THE CRYSTALLINE LENS. 429 



many as twelve. The lens consists of about 60 per cent, of water ; 

 the solid matter is chiefly albuminoid. 



Before birth, the lens is nearly spherical, very soft, and not quite 

 transparent, its fibres being then imperfectly developed. At this 

 period, a small artery traverses the vitreous humor to the back of the 

 lens ; here its branches form a plexiform network on the back of the 

 capsule. In front of the lens, this network of vessels is met by a vas- 

 cular extension from the pupillary margin of the iris, constituting the 

 pupillary membrane, which then closes the pupil. By means of these 

 vascular membranes, the humors of the eyeball are nourished during 

 their growth. Shortly before birth, the pupillary membrane gradually 

 undergoes absorption, so that, at birth, almost all traces of it have 

 disappeared. In some rare instances, however, this structure is not 

 absorbed, but remains as a permanent part of the eyeball ; in such 

 cases, sight is necessarily interrupted. 



Between the front of the crystalline lens and the posterior surface 

 of the cornea, is a small space, measuring, from before backwards, 

 about Jjyth of an inch, and occupied by the aqueous humor. This in- 

 terval is divided, before birth, by the pupillary membrane, into two 

 parts, called the anterior and posterior chambers. In the fully-formed 

 eyeball, the iris is often described as also imperfectly dividing the 

 space occupied by the aqueous humor, into an anterior and posterior 

 chamber ; but it has been shown that the iris rests immediately upon 

 the anterior surface of the capsule of the lens, so that there is no 

 posterior chamber, or interval between them. The aqueous humor 

 consists of about five drops of a limpid fluid, resembling pure water ; 

 it contains a little salt, and a trace of animal matter. It is probably 

 secreted by the posterior surface of the cornea, and by the vessels of 

 the iris and of the points of the ciliary processes. 



The following table shows, in decimal parts of an inch, various 

 measurements of the eyeball and its parts (Krause): 



a. Diameters of the eyeball, through its centre : 



Inch. 



Antero-posterior, . * , . . . ' . -. - .944 

 Vertical, . . .-.. .' . >. r$. &. , , . .944 

 Horizontal,. . ...: .. , ..... . . . 1.003 



b. Thickness of the various parts in the direction of the antero- 

 posterior axis : 



Cornea, 



Aqueous humor,. 

 Crystalline lens, . 

 Vitreous body, . 

 Retina and choroid, 

 Sclerotic, ... . 



.0393 

 .0984 

 .2755 

 .4921 

 .0078 

 .0511 



c. Radii of the curves of the surfaces of the refracting media: 



Cornea, . . . '. 275 to .314 



Anterior surface of lens, 275 to .393 



Posterior " " .236 



