XVII 



THE SENSE ORGANS 



329 



9. The layer of rods and cones. 



The latter layer lies next to the pigmented epithelium of 

 the retina, and processes from the pigmented cells extend 

 between the rods and cones. It forms 

 the receptive surface of the retina, al- 

 though the hght must pass through all 

 of the other layers before reaching it. 

 The rods, which are larger and more 

 numerous than the cones, are very much 

 elongated cells ; their outer ends are cy- 

 lindrical and show a cross striation which 

 marks their division into transverse disks, 

 into which they may be separated by 

 treatment with certain reagents. Behind 

 the cylindrical portion comes a lens- 

 shaped, refractive body with a flat inner 

 and a very convex outer surface. The 

 nucleus lies in the part of the cell on the 

 inner side of the outer limiting mem- 

 brane. The inner end of the cell tapers 

 into a fiber which extends into the outer 

 reticular layer. The rods are of two 

 kinds, the red and the green. The for- 

 mer are more abundant and have a much 

 longer cylindrical segment, which is per- 

 meated by a peculiar reddish purple pig- 

 ment which is known as visual purple. The 

 green rods have a short, cylindrical seg- 

 ment which is followed by an elongated 

 narrow neck ; the cylindrical portion con- 

 tains a green substance {visual greeii). 



The cones have a conical outer seg- 

 ment which shows a cross striation like the corresponding 



Fig. 90. — Rods from 

 the retina of the 

 frog : a green rod 

 on the right, a 

 red one on the 

 left, a, outer seg- 

 ment; b, refractive 

 body; c, inner seg- 

 ment; ^.intermedi- 

 ate disk; e, inner 

 enlargement con- 

 taining the nucleus ; 

 /, external limiting 

 membrane. (From 

 Gaupp, after Greeff.) 



