The more essential salts of the food seem to be the chlorides and phosphates of the alkalies ; and especially, the chloride of so dium, and the phosphate of soda. Lime and iron are also important bases. All of these ingredients are present in the salts of the milk ; together with some free soda and potash, which are probably combined with its casein. The phosphates are in large quantity ; espe cially the phosphate of lime — the predo minance of which is doubtless connected with the exigencies of ossification in the fcetus.
Varieties of food.— The above grouping of the various constituents of the food, will afford us a valuable clue to the composi tion of its principal varieties. For however widely these varieties may differ from each other, they always contain representatives from each of the preceding classes. And the best food for any particular animal will always consist of such a proportion of all these con stituents, as bcst corresponds to the demands made by the waste of its whole body, and to the peculiarities of its organs of digestion.
The food most natural to Man is a mixed diet. But though thus far omnivorous, he readily adopts an exclusively animal or vege table food, according to the circumstances in which he is placed. And there are probably but few of the carnivorous and herbivorous animals, most properly so terined, in whom careful experiments would not detect a shni lar, though scarcely equal, capacity for such a change of diet. Thus the herbivorous Horse and Cow may be brought to eat fish and flesh ; and the carnivorous sea-birds can be gradually habituated to the far more diffi cult change implied in their feeding on grain. But many of the frugivorous Quadrumana seem little susceptible of such alterations of diet. While there seem to be numerous Insects, which are not only strictly limited to a vege table food, but even to certain species of plants, or particular parts of their structure.
The influence of any special variety of food on the human organism depends chiefly, on its physical and chemical properties : — in other words, on its mechanical arrangement and admixture ; and on the constituents which it presents ; either originally, or as modified by the operations of cooking. Hence these are the chief points which will be noticed in the following short description.
It is obvious that the division of the various alimentary substances into solid and liquid, or food and drink, is an incorrect one. For, on the one hand, even the driest articles of solid food contain a large proportion of water of composition. And conversely, the purest liquids ordinarily made use of contain a certain quantity of solids, in the shape of dissolved salts, which are by no means in different to the organism.
In the following cursoryview of the ordinary articles of diet, we shall begin by contrasting the general characters of animal and vegetable food. We shall then sketch the chief va rieties of each generally made use of. And, finally, we shall attempt to estimate the pro portions of each contained in a suitable die tary of ordinary mixed food.
It is to animal food that we must on the whole assign the first rank as an article of diet. For not only do the tissues of one animal necessarily contain most, if not all. of the organic and inorganic substances required for the construction of another, and in sorne thing like the proper proportions of their respective ingredients, but they are generally devoid of all noxious constituents. Besides these advantages, they offer the equally im portant ones of possessing such a structure, arrangement, and solubility, as materially aid their entry into the organism. Hence they are not only much more nutritious than an equal quantity of vegetable food, but are also di gested and assimilated with far greater ease and rapidity. It is for this reason that the use of animal food is so much to be preferred in all circumstances where it is our object to avert the speedy exhaustion of the vital powers.
Against these advantages, possessed by animal food, we must, however, set off the disadvantages, that it not only contains some substances which (like gelatine and the horny tissues) appear to be either useless or even to require a speedy excretion ; but that, as a rule, it is deficient in those non-azotized elements, which are so important to the main tenance of the combustion and heat of the organism. For the limited quantity of fatty matters which it generally includes rarely suffices to make these hydrocarbons a proper substitute for the copious am3laceous and saccharine constituents of vegetable food.