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.

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:

I. Proteids, or Nitrogenous Foods.
II. Starches and Sugars, or Carbohydrates.
III. Fats and Oils.
IV. 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