3. Where a new link has been introduced into a circle of actions by some
accidental defect of stimulus; if that defect of stimulus be repeated at
the same part of the circle a second or a third time, the defective motions
thus produced, both by the repeated defect of stimulus and by their
catenation with the parts of the circle of actions, will be performed with
less and less energy. Thus if any person is exposed to cold at a certain
hour to-day, so long as to render some part of the system for a time
torpid; and is again exposed to it at the same hour to-morrow, and the next
day; he will be more and more affected by it, 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