770 



THE EYE AND VISION 



[CH. LVI. 



by a scanty amount of cement substance. The central portion 

 (nucleus) of the lens is the hardest. 



The epithelium of the lens consists of a layer of cubical cells 

 anteriorly, which merge at the equator into 

 the lens fibres. The development of the 

 lens explains this transition. The lens at 

 first consists of a closed sac composed of 

 a single layer of epithelium. The cells of 

 the posterior part soon elongate forwards 

 and obliterate the cavity ; the anterior cells 

 do not grow, but at the edge they become 

 continuous with the posterior cells, which 

 are gradually developed into fibres (fig. 

 570). The principal chemical constituent 

 of the lens is a proteid of the globulin class 

 called crystallin. 



Corneo-scleral junction. At this junction 

 the relation of parts (fig. 567) is so important 

 as to need a short description. In this neigh- 

 bourhood, the iris and ciliary processes join with the cornea. The 

 proper substance of the cornea and the posterior elastic lamina 

 become continuous with the iris, at the angle of the iris, and the iris 

 sends forwards processes towards the posterior elastic lamina, form- 



Fio. 569. Laminated structure of 

 the crystalline lens. The laminae 

 are split up after hardening in 

 alcohol. 1, The denser central 

 art or nucleus ; 2, the succes- 

 sive external layers. }. 



(Arnold.) 



FIG. 570. Meridional section through the lens of a rabbit. 1 , Lens capsule ; 2, epithelium of lens ; 

 3, transition of the epithelium into the fibres ; 4, lens fibres. (Bubuchin.) 



ing the ligamentum pectinatum iridis, and these join with fibres of 

 the elastic lamina. The epithelial covering of the posterior surface 

 of the cornea is, as we have seen, continuous over the front of the 

 iris. At the iridic angle, the compact inner substance of the cornea 

 is looser, and between the bundles are lymph spaces called the spaces 

 of Montana. They are little developed in the human cornea. 



The spaces which are present in the broken-up bundles of corneal 

 tissue at the angle of the iris are continuous with the larger 

 lymphatic space of the anterior chamber. Above the angle at the 

 corneo-scleral junction is a canal, which is called the canal of 

 Schlemm. It is a lymphatic channel. 



The retina (fig. 571) apparently ends in front, near the outer 

 part of the ciliary processes, in a finely-notched edge, the ora 



