10 
THE ORGANS OF ANIMAL BODIES— THEIR FUNCTIONS. 
is comparatively simple ; and a supply of abundant nourishment is readily procured from 
animal food. 
After passing through the stomach, the food is received into the remain- 
ing part of tlie canal, where it is acted upon by other juices destined to 
complete its preparation. 
The chyme formed in the stomach having passed into the intestine, comes in contact 
with the bile and the pancreatic juice. An immediate change takes place. The chyme 
acquires the yellow colour and bitter taste of bile, and at length divides into two por- 
tions; the one, a white tenacious liquid called chyle, and the other, a yellow pulp. 
The coats of the intestinal canal are supplied with pores, which imbibe 
that portion of the alimentary mass adapted for the nutrition of the body 
[being the chyle], while the useless residue is finally conveyed away and 
ejected. 
The canal in which this first function of nutrition is performed, appears 
to be a continuation of the skin, and it is composed, in a similar manner, 
of lamince. Even the surrounding fibres are analogous to those adhering 
to the internal surface of the skin, and called the fleshy pannicle. A 
mucous secretion takes place throughout this canal, which seems to have 
some connexion with the perspiration from the surface of the skin ; for, 
when the latter is suppressed, the former becomes more abundant. The 
skin exercises a power of absorption very much resembling that possessed 
by the intestines. 
The whole length of the intestinal canal is much greater in herbivorous, than in 
carnivorous animals. 
It is only in the very lowest tribes of animals that the same orifice is 
applied to the double purpose of receiving fresh supplies of aliment, and 
of ejecting the substances unfitted for nutrition. Their intestines assume 
the appearance of a sack with only one entrance. But in a far greater num- 
ber of animals, having the intestinal canal supplied with two orifices, the 
nutritive juice [or chyle] is absorbed through the coats of the intestines, 
and immediately diffused [by the lacteals] through all the pores of the 
body. This arrangement appears to belong to the entire class of insects. 
If we commence from the arachnides [or spiders] and the worms, and 
then examine all animals higher in the scale of cre.ation, it will be found 
that the nutritive fluid circulates through a system of cylindrical vessels; 
and that it only' supplies the several parts requiring nourishment by means 
of their ramified extremities [or lacteals], through which the nutriment 
is deposited in the places requiring sustenance. These vessels, which 
distribute the nutritive fluid or blood to all parts of the body, receive the 
name of arteries. Those, on the contrary, are called veins, which restore 
the blood to the centre of the circulating system. This motion of the 
nutritive fluid is sometimes performed simply in one circle ; often there are 
two circular motions, and even three, if we include that of the vena-portm 
[which collects the blood of the intestines, and conveys it to the liver.] 
,The velocity of its motion is frequently assisted by certain fleshy organs 
called hearts, which are placed at some one centre of circulation, often at 
both. 
In the vertebrated and red-blooded animals, the nutritive fluid, or chyle, 
leaves the intestines either white or transparent; and is conveyed into 
the venous system, by means of particular vessels called lacteids, where 
it mixes with the blood. Other vessels similar to the lacteals, and com- 
posing with them one arrangement, called the lymphatic system, convey 
into the venous system those nutritive particles which have either 
escaped the lacteals, or have been absorbed tlnough the cuticle or outer 
skin. 
Before the blood is fitted to renovate the substance of the several parts 
of the body, it must receive, from the surrounding element, through the 
medium of respiration, that modification which we have already noticed. 
One part of the vessels belonging to those animals, which possess a cir- 
culating sy stem, is destined to convey the blood to certain organs, where 
it is distributed over a large extent of surface, in order that the action of 
the surrounding element may be the more energetic. When the animal 
is adapted for breathing the air, this organ is hollow, and called lungs. 
But when the animal only breathes [the air dissolved] in water, the organ 
projects, and is called branchim, or gills. Certain organs of motion are 
always arranged so as to draw the surrounding element cither within or 
upon the organ of respiration. 
In animals which do not possess a circulating system, the air penetrates 
into every part of the body, through elastic vessels called trachece; or 
else water acts upon them, either by penetrating, in a similar manner, 
through vessels, or simply by being absorbed through the surface of the 
skin. 
In Man, respiration is porfonned by means of the pressure and clastic force of the air, 
which rushes into the lungs, where a vacuum would otherwise have been formed by the 
elevation of the ribs, and the depression of the diaphragm. Muscular force then expels 
the air, after the necessary puritication of the blood existing in the lungs has been 
performed; and the same actions arc again repeated. The blood, which was of a dark 
purple colour, while slowly travelling from all parts of the body to the heait, has no 
sooner been purified by yielding its excess of carbonic acid to the surrounding air, 
and by absorbing oxygen, than its colour changes into a bright vermilion. 
In Birds, it was necessary to combine lungs of small bulk with an extensive aera- 
tion of the blood ; and, accordingly, the blood ni.t only passes into the lungs, but 
through them into capacious air ceUs; from which, by the action of the chest, it is 
again expelled. The lungs thus act twice upon the same portion of air. 
The change of the tadpole into the frog is accompanied by extraordinary alterations 
in its respiratory organs, which will be more fully explained hereafter. In the first, 
or tadpole state, the organs are branchial, in the frog they are puhuonary. The ar- 
rangements are striking and singular. 
All respiration must bo either aquatic or atmospheric. In the former case, the 
respiration is said to bo cutaneous or branchial, according as it is performed through 
the skin or through gills. On the other hand, atmospheric respiration may be cither 
tracheal or pulmonary, according as it is performed through the air-tubes called 
trachea:, or by means of lungs. 
After the blood has been purified by respiration, it is fitted to restore 
the composition of all parts of the body, and to execute the function of 
nutrition properly so called. The wonderful property, possessed by the 
blood, of decomposing itself so as to leave precisely, at each point, those 
particular kinds of particles which are there most wanted, constitutes the 
mysterious essence of vegetative life. We lose all traces of the secret 
process by which the restoration of the solids is performed, after having 
arrived at the ramified extremities of the arterial canals. But in the pre- 
paration of fluids we are able to trace appropriate organs, at once varied 
and complicated. Sometimes the minute extremities of the vessels are 
simply distributed over extended surfaces, from which the liquid exudes; 
and sometimes the liquid runs from the bottom of minute cavities. But 
the more general arrangement is, that the extremities of the arteries, be- 
fore changing into veins, form particular vessels called capillary, which 
produce the requisite fluid at the exact point of union between these two 
kinds of vessels. The blood-vessels, by interlacing with the capillary 
vessels which we have just described, form certain bodies called conglo- 
merate or secretory glands. 
With all animals destitute of a circulation, and especially with Insects, 
the nutritive fluid bathes the solid parts of the body; and each of them 
imbibes those particles necessary for its sustenance. If it become requi- 
site that any particular fluid should be secreted, capillary vessels, adapted 
for this purpose, and floating in the nutritive fluid, imbibe, through their 
pores, the elements necessary for the composition of the fluid to be se- 
creted. 
It is thus that the blood continually renovates all the component parts 
of the body, and repairs the incessant loss of its particles, resulting ne- 
cessarily from the continued exercise of the vital functions. The gene- 
ral idea which we are able to form of this process is sufficiently distinct, 
although the details of the operations performed at each particular point 
are involved in obscurity, from our ignorance of the precise chemical 
composition of each part, and our consequent inability to determine the 
exact conditions necessary for their reproduction. 
In addition to the secretory glands necessary for performing a part in 
the internal economy of the system [such as the liv'er and the pancreas], 
there are others which secrete fluids destined to be rejected, either as 
being superfluous, or for some purpose useful to the animal. Of the lat- 
ter we may mention the black fluid secreted by the Cuttle fish [with 
which, when pursued, he obscures the water to cover his retreat], and 
the purple m:itter of several Mollusca. 
The function of gkner.vtion is involved in much greater obscurity and 
difiiculty than that of simple secretion; and this difficulty attaches chiefly 
to the production of the germ. We have already explained the insuper- 
able difficulties attending the pre-existence of germs ; yet, if once we as- 
sume their existence, no particular difficulty remains attached to genera- 
tion [which is not equally applicable to ordinary secretion.] While the 
germ adheres to the mother, it is nourished as if it formed a part of her 
own body; but when the germ detaches itself, it possesses a distinct life 
of its own, essentially similar to that of an adult animal. 
The form of the germ, in its passage through the several progressive 
states of development, successively termed the embryo, the foetus, and, 
finally, the new-born animal, never exactly resembles that of the parent; 
and the difference is often so very great that the change has received the 
name of metamorphosis. Thus, no person could ever anticipate that the 
caterpillar would finally be transformed into the butterfly, until ho had 
either observed or been informed of the fact. 
