erve of _sensation_ or
_feeling_, the other, the nerve of _motion_. The nerves of sensation are
those which are affected by the emotions and volitions of the mind; and
the nerves of motion are those which impart moving power to the muscles.
Experiments show, that, where the nerves issue from the spine, the
nerve of sensation may be cut off without severing the nerve of motion,
and then the parts, to which this nerve extends, lose the power of
feeling, while the power of motion continues; and so, on the other hand,
the nerve of motion may be divided, and, the nerve of sensation
remaining uninjured, the power of feeling is retained, and the power of
motion is lost.

In certain nervous diseases, sometimes a limb loses its power of
feeling, and yet retains the power of motion; in other cases, the power
of motion is lost, and the power of sensation is retained; and in other
cases, still, when a limb is _paralysed_, both the power of motion and
of sensation are lost.

Now, the nerves, like all other parts of the body, gain and lose
strength, according as they are exercised. 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 feeling 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 a 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. By this is shown the
importance of furnishing, for youn