CHAPTER X.

            ELEMENTARY PHYSIOLOGY—continued.

Food and digestion.—The principal use of food is to supply
energy to the body, the muscles of the body being constantly active
or continually liberating energy by breaking down the complex
proteins, carbohydrates and fats, and therefore fresh supplies of
these are necessary to prevent the body living on its own material
and wasting away.

Again during growth, the material from which the body is built
up must be supplied by the food, so that a suitable food must be
able to yield the necessary amount of energy and also supply the
materials necessary for growth and repair.

Before food can be utilised by the body it must be broken down
to its simplest form and rendered soluble, and the process of
preparing the food for absorption is called digestion.

Before considering the process of digestion some idea of the
composition of food must be given. Foodstuffs are classified into
those not yielding energy and those that do yield energy—the former
acting chiefly as solvents, e.g., water and the inorganic salts. The
foodstuffs yielding energy are complex combinations of the chemical
elements—carbon, hydrogen and oxygen, with or without nitrogen,
sulphur, phosphorus and iron—that is to say, they are of the same
nature as the materials found on the analysis of dead protoplasm.
The energy-yielding foodstuffs are further subdivided into nitrogen-
containing compounds and non-nitrogen-containing compounds.
The proteins belong to the former, and the carbohydrates and fats
to the latter. The proteins are important in that they are the only
source of the nitrogen and sulphur necessary for the construction
and repair of living tissues ; they are therefore the essential organic
constituents of the food. Proteins leave the body as carbonic acid,
water and urea.

The difference between the carbohydrates and fats is that in the
former the oxygen and hydrogen exist in the same proportion as in
water ; carbohydrates and fats leave the body as carbonic acid and
water.

Digestion.—In the elephant the food is grasped by the trunk
and placed in the mouth where, by the act of chewing, it is
thoroughly broken up and mixed with the saliva.