s, are rich in a kind
of proteid which is called _legumen_.

Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small
per cent of sulphur. Certain ones (the nucleo-proteids from grains) also
contain phosphorus. All of the proteids are highly complex compounds and
form a most important class of nutrients.

*Purposes of Proteids.*--The chief purpose of proteids in the body is to
rebuild the tissues. Not only do they supply all of the main elements in
the tissues, but they are of such a nature chemically that they are
readily built into the protoplasm. They are absolutely essential to life,
no other nutrients being able to take their place. An animal deprived of
them exhausts the proteids in its body and then dies. In addition to
rebuilding the tissues, proteids may also be oxidized to supply the body
with energy.

*Albuminoids* form a small class of foods, of minor importance, which are
similar to proteids in composition, but differ from them in being unable
to rebuild the tissues. Gelatin, a constituent of soup and obtained from
bones and connective tissue by boiling, is the best known of the
albuminoid foods. On account of the nitrogen which they contain, proteids
and albuminoids are often classed together as _nitrogenous foods_.

*Carbohydrates.*--While the carbohydrates are not so essential to life as
are the proteids, they are of very great value in the body. They are
composed of carbon, hydrogen, and oxygen, and are obtained mainly from
plants. There are several varieties of carbohydrates, but they are similar
in composition. All of those used as food to any great extent are starch
and certain kinds of sugar.

*Starch* is the carbohydrate of greatest importance as a food, and it is
also the one found in the greatest abundance. All green plants form more
or less starch, and many of them store it in their leaves, seeds, or roots
(Fig. 60). From these sources it is obtained as food. _Glycogen_, a
substance closely resembling starch, is found in the body of the oyster.
It is also formed in the liver and muscles of the higher animals, being
prepared from the sugar of the blood, and is stored by them as reserve
food (Chapter XI). Glycogen is, on this account, called _animal starch_.
Starch on being eaten is first changed to sugar, after which it may be
converted into glycogen in the liver and in the muscles.

[Fig. 60]

Fig. 60--*Starch grains* in cells of potat