VISION. 



785 



ganglion-cell first disappear. At a certain distance from the ora 

 serrata the rods disappear, and then the cone-cells change and 

 become a layer of cylindrical epithelium. 



The Pigmentary Layer. It is composed of hexagonal pigment- 

 cells. The outer surface of the cell, turned towards the choroid, is 

 smooth and flattened, and the part of the cell near this surface is 

 without pigment and is nucleated. The inner boundary of the cell 

 is loaded with pigment and is prolonged into fine, straight, filament- 

 ous processes, which reach for a certain distance into the outer seg- 



A B 



Fig. 336. Action of the Light on Retina. Section of retina of frog. 

 (ENGLEMANN.) (From Tigerstedt's "Human Physiology," copyright, 

 1906, by D. Appleton and Company.) 



A, After two days of rest, animal in dark, pigment concentrated, cones 

 protruded. B, After diffuse daylight, cones retracted, pigment diffused. 



ments of the rods and cones, which are imbedded in the pigment- 

 cells. The pigment is in the form of small, dark-brown granules and 

 rods. In the dark, the pigment is mainly heaped up in the body 

 of the cell; but when light strikes the pigment, it is drawn in 

 between the rods. The pigment seems to renew the visual purple of 

 the outer segment of the rods after they have been bleached by the 

 light. The eyes of albinos have no pigment in the cells. 



Macula Lutea. The yellow spot of Soemmering is an oval de- 

 pression in the center of the retina. It measures one-twentieth of 



M 



