hey attend to the apparent motions, and to the
battement of sounds of the bodies around them, and for a moment mistake
them for those real motions of the ship, and noise of wheels, which they
had lately been accustomed to: or at these intervals of reverie, or on the
approach of sleep, these supposed motions or sounds may be produced
entirely by imagination.

We may conclude from this account of vertigo, that sea-sickness is not an
effort of nature to relieve herself, but a necessary consequence of the
associations or catenations of animal motions. And may thence infer, that
the vomiting, which attends the gravel in the ureter, inflammations of the
bowels, and the commencement of some fevers, has a similar origin, and is
not always an effort of the vis medicatrix naturae. But where the action of
the organ is the immediate consequence of the stimulating cause, it is
frequently exerted to dislodge that stimulus, as in vomiting up an emetic
drug; at other times, the action of an organ is a general effort to relieve
pain, as in convulsions of the locomotive muscles; other actions drink up
and carry on the fluids, as in absorption and secretion; all which may be
termed efforts of nature to relieve, or to preserve herself.

11. The cure of vertigo will frequently depend on our previously
investigating the cause of it, which from what has been delivered above may
originate from the disorder of any part of the great tribes of irritative
motions, and of the associate motions catenated with them.

Many people, when they arrive at fifty or sixty years of age, are affected
with slight vertigo; which is generally but wrongly ascribed to
indigestion, but in reality arises from a beginning defect of their sight;
as about this time they also find it necessary to begin to use spectacles,
when they read small prints, especially in winter, or by candle light, but
are yet able to read without them during the summer days, when the light is
stronger. These people do not see objects so distinctly as formerly, and by
exerting their eyes more than usual, they perceive the apparent motions of
objects, and confound them with the real motions of them; and therefore
cannot accurately balance themselves so as easily to preserve their
perpendicularity by them.

That is, the apparent motions of objects, which are at rest, as we move by
them, should only excite irritative ideas: but as these are now become less
distinct, owing to the beginning imperfe