THE EYE. 343 



of the iris, where they are woven into a thick anastomosing 

 circle. 



The nerves of the iris are derived from the ciliary plexus ; 

 at the periphery they divide and scatter in various directions : 

 the pale fibres to the posterior layers, forming a fine network 

 about the dilator muscle; the fibres with a medullary sheath 

 to the anterior portion ; another set supplies the sphincter 

 muscle these being, in the order of description, the branches 

 possibly of the sympathetic, sensory, and of the third pair. 



The posterior surface of the iris, which, near the pupil, rests 

 upon the anterior capsule of the lens, is covered with a thick 

 layer of densely pigmented cells, the uvea, which can rarely be 

 so separated as to determine their shape, and which appear to 

 have no distinct limiting membrane behind them. 



This layer extends from the pupil, where it meets the endo- 

 thelium of the anterior surface, back to the pigment of the 

 ciliary body, with which it is continuous and from which it can 

 be distinguished by having no connective tissue covering it. 

 The pigmented cells, which are more or less thickly scat- 

 tered through the stroma of the iris, determine the color of 

 the anterior surface. 



Transverse sections through the sclera and choroid are best 

 made from eyes hardened in Muller's fluid. An eye which has 

 been injected with colored gelatine, introduced through the 

 aorta after that vessel has been tied beyond the carotids, will 

 show the fine meshes of the chorio-capillaris, when the pigment- 

 layer covering the choroid has been brushed away under gly- 

 cerine. Such injections are best made on albinotic rabbits. 



Good sections of the ciliary body can be made from eyes 

 hardened in alcohol or Muller's fluid, and the blood-vessels 

 can be easily seen in injected specimens. The muscular tissue 

 of this body and the iris may be examined in specimens treated 

 with a 30 to 40 per cent, solution of potash. Carmine may 

 then be used to color. The vessels of the iris are best seen in 

 the eyes of a young albino rabbit, injected with colored 

 glycerine or Berlin blue. 



The retina lines the whole inner surface of the choroid as 

 far as the ora serrata ; it is composed of nervous elements, 

 connective tissue, and blood-vessels. 



The following division into well-marked layers from within 

 outward has been generally adopted. (See Fig. 157). 



