The brain has, as it were, a fountain of supply for the nervous fluid,
which flows to all the nerves, and stimulates them to action. Some
brains have a larger, and some a smaller fountain; so that a degree
of mental activity that would entirely exhaust one, would make only
a small and healthful drain upon another.
The excessive use of certain portions of the brain tends to withdraw
the nervous energy from other portions; so that when one part is
debilitated by excess, another fails by neglect. For example, a person
may so exhaust the brain power in the excessive use of the nerves of
motion by hard work, as to leave little for any other faculty. On the
other hand, the nerves of feeling and thinking may be so used as to
withdraw the nervous fluid from the nerves of motion, and thus
debilitate the muscles.
Some animal propensities may be indulged to such excess as to produce
a constant tendency of the blood to a certain portion of the brain,
and to the organs connected with it, and thus cause a constant and
excessive excitement, which finally becomes a disease. Sometimes a
paralysis of this portion of the brain results from such an entire
exhaustion of the nervous fountain and of the overworked nerves.
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