CRYSTALLINE LENS. 441 



When the fluid contained in the tunica hyaloidea is discharg- 

 ed and collected, it appears to resemble, in all its properties, 

 the fluid in the anterior chamber of the eye ; which, from its 

 consistence, has been called the Aqueous Humor. This cir- 

 cumstance proves that the particular consistence of the vitre- 

 ous humor is derived from the arrangement of its internal 

 membranes. 



The fluid, thus obtained, consists of water slightly impregnat- 

 ed ; 1st, with albumen, 2dly, with gelatine, and 3dly, with 

 muriate of soda. 



The vitreous humor appears necessary, to give the ball 

 of the eye the necessary size, for the performance of its 

 optical functions ; to keep the retina properly distended, and 

 to retain the crystalline lens at the proper focal distance from 

 the retina. 



The Crystalline Lens 



Is a solid body ; although it is considered as one of the hu- 

 mors of the eye. It is of a softish consistence, and has been 

 compared to gum half dissolved ; but is more firm in the cen- 

 tre than about the circumference. 



When sound it is perfectly transparent in young and mid- 

 dle-aged persons ; but is yellowish in old age. It is convex 

 on both surfaces, but the convexity of the different surfaces is 

 different. The anterior surface, which is the least convex, is 

 the segment of a sphere, whose diameter varies from six to 

 nine lines. The posterior surface, which is most convex, is the 

 segment of a sphere, whose diameter varies from four lines and 

 a half, to five lines and a half. Tin's lens is most convex in 

 young subjects. 



It is invested with a tunic of the same lenticular form with 

 itself, which has some firmness ; but in a healthy state, is 

 also perfectly transparent. The lens either does not adhere at 

 all to the tunic, or so slightly, that it projects from it with 

 a very slight pressure, as in the operation of extracting the 

 cataract. 



