al to its waste or
expenditure in the perpetual movement of the vascular organization.

3. When a stimulus is repeated at uniform intervals of time with such
distances between them, that the expenditure of sensorial power in the
acting fibres becomes completely renewed, the effect is produced with
greater facility or energy. For the sensorial power of association is
combined with the sensorial power of irritation, or, in common language,
the acquired habit assists the power of the stimulus.

This circumstance not only obtains in the annual and diurnal catenations of
animal motions explained in Sect. XXXVI. but in every less circle of
actions or ideas, as in the burthen of a song, or the iterations of a
dance; and constitutes the pleasure we receive from repetition and
imitation; as treated of in Sect. XXII. 2.

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diminished, or totally withdrawn, and the action of the organ will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