capacity than the system of arteries.

[Fig. 20]


Fig. 20—Vein split open to show the valves.


*Why the Arteries are Elastic.*—The elasticity of the arteries serves a
twofold purpose. It keeps the arteries from bursting when the blood is
forced into them from the ventricles, and it is a means of _supplying
pressure to the blood while the ventricles are in a condition of
relaxation._ The latter purpose is accomplished as follows:

Contraction of the ventricles fills the arteries overfull, causing them to
swell out and make room for the excess of blood. Then while the ventricles
are resting and filling, the stretched arteries press upon the blood to
keep it flowing into the capillaries. In this way _they cause the
intermittent flow from, the heart to become a steady stream in the
capillaries_.

The swelling of the arteries at each contraction of the ventricle is
easily felt at certain places in the body, such as the wrist. This
expansion, known as the "pulse," is the chief means employed by the
physician in determining the force and rapidity of the heart’s action.

*Purpose of the Valves in the Veins.*—The valves in the veins are not used
for directing the _general_ flow of the blood, the valves of the heart
being sufficient for this purpose. Their presence is necessary because of
the pressure to which the veins are subjected in different parts of the