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 the 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 

 nvith 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 hquid 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 laminae. Even the surrounding fibres are analogous to those adhering 

 to the internal surface of the skin, and called tlie 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 resembhng 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 difl^used [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 creation, it will be found 

 that the nutritive fluid circulates through a system of cylindrical vessels; 

 and that it only suppUes 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 oi 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-portcB 

 [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 lacteals, 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 through 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- 

 ciUating system, 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 branchice, or gills. Certain organs of motion are 

 always arranged so as to draw the surrounding element either 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 trachem; 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 performed by means of the pressure and elastic 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 purification of the blood existing in the lungs has been 

 performed ; and the same actions are again repeated. The blood, which was of a dark 

 purple colour, while slowly travelling from all parts of the body to the heart, 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 not only passes into the lungs, but 

 through them into capacious air cells; 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 pulmonary. The ar- 

 rangements are striking and singular. 



All respiration must be either aquatic or atmospheric. In the former case, the 

 respiration is said to be cutaneous or branchial, according as it is performed through 

 the skin or through gills. On the other hand, atmospheric respiration may be either 

 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 sohds 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 sohd 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 suflSciently 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 liver 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 matter of several Mollusca. 



The function of generation is involved in much greater obscurity and 

 difficulty than that of simple secretion ; and this difficulty attaches ciiiefly 

 to the production of the germ. We have already explained the insuper- 

 able difliculties 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 woidd finally be transformed into the butterfly, until he had 

 either observed or been informed of the fact. 



