4. When a stimulus has been many times repeated at uniform intervals, so as
to produce the complete action of the organ, it may then be gradually
diminished, or totally withdrawn, and the action of the organ will
continue. For the sensorial power of association becomes united with that
of irritation, and by frequent repetition becomes at length of sufficient
energy to carry on the new link in the circle of actions, without the
irritation which at first introduced it.

Hence, when the bark is given at stated intervals for the cure of
intermittent fevers, if sixty grains of it be given every three hours for
the twenty-four hours preceding the expected paroxysm, so as to stimulate
the defective part of the system into action, and by that means to prevent
the torpor or quiescence of the fibres, which constitutes the cold fit;
much less than half the quantity, given before the time at which another
paroxysm of quiescence would have taken place, will be sufficient to
prevent it; because now the sensorial power, termed association, acts in a
twofold manner. First, in respect to the period of the catenation in which
the cold fit was produced, which is now dissevered by the stronger stimulus
of the first doses of the bark; and, secondly, because each dose of bark
being repeated at periodical times, has its effect increased by the
sensorial faculty of association being combined with that of irritation.

Now, when sixty grains of Peruvian bark are taken twice a day, suppose at
ten o'clock and at six, for a fortnight, the irritation excited by this
additional stimulus becomes a part of the diurnal circle of actions, and
will at length carry on the increased action of the system without the
assistance of the stimulus of the bark. On this theory the bitter
medicines, chalybeates, and opiates in appropriated doses, exhibited for a
fortnight, give permanent strength to pale feeble children, and other weak
constitutions.