The great physiologist, Claude Bernard,[3] has shown bow 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.
[2] Muller remarks (`Elements of Physiology,' Eng. translat. vol. ii. p.
934) that when the feelings are very intense, "all the spinal nerves become
affected to the extent of imperfect paralysis, or the excitement of trembling
of the whole body."
[3] `Lecons sur les Prop. des Tissus Vivants,' 1866, pp. 457-466.
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.
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