dextrose to glycogen and _vice versa_, that break down and build up the
proteids, and that aid in the oxidations at the cells. The necessity for
such enzymes is quite apparent.

*Summary.*—The digested nutrients are taken up by the capillaries and the
lymph vessels and transferred by two routes to the circulation. In passing
from the alimentary canal into the circulation the more important of the
foods undergo changes which adapt them to the needs of the body. Since
materials are absorbed more rapidly than they are used, means are provided
for storing them and for supplying them to the cells as their needs
require. _Capability of storage is an essential quality of energy-yielding
foods_; and substances, such as alcohol, which lack this quality are not
adapted to the needs of the body. For causing the chemical changes that
occur in the storage of foods, as well as the oxidations at the cells, the
presence of active agents, or enzymes, is necessary.

*Exercises.*—1. In what respects does the absorption of food materials
from the alimentary canal differ from the absorption of a simple liquid by
a solid?

2. In what different ways is the small intestine especially adapted to the
work of absorption?

3. What are the parts of a villus? What are the lacteals? Account for the
name.

4. What part is played by the capillaries and the lacteals in the work of
absorption? How does their work differ?

5. What changes, if any, take place in water, common salt, fat, proteids,