101. Classification of Foods. Foods may be conveniently divided into four great classes, to which the name food-stuffs or alimentary principles has been given. They correspond to the chief “proximate principles” of which the body consists. To one or the other of these classes all available foods belong[16]. The classification of food-stuffs usually given is as follows: Proteids, or Nitrogenous Foods. Starches and Sugars, or Carbohydrates. Fats and Oils. Inorganic or Mineral Foods,—Water, Salt. 102. Proteids; or Nitrogenous Foods. The proteids, frequently spoken of as the nitrogenous foods, are rich in one or more of the following organic substances: albumen, casein, fibrin, gelatine, myosin, gluten, and legumin. The type of this class of foods is albumen, well known as the white of an egg. The serum of the blood is very rich in albumen, as is lean meat. The curd of milk consists mainly of casein. Fibrin exists largely in blood and flesh foods. Gelatine is obtained from the animal parts of bones and connective tissue by prolonged boiling. One of the chief constituents of muscular fiber is myosin. Gluten exists largely in the cereals wheat, barley, oats, and rye. The proteid principle of peas and beans is legumin, a substance resembling casein. As the name implies, the proteids, or nitrogenous foods, contain nitrogen; carbohydrates and fats, on the contrary, do not contain nitrogen. The principal proteid food-stuffs are milk, eggs, flesh foods of all kinds, fish, and the cereals among vegetable foods. Peas and beans are rich in proteids. The essential use of the proteids to the tissues is to supply the material from which the new proteid tissue is made or the old proteid tissue is repaired. They are also valuable as sources of energy to the body. Now, as the proteid part of its molecule is the most important constituent of living matter, it is evident that proteid food is an absolute necessity. If our diet contained no proteids, the tissues of the body would gradually waste away, and death from starvation would result. All the food-stuffs are necessary in one way or another to the preservation of perfect health, but proteids, together with a certain proportion of water and inorganic salts, are absolutely necessary for the bare maintenance of animal life—that is, for the formation and preservation of living protoplasm. 103. Starches and Sugars. The starches, sugars, and gums, also known as carbohydrates, enter largely into the composition of foods of vegetable origin. They contain no nitrogen, but the three elements, carbon, hydrogen, and oxygen, the last two in the same proportion as in water. The starches are widely distributed throughout the vegetable kingdom. They are abundant in potatoes and the cereals, and in arrowroot, rice, sago, and tapioca. Starch probably stands first in importance among the various vegetable foods.