And not only is there no identity in the composition of the organism and the ingesta, but it would seem that there are some tissues of the body which have absolutely no repre sentative in the food : no kindred substance to which their formation can possibly be referred. Such are the various tissues that yield gela tine ; a substance which, though it appears to escape assimilation when introduced into the organism from without, is yet constantly formed within it, from the metamorphoses of other parts of its substance.
The chemistry of nutrition therefore implies neither construction, on the one hand, nor identity, on the other ; but something mid way between these two extremes. Its forces occupy, so to speak, a debateable ground between the prehension of old materials, and the formation of new ones. And the food submitted to its action is only required to possess such a similarity of composition with the body, as will concede these limited changes, without implying any wider process of metamorphosis.
Any exact definition of the degree of re semblance thus requisite, would be foreign to our present object. Indeed, in the existing state of our knowledge, it is impossible to specify the precise nature of those metamor phoses, which accompany the digestive act, and are bounded by the food and the organ ism as their respective beginning and end. It is enough to indicate, that they appear to be intermediate between the forces of che mical affinity on the one hand, and homoge neous and heterogeneous adhesion on the other; and that while they are sometimes * akin to the formation of hydrates, they oc casionally resemble those still more recon dite phenomena which are concerned in the production of isomeric or isomorphous com pounds :— substances which, though identical in their composition, offer striking differences in their solubility, as well as in many of their chemical properties and reactions.
This very limited convertibility of the main components of the food, renders their variety almost as essential, as though each different tissue of the body had required the entry of its corresponding substance from without. In other words, within the range of the chemical parallelism just mentioned, the organism de mands alimentary compounds containing all the different ingredients necessary to cover its own waste.
This fact receives a good illustration from that selection which the instinct of most per sons would impel them to make. Left to himself, Man always chuses a mixed diet, composed of proper quantities of animal and vegetable, liquid and solid, matter. Nay more,
that almost equally imperious instinct which urges him to vary his diet, though often con fused with the morbid cravings of luxury, is essentially nothing less than an expression of the natural wants of a healthy organism.
Obscured, however, as these really natural instincts often are by the stereotyped tastes and habits of a highly artificial state of society, we gain a far better insight into the proper composition of food, by examining that store of nutriment which, in the shape of the yolk of the Bird's egg, or the milk of the Mammal, Nature herself provides for the maintenance of the young of these classes. Of these two substances, the milk is justly regarded as forming the very best example of a proper food :—both as regards the nature of its several ingredients, and the proportions in which they are mingled with each other.
Milk.— The alimentary properties of the milk are due to the presence of a number of proximate constituents, the more impor tant of which may be enurnerated as fol lows. — (1) A protein-compound, casein ; (2) a hydro-carbon or fat ; (3) a hydrate of carbon or sugar ; (4) certain salts; and (5) the water in which the whole of these materials are suspended or dissolved. Of these five groups of substances, at least four are indispensable ingredients of every proper food. The hydrate of carbon and the hydro carbon are, to some extent, capable of forming substitutes for each other. But with this ex ception, (an explanation of which will be attempted by and by), the absence of any one of these constituents, or even its presence in insufficient quantity, suffices to destroy. the capacity of any particular food for maintaining life ; so that an animal limited to such a diet ultimately dies with appearances of inanition. And ti fortiori, the ingestion of but one of these alimentary ingredients,— such as albu men, fat, or sugar, — is soon attended with effects which still more closely resemble those of starvation. Such a diet does indeed essen tially starve the entire organism, even while it supplies some of the constituents of its lost substance. For although the unchecked waste of the remaining constituents of its mass tells upon certain of its textures with greater rapidity and energy than on others, still it ultiniately involves the whole in a common destruction :— a fact which need little surprise us, when we recollect the mixed composition of the simplest tissues, and the intimate mutual dependence of the most distant and isolated parts.