t, till at length a cold fit of
fever is completely formed, as happens at the beginning of many of those
fevers, which are called nervous or low fevers. Where the patient has
slight periodical shiverings and paleness for many days before the febrile
paroxysm is completely formed.

4. On the contrary, if the exposure to cold be for so short a time, as not
to induce any considerable degree of torpor or quiescence, and is repeated
daily as above mentioned, it loses its effect more and more at every
repetition, till the constitution can bear it without inconvenience, or
indeed without being conscious of it. As in walking into the cold air in
frosty weather. The same rule is applicable to increased stimulus, as of
heat, or of vinous spirit, within certain limits, as is applied in the two
last paragraphs to Deficient Stimulus; as is further explained in Sect.
XXXVI. on the Periods of Diseases.

5. Where irritation coincides with sensation to produce the same
catenations of motion, as in inflammatory fevers, they are excited with
still greater energy than by the irritation alone. So when children expect
to be tickled in play, by a feather lightly passed over the lips, or by
gently vellicating the soles of their feet, laughter is most vehemently
excited; though they can stimulate these parts with their own fingers
unmoved. Here the pleasureable idea of playfulness coincides with the
vellication; and there is no voluntary exertion used to diminish the
sensation, as there would be, if a child should endeavour to tickle
himself. See Sect. XXXIV. 1. 4.

6. And lastly, the motions excited by the junction of voluntary exertion
with irritation are performed with more energy, than those by irritation
singly; as when we listen to small noises, as to the ticking of a watch in
the night, we perceive the most weak sounds, that are at other times
unheeded. So when we attend to the irritative ideas of sound in our ears,
which are generally not attended to, we can hear them; and can see the
spectra of objects, which remain in the eye, whenever we please to exert
our voluntary power in aid of those weak actions of the retina, or of the
auditory nerve.

7. The temporary catenations of ideas, which are caused by the sensations
of pleasure or pain, are easily dissevered either by irritations, as when a
sudden noise disturbs a day-dream; or by the power of volition, as when we
awake from sleep. Hence in our waking hours, whenever an idea occurs