256 ANATOMY AND PHYSIOLOGY FOR NURSES 



angular folds, formed, apparently, by the plaiting of the 

 internal layer of the choroid coat; they are about sixty 

 in number. Their external border is continuous with the 

 internal layer of the choroid coat. The central border is 

 free, and rests against the circumference of the crystalline 

 lens. These processes are covered by a layer of pigmen- 

 tum nigrum. 



The retina is composed of three layers: the external, 

 the middle or nervous, and the internal or vascular. 



The external layer is extremely thin, and the membrane 

 is seen as a flocculent film when the eye is suspended in 

 water. 



The nervous membrane is the expansion of the optic 

 nerve, and forms a thin, semitransparent, bluish-white 

 layer. The vascular membrane consists of the ramifica- 

 tions of a minute artery and its accompanying vein. 

 The vascular layer forms distinct sheaths for the nervous 

 papillae, which constitute the inner surface of the retina. 



The aqueous humor fills the anterior and posterior 

 chambers of the eye. It is an albuminous fluid, having 

 an alkaline reaction. Its specific gravity is a very little 

 greater than that of distilled water. The anterior chamber 

 is the space intervening between the cornea in front, 

 and the iris and the pupil behind ; the posterior chamber is 

 the narrow space, less than half a line in depth, bounded 

 by the posterior surface of the iris and pupil in front, and 

 by the ciliary processes and crystalline lens behind. The 

 two chambers are lined by a thin layer the secreting 

 membrane of the aqueous humor. 



The crystalline humor, or lens, is situated imme- 

 diately behind the pupil, and is surrounded by the ciliary 

 processes. This humor is more convex on the posterior 

 than on the anterior surface, and in different portions of 

 the surface of each the convexity varies. The lens is 

 embedded in the anterior part of the vitreous humor, from 



