STRUCI'URE OF RETINA PHRYNOSOMA 



351 



is only 40 m- Here the retinal layers, which around the periphery 

 of the depression are highly thickened, become greatly thinned 

 out; in fact, at the deepest point of the foveal depression the 

 ganglionic laj^er, the internal molecular layer, the internal nu- 

 clear layer, and the external molecular layer are entirely inter- 

 rupted, the apex of the depression being sm-mounted by the 

 external nuclear la3^er which, at this point, is relatively thin 

 (figs. 1 and 3). 



The foveal region shows a marked alteration in the type of 

 visual elements as compared with the remamder of the retina. 



Fig. 4 Diagrammatic drawing of the foveal cones, e.l.vi., external limiting 

 membrane. 



Fig. 5 A portion of the retina, 1.5 mm. below the entrance of the optic nerve. 

 X 935. 



As the fovea is approached the cones become gradually more and 

 more attenuated. Opposite the apex they are so slender that with 

 microscopic magnification of 3000 diameters their width cannot 

 be measured with certainty. Concomitant with these changes 

 is a marked increase in the number of cones and a pronounced 

 thickening of the external nuclear layer which, at its widest por- 

 tion (about 0.4 mm. from the apex of the fovea), is from eight to 

 ten cell laj^ers thick. At the apex, however, the external nuclear 

 layer is greatly thinned out and is entirely devoid of cone nuclei. 

 The nuclei of the cones of this region lie in a more peripheral 

 position — the cones being connected with their respective nuclei 

 by long attenuated central processes as is illustrated in figm'e 4. 



