230 THE NERVOUS SYSTEM. 



The vitreous body fills the large cavity of the eye. It con- 

 sists of a soft gelatinous substance, scattered about in which 

 are stellate connective-tissue cells and scanty fibrous elements 

 a sort of mucous tissue. It is surrounded by a homogene- 

 ous membrane, the hyaloid membrane, which separates it from 

 the retina and lens. 



The retina, the inner tunic of the eye, lies between the 

 choroid and the vitreous. It is divisible into three parts, the 

 optical, ciliary, and iridic portions ; the two latter being rudi- 

 mentary anterior portions. 



The pars retince iridica covers the posterior surface of the 

 iris, and consists of densely pigmented epithelium-cells covered 

 by a limiting membrane. 



The pars retince ciliaris covers the posterior surface of the 

 ciliary body and processes, and consists of layers of pigmented 

 and columnar epithelium and an internal limiting membrane. 



A short distance behind the ciliary body the pars retince 

 optica or main functional portion of the retina, the retina 

 proper, begins by an abrupt thickening or fold, the ora serrata. 



The retina proper exhibits the following layers, from within 

 outward (Fig. 92) : internal limiting membrane ; nerve-fibre 

 layer ; ganglion-cell layer ; inner reticular layer ; inner nuclear 

 layer ; outer reticular layer ; outer nuclear layer ; external 

 limiting membrane ; layer of rods and cones, and pigment- 

 layer. 



The elements making up the retina are nervous, neuro- 

 epithelium, pigmented epithelium, and sustentacular. 



Conspicuous among the sustentacular elements are the 

 radiating fibres of MiQler, specialized neuroglia-cells. Their 

 inner ends are expanded and by their junction form the so- 

 called internal limiting membrane of this portion of the retina ; 

 from these pyramidal ends the cells or fibres, more attenuated, 

 pass perpendicularly outward through the various layers to 

 the external limiting membrane, giving off lateral sustentacular 

 branches and reticula ; in the inner nuclear layer the fibre ex- 

 hibits a nucleus, and externally the fibres terminate in fine 

 processes which pierce the external limiting membrane and 

 lie among the bases of the rods and cones. Other branched 

 neuroglia-cells lie in the outer reticular layer. 



The nerve-fibre layer is made up of non-medullated nerve- 



