THE INTERNAL OR NERVOUS TUNIC OF THE EYE. 461 



of the cones, and since the cone-fibers, in order to reach the outer 

 molecular layer, must here describe a curve, there arises a peculiar 

 layer, composed of obliquely directed fibers, known as the outer 

 fiber-layer or Henle's fiber layer. In other words, the fibers of this 

 region are more distinctly seen because they are not covered by the 

 rod-nuclei and rod-fibers. 



Fovea centralis. 



Layer of ~^MBHHHM^^ i 



nerve-fibers. ', 

 Ganglion-cell .J <?' 



layer. 



Inner molecu-j 



lar layer. r afc*^ 

 Inner nuclear 



layer. 

 Outer molec-_} 



ular layer, r--? 

 Outer fibrous I - 



layer. 



Outer nuclear JB 

 layer. 



Cones. J 



Fig. 360. Section through human macula lutea and fovea centralis ; X I 5- 

 a result of treatment with certain reagents, the fovea centralis is deeper and the margin 

 more precipitous than during life. 



The yellowish color of the fovea centralis is due to pigment held 

 in solution within the layers of the retina. The cone-visual cells 

 themselves contain no pigment. 



5. ORA SERRATA, PARS dLIARIS RETINAE, AND PARS IRIDICA 



RETINAE* 



In the region of the ora serrata the retina suddenly becomes 

 thinner. As seen from the inner surface of the retina, its decrease 

 presents the appearance of an irregular curve rather than of the 

 segment of a sphere. Shortly before the retina terminates, its layers 

 become markedly reduced, certain ones disappearing entirely ; first 

 the nerve-fiber layer, then the ganglion-cell layer and cone- and rod- 

 visual cells, their place being taken by an indifferent epithelium. 

 The inner molecular layer of the retina gradually loses the pro- 

 cesses which penetrate inward. In the region of the ora serrata the 

 sustentacular fibers are markedly developed. Relatively large hol- 

 low spaces are often found in the retina at the ora serrata ; they are 

 thought to be due to edema. 



The pars ciliaris retinae consists essentially of two simple 

 layers of cells, of which the external represents the pigment layer 

 and the internal the inner epithelium of the secondary optic vesicle. 

 In the pars iridica retinae the arrangement is similar ; here both 

 layers are pigmented. 



