nected together,
and make a diurnal circle of irritative action, and if one of this chain be
disturbed, the whole is liable to be put into disorder. See Sect. XX. on
Vertigo.

5. When the stomach and intestines receive a quantity of food, whose
stimulus is greater than usual, all their motions, and those of the glands
and lymphatics, are stimulated into stronger action than usual, and perform
their offices with greater vigour and in less time: such are the effects of
certain quantities of spice or of vinous spirit.

6. But if the quantity or duration of these stimuli are still further
increased, the stomach and throat are stimulated into a motion, whose
direction is contrary to the natural one above described; and they
regurgitate the materials, which they contain, instead of carrying them
forwards. This retrograde motion of the stomach may be compared to the
stretchings of wearied limbs the contrary way, and is well elucidated by
the following experiment. Look earnestly for a minute or two on an area an
inch square of pink silk, placed in a strong light, the eye becomes
fatigued, the colour becomes faint, and at length vanishes, for the
fatigued eye can no longer be stimulated into direct motions; then on
closing the eye a green spectrum will appear in it, which is a colour
directly contrary to pink, and which will appear and disappear repeatedly,
like the efforts in vomiting. See Section XXIX. 11.

Hence all those drugs, which by their bitter or astringent stimulus
increase the action of the stomach, as camomile and white vitriol, if their
quantity is increased above a certain dose become emetics.

These inverted motions of the stomach and throat are generally produced
from the stimulus of unnatural food, and are attended with the sensation of
nausea or sickness: but as this sensation is again connected with an idea
of the distasteful food, which induced it; so an idea of nauseous food will
also sometimes excite the action of nausea; and that give rise by
association to the inversion of the motions of the stomach and throat. As
some, who have had horse-flesh or dogs-flesh given them for beef or mutton,
are said to have vomited many hours afterwards, when they have been told of
the imposition.

I have been told of a person, who had gained a voluntary command over these
inverted motions of the stomach and throat, and supported himself by
exhibiting this curiosity to the public. At these exhibitions he swallowed
a p