upon
the surface. On the contrary, when the chilled extremities are suddenly
exposed to heat, the rapid enlargement of the contracted blood-vessels
excites the nerves unduly, which causes the pain experienced on such
occasions.

The sensibility of the nerves depends much upon the habits of persons.
Suppose two boys go out to play when the thermometer stands at the
freezing point, and that one of them has been accustomed to exercise in
the open air, and to practice daily ablution, while the other one has
been confined most of the time to a warm room, and has been accustomed
to wash only his hands and face. The skin of the former, other things
being equal, will be active and healthy, while that of the latter will
be enfeebled and diseased. The organs of touch diffused over the body at
the surface will be very differently affected in these two boys, and the
perceptions of their minds will be alike dissimilar. One will be roused
to action, and will feel just right for some animating game. Both body
and mind will be elastic and joyous. He will bound like the roe, make
the welkin ring with his merry shout, and return to the bosom of his
family with a gladdened heart, ready to impart and receive pleasure,
while the other boy will be too keenly affected by the contact of the
air, and think it too cold to stay out of doors. He will thrust his
hands into his pockets, and curl himself up like one decrepit with age.
His teeth will chatter and his whole frame tremble. Of course, very
different reflections will be awakened in his mind. He will hurry back
to the fireside, thinking winter a very dismal season, and will be apt
to fret himself and all about him, because of the confinement from which
he has not the resolution to break out.

The sensibility of the cutaneous nerves in these two cases depends upon
the habits of the persons. If the latter would practice frequent
ablutions, and excite a healthy action in the skin by friction and
exercise, and conform to other laws of health, he would experience all
that gladness of heart, and elasticity of body and mind, which the other
is supposed to enjoy. Hence the advantages resulting from a strict
conformity to the laws of health in this particular as well as in others
that are generally regarded as more important.

The general law that the exercise of a faculty increases its power is
applicable to the senses. We have referred to the blind, who read as
rapidly as seeing persons by pass