Animals are furnished with the means of this supply, by their power of converting animal and vegetable substances into the nature of blood, by a process called digestion. Some animals are led by their nature to live on vegetable food, others on animal food only, whilst others can subsist on either, or any mixture of both.[193] The digestive powers of man fit him for any proportion of animal or vegetable foods, and are the most perfect of all animals. Other creatures may be said to be confined to a certain district, but the curiosity of man is to lead him over the whole world, and frequently place him in situations where only one kind of food is attainable.
The first change which takes place in the food, in order that it shall be converted into the nature of the blood, is its division into smaller parts, by the teeth or gizzards of animals. It is then passed into the stomach, where it remains for some time exposed to the action of a fluid, formed in the stomach, which is called gastric juice. This possesses a very strong power of coagulating and dissolving various animal and vegetable substances. As far as we know, it acts on the principle of any other solvent, for it produces the same change in substances out of the body, or even within the body after death. It frequently happens, for instance, when a person has been killed, by accident, in full health, that, on inspection, the stomach is found dissolved, and reduced to a gelatinous mass in several parts, arising from the action of the gastric juice, which had been formed in it before death. The gastric juice, however, cannot act upon living substances: hence the stomach resists its action, and worms sometimes reside and are even generated in the stomach. Every substance capable of being acted upon in the stomach, is reduced, by the solvent power of the gastric juice, into a pulpy mass, which has been called chyme, the exact chemical properties of which have not been ascertained; in this state it is by degrees transferred into the beginning of the small intestines, where it is mixed with the bile and pancreatic fluid, and undergoes a change into a milky fluid, which is called chyle. It is then diffused by an undulating motion of the intestines over their inner surface, that it may be absorbed, and carried into the general mass of blood.
As far as has yet been ascertained by experiment, the chyle of animals, most opposite to each other in their food, structure, and habits of life, is so much alike as to have no distinguishable difference. The chyle of a Dog, or Wolf, differs in nothing from that of a Sheep or an Ox. This would appear surprising, were it not ascertained that almost every alimentary matter undergoes a chemical change before it is converted into chyle, and that the ultimate analysis of either animal or vegetable matter presents us with the same elements as those of the blood, which, though only three or four in number, are capable of forming the various substances of which the body is composed, by combining with each other, and in different proportions. There is, however, this difference observable in the chyle, that in reptiles and insects it is transparent like lymph.
The lacteals are the vessels by which the chyle is absorbed from the intestines: they form small processes on the internal surface of the intestines like the pile of velvet, which are hence called villi. A small portion of chyle being received into their open mouths, is propelled by successive contractions of these vessels into their large trunk, the thoracic duct, from whence it is poured into a great vein near the heart, and, by circulating through the lungs, probably receives its final change into blood; and this change would seem to be easily effected, as the chyle already possesses the principal properties of blood, being formed of particles swimming in a thinner fluid, and having a power of coagulating spontaneously.[194]
This is the apparatus by which the food is digested in man so as to replenish the blood; but the digestive organs of different animals exhibit considerable varieties, some being more simple, others more complex in their structure, adapted to the kind of food with which the animal is nourished. Ruminating animals, or animals which chew the cud, such as the Cow, have several stomachs, and the food undergoes mastication several times, at each time being passed into a different stomach, before being finally acted upon by the gastric juice, after which it is transmitted through a long tract of intestines. This is an example of the most complex digestive organ fitted to act upon hard and fibrous food, which must be subjected to the action of several menstrua preparatory to its being acted upon by the gastric juice.
In birds who live on grain as has been noticed, we meet with a different apparatus to prepare it to be acted upon by the gastric juice. The food first passes into the crop, which forms a kind of reservoir from whence it may pass by degrees into the gizzard, by which the grain is ground into small particles, before it is transmitted into the stomach: and it is surprising with how great power the gizzard acts for this purpose. The Abbé Spallanzani introduced a garnet, which is a very hard and angular stone, into the gizzard of a Wood-Pigeon, and, in the course of a day, it was ground perfectly smooth, by the action of the gizzard. He also introduced a leaden ball stuck full of tin points, and another with fine lancets, into the gizzard of a Turkey, and in about 18 hours, the whole of the points were rubbed down. The gizzard also possesses an amazing power of compression. Raumeur introduced into the gizzard of a Turkey tubes of tinned iron, seven lines in length, and two in diameter, closed with solder at each end; some were indented by the action of the gizzard, and others crushed flat. Similar tubes, introduced into the teeth of a vice, required the weight of about 440 lb. to produce the same effect. The gizzard thus reduces into small particles whatever food the animal selects, that it may be more readily acted upon by the gastric juice in the stomach; for the gastric juice acts like any other solvent, and therefore acts most advantageously when the food is reduced into small parts.—The digestive organs of some of the lower orders of animals form a striking contrast to these. In the most simple apparatus with which we are acquainted, the stomach and the intestines are composed of a simple bag which has but one opening, which serves both to receive the food, and discharge the excrement. It composes in fact the whole bulk of a fresh-water Polypus. In these animals the chyle is absorbed by small vessels in the sides of the bag, and is conveyed to every part of the body.
Thus we find that the supply of materials to the blood is commensurate to its exhaustion, that in young animals where a more active process of formation is going on, a larger proportion of food is requisite, and more chyle formed; this, however, is not all that is necessary to prepare the blood for its important purposes within the body. The blood, by passing through the various parts of the body, is so changed by the abstraction of certain properties, as to render it unfit for circulation, which implies the necessity of an organ, which may restore to the blood its requisite qualities. This office is performed by respiration, that function in animals by which the blood receives the influence of atmospherical air.
There is a great variety in the structure of the organ for exposing the blood to the air, suited to the mode of life in different animals. In man and quadrupeds generally the lungs serve this purpose; they are composed of a number of blood vessels spread out upon minute air cells, which communicate with and receive the air by means of the trachea or windpipe, in consequence of the expansion of the chest by certain muscular powers. These vessels and cells are connected together by cellular membrane, so as to form a spongy mass called lungs, which are commonly placed in the chests of animals.—But besides this kind of organ, which in birds is very large, they have air bags, or appendages to the lungs, diffused through various parts of the body; even some of their long bones contain nothing but air, and communicate with the lungs. It was from a knowledge of this fact that Mr. J. Hunter made a Turkey breathe by its wings, by making an opening into their large bones, and closing the animal’s mouth.
In Fish, the gills serve the purpose of lungs. They are composed of a number of processes arising from cartilages, having distributed upon them minute blood-vessels, which receive the influence of air contained in water: and hence distilled water, which contains little air, destroys fish, in the same manner as the exhausted receiver of an air pump does a breathing animal.
There is another mode of conveying air for the use of the blood in many insects, by means of a number of tubes or spiracula: these receive the external air, and, by ramifying in the body of the animal, convey its influence to the blood. Thus these animals may be said to respire like vegetables, throughout the whole of their surface, by vessels which introduce the air at different points into their bodies. In some insects the rectum forms the principal organ of respiration, and, in the class of animals called Zoophites, there are no visible organs of respiration.