The nitrogen compounds of the vegetable and animal kingdoms have received much attention from scientific investigators and writers during the past fifty years. It is quite the custom to declare that certain members of this class of substances are the ones most important in the domain of animal nutrition, and many writers have given to them so prominent a place in discussing nutrition problems as to almost ignore the other nutrients. Certain investigators claim, on the other hand, that from the economic point of view the function and relative value of protein have been unduly magnified. Whatever may be the correct opinion, it is very evident that the present tendency is towards a fuller discussion of the office and value of the other classes of nutrients.

There can scarcely be any disagreement, however, concerning the general proposition that the compounds of nitrogen play a leading part in the processes and economy of human nutrition. This is true for several reasons: (1) The nitrogen compounds are those fundamental to the energies of the living cells which make up the tissues of plants and animals. The basic substance of the active cell is protoplasm, a complex nitrogenous body, which Huxley called "the physical basis of life." Around this primal substance seem to center all vital activities, especially the transformation of the raw materials of the inorganic world into the organized structures of plant life and the transformations of food compounds into the tissue substance of the human body.

(2) These compounds are structurally essential to the growth of living tissues and to the formation of milk.

(3) Foods rich in nitrogen have reached a position of great commercial importance, and they bear relatively high market prices.