s, whether those of the muscles or organs of
sense, are probably simple contractions; the fibres being again elongated
by antagonist muscles, by circulating fluids, or sometimes by elastic
ligaments, as in the necks of quadrupeds. The sensorial motions, which
constitute the sensations of pleasure or pain, and which constitute
volition, and which cause the fibrous contractions in consequence of
irritation or of association, are not here supposed to be fluctuations or
refluctuations of the spirit of animation; nor are they supposed to be
vibrations or revibrations, nor condensations or equilibrations of it; but
to be changes or motions of it peculiar to life.

* * * * *

SECT. VI.

OF THE FOUR CLASSES OF FIBROUS MOTIONS.

I. _Origin of fibrous contractions._ II. _Distribution of them into
four classes, irritative motions, sensitive motions, voluntary motions,
and associate motions, defined._

I. All the fibrous contractions of animal bodies originate from the
sensorium, and resolve themselves into four classes, correspondent with the
four powers or motions of the sensorium above described, and from which
they have their causation.

1. These fibrous contractions were originally caused by the irritations
excited by objects, which are external to the moving organ. As the
pulsations of the heart are owing to the irritations excited by the
stimulus of the blood; and the ideas of perception are owing to the
irritations excited by external bodies.

2. But as painful or pleasurable sensations frequently accompanied those
irritations, by habit these fibrous contractions became causeable by the
sensations, and the irritations ceased to be necessary to their production.
As the secretion of tears in grief is caused by the sensation of pain; and
the ideas of imagination, as in dreams or delirium, are excited by the
pleasure or pain, with which they were formerly accompanied.

3. But as the efforts of the will frequently accompanied these painful or
pleasureable sensations, by habit the fibrous contractions became causable
by volition; and both the irritations and sensations ceased to be necessary
to their production. As the deliberate locomotions of the body, and the
ideas of recollection, as when we will to repeat the alphabet backwards.

4. But as many of these fibrous contractions frequently accompanied other
fibrous contractions, by habit they became causable by their association