The gluten of wheat is formed from the chemical union of nitrogen in the air and nitrogen in the soil with other substances. The starch of wheat and other grains is from carbon which the plant has taken from the soil and combined with other substances. All meats are largely derived from plants which have appropriated the elements from the soil, water, and air. The chemical processes of the animal convert the energy latent in the plant foods into the more concentrated form of meat. The animal thus performs a part of the chemical work for man—the digestive organs of one animal convert the food contained in certain plants, into a substance more easily assimilated by another animal. Man would need to eat a large amount of nitrogen-containing plants in order to get as much protein as is contained in one egg or in a piece of lean meat the size of an egg. It is because the nitrogen is in such condensed form in meat and eggs that one is likely to take more than the system can handle, if he eats too freely of these two foods, particularly of meat. We will discuss this question more fully under “Proteins.” Most domestic animals take their food elements from air and water, as well as from the compounds which the plants have formed, while wild animals and some domestic ones, such as hogs and chickens, make use of meat as well. The greater part of muscle, nerve, and gland is composed of _protein_. When the muscles are exercised constantly they use up their protein and must have it resupplied, or the muscle substance will waste. When the muscles are exercised freely, as in the laborer, or the athlete, they need more building material. The skeleton is composed largely of deposited _salts_, as _calcium_. If, therefore, the growing child be not supplied with a sufficient amount of this substance, the bones will be weak and liable to deformity and the teeth will be slow in coming or will be small and malformed. Children need foods rich in lime. The elements which supply heat and keep up muscular activity are _starches, fats, and sugars_. It must be apparent to every thoughtful person, that, since the nerves, muscles, and glands are composed largely of protein and the skeleton largely of calcium salts, in order to furnish the body with the elements necessary for growth and repair, all of these elements, as also the energy-producing substances, must be provided. _Each individual, therefore, should learn how much he requires to replace his daily waste, both for rebuilding tissue and for supplying heat and energy._ The day laborer, though he may do more muscular work than an athlete in training, expends scarcely any nervous energy. Therefore he needs less protein in his diet than one does who expends both nervous and muscular activity, as does the athlete. CHAPTER II CLASSIFICATION OF FOOD ELEMENTS By foodstuffs are meant the chemical elements appropriated by the animal for the use of the body, as previously described.