d, and another fit
of exertion follows in train. These vicissitudes of exertion and inertion
of the arterial system constitute the paroxysms of remittent fevers; or
intermittent ones, when there is an interval of the natural action of the
arteries between the exacerbations.

In these paroxysms of fevers, which consist of the libration of the
arterial system between the extremes of exertion and quiescence, either the
fits become less and less violent from the contractile fibres becoming
coming less excitable to the stimulus by habit, that is, by becoming
accustomed to it, as explained below XII. 3. 1. or the whole sensorial
power becomes exhausted, and the arteries cease to beat, and the patient
dies in the cold part of the paroxysm. Or secondly, so much pain is
introduced into the system by the violent contractions of the fibres, that
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,