The manner in which the exercise of the muscles quickens the circulation
of the blood will now be explained. The veins abound in every part of
every muscle, and the large veins have _valves_ which prevent the
blood from flowing backward. If the wrist is grasped tightly, the veins
of the hand are immediately swollen. This is owing to the fact that
the blood is prevented from flowing toward the heart by this pressure,
and by the vein-valves from returning into the arteries; while the
arteries themselves, being placed deeper down, are not so compressed,
and continue to send the blood into the hand, and thus it accumulates.
As soon as this pressure is removed, the blood springs onward from the
restraint with accelerated motion. This same process takes place when
any of the muscles are exercised. The contraction of any muscle presses
some of the veins, so that the blood can not flow the natural way,
while the valves in the veins prevent its flowing backward. Meantime
the arteries continue to press the blood along until the veins become
swollen. Then, as soon as the muscle ceases its contraction, the blood
flows faster from the previous accumulation.
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