FATS, ANIMAL. There is considerable difference of opinion amongst chemists regard ing the exact nature of 'the fats occurring in the animal body. According to most chemists, they are composed of an admixture of three separate fats—margarine, stea rine, and oleine, of which the two former are solid, and the latter fluid, at ordinary tem peratures. Heintz, who has carefully studied these bodies, declares, however, that margarine is not a simple fat, but a mixture of stearine and palmitine (a solid fat occur ring in palm-oil); and he considers human fat to be a mixture of stearine, palmitine, and oleine. For the chemical characters of these substances, we refer to the articles MAR GARINE, OLEINE, PALMITINE, and STEARIC ACID, and we proceed to the consideration of the physiological relations of the fat.
Fat, usually inclosed in vesicles, is found very extensively in the animal kingdom. It is abundant in many larvae, and occurs more scantily in most insects. It is met with in the mollusca, and is comparatively abundant in all the divisions of the vertebrata. In most fish, it occurs throughout the body, but is especially abundant in the liver, where it is found in the hepatic cells, and not in its own characteristic vesicles. In reptiles, it chiefly exists in the abdomen. In birds, we especially find it about the peritoneum, and under the skin. In mammals, it is very generally diffused, but the greatest quantity is under the skin, in the omentum, and round the kidneys.
The quantity of fat in the human body varies considerably at different periods of life. In the earlier stages of fetal existence, we find scarcely any fat; in new-born children, there is usually a considerable quantity of this substance deposited under the skin, and the organism continues rich in fat till the age of puberty, when a marked diminution of the substance occurs. It again increases about middle life, and then occasionally occurs in great excess; for example, 3 or 4 in, of fat are not unfrequently found under the skin of the abdomen in corpulent persons.
Extraordinary deposits of fat in some particular part of the body are observed in certain races of men and animals. One of the most remarkable examples of this peculi arity is afforded by the Hottentot women, in whom the fat accumulates in the gluteal region to such an extent as to give a most remarkable prominence to that part of the body; and a somewhat analogous deposit exists in a variety of sheep (ovis steatopyga, the fat-buttocked sheep), in which a large mass of fat, sometimes attaining a weight of 40 lbs., is developed on the buttocks, and takes the place of a tail.
The origin of the fat in the animal body must undoubtedly be chiefly referred to the fat taken with the food. It has, however, been proved by the most careful investiga tions on various animals submitted to the process of fattening, on bees fed with cane sugar, or with honey containing scarcely any wax, and on the larvae of the insects inhabiting galls, that the animal, like the vegetable organism, has the power of forming or producing fat, far more fat being found, in these experiments, in the body of the animal, than could be referred to the fat taken in the food. The excess must therefore have been formed either from the non-nitrogenous portion of the food, such as starch and sugar; or from the nitrogenous matters, such as fibrin, albumen, etc. In the case of the bees, it was distinctly proved that the fat was formed from sugar; while in the case of the larvle of the gall-insect, it was similarly shown that it was produced from the starch which forms the interior of the gall in which the animal lives; and as we have no corresponding evidence of the convertibility of fibrin, albumen, etc., into fat (although such a conversion is by no means improbable), we must for the present regard the non-nitrogenous foods as the chief fat-formers next to fat itself.
The physiological value of the fats is partly due to their physical, and partly to their chemical characters.