430 THE RETINA. 



whitish color, the processes, at their central extremities, have 

 a whitish cast. 



The black pigment is also spread over the anterior surfaces 

 of the processes ; on all that part which is loose, and not in 

 contact with the posterior surface of the iris. 



When the pigment is completely washed away, the color 

 of the ciliary body appears grayish ; but if the eye has been 

 successfully injected, it will appear to be composed almost 

 entirely of vessels ; and to have a villous surface. 



The Retina. 



Within the tunica choroides, and in contact with its internal 

 surface, is a third coat of the eye, the Retina. This coat is 

 evidently derived from the optic nerve, although its texture 

 appears somewhat different. 



Before the optic nerve arrives at the ball of the eye, a small 

 branch of the ophthalmic artery penetrates its coats ; and when 

 the nerve perforates the tunica sclerotica, as described in page 

 410, this artery passes with it. 



On the internal surface of the choroides, the nerve forms a 

 small prominence ; and from this, the retina expands, with 

 many ramifications of the aforesaid artery in it. 



The retina has the appearance of mucus, and the semi- 

 transparency of a surface of ground glass ; but when immersed 

 in water, it floats like a membrane. 



By particular management, when the retina floats in water, 

 a considerable quantity of the soft substance may be removed, 

 and a delicate, soft, transparent, membranous substance, with 

 the vessels, will remain. 



The retina, therefore, seems to consist of a delicate, vascular, 

 membranous web, with a medullary pulpy matter spread upon 

 it, and supported by it. 



Thus constituted, the retina extends from its origin, at the 

 optic nerve, to the commencement of the ciliary processes. 



It lines the choroid coat, and is, therefore, in contact with 

 the pigmentum nigrum, on its internal surface, but it is simply 



