5. When a defect of stimulus, as of heat, recurs at certain diurnal
intervals, which induces some torpor or quiescence of a part of the system,
the diurnal catenation of actions becomes disordered, and a new association
with this link of torpid action is formed; on the next period the quantity
of quiescence will be increased, suppose the same defect of stimulus to
recur, because now the new association conspires with the defective
irritation in introducing the torpid action of this part of the diurnal
catenation. In this manner many fever-fits commence, where the patient is
for some days indisposed at certain hours, before the cold paroxysm of
fever is completely formed. See Sect. XVII. 3. 3. on Catenation of Animal
Motions.

6. If a stimulus, which at first excited the affected organ into so great
exertion as to produce sensation, be continued for a certain time, it will
cease to produce sensation both then and when repeated, though the
irritative motions in consequence of it may continue or be re-excited.

Many catenations of irritative motions were at first succeeded by
sensation, as the apparent motions of objects when we walk past them, and
probably the vital motions themselves in the early state of our existence.
But as those sensations were followed by no movements of the system in
consequence of them, they gradually ceased to be produced, not being joined
to any succeeding link of catenation. Hence contagious matter, which has
for some weeks stimulated the system into great and permanent sensation,
ceases afterwards to produce general sensation, or inflammation, though it
may still induce topical irritations. See Sect. XXXIII. 2. 8. XIX. 9.

Our absorbent system then seems to receive those contagious matters, which
it has before experienced, in the same manner as it imbibes common moisture