_Proximate Composition of Animal Substances._--The differences between vegetable and animal substances are often more apparent than real. Indeed many of the more important of these substances are almost identical in composition. The albumen which coagulates when the juices of vegetables are boiled, is identical with the albumen of the white of eggs; the fibrine of wheat is in no respect chemically different from the fibrine, or clot, of the blood; and, lastly, the legumine, or _vegetable caseine_, of peas is almost indistinguishable from the curd of milk, or _animal caseine_. But not only has chemical research demonstrated the identity of the albumen, fibrine, and caseine of vegetables with three of the more important constituents of animals, it has gone a step further, and proved that they differ from each other in but a few unimportant respects. They are unquestionably convertible into each other[2] within the animal organism; and their functions, as elements of nutrition, are almost, if not quite, identical. Exclusive of the blood, which contains the elements of every part of the body, the animal organism is composed of three distinct classes of substances--namely, _nitrogenous_, _non-nitrogenous_, and _mineral_. All of these constituents, or substances capable of being converted into them, must exist in the food. Certain articles, for example, milk, contains all of them; but in others, for instance, butter, only one of these substances is found. The nitrogenous part of the body embraces the muscles, or lean flesh, the gelatine of the bones, and the skin and its appendages--such as hair and horns; the non-nitrogenous constituents are its fat and oil; and its mineral matter is found chiefly in the bony framework. These constituents are not, however, isolated: the mineral matter, no doubt, accumulates in certain parts, but in small quantities it is found in every portion of the body; and although the fat forms a distinct tissue, the muscles of the leanest animal are never free from a sensible proportion of it. Albumen, fibrine, and caseine are the principal nitrogenous constituents of food, and as they are employed in the reparation of the nitrogenous tissues of the animal body, they have been termed _flesh-formers_. The fat and oil of animals are derived either from vegetable oil and fat, or from some such substance as starch or sugar. The constituents of food which form fat are termed _fat-formers_, and sometimes _heat-givers_ or _respiratory elements_, from the notion that their slow combustion in the animal body is the chief cause of its high temperature. The mineral elements of the body are furnished principally by the varieties of food which contain nitrogen. The whey of milk is rich in them; but they do not exist in pure butter, in starch, or in sugar.