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. Such spaces would
seriously interfere with the circulation, since the heart pressure could
not then reach all parts of the blood stream. This is prevented by the
contracted state, or "tone," of the blood vessels, due to the muscular
coat.

In the second place, the muscular coat serves the purpose of _regulating_
the amount of blood which any given organ or part of the body receives.
This it does by varying the caliber of the arteries going to the organ in
question. To increase the blood supply, the muscular coat relaxes. The
arteries are then dilated by the blood pressure from within so as to let
through a larger quantity of blood. To diminish the supply, the muscle
contracts, making the caliber of the arteries less, so that less blood can