to the auricles. The heart is aided in its work by the elasticity of the
arteries, which keeps the blood under pressure while the ventricles are in
a state of relaxation. It is also aided by the muscles and elastic tissue
in all of the blood vessels. These, by keeping the blood vessels in a
state of "tone," or so contracted that their capacity just equals the
volume of the blood, enable pressure from the heart to be transmitted to
all parts of the blood 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