d stream. A further aid to the circulation is found
in the valves in the veins, which enable muscular contraction within the
body, and variable pressure upon its surface, to drive the blood toward
the heart. The heart is also aided to some extent by the movements of the
chest walls in breathing. The organs Of circulation are under the control
of the nervous system (Chapter XVIII).

HYGIENE OF THE CIRCULATION

*Care of the Heart.*--The heart, consisting largely of muscle, is subject
to the laws of muscular exercise. It may be injured by over-exertion, but
is strengthened by a moderate increase in its usual work.(23) It may even
be subjected to great exertion without danger, if it be trained by
gradually increasing its work. Such training, by giving the heart time to
gain in size and strength, prepares it for tasks that could not at first
be accomplished.

In taking up a new exercise requiring considerable exertion, precautions
should be observed to prevent an overstrain of the heart. The heart of the
amateur athlete, bicyclist, or mountain climber is frequently injured by
attempting more than the previous training warrants. The new work should
be taken up gradually, and feats requiring a large outlay of physical
energy should be attempted only after long periods of training.

Since the heart is controlled by the nervous system, it frequently becomes
irregular in its action through conditions that exhaust the nervous
energy. Palpitations of the heart, the missing of beats, and pains in the
heart region frequently arise from this cause. It is through their effect
upon the nervous system that worry, overstudy, undue excitement, and
dissipation cause disturbances of the heart. In all such cases the remedy
lies in the removal of the cause. The nervous system should also be "toned
up" through rest, plenty of sleep, and moderate exercise in the open air.

*Effect of Drugs.*--A number of substances classed as drugs, mainly by
their action on the nervous system, produce undesirable effects upon the
organs of circulation. Unfortunately some of these are extensively used,
alcohol being one of them. If taken in any but small quantities, alcohol
is a disturbing factor in the circulation. It increases the rate of the
heart beat and dilates the capillaries. Its effect upon the capillaries is
shown by the "bloodshot" eye and the "red nose" of the hard drinker.
Another bad effect from the use of much alcohol is the weakening of