The _back_ portion of the spinal marrow and the nerves that run from it
are employed in _sensation_, or the _sense of feeling_. These nerves
extend over the whole body, but are largely developed in the network of
nerves in the skin. The _front_ portion of the spinal marrow and its
branches are employed in moving those muscles in all parts of the body
which are controlled by the _will_ or _choice_ of the mind. These are
called the _nerves of motion_.
The nerves of sensation and nerves of motion, although they start from
different portions of the spine, are united in the same _sheath_ or
_cover_, till they terminate in the muscles. Thus, every muscle is moved
by nerves of motion; while alongside of this nerve, in the same sheath,
is a nerve of sensation. All the nerves of motion and sensation are
connected with those portions of the brain used when we think, feel, and
choose. By this arrangement the mind _knows_ what is wanted in all parts
of the body by means of the nerves of sensation, and then it _acts_ by
means of the nerves of motion.
For example, when we feel the cold air on the skin, the nerves of
sensation report to the brain, and thus to the mind, that the body is
growing cold.
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