If, then, we use a number of muscles, and use them strongly and quickly,
there are so many veins affected in this way as to quicken the whole
circulation. The heart receives blood faster, and sends it to the lungs
faster. Then the lungs work quicker, to furnish the oxygen required
by the greater amount of blood. The blood returns with greater speed
to the heart, and the heart sends it out with quicker action through
the arteries to the capillaries. In the capillaries, too, the decayed
matter is carried off faster, and then the stomach calls for more food
to furnish new and pure blood. Thus it is that exercise gives new life
and nourishment to every part of the body.
It is the universal law of the human frame that _exercise_ is
indispensable to the health of the several parts. Thus, if a
blood-vessel be tied up, so as not to be used, it shrinks, and becomes
a useless string; if a muscle be condemned to inaction, it shrinks in
size and diminishes in power; and thus it is also with the bones.
Inactivity produces softness, debility, and unfitness for the functions
they are designed to perform.
Now, the nerves, like all other parts of the body, gain and lose
strength according as they are exercised.
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