If they have too much or too
little exercise, they lose strength; if they are exercised to a proper
degree, they gain strength. When the mind is continuously excited, by
business, study, or the imagination, the nerves of emotion and sensation
are kept in constant action, while the nerves of motion are unemployed.
If this is continued for a long time, the nerves of sensation lose
their strength from over-action, and the nerves of motion lose their
power from inactivity. In consequence, there is a morbid excitability
of the nervous, and a debility of the muscular system, which make all
exertion irksome and wearisome.
The only mode of preserving the health of these systems is to keep up
in them an equilibrium of action. For this purpose, occupations must
be sought which exercise the muscles and interest the mind; and thus
the equal action of both kinds of nerves is secured. This shows why
exercise is so much more healthful and invigorating when the mind is
interested, than when it is not. As an illustration, let a person go
shopping with a friend, and have nothing to do but look on. How soon
do the continuous walking and standing weary! But, suppose one, thus
wearied, hears of the arrival of a very dear friend: she can instantly
walk off a mile or two to meet her, without the least feeling of
fatigue.
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