches in length, and, at the widest part, three
inches in diameter; in addition to which, there arc 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 Mam-
malia, we find them holding a place in the series intermedi-
ate between those of tlie purely carnivorous, and exclusively
herbivorous tribes, and, in some measure, uniting the cha-
racters 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 ex-
tract nourishment from a wider range of alimentary sub-
stances, than the digestive organs of almost any other animal.
This adaptation to a greater variety of food may also be in-
ferred from the form and disposition of the teeth, which
coml)inc those of different kinds more completely than in
most mammalia, excepting, perhaps, the Quadrumana, in
whicii, however, the teeth do not form, as in man, an unin-
terrupted series in both jaws. In addition to these pecu-
liarities, 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 arc concurring indications that nature, in thus render-
ing man omnivorous, intended to qualify him for maintain-
ing life wherever he could procure the materials of subsist-
ence, whatever might be their nature, whether animal or
vegetable, or a mixture of both, and in whatever soil or

* Lectures, &c. I. 470. In the account above given of the digestive or-
gans I have pui-poscly omitted all mention of the spleen; because, although
it is probably in some way related to digestion, the exact nature of its func-
tions has not yet been determined with any certainty.

DIGESTIVE ORGANS OP MAN.

1G3

climate they may be produced; and for endowing liini with
the power of spreading his race, and extending liis dominion
over every accessible region of the globe. Thus, then,
from the consideration of the peculiar structure of the vitiil,
as well as the mechanical organs of his frame, may be de-
rived additional proofs of their being constructed with re-
ference to faculties of a higher and more extensive range
than those of any, even the most favoured species of the
brute creation.

( 164 )

CHAPTER IX.

LACTEAL ABSORPTION.

The Chyle, of which wc have nowtraccd the formation,
is a fluid of uniform consistence, perfectly hland and unirri-
tating in its properties, 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 sys-
tem for the purposes of nourishment. In all the lower or-
ders 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
converted. From some experiments made by my late much
valued friend Dr. INIarcet, 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 intes-
tines by the Lacteals, which commence by very minute ori-
fices, in incalculable numbers, and unite successively into
larger and larger vessels, till they form trunks of considera-
ble size. They pass between the folds of a very fine and
delicate membrane, called the Tuesentery^ which connects
the intestines to the spine, and which appears to be inter-
posed in order to allow them that degree of freedom of mo-
tion, which is so necessary to the proper performance of
their functions. In the mesentery, the lacteals pass through
several glandular bodies, germed the vicsenleric glands,
where it is probable that the chyle undergoes some modifi-
cation, preparatory to its conversion into blood.

• Medico-Chii'urgical Transactions, vi. 630.

LACTEAL ABSORPTION. 1G5

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

It is only among the Vertebrata that lacteal vessels are
met with. Those of Fishes are simple tul)es, either wholly
without valves, or if there be any, they are in a rudimcntal
state, and not sufficiently extended to prevent the free pas-
sage 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
and in Mammalia these valves are perfectly eflcctual, and arc
exceedingly numerous, giving to the lacteals, when distend-
ed with fluid, the appearance of strings of beads. The ef-
fect 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 intend-
ed to move. Hence it is easy to understand how exer-
cise must tend to promote the transmission of the chyle.
The gflands are more numerous and concentrated in the
Mammalia, than in any other class.

From the mesenteric glands the chyle is conducted, by
the continuation of the lacteals, into a reservoir, which is
termed the receptacle of the chyle; whence it ascends through
the thoracic diict^^ which passes along the side of the spine,
in a situation afibrding the best possible protection from in-
jury 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 conversion into
blood, takes place, during the course of the circulation, and

• This duct Is occasionaUy double.

IGG THE VITAL FUNCTIONS.

is principally cfTectcd by the action of atmospheric air in
certain organs, hereafter to be described, where that ac-
tion, or aeration as it may be termed, in common with an
analogous process in vegetables, takes place. In all verte-
brated animals the blood has a red colour, and it is also red
in most of the Annelida; but in all other invertebratcd ani-
mals, it is cither white or colourless.* We shall, for the
present, then, consider it as having undergone this change,
and j)roceed to notice the means employed for its distribu-
tion and circulation throughout the system.

• Vauquclin has observed that the chyle has often a red tinge in animals.

( 1G7 )

CHAPTER X.

Circulation.

§ 1. Diffused Circulation.

Animal life, implying mutual actions and reactions be-
tween the solids and fluids of the body, requires for its
maintenance the perpetual transfer of nutritive juices from
one part to another, corresponding in its activity to the ex-
tent of the changes which are continually taking place in
the organized system. For this purpose we almost con-
stantly find that a circulatory motion of the nutrient fluids
is established; and the function which conducts and regu-
lates their movements is emphatically denominated the Cir-
culation. Several objects of great importance are answered
by this function; for in the first place, it is through the cir-
culation that every organ is supplied wiih the nutritive
particles necessary for its development, its growth and the
maintenance of its healthy condition; and that the glands, in
particular, as well as the other secreting organs, arc fur-
nished with the materials they require for the elaboration of
the products, which it is their peculiar ofiice to prepare. A
second essential object of the circulation, is to transmit the
nutritive juices to certain organs, where they are to be sub-
jected to the salutary influence of the oxygen of the atmo-
sphere; a process which in all warm-blooded animals, com-
bined with the rapid and extensive distribution of the blood,
difl'uses and maintains throughout the system the high tem-
perature required by the greater energy of their functions.
Hence it necessarily follows that the particular mode in
which the circulation is conducted in each respective tribe,

168 THE VITAL FUNCTIONS.

must influence every other function of the economy, and
must, therefore, constitute an essential element in deter-
mining the physiological condition of the animal. We fmd,
accordingly, that among the characters on which systematic
zoologists have founded their great divisions of the animal
kingdom, the utmost importance is attached to those de-
rived from differences of structure in the organs of circula-
tion.

A comprehensive survey of the different classes of ani-
mals 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 or-
ders; 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
vegetable kingdom, where, as we have already seen, the nu-
tritive fluids, instead of being confined in vessels, appear to
permeate the cellular tissue, and thus immediately supply
the solids with the materials they require; for, in the sim-
pler kinds of Polypi, of Infusoria, of JVIedusee, and of Ento-
zoa, tlie nourishment which has been prepared by the di-
gestive 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 de-
nominated nourishment by imbibition, in contradistinction
to that by circulation^ a term, which, as we have seen, im-
plies, not merely a system of canals, such as those existing
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 whicli has thus been drawn, however, is
one on which wc should be careful not to place undue reli-

DIFFUSED CIRCULATION. 1G9

nnce, for It is founded, perhaps, more on our Imperfect means
of investigation, tlian on any real diflercnces in the ])ro-
cedurcs of nature relative to this function. When the juices,
either of plants, or of animals, are transparent, their motions
are imperccptihlc to the eye, and can be judged of only by
other kinds of evidence; 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 tlie sap of
the Chara and Caulinia, we have ocular proof of the ex-
istence of currents, which, as long as the plant is Jiving and
in health, pursue a constant course, revolving in a regular
and deflned circuit; and all ])lants 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 examj)lc of the tendency
which the nutrient fluids of organized structures have to
move in a circuit, even when not confined within vessels or
narrow channels; for this movement of rolalion, or cf/clo.siSj
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 movements occasionally pro-
duced in fluids by electro-magnetism. t

Movements, very similar in their appearance and cha-
racter to those of vegetable cyclosis, have been recently dis-
covered in a great number of polyferous 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 Tubulai'ia 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 circulation in the

* See pages 41 and 42 of this volume.

f So great is this resemblance, that it has led several physiologists to as-
cribe these movements to the agency of electricity; but there docs not, as
yet, appear to be any substantial foundation for this hypothesis.

Vol. II. 22

170 THE VITAL FUNCTIONS.

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; each of the
opposite currents occu])ying one-half of the circumference
of the cylindric cavity. At the knots, or contracted parts
of the tube, sliglit eihlies were noticed in the currents; and
at each end of the tube the particles were seen to turn round,
and pass over to the otiier side. In various species of *S'er-
tiilariic the stream does not flow in the same constant di-
rection; but, after a time, its velocity is retarded, and it
then either stops, or exhibits irregular eddies, previous to
its return in an op])osite course; and so on alternately, like
the ebb and flow of the tide. If the currents be designedly
obstructed in any part of the stem, those in the branches
go on without inteiTuptlon, and independent!}^ of the rest.
The most remarkable circumstance attend In 2; these streams
of fluid is that they appear to traverse the cavity of the sto-
mach itself, flowing from the axis of the stem into that or-
gan, and returning into the stem without any visible cause
determining these movements. Similar phenomena were
observed by Mr. Lister in Campanxdarhe and Flumula-
rix.

In some of the minuter species of Crustacea the fluids
have been seen, by the aid of the microscope, moving with-
in the cavities of the body, as if by a spontaneous impulse,
without the aid of a propelling organ, and apparently with- '
out being confined in membranous channels, or tubes of any
sort. This kind of diff'used circulation is also seen in the
embryos of various animals, at the earliest periods of their
development, and before any vessels are formed.

§ 2. Vascular Circulation.

The next step in the gradation of structures consists in
the presence of vessels, within which the fluids arc confined,
and by which their course and their velocity are regulated j

VASCULAR CIRCULATION. 171

and, in general, these vessels form a complete circuit. The
first rudiments of a vascular organization arc those observed
and described by I'iedemann, in the Jl^tcrivc, which are si-
tuated higher in the animal scale than McduscT; but whether
any actual circulation takes place in the channels constituted
by these vessels, which communicate both with the cavitv
of the intestine, and with the respiratory organs, is not yet
determined with any certainty. The ITolothuriin, which
also belong to the order of Echinodcrmata,are furnisiied with
a complex apparatus of vessels, of which the exact functions
are still unknown. In those species of Entozoa which ex-
hibit a vascular structure, the canals appear rather to be ra-
mifications of the intestinal tube, than proper vessels, for no
distinct circulation can be traced in them: an organization of
this kind has already been noticed in the Txnix:*

It was, till very lately, the prevailing opinion among na-
turalists that all true insects are nourished by imbibition,
and that there exists in their system no real vascular circu-
lation of juices. In all the animals belonging to this class
and in every stage of their development, 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 dilatations, occurring in succession in
different parts of the tube; and it is also connected with
transverse ligamentary bands, apparently containing muscu-
lar fibres, capable, by their action, of producing, or, at least,
of influencing these pulsatory movements. An enlaro-ed re-
presentation of the dorsal vessel of the Mclolontha vulgaris,
or common cockchaffer, isolated from its attachments, 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

* Page 64, in this volume, Tit^. 247. The family of rianarix present ex-
ccptions to this genenxl rule: for many species possess a system of circu-
lating vessels. Sec Duges, Amiales des Sciences Naturellesj xv. 161.

172

THE VITAL FUNCTIONS.

and ninscular apparatus which surrounds it, seen from the
lower side. In the hist of these figures, a is the tapering

336

^ )i if

f

prolongation of the tube, proceeding towards the head of tlie
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 ligamentary 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 rliomboidal shape, and attached at h, to the su-
jierior segments of the abdomen. At i is seen a layer of the
same fibres, wliich are partly ligamentous and partly muscu-
lar, passihg 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 in-
ferior layers have been removed from the other parts of the
vessel, to allow the upper layers to be seen, as is the case at

CIRCULATION IN INSECTS. 173

L. Fig. 335 gives a side view of the anterior extremity of
the same vessel, showing tlie curve (a) which it dcscrihcs
as it bends downwards in its course towards the head.

The function performed by tlie dorsal vessel, which,
judging from the universal presence of this organ in insects,
must be one of great importance in their economy, was long
a profound mystery. Its analogy in structure and position
to the dorsal vessels of the Arachnida and the Annelida,
r where it evidently communicates with channels of circu-

lation, 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 circulation;
but then, again, this hypothesis appeared to be overturned
by the fact that no vessels of any kind could be seen extend-
ing 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, 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 to-
tally 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 detecting 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 des Serres, who bestowed great pains in investi-
gating this subject, 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 accumu-
lated particularly around the dorsal vessel."^ A more at-
tentive examination of the structure of the vessel itself
brought to light a valvular apparatus, of which the only con-

* See liis vai'ious papers in the Memolrcs clu Museum d'liist. Nat.; torn.
iv. and v.

174 THE VITAL FUNCTIONS.

ceivable purpose is that of determining the motion of the
contained fluid in one constant course; a purpose necessarily-
incompatible with its supposed alternate undulation in op-
posite directions, from one end of the tube to the other.
These valves are exhibited in Fig. 336, in a still more mag-
nified view of a longitudinal section of the dorsal vessel,
showing tlie semicircular folds (s, s) of its inner membrane,
which perform the function of valves by closing the passage
against any retrograde motion of the fluid. This discovery
of valves in the dorsal vessel, again made the balance of pro-
bability 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 discoveries of Professor Ca-
rus, who, in the year 1824, first observed this phenomenon
in the larva of the Agrion puella. In the transparent parts
of this insect, as w^ell 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 in-
tervene 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 back-
wards 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 pulsations towards its anterior extremi-
ty, where they again issue from it, and are subsequently di-
vided into the scattered streams, which descend along the
sides of the body, and which, after having thus completed
their circuit, return into the pulsating dorsal vessel.

This mixed kind of circulation, partly difl'used and partly
vascular, is beautifully seen in the larva of the Ephemera
marginata,'^' where, besides the main current, which, after

• This insect is figured and described in Dr. Goring- and Mr. Pritchard's
*' Microscopic Illustrations," and its circulation is very fully detailed, and il-
lustrated by an engraving- on a large scale, by Mr. Bowerbank, in the Ento-
mological Magazine, i. 239; plate ii.

CIRCULATION IN INSECTS.

175

being discharged 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 con-
veyed 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 vessels, and not portions
of the great abdominal cavity, for their boundaries arc
clearly defined; yet they allow the blood contained in them
to escape into that cavity, and mix with the portion previ-
ously 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 valves. In the legs, the
tail, and the antennas, the circulation is carried on by means
of vessels, which are continuous with the lateral vessels of
the body, branching off from them in the form of loops, as-
cending on one side, and then turning back to form the de-
scending vessel, so that the currents in each move in con-
trary directions. Fig. 337 represents the appearance of

c.-.. - -^x ^ — *

^^^::":^>'''^•r■^r■•■.,^^•^^^'::i'■^^^^^^^

these parallel vessels in one of the antennae, of the Seynhlis
viridis, magnified thirty times its natural size. The whole

176 THE VITAL PTTNCTIONS.

system of circulating vessels in tliat insect, of which the for-
mer is only a dctaclied part, is sliown in Fig. 33S, where
the course of the Ijlood is indicated h}' arrows; A, repre-
senting the currents in the antenna:; w, those in the rudi-
mental wings; and t, those in the tail; in nil which parts
the vessels form loops, derived from the main vessels of the
trunk. In some larvre tlie vascular loops, conveying tliesc
collateral streams, pass only for a certain distance into the
legs; sometimes, indeed, they proceed no farther than the
haunches. The currents of hlood in these vessels have not
a uniform velocity, heing accelerated hy the impulsions
they receive from the contractions of the dorsal vessel,
which appears to he the prime agent in tlicir motion.

As the insect advances to maturity, and passes through
its metamorphoses, considerahle 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 ap-
pearing to retire into the more internal parts. In the wings,
on the other hand, where the development proceeds rapidly,
the circulation becomes more active; and even after tliey
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 dis-
tinctly visible:^ but afterwards, as the wings become dry,
it ceases there also, and is then confined to the vessels of the
trunk. In proportion as the insect approaches to the com-
pletion of its development, 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 \vc observe this vessel still con-
tinuing its pulsatory movements, we may fairly infer that
they are designed to maintain some degree of obscure and
imperfect circulation of the nutrient juices, through vessels,

• These currents in the wing- of the Semblls hilineata have been described
and delineated by Carus, in the Acta Acad. Cxs. Lcop. Carol. Nat. Cur. vol.
XV. part ii. p. 9.

CIRCULATION IN INSECTS.

177

which may, in their contracted state, corresponding to the
diminished demands of the system, have generally escaped
detection. In confirmation of these views, it may be stated,
that several observers have, at length, succeeded in tracing
minute branches, proceeding in diflcrent directions, from the
dorsal vessel, and distributed to various organs. Tiie divi-
sion 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 spe-
cies of Phalcnx, and farther ramifications in that of the
Gryllus lineola: and Audouin has traced them in many of
the Hymenoptera.*

• Annales des Sciences Natiirelles, xv. 308.

The figures whicli follow (from 339 to 345) are representations, of the na-
tural size, of the doi-sal vessel of the Sphinx ligustri, or Privet Hawk-moth,
which has been dissected in its three different stages, with great care, by
Mr. Newport, from whose drawings these figures have been engraved, and
to whom I am indebted also for the description which follows: —

The doreal vessel of this,insect is an elongated and gradually tapering ves-

339

sel extending from the hinder part of the abdomen, along the back, towards
the head; and furnished with valves, which con-espond very nearly in their
situation to the incisions of the body. During the changes of the insect
from the lai-va 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 aor-
Vol. II. " 23

178 THE VITAL FUNCTIONS.

The discovery of the circulation in insects, and of its va-
rying energy at different periods of growth, has elucidated
many obscure points in the physiology of this important

ial. The dorsal portion, which is the ojie in which a pulsation is chiefly ob-
servable, is furnished with distinct valves, is attached 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 tlie 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 fur-
nished with eight double valves, which are formed, as Mr. Bowerbank has
correctly described them in the Ephemera marginala — namely, the upper
valve " by a reflecting inwards and upwards of the inner coat, or coats of the
artery," (by which he means the dorsal vessel) "and the under one by a con-
traction 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, consti-
tute 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 reflexions 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 por-
tion 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 di-
vision takes place after it has passed the oesophagus, and at a point immedi-
ately beneath the supra-cesophageal 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 commencement 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 exceedingly 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 herves of this spe-
cies of Sphinx: (l^hil. Trans. 1832, p. 385.) These branches are best ob-
served in the chrysaUs (Fig. 339:) in all the stages they maybe divided into
three sets; the first is given off immediately after the vessel has passed be-
neath tlic ganglion; and consists of two lateral tnanks, the united capacity of

CIRCULATION IN INSECTS. 179

class. It explains why insects, after they have attained their
imago state, and the circulation, is nearly ohliterated, 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 deve-
lopment 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
maintaining 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 {gw hours after
it has acquired wings, and reached its perfect state of ma-
turity.

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 se-
cond set consists of two pairs of branches, one going- apparently to the tongue,
the other to the antenna:. The third set is formed by two branches, which
pass upwards, and are the continuations of the aorta; tliey divide into branches,
and are lost in tlie 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 al-
tered; 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 division between the tiio-
rax and abdomen in the perfect insect. It then ascends in the fourth seg-
ment, 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 tlie insect, which
shows, still more distinctly, the vessels entering it laterally, intermixed witli
the lateral muscles. Fig. 343 is a magnified lateral view of the anterior ex-
tremity of the dorsal vessel, corresponding to Fig. 341; and Fig. 344, a simi-
larly magnified view of the same portion of the vessel seen from above, cor-
responding to Fig. 342. Fig. 345 shows the mode in which the valves ai'C
formed by a duplicatare of the inner membrane in the perfect insect.

ISO

THE VITAL FUNCTIONS.

In proportion as the cliangcs of form which the invSect un-
dergoes arc less consitlcr;ihle, llie evidences of a circulation
become more distinct. Such is the case in many of the Ap-
terous Insects, comj)osing the family of Myriapoda: in the
Scolopendra inorsitnns, (l^inn.,) for instance, Duges ob-
served the dorsal vessel dividing into three large branches.

INIost of tlic tribes belonging to the class of Arachnida
liave, likewise, a dorsal vessel, very analogous in its struc-
ture and situation to that of insects; and, as none of them
undergo any metamorphosis, their vascular system admits
of being considerably developed, and becomes a permanent
part of the organization. Fig. 346 shows the dorsal vessel

of the tJSrcniea domestica, or house spi-
der, with some of the arterial trunks
arising from it, lying embedded in a thick
mass of substance, having a similar oily
character to that which is contained, in
large quantities, in the principal cavities
of insects. It is, in general, difBcult to
obtain a view of the circulation in the
living spider, on account of the thick co-
vering 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 larva? of insects.
A complete vascular circulation is established in all the
animals which compose the class of Annelida; the vessels
being continuous throughout, and having sufficient power to
propel the blood through the whole of its circuit. Great va-
rietv exists in the arrangement and distribution of these
vessels, depending on the form of the animal, the compli-
cation of its functions, and the extent of its powers. The
first rudiment of a distinct system of circulating vessels, in-
dependent of the ramified tubes proceeding from the inte^

CIRCULATION IN THE ANNELIDA.

181

tinal canal, occurs in the Plnncmre^ which are a trihc of flat
vermiform animals^ in many respects allied to the more
developed Entozoa, and ap])earing placed as an intermediate
3-16* link between them and the Annelida. In

many species such as the Vhinaria nigra^
fitsca, and tremellaris, {Muller,) Duges ob-
served two longitudinal trunks (Fig. 346*)
running along tlie sides of the under surface of
the animal, and joining together, both at their
fore and hind extremities, so as to form a
continuous channel of an oval form.f A great
number of smaller vessels branch off from
these main trunks in every direction, and
ramify extensivel}^, often uniting with those
from the opposite side, and establishing
the freest communications between them.
In the Annelida which have a more lengthened and cy-
lindric form, the principal vessels have a longitudinal course,
but are differently 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 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 superficial, and ano-
ther 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 alons: the
sides of the body, and are therefore called the lateral ves-
sels. In every case there are, as we have seen in the Plana-
ria, numerous branches, and collateral communications be-
tween the lateral, the abdominal, and dorsal vessels; more
especially at the two extremities of the bod}-, where the great
mass of blood, which has been flowing in one direction in
one set of vessels, is transferred into others, which convey

f De Blainvillc has described a structure similar to this In a Planaria from
Brazil. Diet, dcs Sc. Nat t. xli. 216.

182 THE VITAL FUNCTIONS.

it in the contrary direction, and complete the circuit of its
course. The ramifications and kitcral connexions 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 appli-
cable to the tribes of Annelida possessing the simplest struc-
ture, such as the Nais, 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 the real course which the blood
takes while circulating within them; and we accordingly
find very great discordance in the reports of different phy-
siologists on this subject. De Blainville asserts that in all
the Annelida, the blood in the dorsal vessel is carried back-
wards, that is, from the head to the tail; a motion, which, of
course, implies its return in the contrary direction, either in
the lateral or the abdominal 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 towards the tail, in the abdominal ves-
sels; 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 vulga-
ris* an animal allied to the Leech, and already noticed in
the account of the mechanical functions of this tribe :t but he
considers the abdominal vessel as performing also the same
function of carrying the blood forwards towards the head,
and the two lateral vessels as conveying it backwards, thus
completing the circuit. This is illustrated by the diagram

• Ilimdo vulgaris, (Linn.) Nephelis vulgaris. (Savigny.)

I Vol. i. p. 195, where a delineation of this animal was given, Fig. 130.

CIRCULATION IN THE ANNELIDA.

1S3

(Fig. 347;) where a is the anterior, and p the posterior ex-
tremity of the animal, the dorsal vessel occupying the mid-

dle straight line between the two lateral vessels, and the di-
rection 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
an inferior view of the Erpobdella, showing the numerous
ramifications of the abdominal vessel; the lesser branches
encircling the nervous ganglia, and accompanying the prin-
cipal nervous filaments which proceed from them: while the
lateral vessels are seen pursuing a sliglitly serpentine
course.*

The tribe of Lumbrici, which includes the earth-worm,

♦ Duges represents the blood of this animal as moving- in different direc-
tions in the rig-ht and in tlie left lateral vessels; g-enerally 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 curi-
ous to find an example of a simiUu* transverse circulation, in the vegetable
kingdom; this has recently been observed by Mr. Solly and Mr. Vai-ley, in a
sprout of the Chara vulgaris, near the end of which the enclosed fluid re-
volves continually on its own axis, instead of following the ordinary course
of ascent and descent along the sides of tlic cylintkic cavity.— Sec Trans, of
the Society of Arts, xlix. 180.

1S4

THE VITAL FUNCTIONS.

is distinguished from the annclida ah'cady noticed, by being
more highly organized, and possessing a more extensive cir-
culation, and a more comj)licated apparatus for the perform-
ance of this function. Tlic greater extent of vascular rami-
fications appears to require increased powers for carrying
the blood through the numerous and intricate passages it has
to traverse; and these are obtained by means of muscular
recc})taclcs, capable, by their successive contraction, of add-
ing to the impulsive force witli which the blood is driven
into the trunks that distribute it so extensively. These mus-
cular 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 Duges as consisting, in the Linyibricus gigas,
of seven vessels on each side, forming a series of rounded
dilatations, about twelve in number, resembling a string of
beads.*

In the Lnmhricus ierresiris, or common earth-worm,
there are only five pairs of these vessels; they have been de-
scribed and figured by Sir E. Home if but the most full and
accurate account of their structure has been given by Mor-
ren, in his splendid work on the anatomy of that animal. J
Fig. 349, which is reduced from his plates, represents these

• They arc termed by Duges, Vaisseaux monlliformes, ou dorso-abdor^i-
naux. — Annalcs dcs Sciences Natiirelles, xv. 299.

f Pliilos. Transact, for 1817, p. 3: and Pi. iii. Fig. 4.

t " De Lunibrici tcrrcstris Ilibtom naturalis, nccuon Anutomia Tractatus."
Qto. Bruxelles, 1829.

CIRCULATION IN THE CRUSTACEA. 185

Singular appendages to the vascular system of the cartli-
worm, separated 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 r., into a globu-
lar bulb (c,) which is followed, at equal intervals, by four
others (c, c.) From each of these bulbs, or ventricles, 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 completes the se-
micircular 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, presenting
similar dilatations, terminate. Sir E. Home is of opinion
that these dilated portions of the vessel are useful as reser-
voirs of blood, for supplying it in greater quantity to the
neighbouring 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 pul-
sations, constantly visible in these bulbs, render extremely
probable, this structure would offer the first rudiments of
the organ which, in all the superior classes of animals, per-
forms so important an office in the circulation of the blood,
namely, the heart: and this name, indeed, is given by Cu-
vier, Morren, and others, to these dilated portions of the
vascular systems of the higher orders of Annelida."*

Here, also, the statements of different anatomists are at
variance, with regard to the direction taken by the blood
while circulating in the vessels: Home and Duges represent
it as proceeding forwards in the dorsal, and backwards in
the abdominal vessels; a course which implies its descent

• It is remai'kable tliat tlic blood in most of the Annelida has a bright
scarlet colour, and resembles, in this respect, the blood of vcrtcbratcd ani-
mals.

Vol. H. 24

186 THE VITAL FUNCTIONS.

alone; the lateral communicating vessels just clescril)ecl; while
De jjlainvillc and Morrcn ascribe to it a course precisely the
reverse. Amidst these conflicting testimonies, it is extreme-
ly difficult to determine on which side the truth lies; and a
suspicion will naturally arise, that tlie course of the blood in
the vessels may not be at all times uniform, but may be lia-
ble to ])artial oscillations, or be even completely reversed, by
the operation of particular distiirbinii; causes.

The larger Crustacea possess a circulatory apparatus still
more extensive and complete, accompanied by a correspond-
ing increase in the energy of the vital functions. As we
follow this system in the more higlily organized tribes of
this class, we find the powers of the dorsal vessel becoming
more and more concentrated in its anterior extremity; till,
in the Decapoda, a family which comj^rchends the Lobster
and the Crab, we find this part dilated into an oval or globu-
lar organ, with very muscular coats, capable of vigorous
contractions, propelling its contents with considerable force
into the vessels, and therefore clearly entitled to the appel-
lation of heart. The distinction between arteries and veins,
which can scarcely be made with any precision in the sys-
tems of the inferior tribes, is here perfectly determined by
the existence of this central organ of propulsion: for the ves-
sels 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 carajDace, or shell of the
dorsal region of the thorax, directly over the stomach; its
pulsations are very distinct, and are performed 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 continual and rapid renewal of nutrient fluid, and render
necessary the introduction of farther refinements into its

CIRCULATION IN THE VERTEBKATA.

1S7

structure. The supply of blood to the heart, beins; in a con-
stant stream, produces a gradual dilatation of the cavity which
receives it; and the muscular fibres of that cavity are not ex-
cited to contraction, until tlicy are stretched to a certain
point. But in order effectually to drive the blood into eve-
ry part of the arterial system, where it has great resistances
to overcome, a considerable impulsive force is required, im-
plying 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, be-
ing rapidly distended, is excited to a much more quick and
forcible contraction than the auricle, and propels the blood
it contains into the artery, with an impetus incomparably
greater than could have resulted from the action of the au-
ricle alone. Fig. 350 represents the heart with its two ca-

vities; D being the auricle, and e the ventricle; together'witli
the main trunks of the veins (c, c,) wliich convey 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 i the
heart being tlius increased, it becomes necessary to provide

188 THE VITAL FUNCTIONS.

additional security against the retro2;rade 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

appearance (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 cavre, and a, the main trunk of the
aorta. These valves are composed of two loose membranes,
the fixed edges of which are attached circularly to the aper-
ture 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 pur-
pose thci-e 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 ventri-
cle, so that the valve is ahvays 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.

CIRCULATION IN THE VERTEBRATA.

189

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 a set of valves {v, Fig. 351,) is placed
at the beginning of these tubes where they arise from the
ventricle. These valve^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 sys-
tem, I have drawn the diagram,
Fig. 352, exhibiting the gene-
ral arrangement of its compo-
nent parts. The main arterial
trunk, or Aorta (a,) while pro-
ceeding in its course, gives off
numerous branches, (b,) which
divide and subdivide, till the
ramifications (p) arrive at an
extreme degree of minuteness;
and they are finally distributed
to every organ, and to the re-
motest extremities of the body.
They frequently, during their course, communicate with
one another, or anastomose, as it is termed, by collateral
branches, so as to provide against interruptions to the circu-
lation, which might arise from accidental obstructions in any
particular branches of this extended system of canals. The
minutest vessels (p,) which, in incalculable numbers, pervade

* In the arteiy of the shark, and other cartilag-lnous fishes, where the ac-
tion of the vessel is very powerful, these valves are mucli more numerous,
and arranged in rows, occupying' several parts of the artery. Additional
valves are also met with in other fishes at tlie branching of largo arteries.

190 THE VITAL FUNCTIONS.

every part of the frame, are named, from their behig finer
than hciirs, cajnllary vessels.

After the blood, thus transmitted to tlie dificrent parts of
the body by the arteries, has supplied tlicm with the nourish-
ment 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 directed tow^ards
the heart. Tlic 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 Venx cavx, (c,) which
pour their accumulated torrent of blood into the general re-
servoir, the heart; entering first into the auricle (d,) and
thence being carried forward into the ventricle (e,) which
again propels 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 reconveying 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 xnicroscope, in the trans-
parent 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 feel-
ing which must be exalted to the highest admiration, on re-
flecting that what we there behold is at all times going on

♦ Lcwcnlioeck, speaking- of the dclig'ht he experienced on viewhig the
circulation of the blood in tadpoles, uses the following expressions: " This
pleasure has oftentimes been so recreating to me, that I do not believe that
all the pleasure of fountains, or water-works, either natural or made by art,
covdd have pk-ascd my sight so well, as the view of these creatures has given
me."— rhil. Trans, xxii. 453.

RESf»IRATOnY CIRCULATION.

191

within us, during the whole period of our lives, in every,
even the minutest, portion oC our frame. How inadequate,
then, must be any ideas we are capable of forming of the in-
calculable number of movements and of actions, which are
conducted in the living system; and liow infinite must be
the prescience and the wisdom, by whicii these multifarious
and complicated operations were so deeply planned, and so
harmoniously adjusted!

§ 3. Respiratory Circulation.

The object of the circulation is not merely to dislributc
the blood through the general system of the body; it has,
also, another and a very important ofTice to perform. The
blood undergoes, in the course of its circulation, considera-
ble changes, both in its colour and its chemical composition.
The healthy l^lood transmitted by the arteries is of a bright
scarlet hue; that brought back by the veins is of a dark pur-
ple, from its containing an excess of carbon, and is conse-
quently unfit to be again circulated. Whenever, from some
derangement in the functions, 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 circu-
late, destroy life. Hence, it is ne-
cessary that the blood f^hich returns
by the veins should undergo purifi-
cation, by exposure either to the air
itself, or to a fluid containing air, for
the purpose of restoring and pre-
serving its salutary qualities. The
heart and vascular system have,
therefore, the additional task as-
signed them of conveying the vi-
tiated venous blood to certain or-
gans, where it may have access to the air, and receive its

102 THE VITAL FUNCTIONS.

vivifyino; influence; and to this oflicc a distinct set of arte-
rics and veins is appropriated, constituting a distinct circu-
lation. This I have endeavoured to illustrate by the dia-
gram, 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 diffe-
rent parts of the diagram. The vessels which pervade the
body generally (li,) and are subservient to nutrition, belong
to what is termed the greater^ or systemic circulation: 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 in-
terest than the comparison of the modes in which these two
great functions are, in all the various classes of animals, ex-
actly adjusted to each other. So intimately are the organs
of circulation related to those which distribute the blood to
the respiratory organs, that we never can form a clear idea
of the first, without a close reference to the last of these sys-
tems. While describing the several plans of circulation
presented to us by the diflcrent classes, I shall be obliged to
assume both the necessity of the function of respiration, and
of a provision of certain organs for the reception of air, ei-
ther in its gaipous form, or as it is contained in the water,
where the blood may be subjected to its action. It is ne-
cessary, also, to state that the organs for receiving atmosphe-
ric air, in its gaseous state, are either lungs, or pulmonary
cavities, while' those which are constructed for aquatic res-
piration are termed gills, or hranchicn; the arteries and the
veins which carry on this respiratory circulation, being
termed pulfnoiiary, 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 circulating blood
which undergoes aeration; the pulmonary or branchial arte-
ries and veins being merely branches of the general system

RESPIRATORY CIRCULATION.

193

of blood vessels: so that in this case, which is that repre-
sented in the preceding figure (353,) the lesser circulation is
included as a part of the general 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 An-
nelida, for instance, the venas cav3e, which bring back the
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 systemic circulation.

Most insects, especially when arrived at the advanced stages
of their development, have too imperfect a circulation to ef-
fect the thorough aeration of the blood: and indeed a greater
part of that fluid is not contained within the vascular sys-
tem, 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 inde-
pendent af 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 Arachni-
da, where the circulation is vigorous, and continues during
the whole of life: here, then, we again meet with a pulmo-
nary 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 Venae Cava^, which arc

exceedingly capacious, and
extend, on eacli side, along
the lower surface of the ab-
domen. They send out
branches, whicli distribute
the blood to the gills; but
these branches, at their ori-
gin, suddenly dilate, so as to

Vol. II.

25

194 THE VITAL FUNCTIONS.

form large receptacles, which are called sinuses, where the
blood is allowed to accumulate, and where, by the muscularity
of the expanded coats of the vessels, it receives an additional
force of propulsion. From the branchias the blood is re-
turned by another set of veins to the elongated heart for-
merly described, and propelled by that organ into the sys-
temic arteries. Fig. 354 shows the relative situation of
these vessels, when isolated and viewed from behind in
the Muja squinado. c, c, are the venae cavse; e, e, the ve-
nous sinuses above-mentioned; f, f, are the branchial ar-
teries; G, the gills, or branchiae; and i, i, the branchial
veins terminating 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. In the de-
velopment of its structure, in the different orders of this class,
a beautiful gradation may be perceived: the Branchiopoda
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 Jicephala
being provided with a heart, which has a single ventricle,
but two auricles, corresponding to the two trunks of the
branchial veins.t

The most remarkable variety of structure is that exhibited
by the Cephalopoda. We have already seen, in the Crusta-
cea, dilatations of the venae cavse, at the origin of the branch-
ial arteries: but in the Nautilus the dilatations of the branch-
ial veins are of such a size, as to be almost entitled to the
appellation of auricles. The Sepia, in whose highly organ-
ized system there is required great additional power to pro-
pel the blood with sufficient force through the gills, is pro-
vided with a large and complicated branchial apparatus; and

• A minute account of the organs of circulation in the Crustacea is g-iven
by Audouin and Milne Edwards, in the Annates des Sciences Naturelles,
xi. 283 and 352, from which work the above figure is taken.

f A great number of bivalve Mollusca exhibit the singular peculiarity of
the lower portion of the intestinal tube traversing through the cavity of the
heart.

RESPIRATOnY CIRCULATION IN FISHES. 195

the requisite power is supplied by two additional hearts,
situated on the venae cavae, of which they appear as if they
were dilatations, immediately before the branchial arteries
are sent ofT."^ They are shown at e, e, Fig. 355, which re-

presents this part of the vascular system of the Loligo, de-
tached 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 descends, and terminating, conjointly
with the two venous trunks (d,) which are cominir from be-
low, into the lateral or branchial hearts (e, e,) already men-
tioned. 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 sys-
temic arteries, 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 circulation: none of these
hearts are furnished with auricles.

The remarkable distribution of the muscular powers
which give an impulse to the circulating fluids, met with in

* These veins arc surrounded by a great number of blintl pouches, wliich
have the appearance of a fringe; the use of this singular structure is un-
known.

196

THE VITAL FUNCTIOlSrS.

the Sepia, constitutes a step in the transition from Mollusea
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 consist of
four cavities, c, d, e, f, opening
successively the one into the other.
The heart belongs exclusively to
the gills; and there proceeds 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 performing, without any
intermediate heart, the office of an aorta; that is, it divides
into innumerable branches, and distributes the blood to
every part 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 considerably dilated at
its termination, or just before it opens into the auricle, con-
stituting what has been termed a venous sinus (s.) This,
then, is followed 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 biilbus arterlosnSy contributing, doubtless, to

• The caudal brancli of the aorta is protected by the roots of the inferior
spinous processes, joining- to form arches through which it passes; and fre-
quently the artery is contained in a bony channel, formed by the bodies of
the vertebr?e, 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.

357 H

M, '/,'''

RESPIRATORY CIRCULATION IN REPTILES. 197

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 in-
fluence 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 arc as-
tonishingly tenacious of life. The simplest form in which we
meet with this mode of circuhition is in the Batrachia; it is
.shown in the diagram, Fig. 357. The heart of the Frog,
for example, may be considered as consisting of a single

ventricle (e,) and a single auricle (d.*)
From the former there proceeds one
f|\N S^^^^ arterial trunk, which is proper-
%\ ly the aorta. This aorta soon divides
\iij into two trunks, which, after sending
branches to the head and neck, bend
downwards (as it is seen at o, p,) and
unite to form a single trunk (a,) which
is the descending 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 ramifica-
tions are continued into the great ve-
nous trunks, which, as usual, constitute the vena? cavce (c,)
and terminate 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 (h,) and convey to those
organs a part only of the mass of circulating blood. To

* Dr. Davy has observed that although the auricle appeare sing-le, wlien

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 semilunai- valves. Edin. Phil. Joumal; xix.
161.

198 THE VITAL FUNCTIONS.

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, during the transformations of this animal, Nature
conducts tbe gradual transition 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 conse-
quently 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 aor-
ta, are distributed to those organs, and in proportion as these
arteries enlarge, the branchial arteries diminish; until, on
their becoming entirely obliterated, the course of the blood
is wholly diverted from them, and flows through the en-
larged 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 be-
ing 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 dis-
tended 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.

WARM-BLOODED CIRCULATION.

199

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.'* The heart of the Crocodile has not only two au-
ricles, but its ventricle is divided by two partitions, into
three chambers: each of the partitions is perforated to allow
of a free communication between the chambers; and the pas-
sages 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.t

It is in warm-blooded animals that the two offices of the
circulation are most efficiently performed; for the whole of
the blood passes, alternately, through the greater and the
lesser circulations, and a complete apparatus is provided for
each. There are, in fact, two hearts, the one on the left side
impelling the blood through the greater or systemic circu-
lation ; the other, on the right side, appropriated to the lesser,
or pulmonary circulation. The annexed diagram, (Fig. 35,9,)
359 .,-.^<:^ H illustrates the plan of the cir-

^^ culation in warm-blooded ani-

mals. From the left ventri-
cle (l) the blood is propelled
into the aorta (a,) to be dif-
fused through the arteries of
the system (b) to every part,
and penetrating into all the
capillary vessels; thence it is
returned by the veins, through
the venae 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

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

f It would appear, from this arrangement of the vessels, that the brain, or

200

THE VITAL FUNCTIONS.

(f,) into the lungs (at h,) where it is aerated, and whence it
is reconveyed by the pulmonary veins (i,) into the left au-
ricle (k,) which immediately pours it into the left ventri-
cle (l,) the point from whence we had set out.

Both the right and the left heart have their respective au-
ricles and ventricles; but they are all united in one envelope,
so as to compose, in appearance, but a single organ:* still,
however, tlic riglit and left cavities are kept perfectly dis-
tinct from one another, and are separated 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 between them. This junction of the two hearts

central organ of the nervous system, requires, more than any other part, a
supply of oxyg-enated blood for the due performance of its functions. The
cui-ious 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 Uicse 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 vertebratcd animals, as, for instance, among the MammaUa, in the
Monotremaia and the Marsupialia.

* A remai'kable exception to this
general law of consolidation occurs
in the heart of the Bugong, repre-
sented in Fig. 360, in which it may
be seen that the two ventricles, e and
L, are almost entirely detached from
each other. In tliis figure, which is
taken from the Philosopliical Trans-
actions for 1820, T) is the systemic
auricle, k the riglit or pulmonary
ventricle, f the pulmonary arteiy, k
the left or pulmonary auricle, l the
left or systemic ventricle, and a the
aorta.

DISTRIBUTION OF BLOOD VESSELS. 201

is conducive to their mutual strength: for the fibres of each
intermix and even co-operate in their actions, and both cir-
culations are carried on at the same time; that is, both ven-
tricles 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 cavic, the latter by the pulmonary veins, fdl their re-
spective auricles at the same instant; and both auricles, con-
tracting at the same moment, discharge their contents simul-
taneously 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 likewise driving the vitiated
blood into the pulmonary artery, in order that it may be pu-
rified 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 tiie
body; and thus do the contractions of the veins, auricles,
ventricles, and arteries, all concur in the same general end,
and establish the most beautiful and perfect harmony of ac-
tion.*

§ 4. Distribution of Blood Vessels,

In the distribution of the arteries in the animal system,
we meet with numberless proofs of wise and j)rovident ar-
rangement. 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 adja-

* Evidence is afforded of the human conformation being expressly adapted
to tlie erect position of the bod}' 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 dia-
phragm, with the apex turned towards the left side.

Vol. II. 2r>

202 THE VITAL FUNCTIONS.

cent bones. Grooves are formed in many of the bones,
where arteries are lodged, with the evident intention of af-
fording them a more secure passage. Thus, the principal
arteries which supjily the muscles of the chest, proceed
alonn- the lower ed";es of the ribs, in deep furrows formed
for their protection. Arteries are often still more effectu-
ally guarded against injury or obstruction by passing through
complete tubes of solid bone. An instance occurs in the ar-
teries supplying the teeth, which pass along a channel in the
lower jaw, excavated througli tlie whole length of the bone.
The aorta in fishes, after having supplied arteries to the vis-
cera of the abdomen, is continued to the tail, and passes
through a channel, formed by bony processes from the ver-
tebra; and the same kind of protection is aflbrded to the
corresponding artery in the Cetacea. In 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 contrac-
tions of the muscles: in order, therefore, to guard against
this inconvenience, it is made to pass through a perforation
in the bone itself, where it is completely secure from pres-

sure.*

The energy of every function is regulated in a great mea-
sure by the quantity of blood which the organs exercising
that function receive. 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 habi-
tually more used than the left. But still the different mode
in which the arteries are distributed to the two arms consti-
tutes a natural source of inequality. The artery supplying
the risht arm willi blood is the first which arises from the

• In like manner the coffin bone of the Horse is perforated for tlie safe
conveyance of tlie arteries going to the foot.

DISTRIBUTION OP BLOOD VESSELS. 203

aorta, and it proceeds in a more direct course from the heart
than the artery of the left arm, whicli has its ori^ijin in com-
mon with the artery of that side of the head. Hence it has
heen inferred that the right arm is originally better supplied
with nourishment than the left. It may be alleged, in con-
firmation of this view, that in birds, where any inerpiality
in the actions of the two wings would have disturbed the
regularity of flight, the aorta, when it lias 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 nou-
rished, preserve that equality of strength, which their func-
tion rigidly demands.

When a large quantity of blo^od is wanted in any particu-
lar 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 subdividing them, before they
reach their destination, into a great number of smaller arte-
ries. 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 re-
quired by that organ for the due performance of its func-
tions. Accordingly we find that all the arteries which go
to the brain are very tortuous in thcii- course; 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 extraor-
dinary provision is made to obviate this danger. The arte-
ries which supply the brain, on their entrance into the basis
of the skull, suddenly divide into a great number of minute
branches, forming a complicated net- work of vessels, an ar-

204 THE VITAL FUNCTIONS.

rangemcnt wliich, on the well known principles of hydrau-
lics, must greatly check the velocity of the hlood conducted
through them. That such is the real purpose of this struc-
ture is evident from the hranchcs afterwards uniting into
larger trunks when they have entered the brain, through tiic
substance of which they are then distributed exactly as in
other animals, where no sucli previous subdivision takes
place.

In the Bradijpxis tridacii/lus^ or great American Sloth,
an animal remarkable for the slowness of its movements, a
plan somewhat analogous 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 extremities,
suddenly divide into a great number of cylindric vessels of
equal size, communicating in various places by collateral
branches. These curiously subdivided arteries are exclu-
sively 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 discovered by Sir A. Carlisle,* is
not confined to the Sloth, but is met with in other animals,
as the Lcinxir tardigradus, and the Lemur lo?'is, which re-
semble the sloth in the extreme sluggishness of their move-
ments. It is extremely probable, therefore, that this pecu-
liarity in the muscular power results from, or is at least in
some way connected with this remarkable structure in the
arteries. In the Lion, and some other beasts of prey, a simi-
lar construction is adopted in the arteries of the head, pro-
bably with a view to confer a power of more permanent
contraction in the muscles of the jaws for holding a strong
animal, such as a buffalo, and carrying it to a distance.

That we may form an adequate conception of the im-
mense power of the ventricle, or prime mover in the circu-
lation of the blood, we have but to reflect on the numerous
obstacles impeding its passage through the arterial system.
There is, first, the natural elasticity of the coats of the ar-

• Pliil. Tr.ins. for 1800, p. 98, and for 1804, p. 17.

FORCE OP THE HEART. 205

teries, which must be overcome before any blood can enter
them. Secondly, the arteries are, in most places, so con-
nected with many heavy parts of the body, that their dila-
tation cannot be effected without, at the same time, commu-
nicating motion to them. Thus, when we sit cross-legi^cd,
the pulsation of the artery in the ham, which is pressed upon
the knee of the other leg, is sufficiently strong to raise tlie
whole leg and foot at each beat of the pulse. If we con-
sider 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 cur-
rent 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 not-
withstanding 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
velocity into the aorta. The ventricle of the human heart
does not contain more than an ounce of blood, and it con-
tracts 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 at London Bridge; and the water roaring
in its passage through that pipe is inferior in impetus and ve-
locity to the blood gushing through the whale's heart. An
anatomist who understood 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 derange-
ment, or that it would soon work itself out. Yet shall this
wonderful machine 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

206

THE VITAL FUNCTIONS.

of time, witliout disorder and without weariness. To those
who venture tlieir lives in a ship, it has often been said that
there is only a plank between 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 provi-
dentiall}' 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 expended by the time it has
reached the veins: nature has accordingly furnished them
with numerous valves, all opening, of course, in a direction
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 pressure, occasionally ap-
plied to the veins, assists in promoting the
flow of blood towards the heart; and hence
the effects of exercise in accelerating the cir-
culation. Valves are more especially pro-
vided in the veins which pass over the mus-
cles of the extremities, or which run imme-
diately beneath the skin; while they are not
found in the more internal veins beloncrino; to
the viscera, which are less exposed to une-
qual pressure. These valves are delineated in Fig. 365,
which represents the interior of one of the larger veins.

The situation and structure of the valves belonjiino; to
the hydraulic apparatus of the circulation furnish as une-
quivocal proofs of design as any that can be adduced. It
was the observation of these valves that first suc-o-ested to
the mind of Harvey the train of reflections 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 in-
quiry, which, at the era of Bacon^s writings, was beginning
to awaken the human mind from its long night of slumber,

VALVES OF THE VEINS. 20'

and to dissipate the darkness which had, for so many ages,
overshadowed the regions of philosophy and science. Wo
cannot but feel a pride, as Englishmen, in the recollection,
that a discovery of such vast importance as that of the cir-
culation of the blood, which has led to all the modern im-
provements in the medical art, was made by our own coun-
tryman, whose name will for ever live in the annals of our
race as one of its most distinguished benefactors. Tlic con-
sideration, also, that it had its source in the study of com-
parative anatomy and physiology, affords us a convincing
proof of the great advantage that may result from the culti-
vation of those sciences; to which Nature, indeed, seems,
in this instance, expressly to have invited us, by displaying
to our view, in the organs of the circulation, an endless di-
versity of combinations, as if she had ])urposcly designed to
\ elucidate their relations with the vital powers, and to assist

our investigations of the laws of organized beings.

( 208 )

CHAPTER XL

Respiration.
* § 1. Respiration in General.

The action of atmospheric air is equally necessary for
the maintenance of animal, as of vegetable life; and as the
ascending sap of the one cannot be perfected unless exposed
to the chemical agency of air in the leaves, in like manner
the blood of animals requires the perpetual renovation of
its vital properties by the purifying influence of respiration.
The great importance of this function is evinced by the con-
stant 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 circula-
tion of these juices is an object of inferior importance, com-
pared with their aeration; for we fmd 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 sub-
sisting for a considerable time without nourishment, but all
speedily perishing when deprived of air. The influence of
this clement is requisite as well for the production and de-
velopment, as for the continuance 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 Spallanza-
ni, who found that insects under an air pump, conflned in
rarefied air, in general lived for shorter periods in propor-

RESPIRATION. 209

tioii to the degree to which the exhaustion of air had been
carried. Those species of infusoria, wliich are most tena-
cious 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, tiicy were
seen still to continue their accustomed evolutions, wheeling
in circles, darting to the suriace, or diving to the bottom of
the fluid, and producing vortices by the rapid vibration of
their cilia, to catch the floating particles which serve as their
food: in course of time, however, they invariably gave in-
dications of uneasiness; their movements became languid, a
general relaxation ensued, and they at lengtii expired. But
when the vacuum was rendered perfect, none of the infu-
sions of animal or vegetable substances, whicli, under ordi-
nary circumstances, soon swarm with millions of these mi-
croscopic beings, ever exhibited a single animalcule; although
these 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 di-
rect action of atmospheric air in its gaseous state. But as
all water exposed to the air soon absorbs it in large quanti-
ties, it becomes the medium by which that agent is applied
to the respiratory organs of aquatic animals; and the oxy-
gen it contains may thus act upon the blood with considera-
ble efi'ect; though not, perhaps, to the same extent as when
directly applied in a gastious state. The air which is pre-
sent 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 ani-
mals, 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.

Vol. II. 27

210 THE VITAL FUNCTIONS.

ResjDiration, in its dlfiferent modes, may be distinguished,
according to the nature of the medium which is breathed,
into aqualic or atmospheric; and in the former case, it is
either cutaneous, or bi^anchial, according as the respiratory-
organs are external or internal. Atmospheric respiration,
again, is cither tracheal, or "puhnonary , according as the
air is received by a system of air tubes, or trachex, or into
pulmonary cavities, or lungs.

§ 2. •Aquatic Respiration,

Zoophytes appear in general to be unprovided with any
distinct channels for conveying aerated w^ater into the inte-
rior of the bodies, so that it may act in succession on the nu-
tritive juices, and after performing this office, may be ex-
pelled, and exchanged for a fresh supply. It has according-
ly 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 ex-
erted 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 respiration : and no other mode of accomplishing
this function has been distinctly traced in the Acalepha. Me-
dusae, indeed, appear to have a farther object than mere pro-
gression in the alternate expansions and contractions of the
floating edges of their hemispherical bodies; for these move-
ments are performed with great regularity under all circum-
stances 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 Medu-
sa pulnio'* (the Pulmone Marino of the Italians,) is derived
from the supposed resemblance 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 conje.cture,d to be res-

• See the delineation of this animal in Fig. 135, vol. i. p. 198.
•j- Pages 67" and 68 of this volume.

AQUATIC RESPIRATION. 211

piratory organs, chiefly, I believe, because they are not
known to serve any other purpose.

The Enlozoa, m like manner, present no appearance of
internal respiratory organs; so that they probal)ly receive
the influence of oxygen only through the medium of tlie
juices of the animals on which they subsist. Planar ix,
which have a more independent existence, though endowed
with a system of circulating vessels, have no internal respira-
tory organs; and whatever respiration they perform must be
wholly cutaneous. 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 external to the body, and appears as an ap-
pendage to it, consisting generally of tufts of projecting
fibres, branching like a plume of feathers, and floating in the
surrounding fluid. The Lumbricns marinus^ or lob-worm,*
for example, has two rows of branchial organs of this de-
scription, one on each side of the body; each row being com-
posed of from fourteen to sixteen tufts. In the more sta-
tionary Annelida, which inhabit calcareous tubes, as the
Serpula and the Teredo^ these arborescent tufts are protected
by a sheath which envelops their roots; and they are placed
on the head, as being the only part w'hich 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 appear-
ance 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 specific description of them in this place: but
amidst these varieties, it is sufficiently apparent that their
construction has been, in all cases, designed to obtain a con-
siderable extent of surface over which the minute subdivi-

* Arenlcola piscatomm (Lam.) See a deHncation of this marine worm in
Fig. 135, vol. i. p. 198.

212 THE VITAL FUNCTIONS.

sions of the blood vessels might be spread, in order to ex-
pose 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 few
exceptions, adapted to aquatic respiration. In many of the
tribes which have no shell, as the Thelis, the Doris, and the
Tritoiiia, 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 ofgans
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 Holothicria, where there is
an organ composed of ramified tubes, situated in a cavity
communicating with the intestine, and having an external
opening for the admission of the aerated water, which is
brought to act on a vascular net-work, containing the nutri-
tive juices of the animal, and apparently performing a par-
tial circulation of those juices. A still more complicated
system of respiratory channels occurs, both in the Echinus
and Ssterias^ where they open by separate, but very minute
orifices, distinct from the larger apertures 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 orojans.

The animals eomiposing the family of Jiscidix 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

AQUATIC RESPIRATION. 213

respiratory cavity, before it can be swallowed, and received
into the stomach.

In several of the Annelida, also, we find internal organs
of respiration. The Lumhricus lerresiris, or common
earth-worm, has a single row of apertures, al)out 120 in num-
ber, placed along the back, and opening between the seg-
ments of the body: they each lead into a respiratory vesicle,
situated between the integument and the intestine.* The
Leech has sixteen minute orifices of this kind on each side
of the body, opening internally into the same number of oval
cells, which are respiratory cavities; the water passing both
in and out by the same orifices.t

The *Bphrodita aculeata has thirty-two orifices on each
side, placed in rows, opening into one large respiratory sac,
which is situated immediately under the muscles of the back,
but separated by a membrane from the abdominal cavity.
Projecting into this sac, are seen several membranous vesi-
cles, fifteen in number on each side, which have no external
opening, but which receive, on the inner part, the ends of
certain tubes, or caeca, sent off from the intestinal canal; 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 cir-
culation 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 belonging to the respiratory
system are extensively distributed over the whole surface of

♦ A minute description of these organs is given by Morrcn, in pages S'2>
and 148 of his work, ah-eady quoted.

■j- The blood, after being aci-atcd in these cells, is conveyed into the large
lateral vessels, by means of canals, which pass transversely, forming loops, si-
tuated between tlie c?eca of the stomach. These loops are studded with an
immense number of small rounded bodies of a glandular appearance, resem-
bling those which convey the vcnx cavae of the cephalopoda.

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

214 THE VITAL FUNCTIONS.

these filaments. Organs of this description are denominated
Branchice, 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 tiie body, not only in order to obtain protec-
tion 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 move-
ments 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 mi-
nute filaments, proceeding from their respective stems, like
the fibres of a feather; and each group having a triangular,
or a pyramidal figure. The number of these pyramidal bo-
dies 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 moved
by appropriate muscles, and arc kept in incessant motion,
producing strong currents of water, evidently for the pur-
pose of obtaining the full action of the element on every por-
tion of the surface of the branchiae.

In the greater number of INlollusca, these important or-
gans, although external with respect to the viscera, are with-
in 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.t In the

* Page 193, of this volume.

■f- These filaments appear, in many instances, to have the power of pro-
ducing* currents of water in their vicinity by the action of minute cilia, similar
to those belong-ing" to the tentacula of many polypi, where the same pheno-
menon is obseiTablc. 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 tlic tentaculum of a polype, under

AQUATIC RESPIRATION. 215

Gasteropoda, or inhabitants of univalve shells, this opening
is usually wide. In the ^cephalu, or bivalve molhisca, the
gills are spread out, in the form of laminae, round the mar-
gin of the shell, as is exemplified in the oyster, where it is
commonly known by the name of beard. The aerated wa-
ter is admitted through a fissure in the mouth, and when it
has performed its office in respiration, is usually expelled by
a separate opening. The part of the mouth through which
the water is admitted to the branchiae is sometimes prolonged,
forming a tube, open at the extremity, and at all times al-
lowing 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, Buccinuui, and Sirombus;
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 Sepia: have
their gills enclosed in two lateral cavities, which communi-
cate with a funnel-shaped opening in the middle of the neck,
alternately receiving and expelling the water by the muscu-
lar action of its sides. The forms assumed by the respira-
tory organs in 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 respiratory func-

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 Ascidix.

216

THE VITAL FUNCTIONS.

tion assumes a higher importance. In fishes the gills form
large organs, and the continuance of their action is more es-
sential 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 tlie immediate vicinity of the heart. Their
usual form is shown at g g, Fig. 367, where they are repre-

sented 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 artery (f.) They
have the same fringed structure 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 con-
nected 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 support, and
enabling it to preserve its shape while moved by the streams
of water which are perpetually rushing past. When their

RESPIRATION IN FISHES. 217

surfaces are still more minutely examined, they are fountlto
be covered with innumerable minute processes, crowded to-
gether 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, and leaving below a
wide fissure for the escape of the water, whicli has per-
formed its office in respiration. 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 ca-
vities: in this action the branchial arteries are brought for-
wards and separated to a certain distance from each other;
and the rush of w^ater 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 i)rincipal mechanical ob-
ject of the act of respiration in this class of animals; and it
is an oljject 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 w^a-
ter, this effect can no longer be produced; in vain the ani-
mal reiterates its utmost eflforts to raise the branch ia2, and
relieve the sense of suflbcation it experiences in consequence
of the general collapse of the filaments of those organs, wliich
adhere together in a mass, and can no longer receive the vi-
vifying influence of oxygen. t Death is, in like manner, the
consequence of a ligature passed round the fish, and prevent-
ing the expansion of the branchiai and the motion of the
. opercula.

• Dr. Monro computed that in the state, the surface of the g-llls is, at the
least, equal to the whole surface of the human body.

•j- It lias been generally stated by physiolog-ists, even of the highest au-
thority, such as Cuvier, that the principal reason why fibhcs cannot maintain
Vol. II. 28

218 THE VITAL FUNCTIONS.

In all osseous fishes the opening under the operculum for
the exit of the respired water, is a simple fissure; hut in
most of the cartilaginous trihes, there is no operculum, and
the water escapes tiu'ough a series of apertures in the side of
the throat. Sharks have live ohlong orifices of this descrip-
tion, as may he seen in Fig. 367.*

As the Lcni} ])7'cy cm\)\o\s its mouth more constantly than
other fisli in laying hold of its prey, and adiiering to other
bodies, the organs of respiration are so constructed as to be
indc])endcnt of the mouth in receiving tho water. There
are seven external openings on each side (Fig- 3GS,) lead-
ing 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 scale
than in the preceding figure, in Fig. 369. There is also an
equal number of internal openings, 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 la-
teral openings, enters at one side, and after it has acted uj)on
the gills, passes round the projecting membranes. The
greater part makes its exit by the same orifices; but a por-
tion escapes into the middle tube, and thence passes, either
into the other cavities, or into the oesophagus.!

life, when suiToundeJ by air instead of water, is tliat the branchix become
diy, unJ lose the power of acting" when thus deprived of their natural mois-
ture: for it might otherwise naturally be expected that the oxygen of atmo-
spheric air woukl exert a more powerful action on the blood whicli circulates
in the branchix, than that of merely aerated water. The rectification of this
en'or is d\ie to Flourens, who pointed out the true cause of sulFocation, stated
in the text, in a Memoir entitled " Experiences sur le Mcchanisme de la Res-
piration des Poissons." — Annates des Sciences Naturelles, xx. 5.

• They arc also visible in Fig. 293, (page 122,) which is that of the«S'^ua-
lus prhtis, a species belonging to this tribe.

■j- It was commonly supposed that the respired water is ejected through
the nostril; but this is certainly a mistake, for the nostril has no communi-
cation through the mouth, as was pointed out by Sir E. Home. Phil. Trans,
for 1815, p. 259. These oj^'ans have also been described by Bloch and
Gaertncr.

RESPIRATION IN PISHES. 219

In the Myxine, which feeds upon the internal parts of its
prey, and huries its head and part of its body in the flesli,
. the openinpjs of the respiratory organs are removed suffi-
ciently far from the head to admit of respiration going on
■while the animal is so employed; and tlicre are only two
external openings, and six lateral pouches on each side, with
tubes similar to those in tlic lamprey.

The Ferca scandens (DaldorlT, ' ) whicli is a fisli inhabiting
the seas of India, has a very remarkable structure adapting
it to the maintenance of respiration, and consequently to tlic
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.t The pharyngeal bones of this fish
have a foliated and cellular structure, which gives them a
capacity for retaining a sufficient quantity of water, not
only to keep the gills moist, but also to enable them to per-
form their proper office; while not a particle of water is suf-
fered 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 respiration when out of water, for a certain pe-
riod, in consequence of the narrowness of the aperture for
the exit of the water from the branchial cavity, wliich en-
ables it to be closed, and the water to be retained in tliat
cavity, t

I have already stated that, in all aquatic animals, the water
which is breathed is merely the vehicle by which the air it
contains is brought into contact with the organs of respiration.
This air is constantly vitiated by the respiration of these
animals, and requires to be renewed by tlie absorption of a

* Jlnthlas testudinus (Rloch:) Anahs (Cuv.)

f This peculiar faculty has been ah'cady alluded to In volume i. p. oOl.

t Dr, Hancock states that the Jhras cuslalus, (Siluriis costaltis, Linn.) or
Hassar, in very dry seasons, is sometimes seen, in i^rcat nuniliers, making- long-
marches over land, in search of water. Edin. Phil. Journal, xx. 396.

220 THE VITAL FUNCTIONS.

fresh portion, which can only take place whpn the water
freely communicates with the atmosphere: and if this re-
newal be by any means prevented, the water is no longer
capable of sustaining life. Fishes are killed in a very [ew
hours, if confined in a limited portion of water, which has no
access to fresh air. When many fishes arc enclosed in a
narrow vessel, they all struggle for the uppermost place,
(where the atmospheric air is first absorbed,) like the unfor-
tunate men imprisoned in the black-hole at Calcutta. AVhen
a small fisli pond is frozen over, the fishes soon perish, un-
less holes be broken in the ice, in order to admit air: they
may be seen flocking towards these holes, in order to re-
ceive the benefit of the fresh air as it is absorbed by the
water; and so great is their eagerness on these occasions,
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 respiration. They found that river-
water generally contains about one 3Gth of its bulk of air,
of which quantity one-third consists of oxygen, being about
one per cent., of the whole volume. A tench is able to
brQ^the when the quantity of oxygen is reduced to the
5000th part of the bulk of the water, but soon becomes ex-
ceedingly feeble by the privation of this necessary element.
The fact, however, shows the admirable perfection of the or-
gans 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 with the respiratoiy function, and as be-
ing the rudiment of the pulmonary cavity of land animals; the passage of
communication wit'i the oesophagus being conceived to represent the trachea.
The air contained 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, liiot concluded from his experiments, that in the air-

ATMOSPHERIC RESPIRATION. 221

§ 3. •^tmosjiheric licsjjiration.

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 hy whicli water
is respired.

Air may be respired by irachese, or by pulmonary cavi-
ties; 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 cir-
culating in them, the salutary influence of the air could not
have been generally extended to that fluid by any of the or-
dinary 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 trachex, distributing air to every
part of the body. The external orifices, from which these
air tubes commence, are called spiracles, or stiginaia, and

bladder of fishes inliabiting" the greatest depths of the ocean, the quantity of
oxyg-en is greater, while in those of fishes which come often to the surface,
the nitrog-en is more abundant; and De k lloche came to the same concUision
from his researches on the fishes of the Mediterranean. From the exjieriments
of Humboldt and Provencal, on the other hand, we may conclude, that the
quality of the air contained in the air-bladder is but remotely coniiected with
respiration. (Memoires de la Socicte d'Arcueil, ii. 359.)

According- to Ehrmann, the Colitis, or Loche, occasionally swallows air,
which is decomposed in the alimentary canal, and effects 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. Cuvicr, sur les
Poissons, I. 383.

222

THE VITAL rUNCTIOWS.

are usually situated in rows on each side of the bodj^, as is
shown in Fig. 370, which represents the lower abdominal

surface of the Dytisciis marginalis. They are seen very
distinctly in the caterpillar, which has generally ten on each
side, corresponding to the number of abdominal segments.
In many insects we find them 2:uardcd by bristles, or tufts
of hair, and sometimes by valves, placed at the orifice, to
prevent the entrance of extraneous bodies. The spiracles
are opened and closed by muscles provided for that purpose.
Fig. 371 is a magnified view of spiracles of this description,
from the Cerainhyx heros, (Fab.) They are the begin-
nings of short tubes, which open into large trunks, (as shown
in Fig. 372,) extending longitudinally on each side, and
sending off radiating branches from the parts which are op-
posite to the spiracles; and these branches are farther subdi-
vided, in the same manner as the arteries of the larger ani-
mals, so that their minute ramifications pervade every organ
in the body. This ramified distribution has frequently oc-
casioned their being mistaken for blood vessels. In the
wings of insects, the nervures, which have the appearance
of veins, are only large air-tubes. Jurlne asserts that it is
by forcing air into these tubes that the insect is enabled sud-
denly to expand the wings in preparing them for flight,

RESPIRATION IN INSECTS. 003

giving them, by this means, greater buoyancy, as well as
tension.

The trachea} 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 tra-
cheas have a shining silvery appearance, from the air they
contain. This structure has a remarkable analogy with that
of the air vessels of plants, which also bear the name of tra-
chea; and in both similar variations are observed in the con-
texture of the elastic membrane by which they are kept
pervious.* i.

The tracheae, in many parts of their course, present re-
markable dilatations, wdiich apparently serve as reservoirs
of air: they are very conspicuous in the Dytiscus margina-
lis, which resides principally in water; but they also exist
in many insects, as the Melolontha and the Cerambijx,
which live wlK)lly in the air.t Those of the Scolia horto-
rum (Fab.) are delineated in Fig. 373, considerably magni-
fied.

If an insect be immersed in water, air will be seen es-
caping in minute bubbles at each spiracle; and in proportion
as the water enters into the tubes, the sensibility is de-
stroyed. If all the spiracles be closed by oil, or any other
unctuous substance, the insect immediately dies of suflbca-
tion; but if some of them be left open, respiration is kept
up to a considerable extent, from the numerous communi-
cations which exist among the air vessels. Insects soon

• According' to the observation of Dr. Kidd these vessels are often annular
in insects, as is also the case witli those of plants. He considers the long-i-
tudinal trachex as connecting channels, by which the insect is enabled to
direct the air to particular pai'ts for occasional purposes. Phil. Trans, for
1825, p. 234.

t L^on Diifour, Annalcs dcs Sciences Naturellesj viii. 26.

024 THE VITAL FUNCTIONS.

perish when placed in the receiver of an air-pump, and the
air exhausted; but they are generally more tenacious of life
under these circumstances than the larger animals, and often,
after being apparently dead, revive on the readmission 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 surface of the water, where they absorb
air for respiration. In a few tribes a complicated mode of
respiration is practised; aerated water is taken into the body,
and introduced into cavities, when the air is extracted from
it, and transmitted by the ordinary tracheai to the different
parts of the system.*

Such, then, is the extensive apparatus for aeration in ani-
mals!^ which have either no circulation of their nutritious
juices, or a very imperfect one; but no sooner do we arrive
at the examination of animals possessing an enlarged sys-
tem of blood vessels, than we find nature abandoning the
system of tracheae, and employing more simple means of
effecting the aeration of the blood. Advantage is taken of
the facility afforded by the blood vessels of transmitting the
blood to particular organs, where it may conveniently re-
ceive 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 cavi-
ties by means of two sets of muscles, the one set (m, m) be-
ing longer than the other (m, m, m.) The branchial arte-
ries (v) are seen ramifying over the inner surface of the

• Mr. Dutrochct conceives that the principle on which this operation is
conducted is the same with that by which g-ascs are reciprocally transmitted
througli moistened membranes; as in the experiments of Humboldt and Gay
Lussac, who, on enclosing- mixtures of oxygen, nitrogen, and carbonic acid
gases, iTi any proportion, in a membranous bladder, which was then im-
mersed in aerated water, found that there is a reciprocal transit of the g-ases;
until at leng-th pure atmospheric air remains in the cavity of the bladder.

RESPIRATION IN INSECTS.

225

pulmonary cavities (r) on tlic right side, whence the blood
is conveyed by a corresponding set of branchial veins to
the dorsal vessel: and other vessels, which are oi'dinary

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, presenting the ap-
pearance of the teeth of a comb, and partaking of the struc-
ture of gills; and on this account these organs are termed
by Latreille pneumo-hranchix. Organs of a similar de-
scription 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 diflcrence ex-
ists in the structure of the respiratory organs, according to
the nature of the medium which is to act upon them: for in

Vol. II. 29

226 THE VITAL FUNCTIONS.

aquatic respiration tlie 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 at-
mospheric resj)iration 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, are distri-
buted. It is not difllcult to assign the final cause of this
change of jilan; for in each case the structure is accommo-
dated to the mechanical properties of the medium respired.
A liquid, being inelastic and ponderous, is adapted, by its
momentum alone, to separate and surround the loose float-
ing filaments composing the branchiae; but a light gaseous
fluid, like air, is, on the contrary, better fitted to expand di-
latable cavities into which it may be introduced.

Occasionally, however, it is found that organs constructed
like branchiae, and usually performing aquatic respiration,
can be adapted to respire air. This is the case with some
species of Crustacea, of the order Decapoda, such as the
Crab, which, by means of a peculiar apparatus, discovered
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 development, 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 Testucella, the Claiisilia, and many
others, which, though partial to moist situations, are, from
the conformation of their 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 require, but which is
sometimes completely concealed from view by the mouth
folding over it. The cavity, to which this opening leads,
is lined by a membrane delicately folded, and overspread

RESPIRATION BY LUNGS. 221

with a beautiful net-work of pulmonary vessels. Other
mollusca of the same order, which arc more aquatic in their
habits, have yet a similar structure, and are ol)liged at in-
tervals to come to the surface of the water in order to breatiie
atmospheric air: this is the case with the Onchidium, the
Planorbis, the Lymnsea, &c.

The structure of the pulmonary organs becomes 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 vesi-
cles, 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
muscles that perform deglutition, and which then closes ac-
curately the entrance into the air-tube. It is an exceedingly
beautiful contrivance, both as to the simplicity of the me-
chanism, 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 inter-
position of a series of firm cartilaginous ringlets, interposed
between their inner and outer coats, and placed at small and
equal distances 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 analo-
gous to that of the trachea of insects, or of the vessels of the
same name in plants.

The lungs of Reptiles consist of largo sacs, into the cavity
of which the bronchia, proceeding from the bifurcation of
the trachea, open at once, and without farther subdivision.
Cells are formed within the sides of this great cavity, by
fine membranous partitions, as thin and delicate as soap

228 THE VITAL FUNCTIONS.

bubbles. The lungs of serpents have scarcely any of these
partitions, but consist of one simple pulmonary sac, situated
on the rijj^ht side, having the slender elongated form of all
the other viscera, and extending nearly the whole length of
the body. The lung on the left side is in general scarcely
discernible, being very imperfectly developed. In the cha-
meleon the lungs have numerous processes which project
from them like ca^ca. 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, wc cannot readily
discover that it breathes at all, for it never opens its mouth
to receive air, and there is no motion of the sides to indi-
cate 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 mo-
tion, as if the frog were economizing its mouthful of air,
and transferring it backwards and forwards between its
mouth and lungs; but if we direct our attention to the nos-
trils, we may observe in them a twirling motion, at each
movement of the jaws; for it is, in fact, through the nostrils
that the fros: 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 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 performed 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

RESPIRATION IN REPTILES.

229

of the parts which surround the cavity of lungs; for even
the ribs of serpents contribute but little, by their motion,
to this effect, being chiefly useful as organs of progressive
motion.

The Chelonia have lungs of great extent, passing back-
wards under the carapace, and reaching to the posterior
part of the abdomen. Turtles, which are aquatic, derive
great advantages 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 swim-
ming bladder. That this use was contemplated in their
structure is evident from the volume of air received into
the lungs, being much greater than is required for the sole
purpose of respiration. The section of the lungs of the tur-
tle (Fig. 375,) shows their interior structure, composed of
large cells, into which the trachea (t) opens.

Few subjects in animal physiology are more deserving
the attention of those whose object is to trace the operations
of nature in the progressive development of the organs, than
the changes which occur in the evolution of the tadpole trom
the time it leaves the egg till it has attained the form of the

230 THE VITAL FUNCTIONS.

perfect frog. We have already had occasion to notice seve-
ral of these transformations 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, corresponding with the opposite na-
ture of the elements which the same animal is destined to
inhabit in the different stages of its existence. No less than
three sets of organs are provided for respiration; the two
first 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 ob-
serve that this animal at first breathes by gills, wdiich pro-
ject in an arborescent form from the sides of the neck, and
float in the water; but that these structures are merely tem-
porary, being provided only to meet the immediate exigen-
cy of the occasion, and being raised at a period when none
of the internal organs are as yet perfected. As soon as ano-
ther set of gills, situated internally, can be constructed, and
are ready to admit the circulating blood, the external gills
are superseded in their office; they now shrivel, and are re-
moved, and the tadpole performs its respiration by means of
branchiae, formed on the model of those of fishes, and acting
by a similar mechanism. By the time that the system has
undergone the changes necessary for its conversion into the
frog, a new apparatus has become evolved for the respira-
tion 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 of the office of respiration.
The branchiae, in their turn, become useless, are soon obli-
terated, and leave no other trace of their former existence
than the orio;inal division of the arterial trunks, which had
su])plied 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, be-

• See Fig. 357, p. 197.

RESPIRATION IN REPTILES. 231

longing to this class, which, instead of undergoing all the
changes I have been describing, present, during their whole
lives, a great similitude to the first stage of the tadpole.
This is the case with the */lxolotl, the Proteus angimius,
the Siren lacertina, and the Menobranchiis lateralis, which
permanently retain their external gills, while at the same
time they possess imperfectly developed lungs. It would
therefore seem as if, in these animals, the progress of deve-
lopment had been arrested at an early stage, so that their
adult state corresponds to the larva condition of the frog.*

In all warm-blooded animals respiration becomes a func-
tion of much greater importance, the continuance of life
being essentially dependent 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 pos-
sible, and presenting a vast extent of internal surface. The
thorax, or cavity in which the lungs, together with tlic heart
and its great blood vessels, are enclosed, has somewhat the
shape of a cone; and its sides are defended from compres-
sion by the arches of the ribs, which extend from the spine
to the sternum, or breast-bone, and produce mechanical sup-
port on the same principle that a cask is strengthened by
being girt with hoops, which, though composed of compara-
tively weak materials, are yet capable, from their circular
shape, of presenting great resistance to any compressive
force.

While Nature has thus guarded the chest, with such pe-
culiar solicitude, against the efibrts of any external force,
tending to diminish its capacity, she has made ample provi-
sion for enlarging or contracting its diameter in the act of

* GeofFroy St. Hilaire thinks there is g-vound for believing that Crocodiles
and Turtles possess, in addition to the ordinary pulmonary respiration, a par-
tial aquatic abdominal respiration, cficcted 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 l)e traced between this aquatic respiration in reptiles, by these pcrituncal
canalSi and tlie supposed function of the swimming bladder of fishes, \\\ sub-
serviency to a species of aerial rcspimtion.

232 THE VITAL FUNCTIONS.

respiration. First, at tlie lower part, or that which cor-
responds to the hasis of the cone, the only side, indeed,
which is not defended by bone, there is extended a thin ex-
pansion, partly muscular, and partly tendinous, forming a
complete partition, and closing the cavity of the chest on the
side next to the abdomen. This muscle is called the Dia-
phragm: 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 con-
vex 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
interfering with this action,) and thus, the cavity of the tho-
rax may be considerably enlarged. It is obvious that if,
upon the descent of the diaphragm, the lungs were to re-
main in their original 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 al-
ways contain, even in their most compressed state, a certain
quantity of air: and this air will tend, by its elasticity, to ex-
pand 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, concurring also in the same effect;
and thus expiration is accomplished.

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 motion, as-
suming the chest to be in the erect position, as in man, is
chiefly upwards and downwards. 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 to-
wards the breast; and the uppermost rib being a fixed point.

RESPIRATION IN MAMMALIA.

233

the action of the intercostal muscles, which produces an ap-
proximation of the ribs, tends to raise them, and to bring
them more at riglit 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 being to increase the capacity of the chest.

Thus, there are two ways in which the cavity of the
thorax can be dilated; namely, by the action of the dia-
phragm, and by the action of the intercostal muscles. It is
only in peculiar exigencies that the whole power of this ap-
paratus is called into action; for in ordinary respiration the

diaphragm is the chief agent employed, and the princi])al
effect of the action of the intercostal muscles is simply to fix
the ribs, and thus give greater purchase to tlic diapliragm.
The muscles of the ribs arc employed chiefly to give active
Vol. II. 30

234 THE VITAL FUNCTIONS.

assistance to the diaphragm, when, from any cause, a diffi-
culty arises in dilating the chest.

In Birds the mechanism of respiration proceeds upon a
different plan, of which an idea may he derived from the
view given of the lungs of the Ostrich, at l. l.. Fig. 377.
The construction of the lungs of hirds 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 up-
per parts of the chest, by firm membranes, as to preclude all
possibility 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 transparent membrane which
covers the lower surface of the lungs. They have the ef-
fect of lessening the concavity of the lungs towards the abdo-
men at the time of inspiration, and thereby assist in dilating
the air-cells.* The bronchia, or divisions 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 ad-
mitted, through these apertures into several large air-cells
(c c c,) which occupy a considerable portion of the body,
and which enclose most of the large viscera contained in the
abdomen, such as the liver, the stomach, and the intestines;!
and there are, besides, many lateral cells in immediate com-
munication with the 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 ma-

* Hunter in the Animal Economy, p. 78.

-}■ It was asserted by the Parisian Academicians, that the air got admission
into the cavity of the pericardium, in which the heart is lodged. This error
was first pointed out by Dr. Macartney, in Rees's Cyclopaedia.— Art. Bird,

RESPIRATION IN BIRDS. 235

terially to the litghtness of the fahric* All these cells are
veryjarge 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 Oi^;/, are,in like man-
ner, 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, bein|[ drawn for-
wards 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
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 consequently must have been twice as heavy,
which would have been a serious inconvenience in an ani-
mal formed for flying.t 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 confor-
mation of insects, where the same object is effected by means
of tracheae.:}:

* In birds, not formed for extensive flight, as the gallinaceous tribes, tlie
humerus is the only bone into which air is introduced. — Hunter on tlie
Animal Economy, p. 81.

\ I must mention, however, that the correctness of this view of the sub-
ject is contested by Dr. Macartney, who thinks it probable that the air, on
its return from the large air-cells, passes directly by the large air-holes into
the bronchia, and is not brought a second time into contact with the blood.

\ The peculiarities of structure in the rcspiratoiy 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 heiglit of
above 20,000 feet, to the edge of the sea, where the air is more than twice

236 THE VITAL FUNCTIONS

Thus, has the mechanism of respiration been varied in
the diflferent classes of animals, and adapted to the particu-
lar element, and mode of life designed for each. Combined
with the peculiar mode of circulation, it affords a tolerably
accurate criterion of the energy of the 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
circulating blood in the pulmonary organs. The Mamma-
lia rank next below birds, in the scale of vital energy; but
they still p(^sess a double circulation, and breathe atmo-
spheric air. The torpid and cold-blooded reptiles are sepa-
rated from mammalia by a very wide interval, because, al-
though they respire air, that air only influences a part of
the blood; the pulmonary, being only a branch of the gene-
ral circulation. In fishes, again, we have a similar result,
because, although the w^hole 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 man-
ner, 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 the extensive
distribution of trachea? through every part of their system.

§ 4. Chemical Changes effected by Resjnration.

We have next to direct our attention to the chemical of-
fices which respiration performs in the animal economy. It

the density of that which the bird had been breathing-. We are, as yet, una-
ble to trace the connexion which probably exists between the structure of
the lung^, and this extraordinary power of* accommodation tq^such great
and sudden variations of atmospheric pressure.

CHEMICAL EFFECTS OF RESPIRATION. 237

is only of late years that we may be said to have obtained
any accurate knowledge as to the real nature of this impor-
tant function; and there is perliaps no brancli 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 hypo-
theses, devoid of all solid foundation, and perplexed by the
most inconsistent reasonings, which formerly prevailed with
regard to the objects and the processes of respiration. To
give an account, or even a brief enumeration of these thco-
ries, now sufficiently exploded, would be incompatible with
the purpose to which I must confine myself in this treatise.*
I shall content myself, therefore, with a concise statement
of such of the leading facts relating to this function, as have
now, by the labours of modern physiologists, been satisfac-
torily established, and which serve to 'elucidate the benefi-
cent intentions of nature in the economy of the animal sys-
tem.

Atmospheric air acts without difficulty upon the blood
w^hile it is circulatins; through the vessels which are rami-
fied over the membranes lining the air cells of the lungs;
for neither these membranes, nor the thin coats of the ves-
sels themselves, present any obstacle to the transmission of
chemical elements from the one to the other. The blood
being a highly compound fluid, it is exceedingly difficult to
obtain an accurate 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 determine, by direct observation, what
are the exact chemical changes, which that fluid undergoes

* For an account of the history of the various chemical theories which
have prevailed on this "interesting department of Physiolog-y, 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 comprehensive and accurate compendium of the science which has
yet appeared.

23S THE VITAL FUNCTIONS.

during Its passage through the lungs; and we have only col-
lateral evidence to guide us in the inquiry.*

The most obvious effect resulting from the action 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
vermilion colour, wlilch it is found to assume in those or-
gans, and which accompanies its restoration to the qualities
of arterial blood. In what the chemical difference 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 returned from the
lungs, it is found that a certain proportion of 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 ap-
pears also probable that a small portion of the nitrogen gas
is consumed during respiration.

For our knowledge of the fact of the disappearance of ox-
ygen we are indebted to the labours of Dr. Priestley. It had,
indeed, been long before suspected by Mayovv, 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 consumed is unquestionably due to Dr.

* 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 lai'g-er proportion of oxy-
g-en than the latter; and that the latter contains a larger proportion of carbon
than the former: the proportions of nitrogen and hydrogen being the same
in both. The following are the exact numbers expressive of these propor-
tions:

Carbon. Oxygen. JVitrogen. JJydrogen.

Arterial blood 50.2 . . . 26.3 . . . 16.3 ... 6.6

Venous blood 55.7 . . . 21.7 . . . 16.2 ... 6.4

Memoires de la Society de Physique ei d'JIist. Naturelk de Geneve. T. v.
p. 400.

CHEMICAL EFFECTS OF RESPIRATION. 239

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 in-
stance, consume larger quantities of oxygen by their res-
piration; and hence require, for the maintenance of life, a
purer air than other vertebrated animals. Vauquelin, how-
ever, found that many species of insects and worms pos-
sess the power of abstracting oxygen from the atmosphere
in a much greater degree than the larger animals. Even
some of the terrestrial mollusca, such as snails, are capa-
ble of living for a long time in the vitiated air in which
a bird had perished. Some insects, which conceal them-
selves in holes, or burrow under ground, have been known
to deprive the air of every appreciable portion of its oxygen.
It is observed by Spallanzani, that those animals, whose
modes of life oblige them to remain for a great length of
lime in these confined situations, possess this power in a
greater degree than others, which enjoy more liberty of
moving in the open air: so admirably have the faculties of
animals been, in every instance, accommodated to their re-
spective w^ants.

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 has disappeared 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
vermilion colour, and is now qualified to be again transmit-
ted 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 accumulates in
proportion as its other principles, (namely, oxygen, hydro-
gen, and nitrogen) are abstracted from it by the processes of
secretion and nutrition. By the time it has returned to the

240 THE VITAL FUNCTIOJTS.

heart, therefore, it is loaded with carbon, a principle, which,
when in excess, becomes noxious, and requires to be re-
moved from the blood, by combining it with a fresh quan-
tity of oxygen obtained from the atmosphere. It is noty^t
satisfactorily determined whether the whole of the oxygen,
which disappears during respiration, is employed in the for-
mation of carbonic acid gas: it appears, probable, however,
from tlie concurring testimony of many experimentalists,
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, ap-
pears, on the whole, to be tolerably well ascertained by the
inquiries of Davy, Pfaflf, and Henderson. With regard to
the respiration of cold-blooded animals, it has been satisfac-
torily established by the researches of Spallanzani, and more
especially by those of Humboldt and Provengal, on fishes,
that nitrogen is actually absorbed. A confirmation 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 the air of the atmosphere, to
which it is exposed in its passage through the pulmonary
vessels.*

According to these views of the chemical objects of res-
piration, the process itself is analogous to those artificial
operations which effect the combustion of charcoal. The
food supplies the fuel, which is prepared for use by the di-

* See the note at pag-e 238.

CHEMICAL EFFECTS OF RESPIRATION. 241

gestive organs, and conveyed by the pulmonary arteries to
the place where it is to undergo combustion: the diaphragm
is the bellows, which feeds the furnace with air; and the tra-
chea is the chimney, through which the carbonic acid, which
is the product of the combustion, escapes.

It becomes an interesting problem to determine whetlier
this analogy may not be farther extended; and whether the
combustion of carbon, which takes place in respiration, be
not the exclusive source of the increased temperature, which
all animals, but more especially those designated as warin-
bloodedy 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 ener-
gy of the respiratory function, or, rather, the amount of the
chemical changes induced by that function, affords very
strong pvidence in favour of this hypothesis. The coiiici-
dence, indeed, is so strong, that, notwithstanding the objec-
tions that have been raised against the theory founded upon
this hypothesis, from some apparent anomalies which occa-
sionally present themselves, we must, I think, admit that it
affords the best explanation of the phenomena of any theory
yet proposed, and that, therefore, it is probably the true one.

The maintenance of a very elevated temperature appears
to require the concurrence of two condi|ions; namely, first,
that the whole of the blood should be subjected to the influ-
ence of the air, and, secondly, that the air should be pre-
sented to it in a gaseous state. These, then, are the circum-
stances which establish the great distinction 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 quadru-
ped and the bird to a rank, in the scale of vitality, so far
above that of the reptile: it is this which places an insupera-
ble 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 insti'umcnts of motion, to the other iniiabitants

Vol II. 31

242 THE VITAL FUNCTIONS.

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, hut for a short time,
under the surface of the water, it would perish with the
same certainty as any otiicr of the mammalia, placed in the
same situation. We ohserve them continually rising to the
surface in order to hreathe, under every circumstance of pri-
vation 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 sur-
face 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 ne-
cessity recurs, and compelling him again to present himself
to his foes, exposes him to their renewed attacks, till he falls
in the unequal strtMgle. 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 superior intellect.

( 243 )

CHAPTER XII.

SECRETION.

The capability of effecting certain chemical changes in the
crude materials introduced into the body, is one of the
powers which more especially characterize life; but although
this power is exercised both by vegetable and by animal or-
ganizations, 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 combina-
tions of elementary bodies, which are elaborated in celhi-
lar or vascular textures, and converted into various pro-
ducts. The oak, for example, forms, by the powers of ve-
getation, 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 com-
bination, 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 the bland farina of the potato,
will produce in another the poisonous extract of the night-
shade. Yet all these, and thousands of other vegetable pro-
ducts, differing widely in their sensible qualities, agree very
nearly in their ultimate chemical analysis, and owe their pe-
culiar 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.

244 THE VITAL FUNCTIONS.

In the animal kingdom we observe these processes mul-
tiplied to a still greater extent; and the resulting substances
are even farther removed from the oriiirinal condition of un-
organized matter. In the first place, the food of animals,
instead of being simple, like that of plants, has always un-
dergone previous preparation; for it has either constituted
a portion of some otlier organized being, or it has been a
product of organization; in each case, therefore, partaking
of the complexity of composition which characterizes or-
ganized bodies. Still, whatever may be its qualities when
received into the stomach, it is soon converted by the pow-
ers of digestion into a milky, or transparent fluid, having
nearly the same uniform properties. We have seen that
there is scarcely any animal or vegetable substance, how-
ever dense its texture, or virulent its qualities, but is capa-
ble of affording nourishment to various species of animals.
Let us take 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, not-
withstanding 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, arc perfectly bland,
and devoid of all acrimony. Cantharides are also, accord-
ing 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, even in
the minutest quantity, into a wound, prove instantly fatal,
may be taken into the stomach without producing any de-
leterious effects. These, and a multitude of other well-
known fiicts, fully prove how completely substances re-
ceived as aliment may be modified, and their properties
changed, or even reversed, by the powers of animal diges-
tion.

No less remarkable arc the transmutations, which the
blood itself, the result of these previous processes, is subse-

SECRETION. 245

quently made to undergo in the course of circulation, and
when subjected to the action of the nutrient vessels and se-
creting organs; being ultimately converted into the various
textures and substances which compose all the parts of the
frame. All the modifications of cellular substance, in its
various states of condensation; the membranes, the liga-
ments, the cartilages, the bones, the marrow; the muscles,
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 ofiices, are
derived from the same nutrient fluid, and may be considered
as being merely modified arrangements of the same ultimate
chemical elements.

In what, then, we naturally ask, consists this subtle che-
mistry of life, by which nature effects these multifarious
changes; and in what secret recesses of the living frame has
she constructed the refined laboratory in which she operates
her marvellous transformations, far surpassing even those
which the most visionary alchemist of former times had
ever dreamed of achieving? Questions like these can only
be fairly met by the confession of profound ignorance; for,
although the subject of secretion has long excited the most
ardent curiosity of physiologists, and has been prosecuted
with extraordinary zeal and perseverance, scarcely any po-
sitive information has resulted from their labours, and the
real nature of the process remains involved nearly in the
same degree of obscurity as at first. '" It was natural to ex-

♦ It is not yet precisely determined to what extent the organs of secretion
are immediately instrumental in producing the sL'bstance which is secreted;
and it has been even suggested that possibly their office is confined to the
mere separation, or filtration from the blood, of cert:un animal products,
which are always spontaneously forming in that fluid in the course of its
circulation. This hypotliesis, in which the glands, and other secreting ap-
paratus are regarded as only very fine strainers, is supported by a few facts,
which seem to indicate the presence of these products in the blood, inde-
pendently of the secreting processes by which they are usually supposed to
be formed; but the evidence is as yet too scanty and equivocal to warrant
the deduction of any general theory on the subject.

246 THE VITAL FUNCTIONS.

pect that in this inquiry material assistance would be de-
rived from an accurate anatomical examination of the or-
gans 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
secretions of a similar nature; and, on the other hand, sub-
stances of very different properties produced by organs,
which, even in their minutest details, appear to be identical
in their structure. Secretions are often found to be poured
out from smooth and membranous surfaces, such as those
which line the cavities of the abdomen, the chest, and the
head, and which are also reflected inwards, 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 mi-
nute 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
structure 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 convcy-

• Sometimes the secreting- organ appears to be entirely composed 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. Verte-
brated animals present us with numerous instances of glandular organs em-
ployed for special purposes of secretion: thus, in the eyes of fishes there ex-
ists 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. Appa-
rently sci-ving 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.

SECRETION. 247

ing 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 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, consists of short,
narrow and undivided tubes; we next find tubes which are
elongated, tortuous or convoluted, occasionally presenting di-
lated portions, 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 se-
parate tubes admit of being unravelled. The secreting fila-
ments 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 membrane;,
forming what is termed diCapsule, and assuming the appear-
ance 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

Great variety is observable both in the form and struc-
ture of difl'erent glands, and in the mode in which their
blood vessels are distributed. In animals which are fur-
nished 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 particular secretion to

* See p. 135 of this volume; and in particular Fig. 305. Sebaceous folli-
cles are also noticed in Vol. i. p. 91.

•j- Dr. Kidd, however, describes bodies apparently of a glandular charac-
ter, disposed in rows on the inner surface of the intestinal canal of the Gryl'
lotalpa, or mole-cricket. Phil. Tran. for 1825, p. 227.

248 THE VITAL FUXCTIOiNS.

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 or-
gan, suddenly divide into a great number of smaller branch-
es, like the fibres of a camel-hair pencil: this is called the
pencillaled structure. Sometimes the minute branches, in-
stead of proceeding parallel to each other after their divi-
sion, separate like rays from a centre, presenting a stel-
lated, or star-like arrangement. In the greater number of
instances, the smaller arteries take a tortuous course, and
are sometimes coiled into spirals, but generally the convo-
lutions are too intricate to admit of being unravelled. It is
only by the aid of the microscope that these minute and
delicate structures can be rendered visible; but the fallacy,
to which all observations requiring the application of high
inagnifying powers are liable, is a serious obstacle to the ad-
vancement of our knowledge in this department of phy-
siology. Almost the only result, therefore, which can be
collected from these laborious researches in microscopic ana-
tomy, 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 structure to its respective object, and as to the
nice adjustment of chemical affinities which has been pro-
vided in order to accomplish the intended effects."* Elec-

* 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, imlike all the other glands, has venous,
instead of arterial blood, sent to it for that purpose. The veins, which re-
turn the blood that has circulated through the stomach, and other abdominal
viscera, are collected into a large trunk, called the vena portas, 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 Phy-
siology of the Liver, Phil. Trans, for 1833, p. 711.) A similar system of ve-

• SECRETION. 249

tricity is, no doubt, an important agent in all these processes,
but 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 nutri-
tion, 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.

nous ramifications, though on a much smaller scale, has been discovered by
Jacobson, in the kidneys of most fishes and reptiles, and even in some birds.

Vol. II. 32

( 250 ) .

CHAPTER XIII.

ABSORPTION.

Absohption is another function, related to nutrition,
which deserves special notice. The principal object of this
function is the removal of such materials as have been al-
ready deposited, and have become either useless or injurious,
and their conveyance into the general mass of circulating
fluids; purposes which arc accomplished by a peculiar set of
vessels, called the Lymphatics. These vessels contain a
fluid, which, being transparent and colourless like water, has
been denominated the lymjyh. The lymphatics are perfect-
ly similar in their structure, and probably, also, in their mode
of action, to the lacteals, which absorb the chyle from the
intestinal cavity: they are found in all the classes of verte-
brated animals, and pervade extensively every part of the
body. Exceedingly minute at their origin, they unite to-
gether as they proceed, forming larger and larger trunks,
generally following the course of the veins, till they finally
discharge their contents either into the thoracic duct, or into
some of the large veins in the vicinity of the
heart. Throughout their whole course, they
are, like the lacteals, provided with numerous
valves, which, when the vessel is distended
with lymph, give it a resemblance to a string
of beads, Fig. 37S."* In the lower animals, it
appears that the veins are occasionally en-
dowed with a power of absorption, similar to that possessed

• In warm-blooded animals, the lymphatics are made to traverse, in some
part of their coui-se, certain bodies of a compact stmcture, resembling- glands,
and termed, accordingly, tlic lymphatic glands. One of these is represented

ABSORPTION. 251

by the lymphatics. None of the invcrtcbrata, indeed, pos-
sess lymphatics, and absorption must consequently be per-
formed by the veins, when these latter vessels exist. The
addition of the system of lymphatic vessels, as auxiliaries to
the veins, may therefore be regarded as a refinement in or-
ganization, peculiar to the higher classes of animals.*

Professor JVIuller, of Bonn, has lately discovered that the
frog, and several other amphil)ious animals, are provided
with large receptacles for the lymph, situated immediately
under the skin, and exhibiting distinct and regular pulsa-
tions, 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 pulsations 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 discovered similar organs in the toad, the sala-
mander, and the green lizard, and thinks it probable that
they exist in all the amphibia. f

in Fig. 378. They correspond in structure, and probably also in their func-
tions, to the mesenteric ghuids, through which, in the nianimaha, the lacteals
pass, before reaching- the thoracic duct. It is chiefly in the mammalia, in-
deed, that these glands are met with? for they are rare among birds, and still
more so among fishes and reptiles.

* Fohmann, who has made extensive researches on the absorbent vessels
throughout all the classes of vertebrated animals, has found that they termi-
nate extensively in the veins. Sec his work, entitled " Anatomischc Untcr-
suchungcn uber die Verbindung dcr Saugadcrn mit den Vencn."

t Phil. Trans, for 1833, p. 89.

( 252 )

CHAPTER XIV.

NERVOUS POWER.

The organs which are appropriated to the performance
of the various functions conducive to nutrition, are generally
designated the vital ors;ans^ in order to distinguish them
from those which are subservient to sensation, voluntary
motion, and the other functions of animal life. The slight-
est 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 contractions of
which effect the intermixture and propulsion of the con-
tents of these cavities, in the manner best calculated to fa-
vour the chemical operations to which they are to be sub-
jected, and to extract from them all the nourishment they
may contain. In like manner, all the tubular vessels, which
transmit fluids, are endowed w^th muscular powers adapted
to the performance of that office. The heart is a strong hol-
low 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 completing its circulation.
The quantity of blood circulating in each part, the velocity
of its motion, and the heat which it evolves, are regulated

NERVOUS POWER. 253

in a great measure by the particular mode of action of the •
blood vessels of that part. The quantity, and sometimes
even the quality 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 re-
spective destinations, is also resolvable into the cflfccts 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 mechanically distended,
beyond a certain degree, by the presence of contained fluids,
or other substances. At other times, the chemical quality
of their contents appears to be the immediate stimulus in-
citing them to contraction. But numerous instances occur,
in the higher orders of animals, in which these causes alone
are inadequate to explain the phenomena of the vital func-
tions. No mechanical hypothesis will suffice to account for
the infinite diversity in the modes of action of the organs
which perform these functions, or aflford any clew to the
means by which they are made to co-operate, with such
nicety of adjustment, in the production of the ultimate ef-
fect. 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 produce its visible efiects 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 mo-
ment, to act in concert, in the production of a common ef-
fect. Yet such co-operation must, in innumerable cases, be
absolutely indispensable to the perfect accomplishment of
the vital functions of animals.

Nature has not neglected objects so important to the suc-
cess of her measures, but has provided, for the accomplish-
ment of these purposes, a controlling faculty, residing in the
nervous system, and denominated the 7iervous power. Ex-

254 THE VITAL FUNCTIONS.

pcriments have shown that the due performance of the vital
functions of digestion, of circulation, and of secretion, re-
quires the presence of an ai^ency, derived from different
parts of tlie hrain and spinal marrow, and rci^ulatino; the or-
der and combinations of the actions of the organs which are
to perform those functions. The same influence, for exam-
ple, which increases the power of secretion in any particu-
lar gland, is found to increase, at the same time, the action
of those blood vessels which supply that gland with the ma-
terials 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 ner-
vous power combines together different operations, adjusts
their respective degrees, and regulates their succession, so as
to ensure that perfect harmony which is essential to the at-
tainment of the objects of the vital functions; 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 nutrition, deve-
lopment, and growth of each part, are placed under the con-
trol of the nervous povver.^

Although we are entirely ignorant of the nature 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 fflaments of this
class of nerves compose what is called the sympathetic
nerve, from its being regarded as the medium of extensive

* As the functions of plants arc sufficiently simple to admit of being con-
ducted witliout the aid of muscidar power, still less do tiiey require the as-
sistance of tlie ncn'ous encrg-y: both of which properties are the peculiar at-
tributes of animal vitality. We accordingly find no traces citlier of nervous
or of musctdar fibres in any of the vegetable structures.

NERVOUS POWER.

255

sympathies among the organs; but the wliole assemblage of
these nerves is more commonly known by the name of the
ganglionic system^ from tlie circumstance of their being
connected with small masses of nervous substance, termed
ganglia, which are placed in difTcrent 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 inter-
changes of filaments, which subsequently take place at vari-
ous parts, forming what is called ^plexus, are shown in Fig.
380: where four trunks (t, t,) divide into branches, which
are again separated, and variously reunited in their course,
like a ravelled skein of thread, before they proceed to their
respective destinations.

The ganglia are connected by nervous filaments with
every part of the brain and spinal marrow, the great central
organs of the nervous system; and they also send out innu-
merable branches, to be distributed all over the body. All
the parts receiving blood vessels, and more especially the
organs of digestion, are abundantly supplied with ganglionic
nerves; so that, by their intervention, all these parts have
extensive 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

256 THE VITAL FUNCTIONS.

many different parts: they may be considered, therefore, as
performing, with regard to the vital functions, an office ana-
logous to that which the brain and spinal marrow perform
witli regard to tlic otlicr nerves, or as being secondary cen-
tres of nervous power. Thus, there are two important ob-
jects for whicli tlic nerves belonging to the ganglionic sys-
tem have ])ccn provided; fil'st, to serve as the channels
through which the affections of one organ miglit be enabled
to influence a distant organ; and secondly, to be the medium
through which the powers of several parts might be com-
bined and concentrated for effecting particular purposes, re-
quiring such co-operation. Hence it is by means of the gan-
glionic nerves that all the organs and all the functions are
rendered efficient in the production of a common object, and
are brought into one comprehensive and harmonious system
of operation.

The nervous power, the effects of which we are here con-
sidering, should be carefully distinguished from that power
which is an attribute of another portion of the nervous sys-
tem, and which, being connected with sensation, volition,
and other intellectual operations, has been denominated sen-
so7'ial poicer.^ The functions of digestion, circulation, ab-
sorption, secretion, and all those included under the class of
nutrient or vital functions, are carried on in secret, are not
necessarily, or even usually attended with sensation, 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, an4 whether
our attention be directed to them or not; and though occa-
sionally influenced by strong emotions, and other affections
of mind, they arc in general quite independent of every in-
tellectual process. In the natural and healthy condition of

• TlVis distinction has been most clearly pointed out, and illustrated by Dr.
A. P. W. Philip. Sec his <* Experiiiicntal Inquiry into tlic Laws of the Vi-
tal Functions."

NERVOUS POWER. 257

the system all its internal operations proceed quietly, stea-
dily, and constantly, whether the mind be absorbed in thought
or wholly vacant. The kind of existence resulting from
these functions alone, and to which our attention has hither-
to 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.

Vol. II. 33

( 258 )

PART III.

THE SENSORIAL FUNCTIONS.

CHAPTER I.

SEiVSATION.

The system of mechanical and chemical functions which
we have been occupied in reviewing, has been established
only as a foundation for the endowment of those higher fa-
culties which constitute tlie great objects of animal exist-
ence. 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 its grandeur. The whole of
the phenomena we have hitherto considered concur in one
essential object, the maintenance of a simply vital existence.
Endowed with these properties alone, the organized system
would possess all that is absolutely necessary for the conti-
nuance and support of mere vegetative life. The machine-
ry provided for this purpose is perfect and complete in all
its parts. To raise it to this perfection, not 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 energies 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

SENSATION. 259

superadded the still more subtle and potent agencies of ca-
loric and of electricity: every resource has been employed,
every refinement studied, every combination exhausted that
could ensure the stability, and prolong the duration of the
system, amidst the multifarious causes which continually
menace it with destruction. It has been supplied vvitii am-
ple means of repairing the accidents to which it is ordinarily
exposed; it has been protected from the injurious influence
of the surrounding elements, and fitted to resist for a length-
ened period the inroads of disease, and the progress of
decay.

But can this, which is mere physical existence, be the sole
end of life? Is there no farther purpose to be answered by
structures so exquisitel)'' contrived, and so bountifully pro-
vided with the means of maintaining an active existence, than
the mere accumulation and cohesion of inert materials, dif-
fering from the stones of the earth only in the more arti-
ficial 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 concre-
tion 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 directed like our own, to obvious
ends, proceed from voluntary acts, and imply the operation
of an intellect, not wholly dissimilar in its spiritual es-
sence from our own? In vain may Descartes and his fol-
lowers labour to sustain their paradox, that brutes are only
automata, — mere pieces of artificial mechanism, insensible
either to pleasure or to pain, and incapable of internal af-
fections, analogous to those of which we arc conscious in our-
selves. Their sophistry will avail but little against the plain
dictates of the understanding. To those who refuse to admit
that enjoyment, which implies the powers of sensation, and
of voluntary motion, is the great end of animal existence, the
object of its creation must for ever remain a dark and im-
penetrable mystery; by such minds must all farther inquiry

260 THE SENSORIAL FUNCTIONS.

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; and
no artificial logic or scholastic syllogisms will long prevail
over the natural sentiment, which must ever guide our con-
duct, that animals possess powers of feeling, and of sponta-
neous action, and faculties appertaining to those of intellect.

The functions of sensation, perception, and voluntary mo-
tion re(juire 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 medullary sub-
stance; 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 affections of particular portions of this me-
dullary substance, generally occupying some central situa-
tion, are, in a way that is totally inexplicable, connected with
affections of the sentient and intelligent principle; a princi-
ple which we cannot any otherwise conceive than as being
distinct from matter; although we know that it is capa-
ble of being affected by matter operating through the me-
dium 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 con-
veniently come under our notice at a subsequent period of
our inquiry, I shall postpone their consideration; and pro-
ceeding upon the assumption that this connexion exists, shall
next inquire into the nature of the intervening steps in the
process, of which sensation and perception are the results.

Designating, then, by the name oi brain this primary and
essential organ of sensation, or the organ whose physical af-
fections are immediately attended by that change in the
percipient being which we term sensation; let us first in-
quire what scheme has been devised for enabling the brain
to receive impressions from such external objects, as it is

NERVOUS SYSTEM. 261

intended that this sentient being shall be capable of per-
ceiving. 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 com-
munication should be provided between the external organ
and the brain. Such a medium is found in the nerves^ which
are white cords, consisting of bundles of threads or fila-
ments of medullary matter, enveloped in sheaths of mem-
brane, and extending continuously from the external organ
to the brain, where they all terminate. It is also indispen-
sably requisite that these notices of the presence of objects
should be transmitted instantly to the brain; for the slightest
delay would be attended with serious evil, and might even
lead to fatal consequences. The nervous power, of which,
in our review of the vital functions, we noticed some of the
operations, is the agent employed by nature for this import-
ant office of a rapid communication of impressions. The ve-
locity with which the nerves subservient to sensation trans-
mit 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 elec-
tricity passing along a conducting wire.

It is evident, therefore, that the brain requires to be fur-
nished 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 espe-
cial 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 abun-
dantly supplied with nerves; each nerve, however, commu-
nicating a sensation distinguishable from that of every other,
so as to enable the mind to discriminate between them, and

262 THE SENSORIAL FUNCTIONS.

refer them to their respective origins in different 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 injurious: hence, the nerves of sensation
are distributed in less abundance to these organs.

It is not sufficient that the nerves of touch should com-
municate tlie perccj)tions of the simple pressure or resistance
of the bodies in contact with the skin: they should also fur-
nish indications 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 conveyed by the same nervous fibres, it
is difficult, and, perhaps, impossible to determine.

When these nerves are acted upon in a way which threat-
ens to be injurious to the part impressed, or to the system
at large, it is also their province to give warning of the im-
pending 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 oc-
casions. They act tlie part of sentinels, placed at the out-
posts, to give signals of alarm on the approach of danger.

Sensibility to pain must then enter as a necessary consti-
tuent among the animal functions; for, had this property
been omitted, the animal system would have been but of
short duration, exposed, as it m.ust necessarily be, to perpe-
tual casualties of every kind. Lest any imputation should
be attempted to be tlirown on the benevolent intentions of •
the great Author and Designer of this beautiful and wondrous
fabric, so expressly formed for varied and prolonged enjoy-
ment, 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
consc(iuences arising from the capacities for pleasure, with
which it has been beneficently ordained that the healthy ex-
ercise of the functions shall be accompanied. Enjoyment

NERVOUS SYSTEM. 263

appears universally to be the main end, the rule, the ordi-
nary and natural condition: while pain is but the casualty,
the exception, the necessary remedy, which is ever tending
to a remoter good, in subordination to a higher law of crea-
tion.

It is a wise and bountiful provision of nature that each of
the internal parts of the body has been endowed with a par-
ticular sensibility to those impressions which, in the ordina-
ry course, have a tendency to injure its structure; while it
has, at the same time, been rendered nearly, if not complete-
]}'', insensible to those which are not injurious, or to which
it is not likely to be exposed. Tendons and ligaments, for
example, are insensible to many causes of mechanical irrita-
tion, 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 com-
municate a feeling of acute pain. The bones, in like man-
ner, scarcely ever communicate pain in the healtliy state,
except from the application of a mechanical force which
tends to fracture them.

The system of nerves, comprising those which are de-
signed to convey the impressions of touch, is universally
present in all classes of animals; and among the lowest or-
ders, they appear to constitute the sole medium of commu-
nication with the external world. As we rise in the scale
of animals, we find the faculties of perception extending to
a wider range, and many qualities, depending on the chemi-
cal 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 impressions to be conveyed, renders it ne-
cessary 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.

As the sphere of perception is enlarged, it is made to

264 THE SENSORIAL FUNCTIONS.

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 im-
portant that the animal, of whose sensitive faculties we are
examining the successive endowment, should be apprized.
It is more especially necessary that he should acquire an
accurate 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 sup-
ply his wants, or procure him gratification; and which also
apprize him of danger while it is yet remote, and may be
avoided. Endowed with the power of combining all these
perceptions, he commences his career of sensitive and intel-
lectual existence; and though he soon learns that he is de-
pendent 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 control over
many of those changes, and that he moves his limbs by his
own voluntary act; movements which originally, and of
themselves, appear, in most animals, to be productive of
great enjoyment.

To a person unused to reflection, the phenomena of sen-
sation 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 sim-
ple 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 be-

SENSATION. 265

ing 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 phe-
nomena! Science has taught us that these perceptions of
external objects, far from being direct or intuitiv^e, 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, disposed in
particular situations in our bodies, and adjusted with admi-
rable art to receive their impressions, to modify and com-
bine them in a certain order, and to convey them in regular
succession, and without confusion, to the immediate seat of
sensation.

Yet this process, complicated as it may appear, constitutes
but the first stage of the entire function o{ pei^ception: for
ere the mind can arrive at a distinct knowledge of the pre-
sence and peculiar qualities of the external object which
gives rise to the sensation, a long series of mental changes
must intervene, and many intellectual 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 ana-
lysis of the phenomena, however, as I shall afterwards at-
tempt to show, we find that no less than twelve distinguish-
able kinds of changes, or rather processes, some of which
imply many changes, must always intervene, in regular
succession, between the action of tiie 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 system, so as to produce sen-
sation, are of different kinds, and are governed by laws pe-
culiar to themselves. The structure of the organs must,
accordingly, be adapted, in each particular case, to receive

Vol. II. 34

266 THE SENSORIAL FUNCTIONS.

the impressions made l)y these agents, and must be modi-
fied in exact conformity with the physical laws they obey.
Thus, the structure of that portion of the nervous system
which receives visual impressions, and which is termed the
retina, must Ix; adapted to the action of light; and the eye,
throusili which the ravs 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 imj)ressions 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 delicate 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 ap-
propriate 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 functions are not
interchangeable, as is the case with many others in the ani-
mal system. No nerve, but the optic nerve, and no part
of that nerve, except the retina, is capable, however im-
pressed, of giving rise to the sensation of light: no part of
the nervous system, but the auditory nerve, can convey that
of sound; and so of the rest. The credulity of the public
has sometimes been imposed upon by persons who pretend-
ed to see by means of their fingers: thus, at Liverpool, the
celebrated Miss M'Avoy contrived for a long time to per-
suade 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 mag-
netism, hearing sounds addressed to the pit of the stomach,
and reading the pages of a book applied to the skin over

that organ.

In almost every case the impression made upon the sen-
tient extremity of the nerve which is appropriated to sen-
sation, is not the direct effect of the external body, but re-
sults from the agency of some intervening medium. There
is always a portion of the organ of sense interposed hetwcen

SENSATION. 267

the object and the nerve on which the impression is to be
made. The object is never allowed to come into direct con-
tact with the nerves; not even in the case of touch, where
the organ is defended by the cuticle, through which the im-
pression is made, and by which that impression is modified
so as to produce the proper effect on the subjacent nerves.
This observation 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 expressly for that purpose. In the senses of hearing
and of vision, the changes which take place in the organs
interposed between the external impressions and the nerves,
are still more remarkable and important, and will be re-
spectively the subjects of separate inquiries. The objects of
these senses, as well as those of smell, being situated at a dis-
tance, produce their first impressions by the aid of some me-
dium 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 phenemena be-
longing to sensation, regard must be had to tlie 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.

( 2GS )

CHAPTER IT.

TOUCH.

I HAVE already had occasion to point out the structure of
the integuments, considered in their mechanical ofTice of
protectinp; the general frame of the body;* but we are not
to view them in their relation to the sense of touch, of which
they are the immediate organ. It will be recollected that
the corutm forms the principal portion of the skin; that the
cuticle composes the outermost layer; and that between these
there occurs a thin layer of a substance, termed there/e mu-
cosum. The corium is constructed of an intertexture of
dense and tough fibres, through which a multitude of blood
vessels and nerves are interspersed; but its external sur-
face 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 sensible in every point: hence we may-
infer that it contains the terminations of all the nervous fila-
ments distributed to this organ, and which are here found to
divide to an extreme degree of minuteness.

When examined with the microscope, this external sur-
face presents a great number of minute projecting filaments.
ISIalpighi first discovered this structure in the foot of a pig;
and gave these prominences the name of papillae. It is pro-
bable that each of these papilla? contains a separate branch
of the nerves of touch, the ultimate ramifications of which
are spread over the surfiice: so that we may consider these
papiliaN of which the assemblage has been termed the cor-
pus 2)upillare, as the principal and immediate organ of

• Vol. I. p. 90.

TOUCH. 269

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 sensible to all ir-
ritations, whether proceeding from the contact of foreign
bodies, or from the impression of atmospheric air. This ex-
treme sensibility 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, admi-
rably calculated to afford protection to the fmely organized
corium.

Although the cuticle exhibits no traces of vascularity, it
is by no means to be regarded as a dead or inorganic sub-
stance, 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 de-
cay and renovation; its external surface drying off in mi-
nute scales, and in some animals peeling off in large por-
tions. When any part of the human skin is scraped with a
knife, a gray dust is detached from it, which is found to con-
sist 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 thick-
ness in the parts designed by nature to suffer considerable
pressure, is not entirely the eflect of education; for the cuti-
cle, 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 origi-

270 THE SENSORIAL FUNCTIONS.

nally adjusting the structures of parts to the circumstances
in which 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. liut the proofs of design in the adaptation
of organs to their respective purposes multiply upon us in
such profusion, as we study in detail each department of the
animal economy, that we are apt to overlook individual in-
stances, unless they are particularly brought before our no-
tice. IIow often have we witnessed and profited by the
rapid renewal of the cuticle, when by any accident it has
been destroyed, without adverting to the nature of the pro-
cess 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 elfect their
combination in the requisite proportions, and must deposite
them in the precise situations in which they are wanted!

Different animals present remarkable differences 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 ex-
posure 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 effects, 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 employed 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 arc, first, an abundant provision of soft pa-
pillae 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 resist-
ing basis, such as that which is provided in the nails of the

TOUCH. 271

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 percep-
tions 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 mobili-
ty, the capability of flexion, and the figure of the parts em-
ployed as organs of touch.

As touch is the most important of all the senses, in^much
as it is the foundation of all our knowledge of the material
world, so its relative degrees of perfection establish marked
differences in the intellectual sagacity of the several tribes,
and have a considerable influence on the assignment of their
proper station in the scale of animals.

Although the power of receiving obscure impressions
from the contact of external bodies, and of perceiving varia-
tions of temperature, is probably possessed by all animals,
a small number only are provided with organs specially ap-
propriated 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 w^ith them; and the same observation applies, with
even greater force, to the Crustacea. The scales of Fishes,
and of Reptiles, the solid incasements of the Chelonia, the
plumage of Birds, the dense coating of the Armadillo, the
thick hides of the Rhinoceros, and other Pachydermata, arc
evidently incompatible with any delicacy of touch. This
nicer faculty of discrimination can be enjoyed only by ani-
mals having a soft and flexible integument, such as all the
naked Zoophytes, Worms, and Mollusca, among the lower
orders, and Serpents, among the higher. The flexibility of
the body or limbs is another condition which is extremely
necessary towards procuring extensive and correct notions

272

THE SENSORIAL FUNCTIONS.

of the relative positions of external objects. It is essential
therefore that those instruments which are more particularly
intended as organs of toucii. should possess this property.

It will not be necessary to enter into a minute descri])tion
of these organs, because they have, for the most part, been
already noticed as instruments of prehension; for the sense
of touch is in general exercised more particularly by the
same ])arts 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 arc, in like man-
ner, subservient botii to tlie sense of touch, and to the fa-
culty of progressive motion. Tl^e feet of Insects and of
Crustacea are well calculated, indeed, by their jointed struc-
ture, for being applied to the surfaces, and to different sides
of bodies; but they are scarcely ever employed in this capa-
city; being superseded by the palpi, which are situated near
the mouth. When insects are walking, the palpi are inces-

santly applied to the surface on which they advance, as if
these organs were especially employed to feel their way.
There can be little doubt, however, that, in most insects,

TOUCH. 273.

the principal organs of touch are the sflnlcnnx^ also deno-
minated, from their supposed oilice, \\\q. feelers.^

Some idea of the great variety in the forms of the anten-
nae of insects may be obtained from the specimens deh'ne-
ated in Fig. 3S1, which shows a few of the most remarka-
ble.f

The universality of these organs among every species of
this extensive class of animals, their great flexibility, arising
from their jointed structure,! their incessant motion when
the insect is walking, and their constant employment in exa-
mining 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 par-
ticularly necessary to insects, since the horny nature of the

• The German name for ^\em, fiihlh'dmer, or the feeling horns, is founded
on the same notion. i

f In this figure, A represents the form of antennae, technically denomi-
nated Antenna capitulo uncinato, as exemplified in the Pausus.

B . is the A . piloso-verticillata, as in the Psychoda ocellaris.

C . . A . biclavata, (Clavlger longicornis.)

D . . A . triang-ularis, (Lophosia.)

£ . . A . clavata, (Masaris.)

F . . A . capit. lamellato, (Melolontha mas.)

G . . A . capit. fissile, (Jphodlusfossor.)

H . . A . fusiformis, {Zygaena.)

I . . A . capitata, (Ascalaphus.)

K . . A . furcata, (Nepa.)

L. . A . bipectinata, {Bombyx.)

M . . A . irregularis, {Agaon paradoxum.)

N . . A . cordata, (Diaperis boleti.)

O . . A . bipectinata, (Ctenophora.)

P . . A . palmataj (Nepa cinerea.)

Q . . A . ensiformis, (TVuxalis.)

R . . A . setacea, (Cerambyx.)

i The number of segments into which these organs are divided is often
very great. In the Grylloialpa, 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, except-
ing at their very commencement. These are extremely sensible, and sei-ve,
probably, to give the animal notice of the approach of any annoyance from
behind, lb. p. 216.

Vol. II. 35

274 THE SENSORIAL FUNCTIONS.

intcoruments of the greater number, precludes them from im-
parting any accurate perceptions 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 antennas, it either remains motionless, or if it
attempt to walk or fly, appears bewildered, and moves with-
out any apparent object. Ilubcr found that bees are ena-
bled, 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 these organs
perpetually while building the combs, pouring honey into
the magazines, ascertaining the presence of the queen, and
feeding and tending the larvae. The same naturalist ob-
serves, also, that it is principally by means of the antennae
that these social insects communicate to one another their
impressions and their wants.

The different modes in which ants, when they happen to
meet during their excursions, mutually touch one another
with their antennae, appears to constitute a kind of natural
language understood by the whole tribe. This contact of
the antennse evidently admits of a great variety of modifica-
tions, and seems capable of supplying all the kinds of in-
formation which these insects have occasion to impart. It
would seem impossible, indeed, for all the individuals com-
posing these extensive societies to co-operate effectually in
the execution of many works, calculated for the general be-
nefit of the community, 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 emitted by these insects. Their mode
of conversing together appears to be simply by touching one
another in different ways with the antennae. Iluber's ob-
servations on this subject are exceedingly curious.* He re-
marks that the signal denoting the apprehension of danger,
is made by the ant striking its head against the corselet of

• See his '*Rcchcrches sur les mccursdcs foiirmis indigenes."

TOUCH. 275

every ant which it chances to meet. Each ant, on receiving
this intimation, 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 tliroughoutthc
whole society. Sentinels are at all times stationed on the
outside of the nests, for the purpose of apprizing the inha-
bitants 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 intrusted with the office of guarding the eggs and the
larvae, hasten to remove their charge to places of greater se-
curity.

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 absence, and the labours of the hive are car-
ried 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 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 occupied be-
fore 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 cross-
ing their antennae, and striking lightly with them. The
bees which receive the news become, in their turn, agitated,
and conveying this feeling wherever they go, the alarm is
soon participated by all the inhabitants of the hive. All
rush forwards with tumultuous precipitation, eagerly seek-
ing 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 hubbub 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 tlie
bottom of the hive, seems attracted only by the light, and

276 THE SENSORIAL FUNCTIONS.

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 incapa-
ble of directing her trunk with precision to the food which
was offered to her. Latreille relates that, having deprived
some labouring ants of their antennae, he replaced them near
the nest; but they wandered in all directions, as if bewil-
dered, and unconscious of what they were doing. Some of
their companions were seen to notice their distress; and, ap-
proaching them with apparent compassion, 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 investment, 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 exercise it by
means of their long and flexible tentacula. 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 exer-
cise of this sense; and their fins are used for no other pur-
poses than those of progressive motion. That part of the
surface which possesses 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 supplied with nerves; and perhaps
the cirrhi^ or little vermiform processes called bm^bcls, which
in some species are appended to the mouth, may be subser-
vient to this sense. * These processes in the Silurus glanis
are moved by particular muscles.

♦ These kind of tentacula are remarkable for their length and mobility in
the Lnphhvf piscatorius, or Angler; and it is said that they are employed by

TOUCH. 277

Serpents, from the great flexibility of their spine, are ca-
pable 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 acqui-
sition of correct perceptions of touch. As it is these per-
ceptions, which, as we shall afterwards find, lay the founda-
tion of the most perfect acquaintance with the tangible pro-
perties of surrounding 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 instru-
ment. Hellman has shown that the slender bifurcated
tongue of these animals is used for the purposes of touch.*

In those species of Lizards which are enabled by the
structure of their feet to clasp the branches of trees, as the
Gecko and the Chameleon, and whose tails also are prehen-
sile, 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 articulations and
their divergent position, and from the papillae with which
their skin abounds, accompanied as they are with a large
supply of nerves. Those birds, which, like the Parrot, em-
ploy the feet as organs of prehension, probably enjoy a
greater development of this sense. The skin which covers
the bills of aquatic birds is supplied by very large nerves,
and consequently possesses great sensibility. This struc-
ture 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 Ornithorhyncus.

Among Mammalia, we find the seat of this sense frequent-
ly transferred to the lips, and extremity of the nostrils, and

the fish, while lurking" in ambush, as a decoy to other fishes, which they en-
tice by their resemblance to worms.
* tiuoted by Blumenbach.

278 THE SENSORIAL FUNCTIONS.

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
with the Pachydermata, where this greater sensibility of the
parts about the face seems to have been bestowed as some
compensation for the general obtuseness of feeling resulting
from the tliickness of the hide which covers the rest of the
body. Thus, the Rhinoceros has a soft, hook-shaped exten-
sion 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 ob-
jects, 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 Fe-
lls, 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 doubt-
less fitted to convey accurate perceptions 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 instruments of prehension. But as the struc-
ture and functions of this organ are the exclusive subjects of
another of these treatises, I shall refrain from any farther
remarks respecting them.

( 279 )

CHAPTER III.

TASTE.

The senses of taste and smell are intended to convey im-
pressions 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 furnished with a large propor-
tion of blood vessels and nerves. The vascular plexus im-
mediately covering the corium is here very visible, and forms
a distinct layer, through which a great number of papillse
pass, and project from the surface, covered with a thin cuti-
cle, like the pile 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.t They
are of different kinds; but it is only those which are of a co-
nical shape that are the seat of taste. If these papilla be
touched with a fluid, which has a strong taste, such as vine-
gar, applied by means of a camel-hair pencil, they will be
seen to become elongated by the action of the stimulus, an
effect which probably always accompanies the perception of
taste.

• Bellini contended that the different flavors of saline bodies were owing-
to the peculiar figures of their crystalline particles. It is strang-e tliat Dumas
should have thouglit it worth while seriously to combat this exti-avag-ant liy-
pothesis, by a laboured refutation.

j- This is particularly the case in scarlatina, In the early stag-e of wliich dis-
ease they are elongated, and become of a bright red colour, from their mi-
nute blood vessels being- distended with blood. As the fever subsides, tbe
points of the papilla; collapse, and acquire a brown hue, giving- rise to tlie
appearance known by the name of the strawberry tongue.

280 THE SENSORIAL FUNCTIONS.

The primary use of this sense, the organ of which is
plaped at the entrance of the alimentary 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 sto-
mach. With respect to the human species, this use has been,
in the present state of society, superseded by many acquired
tastes, which have supplanted those originally given to us
by nature: but in tlie inferior animals it still retains its pri-
mitive oflice, and is a sense of great importance to the safety
and welfare of the individual, from its operation being coin-
cident 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 vertebrated ani-
mals, namely, Fishes, Reptiles, and Birds, is scarcely ever
constructed with a view to the reception of delicate impres-
sions 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 a large propor-
tion of quadrupeds, which swallow their food entire, and
which cannot, therefore, be supposed to have the sense of
taste much developed.

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 motion, 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 must be obvious that
in this, as in every other instance in which our researches
extend to beings of such minute dimensions, and which oc-
cupy a station, in the order of sensitive existence, so remote
from ourselves, we are wandering 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.

( 281 )

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 for-
mer, 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 beginning of the passages for res-
piration, where a distinct nerve, named the olfactory, ap-
propriated 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, are
certain effluvia, or particles of extreme tenuity, which are
disseminated very quickl}'- through a great extent of atmo-
spheric air. It is exceedingly difficult to conceive how mat-
ter so extremely rare and subtle as that which composes
these odorous effluvia can retain the power of producing any
sensible impression on the animal organs: for its tenuity is
so extraordinary as to exceed all human comprehension.
The most copious exhalations from a variety of odoriferous
substances, such as musk, valerian, or asafoetida, will be
continually emanating 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

Vol. II. 36

282 THE SENSORIAL FUNCTIONS.

the nicest balance will not show the smallest increase of its
wei*'ht from this impregrnation. No facts in natural philo-
sophy afford more striking illustrations of the astonishing^
and indeed inconceivable divisibility of matter, than those
relatinc; to odorous effluvia.

It would apj)ear that most animal and vegetable bodies are
continually emitting these subtle effluvia, of which our own
organs are not sufficiently delicate to apprize us, unless
wlicn they arc much concentrated, but which are readily
perceived and distinguished by the lower animals; as may
be inferred from their actions. A dog is known to follow
its master by the scent alone, through the avenues and turn-
ings of a crowded city, accurately distinguishing his track
amidst thousands of others.

The utility of the sense of smell is not confined to that of
being a check upon the respiration 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 necessity for attending to its per-
ceptions, and our sensibility to odours may perhaps have di-
minished in the same proportion. It is asserted both by
Soemmerring and Blumenbach that the organ of sm.ell is
smaller in Europeans, and other civilized 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 carni-
vorous tribes, especially, it is highly 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,

SMELL.

283

called the Schneiderian membrane,* which is constantly-
kept moist, is supplied with numerous hlood vessels, and
upon which are spread the ultimate ramifications of the ol-
factory nerves. The relative magnitude of these nerves is
much greater in carnivorous quadrupeds than in those which
subsist on vegetable ^ood. In quadrupeds as well as in man,
these nerves are not collected into a single trunk in tlicir
course towards the brain, but compose a great number of fila-
ments, which pass separately through minute perforations
in a plate of bone, (called the ethm,oid boiie) before they en-

ter into the cavity of the skull, and join that part of the ce-
rebral substance with which they are ultimately connected.
The surface of the membrane which receives the impres-
sions from odorous effluvia, is considerably increased by
several thin plates of bone, which project into the cavity of
the nostrils, and arc called the turbinated hones. These are
delineated at t, t, in Fig. 3S2, as they appear in a vertical
and longitudinal section of the cavity of the human nostril,
where they are seen covered by the Schneiderian mem-

* It has been so named in lionour of Schneider, the first anatomist who
gave an accurate description of this membrane.

284

THE SENSORIAL FUNCTIONS.

brane."* A transverse and vertical section of these parts is
given in Fig. 3S3.t The turbinated bones are curionsly
folded, and often convoluted in a spiral form, with the evi-
dent design of obtaining as great an extent of surface as pos-
sible within the confined space of the nasal cavity. This tur-

binated, or spiral shape, chiefly cliaracterizes 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 arc pierced on
all their internal sides with numerous perforations, through

• This fig-ure sliows the branches of the olfactory nerve (o,) passing
through tlie thin cribriform plate of the ethmoid bone, and distributed over
that membrane. Several of the cells, which open into the cavity, are also
vSecn; such as the large sphenoidal sinus (s,) the frontal sinus (f,) and one of
the ethmoidal cells (c.) s, is the nasal bone; p, the palate; and e, the
mouth of the Eustachian tube, which leads to the ear.

■|- 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 An-
trum maxillare (a,) which is another large cavity communicating with the

<

iiostrils.

SMELL.

285

which the membrane, together with the fine branches of the
nerves, passes freely from one side to the other. The ca-
vities resulting from the convolutions are inlersccted by un-
perforated partitions of extraordinary tenuity, serving both
to support the arches of bone, and 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 si-
milar to that just described; but the convolutions are double,
with an intermediate partition, so as to resemble in its trans-
verse section the capital of an Ionic column.* They are
shown at (t) in the transverse section of the nostrils of a
sheep in Fig. 384.

In carnivorous quadrupeds the structure of these bones is
still more intricate, and is calculated to afford a far more ex-
tensive surface for the distribution of the olfactory nerve.
In the Seal this conformation is most fully developed, and
the bony plates are here not turbinated, but ramified, as

* In a species of Antelope described by Mr. Hodgson, cavities exist, si-
tuated immediately behind the ordinary nostrils, and communicating- with
them. The accessory nostrils are conjectured to be useful to this excceding--
ly 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 So-
ciety, Feb. 1832, p. 59.

286 THE SENSORIAL FUNCTIONS.

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 luindred 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 nos-
tril. An organ of such exquisite sensibility requires an ex-
traordinary 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 boo;, which, in its natural state, subsists wholly on ve-
getable 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 sin-
gle, and slightly convoluted plates, without partitions or per-
forations. In this respect, they approach to the human
structure, which is even less complicated, and indicates a
greater affinity with vegetable than with animal feeders.
Man, indeed, distinguishes more accurately vegetable odours
than those proceeding from animal substances; while the
reverse is observed with regard to quadrupeds whose habits
are decidedly carnivorous. A dog, for instance, is regard-
less 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. Predacious 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 generally 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

SMELL.

287

the weak impressions of particles widely diffused through
thl5 surrounding medium, or slightly adhering to those bo-
dies, with which the ohjcct 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 cxhi-

386

bits their appearance in the Turkey: but the size of the ol-
factory nerves of birds of prey greatly exceeds 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 de-
ceived 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 con-
tents 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 poison-
ous composition taken from each; and the crops, either na-
turally, 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 recovered.

The olfactory nerves are conspicuous in the Duck, both
from their size and mode of distribution. They are seen

288

THE SENSORIAL FUNCTIONS.

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 wathin and without. For the protection of the highly
sensible extremity of the beak against the injurious impres-
sions of hard bodies, a horny process (p,) similar, both in
form and office, to the human nail, is attached to it, and its
edtJ-es guarded by a narrow border of the same horny mate-
rial; these receive a first, and fainter impression, and admo-
nish the animal of approaching danger; if none occur, the
matter is then submitted to the immediate scrutiny of the
n.erves 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 suf-
ficiently established by the observations and experiments of
Mr. Audubon, that these birds in reality possess the sense
of smell in a degree very inferior to carnivorous quadru-
peds; 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 following experiments appear to
be perfectly conclusive 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

* Such is tlie account given by Sir Busick Harwood, in his *' System of
Comparative Anatomy and Physiology," p. 26.

SMELL. 289

its back, in the attitude of a dead animal. In the course of
a few minutes afterwards, he observed a vulture flying to-
wards it, and alighting near it. Quite unsuspicious of the
deception, the bird immediately proceeded to attack it, as
usu^l, in the most vulnerable points. Failing in his object,
he next, with much exertion, tore open the scams 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 efibrts,
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, 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, sail-
ing in all directions over the spot where the putrid carcass
was lying, covered only with twigs of cane, none ever dis-
covered it; but in the mean while, several dogs had found
their way to it, and had devoured large quantities of the
flesh. In another 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 ani-
mals, possess the sense of smell; in some of the whale tribe,

♦Edinburgh New Journal of Science, ii. 172. The accuracy of these re-
sults, which had been contested by Mr. Waterton, is fully established by the
recent observations and experiments of M. Bachman, which are detailed in
Loudon's Magazine of Nat. Hist. vii. 167.

Vol. II. 37

290 THE SENSORIAL FUNCTIONS.

indeed, neither the organ of smell nor the olfactory nerves
are found.* Some physiologists have gone the length of de-
nvin«j; the capability of water to serve as the vehicle of odo-
rous 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 peculiar agents which are calculated to excite perceptions
of smell. Fishes arc, 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 predilec-
tion for all highly odoriferous substances. Baits used by
ansjlers are rendered more attractive by being impregnated
with volatile oils, or other substances having a powerful
scent, such as asafoetida, camphor, and musk. Mr. T. Bellt
has discovered in the Crocodile and Alligator, a gland, which
secretes an unctuous matter, of a strong, musky odour, si-
tuated beneath the lower jaw, on each side. The external
orifice of this gland is a small slit, a little within the lower
edo-e of the jaw; and the sac, or cavity containing the odo-
riferous 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 rea-
dily seized in the mode usual to the alligator, which is that
of snapping sideways at the objects he aims at devouring.

The organs of smell in Fishes are situated in cavities,
placed one on each side, in front of the head: they are mere-
ly blind sacs, having no communication with the mouth or
throat, and, indeed, no other outlet but the external open-
ings, which are generally two to each sac. The principal
entrance is furnished with a valve, formed by a moveable
membrane, appearing like a partition dividing each nostril

* Home; Lectures on Comparative Anatomy, i. 17.
j IMiil. Trans, for 1827, p. 132.

SMELL.

291

into two cavities, and serving the purpose of preventing the
introduction of any foreign body. The organ itself is si-
tuated behind this valve, and consists either of a membrane,
curiously plaited into numerous semicircular folds, or of
tufted or .arborescent filaments. Fig. 3SS shows this cavity

(s,) with its plaited membrane in the Perch; and Fig. 3S9, in
the Skate; the laminae in the former being radiated, and in
the latter, foliated, or parallel to each other. On the sur-
face of these organs, whatever be their. shape, the olfactory
nerves (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 acuteness 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. According to Gcof-
froy 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 Dumerll,
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

292 THE SENSORIAL FUNCTIONS.

o

those of the mammalia, although they do not, as in the latter
class, open into the the respiratory organs, is shown by the
curious transformation of the one into the other during the
development of the tadpole, both of the frog and of the sa-
lamander. During the first periods of their existence, these
animals arc perfectly aquatic, breathing water by means of
gills, and having all tlicir 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, swallow-
ing it in small portions at a time, and afterwards throwing
it out in bubbles by the same channel. But when they quit
the water, and become land animals with pulmonary respi-
ration, 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 as-
signed by many to the antennae; while other entomologists
have supposed that the palpi are the real organs of smell.*
Experiments on this subject are attended with great diffi-
culty, and their results must generally be vague and incon-
clusive. Those which Mr. P. Huber made on bees, seem,
however, to establish, with tolerable certainty, that the spira-
cles are insensible to strong odours, such as that of oil of
turpentine, which is exceedingly offensive to all insects. It
was only when a fine camel-hair pencil containing this pun-
gent fluid was presented near the cavity of the mouth, above
the insection of the proboscis, that any visible effect was pro-
duced upon the insect, which then gave decisive indications

• On the subject of this sense in insects. See Kirby and Spence's Introduc-
tion to Entomology, vol. iv. p. 249.

SMELL. 293

of strong aversion. Mr. Kirby has discovered in the ante-
rior part of the nose of the Necrophorus vespillo^ or bu-
rying beetle, which is an insect remarkable for the acute-
ness of its smell, a pair of circular pulpy cushions, covered
with a membrane, 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 discovered in any
of the Mollusca; but as there is evidence that some of the
animals belonging to that class possess this sense, it has
been conjectured that it resides either in the whole mucous
surface of the mantle, or in the respiratory organs. Swam-
merdam observed, long ago, that snails are evidently af-
fected by odours; and cuttle-fish are said to show a decided
aversion to strongly scented plants.

* Kirby and Spence's Introduction to Entomology, vol. iii. 481? and iv.
254.

( 294 )

CHAPTER V.

HEARING.

§ 1. Acoustic Principles.

The knowledge acquired by animals of the presence and
movements of distant objects is derived almost wholly from
the senses of hearing and of sight; and the apparatus, ne-
cessary for the exercise of these senses, being more elabo-
rate and refined than any of the organs we have yet exa-
mined, exhibit still more irrefragable evidence of those pro-
found designs, and that infinite intelligence, which have
guided the construction of every part of the animal frame.

Sound results from certain tremulous or vibratory 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 transmitted with great velocity
through these 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 filaments of a par-
ticular nerve called the acoustic, or auditory nerve, of
which the structure is adapted to receive these peculiar im-
pressions, 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 ve-
hicle for its transmission; and, accordingly, a bell suspended
in the vacuum of an air-pump, gives, when struck, no audi-

HEARING. 295

ble 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 dis-
tinct; 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 density.*

The impulses given by the sounding body to the contigu-
ous 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 reaction of the particle
against which it strikes. Hence, after moving a certain de-
finite distance, a distance, indeed, which is incalculably small,
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,t suffers a momentary com-
pression, 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 sono-
rous undulations have an analogy with 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 so-
norous undulations spread out in all directions, forming, not
circles in one plane, but spherical shells; and, whatever be
the intensity of the sounds, the velocity with which the un-
dulations 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

* These facts were first ascertained by Dr, Ilauksbee. See Philosophical
Transactions for 1705, vol. xxiv., p. 1902, 1904.

■|- The particles of water are as elastic, within a limited distance, as those
of the most solid body, although, in consequence of their imperfect cohe-
sion, or, rather, theii* perfect mobility in all directions, this property cannot
be so easily recog^iised in masses of fluids, as it can in solids.

296 THE SENSORIAL FUNCTIONS.

miles in a minute: it is greater in dense, and smaller in rare-
fied air; being, in the same medium, exactly proportioned to
the elasticity of that medium.

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 account of the greater elasticity of the constituent parti-
cles 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 as great as it is in air.

Solid bodies, especially such as are hard and elastic, and
of uniform substance, are also excellent 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 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 con-
veyed by the ice, and the second by the air. In like man-
ner, 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,

HEARING. 297

will hear two sounds after each hlow, the first descending
through the wall, and the second through the air.

As sounds are weakened by diflfusion over a larger sphere
of particles, so they are capable of having tlieir intensity in-
creased by concentration into a smaller space; an cflect
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 persons dull of hearing, is construct-
ed: 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, the impressions become so blended together as not to
be distinguishable 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 in all probability were originally derived
from the visible motions of the throat of a person who is
singing these diff'erent notes; for, independently of this cir-
cumstance, the terms of high and low are quite arbitrary;
and it is well known that they were applied by the ancients
in a sense exactly the reverse of that in which we now use
them.

The difierent degrees of tension given to the cord or wire
of a stringed musical instrument, as well as its diflferent
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 con-
trary, the note is rendered more grave by either lessening
the tension, or lengthening the cord or wire. In a wind
instrument, the tone depends altogether upon the length of
the tube producing the sound.

Vol. II. 3S

298 THE SENSORIAL FUNCTIONS.

Tlicre are, therefore, two qualities in sound recognisable
by the ear, namely, loudness, or intensity, and quality, or
tone; the former depending on tlie force of the vibrations;
the latter, on tlieir frequency. These acoustic principles
arc to l)e borne in mind in studying the comparative physi-
ology of hearing; and since the functionsof the different parts
of the organ of this sense are, as yet, but imperfectly under-
stood, 1 sliall, in treating of this subject, deviate from the
plan I liave hitherto followed, and premise an account of the
structure of the ear in its most perfectly developed state,
wluch 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 acous-
tic nerve, of which the fibres are expanded, and spread over
the surface of a fine membrane, placed in a situation adapt-
ed to receive the full impression of the sonorous undulations,
which are conveyed to them. This membrane, then, with
its nervous filaments, may be regarded as the immediate or-
gan of the sense; all the other parts being merely accessory
apparatus, designed to collect and to condense the vibrations
of the surrounding medium, and to direct their concentrated
action on the auditory membrane.

I have endeavoured, in Fig. 390, to exhibit, in one view^,
the principal parts of this complicated organ, as they exist
in man, in their relative 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 ap-
preciated.*

The Coiicha, or external ear (c,) is formed of an elastic
plate of cartilage, covered by integument, and presenting va-

• In lliis and uU the following' figures, the parts of the right car are shown,
and similar parts arc always indicated by the same letters.

HEARING.

299

rious elevations and depressions, which form a series of pa-
rabolic curves, apparently for the purpose of collecting the
sonorous undulations of the air, and of directing them into
a funnel-shaped canal (m,) termed the meatus auditoriiis,

which leads to the internal ear. This canal is composed
partly of cartilage, and partly of bone; and the integument
lining it is furnished with numerous small glands, which
supply a thick oily fluid, of an acrid quality, apparently de-
signed to prevent the intrusion of insects: the passage is also
guarded by hairs, which appear intended for a similar pur-
pose.

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 of-
fice corresponding to its name; for the sonorous undulations
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

* The Inner surface of the ear-drum is shown in this figure, the cavity of
the tympanum, which is behind it, being- laid open.

300 THE SENSORIAL FUNCTIONS.

composed of an external membrane, derived from the cuti-
cle whicli lines the meatus; an internal layer, which is con-
tinuous witli that of the cavity beyond it; and a middle
layer, wiiich consists of radiating muscular fibres, proceed-
ing from the circumference towards the centre, where they
are inserted into the extremity of a minute bony process
(h,) presently to be described.* This muscular structure
apj)ears designed to vary the degree of tension in the ear-
drum, and tlius adapt the rate of its vibrations to those com-
municated 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 (t,) called the cavity of the tympanum^
of an irregular shape, scooped out of the most solid part of
the temporal bone, which is here of great density and hard-
ness. This cavity is always filled 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 occa-
sionally 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
w^ell known, is subject to great variations, indicated by the
rise and fall of the barometer. These variations would ex-
pose 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
outwards or inwards, which would completely interfere with
the delicacy of its vibrations. Nature has guarded against

• In many quadrupeds their insertion into this process is at some distance
from the centre of the membrane. These muscular fibres arc dcUncated in
Fig. 45, vol. i. p. 105.

f Home, Lectures, &c., iii. 268.

HEARING.

301

these evils by establishing a passage of communication be-
tween the tympanum 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 tympa-
num, and opens by a wide mouth at the back of the nos-
trils.* 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 obstructed, which
may happen either from the swelling of the membrane
lining it, during a cold, or from the accumulation of secre-
tion in the passage. It is also occasionally useful as a chan-
nel through which sounds may gain admittance to the inter-
nal ear; and it is perhaps for this reason that we instinct-
ively 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 op-
posite to the opening of the Eustachian 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

;393

1

a

filled with air. The innermost side of the same cavity, that
is, the side opposite to the ear-drum, and which is shown in

• This opening is seen at e, in Fig". 283, p. 383, representing a vertical and
longitudinal section of the right nostril.

302 THE SENSORIAL FUNCTIONS.

Fie;. 391, is occupied by a rounded eminence (p,) of a tri-
ane;ular shape, termed the pro77iojitor}j ; on each side of
which there is an opening in the bone, closed, however, by'
the membrane lining the whole internal surface of the cavi-
ty. 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 (r,) is the fenestra
rotunda, or round window.

Connected with the membrane of the ear-drum, at one
end, and with the fenestra ovalis at the other, there extends
a chain of very minute moveable bones, seen at (b,) in Fig.
390; but more distinctly in Fig. 392, which is drawn on a
somewhat larger scale, and in which as before (d) is the
ear-drum; (p) the promontory, (o,) the fenestra ovalis; and
(r) the fenestra rotunda. These bones, which may be called
the tymjjanic ossicula, are four in number, and are repre-
sented, enlarged to twice the natural size, in Fig. 393.. The
names they have received are more descriptive of their
shape than of their office. The first is the malleus, or ham-
mer (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
regularly articulated together, with all the ordinary appa-
ratus 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.

The more internal parts of the ear compose what is de-

• lilumenbach, 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 sup-
ported by the observations of Mr. Shrapnell, detailed in the Medical Ga-
zette, xii., 172.

HEARING.

303

signaled, from the intricacy of its winding passages, the la-
byrinth. It is seen at (s v k) in
Fig. 390, in connexion with the
tympanum; but in Fig. 394, it is
represented, 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 portion,
termed the vestibule (v,) from
which, on its upper and posterior
side, proceed the three tubes (x,
T, 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 ap-
pearance the shell of a snail, and on that account denomi-
nated the Cochlea (k.) All these bony cavities are lined
with a very delicate membrane, or periosteum, and are fdled
with a transparent watery, or thin gelatinous fluid, which is
termed by Breschet, Wiq perilym,ph.^

Within the cavity of the osseous labyrinth,, now de-
scribed, are contained membranes having nearly the shape of
the vestibule and semicircular canals, but not extending into
the cochlea. These membranes, which compose what has
been termed, for the sake of distinction, the membranous
labyrinth, form pne continuous, but closed sac, containing a
fluidjf perfectly similar in appearance to the perilymph,
which surrounds it on the outer side, and intervenes be-
tween it and the sides of the osseous labyrinth, preventing
any contact with those sides. In Fig. 395, which is on a
still larger scale than the preceding figure, the osseous laby-
rinth is laid open, so as to show the parts it encloses, and

* Annales des Sciences Naturelles, xxix. 97. It has also been called the
Aqua lahyrinnd, and the Jliiid of Cotunnius, from the name of the Anato-
mist who first distinctly described it.

f De Blainville has termed tiiis fluid " la vitrine auditive," from its sup-
posed analogy with the vitreous humour of the eye.

304

THE SENSORIAL FUNCTIONS.

mor§ especially the membranous labyrinth, floating in the
perilymph (p.) The form of this latter part is still more
distinctly seen, in Fig. 39G, where it is represented in a po-
sition exactly corresponding to the former figure, but whol-
ly detached from the bony labyrinth, and connected only
with the nervous fdaments which arc proceeding to be dis-
tributed to its diflerent 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 semi-
circular canals, (x, y, z,) each of which present, at their ori-
gin from the vestibule, a considerable dilatation, termed an
cnnjmlla (a, a, a,) while, at their other extremities, where
they terminate in the vestibule, there is no enlargement of
their diameter: and it will also be seen that two of these ca-
nals (x and y) unite into one before their termination. The

HEARING. 305

same description applies in all respects botli to the osseous
and to the membranous canals contained witliin 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 imi-
tation of that 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 sacculus. Each
sac contains in its interior 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 interme-
dium 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 la-
byrinth of all the mammalia, and from their much greater
size and hardness in aquatic animals, there can he little
doubt that they perform some office of great importance in
the physiology of hearing.t Their size and appearance in
the Dog is shown in Fig. 397: and in the Hare, in Fig. 398.
The Cochlea, again, is an exceedingly curious structure,
being formed of the spiral convolutions 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 cone, where it
suddenly terminates in a curved point, or hook (h,) leaving
an aperture by which the two portions of the tube commu-
nicate together. 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 distin-
guished by the name of the vestibular tiibe,X in consequence

• Scarpa and Weber term it the sinus or alveus titrlculosus ,- it is called by
others the sacculus vestihuU. Brcschct ^ives it tlie name of Ic sinus median.
See the Memoir already quoted, p. 98.

I These cretaceous bodies are termed by IJrcschct otolithcs, and otoconicSy
according as they are of a hard or soft consistence. Ibid. p. 99.

\ Sea la vestibuli.

Vol. II. 39

306 THE SENSORIAL FUNCTIONS.

of its arising from the cavity of the vestibule; and the other
by that of the tympanic tube,'^ because it begins from the
inner side of the membrane which closes the fenestra ro-
tunda, and forms the only separation between the interior of
that tube, and the cavity of the tympanum. The trunk of
the auditory nerve occupies a hollow space immediately be-
hind 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 different parts of the mem-
branous lal)yrinth.t

Great uncertainty prevails with regard to the real func-
tions performed by the several parts of this very complex
apparatus. It is most probable, however, that the sonorous
viljrations of the air which reach the external ear, are di-
rected down the meatus, and striking against the ear-drum
which closes the passage, throw that membrane into vibra-
tions of the same frequency; to which the action of its mus-
cles, which appear intended to regulate its tension, may also
contribute. The vibrations of the ear-drum, no doubt, ex-
cite corresponding motions in the air contained in the cavity
of the tympanum; which, again, communicates them to the
membrane of the fenestra rotunda; while, on the other hand,
the membrane 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 con-
tained in the labyrinth, is affected by each external sound,
both tlirough the medium of the air in the tympanum, and
by means of the ossicula: the undulations thus excited pro-

• Scala tympani.

f In Fig. 396, the anterior tmnk of the auditory nerve is seen (at g) dis-
tributing branches to the ampullse (a, a,) the utricle (u,) and the calcareous
body it contains; while the posterior trunk (u) 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 nei*ve called the por-
iio duruy which merely accomi)anies the auditoiy nerve, but has no relation
to the sense of hearing. In Fig. 390, the auditory nerve (n) is seen enter-
ing at the back of the vestibule.

HEARING. 307

duce impressions on the extremities of the nervous filaments,
which are spread over the membranous labyrinth; and these
impressions being conveyed to the brain, are immediately
followed by the sensation of sound.

With regard to the purposes which are answered by the
winding passages of the semicircular canals, and cochlea,
hardly any plausible conjecture has been oflered; yet no
doubt can be entertained that the uses of all these parts are
of considerable importance, both as to delicacy and correct-
-ness of hearing. There is an obvious correspondence be-
tween 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 another,) and the
three dimensions by which the geometrical relations of space
are estimated; and it might hence be conjectured that the ob-
ject of this arrangement is to allow of the transmission of vi-
brations of every kind, in whatever direction they may ar-
rive. It is not an improbable supposition tliat tiie return
into the vestibule, of undulations which have passed through
these canals, has the effect of at once putting a stop to all
farther motion of the fluid, and preventing 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 tlie mem-
brane 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 pro-
duce but a single momentary impression; the prolongation
of the undulations of the fluid of the labyrinth being pre-
vented by their mutual collision and neutralization.*'

* The preliminary steps in the process above described are not absolutely
essential to hearing-, for many instances have occuiTed in which the power of
bearing 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

808

THE SENSORIAL FUNCTIONS.

§ 3. Comjjarative Physiology of Heariiig.

The structure of the organs of hearing in the lower ani-
mals 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 or-
gan, through the successive additions of semicircular canals,
fenestra ovalis, tympanic cavity, ossicula, ear-drum, meatus
auditorius, cochlea, and concha, till we arrive at the combi-
nation of all these parts in the higher orders of the Mam-
malia. The simpler forms are generally met with in aqua-
tic animals, probably because the sonorous undulations of
water are communicated more readily, and with greater
force, than tiiose of air, and require no accessory apparatus
for their concentration. The loftster, for instance, has a ves-
tibular cavity (seen at v, in Fig. 399,) containing a membra-
nous sac, with a striated groove (g,)* and receiving the fila-
ments of the auditory nerve. This vestibule is protected
by tho 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 membrane in the Jlstaciis Jluviatilis, or
cray-fish, is seen at f in Fig. 401; while Fig. 402, shows an

ear-drum be deslroyed, and the ossicula lost, an almost total deafness g-ene-
rally ensues. After a time, however, the hearing- may be in a great measure
recovered, with an undiminished power of distinguishing musical tones. See
two papers by Sir Astlcy Cooper, in the Phil. Trans, for 1800, p. 151; and
for 1801, p. 437.

♦ This g-roove is represented magnified in Fig. 400.

HEARING. 309

interior view of the same membrane (f,) with the vestibule
(v) laid open, and the auditory nerve (n) passing through
the shell to be distributed on the sacculus.

It appears from a variety of observations that Insects, both
in their larva and their perfect state, possess the faculty of
hearing; but no certain knowledge has been obtained of tbe
parts which exercise this sense. Tlic prevailing opinion
among entomologists is that it resides in some part of the
antennae; organs, which are supposed to have a peculiar sen-
sibility to aerial undulations. This hypothesis is founded
principally 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 Mollusca appear to possess, even in the small-
est degree, the sense of hearing, if we except the highly or-
ganized Cephalopoda; for in them we find, at the lower part
of the cartilaginous ring, which has been supposed to exhi-
bit the first rudiment of a cranium, a tubercle, containing in
its interior two membranous vesicles, contiguous to each
other, and surrounded by a fluid. They evidently corre-
spond to the vestibular sacs, and contain each a small calca-
reous body, suspended from the vesicles by slender nervous
filaments, like the clapper of a bell, and probably performing
an office analogous to that instrument; for, being thrown
into a tremulous motion by every undulation of the sur-
rounding 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,

• Camparetti has described structures in a great number of insects, which
he imagined were organs of hearing; but liis observations have not been con-
firmed by subsequent inquirers, and their accuracy is therefore doubtful.
See De Blainville " De I'Organisation des Animaux," i. 565.

310

THE SENSORIAL FUNCTIONS.

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 mo-
tion to tiie 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 arc, in general, greatly
developed.* An enlarged view of the membranous laby-
rinth of the Lophiiis piscatoruis is given in Fig. 403, show-
ing the form and com})lication of its parts, which are repre-
sented of twice the natural size, x, y, z, are tlie semicircular
canals, with their respective ampullae (a, a, a.) m is the
Sinus rtiedianus^ or principal vestibular sac, with its ante-
rior expansion, termed the Utricle (u.) The Saccidus (s)
has, in like manner, a posterior appendage (c) termed the
Cysticule. The hard calcareous bodies (o, o, o) are three
in number; and tlie branches of nerves (i, i, i) by which
they are suspended in the fluid contained in the membranes,
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 enclosed in the bmies of the cranium, but
projects into its cavity; but in the larger cartilaginous fishes,

♦ In the lamprey, these canals exist only in a rudimental state, appearing"
as folds of tlie membrane of the vestibule; and there are also no cretaceous
bodies in the vestibular sac.

HEARING.

311

as the ray and shark tribes, it is surrounded 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 mem-
brane, which is covered by the skin.

Aquatic reptiles have ears constructed nearly on the same
plan as those of fishes: thus, the Triton or Newt has a vesti-
bule containing only one cretaceous body, and three semi-
circular canals, unprotected by any surrounding bone. In
the Frog, however, we first perceive the addition of a dis-
tinct cavity, closed by a membrane, 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

unci

the Columeim. This bone is seen at c in Fig. 404, attached

by its base (b) to the fenestra ovalis of the vestibule (v,)
which contains 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 the tympanum and columella are of greater
length. In the saurian reptiles the cavity of the tympanum
is still more capacious, and the ear-drum very distinctly
marked, and these animals possess great delicacy of hearing.
The labyrinth of tlie Crocodile is enclosed in bone, and con-
tains three calcareous bodies: it presents also an appendage

312 THE SENSORIAL FUNCTIONS.

which has been regarded as tlie earliest rudiment of a coch-
lea; 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 development of an external ear.

The structure of the car in the Crocodile is but an ap-
proximation to that wliich we fmd prevailing in Birds,
where the organ is of large size compared with that of the
head. The rudimcntal 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
(t) 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
quadratum (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.
In Owls these feathers are a prominent ancjfcharacteristic
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 internal ears of
]\Iammalia is the great development of the cochlea, the
tubes of wliich are convoluted, turning in a spiral, and as-
suming the figure of a turbinated shell. From an extensive
comparison of the relative size of the cochlea in different
tribes of quadrupeds, it has been inferred that it bears a to-
leraldy constant proportion 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 coch-
lea of extraordinary size, compared with the other parts of
the ear. The tympanic ossicula are completely developed
only in the JMammalia.* It is also in this class alone that

• TIjcsc tympanic ossicula are reg-arded by GcofFroy St. liilaire as cor-
responding to the opercular hones of fishes, where, according to his theory,
they have attained the highest degree of development.

HEARING. 313

we meet with a concha, or external ear, distinctly marked;
and the utility of this part, in catching and coliectinj^ the
sonorous undulations of the air, may be inferred from the
circumstance, that a large and very moveable concha is ge-
nerally attended with great- acuteness of hearing. This is
more particularly the case with feeble and timid quadrupeds,
as the hare and rabbit, 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 contrary, 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
tympanum.

The Cetacea, being strictly inhabitants of the water, have
no external ear; and the passage leading to the tympanum is
a narrow and winding 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 en-
trance of any quantity of water.* It is apparently with the
same design that in the Seal the passage makes a circular
turn; and that, in the Ornithorhyncus paradoxus, it winds
round the temporal bone, and has its external orifice at a
great distance from the vestibule. The internal parts of the
organ of hearing in the Whale, and other cetacea, are en-
closed in a bone of extraordinary 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 ca-
vity, formed by the adjacent bones. The cochlea is less de-
veloped than in quadrupeds, for it only takes one turn and
a half, instead of two and a half. The existence of the se-

* It is probable that in these animals the principal channel by which
sounds reach the internal organ is the Eustachian tube.

Vol. II. 40

314 THE SENSORIAL FUNCTIONS.

micircular 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 Soy^ex fodiens, or water-shrew, are fur-
nished with a valve, composed of a double membrane, capa-
ble of accurately 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 fmd, indeed, the same provident care displayed in
this as in every other department of the animal economy:
every part, however minute, of the organ of this important
sense, being expressly adapted, in every species, to the par-
ticular circumstances of their situation, and to that degree of
acuteness of perception, which is best suited to their respec-
tive wants and powers of gratification.!

• GeofTroy St. Hllaire; Memoires du Museum, i. 305.

f The Comparative Physiology of 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, feeling's, desires and passions, forms a natural se-
quel to that of Hearing-; but Sir Charles Bell having- announced his intention
of inti'oducing- it in his Treatise on the Hand, I have abstained from enter-
ing into tliis extensive subject.

( 315 )

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 instruc-
tive than the physiology of Vision, the most refined and
most admirable of all our senses. However well we may
be acquainted with the construction of any particular part of
the animal frame, it is evident that we can never form a
correct estimate of the excellence 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 accom-
plished. Innumerable are the works of creation, the art and
contrivance of which we are incompetent to understand,
because we perceive only the ultimate effects, and remain ig-
norant 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 en-
gaged in the perusal of a volume, written in some unknown
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 sur-
passing 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 deciphering any continued portion of the dis-
course, we find it characterized by a perfection of style, and
grandeur of conception, whicli at once reveal a master's hand.,

316 THE SENSORIAL FUNCTIONS.

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 suhject which now claims
our attention we have hccn permitted to trace, for a consi-
derable extent, the continuity of the design, and the length-
ened series of means employed for carrying that design into
execution; and the view which is thus unfolded of the mag-
nificent scheme of creation is calculated to give us the most
sublime ideas of the wisdom, the power, and the bene-
volence OF God.

On none of the works of the Creator, which we are per-
mitted to behold, have the characters of intention been more
deeply and legibly engraved than in the organ of vision, where
the relation of every part to the efifect 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 comprehen-
sion. 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 re-
fined optical instrument, the perfection of which can never be
fully appreciated until we have instituted such a comparison;
and the most profound scientific investigations of the anato-
my and physiology of the eye concur in showing that the
whole of its structure is most accurately and skilfully adapt-
ed to the physical laws of light, and that all its parts are
finished with that mathematical exactness which the preci-
sion of the effect requires, and which no human effort can
ever hope to approach, — far less to attain.

To the prosecution of this inquiry we are farther invited by
the consciousness of the incalculable 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 consider of how large
a portion of our sensitive and intellectual existence it is the
intermediate source. Not only has it given us extensive
command over the objects which surround us, and enabled
us to traverse and explore the most distant regions of the

VISION. 317

globe, but it has introduced us to the knowledge of the bo-
dies 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 confines of crea-
tion. 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, fur-
nish the principal means of developing the powers of the un-
derstanding: it is to this class of perceptions that the philoso-
pher resorts for the most apt and perspicuous illustrations 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 posses-
sor a knowledge of the presence, situation, and colour of
external and distant objects, by means of the light which
those objects are continually sending off, either spontane-
ously, or by reflection from other bodies. It would appear
that there is only one part of the nervous system so pecu-
liarly organized as to be capable of being affected by lumi-
nous rays, and conveying 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 extre-
mities of the optic nerves^ establishing an immediate com-
munication 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 per-
ception 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, however, 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, produce its impression exclusively

318 THE SENSORIAL FUNCTIONS.

on some particular 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 ac-
tions would interfere with one another, and only a confused
impression would result. The objects in a room, for exam-
ple, 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 paper, furnishes no means of distinguish-
ing 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
tlicse objects.

§ 2. Modes of accomjjlishing 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 concentra-
tion 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 exclusion of all other rays; and also
to contrive that the points of the retina, so illuminated, shall
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 deli-
neation, in miniature, of the external scene.

As it is a fundamental law in optics that the rays of light,
while tliey arc transmitted through the same medium, pro-
ceed in straiglit lines, the simplest mode of accomplishing
the proposed end would be to admit into the eye, and con-
vey 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 exclude all other rays. For car-

VISION.

319

rying this design into effect we have the choice of two me-
thods, 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 re-
spective 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 convex hemi-
sphere; 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 arrangement will be understood by
reference to Fig. 406, which represents
a section of the whole organ: (t, t,) being
the tubes disposed in radii every where
perpendicular to the convex hemispheri-
cal 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 themselves, the same rela-
tive situation as the external objects, from which they ori-
ginally proceeded, and which they will accordingly faith-
fully represent.

The second method, which is nearly the inverse 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
hemispherical surface. The mode in which this arrange-
ment is calculated to answer the intended purpose will be
easily understood by conceiving a chamber (as represented
in Fig. 407,) into 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

320

THE SENSORIAL FUNCTIONS.

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 whicli 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,
througli which they are admitted.

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One inconvenience attending the limiting of the illumina-
tion of each point of the wall to that of a single ray, in the
mode last pointed out, is that the image produced must ne-
cessarily 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 interfe-
rence of adjacent rays; for all the lines would be spread out,
the outlines shaded off, and the whole picture confused.

The only mode by which distinctness of image can be ob-
tained with increased ilhimination, is to collect into one
poin^ a great number of rays proceeding from the corre-
sponding point of the object to be represented. Such a col-
lection of rays proceeding from any point, is termed, in the
language of optics, a pencil of rays; and the point into

VISION.

321

niilch Ihey arc collected is called n 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 ohject to that focus, must be dejlected, or bent from
their rectilineal course. This effect may be produced b\-
refractio7i, which takes place according to another optical
law; a law of which the following is the exposition.

It is only when the medium which the rays are traversing
is of uniform density tiiat their course is constantly recti-
lineal. If the density change, or U the rnys pass obliquely
from one medium into another of a difierent density, they
are refracted; each ray being deflected towards a line situ-
ated 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. 40S, striking
obliquely 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

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perpendicularly to the surface of the medium, at the point s,
and within that medium. When the ray arrives at t, and
meets the posterior surface of the dense medium, passing
thence into one that is less dense, it is again refracted ac-
cording to the same law; that is, it inclines towards the per-
pendicular line T I, drawn from t, within the denser me-
dium, and describes the new course t u instead of t v. The
amount of the deflection corresponds to the degree of ob-
VoL. II. 41

THE SENSORIAL FUXCTIO^fS.

liquity of the ray to the surface which refracts 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 refrac-
tion 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 per-
pendicular line to that 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 xis;
that is, shall lie on the other side of s t. The same rule ap-
plies 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 aj)ply 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, proceed-
ing from a distant object, accurately to a focus. We shall

409

•B

suppose the object in question to be very remote, so that
the rays composing the pencil may be considered as being
parallel to each other; for at great distances their actual de-

VISION. 32-3

viation from strict parallelism is wholly insensible; and let 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) wliich 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 refrac-
tion w^ill be effected by a slight degree of obliquity in the
medium at the points (h and k) where those rays meet it.
In proportion 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 required, in order to bring them to the same
focus.

The convergence of these rays, after they have passed
this first surface, may be farther increased 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. This last case is il-
lustrated in the figure, where m, n, o, p, q, represent the in-
clinations of the surfaces of a rarer medium, calculated to
increase 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
surface, which, in the present case, approaches very nearly
to that of a sphere. Hence by giving these refractive me-
dia spherical surfaces, we adapt them, with tolerable exact-
ness, 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 pro-
ducing the desired efiects. Such an instrument is termed

324

THE SENSORIAL FUNCTIONS.

a double convex lens; anil it has the property of collecting
into a focus rays proccedinj; from distant points."^

410

Ilavinii: obtained this instniincMil, wc mav now venture
to enlarge the a])ertui'e tlii-oni;h wlilch the light was admit-
ted into our dark chamber, and fit into the apertivrc a dou-
ble convex lens. Vv'e hnve thus constructed the well known
optical instrument called the Camera Ohscura, in which
the imaj];es of external objects are formed upon a white sur-
face of paper, or a 5cnii-transj)arent ])latc of glass; and these
images must evidently bo in an inverted position with re-
•pect to the actual objects which they represent.

Such is precisely the construction of the eye, which is, to

• The refraction by spherical surfiiccs docs not, strictly speakint^, unite x
pencil of pnrallel or divergent rays into a muthematicul point, or focus; for
in reality the rays wiiich are near the central line are made to converg-e to a
point a little more distant than that to which tiie remoter rays converg-e: an
•ffect which 1 have endeavoured to ilUistrate by the diagram Fig-. 411} where,

in order to render it obvious to tlie eye, the disparity is much exaj^fgerated.
But, on ordiuMiy occiisions, where great nicety is not required, this differ-
ence in the degree of convcrgiMicc l)et\vcen the central rays and tliose near
the circumference of titc lens, giving r'.sc to \vi;at is termed the Aberration
ef Sphericity, is too small to attract notice.

VISION.

325

all intents, a camera obscura: for in both these instrumenls,
the objects, the principles of consti^ction, and the mode of
operation are exactly the same; and tlie only difference is,
that the former is an infinitely more perfect instrument thart
the latter can ever be rendered by the utmost efforts of hu-
man 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 remote in proportion as the rays are more divergent
or, in other words, proceed from nearer objects. This is illustrated by Fi-
gures 412, 413, and 414j to which I shall again have occasion to refer in the
sequel.

§ 3. Structure of the Eye,

One of the many points of superiority which the eye pos-
sesses 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 the camera obscura, a considerable
portion of the image would have been indistinct. This sphe-
rical 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

326

THE SENSORIAL FUNCTIONS.

the convergence of the rays, are termed the Humours of
the Eye. There are in tliis 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 liorizontal section of the right eye, exhibits distinctly
all these parts.

Tiie outermost coat (s,) which is termed the Sclerotica, is
exceedingly firm and dense, and gives to the globe of the
eye the mechanical support it requires for the performance
of its delicate functions. It is perforated behind by the op-
tic nerve (o,) which passes onwards to be expanded into the
retina (ii.) The sclerotica docs not extend farther than about
four-fifths of the globe of the eye; its place in front being
supplied 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

VISION. 327

(x,) which is chiefly made up of a tissue of 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 Pigrnenliim nigritvi, or black pigment.
Its use is to absorb all the light which may happen to be ir-
regularly scattered througli the eye, in consequence of re-
flection from different quarters; and it serves, theiefore, the
same purpose as the black paint with which the inside of
optical instruments, sucli as telescopes, microscopes, and ca-
meraB obscurae, is darkened. Within the pigmentum nigrum,
and almost in immediate contact with it,* the Retina (r) is
expanded, forming 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 vilreous humour (v,) which has the appearance of
a pellucid and elastic jelly, contained in an exceedingly de-
licate texture of cellular substance. The Crystalline hu-
motcr, (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, immedi-
ately in front of the vitreous humour, which is there hol-
low^ed to receive it. The space which intervenes between
the lens and the cornea is filled with a watery secretion (a,)
called the tdqueous /tumour. This space is divided into an
anterior and a posterior 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 choroid coat, by a white
elastic ring (q,) called the Ciliary Ligament. The poste-
rior 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 contractile fibres;
the one, forming concentric circles; the other, disposed like
radii between the outer and inner margin.! When the

* Between the pigmentum and the retina there is found a very fine mem-
brane, discovered by Dr. Jacobson: its use has not been ascertained,
.f See Fig. 47, vol. i.p. 105.

323

THE SENSORIAL FUNCTIONS.

former act, the pupil is contracted: when the latter act, the
breadth of tlie iris is diminished, and the pu])il is, of course,
dilated. By varying the size of the pupil the quantity of
li^ht admitted into tlie interior of the eye is regidated, and
accommodated to tlie sensibility of the retina. When the
intensity of the light would be injurious to that highly deli-
cate organ, the pupil is instantly contracted, so as to exclude
the greater portion; and, on the contrary, when the light is
too leeblc, it is dihUed, in order to admit as large a quantity
as possible. The iris also serves to intercept such rays as
woukl 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 engaging 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 appendages.

The purposes answered by the parts exterior to the eye-
ball are chiefly those of motion, of lubrication, and of pro-
tection.

As it is the central part of the retina which is endowed
with tlic greatest share of sensibility, it is necessary that the
images of the objects to be viewed should be made to fail
on this part; and, consequently, that the eye should be ca-
pable of having its axis instantly directed to those objects,
wherever they may be situated. Hence, muscles are pro-
vided within the orbits, for effecting the motions of the eye-
ball. A view of these muscles, with their attachments 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 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

VISION. 329

fore part of the sclerotic coat. Three of these arc 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, surround the optic nerve and the eye-hall, forming
four longitudinal bands; one (a) being situated above for the
purpose of turning the eye upwards; a second (c,) situated
below, for turning it downwards; and the two others, on
either side, for performing its lateral motions to the riglit or
left. The cavity of the orbits being considera])ly larger tlian
the eye-ball, the intervening space, especially at the back
part, is filled up by fat, which serves as a soft cushion for
its protection, and for enabling it to roll freely in all direc-
tions.

J3esides these straight muscles, there are also two others
(s and i) termed the ohlique muscles, which give the eye-
ball a certain degree of rotation on its axis. When these act
in conjunction, they draw the eye forwards, and serve as an-
tagonists to the combined power of the straight muscles.
The upper oblique muscle (s) is remarkable 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 effect pro-
duced by ^he 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, afibrds one of the most palpable
instances that can be adduced of express contrivance; for in
no other situation could the muscle have been so conve-
niently lodged as within the eye-ball; and in no other way
could its tendon have been made to pull in a direction con-
trary 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 surrounding integuments by a
membrane, termed the Conjunctiva* This membrane, on

* An abundant supply of nerves has been bestowed on this membrane for
the purpose of conferring upon it that exquisite degree of sensibility which
Vol. II. 42

330

THE SEKSOllIAL FUNCTIONS.

arriving at the base of the eye-lids, is folded forwards so as
to line their inner surfaces, and to be continuous with the
skin whicli covers their outer sides. The surfaces of the
conjunctiva and of tiie cornea are kept constantly moist by
the tears, which are as constantly secreted by the Lacry-
mal inlands. Each i»;hind, (as shown at l, Fig. 417,) is si-

tuated above the eye, in a hollow of the orbit, and the ducts
(d) proceeding from it open upon the inner side of the up-
per eye-lid (e.) This fluid, the uses of which ar(f obviously
to wash away dust, or other irritating substances which may
happen to get introduced, is distributed over the outer sur-
face of the eye by means of the eye-lids. Each lid is sup-
ported by an clastic plate of cartilage, shaped like a cres-
cent, 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

was necessary to give immediate warning of the slightest clanger to so im-
portant an organ as the eye from the intrusion of foreign bodies. That this
is the intention is apparent from the fact that the internal parts of the eye
possess but little sensibility compared with the external surface.
• Sec Fig. 46, vol. i. p. 105.

VISION. * 331

upper straight muscle of the eye-ball. 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 substances,
such as hairs, dust, or perspiration, and also in shading the
eye from too strong impressions of light, is sufiiciently ap-
parent. 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 arc 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 separated by a round projecting
body (c,) connected with a fold of the conjunctiva, and
termed the lacryrnal 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 be-
low (at n) in the cavity of the nostrils, into which the tears
are ultimately conducted. When the secretion of the tears
is too abundant to be carried off by this channel, they over-
flow upon the cheeks; but when the quantity is not exces-
sive, the tendency to flow over the eye-lid is checked by an
oily secretion proceeding from a row of minute glands, si-
tuated at the edge of the eye-lids, and termed the Meibo-
mian glands.

The eye-brows arc a fiU'ther 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 ciiancc to
fall from above.

Excepting in front, where tlie eyes are covered and pro-
tected jjy the eye-lids, these important organs are on all

332 THE SENSORIAL FUNCTIONS.

sides effectually guarded from injury by Ijeing contained in
a hollow bony socket, termed the orbit, and composed of
seven portions of bone. Tliese seven elements may be re-
cognised in the skulls of all the mammalia, and perhaps also
in those of all other vertebrated animals, affording a remark-
able illustration of the unity of the plans of nature in the
construction of the animal fabric.

§ 4. Physiologi/ of perfect Vision.

The rays of light, proceeding from a distant object, strike
upon the convex surface of the cornea, which being of great-
er density than the air, refracts them, and makes them con-
verge towards a distant focus. This effect, however, is in
part counteracted on their emergence from the concave pos-
terior surface of the cornea, when the rays enter into the
aqueous humour. On the whole, however, they are refract-
ed, 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 en-
tered 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 refractions,
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 refrac-
tions 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 humours of the eye compared with one ano-
ther.

VISION. 333

The accurate converp;cnce of all the rays of light, which
enter through the pupil, to their respective foci on the reti-
na, is necessary for the perfection of the images there formed;
but, for the complete attainment of this end, various nice ad-
justments are still requisite.

In the first place, the Aberration of Sphericity * which
is a consequence of the geometrical law of refraction, intro-
duces 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. Na-
ture, 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 spheri-
cal 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 increase in their densi-
ty, as they succeed one another from the surface to the cen-
tre. The refracting power, being proportional to the
density, is thus greatest at the centre, and diminishes as we
recede from that centre. This admirable adjustment exact-
ly corrects the deficiency of refraction, which always takes
place in the central portions of a lens composed of a mate-
rial 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 diffe-
rent distances, and which may be called the Merrations of

* See Fig. 411, and the note referring to it, p. 324.

■j- Sir David Brewster has ascertained that the variations of density pro-
ducing- the doubly refracting structure, in the crystalline lens of fishes, arc
related, not to the centre of the lens, but to the diameter which forms the
axis of vision: an arrangement peculiarly adapted for correcting the splicri-
cal aberrations. Philos. Trans, for 1810, p. 317.

334 THE SENSORIAL FUNCTIONS.

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 arc parallel to each other; the ac-
tual deviation from parallelism being quite insensible. But
if the same object be brought nearer to the eye, the diver-
gence 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.* For every distance of the
object there 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 be-
fore 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
confusion in thd image.

It is found, however, that the eye has a power of accommo-
dating itself to the distinct vision of objects at a great variety
of distances, according as the attention of the mind is di-
rected 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 controversy. The increase of the re-
fracting power of the eye necessary to adapt it to the vision
of near objects is evidently the result of a muscular eifort,
of which we are distinctly conscious when we accurately at-
tend to the accompanying sensations. The researches of

• This is illustrated by Fig. 412, 413, and 414; the first of which shows
the rapid convergence of rays proceeding from a veiy distant object, and
which may be considered as parallel. The second shows that divergent i-ays
unite at a more distint focus; and the third, that the focus is more distant the
greater the divergence.

VISION. 335

Dr. Young have rendered 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 immersed 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, namely, the violet and the blue, are
brought to a nearer focus, than those which are less refran-
gible, which are 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 pro-
duced by refraction, shading off their outlines with various
colours, even when the object itself is colourless. This de-
fect, which is incident to the power of a simple lens, and
which is termed the Chromatic Aberration, is remedied
almost perfectly in the eye, by the nice adjustment of the
powers of the different refracting media, which the rays of
light have to traverse before they arrive at the retina, pro-
ducing what is called an achromatic combination;^' and it is
found that the eye, though not an absolutely achromatic in-
strument, 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 appended to the
optic nerve, is to form inverted images of external objects
on the retina, which, as we have seen, is the expanded ex-

* For the exposition of the principles on which these achromatic combi-
nations of lenses correct this source of aberration, I must refer to works
which treat professedly on Optics.

f For the rectification of this error we are indebted to Ur. Young.

336 THE SENSORIAL FUNCTIONS.

tremity of that nerve. That this cflcct is actually produced,
may be easily shown by direct observation; for if the scle-
rotic and choroid coats be carefully dissected off from the
posterior part of the eye of an ox, or any other large quad-
ruped, leaving ordy the retina, and tlic eye so prepared be
j)laced 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 in-
verted j)osition, on the retina.

Few spectacles are more calculated to raise our admira-
tion than this delicate picture, which nature has, with such
exquisite art, and with the finest touches of her pencil,
spread over the smooth canvass of this subtle nerve; a pic-
ture, which, though scarcely occupying a space of half an
inch in diameter, contains the delineation 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 interference, 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 collected on its
narrow curtain into a luminous focus of inconceivable mi-
nuteness; and yet this almost infinitesimal point shall be
sufllcient to convey to the mind, through the medium of the
optic nerve and brain, a knowledge of the existence and po-
sition of the far distant luminary, from which that light has
emanated. How infinitely 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. 337

§ 5. Comparative Physiology of Vision.

In the formation of every part of the animal machinery
we may generally discern the predominance of the law of
gradation; but this law is more especially observed in those
organs which exhibit, in-their most perfect state, the great-
est complication and refinement of structure: for on follow-
ing 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 con-
structions, successive degrees of refinement, accompanied by
corresponding extensions of power. So gradual is the pro-
gress of this development, that it is not easy to determine
the point where the faculty of vision, properly so called, be-
gins to be exercised, or where the first rudiment of its or-
gan begins to appear.

Indications of a certain degree of sensibility to light are
afibrded 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 they feel the influ-
ence of this agent; for, when confined in a vessel, they al-
ways place themselves, by preference, on the side where
there is the strongest light.* The Veretillum cynomo-
rium, on the other hand, seeks the darkest places, and con-
tracts itself the moment it is exposed to light.t In a per-
fectly calm sea, the Medusae 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

* Such is the uniform report of Trembley, Baker, Bonnet, Goeze, Ha-
now, RcEsel, and Schseffer.

f Rapp; Nov. Act. Acad. Nat. Cur. of Bonn, xlv. 645.

Vol. II. 43

33S THE SENSORIAL FUNCTIONS.

their bodies has come into contact with the atmosphere.*
But, in all these instances a doubt may arise whether the
observed actions be not prompted by the mere sensation of
warmth excited by 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 no-
ticed them in several of the more highly organized Infuso-
ria, belonging to the 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. Planarix
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 torva, of clusters of black
grains, situated underneath the white or transparent integu-
ment.! The eyes of the Nais pi^oboscidea are composed, ac-
cording to Gruithuisen, simply of a small mass of black pig-
ment, attached to the extremity of the optic nerve ;| and or-
gans apparently 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 whether the visual organs
are constructed with any other intention than merely to con-
vey general sensations of light, without exciting distinct per-
ceptions of the objects themselves from which the light pro-
ceeds; this latter purpose requiring, as we have seen, a spe-
cial optical apparatus of some degree of complexity. An ap-
proach to the formation of a crystalline lens takes place in
the genus Eiuiice of Cuvier, {Lycoris, Sav.,) which, from the

• Grant; Edin. Journal of Science: No. 20.

f Nov. Act Acad. Nat. Cur. of Bonn, xiii. 712. See also the Memoir of
Dugt'S, entitled *' Recherches sur I'Organisation etles Mocui'sdes Planaires,"
in the Annales dcs Sc. Nat. xv. 148.

i Nov. Act. Acad. N.it. Cur. of Bonn, xi. 242.

VISION. 339

account given by Professor MuUer,* has four eyes, situated on
the hinder part of the head, and covered with the epidermis,
but containing 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 project-
ing a little from the surface of the integument: they present
externally a convex, and perfectly transparent cornea; while
internally, they are prolonged into cylindrical tubes, con-
taining a black pigment;! 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 MoUusca; but various species of Gasteropoda have

418

organs which appear to exercise this sense, situated sometimes
at the base, sometimes at the middle, and frequently at the ex-
tremity of the tentacula. Of the latter we have examples in
the common slug and snail, where 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,J the eye
of the Helix pomatia, represented at e, (Fig. 41S;) is situ-
ated a little to one side of the rounded extremity, or papilla
(p,) of the tentaculum, and is attached to an oval bulb of a

• Annales des Sciences Naturelles, xxii. 23,

f Meckel, Archiv. fiir Anatomic und Physiologic; 1824, p. 301.

t Annales des Sciences Naturelles; xxii. 12.

340 THE SENSORIAL FUNCTIONS.

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 represented, in this figure, as half retracted within the tu-
bular sheath of the tentaculum (s s;) but it can exercise its
proper function only when fully exposed, by the complete
unfolding and protrusion of the tentaculum. This eye con-
tains, within its choroid coat, a semi-fluid and perfectly
transparent substance, filling the whole of the globe; and
JNIuUer also discovered at the anterior part, another transpa-
rent body, having the shape of a lens.* A structure very
similar to this was found to exist in the eye of the Murex
iritonis, with the addition of a distinct iris, perforated so as
to form a pupil; a part which had also been observed, to-
gether with a crystalline lens of very large size, in the Va-
luta cymbium, by De Blainville.t Thus, the visual organs
of these Gasteropoda appear to possess every requisite for dis-
tinct vision, properly so called. Experiments are said to
have been recently made, both by Leuchs, and by Steifen-
sand,t 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 difficulty, both from "the mi-
nuteness of their parts and from the complication of their
structure; so that notwithstanding the light which has re-
cently been thrown on this interesting subject by the patient
and laborious researches of entomologists, great obscurity
still prevails with regard to the mode in which these dimi-

• MtiUer 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.

■j- Principes d'Anatomic Comparce, i. 445.

% Quoted by MuUer; ibid. p. 16. These results also corroborate the testi-
mony of Swammerdam, who states that he had obtained proofs that the snail
could see by means of these organs.

VISION. 341

nutive beings exercise the sense of vision. Four descrip-
tions of visual organs are met with in the class of Articu-
lated Animals; the first are the simple eyes, or siemmata,
as they are termed, which appear as lucid spots, resembling
those we have noticed in the higher orders of Annelida; the
second, are the conglomcrale eyes, which consist of clus-
ters or aggregations of simple eyes; the third, are the com-
"pound eyes, which are formed of a vast assemblage of small
tubes, each having its respective 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 co-
vered 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 complete metamorphosis have only stem-
mata; but those which are subjected only to a partial change
of form, as the Orthoptera, the Hemiptera, and the aquatic
Neuroptera, have compound as well as simple eyes. Perfect
insects, with the few exceptions above noticed, have always
compound eyes, generally two in number, placed on the
sides of the head: and they are often accompanied by stem-
mata 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 hum.our, and a choroid
coat, with its accompanying black pigment; the whole being
covered externally by a convex cornea. The stemmata of

* This is the case, however, with the genus Claviger, among the Coleoptera
Braula (Nltzch) among Diptera, and also some of the species of Pupipara,
Nyderibia, and 31elophagus, which are all parasitic insects: there are also five
species of ants, whose neuters have no eyes. (Muller, Annales dcs Sc. Nat.
xvii. 366,)

342

THE SENSORIAL FUNCTIONS.

a caterpillar, which has eight of these eyes, are shown in Fig.
419, connected together by a circular choroid membrane

o

o o

e o o

o o o o

o o e o o

O o o e o o

O o o o o O o

(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 facilities for dissection,
which are not met with among proper insects. Their num-
ber 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 tunensis, dissected so as to dis-
play its internal parts; in which are seen the cornea (c,) de-
rived from an extension of the integument (i;) the dense
spherical crystalline lens (l;) the choroid coat with its pig-
ment (x,*) forming a wide opening, or pupil; the vitreous hu-
mour (v,) covered behind 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 Myriapo-
da: 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 terrestriSi or common
Millepede, these eyes, amounting to 28, form a triangle, be-

• Marcel dc Serres states, that some of the stemmata of the insects which
he examined contain a thin choroid, having a silvery lustre, as if intended as
a reflector of the light which falls on it.

VISION. 343

ing disposed in seven rows, the number in each regularly
diminishing from the base to the apex; an arrangement
which is shown in Fig. 421.*

The compound eyes of insects arc formed of a vast num-
ber of separate cylinders or elongated cones,t closely packed
together on the surface of a central bulb, which may be con-
sidered as a part of the optic nerve; while their united bases
or outer extremities constitute the surface of a hemispherical
convexity, which often occupies 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 uni-
form 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 sur-
face 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 Melolontha, when highly magnified, is shown
in Fig. 422.% The internal structure of these eyes will be
best understood from the section of that of the Lihelhda
vulgata, or gray Dragon-fly, shown in Fig. 424, aided by
the highly magnified views of smaller portions given in the
succeeding figures, in all of which the same letters of refe-
rence are used to indicate the same objects.^ The whole
outer layer (c c) of the compound eye may be considered

• Kirby and Spence's Introduction, Sec, ill. 494.

f The number of these cones or cylinders which compose the entire org^n
differs much in different species. In the ant, there are only 50; in a Scara-
baeus, 3180; in the Bomhyxmori^ 6236; in the house-fly (Musca domestica,)
8000; in the Melolontha vulgaris, 8820; in the Phalena cossus, 11,300; in the
Libellula, 12,544; in the Papilio, 17,325; and in the Mordella, 25,088.

^ In the Fhaknae, and other tribes, they are arranged in squares (as shown
in Fig-. 423,) instead of hexagons, and frequently much less regularly; as
must necessarily happen, in many parts, from the curvature of the spherical
surface.

§ 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 Science*
Naturelles, xx. 341.

344

THE SENSORIAL FUNCTIONS.

as corresponding to the cornea: each separate division of
which has heen termed a Corneicle, heing composed of a
horn}^ and perfectly transparent material. Each corneule

422

« • # .# 1i 4
|# # # • #

# # • r

423

424

^^A

J^O

^

1

^

«y-^MMJW»

(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 concave; and
the concavity of the latter being smaller than the con-

vexity of the former, its optical effect is that of a menis-
cus, or concavo-convex lens, with power of converging to
a distant focus the rays of light which traverse it. With-
in these corneules there is extended a layer of an opaque

VISION. 345

black pigment (x,) probably connected with a choroid
coat, which, from the delicacy of its texture, has hitherto
escaped observation. There exists opposite to the centre,
or axis of each corneule, a circular perforation (p,) which
performs the functions of a pupil.* Duges states, in-
deed, that he has witnessed in this part movements of con-
traction and dilatation, like those of the iris in vertebrated
animals. He has likewise found that tliere is a small space
(a) intervening between the extremity of each corneule and
the iris, and filled with an aqueous humour. The compart-
ments formed by the substance of the choroid (x) are conti-
nued inwards towards the centre of the general hemisphere,
the 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 vi-
treous humour. 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, immediately 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 ganglion.^

* This pupillary aperture was discovered by Muller, and had eluded all
the efforts of former observers to detect it; and it was accordingly the pre-
vailing notion that the black pigment lined the whole surface of the cornea,
and interposed an insuperable barrier to the passage of light beyond the
cornea. It was evidently 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 e^h of these constituent parts
occur in different species of insects. Very frequently the vitreous humour
(v,) instead of forming an elongated cylinder, has the shape of a short cone,
terminating in a fine point, as shown in Fig. 426. Straus Durckheim ap-
pears 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, which latter part he considered as properly the crystalline lens.
In his elaborate work on the anatomy of the Melolontha, he describes the

Vol. II. 44

346 THE SENSORIAL FUNCTIONS.

It thus appears that each of the constituent eyes, which
compose this vast agji;regate, consists of a simple tube, fur-
nished with all the elements requisite for distinct vision,
and capable of receivinsj; impressions from objects situated
in the direction of the axis of the tube. The rays tra-
versing adjacent corneules are prevented from mixing them-
selves with those which arc proper to each tube by the in-
terposition of the black pigment, which completely surrounds
the transparent cylinders, and intercepts all lateral or scat-
tered light. Thus has nature supplied the want of mobility
in Ihc eyes of insects, by the vast multiplication of their
number, and by providing, as it were, a separate eye for
each separate point which was to he viewed; and thus has
she realized the hypothetical arrangement, which suggested
itself in the outset of our inquiries, while examining all the
possible modes of eficcting this object.

This mode of vision is probably assisted by the converging
powers of each corneule, although in parts which are so mi-
nute it is hardly possible 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. Wollaston, relative to the eye of the Aslacus
Jhiviatilis, or cray-fish, where the length of each compo-
nent tube is short, compared with that of the Libellula. On
measuring accurately the focal distance of one of the cor-
neules. Dr. Wollaston ascertained that it corresponds with

filaments (f) of the optic nerve, in tlieir progress inwards, as passing through
a second membrane (k, Fig. 428,) which he denominates the common cho-
roid, and afterwards uniting to form an expanded layer, or more general re-
tina (h,) whence proceed a small number of short but thick nervous co-
lumns (>',) still converging towards the large central ganglion (g,) in which
they terminate. The use he ascribes to this second clioroid is to intercept
the light, which, in so diminutive an organ, might otherwise penetrate to
the general retina and produce confusion, or injurious irritation. The co-
lour 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 transvci-sc layer of pigment between the corneule and the vitre-
ous humour is absent.

VISION. 347

great exactness to the length of the tul)e attached to it; so
that an image of an external ohject 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 compound, both of which
are generally possessed by the higher orders of winged in-
sects. 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 objects; but it must be confessed that
the experiments made on this subject have not, hitherto, led
to any conclusive results. Duges found, in his trials, that
after the stemmata had been covered, vision remained appa-
rently as perfect as before, while, on the other hand, when
insects 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 co-
vered with an opaque varnish, the insect loses the power of
guiding its flight, and strikes against walls or other obsta-
cles: 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 tho
eyes of an insect be covered, it will seldom make any at-
tempt whatsoever to fly.

The eyes of insects, whether simple or compound, are
immoveably fixed in their situations; but the compound eyes
of the higher orders of the class of Crustacea, arc placed at
the ends of moveable pedicles, so as to admit of being turned
at pleasure towards the objects to be viewed.f This, how-

* This interesting fact was communicated to me by Captain Kater, who,
together with Mr. Children, assisted Dr. Wollaston in this examination.

f Latreille describes a species of Crab, found on the shores of the Medi-
terranean, having its eyes supported on a long jointed tube, consisting of
two articulations, which enables the animal to move them in various direc-
tions, like the arms of a telegraph.

348 THE SENSORIAL FUNCTIONS.

ever, Is not the case with the Entomostraca^ comprising
the various species of Monocidi, in which the two eyes are
brought so close to one another, as apparently to constitute
a sini!;lc organ, corresponding in its structure to the fourth
class of eyes already enumerated; that is, the separate lenses
it contains have a general envelope of a transparent mem-
brane, or cornea. Muscles are provided for moviiTg 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 tlie latter in the eye of the Cephalopoda;
for Sepix differ from all the tribes belonging to the inferior
orders of mollusca in having large and efficient eyes, con-
taining a hemispherical vitreous humour, placed immediate-
ly before a concave retina, and receiving in front a large
and higlily convex crystalline lens, which is soft at its exte-
rior, but rapidly increases in density, and contains a nucleus
of great hardness: there is also a pigmentum nigrum, and a
distinct iris, with a kidney-shaped pupil. This eye is re-
markable for the total absence of a cornea; the integuments
of the head being continued over tlic iris, and reflected over
the edges of the pupil, giving a covering to the external sur-
face of the lens: there is, of course, no chamber for contain-
ing an aqueous humour. The globe of the eye is nearly
spherical, but the sclerotica is double, leaving, at the poste-
rior part, between its two portions, a considerable space, oc-
cupied by the large ganglion of the optic nerve, with its nu-
merous filaments, which are embedded in a soft glandular
substance.*

The eyes of Fishes differ from those of sepiae 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,

• See Cuvier, sur lea Mollusques; Memoir sur Ic Poulpe, p. 37. In the
Octopus there are folds of the skin, which appear to be nutiments of eye-lids.

^

VISION. 349

however convex, would be inconsiderable; and the chief
agent for performing 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 sphe-
rical, and of great density. The circumstances are shown
in the section of the eye of the Ferch, 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 continually washed
by the water in which it is immersed, requires no provision
43Q of a secreted fluid for that purpose; and

there are consequently 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 oilice of a conjunc-
tiva. The eye retains its form by the support it receives
from the sclerotic coat, which is of extraordinary thickness
and density. In the Shay^k and the Skate the eye is sup-
ported from the bottom of the orbit, by a cartilaginous pe-
dicle, which enables it to turn as on a pivot, or lever.

Sir David Brewster has recently made an interesting ana-
lysis of the structure of the crystalline lens of the Cod, to
which he was led by noticing some remarkable optical ap-
pearances presented by thin layers of this substance when
transmitting polarized light. He found that the hard cen-
tral portion is composed of a succession of concentric, and
perfectly transparent, spheroidal lamina}, the surfaces of
which, though apparently smooth, have the same kind of
iridescence as mother-of-pearl, and arising from the same
cause; namely, the occurrence of regularly arranged lines,

• In this figure, as in the others, c is tlic cornea; l, the lens; v, the vitreous
humour; n, 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 envelops the optic nerve, is shaped like a horse-
shoe, is of a deep red colour, and highly vascular; its use is quite unknown.

350 THE SENSORIAL FUNCTIONS.

or strixJ* These lines, which mark the edges of the sepa-
rate fibres, composing each lamina, converge like meridians
from the equator to the two poles of the spheroid, as is
shown in Fig. 431. The fibres themselves arc not cylindri-

cal, 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 part
of an inch. The observation of another optical phenomenon,
of a still more delicate kind, led Sir David Brewster to the
farther discovery of the curious mode in which, (as is re-
presented 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
02,500,000,000. t

Some fishes, which frequent the depths of the ocean, be-
ing found at between three and four hundred fathoms below
the surface, to which it is impossible that any sensible quan-
tity of the light of day can penetrate, have, like nocturnal
quadrupeds, very large eyes. J In a few species, which

♦ Sec vol. i. p. 169.

f As far as his observations have extended, this denticulated structure ex-
ists 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 Ornithorhyncus; but
he has not been able to find it in any of the Mammalia, not even in the Ce-
iacea. (Phil. Trms. for 1833, p. 323.)

i See *' ObsciTations sur les Poissons recuciUis dans un Voyage aux lies
Baleares ct Pythiuses. Par M. Delaroche."

VISION. 351

dwell in the muddy banks of rivers, as the Csecilia, and
Muraena cxca, or blind eel, the eyes are quite rudimental,
and often nearly imperceptible; and in the Gasi7'ohranchus,
De Blainvillc 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 well as in wa-
ter, have eyes accommodated to these variable circumstances.
By the protrusion of the cornea, and the addition of an aque-
ous humour, they approach nearer to the spherical form than
the eyes of fishes; and the lens has a smaller refractive pow-
er, because the principal refraction is now performed by
the cornea and aqueous humour. Rudiments of eye-lids
are met with in the Salamander, but they are not of suffi-
cient 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, be-
hind 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 pre-
paring to detach itself, air insinuates itself underneath the
external membrane and renders it opaque: so that until this
operation is completed and an entire separation effected, the
serpent is rendered blind. Serpents have no proper eye-
lids; but the cornea is covered by a transparent integument,
which does not adhere to it.* Lizards have usually a sin-
gle 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

* It was the general opinion, until very lately, that serpents are unpro-
vided with any lacrymal apparatus; but a small lacrymal passage has been
recently discovered by Cloquet, leading from the space in the inner corner
of the eye, between the transparent integument and the cornea. This la-
crymal canal opens into the nasal cavity in venomous snakes, and into the
moutli in those that are not venomous.

352 THE SENSORIAL FUNCTIONS.

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 nicitans or third oye-lid.

Birds present a still fartlier development of all these
parts: tlieir eyes are of great size compared with the head,
as may be seen from the large portion of the skull wdiich is
occupied on each side by the orbits. The chief peculiari-
ties of the' internal structure of these organs are apparently
designed to accommodate them to vision through a very
rare medium, and to procure their ready adjustment to ob-
jects situated at very dillerent 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, aflbrds space for a larger quantity of aqueous
humour, and also for the removal of the lens to a greater
distance from the retina, whereby the vision of near ol)jccts
is facilitated, while at the same time, the refracting powers
are susceptible of great variation.

For the purpose of preserving the hemispherical 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 thickness.

There is a dark-coloured membrane, called the Marsu-
pium^ situated in the vitreous humour, the use of which is
unknown, though it appears to be of some importance, as it

VISION.

353

is found in almost every bird having extensive powers of
vision.* The comparative anatomy of the eye offers, in-
deed, a great number of special structures of which we do
not understand the design, and which I have therefore pur-
posely omitted to notice, as being foreign to the object of
this treatise.

In most birds the memhrana nictiians, 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 cor-
ner of the eye, with its loose edge nearly vertical: it is re-
presented at N, Fig. 434, covering half riie 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 de-
scending in a parallel course towards the optic nerve, where

433

434

^Zb

they terminate, by a semicircular edge, in a tubular tendon.
This tendon has no 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
jjyramidalis, and which arises from the lower and back part
of the sclerotica. Its tendon (t,) after passing through the

• It is shown at m, Fig. 433, wliich is a mag-nified 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.

Vol. II. 45

354 THE SENSORIAL FUNCTIONS.

channel above described, which has the effect of a pulley, is
conducted throuf^h a circular sheath, furnished by the scle-
rotica to the under part of the eye, and is inserted into the
lower portion of the loose edge of the nictitating membrane.
lly the united action of these two muscles, the former of
which serves merely to guide tlie 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.

Tlie 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 por-
tion of the whole sphere. This is the case very generally
among fislics, reptiles, and birds. Some exceptions, indeed,
occur in particular tribes of the first of these classes, as in
the Uranoscopus^ where the eyes are directed immediately
upwards; in the /?f/y and the Caliionymus, where their di-
rection is oblique; and in the Pleuroncctes, where there is
a remarkable want of symmetry between the right and left
sides of the bod}-, 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 are nocturnal and predacious,
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 be-
coming smaller, till at length the approximation of the ffelds
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.

VISION. 355.

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 cha-
racter of the animal and the nature of its pursuits. Herbi-
vorous animals, and especially those whose bulk is great, as
the Elephant^ the Rhinoceros ^\^i\ tlie Hippopotamus^ have
comparatively small eyes; for that of the elephant docs not
exceed two inches in diameter. The eye of the Whale is
not much more than the 200th part of the length of tlic body.
When the natural food of an animal is stationary and re-
quires no effort of pursuit, the eye is generally small, and
the sight less keen; while in the purely carnivorous tribes,
which are actively engaged in the chase of living prey, the
organ of vision is large and occupies a considerable 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 at 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 to the expression of the countenance a peculiar fe-
rocity.

While nature has thus bestowed great acuteness 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 impending danger. Thus, large
eyes are bestowed on the Rodentia and the Ruminantia.
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 ex-
ception 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 por-
tion of the choroid coat is highly glistening, and reflects a

356 THE SENSORIAL FUNCTIONS.

great quantity of coloured light: the object of this structure,
which is termed the Tapeiiim, 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 liair, which completely obstructs vi-
sion, and requires to be removed by the action of the subcu-
taneous muscles, in oi;der to enable the animal to derive any
advantage from its eyes. These organs in the Mole are still
more remarkably deficient in their development, 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 earth are
readily transmitted to its ears. The Mus typhlus,ov 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 ani-
mal. The whole side of the head is covered with a conti-
nuous integument of uniform thickness, and equally over-
spread with a thick velvetty hair. It is only after removing
the skin that a black spot is discovered on each side, of ex-
ceedingly small size, and apparently the mere imperfect ru-
diment of an eye, and totally 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 accommodating the refractive power of that
organ to this variation of circumstances. Accordingly, it is
found that in the Seal, and other amphibious tribes, the

• Magcndie asserts that the mole has no optic nei*ve; but G. St. Hilaire
and Carus recognise the existence of a very slender nci*vous filament, arising
from tlic brain, and distributed to the eye of that animal.

VISION. 357

structure of the eye approaches to that of fishes, the lens be-
ing denser and more convex than usual, the cornea thin and
yielding, and both the anterior and posterior segments of
the sclerotic thick and firm; but the middle circle is very
thin and flexible, admitting of the ready separation or ap-
proximating of the other portions, so as to elongate or con-
tract the axis of the eye;" just as a telescope 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 mus-
cles. 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.

( 35S )

CHAPTER VII.

PERCEPTION.

The object of nature in establishing the organizations we
have been reviewing is to produce certain modified impres-
sions on the extremities of particular nervous filaments pro-
vided to receive them; but these impressions constitute only
the commencement of the series of corporeal 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 physical change must
take place, before the resulting 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 Sensorium.. Abundant proofs
exist that all the physical changes here referred to really oc-
cur, and, also, that they occur in this order of succession:
for they are invariably found to be dependent on the healthy
state, not only of the nerve, but, also, of the brain; thus, the
destruction, 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 slightest pressure made on the sensorium itself.

Although the corporeal or physical change taking place in
the sensorium, and the mental affection we term sensation,
arc linked together by some inscrutable bond of connexion,

• It is usual to designate the end of the nerve which is next to the senso-
rium, as the origin of tliat nerve; whereas, it should more properly be re-
garded as its termination; for the scries of changes which end in sensation
commence at the organ of sense, and arc thence propagated along the nerve
to the sensorium.

PERCEPTION. 359

they are, in their nature, as perfectly distinct as the subjects
in which they occur; that is, as mi?id is distinct from mat-
ter; and they cannot, therefore, be conceived by us as
having the slightest resemblance the one to the other. Yet
sensations 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 belief, or rather the irresistible conviction,
thus forced upon us, of the reality of these external agents,
which we conceive as constituting the material world, is
termed P accept ion.

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
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 culti-
vation the combined effi^rts of the physiologist and the me-
taphysician. For although our sensations are purely men-
tal affections, yet inasmuch as they are immediately depend-
ent 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 pro-
cesses, are amenable rather to the laws which regulate men-
tal than physical phenomena. It is certain, from innume-
rable 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 instruments
by which we see and hear, so the brain is the material in-
strument by which we retrace and combine ideas, and by
which we remember, we reason, we invent. Sudden pres-
sure 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 tex-
ture of the brain, recovery may take place; and immediately

360 THE SENSORIAL FUNCTIONS.

on the return of consciousness, the person awakes 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 wliich disorder more especially the
circulation in that organ, such as those inducing fever, or
inflammation, produce corresponding derangements of the
intellectual powers; modifying the laws of the association of
ideas, introducing confusion in the perceptions, irregularity
in the trains of thought, and incapacity of reasoning, and
leading to the infinitely diversified forms of mental halluci-
nation, delirium, or insanity. ICven the strongest minds are
subject to vicissitudes arising from slighter causes, which
aff'ect the general tone of the nervous system. Vain, in-
deed, was the boast of the ancient Stoics that the human
mind is independent of the body, and impenetrable to ex-
ternal influences. No mortal man, whatever may be the vi-
gour of his intellect, or the energy of his application, can
withstand the influence of impressions on his external
senses; for, if sufiiciently reiterated or intense, they will al-
ways have power, if not to engross his whole attention, at
least to interrupt the current of his thoughts, and direct them
into other channels. Nor is it necessary for producing this
efiect that cannon should thunder in his ears; the mere rat-
tling of a window, or the creaking of a hinge will often be
sufficient to disturb his philosophical meditations, and disse-
ver the whole chain of his ideas. . "Marvel not," says Pas-
cal, " that this profound 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 distinguishing truth from falsehood, you must first
chase away the insect, holding in thraldom that exalted rea-
son, and that gigantic intellect, which govern empires and
decide the destinies of mankind."

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

PERCEPTION. 3i)l

utter ignorance of the nature of those actions; and all our
knowledge on this suhject is limited to the changes which
we are conscious are going on in the mind. It is to these
mental changes, therefore, that our attention is now to be
directed.

In experiencing mere sensations, whatever be their as-
semblage 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 w^hich are derived from each separate sense,) when
we compare sensations or ideas with one another, when we
analyze a compound idea, 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 al-
though it might be supposed that the diversity in the nature
of our sensations would sufficiently indicate to us a corre-
sponding 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 them-
selves, 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 undulations 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 con-
tact with the auditory nerve. It is not difficult to prove
that the objects of perception are mere creations of the mind,
suggested, probably instinctively, by the accompanying sen-
sations, but having no real resemblance or correspondence
either with the impressions themselves, or with the agencies
Vol. II. 46

362 THE SENSORIAL FUNCTIONS.

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 cifcct on the retina, the sensation of vivid
light. That tiiis sensation is present in the mind we are
certain, because we are conscious of its existence: here there
can be no fallacy. But the perception of light, as a cause
of this sensation, being inseparably associated with such sen-
sation, 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 cer-
tain 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.

INIany other examples might be given of fallacious per-
ceptions, arising from impressions made in an unusual man-
ner on the nerves of the senses. One of the most remark-
able is the appearance of a flash of light from the transmis-
sion 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 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 communication be then made be-
tween 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 mechanical
impulse, such as a blow directed to the same part of the ner-
vous 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,

PERCEPTION. 363^

and making them communicate, when a pungent and disa-
greeable 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 w^hich 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
upon as being always exactly correspondent with the quali-
ties of the external agent which excites them.

Evidence to the same effect may also be gathered from
the consideration of the narrowness of those limits within
which all our senses are restricted. It requires a certain in-
tensity 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 referred to the sense of
touch, there are many which convey no perception of the
cause producing them. Thus, a slighter impression than
that which gives the feeling of resistance produces the sensa-
tion 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; al-
though we know that it is only in its degree that the exter-
nal cause of each of these sensations differs.

The only distinct notions we are capable of forming re-
specting Matter, are that it consists of certain powers of
attraction and repulsion, occupying certain portions of space,
and capable of moving in space; and that its parts thereby
assume different relative positions or configurations. But of
mind, our knowledge is more extensive and more precise,
because we are conscious of its existence, and of many of
its operations, which arc comprised in the general term
thought. To assert that thought can be a property of mat

364 THE SENSORIAL FUNCTIONS.

ter, is to extend the meaning of the term matter to that with
which we cannot perceive 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 proposition that
thought is :{ function of the hrain. 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
these respective effects. That which sees and which thinks
is exclusively the mind; although it is by the instrumentali-
ty of its bodily organs that these changes take place. At-
tention to this fundamental distinction, which, although ob-
vious when explicitly pointed out, is often lost sight of in
ordinary discourse, will furnish a key to the solution of
many questions relating to perception, 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 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 incongruous elements, has the power of
giving them, as it were, cohesion, of comparing them with
one another, of uniting them into combinations, and of form-
ing them into ideas of external objects. All that nature
presents is an infinite number of particles, scattered in diffe-
rent parts of space; but out of these the mind forms indivi-
dual groups, to which she gives a unity of her own crea-
tion.

All our notions of material bodies involve that of space;
and we derive this fundamental idea from the peculiar sensa-
tions which attend the actions of our voluntary muscles.
These actions first give us the idea of our own bodies, of its
various parts, and of their figure and movements; and next
teach us the position, distances, magnitudes, and figures of

PERCEPTION. 365

adjacent objects. Combined with tbese ideas are the more
immediate perceptions of touch, arising from contact with
the skin, and especially with the fingers. All these percep-
tions, variously modified, make us acquainted with those
mechanical properties of bodies, which have been regarded
by many as primary or essential qualities. The perceptions
derived from the other senses can only add to the former
the ideas of partial, or secondary qualities, such as tempera-
ture, the peculiar actions which produce taste and smell, the
sounds conveyed from certain bodies, and, lastly, their visi-
ble appearances.

The picture formed on the retina by the refracting power
of the humours of the eye, is the source of all the perceptions
which belong to the sense of vision: but the visible appear-
ances which these pictures immediately suggest, when taken
by themselves, could have given us no notion of the situa-
tion, distances, or magnitudes of the objects they represent;
and it is altogether from the experience acquired by the ex-
ercise 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 ab-
stractedly, 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 correspond-
ing 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 to it. This
association is, in man at least, not original, but acquired. The
objects of sight and touch, as Bishop Berkeley has justly ob-
served, 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

366 THE SENSORIAL FUNCTIONS.

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 wliich she conveys informa-
tion most important to our welfare. As, in any language,
the words or sounds bear no resemblance to the things
they denote, so in this particular language the visible ob-
jects 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 tact ualim-pression.

This theory also affords a satisfactory solution of a ques-
tion which has frequently been supposed to involve consi-
derable difficulty; namely, how it happens that we see ob-
jects 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 pro-
duce the perception of a similar position in the object viewed,
is to commit 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
€ye bears to a camera obscura has perhaps contributed to the
fallacy in question; for, in using that instrument, we really
contemplate the image which is received on the paper, and re-
flected 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 ut-
terly unconscious that such an image exists, and still less
can we be sensible of the position of the image with respect

VISUAL PERCEPTIONS. 367.

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 vi-
sion; 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
particular direction with reference to our own body. 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 inter-
fere with our habitual, and perhaps instinctive 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 lumi-
nous appearance excited by the pressure is immediately re-
ferred 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. Many other examples might be given of similar
fallacies in our visual perceptions.

All impressions made on the nerves of sensation have a
definite duration, and continue for a certain interval of time
after the action of the external agent has ceased. The ope-
ration of this law is most conspicuous in those cases where
the presence or absence of the agent can readily be deter-
mined. Thus, we retain the sensation of a sound for some

353 THE SENSORIAL FUNCTIONS.

time after the vibrations of the external medium have ceased;
as is shown by the sensation of a musical note being the re-
sult of the rco-ular succession of aerial undulations, when the
impression made by each continues during the whole inter-
val between two consecutive vibrations. The impulses of
lio-ht on the retina are unquestionably consecutive, like those
of sound, but being repeated at still shorter intervals, give
rise to a continuous impression. A familiar instance of the
same principle occurs in the appearance of an entire lumi-
nous circle, from the rapid whirling round of a piece of
liglUed charcoal; for the part of the retina which receives
the brilliant image of the burning charcoal, retains the im-
pression with nearly the same intensity during the entire
revolution of the light, when the same impression is re-
newed. For the same reason a rocket, or a fiery meteor,
shooting across the sky in the night, appears to leave behind
it a long luminous train. The exact time, during which
these impressions continue, after the exciting cause has been
withdrawn, has been variously estimated by different experi-
mentalists, and is very much influenced, indeed, by the in-
tensity of the impression.*

* Many curious visual illusions may be traced to the operation 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 bare, such as those of a palisade, or Vene-
tian window-blind. On studying the circumstances of this phenomenon, t
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 appearances in question. A paper, in which I gave
an account of the details of these observations, and of the theory by which I
explained them, was presented to the Koyal Society, and published in the
Philosophical Transactions, for 1825, p. 131. About three years ago, Mr.
Faraday prosecuted the subject with tlie usual success which attends all his
philosophical researches, and devised a great number of interesting experi-
ments on the appearances resulting from combinations of revolving wlipels;
the details of which are given in a paper contained in the first volume of the
Journal of tlie Koyal Institution of Great Britain, p. 205. Tliis again direct-
ed my attention to the subject, and led me to the invention of the instrument
which has since been introduced into notice under the name of the Phantas-

VISUAL PERCEPTIONS. 369

When the impressions are very vivid, another phenome-
non often takes place; namely, their suhsequent recurrence,
after a certain interval, during whicli tliey are not felt, and
quite independently of any renewed aj)plicatioii of the cause
which had originally excited them. If, for example, we
look steadfastly at the sun for a second or two, and then im-
mediately 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; hut if we continue to keep the eyes shut, the
same impression will, after a certain time, recur, and again
vanish; and this phenomenon will be repeated at intervals,
the sensation becoming fainter at each renewal. It is pro-
bable that these reappearances of the image, after the light
which produced the original impressioji has been withdrawn,
are occasioned 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 senso-
rium. These spectral appearances generally undergo vari-
ous changes of colour, assuming first a yellow 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 exliaust their sensibility,
so that the continued or renewed action of the same exter-
nal cause produces a less effect than at first: while, on tiie
other hand, the absence or diminution of the usual excite-
ment 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 tlic case of the sensations
of temperature. The very same body may appear warm to

mascope or Pheniiklsticope. I constructed several of these at that period,
(in the spring- of 1 831 ) wiiich I showed to many of my friends; but in conse-
quence of occupations and cares of a more serious kind, I did not publish any
account of this invention, which was last year reproduced on Uic continent.

Vol. II. 47

370 THE SENSORIAL FUNCTIONS.

the touch at one time, and cold at another, (although its real
temperature has not varied,) according to the state of the
or«'an induced hy previous impressions: and a very different
judgment will be formed of its temperature, when felt by
each hand in succession, if the one has immediately before
been exposed to cold, while the other has retained its natu-
ral warmth. Similar phenomena may be observed with
regard to all the other senses: thus, the flavour of odorous,
as well as sapid bodies, depends 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, render-
ing them, for some time, nearly insensible to weaker tastes.
Sounds, wliich make a powerful impression on the auditory
nerves, will, in like manner, occasion temporary deafness
with rcc;ard to faint sounds. The converse of this is ob-
served when hearing has been suddenly restored in deaf
persons, by the operation of perforating the ear-drum.* The
sensibility of the auditory nerves, which had not been ac-
cessible 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 withdrawm, 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 im-
moveable 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 sur-
geon,t 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 apply-
ing his hand instinctively to cover his ear; but in doing this,

• Sec the note in p. 307 of this volume.

f M. Maunoir, of Geneva, on whose authority I have given this account.

VARIATIONS OF SENSIBILITY. 371

the sound of his fingers coming in contact with his head was
a fresh source of pain, producing an efTcct 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 loudness of the sounds; and it was several
weeks before this excessive sensibility of the auditory nerves
wore off: by degrees, however, tlicy accommodated them-
selves 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 resto-
ration 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 impressions which it then
receives. It might be supposed 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 re-
tina, is a cause adequate to account for this phenomenon:
hut careful observation will show that the pupil very rapid-
ly enlarges to its full expansion when not acted upon by
liffht; while the insensibilitv of the retina continues for a
much longer time. It regains its usual sensibility, indeed,
only by slow degrees. ]5y remaining in the dark its sensi-
bility is still farther increased, and a faint light will excite
impressions equal to those produced in Ihc 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 vSir William
Herschel in training his eye to the acquisition of extraordi-

372 THE SENSORIAL FUNCTIONS.

nary sensibility, for the purpose of observing very faint ce-
lestial objects. It often happened to him, when, in a fine
winter's night, and in the absence of the moon, he was oc-
cupied during four, five, or six hours in taking sweeps of
the heavens with his telescope, that, by excluding from the
eye the liglit of surrounding objects, by means of a black
hood, the sensibility of the retina was so much increased,
that when a star of the third magnitude approached the
field of view, he found it necessary immediately to with-
draw 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 retreat from the beautiful but over-
powering spectacle.

The peculiar construction of the organ of vision allows of
our distinguishing the effects of impressions made on parti-
cular parts of the retina from those made on the rest, and
from their general effect on the whole surface. These par-
tial variations of sensibility in the retina give rise to the phe-
nomena of ocular spectra^ as they are called, which were
first noticed by Buffon, and afterwards more fully investi-
gated by Dr. Robert Darwin. A white object on a dark
ground, after being viewed steadfastly till the eye has be-
come fiitigued, produces, when the eye is immediately di-
rected to another field of view, a spec^um of a darker co-
lour than the surrounding space, in consequence of the ex-
haustion 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 sensibility of the re-
tina to any particular kind of light, may, in like m.anner, be
increased or diminished, without any change taking place in
its sensibility to other kinds of light. Plence the spectrum

OCULAR SPECTRA. 373

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, while the
yellow and the blue rays still continue to produce their usual
effect; and these, by combining 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 re-
tina, previously rendered less sensible to the yellow and the
blue rays composing the green liglit it had received from the
object viewed.

The judgments we form of the colours of bodies are in-
fluenced, in a considerable degree, by the vicinity of other
coloured objects, which modify the general sensibility of the
retina. When a white or gray object of small dimensions,
for instance, is viewed on a coloured ground, it generally
appears to assume a tint of the colour which is complemen-
tary 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 examina-
tion he found that the marks were also black. The green
appearance of the letters, in this case, was an optical illu-
sion, arising from the tendency of the retina, which had
been strongly impressed with red light, to receive impres-
sions 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 as-
signed to our powers of perception; but the subject is much

• Any two colours which, when combined tog-cther, produce white li^ht,
are said to be compkmentary to one another.

374 THE SENSORIAL FUNCTIONS.

too extensive to be treated at len^jth 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 circumstances, direct our judgment correctly,
but are then acting under unusual or irregular combinations
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 chiss includes those illusions in which an impres-
sion is really made on tlic organ of sense by an external
cause, but in away to wiiicli we have not been accustomed. To
this class belong the acoustic deceptions arising from echoes,
and from the art of ventriloquism; the deceptive appear-
ances of the mirage of the desert, the looming of the horizon
at sea, the Fata Morgana of the coast of Calabria, the gi-
gantic spectre of the Brockcn in the Hartz, the suspended
images of concave mirrors, the visions of the phantasmago-
ria, 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 spectra of various kinds, impressions on the tongue
and the eye from galvanism, and those which occasion sing-
ing 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 niental in their origin, and are the consequences
of errors in our reasoning powers. Some of these have al-
ready been pointed out with regard to the perceptions of
vision and of hearing, the formation of which is regulated

• In the Gulstonian Lectures, whicli I was appointed to read to the Royal
College of Physicians, in May, 1832, 1 took occasion to enlarge on this sub-
ject. A summary of these lectures was given in the London Medical Ga-
zette, vol. X. p. 273,

Illusions of the senses. 375

by the laws of the association of ideas. But even the sense
of touch, which has been generally regarded as the least lia-
ble to fallacy, is not exempt from this sourceof error, as is
proved by the well known experiment of feeling 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 arc 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 re-
spective wants and necessities of each species, in the situa-
tions and circumstances where it has been placed by the gra-
cious and beneficent Author of its being. ,

If the sensorial functions had been limited to mere sensa-
tion and perception, conjoined with the capacity of passive
enjoyment and of suffering, the purposes of animal existence
w^ould have been but imperfectly accomplished; for, in or-
der that the sentient being m^ay secure the possession of
those objects which are agreeable and salutary, and avoid or
reject those which are painful or injurious, it is necessary
that he possess the power of spontaneous action. Hence,
the faculty of Voluntary 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 ^vith the external world,
are, through the intermedium of a separate set of nervous
filaments, totally distinct from those which are subservient
to sensation,^' made to communicate directly with the senso-
rium, and are thereby placed under the direct control and
guidance of the will. The mental act of volition is doubt-
less accompanied by some corresponding physical change in
that part of the sensorium, whence the viotor nerves^ or

* On this subject I must refer the reader to the researches of Sir Charles
Bell, and Magcndie, who have completely cstablisiicil the disitniction be-
tween these two classes of nerves.

376 THE SENSORIAL FUNCTIONS.

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
required 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 treated at suITicient length in the first part of
this work.

Every excitement of the sensorial powers is, sooner or
later, followed by a proportional degree of exhaustion; and
when this has reached a certain point, a suspension of the
exercise of these faculties takes place, constituting the state
of sleej:), during which, by the continued renovating action
of the vital functions, these powers are recruited, and ren-
dered 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 pre-
clude 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, suggested by
internal irritations, and which the mind is bereft of the
power to control, in consequence of the absence of all im-

♦ A voluntary action, occurnng' as the immediate consequence of the ap-
plication of an external agent to an organ of the senses, thougli apparently
a simple phenomenon, implies the occurrence of no less than twelve succes-
sive processes, as may be seen by the following enumeration. First, there
is the modifying action of the organ of tlie 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, associa-
tion, and volition. Then, again, there is another physical change taking
place in the sensorium, immediately consequent on the mental act of voli-
tion: this is followed by the propagation of the impression downwards along
tlie motor nei've; 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.

VOLUNTARY MOTION. 377

pressions from the external senses.* In many animals, a
much more general suspension of the actions of life, extend-
ing even to the vital functions of respiration and circulation,
takes place during the winter months, constituting what is
termed Hybernation.

* The only indications of dreaming- given by the lower animals occur in
those possessed of the gi-eatest intellectual powers, such as the I)o^, amonir
quadrupeds, and the Parrot, among birds.

Vol. II. 48

( 378 )

CHAPTER VIII.

COMPARATIVE PHYSIOLOGY OF THE NERVOUS SYSTEM.

§ 1. Nervous Systems of Invertehrated Animals.

Our knowledge of the exact uses and functions of the
various parts which compose the nervous system, and espe-
cially 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 di-
versities in the organization. The rapid review 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 multiplied
and developed in proportion as the faculties of the animal
embrace a wider range of objects, and arc carried to higher
degrees of excellence.

In none of the lowest tribes of Zoophytes, such as Spoiiges,
Polypi, and MedusXy have any traces of organs, bearing
the least analogy to a nervous system, been discovered; not
even in the largest specimens of the last named tribe, some
of 'which are nearly two feet in diameter. All these ani-
mals give but very obscure indications of sensibility; for
the contractions they exhibit, when stimulated, appear to
be rather the efifect 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 sensations and volitions, though in a de-
gree 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

NERVOUS SYSTEM OP INVERTEBRATA. 37$)

the Intervention of any common scnsorlnm, or centre of ac-
tion. It is at the same time remarkable that their move-
ments are not effected by means of muscular fibres, as they
are in all other animals, the granular flesh, of which their
whole body is composed, appearing to have a generally dif-
fused irritability, and perhaps also some degree of sensibi-
lity; so that each isolated granule may be supjiosed to be
endowed with these combined properties, performing, inde-
pendently of the other granules, the functions both of nerve
and muscle. Such a mode of existence exhibits apparently
the lowest and most rudimental condition 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 a still more decided manner, those of the Lifusoria.
In the way in which the latter avoid obstacles while swim-
ming 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 volun-
tary action.

To seek for an elucidation of these mysteries in the struc-
ture of animals whose minuteness precludes all accurate ex-
amination, would be a hopeless inquiry. Yet the indefati-
gable Ehrenberg has recently discovered, in some of the
larger species of animalcules belonging to the orticr Roii-
ferUy an organization, which he believes to be a nervous sys-
tem. He observed, in the Hydatina senta, a series of six
or seven gray 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 considers as a ganglion, is much larger than the
others, and gives off slender nerves, which, by joining
another ganglion, situated under the integuments at the
back of the neck, form a circle of nerves, analogous to
that which surrounds the oisophagus in the mollusca: from
this circle two slender nervous filaments are sent off to
the head, and a larger branch to the abdominal surface of

380 THE SENSORIAL FUNCTIONS.

the body. The discovery of a regular structure of muscu-
lar bands of fibres, in these animalcules, is a farther evi-
dence of the connexion which exists between nerves and
muscles.

We again meet with traces of nervous filaments, accom-
panied also witli muscular bands of fibres, in some of the
more highly organized Enlozoci. 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 exhibit-
ing any ganglionic enlargement. This may be considered
as the first step towards the particular form of the nervous
system of the higher classes of articulated animals, where
the principal nervous cord is obviously double throughout
its whole length, or, if partially united at different points, it
is always readily divisible into two, by careful manipula-
tion. In addition to this characteristic 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 cen-
tre, branches in various directions. It is probable that these
knots, or ganglia, perform, in each segment of the worm, an
office analogous to that of the brain and spinal marrow of
vertcbrated animals, serving as centres of nervous, and per-
haps, also, of sensorial powers. 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
w^orms and the myriapoda, has in many respects an inde-
pendent sensitive existence, so that when the body is di-
vided 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 external sense which
is capable of being exercised by this simple form of nervous
system, is that of touch; all the higher senses evidently re-
quiring a much more developed and concentrated organiza-
tion of nervous ganglia.

NERVOUS SYSTEM OF ARTICULATA. 3S'l

In this division of the animal kingdom, the primary ner-
vous cords always pass along the middle of the lower sur-
face of the body, this being the situation which, in the ab-
sence of a vertebral bony column, aflbrds them the best pro-
tection. They may be considered as analogous to the spi-
nal 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 uppermost, or that
nearest the head, is generally of greater size than the rest,
and is termed the oesophogcal, cephalic, or cerebral gan-
glion, being usually regarded as analogous to the brain of
larger animals. Perhaps a more correct view of its func-
tions would be conveyed by calling it the principal brain,
and considering; the other 2;ani2;lia as subordinate bi-ains.
This large ganglion, whicli supplies an abundance of ner-
. vous filaments to every part of the heacl, 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
th.eir 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
*/innelida: but in the higher orders of Jirticulata we find
it exhibiting various degrees of concentration. The pro-
gress of this concentration is most distinctly traced in the
Crustacea* One of the simplest forms of these organs oc-
curs 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 Talitrus locust a, or Sand-
^oo hopper, (Fig. 43S.) The central parts

y^^M^j^fe^ of its nervous system are seen in Fig.

f f^^S|^^ 439, wiiich represents the abdominal

side of this animal laid open, and mag-
nified to twice the natural size. The two primary nervous

* See the account of the researches of Victor Audouin, and II. M. Ed-
wards, on this subject, given in the Ann. dcs Sc. Nat. xix. 181.

382

THE SENSORIAL FUxVCTIONS.

cords, which run in a longitudinal direction, are here per-
fectly 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 united
by transverse threads: and other filaments, diverging late-
rally, proceed from each ganglion. During the progress of
growth, the longitudinal cords approach somewhat nearer to
each other, but still remain perfectly distinct. The first

43.9

440

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 Fhyllosoma
(Leach,) where the ganglia composing each pair in the ab-
domen and in the head, are united into single masses, while
those in the thoracic region are still double. In the Cymo-
ihoa, (Fab.,) which belongs to the family of Onisciis, there
is the appearance of a single chain of ganglia, those on the
one side having coalesced with those on the other; each pair
composing a single ganglion, situated in the middle line;
while the longitudinal cords which connect them still re-

• These segments arc numbered in this and the following- figure in their
proper order, beginning witli that near the head, a is the external antenna?
a, tlic internal antenna; and e, the eye.

NERVOUS SYSTEM OP CRUSTACEA.

383

main 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 abdo-
minal region, though still distinct in the thorax.
, In following the ascending series of crustaceous animals,
we observe also an approximation of the remoter ganglia to-
wards those near the centre of the body: this tendency al-
ready shows itself in the shortening of the hinder part of
the nervous system of the Cymolhoa, as compared with the
Talitrus; and the concentration proceeds farther in other
tribes. In the Palemon, for example, most of the thoracic
ganglia, and in the Palinuriis (Fab.,) all of them, have co-
alesced into one large oval mass, perforated in the mid-
dle, and occupying the centre of the thorax; and, lastly, in
the Maia squinado, or Spider Crab (Fig. 441,"^) this mass

• In this figure are seen the great thoracic g-anglion (n,) from which pro-
ceed the superior thoracic nerves (t,) those to the fore feet (f,) to the liiiulcr

3S4 THE SENSORIAL FUNCTIONS.

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 development of the liigher Crustacea:
thus, in the Lobster, the early condition of the nervous sys-
tem is that of two separate j^arallcl cords, each having a dis-
tinct chain of ganglia, as is the case in the Talitrus: then the
cords are observed gradually to approximate, and the gan-
glia on each side to coalesce, as represented in the Cymo-
thoa: and at the period when the limbs begin to be deve-
loped, the thoracic ganglia approach one another, unite in
clusters, and acquire a rapid enlargement, 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 originall)^ double, and
has coalesced into one, is also greatly developed, in corre-
spondence with the growth of the organs of sense. The next
remarkable change is that taking place in the hinder por-
tions of the nervous cords, which are shortened, at the same
time that their ganglia are collected into larger masses, pre-
paratory 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 development, while
their size is augmented.

All Insects have the nervous system constructed on the
same general model as in the last mentioned classes; and it
assumes, as in the Crustacea, various degrees of concentra-
tion in the different stages of development. As an example
we may take the nervous system of the Sphmx ligusiri, of
which representations are given in the larva, pupa, and ima-

feet (f.) and the abdominal nervous trunk (n;) the cephalic ganglion (c,)
communicating by means of two nervous cords (o,) which surround the
cEsophagus and entrance into the stomach (s,) with the thoracic ganglion
(b;) and sending ofT the optic nerve (e) to the eyes (e,) and the motor
ner\'es (m,) to the muscles of those organs; and also the nerves (a) to the
internal antennse, and the nerves (x) to the external antennsc (a.)

NERVOUS SYSTEM OF INSECTS. 385

go States, wholly detached from the body, and of their na-
tural size, in Figures 442, 443, and 444.*

This system in the larva (Fig. 442) has the same simple
form as in the Annelida, or in the Talitrus, for it consists of

* These figures were drawn by Mr. Newport, from orlj^inal 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 prin-
cipal transformations of the insect. Numbers 1 to 11 indicate the scries 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 cephalic, or cerebral ganglion (17) is
situated above the oesophagus and dorsal vessel, and communicates by two
nervous chords with the first of tlie series, or sub-asopliageal ganglion (1,)
which is, in eveiy stage of the insect, contained witliin the head, and distri-
butes nerves to the parts about tiie mouth. The next ganglion (2) becomes
obhterated 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 ima-
go, the large thoracic ganglion; while the two which follow (6 and 7,) be-
come wholly obliterated before the insect attains the imago stale, the inter-
vening cords becoming shorter, and being, with the nerves they send out,
carried forwards. The last four (8, 9, 10, 11) of the abdominal ganglia re-
main, 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 sup-
ply the wings of the imago, are very small in the larva; and tliey arise by
two roots, one derived from the cord, and one from the ganglion. The
nerves sent to the three pairs of anterior, or true legs, are marked 14, 15, 16.
The nervous system of the larva is exhibited in Fig. 442, that of tlie 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 inter-
vening cord, preparatory to their final obliteration in the imago; a cluuige
which those in front of them (4, 5) have already undergone. The pro-
gressive development 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
ligusiri, and of the changes it endergoes up to a certain period, is given by
Mr. Newport in a paper in the Phil. Trans, for 1832, p. 383. lie has since
completed the inquiry to the last transformation of this and other insects, and
has lately presented to the Uoyal Society an account of his researches.
Vol. II. 49

386

THE SENSORIAL FUNCTIONS.

a longitudinal series of ganglia, usually twelve or thirteen in
number, connected in their whole length by a double fila-
ment. By degrees the difi'erent parts of which it consists
approach each other, the thoracic ganglia, in particular, coa-
lescing into larger masses, and becoming less numerous, some
being apparently obliterated; the whole cord becomes in con-

444

443

442

sequence shorter, and the abdominal ganglia are carried for-
wards. The optic nerves are greatly enlarged during the
latter stages of transformation, and are often each 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 vertcbrated animals, while another nerve resem-
bles in its mode of distribution, the pneumo-gasiric nerve,
or par vagum. Very recently Mr. Newport has distinctly
traced a separate nervous tract, which he conceives gives

NERVOUS SYSTEM OF INSECTS.

387

origin to the motor nerves, while the subjacent column sends
out the nerves of sensation.

In the next great division of the animal kingdom, 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 JJsterias, where the ner-
vous system (Fig. 445) is composed of small ganglia, equal

446

445

448

in number to the rays of the animal, and disposed in a cir-
cle round the central aperture or mouth, but occupying si-
tuations 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 Holothuria there is a similar chain
of ganglia, encircling the oesophagus; and the same mode
of arrangement prevails in all the bivalve Mollusca,
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
oesophageal ganglia, by filaments, so as to form with them
one continuous nervous system. In the Gasteropoda, which
are furnished with a distinct head and organs of the higher

388 THE SENSORIAL FUNCTIONS.

senses, (such as the *^plysia, of which the nervous system is
exhihited 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 sup-
plied by two lateral ganglia (l, l;) and there is besides a
a transverse ganglion (t,) below the oesophagus. The ce-
phalic ganglion, on the other hand, attains a considerable
size in the Cephalopoda (c, Fig. 448,) where it has extensive
connexions 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

§ 2. Nervous System of Vertebrated Jinimals.

The characteristic type of the nervous system of verte-
brated animals is that of an elongated cylinder of nervous
matter, (m, z. Fig. 449,) extending down the back, and
lodged in the canal formed by the grooves and arches of the
vertebrce. 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 to-
gether, but leaving frequently a central canal, which is filled
with fluid. On each side of the spinal cord, and between
all the adjacent vertebras, there proceed two sets of nervous
filaments, those which are continuous with the posterior co-
lumns (p,) being appropriated to the function of sensation;
and those arising from the anterior columns (a,) being sub-

* This figure also shows ag-ang-lion (a,) which is placed higher, and com-
municates by lateral filaments with the cephalic gang-lion (c;) two lateral
gunglia (l, l,) of great size; and a large abdominal ganghon (g.)

f Some peculiarities in the structure of the cephalic ganghon of the Sepia
have been supposed to indicate an approach to the vertebrated structure; for
this ganglion, together with the labyrinth of the ear, is enclosed in a cartila-
ginous ring, perforated at the centre to allow of the passage of the CESopha-
giis, and imagined to be analogous to a cranium.

NERVOUS SYSTEM OP VERTEBRATA. 389

servient to voluntary motion. The former, soon after their
exit from the spine, pass through a small ganglion (g,) and

451

then unite with the nerves from the anterior column, com-
posing, by the intermixture of their fibres, a single nervous

390 THE SENSORIAL FUNCTIONS.

trunk (n,) which is afterwards divided and subdivided in
the course of its farther distribution, both to the muscular
and the sentient organs of the body. Each of these spinal
nerves also sends branches to the ganglia of the sympathetic
nerve, which, as was formerly described, passes down on
each side, parallel and near to the spine.

Enlargements of the spinal marrow are observed 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 uni-
form diameter. In Fishes, these enlargements are in pro-
portion to the relative size and muscularity of the lateral fins,
and correspond to them in their situation. The Piper Gur-
nard ( Tri'^la lyra,) which is a species of flying fish, having
very large pectoral fins, that portion of the spinal marrow
supplying their muscles with nerves (as seen in the space
between m and s. Fig. 451,) has numerous enlargements,
presenting a double row of tubercles. Fishes which possess
electrical organs have a considerable dilatation of the spinal
marrow, answering to the large nerves w^hich are distributed
to those organs. Birds which fly but imperfectly, as the
Gallinaceous tribe and the Scansoi^es, have the posterior
enlargement much greater than the anterior; a disproportion
which is particularly remarkable in the Ostrich. On the
contrary, the anterior enlargement is much more considera-
ble 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) corresponding to the wings and legs respectively,
are nearly of equal size. In Quadrupeds, we likewise find
the relative size of these enlargements corresponding to that
of fore and hind extremities. When the latter are absent,
as in the Cetacea, the posterior dilatation does not exist.

The brain (b) may be regarded as an expansion of the an-
terior or upper end of the spinal marrow; and its magnitude,

NERVOUS SYSTEM OF VERTEBRATA. 391

as well as the relative size of its several parts, vary much in
the different classes and families of vertebrated 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 im-
mediate continuation of the spinal marrow (s,) is termed the
medulla oblongata. The single tubercle (c,) arising from
the expansion of the posterior columns of the spinal mar-
row, is termed the cerehellum, or little brain. Next follow
the pair (t) which are termed the oj^lic tubercles, or lobes,*
and appear to be productions 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 smelling, have been
called the olfactory lobes, or tuberclesA These are the
principal parts of the cerebral mass to be here noticed, for I
purposely omit the mention of the minuter divisions, which,
though they have been objects of much attention to anato-
mists, 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 ani-
mals, a progressive increase 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 hemi-
spheres (h,) which, in the inferior orders of fishes, as in the

* In the Mammalia, and in Man, tliey have been often desig-nated by the
very inappropriate name of Corpora quadriiremina.

■\ Several cavities, termed Ventricles, are occasionally found in the inte-
rior of the principal tubercles of the brain; but their use is unknow"-

392 THE SENSORIAL FUNCTIONS.

«

Trigla lyra, or Piper Gurnard, (Fig. 451,) and in the Mu-
rxna conger, or Conger Eel, (Fig. 452,) are scarcely dis-
cernible. They are very small in the Percct Jluviatilis, or
common Perch (Fig. 453;) but more developed in Reptiles,
as in the Tcstudo inydas, 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 the JNIammalia, as is exemplified in the brain of the Lion,
(Fig. 456.) On the other hand, the optic tubercles (t) are
largest, compared 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 development. 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 scarce-
ly exceed it in diameter, they appear to be mere enlarge-
ments. 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, how-
ever, may be perceived in the brain of the Perch (Fig. 457;)
for the medulla oblongata (m) is doubled underneath the ce-
rebellum (c,) pushing 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 tlie 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 po-
sition, near the lower surface of the brain, lying on each side
of the medulla oblongata, at the part indicated by the letter

NERVOUS SYSTEM OF VERTEBRATA.

393

T. In the Mammalia, as the Lion (Fig. 4G0,) they are
lodged quite in the interior of the organ, and concealed hy
the expanded hemispheres (h;) their position only being
marked by the same letter (t.) These changes are conse-
quences of the increasing development of the brain, com-
pared with that of the cavity in which it is contained, re-
quiring every part to be more closely packed; thus, the lay-
ers of the hemispheres in INIammalia are obliged, from their
great extent, to be plaited and folded on one another, pre-
senting 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 produce, likewise, even in
birds, transverse furrows on the surface; and from the in-
terposition of a substance of a gray colour between the la-
minae of the white medullary matter, a section of the ce-
rebellum presents the curious appearance (seen in Fig. 459,)
denominated, from its fancied resemblance to a tree, the
Jirhor Vitx,

461 .'

Thus far we have followed an obvious gradation in the
development and conccutration of the dilierent parts of the

Vol. II. 50

394 THE SENSORIAL FUNCTIONS.

brain: but on arriving at Man the continuity of the series is
suddenly disturbed by the great expansion of the hemi-
spheres, (Fig. 4f)l,) whicli, compared with those of quadru-
peds, 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 "-rcater than in any other known animal: exceptions, how-
ever, occur in the case of the Elephant, and also in that of
the Whale, wliosc brains arc certainly of greater absolute
bulk than that of man. But all the large animals, with
which we are familiarly acquainted, have brains considera-
bly smaller; as will readily appear from an examination of
their skulls, which are narrow and compressed at the part
occupied by the brain; the greater part of the head being
taken up by the development of the face and jaws. In Man,
on the other hand, the bones of the skull rise perpendicu-
larly from the forehead, and are extended on each side, so
as to form a capacious globular cavity for the reception and
defence of this most important organ. It is chiefly from the
expansion of the hemispheres, and the development of its
convolutions, that the human brain derives this great aug-
mentation of size.*

• This will be apparent from the vertical section of the human brain, Fig-.
461; where, as before, s is the spinal marrow; ji, the medulla oblongata; c,
the cerebellum, with the arbor vitae,- t, the optic tubercles, or corpora quad-
rigemina, 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; a, the corpus callosum; and h, ii, h, the hemi-
spheres.

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 higiier intellectual faculties. The
most celebrated is that devised by Camper, and which he ternied the facial
angle, composed of two lines, one drawn in the direction of the basis of the
skull, from the car to the roots of the upper incisor teeth, and the other from
the latter point, touching the most projecting l^ai-t of the forehead. Cam-
per conceived that th.e magnitude of this angle would correctly indicate the
size of the brain, as compared with the organs of the principal senses which

FUNCTIONS OF THE BRAIN. 395

§ 3. Funciions of the Brain.

Physiologists have m 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 me-
dullary fibres, arranged in the most intricate manner. Great
pains have been bestowed in unravelling the tissue of these
fibres, in the hope of discovering some clew to the perplex-
ing 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 sur-
face of the convolutions of the brain and cerebellum, which
are composed of a softer and more transparent gray matter,
termed the cortical or cineritioiis substance of the brain.

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 arc connect-
ed with these functions are all double, and arranged sym-
metrically 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 sen-
sation, corresponding with our consciousness of individual-
ity, it was necessary that a free communication should be

compose the face: but the Hdlacy of this criterion of animal sag'acity has been
shown in a great many cases.

396 THE SENSORIAL FUNCTIONS.

cstablislicd between llie parts on both sides. For this pur-
pose there is provided a set of medullary fibres, passing di-
rectly across from one side of the brain to the other; these
constitute what are called the Cominissiires of the brain.*

The question, however, still recurs: — What relation does
all this artificial intertexture and accumulation of fibres bear
to the mental operations of which we are conscious, such as
memory, abstraction, judgment, imagination, volition? 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 anivial spirits, circulating with great ve-
locity 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 construct-
ed? But 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 ap-
proach to a solution of the difficulty; for they, in fact, do
not touch the real subject to be explained, namely, how the
affections of a material substance can influence and be influ-
enced by an immaterial agent. All that we have been able
to accomplish has been to trace the impressions from the
organ of sense along the communicating nerve to the senso-
rium: beyond this the clew is lost, and we can follow the
process no farther.

• The principal commissure of the human brain, called tlie corpus callosum,
is seen at a, Fig. 461. Dr. Macartney, in a paper which he read at the late
meeting at Cambridge of the Britisli Association for the Advancement of
Science, described the structure of the human brain, as discovered by his pe-
culiar mode of dissection, to be much more complicated than is generally
supposed. He obsei-ved that its fibres are interlaced in the most intricate
manner, resembling the plexuses met with among the nerves, and establish-
ing the most extensive and general communications between every part of
the cerebral mass.

FUNCTIONS OP THE BRAIN. 397

The exact locality of the sensorium has ])ccn eagerly
sought for by }3hysiologists in every age. It would appear,
from the results of the most recent inquiries, that it cer-
tainly 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 tlie mammalia wliich ap-
proach the nearest to him in their stiucture, 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 descend to the inferior orders of the animal kingdom,
we find it more and more extensively diffused over the spi-
nal marrow; and in the Invertebrata the several ganglia ap-
pear to be endowed with this sensorial property: but, be-
coming less and less concentrated in single masses, the cha-
racter of individuality ceases to attach to the sensorial phe-
nomena; until, in Zoophytes, vv^e lose all traces of ganglia
and of nervous filaments, and every part appears to possess
an inherent power of exciting sensation, as well as perform-
ing muscular contractions.

Beyond this point we can derive no farther aid from Ana-
tomy, since the intellectual operations of which we arc con-
scious bear no conceivable analogy with any of the configu-
rations 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 configu-
ration can have with the functions they respectively per-
form. The map of regions which modern Phrenologists
have traced on the surface of the head, and which they sup-
pose to have a relation to different faculties and propensities,
does not agree either with the natural divisions of the brain
^or with the metaphysical classification of mental phenome-

39R THE SENSORIAL FUNCTIONS.

na.* Experiments and pathological observations, however,
seem to show that the hemispheres of the brain are the chief
instruments by whicli the intellectual operations are carried
on; that the central parts, such as the optic lobes and the
medulla oblongata, are those principally concerned in sen-
sation; 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 ex-
tremely imperfect, since it must be derived from a compari-
son 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 sen-
sations and perceptions, of which our minds have not the
power to form any adequate conception. INIany of the
qualities of surrounding bodies, which escape our more ob-
tuse senses, may be distinctly perceived, in all their grada-
tions, by particular tribes of animals, furnished with more
delicate organs. Many quadrupeds and birds possess pow-
ers 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
quadrupeds which do not far surpass us. The organ of
smell, in particular, is often spread over a vast extent of sur-
face, in a cavity occupying the greatest part of the head;
so that the perceptions of this sense must be infinitely diver-
sified.

• For a summary of the doctrines of EJrs, Gall and Spurzheim, I beg leave
to refer the reader to an account which I drew up, many years ago, for the
Encyclopedia liritannica, and which composed the article "CnASfioscoPY"
in the last supplement to that work, edited by Mr. Napier.

FUNCTIONS OF THE BRAIN. 399.

Bats have been supposed to possess a peculiar, or sixth
sense, enabling them to perceive the situations of external ob-
jects without the aid either of vision or of touch. The prin-
cipal facts upon which this opinion has been founded were
discovered by Spallanzani, who observed that these aniniala
would fly about rapidly in the darkest chambers, although
various obstacles were purposely placed in their way, with-
out striking against or even touching tlicm. They continued
their flight with the same precision as before, threading their
way through the most intricate passages, when their eyes
were completely covered, or even destroyed. Mr. Jurine,
who made many experiments on these animals, concludes
that neither the senses of touch, of hearing or of smell, were
the media through which bats obtain perceptions of the pre-
sence and situation of suri'ounding bodies; but he ascribes
this extraordinary faculty to the great sensibility of the skin
of the upper jaw, mouth, and external ear, which are fur-
nished with very large nerves. '■

The wonderful acuteness and power of discrimination
which many animals exercise in the discoverv and selection
of their food, has often suggested the existence of new senses,
different from those which we possess, and Anveying pecu-
liar and unknown powers of perception. An organ, which
appears to perform some sensitive function of this kind, has
been discovered in a great number of quadrupeds by Jacob-
son. t In the human skeleton there exists a small perfora-
tion in the roof of the mouth, just behind the sockets of the
incisor teeth, forming a communication with the under and
fore part of the nostrils. 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 per-
vious even during life, and is sometimes of considerable
width. Jacobson found, on examinina; this structure with

* Sir Anthony Carlisle attributes this power to the extreme delicacy of
hearing- in this animal.

f See Annales du Miiscej xviii. 412.

400 THE SENSORIAL FUNCTIONS.

uttcnllon, that the canal led to two glandular organs of an
oblong shape, and enclosed in cartilaginous tubes: each gland
has in its centre a cavity which communicates above with
the general cavity of the nostrils. These organs lie can-
cealed in a hollow groove within the bone, where they are
carefully protected from injury: and they receive a great
number of nerves and blood vessels, resembling in this re-
spect 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 Rodentia, and next in that
of the Riiminantia; in the Horse, they are still very large,
but the duct is not pervious; while, in carnivorous quadru-
peds, they are on a smaller scale. In Monkeys, they may
still be traced, although extremely 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 no-
ticed. 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 indi-
cate their performing some sensitive function; and their si-
tuation would Aoint them out as fitting them for the ap-
preciation 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 Shm^ks 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 con-
siders it as conveying sensations intermediate between those
of touch and hearing; while De Blainville and Jacobson re-
gard it merely as the organ of a finer touch.

The perceptive powers of Insects must embrace a very
different, and, iii many respects, more extended sphere than
our own. These animals manifest by their actions that they
perceive and anticipate atmospheric changes, of which our
senses give us no information. It is evident; indeed, that

PERCEPTIONS OP ANIMALS. 401

the impressions made by external objects on their sentient
organs must be of a nature widely difl'erent from those which
the same objects communicate to ourselves. While with re-
gard to distance and mas2;nitudc our perceptions take the
widest range, and appear infinitely extended when com-
pared 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 organization. Thus, the varying degrees of
moisture or dryness of the atmosphere, the continual changes
in its pressure, the fluctuations in its electrical state, and va-
rious other physical conditions, may be objects of distinct
perception to these minuteanimals. Organsmayexistinthem,
appropriated to receive impressions, of which we can have
no idea; and opening avenues to various kinds of knowledge,
to which we must ever remain utter strangers. Art, it is
true, has supplied us with instruments for discovering and
measuring many of the properties of matter, which our un-
assisted senses are inadequate to observe. But neither our
thermometers, nor our electroscopes, our hygrometers, nor
our galvanometers, how^ever skilfully devised or elaborately
constructed, can vie in delicacy and perfection with that re-
fined apparatus of the senses which nature has bestowed on
the minutest insect. There is reason to believe, as Dr. Wol-
laston has showm, that the hearing of insects comprehends a
range of perceptions 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 lighl, 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 suscepti-
VoL. II. , 51

402 THE SENSORIAL FUNCTIONS.

ble of receiving; distinct luminous impressions from a 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, tlie Gymnotiis, and several other fishes,
arc furnislied with an electrical apparatus, resembling the
Voltaic battery, wnich they liave 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 comniunicate perceptions, with regard to electricity,
very different from any that we can feel. In general, in-
deed, it may be remarked, that the more an organ of sense
differs in its structure from those which we ourselves pos-
sess, the more uncertain must be our knowledge of its func-
tions. 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 re-
cesses of the ocean, which we know are tenanted by multi-
tudinous tribes of fishes, zoophytes, and mollusca? How can
we figure to ourselves the sensitive existence of the worm or
the insect, organized in so different a manner to ourselves,
and occupying so remote a region in the expanse of creation?
How can we venture to speculate on the perceptions of the
animalcule, whose world is a drop of fluid, and whose fleet-
ing existence, chec^uered, perhaps, by various transforma-
tions, is destined to run its course in a few hours?

Confining our inquiries, then, to the more intelligible in-
tellectual phenomena displayed by the higher animals, we
readily trace a gradation which corresponds with the de-
velopment of the central nervous organ, or brain. That the

• Encyclopjcdiu McUupolilanu, Article *' Ligut."

PERCEPTIONS OF ANIMALS. 403

comparison may be fairly made, however, it is necessary to
distinguisli those actions which are the result of the exer-
cise of the intellectual faculties, from those which are called
instinctive, and are referrible to other sources. Innumera-
ble are the occasions in which the actions of animals appear
to be guided by a degree of sagacity not derivable from ex-
perience,and apparently implying a foreknowledge of events,
v^rhich neither experience nor reflection could have led them
to anticipate. We cannot sufliciently admire the provident
care displayed by nature in the preservation both of the in-
dividual and of the species, which she has intrusted, not to
the slow and uncertain calculations of prudence, but to in-
nate faculties, prompting, by an unerring impulse, 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 employ-
ing various means of defence against enemies they have ne-
ver 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 unexplained, and, perhaps,
inexplicable facts, we must content ourselves with classing
under the head o^ 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 cflects, while we
search in vain for the eflicient cause.

In all the inferior orders of the animal creation, wliere in-
stincts are multiplied, while tiie indications of intellect are
feeble, the organ which performs the office of the brain is
comparatively small. The sensitive existence of these ani-
mals appears to be circumscribed within the perceptions of
the moment, and their voluntary actions have reference
chiefly to objects which arc present to tlie sense. In i)ro-
portion as the intellectual faculties of animals are multiplied,
and embrace a wider sphere, additional magnitude and com-
plication of structure are given to the nervous substance
which is the organ of those faculties. The greater the power

404 THE SENSORIAL FUNCTIONS.

of combining; ideas, and of retaining them in the memory,
the greater do we find the development of the cerebral he-
mispheres. These parts of ihe 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, sa-
gacity 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 size, but beyond all comparison the
most complicated in its structure.*

A large brain has been bestowed on man, evidently with
the design that he should exercise superior powers of intel-
lect; the great distinguishing features of which are the ca-
pacity 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 ac-
quiring knowledge from experience; and, on some rare oc-
casions, of devising expedients for accom])lishing particular
ends. But still this knowdcdoe and these efforts of intellect
are confined within very narrow^ limits; for nature has as-
signed boundaries to the advancement of the low^er animals,
which they can never pass. If one favoured individual be
selected for a special education, some additional share of in-
telligence may, perhaps, with infinite pains, be infused; but
the improvement perishes with that individual, and is w'holly
lost to the race. By far the greater portion of that know-
ledge which it imports 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

• All Ihc parts nut witli m the brain of animals exist also in the brain of
man; while sevcnil of those foiiiid in man arc either extremely small, or alto-
gether absent in the brains of the lower animals. Soemmerring- has enu-
njerated no less than fifteen material anatomical diflerences between the hu-
man brain and tliat of the ape.

PERCEPTIONS OF ANIMALS. 405

of the lower animals; and, unless cultivated and improved
by reason and education, would, of themselves, produce 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 suffer-
ings, and regardless of the treatment experienced by their
companions. The attachment, indeed, of the mother to her
offspring, as long as its wants and 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 con-
fined, even in the most social tribes, to the period of help-
lessness; and the animal instinct is not succeeded, as in man,
by the continued intercourse 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 de-
fenceless, unprovided with the means of subsistence, and
exposed on every side to destruction, she has, in reality, im-
planted in him the germ of future greatness. The helpless-
ness of the infant calls forth the fostering care and ten-
derest affections of the mother, and lays the deep founda-
tions of the social union. The latent eneruries of his mind
and body are successively, though slowly developed. AVhile
the vital organs are actively engaged in the execution of
their different offices, while the digestive apparatus is exer-
cising its powerful chemistry, while myriads of minute ar-
teries, veins, and absorbents are indefatigably at work in
building and modelling this complex frame, the sentient
principle is no less assiduously and no less incessantly em-
ployed. From the earliest dawn of sensation it is ever busy
in arranging, in combining, and in strengthening the im-
pressions it receives. Wonderful as is the formation of the
bodily fabric, and dilhcult as it is to collect its history, still

406 THE SENSORIAL FUNCTIONS.

more marvellous is the progressive construction of the hu-
man mind, and still more arduous the task of tracing the
finer threads which connect the delicate web of its ideas,
which fix its fleeting perceptions, and which establish the
vast system of its associations, and of following the long se-
ries of gradations by which its affections are expanded, pu-
rified, and exalted, and the soul prepared for its higher des-
tination in a future stage of existence.

Here, indeed, we perceive a remarkable interruption 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 hardlv estimate the magnitude. The func-
tions 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
sensation a greater inequality may be perceived: the intel-
lectual faculties, again, are of a more refined and nobler cha-
racter, 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 infinite number
of intermediate degrees. The more exalted faculties are
possessed exclusively by man, and constitute the source of
the immense superiority he enjoys over the brute creation,
which so frequently excels him in the perfection of subor-
dinate powers. In strength and swiftness he is surpassed
by many quadrupeds. In vain may he wish for the power
of flight possessed by the numerous inhabitants of air. He
may envy that range of sight which enables the bird to dis-
cern 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 dulness of his own senses, w^hen he adverts to
the exquisite scent of the hound, or the acute hearing 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 injurious, man alone seems stinted in his pow-
ers of discrimination, and is compelled to gather instruction

INTELLECTUAL FACULTIES OF MAN. 407

from a painful and hazardous experience. Bui if nature has
created him thus apparently helpless, and denied him those
instincts with which she has so liberally furnished the rest
of her offspring, it was only to confer upon him gifts of in-
finitely 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 compre-
hension, in extent of reasoning, in capacity of progressive
improvement, he leaves all other animals at an- immeasura-
ble 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 ema-
nated, to whom he owes all the blessino-s which attend it,
and by whom he has been taught to look forward to bright-
er skies and to purer and more exalted conditions of exist-
ence. Heir to this high destination, Man discards all alli-
ance 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 development of which the great scheme of Pro-
vidence here commenced, will be carried on, in a future
state of being, to its final and perfect consummation.

( 408 )

PART IV.

THE REPRODUCTIVE FUNCTIONS.

CHAPTER I.

REPRODUCTION.

Limits have been assigned to tlic duration of all living
beings. The same power to vvbom they owe their creation,
their organization, and their endowments, has also subjected
them to the inexorable Law of Mortality; and has ordained
that the series of actions which characterize the state of life,
shall continue for a definite period only, and shall then ter-
minate. The very same causes which, at the earlier stages
of their existence, promoted their development and growth,
and which, at a maturer age, sustained the vigour and ener-
gies 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 destruction of the
faculties they had originally 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 ex-

• See the article '• Age," in the Cydopxdia of Practical Mcdiciney where
I have enlarged on this subject.

EEPRODUCTIO.V. 409

pended, 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 production of new individuals, each spring-
ing from its predecessor in endless perpetuity. The pro-
cess by which this formation, or rather this apparent crea-
tion, of a living being is effected, surpasses the utmost
powers of the human comprehension. No conceivable com-
binations of mechanical, or of chemical powers, bear ihe
slightest resemblance, or the most remote analogy, to or-
ganic reproduction, or can afford the least clew to the solu-
tion of this dark and hopeless enigma. We must be con-
tent to observe and generalize the phenomena, in silent
wonder at the marvellous manifestation of express con-
trivance and design, exhibited in this department of the
economy of created beings.

Throughout the whole, both of the vegetable and animal
world. Nature has shown the utmost solicitude to secure
not only the multiplication of the species, but also the dis-
semination of their numbers over every habitable and acces-
sible region of the globe, and has pursued various plans for
the accomplishment of these important objects.

The simplest of all the modes of multiplication consists
in the spontaneous division of the body of the parent into
two or more parts; each part, when separated, becoming a
distinct individual, and soon acquiring the size and shape of
the parent. We meet with frequent examples of this pro-
cess of fissiparous generation, as it is termed among the
infusory animalcules. Many species of Monads, for in-
stance, which are naturally of a globular shape, exhibit at a
certain period of their development a slight circular groove
round the middle of their bodies, which by degrees be-
coming 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

Vol. II. 52

410 THE REPRODUCTIVE FUNCTIOXS.

the fluid, as if animated by two different volitions; and, ap-
parently for the purpose of tearing asunder the last connect-
ing fibres, darting through the thickest of the crowd of sur-
rounding animalcules; and the moment this slender ligament
is broken, each is seei; moving away from the other, and
beginning its independent existence. This mode of sepa-
ration is illustrated by Fig. 462, representing the successive
changes of form during this progress. In this animalcule
the division is transverse, but in others, for example in the

462

463'

0 8 3 8?

o

Vorticella, (as shown in Fig. 463,) and in most of the larger
species, the line of separation is longitudinal. Each animal-
cule, thus formed by the subdivision of its predecessor, soon
grows to the size which again determines a farther spon-
taneous subdivision into two other animalcules; these, in
course of time, themselves undergo the same process, and
so on, to an indefinite extent. The most singular circum-
stance attending this mode of multiplication is that it is im-
possible to pronounce 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 ex-
istence, we must recognise an unbroken continuity in the
vitality of the animal, thus transmitted in perpetuity from
the original stem, throughout all succeeding generations.
This, however, is one of those metaphysical subtleties for
which the subject of reproduction aflfords abundant scope,
but which it would be foreign to the object of this work to
discuss.

REPRODUCTION. 411

It is in the animal kingdom only that we meet with in-
stances of this spontaneous division of an organic being into
parts, where each reproduces an individual of the same spe-
cies. All plants, however, are capable of being multiplied
by artificial divisions of this kind; thus, a tree may be di-
vided longitudinally into a great number of portions, or
slips, as they are called, any one of which, if planted sepa-
rately and supplied with nourishment, may continue to
grow, and may, in time, reproduce a tree similar in all re-
spects to the one from which it had originated. This inhe-
rent 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, ultimately, a com-
plete plant will be formed.* These facts, which are well
known to agriculturists, exhibit only the capal)ilities of ve-
getative power under circumstances which do not occur in
the natural course of things, but have been the effect of hu-
man interference.

Reproductive powers of a similar kind are exhibited very
extensively in the lower departments of the animal king-
dom. The Hydra, or fresh water polype, is capable of in-
definite multiplication by simple division: thus, if it be cut
asunder transversely, the part containing the head soon sup-
plies itself with a tail; and the detached tail soon shoots forth
a new head, with a new set of tentacula. If any of the ten-
tacula, or any portion of one of them, be cut off, the mutila-

• Among the conditions necessary for these evolutions of organs .ire, first,
the previous accumulation of a store of nourishment in the detached frag-
ment, adequate to supply the growth of the new parts; and, secondly, the
presence of a sufficient quantity of circulating sap, as a vehicle fbr the trans-
mission of 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. Ac-
cording to the observations of Mirandola, the leaf of the BryophyUum, when
simply laid on moist groimd, strikes out roots, which quickly penetrate into
the soil. (De CandoUe, Physiologic Vcgctale, ii. 677.) The leaves of the,
monocotyledonous plants often present the same phenomenon.

412 THE REPRODUCTIVE FUNCTIONS.

tion 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 wliich are wanting to render it a
complete individual. The same phenomena are observed,
and nearly to the same extent, in the Flanaria. The *^5-
terias, the %/lciinia, and some of the lower species of Anne-
lida, as the Kais, arc also capable of being multiplied by ar-
tificial divisions, each segment having the power of supply-
ing others, and containing within itself a kind of separate
and individual vitality.

A power of more partial regeneration of mutilated parts
by new growths, which is very analogous to that of com-
plete reproduction, exists in the higher orders of animals,
though it does not extend to the entire formation of two in-
dividuals out of one. The claws, the feet, and the antennae
of the Crustacea, and the limbs of the Arachnida, are re-
stored, 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 hap-
pens 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 for-
mer. We have seen that the teeth of Sharks and other
fishes are renewed with the utmost facility, when by acci-
cident they have been lost. Among Mammalia, similar
powers exist, although they are restricted within much nar-
rower 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 hap-
pening to various partJ' of our own frame, we have abun-

REPRODUCTION. 413

dant reason to admire and be grateful for the wise and boun-
tiful provisions which nature has made for meeting these
contingencies.

The multiplication of the species by buds, or Cernmipa-
rous reproduction, \s 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 gerins.

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 developed at the axillse of the leaves; that is, at the
angles of their junction with the stem; and also at the ex-
tremities of the fibres of the stems; their development being
determined by the accumulation of nourishment around
them. They first produce buds, which expanding, and put-
ting forth roots, assume the form of shoots; and the succes-
sive 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

* In some rare instances the shoots are removed to a distance from the pa-
rent plant, by a natural process: this occurs in some creeping' plants, which
propagate themselves by tlie horizontal extension of their branches on the
ground wliere thay dip, and strike out new roots, g'iving- rise to stems inde-
pendent of the orig-inal plant. This also sometimes happens in the case of
tuberous roots, as the potato, which contain a number of germs, surround-
ed by nutritive matter, ready to be developed when circumstances are fa-
vourable. These portions are called eyes; and each of them, when planted
separately, are readily evolved, and give rise to an individual plant.

41 t THE REPRODUCTIVE FUNCTIONS.

cease to grow. The Lemna^ or common Duckweed, which,
consists of a small circular leaf, floating on the surface of
sta«-nant pools, presents a singular instance of the develop-
ment of germs from the edges of the leaves, and the suhse-
quent separation of the new plant thus formed. In this re-
spect the process is analogous to the natural mode of multi-
plication met with in the lower orders of Zoophytes, such as
the Hydra. At the earliest period at which the young of
this animal is visihle, it appears like a small tuhercle, 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 nourishment, as well as its mechanical sup-
])ort, from the parent; then occasionally stretching forth its
tentacula, and learning the art of catching and of swal-
lowing its natural prey. The tube, which constitutes its sto-
mach, at first communicates by a distinct opening with that
of its parent: but this opening afterwards closes; and the fila-
ments by which it is connected with the parent becoming
more and more slender, at length break, and the detached
hydra immediately moves away, and commences its career
of independent existence. This mode of multiplication, in
its first period, corresponds exactly with the production of
a vegetable by buds; and may therefore be classed among
the instances of gemmiparous reproduction; although at a la-
ter 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. In this case they are termed gemmules instead of
buds. This mode of reproduction is exemplified in the Vol-
vox, which, as we have already seen, is an infusorial animal-
cule of a spherical form, exhibiting incessant revolving move-
ments.* The germs of this animal are developed, in great
numbers, in its interior, having a globular shape, and visible^

• Vol. i. p. 159. This animal is delineated in Fig. 79.

REPRODUCTION. 415

by the aid of the microscope, through the transparent co-
vering; and while yet retained within the hody of the pa-
rent, other still minuter glohulcs 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 con-
sequence, perishes. Similar phenomena are presented by
many of the Infusoria. In some of the Entozoa, likewise,
as in the Hydatid, the young are developed within the pa-
rent; and this proceeds successively for an indefinite number
of generations.* In most cases of the spontaneous evolution

* The mode in which infiisory 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 fa-
vourable to its operation, tends to form these minute organizations, and in
this manner they explain how the same organic matter which had composed
I former living aggregates, on the dissolution of their union, reappears under

new forms of life, and gives rise to the phenomenon of innumerable animal-
cules, starting into being, and commencing a new, but fleeting career of ex-
istence. Yet the analogy of every other department of the animal and ve-
getable 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. Tiie disco-
veries of Ehrenberg relative to the organization of the Rotifera go far to-
wards 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 assumption on which it rests, is
not required for the explanation of the obsen'cd 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. AVhcn tlicse had
gi'own to the size of the 1700th of an inch, or seven times their original dia-
meter, they were distinctly seen to excite cuirents, and to swallow food.
The same diligent observer detected the young of the Rotifer vulgaris, per-
fectly formed, moving in the interior of the parent animalcule, and excluded
in a living state, thus constituting them viviparous animals, as the fonncr were

416 THE REPRODUCTIVE FUNCTIONS.

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
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 pa-
rent, and docs not receive the special protection of mem-
branous envelopes, containing a store of nourishment for its
subsequent growth. But in all the more perfect structures,
both of animals and vegetables, 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 take 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 exam-
ples of this mode of reproduction, the seeds being formed at
the under surface of the leaves, apparently by a simple pro-
cess of evolution; and when detached and scattered on the
ground, being farther developed into a plant similar to the
parent. The Linnean class of CryptogaJiiia includes all the
plants coming under this description. In Animals, likewise,
it is only in the particular organs termed ovaries, that ova
are formed, and they are generally divided into compart-
ments, the whole being enclosed in a membranous covering,
bearing a great resemblance 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 multipli-
cation by mere slips or buds; and the products of the former

oviparous. Other species, ag'ain, imitate the hydra, in being what is termed
gemmiparousy tliat 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. 417-

retain less of the peculiar characters of the individual from
which they spring, than those of the latter. This is re-
markably exemplified in the case of orchard trees, such as
apples and pears; for all the trees which derive their ori«rin
from shoots, or grafts from the same individual, j)artake of
the same properties, and prochice a fruit of the same flavour
and qualities; whereas, trees of the same sj)ccies, which <rro\v
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. ]5y continued graftings,
after a long period, the vitality of the particular variety is
gradually exhausted, and the grafts no longer bear fruit.
This has already happened with regard to the two varieties
of apples just mentioned. For these curious facts, and the
theory which explains them, we are indebted to the obser-
vation 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 bein"-
the formation of the seed or ovum, and that of the other the
production of the vivifying fluid. 'J'he cfl'ect of this vivi-
fying fluid upon the dormant germ is termed Fecunda-
tion; and the germ, when fecundated, receives the name of
Embryo.

The modes in wliich the fecundation of the germ is ac-
complished are exceedingly various in diflcrcnt classes of
organized beings. In all F haneroganious plaiils , (so named

* See his various papers in the Philosophical Tnnsactions.
Vol. II. 53

418 THE REPRODUCTIVE FUNCTIONS.

in contradistinction to those which are Cryptogavioits,) the
whole of tlie doulile apparatus required for reproduction is
contained in \.\\q. Jlower. One set of organs contains the ru-
diment of the seed, enclosed in various envelopes, of which
the assemblage constitutes an ovary, and to which is ap-
pended a tube, (the pistil,) terminated by a kind of spon-
giole, (the stigma. J The fecundating organs are the sta-
rnenSj which arc columns, {or filaments,) placed generally
near and parallel to the pistil, and terminated by a glandu-
lar organ, (the anther.) This organ, when mature, con-
tains, enclosed in a double envelope, a fine powder, (the
pollen,) consisting of very minute vesicles, filled with a vis-
cous liquor, {ihe/ovillaj in which are seen extremely small
granules. Fecundation takes place by a portion of the pol-
len being received by the stigma, and conveyed through the
tubular pistil to the seed, which it impregnates by impart-
ing to it the fluid it contains.

By far the greater number of plants composing the vege-
table kingdom have these two sets of organs contained in
the same flower; or at least in flowers belonging to the same
individual plant. In the animal kingdom this arrangement
is also adopted, but only in a comparatively 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,
composing the class Dioecia, the individuals of the same
species are distinguished by their bearing flowers which con-
tain 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 ef-
fected only by the transference of the pollen from the for-
mer. A similar separation of ofl^ices 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

REPRODUOTION. 419

impregnation is internal, and their subsequent development
takes place in one or other of the four following ways.

1. The ovum, when defended by a firm envelope, which
contains a store of nutriment, is termed an ej^g, and is de-
posited in situations most favourable for the development of
the embryo; and also for its future support when it emerges
from the egg. Birds, as is well known, produce eggs which
are incased in a calcareous shell, and hatch them by the
warmth they communicate by sitting on them with unwea-
ried constancy. All animals which thus lay eggs are termed
oviparoKS.

2. There are a few tribes, such as the Viper and the Sa-
lamander, 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 ani-
mals are said to be Ovo-viviparous. There are other tribes,
again, which, according to circumstances, are either ovipa-
rous, or ovo-viviparous: this is the case with tiie Shark.

3. Viviparous animals are those in which no egg, pro-
perly so called, is formed; but tlie ovum, after proceeding
through the oviduct, sends out vessels, which form an at-
tachment to the interior of a cavity in the i)ody of the ])a-
rent, whence it draws nourishment, and therefore has at-
tained a considerable size at the time of its birth.

4. 3iar5Z//?/«/ animals are those, which, like the Kam^ti-
roo, and the Opossinn, are provided with abdominal pouclies,
into which the young, born at a very early stage of develop-
ment, are received, and nourished witJi milk, secreted from
glands contained within these pouches. As the young, both
in this and in the last case, are nourished willi milk prepared
by similar glands, or Mammx, the whole class of vivipa-
rous and marsupial animals has received, from tiiis ch.ir.u'-
teristic circumstance, the name of Mainmalia.

( 420 )

CHAPTER II.

ORGANIC DEVELOPMENT.

Although the study of organic structures ia their finished
state must tend to inspire the most suhlime conceptions of
the Great Creator of this vast series of beings, extending
from the obscurest plant to the towering tenant of the fo-
rest, and from the lowest animalcule to the stately elephant
and o-io-antic whale, there yet exists another department of
the science of Nature, removed, indeed, from the gaze of or-
dinary observers, but presenting to the philosophic inquirer
subjects not less repkte with interest, and not less calculated
to exalt our ideas of the transcendent attributes of the Al-
mio-hty. To a mind nurtured to reflection, these divine at-
tributes whether of power, of wisdom, or of beneficence,
are no where manifested with greater distinctness, or ar-
rayed in greater glory, than in the formation of these various
beinf^s 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 changes which consti-
tute the metamorphoses of winged insects,* and of I^atra-
chian 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 ani-
mals, even those belonging to the higher classes, such as birds,
and mammalia, not excepting man himself, undergo, in the
early stages of their development, a series of changes fully
as ^^reat and as remarkable as those which constitute the

• The Researches of Nordmann, on different species of Lernsea, have
orou^ht to li^^ht the most sint^ailar succession of forms during the progress of
development of the same individual animal.

ORGANIC DEVELOPMENT. 421

transformations of inferior animals. They have also ren-
dered it extremely probable that the organs of the system,
r instead of existing simultaneously in tl)e germ, arise in re-

gulated succession, and arc the results not of tlic mere ex-
pansion of pre-existing rudiments, but of a real formation
by the union of certain elements; which elements are them-
selves successively formed by the gradual coalescence or
juxtaposition of their constituent materials. On contem-
plating the infinitely lengthened chain of means and ends,
and of causes and eflfects, which, during the construction
and assemblage of the numerous parts composing the ani-
mal machine, are in constant operation, adapting them to
their various purposes, and combining them into one effi-
cient and harmonious system, it is impossible not to be deep-
ly impi-essed with the extent and the profoundness of the
views of Omniscient Providence, which far exceed the ut-
most boundaries of our vision, and surpass even the powers
of the human imagination.*

The clearest evidence of enlarsjed and provident dcsio-ns
may be collected from observing the order in which the
nascent organs are successively brought forwards, and added
to the growing fabric: such order appearing, in all cases, to
be that best calculated to secure the due performance of their
appointed functions, and to promote the general objects of
the system. The apparatus first perfected is that which is
immediately necessary for the exercise of the vital actions,
and which is therefore required for the completion of all the
other structures; but provision is likewise made for the esta-

* " si Ton applique," says Cuvier, when sijcaking- of the anatomy of in-
sects, «' u chaciine cle ces espcces, par la pensec, ce qu'il seroit bien impos-
sible qu'un homme entreprit de verifier en effet pour toutes, ime orgnmisa-
sation a-peu-pres egale en complication fi cclle qui a etc dccrite dans la
chenille par Lyonet, ct tout rccemmcnt dans le hanneton par M. Straus, ct ce-
pendant plus ou moins dift'crentc dans chaquc insecte, Tiniag-ination com-
mencera ;\ concevoir quelque chos% de cette richessc cfiVa}ante, et de ces
millions de millions dc parties, et de parties de parties, toujours coiTclativcs,
toujours en harmonic, qui constituent le grand oun'ag-c dc la nature." (Ilis-
toire dcs Progres des Sciences NaturcUcs, iv. 145.)

422 THE REPRODUCTIVE FUNCTIONS.

blishment 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 preparing for endowments of
a higher kind, by the early development of the organs of
the external senses, the functions of which so essentially mi-
nister to the future expansion of the intellectual faculties,
embracing'- a wide range of perceptions 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 necessi-
ties of the occasion change. At first, all the energies of vi-
tality are directed to the raising of the fabric, and to the ex-
tension of those organs which are of greatest immediate
utility; but still having a prospective view to farther and
more important ends. For the accomplishment of this pri-
mary 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 essen-
tial 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 functions of nu-
trition; and we accordingly find that whenever the seed be-
gins to "-erminate, the first indication of development is the
appearance of the part called the plumula, which is a col-
lection of feathery fibres, bursting from the enveloping cap-
sule of the germ, and which, whatever may have been its
original position, proceeds immediately to extend itself ver-
tically upwards; while, at the same time, slender filaments,
or radicles, shoot out below to form the roots. Thus early
are means provided for the absorption 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

ORGANIC DEVELOPMENT. 423

to a higher order of endowments, is placed in subordination
to a class of functions, of which there exists no trace in ve-
getables, namely, those of the nervous system. }ly intently
watching the earliest dawn of organic formation, in the trans-
parent 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 fol-
lowing 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 side of this
dark line;* and by the union of their edges, they soon form
a canal, containing a deposite 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 cha-
racter of the being to be formed depends. We may next
discern a number of parallel equidistant dots, arranged in
two rows, one on the outer side of each of the fdaments al-
ready noticed: 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. t

The appearance of the elementary filaments of the spinat
cord is soon followed by the development of its u])j)cr or
anterior extremity, from which there arise three vesicles,
each forming white tubercles; these are the foundations of
the future brain. The tubercles are first arranged in pairs
and in a longitudinal series, like those we have seen consti-
tuting the permanent form of the brain in the inferior fishes:

* The plicae primitivx of Pander; the lamina; dorsales of Bucr. Sec a
paper on embryolog-y by Dr. Allen Thomson, in the Eclin. New Phil. Journal
for 1830 and 1831.

f These ring's have, by speculative physiologists, been supposed to be
analogous to those which form the skeleton of the Annelida.

424 THE REPRODUCTIVE FUNCTIONS.

but, in birds, they arc 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
sino-le 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
leo-s and wings do not show themselves until the brain has
acquired greater solidity and development. The nerves
which are to connect these organs of sensation and of mo-
tion 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 homogenous jelly
by the simpler efforts of the vital powers, the elevation of
the vast superstructure 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 vegetative processes, having similar objects in view,
and the adoption of analogous means for their accomplish-
ment; but so widely different in character is the whole or-
ganic economy of these two orders of beings, that we per-
ceive no resemblance in the mechanism employed for their
formation. For the purposes of animal life the nutrient
juices must be brought into active circulation by means of
vessels extensively pervading the system. Nature, then,
hastens to prepare this important hydraulic apparatus, with-
out which the work of construction could not proceed.
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 con-
solidated in the form of opaque globules. These globules
are at first seen to meander through the mass, unconfined by
investing vessels; presently, however, a circular vessel is
discovered, formed by the foldings of the membrane of the
embryo, along which the fluids undulate backwards and for-

ORGANIC DEVELOPMKXT. 425

wards, without any constancy.-'^ A delicate net-work of ves-
sels is next formed in various pnrts of tlic area of the circle,
which are seen successively to join by the formation of com-
municating branches, and ultimately to compose lar<rcr
trunks, so as to estai)lish a more general system of vascular
organization. But increased power for carrying on this ex-
tended circulation will soon be wanted; and for this purpose
•there must be provided a central organ of proj)ulsion, or
heart, the construction of which is now commenced, at a
central point, by the folding inwards of a lamina of the mid-
dle membrane, forming first a simple groove, but, aflcr a
time, converted, by the union of its outer edges, into a kind
of sac, which is soon extended into a longitudinal tube.t
The next object is to bring this tube, or rudimental heart,
into communication with the neighbouring vascular trunks,
and this is effected by their gradual elongation, till their ca-
vities 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 established in the direc-
tion in which it is ever after to flow; and we may now re-
cognise this central organ as the heart, which, under the
name of the punctnm 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 retain; for
from being at first a mere lengthened tube, presenting three
dilatations, which are the cavities of the future auricle, ven-
tricle, and bulb of the aorta, it assumes in process of time a
rounded shape, by the folding of its parts, the whole of

* These phenomena are similar to those which were noticed as presented
by the larvae of some insects and other inferior animals.

f The discovery of this fact is due to Pander. Sec also the works of Ro-
lando, Wolff, Prevost and Dumas, and Seircs.

^OL. ir. 54

426 THE REPRODUCTIVE FUNCTIONS.

which arc coiled, as it were, into a knot; by wliich means
the difierent cavities acquire relative situations more near-
ly corresponding to their positions in the developed and
finislicd orjran.

Tlie blood vessels, in like manner, undergo a series of
changes quite as considerable as those of the heart, and to-
tally altering their arrangement and distribution. Serres
maintains that the primitive condition of all the organs, even
those which are generally considered as single, is that of be-
ing double, or being formed in pairs; one on the right, and
another exactly similar to it on the left of the middle, or
7nesial plane, as if each were the reflected image of the
other.*" Such is obviously the permanent condition of all
the organs of sensation, and also of the apparatus for locomo-
tion: 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 arc brought together, and con-
joined into single organs. In like manner we have seen that
the constituent laminse of the heart are at first double, and
afterwards form, by their union, a single cavity. The ope-
ration 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 wer^ originally formed by the coalescence of
double vascular trunks running parallel to each other, and
at first separated hy a considerabl-e interval; then approach-
ing each other, adhering together, and quickly converted,

• A remarkable exemplification of this tendency to symmetric duplication
oforg-ans occurs in a very extraordinary parasitic animal, which usually at-
taches itself to the gills of the Cyprinus hrama, and which has been lately
examined by Nordmann, and named by him tl>e Dipluzoon paradoxum, from
its having- the semblance of two distinct animals of u lengthened shape, each
bent at an obtuse angle, aiul joined together in the form of the letter X. The
right and left halves of this cross are perfectly similar in their organization,
leaving each a complete and independent system of vital organs, excepting
that the two alimentary canals join at the centre of the cross to form a single
•cavity, or stomacli, (Aimales des Sciences NatureHes, xxx. o7o.)

ORGANIC DEVELOPMENT. 427

by the obliteration of the parts which are in contact, into
single tubes, throughout a considerable portion of their
length.*

Nature, ever vigilant in her anticipations of the wants of
the system, has accumulated round the embryo amj)le stores
of nutritive matter, sufficient for maintaining the life of tlic
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 foresight of future circum-
stances, she delays not, longer than is necessary for the
complete establishment 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 formation of two laminae, arising from folds of the in-
nermost of the pellicles which invest the embryo; that is, on
the surface opposite to the one which has produced the spi-
nal marrow. These laminae, which are originally separate,
and apart from one another, are brought together, and by
the junction or soldering of their opposite edges, formed into
a tube,t which, from being, at first, uniform in diameter, af-
terwards expands into several dilated portions, correspond-
ing with the cavities of the stomach, crop, gizzard, &c., into
which they are to be converted, when the time shall come for
their active employment. These new organs are, however,
even in this, their rudimental state, trained to the perform-
ance of their proper offices, receiving into their cavities,
through a tube temporarily provided for that purpose, the
fluid of the yelk, and preparing nourishment from it.

In the mean time, early provision is mado for the aera-
tion of the fluids by an extensive but tcm])orary system of

* These facts were fii-st observed by Scn-cs (Annates dcs So. Nut. xxi. 8,)
and their accuracy has been confirmed by the observations ofDr. Allen 'I'honi.
son. In Reptiles this union of the tw( onstituent trunks of the aorta is ef-
fected only at the posterior part, wh' the anterior portion rcmuiijs perma-
nently double. (Sec Fig. '357, vol. li. p. 197.)

f Wolf! is the author of tliis discovery.

428 THE REPRODUCTIVE FUNCTIONS.

vessels, spread over the membrane of the egg, and receiving
tlie influence of atmospheric oxygen tlirough the substance
of tlie shell, which is sufllcicntly porous to transmit it; and
these vessels, being brought into communication with the
circulatory system of the chick, convey to its blood this vi-
vifying agent. As the lungs cannot come into use till after
the bird is emancipated from its prison, and as it was suffi-
cient that they^ be in readiness at that cj^och, these organs
are amonji the last which are constructed: and as the me-
chanism of respiration in this class of animals does not re-
quire the play of the diaphragm, this muscular partition,
though begun, is not completed, and there is no separation
between the cavities of the thorax and the abdomen.

The succession of organic metamorphoses is ecjually re-
markable in the formation of the diversified apparatus for
aeration, which is required to be greatly modified, at differ-
ent 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, emerge from the water
when they have acquired wings; and also in the steps of
transition from the tadpole to the frog. ]3ut similar, though
less conspicuous changes occur in the higher vertebrated
animals, during the early periods of their formation, corre-
sponding to the differences in the modes of aeration em-
ployed at different stages of development. In the primeval
conditions this function is always analogous to that of aqua-
tic animals, and requires for its performance only the sim-
pler 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 gi'adually
adapted to this altered condition; and the heart of the Bird
and the Quadruped must be separated into two compart-
ments, corresponding to the double function it will have to
perform. For this purpose a partition wall must, be built
in its cavity; and this wall is accordingly begun around the
interior circumference of the ventricle, and is gradually car-
ried on towards the centre, there being, for a time, an aper-

ORGANIC DEVELOPMENT. 429

ture of communication between the right and left cavities;
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 by a cord, which was gradu-
ally 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 vessels; that which issues
from the left ventricle being the aorta; and the other, which
proceeds from the right ventricle, being the pulmonary ar-
tery; and each being now prepared to exercise its appropri-
ate function in the double circulation which is soon to be
established.*

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 re-
semble branchiae. These changes may be very distinctly
followed in the Batrachia;\ for we see, in those animals,
the trunk of the aorta undergoing successive subdivisions,
by branches sent off from it, and forming loops, which ex-
tend in length and are again subdivided, in a manner not
unlike the unravelling of the strands of a rope; each subdi-
vision, however, being preceded by the formation of a dou-
ble partition in the cavity of the tube; so that at length the
whole forms an extensive ramified system of branchial arte-
ries and veins. Still all these are merely temporary struc-
tures; for when the period of change approaches, and the
branchiae are to be superseded in their office, every vessel,
one after another, becomes obliterated, and there remain
only the two original aorta, which unite into a single trunk
lower down, and from which proceed the pulmonary arte-
ries, conveying either the whole, or a portion of the blood,
to the newly developed respiratory organs, the lungs.

* The principal authorities for the facts here stated are Bacr and Rolando.
See the paper of Dr. Thomson already quoted.

t See the investigations of Kusconi, and of Bacr, on this subject.

430 THE KEPRODUCTIVE FUNCTIONS.

By a similar process of continued bifurcation, or the de-
tachment of branches in the form of loops, new vessels are
developed in other parts of the body, as has been particu-
larly observed in the finny tail, and the external gills of the
frog, and the newt, parts which easily admit of microscopi-
cal examination.*

Progress is in the mean while making in the building of
the skeleton, the forms of the principal bones being modelled
in a gelatinous substance, which is converted into cartilage,
beginning 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, suit-
ed to the 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 integument. The eyes,
which were hitherto unprotected, receive special means of
defence, by the addition of eyelids, which are formed by a
farther extension and folding of these integuments; 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, however,
as yet too tender to apply the force requisite for that pur-
pose. 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 battering 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 pro-

• Sucli is the result of the concurring observations of Spallanzani, Fonta-
na, and DoUinger.

ORGANIC DEVELOPMENT. 431

vided only for this temporary use appears from the circum-
stance of its falling off spontaneously in the course of three
or four days after it has been so employed.

But though tlic bird has now gained its liberty, it is still
unable to provide for its own maintenance, and requires to
be fed by its parent till it can use its wings, and has learned
the art of obtaining food. The pigeon is furnislied by na-
ture with a secretion from the crop, with which it feeds its
young. In the Mammalia the same object is provided for
still more expressly, by means of glands, whose oflice it is
to prepare milk, 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 mammary glands; bwt as the structure 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 difliculty in
understanding how they obtain the nourishment so pro-
vided. A recent discovery of Geoffroy St. Ililaire appears
to have resolved the mystery with respect to ihc De/p/u?ius
globiceps; for he found that the mammary glands of that ani-
mal contain each a large reservoir, in which milk is accu-
mulated, and which the dolphin is capable, by the action of
the surrounding muscles, of emptying at once 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 deve-
lopment in the higher animals, taken in conjunction with the
facts already adverted to in various parts of this Treatise,
and particularly those relating to ossification, dentition, the
formation of hair, of the quills of the porcupine, of the an-
tlers of the stag, and of the feathers of birds, will suffice to
show that they are regulated by laws which arc definite, and
preordained according to the most enlarged and profound

• The account of this discovery is contained in a memoir wliich was read
at the "Institut." March 24, 1834.

432 THE REPRODUCTIVE FUNCTIONS.

views of the future circumstances and wants of the system.
The double origin of all the parts of the frame, even those
which appear as single organs, and the order of their forma-
tion, wliich, 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 unex-
pected results of this train of inquiries.* We cannot but be
forcibly struck with the numerous forms of transition through
which every organ has to pass before arriving at its ultimate
and comparatively permanent condition: we cannot but won-
der at the vast apparatus which is provided and put in action
for eflecting all these changes; nor can we overlook the in-
stances of express contrivance in the formation of so many
temporary structures, which are set up, like the scaffold of
an edifice, in order to afford the means of transporting the
materials of the building in proportion as they are wanted;
nor refuse to recognise the evidence of provident design in
the regular order in which the w^ork proceeds, every organ
growing at its appointed time, by the addition of fresh par-
ticles brought to it by the arteries, while others are carried
away by the absorbents, and gradually acquiring the form
which is to qualify it for the performance of its proper ofTice
in this vast and complicated system.

• The first of these two laws is termed by Serres, who has zealously pro-
secuted these investigations, " la hi dc symmHrit;'" and the second, "^f hi
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 dis-
course to his *' Anatomie comparee du cerveau," p. 25; and also his several
memoirs in the " Annales des Sciences Naturelles," vols. xi. xii. xvi. and xxu

An excellent summary of the principal facts relating to the development
of the embryo is given by Mr. Herbert Mayo, in the third edition of his
^* Outlines of Human Physiology.'*

( 433 )

CHAPTER III.

DECLINE OF THE SYSTEM.

To follow minutely the various steps by which Nature
conducts the individual to its state of maturity, would en-
gage us in details incompatible with the limits of the pre-
sent 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 abun-
dance of nourishment, so that the average supply of mate-
rials rather exceeds the consumption: but when tlie 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 qua-
lifying the organs for the continued exercise of their re-
spective 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, will inevitably bring on the decline
and ultimate destruction of the system.* The process of

* It would appear, from the researches of De CandoUe, that the vegetab?e
system is not, like the animal, subject to the destructive operation of internal
causes; for the agents which destroy vegetable life are always extitmeous 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 obli-
teration of many of its cells and vessels, the vitaUty 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 umiiininishcd
vigour; and unless 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

Vol. II. 55

434 DECLINE OF THE SYSTEM.

consolidation, begun from the earliest period of development,
is still advancing, and is producing in the fluids greater thick-
ness, and a reduction of their total (juantity; and in the so-
lids, a diminution in the proportion of gelatin, and the con-
version of this element into albumen. Hence, all the tex-
tures acquire increasing solidity, the cellular substance
becomes firmer and more condensed, and the solid structures
more rigid and inelastic: hence, the tendons and ligamentous
fibres growing less flexible, the joints lose their suppleness,
and the contractile power 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 them-
selves grow harder and more brittle; and the cartilages, the
tendons, the serous membranes, and the coats of the blood
vessels, acquire incrustations of ossilic matter, which inter-
fere with their uses. Thus are all the progressive modifica-
tions of structure tending, slowly but inevitably, to disqua-
lify the organs for the due performance of their functions.

Among the most important of the internal changes con-
sequent 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, gra(hially disappear. The parts
of the body, no longer yielding to the power applied to ex-
tend them, oppose a gradually increasing resistance to the
propelling force of the heart: w^hile, 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,

other hand, we were to regard each separate shoot as an individual organic
body, and every layer us constituting a distinct 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. 435

and a cllmhiution of energy becomes apparent in every func-
tion.

Such are the insensible gradations by whicli, while gliding
down the stream of time, we lapse into old age, which in-
sidiously 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 has
conducted the system unimpaired to its maturity, is still ex-
erted in adjusting the conditions under which it is placed
in its season of decline. New arrangements are made, new
energies are called forth, and new resources arc em})loyed,
to accommodate it to its altered circumstances, to pro}) the
shattered fabric, and retard tiie progress of its decay. In
proportion as the supply of nutritive materials has become
less abundant, a more strict economy is practised with re-
gard to their disposal; the substance of the body is husband-
ed 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 len2;th of time without material altera-
tion.

The period prescribed for its duration being at length
completed, and the ends of its existence accomplished, the
fabric can no longer be sustained, and preparation must be
made for its inevitable fall. In order to form a correct
judgment of the real intentions of nature, with regard to
this last stage of life, its phenomena must be observed in cases
where the systeni has been wholly intrusted to the operation
of her laws. When death is the simple consequence of age,
we find that the extinction of the powers of life observes an
order the reverse of that which was followed in their evolu-
tion. The sensorial functions, wliicli were llu; last perfect-
ed, are the first which decav: and their decline is found to
commence with those mental faculties more inimediately
dependent on tiic j)hysical conditions of tlu^ sensorium, and
more especially with tlie memory, which is often much im-

436 DECLINE OF THE SYSTEM.

paired, 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 contrihutes to the decline of the intellectual
powers, hy withdrawing 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 immediate 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 resj)iration. The heart, whose pulsations gave the
first indications of life in the embryo, generally retains its
vitality longer than any other organ; but its powers being
dependent on the constant oxidation of the blood in the
lungs, cannot survive the interruption of this function; and
on the heart ceasing to throb, death may then be considered
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 sti-ange eventful history,"

the power of feeling has wholly ceased, and the physical
struggle is carried on by the vital powers alone, in the ab-
sence of all consciousness of the sentient being, whose death
may be said to precede, for some time, that of the body. In
this, as well as in the gradual decline of the sensorial facul-
ties, and the consequent diminution both of mental and of
physical sensibility in advanced age, we cannot fail to re-
cognise the wise ordinances of a superintending and bene-
ficent providence, 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 de-
parting sensation the tranquillity of approaching sleep.

( 437 )

CHAPTER IV.

UNITY OF DESIGN.

The inquiries on Animal and Vegetable Physiology 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 ex-
tend throughout the boundless regions of space, and whose
comprehensive plans embrace eternity.

In examining the manifold structures and diversified phe-
nomena of living beings, we cannot but perceive that they
are extensively, and perhaps universally connected by cer-
tain laws oiJinalogy; 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 analogies,
into classes, orders and genera, have but followed the foot-
steps of Nature herself, who in all her operations combines
the apparently opposite principles of general resemblance,
and of specific variety; so that the races which she has
united in the same group, though possessed of features in-
dividually dilTerent, may easily be recognised bv their fa-
mily likeness, as the offspring of a common parent.

*• Fades non omnibus una;
Nee (liversa tamcn; c[ualem decet esse sororuin."

We have seen that in each of the two great divisions, or

438 UNITY OF DESIGN.

klno-donis of organic nature, the same general objects are
aimed at, and the same general plans are devised for their
accomplishment; 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 com-
position and properties of their elementary textures, in the
nature of their vital powers, in the arrangement of their or-
gans, and in the laws of their production and development.
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 re-
spect to the organization and functions of the objects com-
prehended in each assemblage, whether we examine the
wonders of their mechanical fabric, or study the series of
processes by which nutrition, sensation, voluntary motion,
and reproduction are effected. To specify all the examples
which misiht be adduced in confirmation of this obvious
truth is here unnecessary; for it would be only to repeat the
numerous facts already noticed in every cha])ter of this trea-
tise, relative to eacli 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 esta-
blished, Nature appears to have kept in view a certain defi-
nite type, or ideal standard, to which, amidst innumerable
modifications, rendered necessary by the varying circum-
stances and different destinations of each species, she always
shows a decided tendency to conform. It would almost
seem as if, in laying the foundations of each organized fab-
ric, she had commenced by taking the exact copy of this
primitive model; and, in building the superstructure, had
allowed herself to depart from the original plan only for the
purpose of accommodation to certain specific and ulterior
objects, conformably with the destination of that particular
race of created beings. Such, indeed, is the hypothetical
principle, which, under the title of iLiiity of composition,
has been adopted, and zealously pursued in all its conse-

UNITY OF DESIGN. 439

quences, by many naturalists, of the liighcst eminence, on
the continent. As the facts on wliicli tliis hypothesis is
supported, and the views wliich it unfolds, are highly de-
serving of attention, I sliall here briefly state them; but in
so doing I shall beg to premise the caution that these views
should, for the present, be regarded as hyj)othctical, and as
by no means possessing the certainty of philosophical ge-
neralizations.

The hypothesis in question is countenanced, in the first
place, by the supposed constancy with which, in all the ani-
mals belonging to the same natural group, we meet with the
same constituent 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 applied. This jjrinciple has been most
strikingly exemplified in the osteology of vertcbrated ani-
mals; but its truth is also inferred from tlie 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 sys-
tem. Thus Nature has provided for the locomotion of the
serpent, not by the creation of new structures, foreign to
the type of the vertebiata, but by empIo3'ing 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 develojied
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 Gradation^ in conformity to which all the
living, together with the extinct races, of organic nature, ar-
range themselves more or less, into certain regular series,
is one of the consequences 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 Jii\culties and endowments by the graduated

440 UNITY OF DESIGN.

development 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 re-
sult merely from the different extent and direction given to
these organic developments; those of inferior races proceed-
ing to a certain point only, and there stopping: while in be-
ings of a higher rank they advance farther, and lead to all
the observed diversities of conformation and endowments.

It is remarked, in farther corroboration of these views,
that the animals which occupy the highest stations in each
series possess, at the commencement of their existence, forms
exhibiting a marked resemblance to those presented in the
permanent condition of the lowest animals in the same se-
ries; and that, during the progress of their development,
they assume, in succession, the characters of each tribe, cor-
responding to their consecutive order in the ascending chain:
so that the peculiarities which distinguish the higher ani-
mal, on its attaining its ultimate and permanent form, arc
those which it had received in its last stage of embryonic
evolution. Another consequence of this hypothesis is that
we may expect occasionally to meet, in inferior animals, with
rudimental organs, which fi'om their imperfect development
may be of little or no use to the individual, but which be-
come 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 An-
nelida 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 or-
ganization, from Vermes of the lower orders; both being
equally destitute of, or but imperfectly provided with ex-
ternal instruments of locomotion; both having a distinct vas-

UNITY OF DESIGN. 441

ciilar circulation, and mulllplc organs of digestion; and the
central filaments of the nervous system in both being stud-
ded with numerous pairs of equidistant ganglia. In tlie
worm all these features remain as permanent characters of
the order: in the insect they arc subsequently modified and
altered during its progressive metamorphoses. The em-
bryo 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 characterize 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 Rep-
tile, the Bird, and the warm-blooded quadruped, may be at
the period of their maturity, they arc scarcely distinguisha-
ble from one another in their embryonic state; and their ear-
ly development proceeds for some time in the same man-
ner. They all possess at first the characters of aquatic ani-
mals; 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 progressive 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 transitions all tliese aquatic cha-
racters are changed for those of a terrestrial quadruped, fur-
nished with limbs for moving on the ground, and with lungs
for breathing atmospheric air; and how the plan of circula-
tion is altered from branchial to pulmonary, in proportion

* This curious analogy is particularly obsciTabIc in the successive forms as-
sumed by the nervous system, which exhibits a gradual passage from that of
the Talitrus, to its ultimate greatest concentration in the Main. (See Fi-
gures 439 and 441, p. 382 and 383.) Milne Edwards has lately traced a si-
milar progression of development in the orgtuis of locomotion of the Crvista-
cea. (Annales des Sciences Naturcllcs, xxx. 354.)

Vol. II. 56

442 UNITY OF DESIGN.

as the gills wither and the lungs are developed. If, while
this change is going on, and while both sets of organs are
too-other executing the function of aeration, all farther de-
velopment were prevented, 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 Pro/ew.v, 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 useless 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 im-
proved constructions. Another, and a still more remarkable
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 development.

Birds, though destined to a very different sphere of ac-
tion 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 tu-
bercles, placed longitudinally, like those of fishes, and only
assuming its proper character at a later period. The res-
piratory 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 lamprey and the shark; and
the heart and great vessels are constructed like those of the
tadpole, with reference to a branchial circulation. In their
conversion to the purposes of aerial respiration, they under-

UNITY OF DESIGN. 443

go a series of changes precisely analogous to those of the
tadpole.

Mammalia, during the early periods of their development,
are subjected to all the transformations which iiave been now
described, commencing with an organization corrcsj)onding
to that of the aquatic tribes, exhibiting not only branchiae,
supported on branchial arches, but also branchial aj)crtures
in the neck, and thence passing quickly to Ihe conditions of
structure adapted to a terrestrial existence. The development
of various parts of the system, more especially of the brain,
the ear, the mouth, and the extremities, is carried still far-
ther than in birds. Nor is the human cmbrj-o exempt from
the same metamorphoses, possessing, at one period, branchiai
and branchial apertures similar to those of the cartilaginous
fishes,* a heart with a single set of cavities, and a brain con-
sisting 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, 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 com-
pletely to cover the cerebellum. Thus does the whole fab-
ric arrive, by a gradual process of mutation, at an extent of
elaboration and refinement, unattained by an}' other race of
terrestrial beings, and which has been justly regarded as
constituting the climax of organic development.t

* These facts are given on the authorities of JIathkc, Baer, Huschke,
Breschet, &c. Ann. des Sc. Naturellcs, xv. 266. See, also, the paper of
Dr. A. Thomson, ah-eady quoted.

t A popular opinion has long prevailed, even among the well informed,
that misshapen or monstrous productions, or lusus naturx, as they were
termed, exhibit but the freaks of nature, who w;is believed, on these occa-
sions, capriciously to abandon her usual course, and to amuse hei-self in the
production of grotesque beings, without any special object. But it is now
found that all defective formations of this kind arc occasioned by the impcr-

444 UNITY OF DESIGN.

It must, I think, be admittetl that the analogies, on which
the hypothesis in question is founded, are numerous and
striking; but great care should be taken not to carry it far-
ther than the just interpretation of the facts themselves may
warrant. It should be borne in mind that these facts are
few, compared with the entire history of animal development;
and that the resemblances which have been so ingeniously
traced, arc partial only, and fall very short of that universa-
lity, which alone constitutes the solid basis of a strictly phi-
losophical theory. Wliatever may be the apparent simila-
rity between one animal and another, during different peri-
ods of their respective developments, there still exists spe-
cific differences, establishing between ihem an impassable
barrier of separation, and elYactually preventing any conver-
sion of one species into another, however nearly the two
may be mutually allied. The essential characters of each
species, amidst occasional varieties, remain ever constant and
immutable. Although gradations, to a greater or less ex-
tent, may be traced among the races both of plants and ani-
mals, 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 in-
stances 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 that the original continuity of the series is indi-
cated by a few species presenting, in some respects, inter-
mediate characters, such as the Ornitfwrhyncus, between

feet development 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 anomahcs are,
in reality, in perfect harmony with the established laws of ort^anic develop-
ment, and afford, indeed, striking confirmations of the truth of the theory
here cxj^lained.

• See a paper on tiiis subject, by Cuvier, in the Arm. dcs Sciences Na-

turcUes, xx. 241.

UNITY OF DESIGN. /J45

birds and mammalia, and the Cetacea, between fislies 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 phy-
siologists, been carried to an unwarrantable and extravagant
length: for the scope which is given to the imagination in
these seductive speculations, by leading us far away from the
path of philosophical induction, tends rather to obstruct than
to advance the progress of real knowledge. I3y confining
our inquiries to more legitimate objects, we shall avoid the
delusion into which one of the disciples of this transcenden-
tal 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 or-
ganized matter an inherent tendency to perfectibility, fan-
cies that he can supersede the operations of Divine agency. t
Very different was the humble spirit of the great New-
ton, who, struck with the immensity of nature, compared
our knowledge of her operations, into which he had himself
penetrated so deeply, to that of a child gathering pebbles on

• *' L'univers est expllque, et nous le voyons; c'est un petit nombre de
prlncipes generaux et feconds qui nous en ont donne la clef." Serrcs, Ann.
des So. Nat. xi. 50.

f Allusion is here made to the celebrated theory of Lamarck, as exposed
in his "Pliilosophie Zoolog-ique. " He conceives that there was orig-inally
no distinction of species, but that each race has, in the coui-se 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 ti-ansformations,
to all other animals now existing on the globe. He believes that tribes, ori-
ginally aquatic, acquired by their own efforts, prompted by their desire to
walk, both feet and legs, fitting them for progression on tlie ground; and
that these members, by the long contiinicd operation of the wish to flv, were
transformed into wings, adapted to gratify that desire. If this be philoso-
phy, it is such as might have emanated from the college of Laputa.

446 UNITY OF DESIGN.

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 a control over
all the bodies in nature! Doubtless, there exist 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 incomprehensible is its
connexion with the body; how mysterious arc its secret
springs, and inmost workings! What ineffable 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 cer-
tain limits to its powers, permits us to acquire, by its exer-
cise, a knowledge of some of the wondrous works of his
creation, to interpret the characters of wisdom and of good-
ness with which they are impressed, and to join our voice
to the general chorus which proclaims " His Might, Ma-
jesty, and Dominion." From the same gracious hand we
also derive that unquenchable thirst for knowledge, which
this fleeting life must ever leave unsatisfied; those endow-
ments of the moral sense, with which the present constitu-
tion 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 coun-
sels, or fathom the designs of Omnipotence; for in directing
his views into futurity, the feeble light of his reason is scat-
tered and lost in the vast abyss. Although we plainly dis-
cern intention in every part of the creation, the grand ob-
ject of the whole is placed far above the scope of our com-

UNITY OF DESIGN. 447

prehension. It is impossible, however, to conceive that
this enormous expenditure of power, this vast accumulation
of contrivances and of machinery, and this profusion of ex-
istence resulting from them, can thus, from age to age, be
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 pathless ocean?
Are the operations of Almighty power to end with the pre-
sent scene? May we not discern, in the spiritual constitu-
tion 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 habitation? 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 illusions of
our senses and the infirmities of our nature, our elevation
to which will eventually prove that all these unsated desires
of knowledge, 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.

f*.

1 IV D E X.

Abdojien of insects, i. 230.
Aberration, chromatic, ii. 385.
Aberration of parallax, ii. 325, SM.
Aberration, spherical, ii. 324, 333.
Absorption, vegetable, ii. 21, 23.
Absorption, animal, ii. 17, 250.
Absorption, lacteal, ii. 104,
Absorption of shell, i. 174.
Acalepha, i. 142 ; ii. 210.
Acarus, i. 212.
Achatina zebra, i. 175.
Achromatic power, ii. 335.
Acephala, i. 159 ; ii. 88, 215.
Acetabulum, i. 282.
Acid secretions, ii. 39.
Ac rid a, ii. 155.
Acridium, i. 236.
Acoustic principles, ii. 294.
Actinia, i. 136, 146 ; ii. 75, 272, 337,

412, 415,
Adipose substance, i. 97.
Adductor muscle, i. 160.
Aeration of sap, ii. 28.
Aeration, animal, ii. 31, 428.
iEschna, i. 247.
Affinities, organic, ii. 13.
Agastric medusER, ii. 70.
Age of trees, i. 73.
Age, effects of, ii. 434.
Agouti, i. 344.
Agrion, ii. 174.
Air-bladder, i. 298.
Air cells of plants, i. 67.
Air cells of birds, ii. 234.
Air, rarefaction of, in birds, i. 384.
Air tubes in plants, i. 05.
Albumen, i. 85.
Alburnum, i. 73; ii. 36.
Algse, ii. 21.
Alimentary canal, ii. 82,
Alimentary canal, Jbrmation of, ii.

427.
Vol. II.

Alitrunk, i. 243-.

Alligator, i. 317, 319 ; ii. 290.

Amble, i. 342.

Ambulacra, i. 148.

Amici, i. 68; ii, 42,

Amphibia, i. 303,336.

Amphisbaena, i. 310, 311.

Amphitrite, i. 201.

Anabas, ii. 219.

Analogy, law of, i. 49; ii. 437,

Anarrhichas, ii. 96.

Anchylosis, i. 267.

Ancillaria, i, 175,

Anemone, sea, i. 146.

Angler, i. 293; ii. 276.

Anguis, i, 310, 315,

Animal functions, i. 42.

Animal organization, i. 79.

Animalcules. See Infusoria.

Annelida, i. 194; ii. ISO, 213, 272^

338.
Annular vessels, i. 66.
Anodon, i. 169.
Ant, ii. 274, 341, 343.
Ant-eater, i. 361; ii, 10().
Antelope, ii, 109, 285,
Antelope, horn of, i, 3.5.5.
Antenna?, i, 206; ii. 273,
Antennulffi, ii. 93.
Anther, ii. 418,
Anthias, ii. 219.
Anthophora, i, 247.
Ant i pathos, i. 125,
Antler of deer, i. 351.
Antrum ma.xillare, ii. 284.
Aorta, ii. 83, 426,
Aphrodite, ii, 77, 91, 213,
Aplysia, ii. 95, 124, 3^-*.
Apodcs, i. 294,
Apterous insects, i. 212,
Aquatic animals, i. 11^^
Aquatic plants, ii. 41.

5/

450

INDEX.

Aquatic larvce, i. 2*20.
Aijualic insects!, i, 2'S7.
Aquatic birJs, i. 407.
Aquatic respiration, ii. 210.
Aqueous humour, ii. ^^27.
Arachnida, i. 202; ii. 91, 235, 270,

34:3, 412.
Aranea. See Spider.
Arbor vitaj, ii. 393.
Arcnicola, i. 199; ii. 211.
Ar^ouauta, i. 165.
Aristotle, ii. 394.
Aristotle, lantern of, ii. 90.
Ann, human, i. 375.
Armadillo, ii. 271.
Arteries, i. 44; ii. 82.
Articulata, i. 193.
Ascaris, ii. 86, 380.
Ascidia, i. 106; ii. 212.
Ass, i. 356.

Assimilation, i. 43; ii. 16.
Astacus, ii. 308, 346.
Asterias, i. 147; ii. 76, 151, 171,

212, 272, 387, 412.
Ateles, i. 278, 368.
Atlas of Lion, i, 365.
Atmosphere, purification of, ii. 32.
Atmospheric respiration, ii. 221.
Atriplex, ii. 40.
AuJouin, i. 207, 228, 230; ii. 177,

226, 381.
Audubon, ii. 288.
Auricle, ii. 82, 187.
Auricula, i. 1*^1.
Avicula, i. 171.
Axilla) of plants, i. 76; ii. 413.
AxelotI, ii. 231.

Babiroussa, ii. 105.
Bacculite, i. 192.
Baer, ii. 338, 429, 443.
Bdker, ii. 337.
Balaina, See Whale.
Balance of affinities, ii. 13.
Balistes, i. 300.
Banks, i. 314.
Barbels offish, ii. 276.
Bark, formation of, i. 73.
Barnacle, i. 185 ; ii. 212.
Bat, i. 380;ii. 101,399.
Batrachia, i. 303; ii. 418.
Batracho.-permum, ii. 41.
Bauer, i. 59.
Bear, ii. 108.
Beard of oyster, ii. 215.

Beaver, i. 361; ii. 110, 135, 141.

Bee, i. 247; ii. 275.

Belchier, i. 269.

Bell (Sir C.) ii. 375.

Bell (Thomas) i. 333 ; ii. 290.

Bellini, ii. 279.

Berberis, i. 100.

Berkeley, ii. 366.

Beroe, i. 144, 149.

Berzelius, ii. 21.

Bicuspid teeth, ii. 107.

Bipes canal iculatus, i. 317.

Birds, i. 382; ii. 97, 234, 287, ct

passim.
Blind-worm, i. 315, 317.
Blood, ii. 237.
Blood vessels, ii. 201.
Blunienhach, ii. 302.
Boa, i. 310, 311.
JJoar, i. 53; ii. 105, 118.
Bombyx, i. 21-5, 217, 222; ii. 343.
Bone, i. 89, 256, 263.
Bonnet, i. 51; ii. 20, 62, 70, 182,

337.
Borelli, i. 405.
Bosc, i. 115.
Boslock, ii. 237.
Bound of deer, i. 342.
Bowerhank, ii. 174.
Boyle, ii. 19.
Bractea), i. 78.
Bradypus, i. 333; ii. 204.
Brain, i. 40; ii. 260, 390, 404.
Brain, formation of, ii. 423.
Branchiffi, ii. 192,210,214.
Brassica, ii. 40, 44.
Braula, ii. 341.
Bresehet, ii. 303.
Brewster, I 169; ii. 333, 349.
Brocken, spectre of, ii. 374.
Broussonnet, ii. 412.
Bru^uiere, i. 115, 179.
Bryophyllum, ii. 411.
Buccinum, i. 158, 167, 175; ii- 95,

215.
Buckland, ii. 149.
Buds,i. 73; ii. 413.
Buffon,'!. 137; ii. 372,415.
Bulb of hair, i. 93.
Bulboffeather, i. 397.
Bulbus arteriosus, ii. 196.
Bulbiilus glandulosus, ii. 135,
Bulimus, I. 180.
Bulla, ii. 123.
Burrowing of the mole, i. 361.

INDEX.

451

Cabbage, ii. 40, 44.

Cachalot, i. 334 ; ii. 105.

Caeca, ii. 78, 150.

Csecilia, ii. 351.

Calamary, i. 182.

Callionymiis, ii. 354.

Calosoma, i. 227.

Cambium, ii. 3.5.

Camel, i. 87; ii. 129, 144.

Cameleopard, i. 332, 344; ii. 101.

Camera obscura, ii. 324.

Camerated shells, i. 100.

Campanularin, ii. 170.

Camper, ii. 310, 314, 394.

Canada rat, ii. 130.

Cancelli, i. 262.

Cannon bone, i. 348.

Capibara, ii. 118.

Capillaries, ii. 190.

Capsular ligaments, i. 86.

Caput Medusae, i. 1-55.

Carapace, i 207, 321.

Carbon, non-absorption of, ii. 20.

Carbonic acid, ii. 28, 239.

Cardia, ii. 133.

Card i urn, i. 102, 162, 163, 164.

Carduus, i. 100.

Carinated sternum, i. 390.

Carlisle, i. 296, 301 ; ii. 204, 399.

Carnivora, i. 364; ii. 52, 108.

Carp, i. 286, 298.

Carpus, i. 282.

Cartilag-e, i. 88.

Caruncle, lacrymal, ii. 331.

Cams, i.257; ii. 151, 158, 174, 182,

356.
Cassowary, i. 404; ii. 161.
Cat, ii. 278, 355.
Caterpillar, i. 217, 224; ii. 342.
Caudal vertebrae, i. 281.
Cavolini, i. 121.
Celandine, ii. 41.
Cells of plants, i. 60, 62.
Cellular texture, animal, i. 81.
Centaurea, i. 100.
Cephalic franrjlion, ii. 381.
Cephalo-thorax, i. 202.
Cephalopoda, i. 186; ii. 159, 388.
Cerambyx, i. 233; ii. 322, 323.
Cercaria, i. 138; ii. 338.
Cerebellum, ii. 391.
Cerebral ganglion, ii. 381.
Cerebral hemispheres, ii. 391.
Cerithium, i. 180.
Ceroxylon, ii. 40.

Cetacea, i. 332, 333 ; ii. 105, 128,

140, 313, :m), 431.
Chnbrier, i. 88,241.
Chain of bcincr, i. 51 ; ii. 439.
Chalcides, i. 311. 317.
Chameleon, i. 320 ; ii. 97, 277, 351.
Chara, ii. 42, 183.
Chelidonium, ii. 41.
Chelonia, i. \\Z\ ; ii. 97, 198, 229,

311.
Chemistry, organic, ii. 12, 2Ji6.
Chcseldcn, ii. 3()6.
C/irvrruil, i. 97.
Children, I 12,226; ii. 347.
Chitinp, i. 226.
Cliladni, ii. 296.
Chondrilla, ii. 43.
Choroid coat, ii, 326.
Choroid gland, ii. 349.
Chromatic aberration, ii. 33.5.
Chronuile, i. 63.
Chrysalis, i. 219.
Chyle, ii. 82, 148.
Chyme, ii. 132.
Cicada, i. 240.
Cicindela, ii. 154.
Cilia, i. 99, 117, 119, 129, 144.

185.
Ciliary ligament, ii. 327.
Cimbex, i. 235.
Cimex, ii. 93,
Cineritious, ii. 39.5.
Circulation, ii. 16, 167.
Cirrhi, ii. 276.
Cirrhopoda, i. 185.
Classification, i. 50; ii. 437.
Clausilia, ii. 226.
Clausium, i. 183.
Clavicle, i. 281, 293, 390.
Claviger, ii. 341.
Claw in lion's tail, i. 36(».
Clio, i. 1%; ii. 103.
Cinque t, ii. 3.51.
Clypcaster, i. 155.
Cobitis, ii. 221.

Cobra do capcllo, i. 378; ii. 121.
Coccygeal bone, i. 281.
Cochlea, ii. 303.
Cockchaffer. See Melolontha.
Cockle, i. 162. See rardium.
Cod, lens of, i. 56; ii. 394.
Coenurus, ii. 65.
Coexistence of forms, i. 49.
Coffin-bone, i. IW).
Coleoptcra, i. 245; ii. 271.

452

rNDEX.

Collar-bone, i. 281.
Colours of insects, i. 226.
Colours, perceptions of, ii- 37^
Coluber, i.:311,:U2; ii. 120.
Columella, i. 170; ii. 311.
Comatula, i. 155.
Commissures of brain, ii. 395.
Coinparctli, ii. 177, 309.
Complementary colours, ii. 373.
Compound eyes, ii. 341.
Concha of the ear, ii. 298.
Condor, li. 235.
Conyer eel, ii. 392.
Conjjflomerate eyes, ii. 341.
Conjunctiva, ii. 329.
Consumption of animal matter, ii.

48.
Contractility, muscular, i. 98.
Conus, i. 181.

Convolutions of the brain, ii. 393.
Convolvulus, ii. 41.
Cooper, ii. 308.
Coracoid bone, i. 281, 390-
Coral, i. 125.
Coral islands, i. 26.
Corium, i. 89.
Cornea, ii. 326.
Corneule, ii. 344.
Cornu Ammonis, i. 192.
Coronet bone, i. 356.
Corpora quadrigemina, ii. 391.
Corpus callosum, ii. 396.
Corpus papillare, ii. 268.
Cortical substance, ii. 395.
Cossus, i. 214, 222, 249.
Cotunnius, ii. 303.
Cowrie, i. 179.

Crab, i. 207; ii. 186, 214, 226, 347.
€ranium, i. 278, 307, 326.
Cranium of insects, i. 228.
Craw, ii. 124.
Cray-fish, ii. 308, 346.
Cribriform plate, ii. 284.
Crinoidca, i. 1.56.
Crocodile, i. 317, 318, 320; ii. 105,

119,199,290,312,392.
Crop, ii. 130.
Cross-bill, ii. 98.
Crotalus, i. 312.
Crust, 1.89.
Crusta pctrosa, ii. 113.
Crustacea, i. 204; ii. 194, 211, 214,

381, 412.
Cryptogamia, i. 64; ii. 416.
fTystalline lens, i. 56; ii. 327.

Crystalline needles in biliary ducts,

ii. 159.
Curculio, i.233.
Cushions of insects, i. 235.
Cuticle, vetretable, i. 67.
Cuticle, animal, i. 90; ii. 268.
Cuttle-fish. <SVe Sepia.
Ciivier, passim.
Cnvier/(F.) I 9.5,396.
Cyclidunn, i. 138.
Cyclocffila, ii. 74.
Cyclosis, ii. 41, 169.
Cyclostomata, ii. 88.
Cymbia, i. 175.
Cymothoa, ii. 382.
Cypra?a, i. 179.
Cyprinus, i. 93, 286.
Cysticule, ii. 310.

Duldorf, I 301 ; ii. 219.

Darwin, i. 7.5.

Darwin (Dr. R.,) ii. 372.

Davy, ii. 20, 240.

Davy (Dr.) ii. 197.

Death, ii. 43.5.

De BlainviUe, i. 59, 179, 257 ; ii.

182, 303, 340, 351, 400.
De Candolle, i. 78 ; ii. 21, 2-5, 27,

28, 34, 43, 433.
De Candolle, (junior) ii. 40.
Decapoda, ii. 186.
Decline of the system, ii. 433.
Decollated shells, i. 180.
Deer, i. 3.50 ; ii. 285.
Dcfrance, i. 184.
De Geer, i. 240.
Deg-lutition, ii. 127.
Delaroclie, ii. 221, 3.50.
De Montegr^, ii. 134.
Dermo-skeleton, i. 2.57.
De Saussure (Th.,) ii. 28.
Des Cartes, ii. 259, 394.
De Serrcs, ii. 1-53, 173, 342.
Desii^n, evidence of, i. 35.
Design, unity of, ii. 437.
Development, vegetable, i. 63.
Development, animal, ii. 420.
Diaphragm, ii. 232, 428.
Diffusion of animals, ii. 51.
Digestion, i. 43 ; ii. 132.
Digitigrada, i. 367.
Diodon, i. 301.
Dioecia, ii. 418.
Diona3a, i. 100.
Diplozoon, ii. 426.

INDEX.

453

Diptera, i. 229, 248; ii. 87.

Diquemare, i. 161.

Distoma, ii. 86.

Divisibility of matter, ii. 281.

Dolling er^ ii. 480.

Dolphin, ii. 105, 31:3, 357, 431.

Doras costatus, ii. 219.

D'Orbigny, i. 190.

Doris, ii. 95,212.

Dormouse, ii. 139.

Dorsal vessel, ii. 171.

Dory, i. 292.

Dove, ii. 390, 392.

Down of plants, i. 78, 79.

Down of birds, i. 394,

Draco volans, i. 53.

Dragon-fly, i. 221, 246; ii. 343.

Dreaming-, ii. 376.

Dromedary, ii. 161.

Duckweed, ii. 414.

Dufour (Leon,) ii. 156, 223.

Duges, ii. 177, 182, 338, 343,

347.
Dupfoncr, ii. 106, 200, 313.
Duhamel, ii, 19, 22.
Dumas, ii. 279.
Dum6ril, ii. 291.
Diimnrtier, i. 257.
Duodenum, ii. 151.
Dutrochet, I 66, 141 ; ii. 224.
Dytiscus, i. 35, 221, 236, 238 ; ii.

222, 223.

Eagle, ii. 97.

Ear, ii. 299.

Ear-drum, ii. 299.

Earle, i. 386.

Earths in plants, ii. 38.

Earth-worm. See Lumbricus.

Echinodermata, i. 147.

Ediinus, i. 149, 153; ii. 76,90, 212,

272.
Edwards, ii. 226, 381, 441,
Eel, i. 294 ; ii. 219.

Eg-rr, ii. 419.

Ehrenberg, i. 25, 138, 140; ii. 71,

338, 415.
Ehrmann, ii. 221.
Elaboration, successive, ii. 17.
Elastic ligaments, i, 87.
Elater, i. 240.
Eleaine, i. 97.
Electric organs, i. 36.
Electricity, ii. 248.
Elements, organic, ii. 12.

Elephant, i. .53, ft7, 339, a5M; jj.

105, 113, 119, 14,5, 161, 278,

355, 394.
Ellis, i. 115.

Elytra, analysis of, i. 226, 246.
f^iiibryo, ii. 417.
Emu, i. 401.
Emydes, i. 328.
Enamel of teeth, ii. 111.
Eddogonous plnnts, i. 71.
Entomolino, i, 92, 226.
Entomostraca, ii. 34^.
Entozoa, i. 202; ii. 64, 86, 171,

211, 380, 415.
Ephemera, i. 221 ; ii. 174.
Epidermis, vegetable, i. 75.
Epidermis, animal, i. 90, 168.
Epiphragma, i. 183.
Equivocal generation, ii. 415.
Equorea, ii. 6.5.
Erato, i. 179.
Erect vision, ii. 367.
Erpobdella, i. 195; ii. 182.
Eryx, i. 310.
Eso.x, i. 296.
Ethmoid bone, ii. 283.
Eudora, ii. 69.
Euler, ii. 335.
Eunice, ii. 338.
Euphorbium, ii. 48.
Euryale, i. 155.
Eustachian tube, ii. 301.
Evil from animal warfare, i. 47 ; ii.

53.
Excretion, ii. 16.
Excretion, vegetable, ii. 39, 43.
Exhalation by leaves, ii. 27.
Exocetus, i. 377.
Exogenous plants, i. 71.
Eye, i. 37; ii. 325, 412, 413.
Eye, formation of, ii. 424.
Eye-lids, formation of, ii. 430.

Fabricius, i. 144.

Facial angle, ii. 394.

Fairy rings, ii. 45.

Fallacies of perception, ii. 362.

Fangs of serpents, ii. 120.

Faraday, ii. 363.

Fasciola, ii. 86.

Fasciolaria, i. 180.

Fat, i. 97.

Fata Morgana, ii. 374.

Feathers, i. 392, 407.

Fccula, i. 63.

454

INDEX.

Fecundation, ii. 417.

Feelers, i. '206 ; ii. 273.

Feet-jaws, i. '207.

Feet of birds, i. 403.

Femur, i. 20.3, 232,282.

Ferieslrae of ear, ii. 302.

Ferns, i. 71 ; ii. 41(5.

Fibre, animal, i. bl, 85.

Fibula, i. 282.

Fig-tree, ii. 41.

Fijr Maryrrold, ii. 40.

Filaments of feathers, i. 392.

Filaria, i. 59.

Filices, i. 71 ; ii. 416.

Final canses, i. 17, 31, et passim.

Fins of fishes, i. 292.

Fins of cetacea, i. 336.

Fishes, i. 88, 284; ii. 86, 196, 276,

290, 348, et passim.
Fissiparons reproduction, ii. 409.
Flea, i. 212.
Flight, i. 242, 376.
Flourens, ii. 218.
Flower, ii. 418.
Fluidity, organic, i. 57.
Flustra, i. 125, 127, 129.
Flying fish, i. 377.
Flying lizard, i. 378.
Flying squirrel, i. 380.
Focus, ii. 320.
Fohmann, ii. 251.

Follicles, i. 91; ii. 135.

Fontana, ii. 430.

Food of plants, ii. 19.

Food of animals, ii. 47.

Foot of mollusca, i. 163.

Forces, physical, i. 20.

Fonhjce, ii. 126.

Fovilla, ii. 418.

French bean, ii. 43.

Frog, i. 303; ii. 96, 160, 197, 311.

Fucus vesiculosus, i. 61.

Functions, i. 39, 42; ii. 55.

Fungi, ii. 4.5.

Furcular bone, i. 390.

Furcularia, i. .58.

Fusiform roots, ii. 22.

Future existence, ii. 407, 447.

Gaede, ii. 66.
Gaimard, i. 80.
Galeopithecus, i. 380.
Galileo, i. 70.
Gallina?, ii. 390.
Gallop, i. '342.

Galvanism, ii. 362.

Ganglion, ii 25.5.

Gasteropoda, i. 166; ii. 128, 215,

339.
Gastric juice, ii. 1.34.
Gastric teeth, ii. 123, 155.
Gastric sflaiids, ii. 134.
Gastrobranchus, i. 283, 289; ii. 88,

351.
Gay L7issnc, ii. 224.
Gecko, i. 319; ii. 277.
Gelatin, i. 8.3.

Gemmiparous reproduction, ii. 413.
Gemmule, i. 119; ii. 414.
Geometer caterpillars, i. 224.
Germs, vegetable, i. 73; ii. 413.
Geronia, ii. 70.
Gillaroo trout, ii. 147.
Gills, i. 304; ii. 199,214.
Gimbals, i. 233.
Gizzard, ii. 124, 155.
Glands, vegetable, i. 67 ; ii. 39.
Glands, animal, ii. 247.
Glands in crocodile, ii. 290.
Glands, gastric, ii. 134.

Gleichen, ii. 71.

Globules, i. 59, 81.

Glossa, ii. 93.

Glossopora, ii. 78.

Gmelin, i. 115.

Gnat, ii. 87.

Goat, ii. 285.

Goeze, ii. 337.

Gonium, i. 138.

Goose, ii. 127, 352.

Gordius, i. 59, 198.

Gorgon ia, i. 12.5.

Gradation of being, i. 51 ; ii. 439.

Grampus, ii. 105.

Grallffi, i. 403, 407.

Grant, i. 113, 115, 127, 129, 131,
144, 149, 185, 404 ; ii. 338.

Gray, i. 161, 174, 183.

Growth, vegetable, i. 72; ii. 21, 420.

Gruithiiisen, ii. 338.

Gryllo-talpa, i. 241 ; ii. 27.3.

Gryllus, ii. 177.

Guinea-pig, i. 344.

Gulstonian lectures, ii. 374.

Gum, ii. 33.

Gurnard, ii. 390.

Gymnotus, i. 294 ; ii. 402.

Hffirnatopus, ii. 98.
Haidinger, i. 151.

I

INDEX.

455

Hair, vegetable, i. 78.

Hair, animal, i. 93, 226.

Hair-worm, i. 198.

Hales, ii. 26.

Haliotus, i. 1G9.

Holler^ i. 81.

Halteres, i. 249.

Hamster, ii. 138.

Hancock, ii. 219.

Hand, i. 375; ii. 278.

Hanow, ii. 337.

Hare, i. 243; ii. 110,139.

Hartley, ii. 396.

Harwood, ii. 286, 287.

Hatchett, ii. 38.

Hauksbee, ii. 295.

Haunch in insects, i. 205, 232.

Hawk, ii. 97.

Head of insects, i. 228.

Hearing, ii. 294, 401.

Heart, i. 43, 107 ; ii. 186, 425.

Hedge-hog-, i. 361, 363.

Hedysarum gyrans, i. 100.

Hedwig, i. 68.

Helix, i. 176, 183; ii. 95, 339.

Hellman, ii. 277.

Hemiptera, i. 220, 247; ii. 87.

Hemispheres, cerebral, ii. 391.

Henbane, ii. 48.

Henderson, ii. 240.

Hepatic vessels, ii. 151, 154.

Herring, i. 292.

Herschcl (Sir W.,) ii. 371.

Herschel (Sir John,) i. 169; ii. 38,

401.
Harvey, ii. 206.
Hesperia, i. 250.
Hexastoma, ii. 86.
Hippopotamus, ii. 105, 112, 119,

140, 314, 353.
Hirudo, i. 106, 201 ; ii. 78, 94, 182,

213, 339.
Hodgkin, i. 81, 99.
Hodgson, ii. 285.
Hog, i. 280, 359 ; ii. 140, 278.
Holothuria, ii. 151, 171, 212, 387.
Home (Sir Everard,) passim.
Honey-comb stomach, ii. 142.
Hooded snake, i. 378.
Hooks on feet of insects, i. 2^34.
Hop, i. 76.
Horn, i, 92, .355.
Horn on beak of chick, ii. 430.
Horse, i. 356 ; ii. 139, 284, 400.
Horse-fly, ii. 88.

Hostilities of animals, i. 47; ii. 53.

Houston, ii. 97.

Huher, ii. 274, 292.

Human fabric, i. 3()9; ii. 303.

Hiimholdt, ii. 220, 224, 240.

Humerus, i. 2*^2.

Humours of the eye, ii. 326.

Hiuitfr, i. H7; ii. 126, 137, 235.

Hyena, i. 344; ii. 49, 110,

Hybernation, ii. 377.

Hydatid, ii. 6.5, 86, 41.5.

Hydatina, ii. 74, 3.3?^, 379.

Hydra, i. 123, 132 ; ii. 58, 337, 379,

411, 414.
Hydrogen, ii. 39.
Hydrophilus, i. 221.
Hydrostatic acalcpha, i. 145,
Hyla, i. ;309.
Hymonoptera, i. 229, 247; ii. 8P,

177.
Hyoid bono, ii. 99, 216.
Hyrax, ii. 139.

Ichthyosaurus, i. 325.

Ilium, i. 281.

Imago, i. 219, 225.

Incisions of insects, i. 231.

Incisor teeth, ii. 106.

Incus, ii. 302.

Indian walrus, ii. 106.

Individuality of polvpp.s i. 130.

Infusoria, i.'l3n; ii! 379, 409.

Injuries, reparation of, ii. 10, 412.

Inorganic world, i, 21.

Insects, i. 24, 88, 212; ii. 1.50, 171,

280, 309, 400.
Insectivora, i. 362.
Instinct, ii. 403.
Integuments, i. 89; ii. '2G8.
Intercellular spaces, i. 63.
Intermaxillary bone, ii. 106,443.
Interspinous bones, i. 276.
Intestine, ii. 77.
Iriartea, ii. 10.
Iridescence, i. 169.
Iris, i. 105; ii. 327.
Ischium, i. 282.
Isis, i. 126.
Ivy, i. 76.

Jarobson, ii. 399, 40(».
.Terboa, i. 343, 371.
Johnson, ii. 79.
.lulus, i. 213; ii. 342.
Jurine, ii. 399.

456

INDEX.

Kaleidescope, ii. 374.

Kan^uroo, i. 278, 343, 371 ; ii. 140,

419.
Kater, ii. 347.
Kcrona, i. 13S.

Kidd, i. 241 ; ii. 223, 247, 273.
KiernarL, ii. 24>l
Kieser, i. 61, 06.
Kirby, i. 229 ; jL 293, 342.
Knight, ii. 417.
Knots in wood, ii. 413.
Koala, i. 303.
Kolpoda, i. 139.

Ijabium of insects, ii. 93.

Labnini of insects, ii. 93.

Labyrinth, ii. 30;i

Lacerta, i. 317.

Lacrymal organs, ii. 330.

Lacteals, ii. 82, 104.

Lamarck, i. 115; ii. 71, 445.

Lamina spiralis, ii. 305.

Lamouroiix, i. 115.

Lamprey, i. 289; ii. 88, 218, 310.

Lancets of diptera, ii. 87.

Language of insects, ii. 274.

Lark, i. 401.

Larva, i. 217, 2ia

Lassaigne, i. 226 ; ii. 134.

Latham, ii. 134.

Latreille, i. 207; ii. 22.5, 276, .347.

Laws of nature, L 20.

Law of mortality, i. 44.

Law of coexistence of forms, i. 50.

liaw of gradation, ii. 439.

Law of analogy, i. 49 ; ii. 437.

Leach, i. 161.

Leaves, ii. 27, 38.

Leech. (»S'ec Hirudo.)

Lemur, i. 367, 380; ii. 204, 3-55.

Lens, crystalline, i. 56; ii. 327, 350,

Lenticclla?, i. 78.

Lepas, i. 185 ; ii. 212.

Lepidoptera, i. 217, 249; ii. 87, 157.

Lepisma, i. 212, 250.

Lernsea, L 216 ; ii. 421, 426.

Leuchft, ii. 340.

Leucophra, ii. 73.

Leuret, ii. 134.

Lewenhoeck, i. 251 ; ii, 190.

Libellula, i. 221, 247; ii. 343.

Liber, i. 74 ; ii. 36.

Lichen, ii. 21.

Life, i. 39, 44.

Ligaments, i. 86.

Ligamentum nuchse, i. 87, 346.

Light on plants, i. 76 ; ii. 27.

Lignine, i. 63; ii. 36.

Lilium, i. 68.

Li max, ii. 95, 226.

Limpet. See Patella.

Link, i. 66.

Lion, i. 87, 342, 365 ; ii. 101, 278,

392.
Lister, ii. 169, 215.
Liver, ii. 159, 248.
Lizard, ii. 97, 277, 351, 412.
Lobster, i. 208 ; ii. 123, 186, 214,

308, 383.
Lobularia, i. 122.
Loche, ii. 221.
liOcomotion, i. 110.
Locusta, ii, 91.
Loligo, i. 188,283; ii. 195.
Longevity of trees, ii. 434.
Lophius, i. 293 ; ii. 276, 310.
Loxia, ii. 98.
Lucanus, i. 252.

Lumbricus marinus, i. 199, 210.
Lumbricus terrestris, ii. 77, 87^ 184,

213.
Lungs, ii. 192,428.
Lycopodium, i. 68.
Lycoris, ii. 338.
Lymphatics, ii. 250.
Lymphatic hearts, ii. 251.
Lyonet, i. 214, 222, 249.

Macaire, ii. 43, 45, 47, 238.

Macartney, i. 406; ii. 234, 235, 396.

Macavoy, ii. 266.

Mackerel, i. 295.

Macleay, i. 52.

Madder, i. 269.

Madrepore, i. 126.

Magendie, ii. 356, 375;

Magilus, i. 180.

Mam, ii. 194, 383.

Malleus, ii. 302.

Malpighi, ii. 268.

Mamrna}, ii. 419, 431.

Mammalia, i. 330 ; ii. 230, 312, 419.

Man, i. 369; ii. 394.

Man of war, Portuguese, i. 145.

Manntus, ii. 105.

Mandible, i. 206.

Mantis, ii. 153.

Mantle, i. 90, 172.

Many-plies stomach, ii. 143.

Marcet, ii. 47, 164, 238, 328.

INDEX.

457

Marginella, i. 179.

Marmot, ii. 110.

Marsigli, i. 115.

Marsnpialia, ii. 199, 419.

Marsupium, ii. 3.52.

Mastication, ii. 104.

Mastoid cells, ii. 301.

Matrix of feather, i. 397.

Matter, ii. 363.

Maunoir, ii. 370.

MaxillfD, ii. 93.

Mayer, i. 310.

Mayo, ii. 432.

Meatus auditorius, ii. 299.

Mechanical functions, i. 41.

Meckel, i. 333 ; ii. 339.

Medulla oblongata, ii. .391.

Medullary substance, ii. 260.

Medullary rays, i. 73.

Medusa, i. 80, 143; ii. 51, 56, 05,

210, 337.
Meibomian glands, ii. 331.
Melolontha, i. 214 ; ii. 92, 154, 171,

223, 343, 34.5.
Melophagus, ii. 341.
Membrana nictitans, ii. 3-52, 353,
Membrane, i. 83.
Menobranchus, ii. 231.
Mercurialis, ii. 44.
Mergys, ii. 98.

Merry-thought of fowl, i. 390.
Mesembryanthemum, ii. 40.
Mesenteric glands, ii. 164.
Mesentery, ii. 82.
Mesothorax, i. 229.
Metacarpus, i. 282.
Metals in plants, ii. 37.
Metamorphoses, i. 216, 303 ; ii. 442,

443.
Metatarsus, i. 282.
Metathorax, i. 229.
Milk, ii. 431.
Millepedes, ii. 342.
Millepora, i. 125.
Mimosa, i. 100.
Mint, ii. 20, 29.
Mirandola, ii. 411.
Mirbel, i. 62, 65.
Mite, i. 212.
Mitra, i. 180.
Modiolus, ii. 30.5.
Molar teeth, ii. 107.
Moldenhawer, i. 66.
Mole, i. 361,362; ii, 278, 356.
Mole cricket, i. 241.

Vol. II.

Mollusca, i. 157; ii. 176,276,387.

Monads", i. 25, 137 ; ii. 78, 409.

Monkey, i. 367; ii. 110, 279, 4(K).

Munoculus, ii. 34H.

Monulhaluinous shell, i. 191.

Monotrcmata, ii. 199.

Munru, i. 97, 103; ii.217.

Mordella, ii. :i43.

Morpho, i. 249.

Morren, ii. 182, 184.

Mortality, i. 44; ii. 408.

Mother of pearl, i. 169.

Motion, voluntary, i. 41 ; ii. 375.

Motion, vegetable, i. 1(X).

Motor nerves, ii. 375.

Mucous membrane, i. 90.

Mucous glands, ii. 135.

Mulberry, ii. 48.

Mailer, i. 136 ; ii. 70, 251, :U0.

Mullet, ii. 147.

Multilocular shells, i. IfX).

Multivalves, i. 185.

Murscna, ii. 351, 31J2.

Murex, i. 178, 182; ii. 95, 21.5, 340.

Mus, ii. 130, 356.

Musca, i. 235.

Muscle, (shell fish,) i. 162, 164.

Muscle, i. 97, 100, 214.

Muscles of eye, ii. 328.

Muscular power in plants, ii. 254-

Muscular power in birds, i. 408.

Mushroom, ii. 21.

Musk shrew, ii. 101.

Musical tone, ii. 297.

My a, i. 163.

Myriapoda, i.212; ii. 180,

Myrmecophaga, ii. 100.

Mysis Fabricii, i. 206.

Mytilus, i. 162.

Myxine, i. 283, 289 ; ii.«88, 351.

Nacreous structure, i. 168.
Nais, ii. 78, 182, 338, 412.
Narwhal, i..53; ii. 105.
Nature, i. 20, 25.
Nautilus, i. 17.5, 191 ; ii. 194.
Necrophorus, ii. 293.
Needles in biliary ducts, ii. 159.
Nereis, i. 19.5, 197, 201 ; ii. 182.
Nerve, i. 40; ii. 261.
Nervous system, ii. 260, 37"^. 3)^.
Nervous ]K)wer, ii. 2-52.
Nettle, ii. 40.
Neuro-skeleton, i- 257.
Neuroptcra, i. 215.
5S

458

INDEX.

Newport, i. 247; ii. 78, 155, 157,

177, :385.
Newt, ii. 311, 412.

Nightshade, ii. 48.
Nitrogen, ii. 18, 240.
Nordmann, ii. 420, 426.
Notonecta, i. 85, 238.
Nursling sap, ii. 25.
Nutrition, ii. 9, 15, 17, 47.
Nutrition in lower orders, ii. 5^.
Nutrition in higher orders, ii. K).
Nutritive functions, i. 42.
Nycteribia, ii. 341.

Ovary, ii. 416, 417.

Oviduct, ii. 418.

Oviparous animals, ii. 419.

Ovo-viviparous animals, ii. 419.

Ovula, i. 179.

Ovum, ii. 416.

Owen, i. 388.

Owl, ii. 235, 312, 354.

Ox, iiorn of, i. 355.

O.xygen, ii. 28.

Oyster, i. 102, 161, 162.

Oyster-catcher, ii. 98.

Octopus, i. 188; ii. 348.
Ocular spectra, ii. 301.
Odier, i. 226.

(Esophagus, ii. 76, 82, 129.
Oken, i. 246, 279.
Olfactory nerve, ii. 281.
Olfactory lobes, ii. 391.
OlivcT, i. 175, 180.
Oniscus, ii. 382.
Onocrotalus, i. 384.
Operculum of Mollusca, i. 182.
Operculum of fishes, ii. 217.
Ophicephalus, ii. 219.
Ophidia, i. 310.
Ophiosaurus, i. 315, 317.
Ophiura, i. 155.
Opossum, ii. 101, 419.
Optic axis, ii. ii54.
Optic ganglion, ii. 345.
Optic lobes, ii. 391.
Opuntia, i. 100.
Orache, ii. 40.
Orbicular bone, ii. 303.
Orbicular muscle, i. 105.
Orchidese, i. 62.
Organic Mechanism, i. 58, 79.
Organic development, ii. 420.

Ornithorhyncus, i. 276; ii. 101, 130,
277, 313, 350.

Orobanche, ii. 45.

Orthoceratite, i. 192.

Orthoptera, i. 220, 245.

Os hyoides, ii. 99, 216.

Osier, i. 152, 162, 163, 199, 201.

Osseous fabric, i. 256.

Ossicula, tympanic, ii. 302.

Ossification, i. 263, 383.

Ostracion, i. 300.

Ostrich, i. 388, 404,407; ii. 135,
162, 234, 390.

Otter, sea, ii. 110.

Paces of quadrupeds, i. 339.

Pachydermata, i. 357; ii. 271, 278.

Package of organs, i. 83.

Pain, ii. 262.

Palcmon, ii. 383.

Paley, i. 83, 394 ; ii. 205.

Palinurus, ii. 383.

P(dlas, i. 115 ; ii. 244.

Palms, i. 72.

Palm squirrel, ii. 130.

Palmer, ii. 28.

Palpi, i. 206; ii. 93.

Pancreas, ii. 160.

Pander, ii. 425.

Panniculus carnosus, i. 363.

Panorpa, i. 231.

Paper nautilus, i. 191.

Papilio, i. 251 ; ii. 343.

Papillce, ii. 269, 279.

Par vagum, ii. 386.

Parakeet, ii. 98.

Parallax, aberration of, ii. 333.

Parrot, ii. 130, 277.

Pastern, i. 356.

Patella, i. 167; ii. 167, 388.

Patella of knee, i. 282.

Patellaria, ii. 39.

Paunch, ii. 142.

Pearl, i. 169.

Peccari, ii. 140.

Pediculus, i. 212.

Pelican, i. 383 ; ii. \^.

Pelvis, i. 281.

Pencil of rays, ii. 320.

Penguin, i. 407.

Penitentiary, ii. 138.

Pennatula, i. 131 ; ii. 63.

Penniform muscle, i. 103.

Pentacrinus, i. 156.

Perca, i. 93, 301 ; ii. 219, 291, 349,

392.
Perception, i. 40; ii. 264, 358.

INDEX.

459

Perch. See Perca.
Perennibranchia, ii. 230.
Perilymph, ii. 303.
Periostracum, i. 172.
Peristaltic motion, ii. 148.
P^ron, i. 80; ii. 56.
Pfaff, ii. 240.
Phalanges, i. 282.
Phalena, ii. 177, 343.
Phanerog-amous plants, ii. 417.
Phantasmagoria, ii. 374.
Phantasmascope, ii. 368.
Phaseolus, ii. 43.
Phenakisticope, ii. 369.
Philip, ii. 138, 256.
Phoca, i. 336.
Pholas, i. 161, 185.
Phosphorescence of the sea, i. 143;

ii. 50.
Phrenology, ii. 397.
Phyllosoma, ii. 382.
Physalia, i. 145.
Physiology, i. 30.
Physsophora, i. 145.
Phytozoa, i. 113.
Pierard, ii. 145.
Pigeon, ii. 131, 431.
Pigmentum of skin, i. 90.
Pigmentum of the eye, ii. 327.
Pike, i. 296.
Pileopsis, i. 182.
Pineal gland, ii. 394.
Pinna, i. 171, 164.
Pistil, ii. 418.
Pith of plants, i. 73.
Pith of qui 11, i. 399.
Placuna, i. 169.
Planaria, ii. 86, 171, 181, 211, 338,

402.
Planorbis, i. 166, 175.
Plantigrada, i. 367.
Plastron, i. 321.
Pleurobranchus, ii. 159.
Pleuronectes, i. 299 ; ii. 354.
Plexus, nervous, ii. 255.
Pliny, ii. 394.
Plumula, ii. 422.
Plumularia, ii. 170.
Pneumo-branchisB, ii. 225.
Pneumo-gastric nerve, ii. 386.
Podura, i. 212.
Poisers, i. 249.
Poison of nettle, ii. 40.
PoU,\. 165, 171.
Pollen, ii. 418.

Polygastrica, ii. 73.

Polypi, i. 122 ; ii. 58, 63, 210, 272.

Poly stoma, ii. 86.

Polythalamous shell, i. 190.

Pontia brassica, i. 249.

Pontobdella, i. 195.

Poppy, ii. 41.

Porcupine quills, i. 95.

Porcupine, i. 363 ; ii. 110, 140.

Porifera, i. 113.

Porpita, i. 144.

Porpus, ii. 10.5, 140.

Porterfield, i. 262.

Potato, ii. 413.

Prehension of food, ii. 86, 89.

Priestley, ii. 28, 2:38, 240.

Pristis, i. 53; ii. 122.

Pritchard, ii. 174.

Privet Flawk moth, ii. 157.

Proboscis of insects, ii. 87.

Proboscis of molltisca, ii. 9.5.

Proboscis of Elephant, i. 359.

Progressive motion, i. 112.

Prologs, i. 223

Promontory of ear, ii. 302.

Proteus, 1. 139 ; ii. 231, 442.

Prothorax, i. 229.

Praia, ii. .33, 36.

Provengal, ii. 220.

Proximate principles, ii. 12.

Pterocern, i. 178.

Pteropoda, i. 18.5.

Pteropus, ii. 101.

Pubic bone, i. 282.

Pulmonary organs, ii. 19'2.

Puncta lacrymalia, ii. 331.

Punctnm .=;iliori=, ii. 425.

Pupa, i. 217, 220.

Pupil, ii. 327.

Pupiparn, ii. ;341.

Pyloric appnndicos, ii. 160.

Pylorus, ii. 82, 133.

Pyramidalis muscle, ii.353.

Python, i. 310.

Quadratus muscle, ii. 353.
(iuadrumana, i. 367; ii. 110.
(iuadrupedis, i. 336.
Quaggn, i. 3,56.
Uuail, i. 401.
Quills of porcupine, i. 95.
Quills of feathers, i. 392.
quny, i. «0.

Rabbit, i. 343 ; ii. IKK 1M>

460

INDEX.

Racoon, i. 90.

Radiata, i. 124.

Radicles, ii. 422.

Radius, i. 282.

Ranunculus, i. 69.

Rapp, ii. 337.

Rat, ii. 110, 140.

Ruthke, ii. 443.

Rattle-snake, i. 312.

Ray, i. 24.

Ray, i. 292, 293, 294 ; ii. 354, 400.

Rays of fins, i. 294.

Razor-shell-fish, i. 162.

Reamitr, i. 147, 149, 165, 173,208;

ii. b7, 125, 134.
Receptacles of food, ii. 130.
Receptaculum chyli, ii. 82, 165.
Reed of ruminants, ii. 143.
Refraction, law of, ii. 321.
Regeneration of claw, i. 212.
Rennet, ii. 143-
Reparation, ii. 10, 14, 412.
Repetition of organs, i. 54.
Reproduction, i. 45; ii. 408.
Reptiles, i. 302; ii. 197.
Resinous secretions, ii. 40.
Respiration, i. 44; ii- 16, 191, 208.
Rete muscosum, i. 90.
Reticulated cells, i. 62.
Reticule of Ruminants, ii. 142.
Retina, ii. 266, 317, 327.
Returning sap, ii. 32.
Revelation, ii. 447.
Reviviscence, i. 58 ; ii. 184.
Rhea, i. 404.
Rhinoceros, i. 3.56; ii. 101, 112, 271,

278, 355.
Rhipiptcra, i. 246.
Rliizostoma, ii. 67.
Rhyncops, ii. 98.
Ribs, i. 280; ii. 233.
Ricinus, i. 212.
Rings of annclida, i. 19.5.
Rodcntia, i. 361; ii. 109, 112, 119,

128, 139, 354.
Rcpsel, ii. 337.
Rogpt, ii. 368, 373, 408.
Rolando, ii. 429.
Roosting, i. 405.
Roots, i. 78 ; ii. 22.
Ross, i, 27.
Rostrum, ii. 93.
Rotifer, i. 5<^, 140; ii. 70, 338, 379,

415.
JRoux, ii. 400.

Rudimental organs, i. 52 ; ii. 442.

Rudolphi, i. 66.

Rumjord, i. 67.

Ruminantia, i. 345; ii. 142, 354.

Rusconi, ii. 429.

Sabella, i. 198.

Sacculusofear, ii. 305.

Sacrum, ii. 281.

St. Ayiirp, ii. 200.

<S7. Jlilaire, passim.

Salamander, i. 309; ii. 96,351, 419.

Salicaria, ii. 45.

Saline substances in plants, ii. 37.

Saliva, ii. 128.

Salmon, ii. 160.

Sand-hopper, ii. 381.

Sap, ii. 25.

Sauria, i. 317; ii. 199.

Savigny, i. 197, 207; ii. 90, 92.

Saw-fish, ii. 122.

Scala tympani et vestibuli, ii. 306.

Scales of lepidoptera, i. 249.

Scales of fishes, i. 92.

Scansores, i. 404; ii. 390.

Scapula, i. 281.

Scarabaeus, ii. 343.

Scarf skin, i. 90.

Scarpa, i.S^; ii. 291, 305.

Schoeffer, ii. 337.

Schneiderian 'membrane, ii. 283.

Schultz, ii. 41.

Sciurus, i. 379; ii. 130.

Sclerotica, ii. 326.

Scolopendra, i. 213; ii. 180, 342.

Scoresby, i. 143.

Scorpion, ii. 224, 34'2.

Scuta, abdominal, i. 314.

Scutella, i. 155.

Scyllsea, i. 167.

Sea, phosphorescence of, i. 143; ii.

50.
Sea-hare, ii. 95, 123, 388.
Sea-mouse, ii. 77, 94, 213.
Sea-otter, ii. 110.
Seal, i. 336; ii. 285, 313, 356-
Sebaceous follicles, i. 91.
Secretion, ii. 16, 38, 243.
Seed, ii. 416.

Segments of insects, i. 227.
Semblis, ii. 176.
Semicircular canals, ii. 303.
Senecio, ii. 44.
Sennehier, ii. 22, 28.
Sensation, ii. 258.

INDEX.

4C1

Sensibility, variations of, ii. 369.

Sensitive plant, i. 99-

Sensorial power, ii. 256.

Sensorium, ii. 358.

Sepia, i. 188; ii. 95, 147, 293, 348.

Seps, i. 317.

Series of organic beings, i. 51.

Serous membranes, i. 83.

Serpents, i. 310; ii. 97, 120, 277.

Serpula, i. 198; ii. 211. ^

Serves, ii. 427, 432.

Sertularia, i. 124; ii. 170.

Serum, i. 83.

Sesamoid bones, i. 282.

Seta3, i. 197.

Shark, ii. 119, 149, 189, 349, 400,

412, 419.
Sheep, ii. 113, 141, 293.
Shell, i. 89, 168.
Sheltopusic, i. 317.
Shrapnell, ii. 302.
Shrew, ii. 110, 278.
Shuttle bone, i. 357.
Silica, ii. 21, 38.
Silk worm, i. 217; ii. 48.
Silurus, ii. 219, 276.
Sinistral shells, i. 176.
Siphonaria, i. 182.
Siren, i. 317; ii.231.
Skate, ii. 216, 291, 349.
Skeleton, i. 257, 279. '
Skeleton, vegetable, i. 76; ii. 36.
Skimmer, ii. 98.
Skin, ii. 268.
Skull. See Cranium.
Slack, i. 61.
Sleep, ii. 376.

Slips, propagation by, ii. 411.
Sloth, i. 333, 344, 361 ; ii. 204.
Slug, ii. 95, 226.
Smell, ii. 281.
Smith, ii. 122.
Snail, ii. 226, 287, 412.
Snake-lizard, i. 311,
Snout, i. 359.
Snow, red, i. 27.
Soemmerringi ii. 404.
Soils, fertility of, ii. 21.
Solar light, ii. 29.
Solen, i. 122.
Solipeda, ii. 256.
Solly, ii. 251.
Sorex, ii. 101, 314, 356.
Sound in fishes, i. 298.
Sound, ii. 294.

Spallanzani, i. 68; ii. 63, 125, 134,

239, 399, 430.
Spatangus, i. 151, 15.5.

Spectra, ocular, ii. 369, .372.
Spectre of the Erocken, ii. 374.
Speed of quadrupeds, i. 313.
Spermaceti, i. 334.
Spherical aberration, ii. 333.
I Sphincter muscle, i. 105.
Sphinx, ii. 1.j7, 2 15, 3^^4.
Spicula, in sponge, i. 118.
Spider, i. 202, 203 ; ii. 179.
Spider-crab, ii. .383.
Spider-monkey, i. 278,368.
Spine, i. 271, 275.
Spinal cord, or Spinal marrow, ii.

388, 42,3.
Spiracles, ii. 221.
Spiral threads in plants, i. 62.
Spiral vessels, i. 65.
Spiral growth of plants, i. 76.
Spiral valve in fishes, ii. 149.
Spirits, animal, ii. 396.
Spirula, i. 17.5.
Spix, ii. 182.
Spleen, ii. 162.
Splint bone, i. 357.
Spokes, curved spectra of, ii. 368.
Sponge, i. 113; ii. 65.
Spongiole,i. 69; ii. 22,23.
Spotted cells of plants, i. 62.
Spring-tail, i, 213.
Spur of cock, i. 404.
Squalus. See Shark.
Squalus pristis, ii. 122.
Squirrel, i. 361,379; ii. 130.
Stability of trees, i. 70.
Stability of human frame, i. 373.
Stag, skeleton of, i. 350.
Stamen, ii. 418.
Stapes, ii. 302.

Star-fish, i. 147. See Asterias.
Starch, i. 63 ; ii. 36.
Staunton, ii, 373.
Stearine, i. 97.
Stcifensand, ii. 340.
Stems; vegetable, i. 70.
Stemmata, ii. JUL
Stentor, ii. 74.
Sternum, i. 280.
Stevens, ii. 134.
Stigma, vegetable, ii. 418.
Stigmata of insects, ii. 221.
Sting of bee, i. 247.
StipuliP, i. 78.

462

INDEX.

Stomach, ii. 57, &.c.

Stomata, i. 6S; ii. 21.

Stones, swallowing of, ii. 125.

Stone- wort, ii. 42.

Stork, i. 406.

Stratiomys, i. 221, 245.

Straus Durckhcim, i. 214, 229; ii.

345
Strepsiptera, i. 246.
Striated structures, i. 169.
Strombus, i. 178 ; ii. 215.
Styloid bone, i. 357.
Subbrachieni, i. 294.
Suckers, i. 105, 187, 235.
Sugar, ii. 12.

Sun, action of, on plants, i. 76.
Surveyor caterpillars, i. 224.
Sus ^thiopicus, ii. 110.
Suture, i. 267.

Swa7nmer(hnn, i. 247; ii. 293, 340.
Swan, i. 385, 408 ; ii. 124.
Swimming of fishes, i. 286.
Swimming bladder, i. 298.
Symmetry, lateral, i. 54; ii. 426.
Sympathy, ii. 41)5.
Sympathy of ants, ii. 276.
Sympathetic nerve, ii. 254.
Synovia, i. 84.
Syphon of shells, i. 192.
Systemic circulation, ii. 192.

Tabanus, i. 235; ii. 88.

Tadpole, i. 303; ii. 161, 229, 441.

Tcenia, ii. 64, 86, 171.

Tail, i. 278, 361, 366, 402; ii. 278,

443.
Talitrus, ii. 383.
Tapetum, ii. 356.
Tapeworm, ii. 64, 86, 171.
Tapir, i. 359; ii. 278.
Tarsus, i. 205, 233, 324, 282.
Taste, ii. 279.
Teeth, ii. 104.

Tegmina of Orthoptera, i. 246.
Telegraphic eyes, ii. 347.
Tellina, i. 164.
Temperature, animal, ii. 241.
Tendons, i. 86, 104.
Tendrils, i. 78.

Tentacula, i. 122, 129; ii. 272.
Terebella, i. 198, 199.
Terebra, i. 180.
Teredo, i. 171 ; ii. 211.
Testacella, li. 226.
Testudo, i. 325; ii. 392.

Tetrodon, i. 292, 300.

Textures, vegetable, ii. 60.

Textures, animal, ii. 74.

Thetis, ii. 212.

Thoracic duct, ii. 82, 165.

Thorax, i. 229; ii. 231.

Thorns, i. 78.

Thought, ii. 364.

Threads, elastic, in plants, i. 62.

Tibia, i. 232, 234, 282.

Tick, i. 213.

Tiedemann, ii. 171.

Tiger, i. 342 ; ii. 101, 108, 109, 278.

Tipula, i. 234.

Tone, musical, ii. 297.

Tongue of insects, ii. 93.

Tongue, strawberry, ii. 279.

Torpedo, i. 36; ii. 402.

Tortoise, i. 321 ; ii. 352.

Tortryx, i. 310, 311.

Toucan, ii. 98, 235.

Touch, ii. 268, 375.

Trachea of animals, ii. 210, 221.

Trachese of plants, i. 65-

Tradcscantia, ii. 42.

Trapezius muscle, i. 105.

7Vem6Ze?/, i. 132 ; ii. 62, 337.

Treviranus, i. 65, 66 ; ii- 400.

Trichecus, i. 336.

Trichoda, ii. 74.

Trigla, ii. 392.

Trionyx, i. 329.

Tristoma, ii. 86.

Triton, i. 182, 310.

Tritonia, ii. 182.

Trituration of food, internal, ii. 122.

Trochanter, i. 232.

Trochilus, ii. 88.

Trophi, ii. 92.

Trot, actions in, i. 341.

Trunk-fish, i. 300.

Trunk of elephant, i. 358.

Tuberose roots, ii. 413.

Tubicolse, i. 199.

Tubipora, i. 124.

Tubularia, ii. 169.

Turbinated shells, i. 159.

Turbinated bones, ii. 284.

Turkey, ii. 127, 312.

Turritella, i. 180.

Turtle, i. 321 ; ii. 146, 392.

Tusks, ii. 105.

Tympanum, ii. 299.

Type, i. 48 ; ii. 439.

Typhlops, i. 310.

INDEX.

463

Ulna, i. 282.
Ungual bone, i. 282.
Unio balava, i. 159.
Unity of design, ii. 437.
Uranoscopus, ii. 3.54.
Urceolaria, i. 139.
Urchin, sea. See Echinus.
Utricle of labyrinth, ii. 3013.
Uvea, ii. 327.

Valves, i. 36,85; ii. 188,206.
Vampire bat, ii. 88.
Van Helmont, ii. 19.
Vane of feather, i. 392.
Variety, law of, i. 23, 48; ii. 438.
Varley, ii. 183.
Vascular circulation, ii. 170.
Vascular plexus, ii. 268.
Vauquelm, ii, 166, 239.
Vegetable kingdom, i. 2.5, 43.
Vegetable organization, i. 60.
Vegetable nutrition, ii. 19.
Veins, i. 44; ii. 82.
Velella, i. 144.
Velocity of fishes, i. 301.
Velvet coat of antler, i. 2.52.
Vena cava, ii. 190.
Ventricle of heart, ii. 82, 187.
Ventricles of brain, ii. 391.
Veretillum, ii. 63, 337.
Vertebra, i. 271 ; ii. 423.
Vertebrata, i. 254.
Verticillated arrangement, i. 76.
Vesicles of plants, i. 61.

Vespertilio, i. 379; ii. 101, 399.

Vessels of plants, i. 64.

Vessels of animals, i. 84; ii. 424,
429, 436.

Vestibule of ear, ii. 303.

Vibrations, ii. 396.

Vibrio, i. 58, 138.

Vicq d'Azyr, i. 141.

Villi, ii. 246.

Viper, i. 310; ii. 419.

Vision, ii. 31.5.

Vision, erect, ii. 366.

Visual perceptions, ii. 366.

Vital functions, i. 42; ii. 55.

Vital organs, ii. 254.

Vitality, i. 29.

Vitreous humour, ii. .327.

Vitreous shells, i. 172.

Viviparous reproduction, ii. 419.
Voltaic battery of torpedo, ii. 4(>2.
Voluntary motion, i. 41 ; ii. 375.
Volute, i. 170; ii. 95, :U0.
Volvo.x, i. 13^, 139; ii. 414.
Voracity of hydra, ii. 61.
Vorticella, i. 58, 136; ii. 74, 410.
Vulture, ii. 131, 288.

Wading birds,*!. 40.3, 408.
Walking, i. 340, .374.
Waller, ii. 100.
Walrus, i. 336; ii. 105.
Warfare, animal, i. 47; ii. 53.
Warm-blooded circulation, ii. 199.
Water, not the food of plants, ii. 19.
Water-beetle. See Dytiscus.
Water-boatman, i. 35, 238.
Wax, vegetable, ii. 40.
Web-footed bird.s i. 408.

Weber, ii. 305, 339.

Whale, i. .53; ii. 130, 313, 35.5,394.

Whalebone, ii. 102.

Wheel animalcule, i. 140.

Wheel spokes, spectre of, ii. 368.

Whelk. See Buccinum.

Whiskers, ii. 278.

Whorls of plants, i. 76.

Whorls of shells, i. 176.

Willow, i. a9.

Wings, i. 243, 391.

Winged insects, i. 214. *

Withers, i. 257.

Wolf-fish, ii. 96.

Wollaston, i. 77 ; ii. 45, 346, 4()1.

Wombat, i. 363.

Woodhoiise, ii. 28.

Woodpecker, ii. 98.

Woody fibres, i. 64, 66.

Worms. See Annelida and Eii-
tozoa.

Yarrell, ii. 98.
Young, ii. 335.

Zebra, i. 356.

Zemni, ii. 3.56.

Zoanthus, i. 123, 136.

Zoocarpia, i. 119.

Zoophytes, i. 113; ii. .3.37. .37^.

Zostira, ii. 146.

Zygodactyli, i. 404.

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