all the pains of the body may be divided into those from excess of motion,
and those from defect of motion; which distinction is of great importance
in the knowledge and the cure of many diseases. For as the pains from
excess of motion either gradually subside, or are in general succeeded by
inflammation; so those from defect of motion either gradually subside, or
are in general succeeded by convulsion, or madness. These pains are easily
distinguishable from each other by this circumstance, that the former are
attended with heat of the pained part, or of the whole body; whereas the
latter exists without increase of heat in the pained part, and is generally
attended with coldness of the extremities of the body; which is the true
criterion of what have been called nervous pains.

Thus when any acrid material, as snuff or lime, falls into the eye, pain
and inflammation and heat are produced from the excess of stimulus; but
violent hunger, hemicrania, or the clavus hystericus, are attended with
coldness of the extremities, and defect of circulation. When we are exposed
to great cold, the pain we experience from the deficiency of heat is
attended with a quiescence of the motions of the vascular system; so that
no inflammation is produced, but a great desire of heat, and a tremulous
motion of the subcutaneous muscles, which is properly a convulsion in
consequence of this pain from defect of the stimulus of heat.

It was before mentioned, that as sensation consists in certain movements of
the sensorium, beginning at some of the extremities of it, and propagated
to the central parts of it; so volition consists of certain other movements
of the sensorium, commencing in the central parts of it, and propagated to
some of its extremities. This idea of these two great powers of motion in
the animal machine is confirmed from observing, that they never exist in a
great degree or universally at the same time; for while we strongly exert
our voluntary motions, we cease to feel the pains or uneasinesses, which
occasioned us to exert them.

Hence during the time of fighting with fists or swords no pain is felt by
the combatants, till they cease to exert themselves. Thus in the beginning
of ague-fits the painful sensation of cold is diminished, while the patient
exerts himself in the shivering and gnashing of his teeth. He then ceases
to exert himself, and the pain of cold returns; and he is thus perpetually
induced to reiterate these exe