t causing any fibrous motions, I conjecture
that it contributes to exhaust or expend the general quantity of sensorial
power; because people are fatigued by enduring pain, till at length they
sleep. Which is contrary to what I had perhaps erroneously supposed in
Sect. XXXV. 2. 3. If it causes fibrous motions, it then takes the name of
sensation, according to the definition of sensation in Sect. II. 2. 9.; and
increased fibrous action or inflammation is the consequence. This
circumstance of the general exhaustion of sensorial power by the existence
of pain will assist in explaining many of the diseases of this genus.

Many of the canals of the body, as the urethra, the bile-duct, the throat,
have the motions of their two extremities associated by having been
accustomed to feel pleasurable or painful sensations at the same time or in
succession. This is termed sensitive association, though those painful or
pleasurable sensations do not cause the motions, but only attend them; and
are thus perhaps, strictly speaking, only catenated with them.

SPECIES.

1. _Torpor genae a dolore dentis._ In tooth-ach there is generally a
coldness of the cheek, which is sensible to the hand, and is attended in
some degree with the pain of cold. The cheek and tooth have frequently been
engaged in pleasurable action at the same time during the masticating of
our food; whence they have acquired sensitive associations. The torpor of
the cheek may have for its cause the too great expenditure of sensorial
power by the painful sensation of the membranes of the diseased tooth;
whence the membranes of the cheek associated with those of the alveolar
process are deprived of their natural share of it, and become torpid; thus
they produce less secretions, and less heat, and the pain of cold is the
consequence. This torpor of the vessels of the cheek cannot be produced by
the activity of the sensorial power of sensation; for then they would act
more violently than natural, or become inflamed. And though the pain by
exhausting so much sensorial power may be a remote cause, it is the defeat
of the power of association, which is the immediate cause of the torpor of
the cheek.

After some hours this pain occasioned by the torpor of the vessels of the
cheek either gradually ceases along with the pain of the diseased tooth;
or, by the accumulation of sensorial power during their state of torpor,
the capillaries of the cheek act with greater violence, and pro