These seven last mentioned senses may properly be termed appetites, as they
differ from those of touch, sight, hearing, taste, and smell, in this
respect; that they are affected with pain as well by the defect of their
objects as by the excess of them, which is not so in the latter. Thus cold
and hunger give us pain, as well as an excess of heat or satiety; but it is
not so with darkness and silence.

IX. Before we conclude this Section on the organs of sense, we must
observe, that, as far as we know, there are many more senses, than have
been here mentioned, as every gland seems to be influenced to separate from
the blood, or to absorb from the cavities of the body, or from the
atmosphere, its appropriated fluid, by the stimulus of that fluid on the
living gland; and not by mechanical capillary absorption, nor by chemical
affinity. Hence it appears, that each of these glands must have a peculiar
organ to perceive these irritations, but as these irritations are not
succeeded by sensation, they have not acquired the names of senses.

However when these glands are excited into motions stronger than usual,
either by the acrimony of their fluids, or by their own irritability being
much increased, then the sensation of pain is produced in them as in all
the other senses of the body; and these pains are all of different kinds,
and hence the glands at this time really become each a different organ of
sense, though these different kinds of pain have acquired no names.

Thus a great excess of light does not give the idea of light but of pain;
as in forcibly opening the eye when it is much inflamed. The great excess
of pressure or distention, as when the point of a pin is pressed upon our
skin, produces pain, (and when this pain of the sense of distention is
slighter, it is termed itching, or tickling), without any idea of solidity