ANIMAL CHEMISTRY. The object of researches into the chemical nature of animal substances is twofold: First, to classify the proximate or immediate component ingre dients of the animal body, study their properties, their mutual relations and metamor phoses, and the ultimate elements of which they are composed; second, to investigate the processes that go on during the elaboration and assimilation of new materials, and the wearing out and excretion of old—processes that, taken together, constitute nutri tion, or the vegetative side of animal life. Without a pretty complete knowledge of the first part, no successful researches can be made in the second; and it is chiefly owing to the great progress that has been made within the last thirty years in the knowledge of the chemical properties of the animal compounds containing nitrogen that we owe the recent advance in our knowledge of the chemical processes of life. That advance is not the less decided that we are still far from a complete understanding of them. The general laws of chemistry are now traced into the province of organic nature much further than formerly, and the abrupt partition between the two is removed. It is still acknowledged that these laws operate differently within the sphere of organic life, from what they do without; but instead of resting contented with saying, that owing to the vital force this could not be otherwise, the aim is now to trace the why and wherefore of this modified action as far as possible.
In the animal body two classes of substances may be distinguished: those that properly compose the body, and those that are on the way either into it or out of it. The former, or actual components of the body, are, again, of two kinds: 1. Substances that compose the actual tissues of the organs, and in which the vital functions seem properly to inhere; the substances, namely, of muscle, of nerve, of brain, of membranes, sinews, and the organic part of•the bones. All these agree in consisting chiefly of carbon, hydrogen, nitrogen, and oxygen, with usually minute proportions of sulphur and phos phorus. But in respect of their mode of composition, they fall into two classes—those that yield gelatine on boiling, and those that do not. To the former belong the 'sub stance of the cartilages, bones, sinews, and skin; to the latter the fibrin of the muscles and of the blood corpuscles, the albumen of the nerves and blood, the caseine of milk, etc. These last are the so-called compounds of proteine (q.v.). In the living tissues all these matters are combined with about 90 per cent of water. 2. Besides the above, which are the real animalized or vital substances, the animal body contains substances which are merely deposited in the cells and interstices of the former for imparting color, solidity, elasticity, etc. Of this kind are fat, the earthy matter of the bones, pigment, etc. Whether the minute quantities of common salt and of phosphates that arc found in all parts of animals essentially belong to the constitution of the substance they are associated with, is not yet made out, but it is extremely probable they do; at all events they play a very important part.
The substances that are on their way into and out of the body form on the one hand the contents of the digestive organs, and on the other those of the organs of excretion. The vascular system forms the means of communication between both and the substance of the body, and the blood is the carrier of all that enters that substance or leaves it. In the digestive organs, accordingly, we find, along with the unaltered materials of the food, the various products of their digestion, and at last the useless refuse, not absorb able by the vascular system, and the various fluids—some acid, some alkaline—added to the food to effect its digestion, such as the saliva, gastric juice, and bile.
The matters prepared in the digestive organs for being taken up into the blood, either enter the venous system directly, or get there by first going through the lymphatic sys tem. This last contains a fluid which is chemically very like the blood, but colorless the chyle, namely. This fluid and the blood contain the so-called proteine compounds derived from the food, partly in solution, and partly solid in the blood corpuscles. Arterial blood contains, besides, all those salts and other substances that must be sup plied for the nourishment of the various organs. The venous system, again, which brings back the blood from the different parts to the central organs, is laden with all the mat ters that are no longer of use, and must therefore be carried to the chief excretory organs the skin, liver, and kidneys. The dark color of venous blood indicates that its compo nents have undergone a change. But all blood that is on its way both to and from the parts of the body, before it can impart nourishment, must pass through the lungs, an organ in which it is brought into extensive contact with atmospheric air, and undergoes a process of oxidation, producing the following palpable results: The disappearance of a portion of the inhaled oxygen, and the substitution of water and carbonic acid in its place; the transformation of the dark venous blood and of the chyle into red arterial blood; lastly, the development of heat. Breathing, then, contributes to nutrition by making the blood fit for that purpose; it is an excretory process, inasmuch as it burns out useless matters and separates them in the form of gases; and at the same time it pro duces heat, without which life could not go on.
Sweat, urine, bile, and emanations from the skin and lungs, contain only products of the decomposition of effete animal substances; many of these products arc highly interest ing in a chemical point of view, especially urea, uric acid, and bile. It is clear, then,! that comparative investigation of the blood in its different states, of the excretions and secretions, can alone give any knowledge of the condition of the vegetative side of the organism; and, accordingly, this kind of investigation has in recent times become of the highest importance forpathology and diagnosis. See Liebig's famous work on A. G., translated by Gregory, and the excellent Lchrbuch der Physiologiehen Chemie (3d ed., Leip. 1854), by Lehmann.