ever,
that the bad effects of alcoholic beverages upon the stomach, the liver,
and the body in general are less pronounced when these are taken as a part
of the regular meals.

*Effects of Tea and Coffee.*--In addition to the stimulating agent
caffeine, tea and coffee contain a bitter, astringent substance, known as
tannin. On account of the tannin these beverages tend to retard digestion
and to irritate the lining of the stomach--effects that may be largely
obviated by methods of preparing tea and coffee which dissolve little of
the tannin. (They should be made without continued boiling or steeping.)
The caffeine may do harm through its stimulating effect upon the nervous
system (page 56) and through the introduction of a special waste into the
body. In chemical composition caffeine closely resembles a waste, called
uric acid, and in the body is converted into this substance. If one is in
a weakened condition, the uric acid may fail to be oxidized to urea, as
occurs normally, or to be thrown off as uric acid. In this case it
accumulates in the body, causing rheumatism and related diseases. It thus
happens that while some people may use tea and coffee without detriment,
others are injured by them.

*Summary.*--The main structure in the digestive system is the alimentary
canal. This provides cavities where important dissolving processes take
place, and tubes for joining these cavities, while glands connecting with
the canal supply the necessary liquids for changing and dissolving the
foods. The general plan of digestion is that of passing the food through
the canal, beginning with the mouth, and of acting on it at various
places, with the final result of reducing most of it to the liquid state.
The digestive fluids supply water which acts as a solvent and carries the
active chemical agents, or enzymes, that convert the insoluble foods into
substances that are soluble. The muscles in the walls of the canal perform
the mechanical work of digestion, while the nervous system controls and
regulates the activity of the various organs concerned in this work.

Exercises.--1. State the general purpose of digestion. How does digested
food differ from that not digested?

2. Name all the divisions of the alimentary canal in the order in which
the food passes through them.

3. What other work besides digestion is carried on by the alimentary
canal?

4. What is gained by the mastication of the food? Why should mastication
prece