128 The Ciliary Muscle and [sony 
This adaptation of the refractive state of the eye is technically 
called its accommodation. It has been at different times attributed 
to a change of the figure of the eye-ball, to an alteration of the 
curve of the cornea, and to a shifting of the position of the lens ; 
but more delicate methods of observation than were formerly at the 
command of the physiologist have shown all these views to be 
erroneous, and by the direct inspection of the human eye with 
instruments specially contrived for this investigation, it has been 
demonstrated that its accommodation for a nearer object is effected 
by increased convexity of the lens, and this chiefly of its anterior 
surface, the curve of its posterior surface being altered in a scarcely 
appreciable degree. With the increased convexity of the lens, its 
axis 1s proportionately lengthened, the pupillary region of the inis 
peenelen the cornea, and the peripheral portion of the iris recedes 
rom it. 
There are not any grounds for supposing that this change of 
figure is wrought by any power inherent in the lens itself, which is 
plastic, but is not endowed with contractile irritability, and is not 
dominated by the will being devoid of nerves. 
The active factor of accommodation must, therefore, be external 
to the lens. Now in close relation to the lens there are two 
muscular organs—the iris and the ciliary muscle. The existence of 
accommodation in persons from whom the iris is congenitally 
absent, and its persistence where the iris has been in part or entirely 
removed, demonstrate its independence in man of this diaphragm. 
There remains, therefore, only the ciliary muscle as the active agent 
of accommodation in the human eye. To the ciliary muscle and 
lens I would invite your attention to-day. I shall take the lens 
first. 
Lens.—The lens of the human adult has a flattened bi-convex 
figure. The anterior surface is less convex than the posterior, the 
radius of curvature of the former being nearly twice as great as 
the radius of the latter surface. The infant’s lens is more nearly 
spherical, which makes the distance between its summit and the 
cornea smaller than in the adult's eye. This circumstance is not 
without influence in the causation of the minute white speck on the 
front of the lens (the central sub-capsular cataract) not unfrequent 
in persons who have suffered from infantile purulent ophthalmia, 
even where this has not been complicated with perforation of the 
cornea. The compression of the cornea by the swollen eye-lids and 
cedematous conjunctiva, and a slight amount of deep congestion push- 
ing forward the lens, may bring the lens and cornea together, and 
thus disturb the nutrition of the growing lens at the point of contact, 
and induce a perverted growth and retrogressive changes in the 
lens-tissue. 
In other mammalia, and in the lower vertebrata, the figure of 
