in pains, they become associated with those pains, and cease
at those times to be subservient to the will; as in coughing, sneezing, and
strangury. Of these motions those which contribute to remove or dislodge
the offending cause, as the actions of the abdominal muscles in parturition
or in vomiting, though they were originally excited by volition, are in
this work termed sensitive motions; but those actions of the muscles or
organs of sense, which do not contribute to remove the offending cause, as
in general convulsions or in madness, are in this work termed voluntary
motions, or motions in consequence of aversion, though in common language
they are called involuntary ones. Those sensitive unrestrainable actions,
which contribute to remove the cause of pain are uniformly and invariably
exerted, as in coughing or sneezing; but those motions which are exerted in
consequence of aversion without contributing to remove the painful cause,
but only to prevent the sensation of it, as in epileptic, or cataleptic
fits, are not uniformly and invariably exerted, but change from one set of
muscles to another, as will be further explained; and may by this criterion
also be distinguished from the former.

At the same time those motions, which are excited by perpetual stimulus, or
by association with each other, or immediately by pleasureable or painful
sensation, may properly be termed involuntary motions, as those of the
heart and arteries; as the faculty of volition seldom affects those, except
when it exists in unnatural quantity, as in maniacal people.

2. It was observed in Section XIV. on the Production of Ideas, that those
parts of the system, which are usually termed the organs of sense, are
liable to be excited into pain by the excess of the stimulus of those
objects, which are by nature adapted to affect them; as of too great light,
sound, or pressure. But that these organs receive no pain from the defect
or absence of these stimuli, as in darkness or silence. But that our other
organs of perception, which have generally been called appetites, as of
hunger, thirst, want of heat, want of fresh air, are liable to be affected
with pain by the defect, as well as by the excess of their appropriated
stimuli.

This excess or defect of stimulus is however to be considered only as the
remote cause of the pain, the immediate cause being the excess or defect of
the natural action of the affected part, according to Sect. IV. 5. Hence