They
are fastened to the bones by strong tendons and cartilages; and around
the wrist, in the drawing, is shown a band of cartilage to confine
them in place. The muscle marked 8 is the extensor that straightens
the fingers after they have been closed by a flexor the other side of
the arm. In like manner, each motion of the arm and fingers has one
muscle to produce it and another to restore to the natural position.
The muscles are dependent on the brain and nerves for power to move.
It has been shown that the gray matter of the brain and spinal marrow
furnishes the stimulating power that moves the muscles, and causes
sensations of touch on the skin, and the other sensations of the several
senses. The white part of the brain and spinal marrow consists solely
of conducting tubes to transmit this influence. Each of the minute
fibrils of the muscles has a small conducting nerve connecting it with
the brain or spinal marrow, and in this respect each muscular fibril
is separate from every other.
When, therefore, the mind wills to move a flexor muscle of the arm,
the gray matter sends out the stimulus through the nerves to the cells
of each individual fibre of that muscle, and they contract.
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