s, ipecacuanha the stomach, aloe the sphincter of the anus,
cantharides that of the bladder, and lastly every gland of the body appears
to be indued with a kind of taste, by which it selects or forms each its
peculiar fluid from the blood; and by which it is irritated into activity.

Many of these external properties of bodies, which stimulate our organs of
sense, do not seem to effect this by a single impulse, but by repeated
impulses; as the nerve of the ear is probably not excitable by a single
vibration of air, nor the optic nerve by a single particle of light; which
circumstance produces some analogy between those two senses, at the same
time the solidity of bodies is perceived by a single application of a solid
body to the nerves of touch, and that even through the cuticle; and we are
probably possessed of a peculiar sense to distinguish the nice degrees of
heat and cold.

The senses of touch and of hearing acquaint us with the mechanical impact
and vibration of bodies, those of smell and taste seem to acquaint us with
some of their chemical properties, while the sense of vision and of heat
acquaint us with the existence of their peculiar fluids.

_Sensation and Volition._

II. Many motions are produced by pleasure or pain, and that even in
contradiction to the power of volition, as in laughing, or in the
strangury; but as no name has been given to pleasure or pain, at the time
it is exerted so as to cause fibrous motions, we have used the term
sensation for this purpose; and mean it to bear the same analogy to
pleasure and pain, that the word volition does to desire and aversion.

1. It was mentioned in the fifth Section, that, what we have termed
sensation is a motion of the central parts, or of the whole sensorium,
_beginning_ at some of the extremities of it. This appears first, because
our pains and pleasures are always caused by our ideas or muscular motions,
which are the motions of the extremities of the sensorium. And, secondly,
because the sensation of pleasure or pain frequently continues some time
after the ideas or muscular motions which excited it have ceased: for we
often feel a glow of pleasure from an agreeable reverie, for many minutes
after the ideas, that were the subject of it, have escaped our memory; and
frequently experience a dejection of spirits without being able to assign
the cause of it but by much recollection.

When the sensorial faculty of desire or aversion is exerted so as to