Why the irritation of a nerve-cell should generate or liberate
nerve-force is not known; but that this is the case seems to be
the conclusion arrived at by all the greatest physiologists,
such as Muller, Virchow, Bernard, &c.[6] As Mr. Herbert Spencer remarks,
it may be received as an "unquestionable truth that, at any moment,
the existing quantity of liberated nerve-force, which in an inscrutable
way produces in us the state we call feeling, MUST expend itself
in some direction--MUST generate an equivalent manifestation
of force somewhere;" so that, when the cerebro-spinal system is
highly excited and nerve-force is liberated in excess, it may be
expended in intense sensations, active thought, violent movements,
or increased activity of the glands.[7] Mr. Spencer further maintains
that an "overflow of nerve-force, undirected by any motive,
will manifestly take the most habitual routes; and, if these do
not suffice, will next overflow into the less habitual ones."
Consequently the facial and respiratory muscles, which are
the most used, will be apt to be first brought into action;
then those of the upper extremities, next those of the lower,
and finally those of the whole body.
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