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 the
animal fibre succeeds after a certain interval; which interval is of
shorter continuance in weak people than in strong ones. This is exemplified
in the shaking of the hands of weak people, when they attempt to write. In
a manuscript epistle of one of my correspondents, which is written in a
small hand, I observed from four to six zigzags in the perpendicular stroke
of every letter, which shews that both the contractions of the fingers, and
intervals between them, must have been performed in very short periods of
time.

The times of contraction of the muscles of enfeebled people being less, and