Constituents of food.-1. The first group, consisting of what are called the protein compounds, includes a number of proxi mate principles, which are derived from both the animal and vegetable kingdoms of nature. The chief of these principles are albumen, fibrin, and casein. By digestion in solution of potash, and precipitation with an acid, either of these yields a substance called pro tein : — a name that alludes to the relation this principle is supposed to bear to all the compounds from which it is thus obtained. It is regarded as their common starting-point (upoyrEinu, prima s partes teneo), and most essen tial component. And the slight differences of composition offered by each particular pro tein-compound, are explained as chiefly due to variations in the nature and amount of certain collateral ingredients, the addition of which to protein imparts the specific charac ters of albumen, fibrin, or the like. Hence the various protein-compounds are supposed to differ, not so much in elementary com position, as in certain characters which might almost be termed morphological : — namely, outward form, physical properties, degree of solubility, and the like.
The exact process by which one of these protein-compounds undergoes conversion into another is still a complete mystery. But that such changes do constantly obtain, cannot be doubted. And hence, while the quantity of albumen in the animal body, and the constancy with which it is present, assign to this proxi mate principle the leading position in the above group of proteinous substances, it is on its generic, and not its specific, properties that our attention ought chiefly to be fixed.
The protein of the food may be regarded as its most essential constituent. The reason why such an importance is ascribed to it becomes sufficiently evident, when we com pare its composition with that of the body iihich it is intended to nourish. The highly azotized constitution it possesses (C 54-7 6.8 N 14-2 ± 0 2,1.3 = 100) closely approaches that of the solids of the organism generally. And it shows an equally important relation to most of the tissues in detail. lt forms a large constituent of the blood; and therefore of the plastic nutritional fluid that exsudes directly from this fluid. It is the main component of the muscles, which execute the various movements of the body. It is an equally important ingrethent in the tissues of both the central and peripheric parts or the nervous system. It is probably the-source of the gelatinous * tissues ; which, in herbivorous animals, can only be derived from a kind of degradation or regression of the albuminous substances. And, finally, its large amount in the structures of the fcetus proves that it is just as important to the evolution and growth of the animal, as it is to its maintenance. In
short, in protein and its various kindred sub stances, we recognize the principle, which forms the material exponent of all the struc tures and functions, and is the chief sub stantive agent of the chemistry of life.
The quantity of protein necessary for the proper maintenance of the healthy animal can only be estimated from very indirect and ap proximate calculations.
In milk, the albuminous compounds are chiefly represented by casein, which forms about 31. per cent. of its total quantity. But we can scarcely guess how much milk is daily consumed by the sucking animal, or what proportion this amount bears to the weight of its whole body. And we are justified in assuming, that a large fraction of the protein thus introduced into the system, is applied to exigencies of growth and deve lopment which have little or no place in the adult animal.
Assuming an exact maintenance of the adult organism, without increase or decrease, we might expect that an examination of its various azotized excretions would teach us how much nitrogen had been discharged from the system within a given time : and hence that, by comparing this quantity with the known elementary composition of protein, we might be enabled to calculate how large a quantity of the azotizedr constituent of the food ought to be added to the system, in order to replace its daily loss.
But here we are met by a difficulty con nected with the process of nutrition itself :— ith that chain of events of which food and waste constitute only the extreme links. The amount of nitrogen given off' by the body does not depend solely upon the quantity excreted by its waste, but also varies in close corre spondence with the quantity taken in its food. It is therefore greater in canaivorous, and less in herbivorous, animals.
Hence the true or essential svaste of the organism, in respect of this constituent, can only be dctennined from an analysis of the ex cretions of animals which have been kept for a day or two, either without food, or on a diet altogether devoid of nitrogen. In both cases the results are the same. The nitrogen of the egesta drops to a certain minimum ; at which it reniains for a considerable period.
The quantity of nitrogen evolved by the lungs and skin is at any rate so small, as scarcely to form an important element of cal culation. And even the larger quantity excreted in the biliary resin, hardly deserves notice. It is in the uric acid, and above all in the urea, of the renal secretion, that this element is chiefly dismissed from the body as an effete compound. And hence it is from the urea found in such experiments that we may best deduce the probable rate of daily waste in the albuminous tissues ; and the cor responding quantity of protein which there fore has to be supplied in the daily food.