424 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



The inner segment shows a superficial longitudinal 

 striation, due to impressions from fiber baskets of the 

 neuroglia network. The cone is 15 n to 25 // long 

 and its inner segment considerably broader than the 

 rod. The rods are more numerous than the cones, 

 three or four of the former intervening between two 

 of the latter. (10) The pigment layer forms a com- 

 pact background to the rods and cones. It consists 

 of hexagonal cells that contain black pigment gran- 

 ules. The inner surfaces of these cells possess thread- 

 like filaments that interlace between the rods and 

 cones. The nuclei of these cells lie in the outer ends 

 of these cells, the so-called basal plates, and are not 

 pigmented. The granules are mobile and their dis- 

 tribution in the cells varies according to the illumi- 

 nation of the retina. In strong light the pigment is 

 evenly distributed throughout the cytoplasm, while 

 in weak light it is collected at the outer portion of 

 each cell. This single row of pigment cells represents 

 the outer layer of the primary optic vesicle, while 

 the other nine layers of the retina develop from the 

 inner layer of this vesicle. 



The neuroglia elements of the retina differ from, 

 those of the brain in that they form radial sustentac- 

 ular fibers, called fibers of Mliller, which penetrate 

 the retina to the rods and cones. Each fiber repre- 

 sents a modified epithelial cell which terminates in 

 basal plates, the latter forming the limiting mem- 

 branes of the retina. The end plates that form the 

 external limiting membrane give off externally short, 

 inflexible fibrils, which form fiber-baskets enclosing 

 the basilar portions of the rods and cones. The 



