When this
is done, the nerve of sensation reports it to the brain and mind. If
the mind desires to return the arm to its former position, then follows
the willing, and consequent stimulus sent through the nerves to the
corresponding muscle; its cells contract, and the limb is restored.
When the motion is a compound one, involving the action of several
muscles at the same time, a multitude of impressions are sent back and
forth to and from the brain through the nerves. But the person acting
thus is unconscious of all this delicate and wonderful mechanism. He
wills the movement, and instantly the requisite nervous power is sent
to the required cells and fibres, and they perform the motions required.
Many of the muscles are moved by the sympathetic system, over which
the mind has but little control.
Among the muscles and nerves so intimately connected, run the minute
capillaries of the blood, which furnish nourishment to all.
[Illustration: Fig. 54]
Fig. 54 represents an artery a _a_, which brings pure blood to a muscle
from the heart. After meandering through the capillaries at _c_, to
distribute oxygen and food from the stomach, the blood enters the vein,
_b_, loaded with carbonic acid and water taken up in the capillaries, to
be carried to the lungs or skin, and thrown out into the air.
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