Hence those who have lately accustomed themselves to large doses of opium
by beginning with small ones, and gradually increasing them, and repeating
them frequently, as mentioned in the preceding paragraph; if they intermit
the use of it for a few days only, must begin again with as small doses as
they took at first, otherwise they will experience the inconveniences of
intoxication.

On this circumstance depend the constant unfailing effects of the various
kinds of stimulus, which excite into action all the vascular systems in the
body; the arterial, venous, absorbent, and glandular vessels, are brought
into perpetual unwearied action by the fluids, which are adapted to
stimulate them; but these have the sensorial power of association added to
that of irritation, and even in some degree that of sensation, and even of
volition, as will be spoken of in their places; and life itself is thus
carried on by the production of sensorial power being equal 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.