The changes induced by boiling meat, partially resemble those which are caused by' roasting it. For both of these processes are probably accompanied by a coagulation of albumen, a solution of osmazom, and a formation of gelatine in the mass itself. But they differ greatly from each other in many respects. From the lower tempera ture applied in boiling, no empyreumatic sub stances are developed ; while the water which conveys the heat to the mass always extracts from it a certain proportion of its soluble constituents. This extraction may be to some extent diminished by suddenly plunging the meat into boiling water, so as to coagulate the albumen of its outer most layers ; and conversely, the extractive process may be favoured, not only by in creasing the surface of contact, but also by delaying the coagulation of the albumen, and prolonging the period of the solvent action. Hence, where it is chiefly the broth or watery solution of the meat which is intended to be used as food, the mass is preferably cut in very small pieces, and the temperature of the water raised very slowly to a degree of heat short of ebullition, and maintained there for a long time.
The various modes of salting and smoking meat are chiefly intended to protect it from decomposition ; hence they scarcely require much notice here. In the former process, however, the qualities of the meat appear seriously damaged*, quite apart from the mechanical disadvantages which both it and smoking often impart.
Fat.—In a purely animal diet, the amount of this oleaginous constituent is of indispensable importance. For, with the exception of that minute quantity of inosit or muscular sugar which is proper to the sarcous substance, the fatty matters contained in the various tissues of the body are the only representatives of the two groups of the hydro-carbons and hy drates of carbon, which this kind of food possesses. Hence the fat of such a diet has to replace, as it were, the starch of the vege tables which usually enter into a mixed diet ; and thus constitutes the sole non-azotized or respiratory element of animal food.
And even in what are often miscalled vege table diets, a large quantity of this animal sub stance is commonly added to the other ingre dients of the food. At least there seems to be a strong impulse towards such an admix ture in most of the vegetarian nations and races of modern times : —an impulse which is well exemplified in the butter or ghee so copiously added by the Hindoo to the rice that forms his staple food: The quantity of fatty matter which may thus be taken into the system can scarcely have any definite limit assigned to it. In the Arctic climates it appears to attain a very large proportion. And it is impossible to
avoid connecting this maximum of fat in the food, with the large amount of heat that has to be evolved from the body in these cold regions, as well as with the energy of the combustion on which this evolution of tem perature depends. As a rule, however, but a small quantity of fatty matter can be really digested at a time. Any excess over this amount is merely expelled from the intes tinal canal with the fxces.
The digestibility of fat depends chiefly on two circumstances:— its mechanical arrange ment, and its chemical composition. In the adipose tissue, the fatty substances are en closed in large nucleated cells ; the membranous walls of which consist of a proteinous sub stance that is rather difficult of solution, and yet requires to be dissolved before its contents can enter the lacteals as chyle. And the three substances (stearine, elain, and marga rine), which form the greater part of the fat of the Mammalia ordinarily slaughtered for food, possess very different degrees of solu bility. Hence they are by no means equally easy of digestion; the first resisting its in fluence much more obstinately than either of the other two.
An animal which is fed exclusively on fat increases in size during a short period. Its nutrition, however, soon suffers; and it finally dies, with those appearances of inanition which have already been mentioned as attend ing all attempts to maintain life by the in gestion of only one iiagredient of the normal food. In the later stages of this process of starvation, its body gives off' a repulsive odour, hich appears to be due to the evolution of volatile fatty acids from the skin and lungs. The production of these acids may be re garded as probably due to an imperfect oxida tion of the hydro-carbons accumulated in the organism.
The alimentary properties of various other tissues and organs of the animal body may be passed over with a very brief notice.
The blood itself appears to be a far less valuable article of food than its composition would lead us to suppose:—abounding, as it does, in the important protein-compounds of fibrin and albumen. Some authors have supposed, that its digestion is rendered diffi cult by the dense state of aggregation which its fibrin is so apt to assume in the act of coagulation. But however this may be, still its lar4e albuminous constituent appears to be in a condition such as would eminently fit it for fulfilling the requirements of the organism. We are thus left to remark upon its almost total want of hydrocarbons*: as well as upon the contrast offered by its salts t to those of the muscular substance.