lism of the protoplasm of the special secreting organs and of the general tissues.
From the common nutritive fluid, the blood, protoplasm is formed in all the tissues of the body, and we must look upon the characteristic elements and products of these tissues as the result of its destructive metabolism. In each organ of the body the protoplasm appears to have special endowments adapted to their specific functions, but these diverse activities are correlated to serve a common purpose in the life of the individual. The contraction of muscles, the specific secretions of the glandular organs, including the salivary glands, the liver, the pancreas, the mammary glands, etc., and, in fact, the products of all the metabolic tissues, as well as their characteristic structural elements, must be considered as the resulting products of the downward steps of the metabolism of protoplasm.
As in plants the food elements are built up into protoplasm before they are converted into the proximate constituents of plant tissues (proteids, fats, starch, etc.), so in animal nutrition it appears that the proteids, fats, and carbohydrates, together with oxygen introduced by the lungs, which constitute their food, must pass through the intermediate phases of blood and protoplasm before they appear as animal proteids and fats, or enter into the composition of the different tissues of the animal body, so that a genetic or specific relation of particular tissues to special food constituents can not be traced.
For example, the muscles, from their comparative bulk, contain a large proportion of the nitrogen of the body, and they are spoken of as nitrogenous tissues, but they are not formed directly from the proteid or nitrogenous constituents of the food. Like all other tissues, they have their origin in protoplasm that is built up from the common nutritive fluid, the blood, which is elaborated, as we have seen, from the disintegrated elements of fats and carbohydrates as well as proteids. Moreover, nitrogen is no more essential to the formation of muscle than carbon or oxygen, or even water, which are more abundant constituents of all living tissues.
It must then be evident that we can not formulate the proportions of the proximate principles of foods that will serve the best purpose in animal nutrition. The extended and profound series of changes that intervene between the food constituents on the one hand and the resulting animal tissues on the other are too complex to enable us to trace any direct chemical relations between the initial elements and their final products. Aside from these physiological considerations there are insuperable obstacles in the way of prescribing diets that are even approximately suited to the requirements of any particular individual, or group of
