inflammation arises, which prevents future cold fits by expending a part of
the sensorial power in the extension of old vessels or the production of
new ones; and thus preventing the too great accumulation or exertion of it
in other parts of the system; or which by the great increase of stimulus
excites into great action the whole glandular system as well as the
arterial, and thence a greater quantity of sensorial power is produced in
the brain, and thus its exhaustion in any peculiar part of the system
ceases to be affected.

4. Or thirdly, in consequence of the painful or pleasurable sensation above
mentioned, desire and aversion are introduced, and inordinate volition
succeeds; which by its own exertions expends so much of the spirit of
animation, that the two other sensorial faculties, or irritation and
sensation, act so much more feebly; that the paroxysms of fever, or that
libration between the extremes of exertion and inactivity of the arterial
system, gradually subsides. On this account a temporary insanity is a
favourable sign in fevers, as I have had some opportunities of observing.

III. _Of repeated Stimulus._

1. When a stimulus is repeated more frequently than the expenditure of
sensorial power can be renewed in the acting organ, the effect of the
stimulus becomes gradually diminished. Thus if two grains of opium be
swallowed by a person unused to so strong a stimulus, all the vascular
systems in the body act with greater energy, all the secretions and the
absorption from those secreted fluids are increased in quantity; and
pleasure or pain are introduced into the system, which adds an additional
stimulus to that already too great. After some hours the sensorial power
becomes diminished in quantity, expended by the great activity of the
system; and thence, when the stimulus of the opium is withdrawn, the fibres