animation or
sensorial power on the fibrous parts of the body, whether it acts in the
mode of irritation, sensation, volition, or association, is a contraction
of the animal fibre, according to the second law of animal causation. Sect.
IV. Thus the stimulus of the blood induces the contraction of the heart;
the agreeable taste of a strawberry produces the contraction of the muscles
of deglutition; the effort of the will contracts the muscles, which move
the limbs in walking; and by association other muscles of the trunk are
brought into contraction to preserve the balance of the body. The fibrous
extremities of the organs of sense have been shewn, by the ocular spectra
in Sect. III. to suffer similar contraction by each of the above modes of
excitation; and by their configurations to constitute our ideas.

3. After animal fibres have for some time been excited into contraction, a
relaxation succeeds, even though the exciting cause continues to act. In
respect to the irritative motions this is exemplified in the peristaltic
contractions of the bowels; which cease and are renewed alternately, though
the stimulus of the aliment continues to be uniformly applied; in the
sensitive motions, as in strangury, tenesmus, and parturition, the
alternate contractions and relaxations of the muscles exist, though the
stimulus is perpetual. In our voluntary exertions it is experienced, as no
one can hang long by the hands, however vehemently he wills so to do; and
in the associate motions the constant change of our attitudes evinces the
necessity of relaxation to those muscles, which have been long in action.

This relaxation of a muscle after its contraction, even though the stimulus
continues to be applied, appears to arise from the expenditure or
diminution of the spirit of animation previously resident in the muscle,
according to the second law of animal causation in Sect. IV. In those
constitutions, which are termed weak, the spirit of animation becomes
sooner exhausted, and tremulous motions are produced, as in the hands of
infirm people, when they lift a cup to their mouths. This quicker
exhaustion of the spirit of animation is probably owing to a less quantity
of it residing in the acting fibres, which therefore more frequently
require a supply from the nerves, which belong to them.

4. If the sensorial power continues to act, whether it acts in the mode of
irritation, sensation, volition, or association, a new contraction of