99. Waste and Repair. A steam-engine from the first stroke of its piston-rod begins to wear out, and before long needs repair. All work involves waste. The engine, unless kept in thorough repair, would soon stop. So with our bodies. In their living cells chemical changes are constantly going on; energy, on the whole, is running down; complex substances are being broken up into simpler combinations. So long as life lasts, food must be brought to the tissues, and waste products carried away from them. It is impossible to move a single muscle, or even to think for one moment, without some minute part of the muscular or brain tissue becoming of no further use in the body. The transformation of dead matter into living tissue is the ever-present miracle which life presents even in its lowest forms. In childhood the waste is small, and the amount of food taken is more than sufficient to repair the loss. Some of the extra food is used in building up the body, especially the muscles. As we shall learn in Chapter VIII., food is also required to maintain the bodily heat. Food, then, is necessary for the production of energy, for the repair of the body, for the building up of the tissues, and for the maintenance of bodily heat. 100. Nature of the Waste Material. An ordinarily healthy person passes daily, on an average, by the kidneys about 50 ounces of waste material, of which 96 per cent is water, and from the intestines, on an average, 5½ ounces, a large proportion of which is water. By the skin, in the shape of sweat and insensible perspiration, there is cast out about 23 ounces, of which 99 per cent is water; and by the lungs about 34 ounces, 10 of which are water and the remainder carbon dioxid. Now if we omit an estimate of the undigestible remains of the food, we find that the main bulk of what daily leaves the body consists of water, carbon dioxid, and certain solid matters contained in solution in the renal secretion and the sweat. The chief of these solid matters is urea, a complex product made up of four elements,—carbon, hydrogen, oxygen, and nitrogen. Water contains only two elements, hydrogen and oxygen; and carbon dioxid also has only two, carbon and oxygen. Hence, what we daily cast out of our bodies consists essentially of these four elements in the form mainly of water, carbon dioxid, and urea. These waste products represent the oxidation that has taken place in the tissues in producing the energy necessary for the bodily activities, just as the smoke, ashes, clinkers, and steam represent the consumption of fuel and water in the engine. Plainly, therefore, if we could restore to the body a supply of these four elements equivalent to that cast out, we could make up for the waste. The object of food, then, is to restore to the body an amount of the four elements equal to that consumed. In other words, and briefly: The purpose of food is to supply the waste of the tissues and to maintain the normal composition of the blood.