e 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
body. The contraction of a muscle will, for example, close the small veins
in its vicinity and diminish the capacity of the larger ones. The natural
tendency of such pressure is to empty the veins in two directions--one in
the same direction as the regular movement of the blood, but the other in
the opposite direction. The valves by closing cause the contracting muscle
to push the blood in one direction only--toward the heart. The valves in
the veins are, therefore, an economical device for _enabling variable
pressure_ in different parts of the body _to assist in the circulation_.
Veins like the inferior vena cava and the veins of the brain, which are
not compressed by movements of the body, do not have valves.

*Purposes of the Muscular Coat.*--The muscular coat, which is thicker in
the arteries than in the veins and is more marked in small arteries than
in large ones, serves two important purposes. In the first place it,
together with the elastic tissue, keeps the capacity of the blood vessels
_equal to the volume of the blood_. Since the blood vessels are capable of
holding more blood than may be present at a given time in the body, there
is a liability of empty spaces occurring in these tubes.