These three main functions constitute the purpose of nutrition. The growth of the adult man from the tiny cell or germ that marks his simple beginning is at the expense of the food material he absorbs and assimilates. The rehabilitation of the cells, or the composite tissues of the fully developed organism, is accomplished through utilization of the daily food, whereby cell substance is renewed and all losses made good. The energy which manifests itself in the form of heat and mechanical or mental work, _i. e._, the energy by which the vital machinery is maintained in ceaseless activity, comes from the breaking down of the food materials by means of which, as the saying goes, the body is nourished. The body thus becomes the centre of different lines of activity, the food serving as the material out of which new cells and tissues are constructed, old cells revivified, and energy for running the bodily machinery derived. Development, growth, and vital activity all depend upon the availability of food in proper amounts and proper quality. The food of man is composed mainly of organic materials, for while, as Dr. Curtis[1] has expressed it, “the plant can make organic matter out of inorganic elements, just this the animal cannot do at all. The thing of legs and locomotion, of spine and speech, can build his organic walls only out of organic bricks ruthlessly ripped from existing walls of other animals or plants.” It is true that man has need of certain inorganic salts in his daily diet, but they are in the nature of aids to nutrition (aside from such as are necessary for the formation of bone and teeth), contributing in some measure toward regulation and control of nutritive processes rather than as a source of energy to the body. Inorganic substances, however, are an integral part of the essential tissues and organs of the body, being combined with the organic constituents of the living cells. Indeed, electrolytes are perhaps the substances that put life into the proteids of the protoplasm, and it is truly important for the integrity and functional power of living cells that the proportion of inorganic constituents therein be kept in a constant condition of quality and quantity. Still, the food of mankind is essentially organic in nature, and while it may be exceedingly varied in character, ranging from the simple vegetable dietary of the natives of India and the Far East to the voluminous admixture of varied forms of animal and vegetable foodstuffs so acceptable to the _bon vivant_ of our western civilization, the principles contained therein are few in number. [1] Edward Curtis, M.D. Nature and Health: Henry Holt & Co., New York. 1906. p. 39.