A most obvious and direct inference from the facts set forth under the preceding section is that the body needs for its growth and repair such elements as enter into its construction. While this is true, we must not lose sight of the fact that while the growing corn plant needs carbon, it cannot use this element in the form of coal. Carbon to be available for the growth of a plant must be in the form of carbon-dioxid gas.

The animal needs nitrogen, but it is quite unable to use nitrogen in the form of the nitrates of the soil or the nitrogen gas of the air. Nitrogen to be available for animal uses must be built up into complex substances called proteins, examples of which would be egg albumen, lean meat, gluten of bread, etc.

While plants are able to build up complex substances out of the simple compounds of the earth and air, the animal is not so endowed, and is obliged to depend upon the substances built up by the plant kingdom, for example: starch, sugar, protein, fat, and salts. If we analyze the animal body, we find it composed largely of these substances. However, the greater part of the body is composed of proteins and salts. The proteins make up muscles, glands, and nervous system, while the skeleton is composed largely of deposited salts. It must be evident that to furnish such a body with the substances needed in growth and repair, it is only necessary to furnish a sufficient amount of proteins and salts to provide materials needed to build up the muscles, glands, nervous system, and skeleton.