canal, may be of real advantage where, from hard work or exposure, the
body requires a large supply of energy for some time. These "stay by" the
laborer, giving him strength after the more easily digested foods have
been used up. Storage by the food canal is limited chiefly to the stomach.

*Regulation of the Food Supply to the Cells.*—The storage of food
materials is made to serve a second purpose in the plan of the body which
is even more important than that of supplying nourishment to the cells
during the intervals when no food is being taken. It is largely the means
whereby the rate of supply of materials to the cells is regulated. The
cells obtain their materials from the lymph, and the lymph is supplied
from the blood. Should food substances, such as sugar, increase in the
blood beyond a low per cent, they are converted into a form, like
glycogen, in which they are held in reserve, or, for the time being,
placed beyond the reach of the cells. When, however, the supply is
reduced, the stored-up materials reënter the blood and again become
available to the cells. By this means their rate of supply to the cells is
practically constant.

We are now in a position to understand why carbohydrates, fats, and
proteids are so well adapted to the needs of the body, while other
substances, like alcohol, which may also liberate energy, prove injurious.
It is because foods are of such a chemical nature that they are adapted in
all respects to the body plan of taking up and using materials, while the
other substances are lacking in some particular.

[Fig. 80]


Fig. 80—*Diagrams illustrating the relation of nutrients* and the