vascular system, as the heart and arteries, after a short quiescence, are
thrown into violent action by their natural quantity of stimulus.

Nevertheless by this accumulation of sensorial power during the application
of decreased stimulus, and by the exhaustion of it during the action of
increased stimulus, it is wisely provided, that the actions of the vascular
muscles and organs of sense are not much deranged by small variations of
stimulus; as the quantity of sensorial power becomes in some measure
inversely as the quantity of stimulus.

3. A quantity of stimulus less than that mentioned above, and continued for
some time, induces pain in the affected organ, as the pain of cold in the
hands, when they are immersed in snow, is owing to a deficiency of the
stimulation of heat. Hunger is a pain from the deficiency of the
stimulation of food. Pain in the back at the commencement of ague-fits, and
the head-achs which attend feeble people, are pains from defect of
stimulus, and are hence relieved by opium, essential oils, spirit of wine.

As the pains, which originate from defect of stimulus, only occur in those
parts of the system, which have been previously subjected to perpetual
stimulus; and as an accumulation of sensorial power is produced in the
quiescent organ along with the pain, as in cold or hunger, there is reason
to believe, that the pain is owing to the accumulation of sensorial power.
For, in the locomotive muscles, in the retina of the eye, and other organs
of senses, no pain occurs from the absence of stimulus, nor any great
accumulation of sensorial power beyond their natural quantity, since these
organs have not been used to a perpetual supply of it. There is indeed a
greater accumulation occurs in the organ of vision after its quiescence,
because it is subject to more constant stimulus.