169 
Mr. Bell on the motions of the eye, &c. 
The purpose of this rapid insensible motion of the eye-ball 
will be understood on observing the form of the eye-lids and 
the place of the lacrymal gland. The margins of the eye- 
lids are flat, and when they meet, they touch only at their 
outer edges, so that when closed there is a gutter left between 
them and the cornea. If the eye-ball were to remain without 
motion, the margins of the eye-lids would meet in such a 
manner on the surface of the cornea, that a certain portion 
would be left untouched, and the eye would have no power 
of clearing off what obscured the vision, at that principal part 
of the lucid cornea which is in the very axis of the eye ; and 
if the tears flowed they would be left accumulated on the 
centre of the cornea, and winking, instead of clearing the 
eye, would suffuse it. To avoid these effects, and to sweep 
and clear the surface of the cornea, at the same time that the 
eye-lids are closed, the eye-ball revolves, and the cornea is 
rapidly elevated under the eye-lid. 
Another effect of this motion of the eye-ball is to procure 
the discharge from the lacrymal ducts ; for by the simul- 
taneous ascent of the cornea, and the descent of the upper 
eye-lid, the membrane on which the ducts open is stretched, 
and the effect is like the elongation of the nipple, facilitating 
the discharge of tears. 
By the double motion, the descent of the eye-lid and the 
ascent of the cornea at the same time, the rapidity with which 
the eye escapes from injury is encreased. Even creatures 
which have imperfect eye-lids, as fishes, by possessing this 
rapid revolving motion of the eye, avoid injury and clear off 
impurities. 
I may observe in passing, that there is a prevision for the 
mdcccxxiii. Z 
