The brain and nerves can be overworked, and can also suffer for want
of exercise, just as the muscles do. It is necessary for the perfect
health of the brain and nerves that the several portions he exercised
sufficiently, and that no part be exhausted by over-action. For
example, the nerves of sensation may be very much exercised, and the
nerves of motion have but little exercise. In this ease, one will be
weakened by excess of work, and the other by the want of it.
It is found by experience that the proper exercise of the nerves of
motion tends to reduce any extreme susceptibility of the nerves of
sensation. On the contrary, the neglect of such exercise tends to
produce an excessive sensibility in the nerves of sensation.
Whenever that part of the brain which is employed in thinking, feeling,
and willing, is greatly exercised by hard study, or by excessive care
or emotion, the blood tends to the brain to supply it with increased
nourishment, just as it flows to the muscles when they are exercised.
Over-exercise of this portion of the brain causes engorgement of the
blood-vessels. This is sometimes indicated by pain, or by a sense of
fullness in the head; but oftener the result is a debilitating drain
on the nervous system, which depends for its supply on the healthful
state of the brain.
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