814 



THE EYE AND VISION 



[CH. LVIII. 



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. 512). The principal chemical constituent 

 of the lens is a protein of the globulin class called crystallin. 



FIG. 512. 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.) 



Corneo-scleral junction. At this junction the relation of parts 

 (fig. 509) 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- 

 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 Fontana. They are but little developed in the human cornea. 



The spaces between the 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. 513) apparently ends in front, near the outer 

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

 serrata, but is really represented by the uvea to the very margin 

 of the pupil. The nerve-cells in the retina remind us that the optic, 

 like the olfactory nerve, is not a mere nerve, but an outgrowth of 

 the brain. 



In the centre of the retina is a round yellowish elevated spot, 

 about 2*4 of an inch (1 mm.) in diameter, having a depression in 

 the centre, called after its discoverer the macula lutea or yellow 

 spot of Scemmering. The depression in its centre is called tihefovea 

 centralis. About ^ of an inch (2 '5 mm.) to the inner side of the 

 yellow spot, is the point (optic disc or white spot) at which the optic 

 nerve leaves the eyeball. The optic nerve-fibres are the axons of the 



