sensorial power.

2. A quantity of stimulus less than natural, applied to fibres previously
accustomed to perpetual stimulus, is succeeded by accumulation of sensorial
power in the affected organ. The truth of this proposition is evinced,
because a stimulus less than natural, if it be somewhat greater than that
above mentioned, will excite the organ so circumstanced into violent
activity. Thus on a frosty day with wind, the face of a person exposed to
the wind is at first pale and shrunk; but on turning the face from the
wind, it becomes soon of a glow with warmth and flushing. The glow of the
skin in emerging from the cold-bath is owing to the same cause.

It does not appear, that an accumulation of sensorial power above the
natural quantity is acquired by those muscles, which are not subject to
perpetual stimulus, as the locomotive muscles: these, after the greatest
fatigue, only acquire by rest their usual aptitude to motion; whereas the
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,