216 



THE HUMAN BODY 



The Microscopic Structure of the Retina. This, the sensitive 

 portion of the eye, has the form of a thin membrane lining the 

 entire back part of the cavity of the eyeball as far forward as 

 the ciliary processes. Although only 0.15 millimeter (0.006 inch) 



thick it presents a very complex 

 structure, ten distinct layers ap- 

 pearing upon microscopic exam- 

 ination. The membrane as a 

 whole includes supporting tissues 

 as well as sensitive and nervous 

 tissues proper; we are concerned 

 only with the latter and shall 

 confine our discussion to them. 

 The retina develops in such a 

 way that the actual sensitive 

 structures instead of being on 

 2^ its front surface where light 



would strike them immediately 



FIG. 82. The right retina as it would ,-.- mariner fV.o rafinr 

 be seen if the front part of the eyeball U P On Caching tne letma, are 

 with the lens and vitreous humor were its posterior Surf ace, next to the 

 removed. J ' . 



choroid coat, and interposing be- 

 tween themselves and the source of light the nerve structures 

 which connect them with the optic nerve, and the supporting 

 tissues and blood-vessels of the retina. Fortunately all these 

 structures are so transparent or so placed as not to interfere ser- 

 iously with vision. In the fovea, where all clear sight is located, 

 blood-vessels are absent and the other structures are much reduced. 

 The sensitive elements of the eye are called, from their shape, 

 rods and cones. The rods consist of basal enlarged portions from 

 which slender rod-like processes project toward the choroid coat. 

 These processes contain a peculiar reddish substance (visual pur- 

 ple), which has the property of bleaching out when exposed to 

 light (R, Fig. 83). The cones have somewhat thicker basal por- 

 tions than the rods and much shorter processes containing no 

 visual purple (C, Fig. 83). Rods and cones make up layer number 

 two of the ten retinal layers. The first layer, which is between 

 the rods and cones and the choroid coat, is a layer of pigment 

 cells which send processes in among the rods, and seem to have 

 something to do with forming the visual purple. 



