The brain and nervous centres are so rich in albumen and filt, as to form highly nutri tious articles of food; especially when they are mixed with other substances, which are more capable of affording the requisite tnechanical stimulus to the digestive organs.
The various glands possess a dietetic value which is derived, partly front their physical structure and arrangement, partly from their cheinical composition. Thus, the dense mechanical texture of the liver and kidney must decidedly oppose their usefulness as food : while the bile and urine which they respectively contain, necessmily superadd the properties of these secretions to those of the proteinous parenchyuia that forms the bulk of their mass. And conversely, from both mechanical and chemical reasons, the panereas is highly digestible and nutritious.
The hard solid texture of bone, and its large gelatinous and calcareous constituents, to gether render it of comparatively little use as an article of food.
The eggs of oviparous animals contain, in addition to the embryo itself, a quantity of nutritive matter, which is destined for its nou rishment during the process of incubation. Hence, the large eggs of many Birds form an excellent article of food, the dietetic virtues of which resemble, to some extent, those pre viously attributed to milk. The white of egg contains about 15 per cent. of albumen. The yolk is composed of about 20 per cent. of the same protein compound ; together with about 30 per cent. of fatty matter—chiefly margarin and elain —in a state of subdivision and ad mixture which eminently adapt it to digestive purposes.
The general composition of tbe milk which forms the food of young Mammalia has already been mentioned. It only remains for us to notice its chief varieties, and the products which its artificial preparation adds to the bill of fare of the adult.
The peculiarities exhibited by the various kinds of milk, are chiefly referrible to the species of the parent animal, the date of its lactation, the nature of its food, and its habits. Thus the milk of the Human female* contains about half the quantity of casein, and two-thirds the butter, of that of the Cow : while that of the Ass, which is still poorer in each of these constituents, greatly surpasses them both in the amount of its saccharine ingre dients (being as 3 to 2). The rich colo
strum which is yielded in the puerperal state t soon gives place to a milk which is poorer in all the solid ingredients ; and the further continttance of lactation appears chiefly to increase its casein and salts, and diminish its sugar. The copious ingestion of fatty or starchy substances seems to increase the buttery constituent. The over-feeding of a wet-nurse causes her to secrete a milk abnor mally rich in butter and casein, and injurious to a delicate child. Finally, vigorous exercise appears to diminish both these constituents, especially the former.
The composition of the butter contained in milk is as yet but imperfectly known. That of the Cow is stated by Bromeis* to consist of about 68 per cent. of margarine, with 30 of elain, and 2 of fatty matters specific to butter. The exact nature of the latter constituents probably varies in different animals, as well as in different speci mens of the secretion ; and also seems very liable to be altered by that rancidity which butter so easily acquires from a short expo sure to the air. Such circumstances quickly give rise to the formation of a variety of vola tile fatty acids :—which are known under the names of butyric, caprylic, capronic, cupric, and vaccinic acids.
The dietetic value of butter can scarcely be rated too highly. It is probably by far the best and most natural form in which hydro carbons can be supplied to the healthy organ ism. It is not only attractive to the taste, but is easily assimilated : — even by children or adults, whose delicate digestive organs resent the introduction of the ordinary adipose tis sue of animal food. The quantity which may be advantageously consumed will of course vary with the nature and ainount of other food, and with the rate at which combustion proceeds in the body. But the very large amount of this substance habitually consumed by the Hindoos, and by the dairy-men in many of the Alpine highlands of Europe — in the latter case often reaching a pound daily —is a striking testimony alike of its harm lessness to the digestive organs, and its value to the system generally.