It is found that certain of these simple elements are used for one part of the body, and others for other parts, and this in certain regular proportions. Thus, carbon is the chief element of fat, and also supplies the fuel that combines with oxygen in the capillaries to produce animal heat. The nitrogen which we gain from our food and the air is the chief element of muscle; phosphorus is the chief element of brain and nerves; and calcium or lime is the hard portion of the bones. Iron is an important element of blood, and silicon supplies the hardest parts of the teeth, nails, and hair. Water, which is composed of the two gases, oxygen and hydrogen, is the largest portion of the body, forming its fluids; there is four times as much of carbon as there is of nitrogen in the body; while there is only two per cent as much phosphorus as carbon. A man weighing one hundred and fifty-four pounds, who leads an active life, takes into his stomach daily from two to three pounds of solid food, and from five to six pounds of liquid. At the same time he takes into his lungs, daily, four or five thousand gallons of air. This amounts to three thousand pounds of nutriment received through stomach and lungs, and then expelled from the body, in one year; or about twenty times the man's own weight. The change goes on in every minute point of the body, though in some parts much faster than in others; as set forth in the piquant and sprightly language of Dr. O. W. Holmes [Footnote: Atlantic Almanac, 1869, p. 40.], who, giving a vivid picture of the constant decay and renewal of the body, says: "_Every organized being always lives immersed in a strong solution of its own elements._" "Sometimes, as in the case of the air-plant, the solution contains all its elements; but in higher plants, and in animals generally, some of the principal ones only. Take our own bodies, and we find the atmosphere contains the oxygen and the nitrogen, of which we are so largely made up, as its chief constituents; the hydrogen, also, in its watery vapor; the carbon, in its carbonic acid. What our air-bath does not furnish us, we must take in the form of nourishment, supplied through the digestive organs. But the first food we take, after we have set up for ourselves, is air, and the last food we take is air also. We are all chameleons in our diet, as we are all salamanders in our _habitats_, inasmuch as we live always in the fire of our own smouldering combustion; a gentle but constant flame, fanned every day by the same forty hogsheads of air which furnish us not with our daily bread, which we can live more than a day without touching, but with our momentary, and oftener than momentary, aliment, without which we can not live five minutes."