800. _The brain should be called into action._ This organ, like the
muscles, should be used, and then allowed to rest, or cease from
vigorous thought. When the brain is properly called into action by
moderate study, it increases in size and strength; while, on the other
hand, if it is not used, the action of this organ is enfeebled,
thereby diminishing the function of all parts of the body.

801. The brain, being an organized part, is subject, so far as regards
exercise, to the same laws as the other organs of the body. If it is
doomed to inactivity, its size diminishes, its health decays, and the
mental operations and feelings, as a necessary consequence, become
dull, feeble, and slow. If it is duly exercised after regular
intervals of repose, the mind acquires readiness and strength. Lastly,
if it is overtasked, either in the force or duration of its activity,
its functions become impaired, and irritability and disease take the
place of health and vigor.

802. The consequences of inadequate exercise will first be explained.
We have seen that by disuse the muscles become emaciated, the bones
soften, and the blood-vessels are obliterated. The brain is no
exception to this general rule. It is impaired by permanent
inactivity, and becomes less fit to manifest the mental powers with
readiness and energy. Nor will this surprise any reflecting person,
who considers that the brain, as a part of the same animal system, is
nourished by the same blood and regulated by the same vital laws as
the muscles, bones, and arteries.

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800. Why should the brain be called into action? 801. What is the
effect if the brain is doomed to inactivity? 802. Show the consequences
of disuse of the organs mentioned in preceding chapters. Does the
same principle apply to the brain?

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803. It is the weakening and depressing effect which is induced by the
absence of the stimulus necessary for the healthy exercise of the
brain, that renders solitary confinement so severe a punishment, even
to the most daring minds. Keeping the above principle in view, we
shall not be surprised to find that _non-exercise_ of the brain and
nervous system, or, in other words, inactivity of intellect and
feeling, is a very frequent predisposing cause of every form of
nervous disease.

804. For demonstrative evidence of this position, we have only to look
at the numerous victims to be found among females of th