hat organ
from its reverse sympathy with the increased actions of the organ of
vision. For the expenditure of sensorial power by the organ of vision is
always very great, as appears by the size of the optic nerves; and is now
so much increased as to deprive the next link of association of its due
share. As mentioned in Article 6 of this Genus.

In the same manner the undulations of water, or the motions of a ship, at
first give pleasure by increasing the irritative motions belonging to the
sense of vision; but produce sickness at length by expending on one part of
the associated train of irritative actions too much of that sensorial
power, which usually served the whole of it; whence some other parts of the
train acquire too little of it, and perform their actions in consequence
too feebly, and thence become attended with disagreeable sensation.

It must also be observed, that when the irritative motions are stimulated
into unusual action, as in inebriation, they become succeeded by sensation,
either of the pleasurable or painful kind; and thus a new link is
introduced between the irritative motions thus excited, and those which
used to succeed them; whence the association is either dissevered or much
weakened, and thus the vomiting in sea-sickness occurs from the defect of
the power of association, rather than from the general deficiency of
sensorial power.

When a blind man turns round, or when one, who is not blind, revolves in
the dark, a vertigo is produced belonging to the sense of touch. A blind
man balances himself by the sense of touch, which being a less perfect
means of determining small quantities of deviation from the perpendicular,
occasions him to walk more carefully upright than those, who balance
themselves by vision. When he revolves, the irritative associations of the
muscular motions, which were used to preserve his perpendicularity, become
disordered by their new modes of successive exertion; and he begins to
fall. For his feet now touch the floor in manners or directions different
from those they have been accustomed to; and in consequence he judges less
perfectly of the situation of the parts of the floor in respect to that of
his own body, and thus loses his perpendicular attitude. This may be
illustrated by the curious experiment of crossing one finger over the next
to it, and feeling of a nut or bullet with the ends of them. When, if the
eyes be closed, the nut or bullet appears to be two, fro