319 



by any power inherent in the lens itself, which is plastic, but 

 not endowed with contractile irritability, and is not dominated 

 by the will, being devoid of nerves. 



The active factor of accommodation must therefore be ex- 

 ternal 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 indej^en- 

 dence in man of this diaj)hragm. There remains, therefore, 

 only the ciliary muscle as the active factor of accommodation 

 in the human eye. To the ciliary muscle and lens I would 

 now invite attention. I shall take the lens first. 



The structure of the Lens. — The lens of the human 

 adult has a flattened convex figure. The anterior surface 

 is less convex than the ]30sterior, 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 SiDhe- 

 rical, which makes the distance between its summit and 

 the cornea smaller than in the adult's eye. This circum- 

 stance is not without influence in the causation of the 

 minute white speck on the front of the lens (the central sub- 

 capsular cataract) not infrequent in persons who have suifered 

 from infantile purulent ophthalmia, even when this has not 

 been complicated with perforation of the cornea. The com- 

 pression of the cornea by the swollen eyelids and oedematous 

 conjunctiva and a slight amount of deej) congestion pushing 

 forwards 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 retro- 

 gressive changes in the lens tissue. 



In other mammalia and in the lower vertebrata the figure 

 of the lens is less flattened than in adult man, resembling 

 more nearly the shape of the human foetal lens. 



The lens of all vertebrate animals is formed almost entirely 

 of a peculiar fibrous tissue with a very scanty formless inter- 

 stitial substance. It is enclosed in a short capsule, the 

 integrity of which is of the highest importance to its trans- 

 parence. This capsule is a perfectly transparent, very elastic, 

 yet brittle membrane. It is little prone to degenerative 

 metamorphoses, and it never undergoes absorption. I have 

 found it transparent and unchanged sixteen years after the 

 extraction of the lens for cataract. Its chemical constitution 

 and reactions resemble those of the other hyaloid membranes ; 

 it is unaffected by weak acids and alkalies, and it resists 

 putrefaction. 



