serviceable associated habit. There is the greatest difference in
different persons in the parts which are thus affected, and in the
degree of their affection.
The heart, which goes on uninterruptedly beating night and day in so
wonderful a manner, is extremely sensitive to external stimulants. The
great physiologist, Claude Bernard,[303] has shown how the least
excitement of a sensitive nerve reacts on the heart; even when a nerve
is touched so slightly that no pain can possibly be felt by the animal
under experiment. Hence when the mind is strongly excited, we might
expect that it would instantly affect in a direct manner the heart; and
this is universally acknowledged and felt to be the case. Claude
Bernard also repeatedly insists, and this deserves especial notice,
that when the heart is affected it reacts on the brain; and the state
of the brain again reacts through the pneumo-gastric nerve on the
heart; so that under any excitement there will be much mutual action
and reaction between these, the two most important organs of the body.
The vaso-motor system, which regulates the diameter of the small
arteries, is directly acted on by the sensorium, as we see when a man
blushes from shame; but in this latter case the checked transmission of
nerve-force to the vessels of the face can, I think, be partly
explained in a curious manner through habit. We shall also be able to
throw some light, though very little, on the involuntary erection of
the hair under the emotions of terror and rage. The secretion of tears
depends, no doubt, on the connection of certain nerve-cells; but here
again we can trace some few of the steps by which the flow of
nerve-force through the requisite channels has become habitual under
certain emotions.
A brief consideration of the outward signs of some of the stronger
sensations and emotions will best serve to show us, although vaguely,
in how complex a manner the principle under consideration of the direct
action of the excited nervous system of the body, is combined with the
principle of habitually associated, serviceable movements.
When animals suffer from an agony of pain, they generally writhe about
with frightful contortions; and those which habitually use their voices
utter piercing cries or groans. Almost every muscle of the body is
brought into strong action. With man the mouth may be closely
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