416 PRACTICAL ANATOMY. 



of the ciliary ligament with the sclerotic. The posterior 

 surface is of a deep-purple color, which is due to a layer of 

 pigment-cells, and is called the uvea. The iris is formed 

 of circular and radiating fibres, and, when viewed poste- 

 riorly, the arrangement resembles a wheel ; the circular 

 or sphincter fibres are arranged hub-like around the 

 pupil, the dilator fibres spoke-like from the pupil to the 

 circumference. The hub fibres of the iris surround the 

 pupil and form its sphincter, which is supplied by the 

 third nerve ; the spoke fibres dilate the pupil ; they are 

 supplied by the sympathetic. The ciliary processes are 

 sixty to eighty minute, pointed processes, each about one- 

 tenth inch long, and formed by an involution of the 

 deeper layer of the choroid. They form a pigmented 

 band, which rests upon and impresses the anterior sur- 

 face of the suspensory ligament of the lens; these im- 

 pressions are frequently pigmented, and constitute the 

 zone of Zinn. 



THE RETINA. 



The retina is the nervous tunic of the eyeball ; it is 

 formed by the expansion of the optic nerve, and termi- 

 nates anteriorly, in an irregular, frayed-out margin, 

 the ora serrate,. The retina presents the optic disk, the 

 entrance of the optic nerve, and which is pierced by 

 the arteria centralis. Vision at this point is wanting. 

 About one-tenth inch to its other side, and at the prin- 

 cipal focus of the rays of light, is the yellow spot of 

 Sommering; at this point vision is most acute. The 

 retina here is very thin, and is formed mainly by the 

 close grouping of the cones. At the centre of the yellow 

 spot is a depression, the fovea centralis. The retina is 

 composed, from without inward, of the following ten 

 layers : 



1. The pigmentary layer is applied to the choroid; 





