RETINAL CHANGES INVOLVED IN VISION 



623 



make up the retina are formed by division and differentiation from 

 the anterior wall. That part of the cup originally derived from the 

 external surface of the body is turned towards the posterior layer 

 or retinal pigment. From it is 

 developed the special end-organ of 

 vision, viz. the rod and cone layer 

 of the retina. Besides this special 

 sensory epithelium, the retina pre- 

 sents two other sets of neurons 

 through which impulses generated 

 in the sensory epithelium must pass 

 before they arrive at the optic nerve. 

 The three relays of nervous elements 

 in the retina have the following 

 arrangement : 



(1) The first relay the sense 

 epithelium consists of rods and 

 cones with their nuclei (Fig. 283), 

 the latter being situated in the 

 outer nuclear layer. Each rod pre- 

 sents an external (a) and an internal 

 limb (b). The former, in the eye 

 which has been kept in the dark, 

 has a purplish colour from the pre- 

 sence of rhodopsin or visual purple. 

 From the inner end of the inner 

 limb a fine fibre c passes to its 

 nucleus in the outer nuclear layer, 

 and from the nucleus a central 

 process g passes into the outer 

 molecular layer where it ends freely 

 in a little knob e. The cones, 

 which are thicker than the rods, 

 also possess outer and inner limbs. 

 From the inner limb a thick process, 

 containing a nucleus, passes through 

 the external nuclear layer and ends 

 with a broad base e in the outer 



molecular layer, from which short fibres are given of! to come in 

 contact with the bipolar cells of the inner nuclear layer. 



(2) The second relay is formed by the bipolar cells of the inner 

 nuclear layer. Each of these sends off one fibre peripherally to 

 make contact with endings of the rod and cone fibres in the outer 

 molecular layer, and another process which passes centrally into the 



FIG. 283. 



i, a rod ; n, a cone of mammalian 

 retina ; h, external limiting mem- 

 brane. (R. Greeff.) 



