444 



HISTOLOGY 



cup, ending near the ciliary body in a macroscopic, sharp, irregular line 

 bounding the ora serrata. The pars ciliaris and the pars iridica retina 

 are the rudimentary layers covering the ciliary body and iris respectively. 

 The pars optica retinae in a fresh condition is a transparent layer 

 colored reddish by the "visual purple." In sections it presents many 

 layers arranged as seen in Fig. 453, the cells of which are related to one 

 another as in the diagram, Fig. 454. The outer layer of the optic cup 

 forms the pigmented epithelium of the retina, which consists of a simple 

 layer of six-sided cells. Toward their outer surface (that next the chorioid, 

 where the nucleus lies) they are poor in pigment, whereas in their inner 

 portion they contain numerous rod-shaped (1-5 p, long) brown granules of 



r 



Chorioid. { 



Pigmented _ 

 epithelium. 

 f 



Layer of rods and 

 cones. 



Membrana^imitans _ 

 externa. 



f 



Outer nuclear I 

 layer. 



Henle's fiber layer j 



Outer reticular 

 layer. 



Inner nuclear 

 layer. 



Inner reticular ,' 

 layer. 



Ganglion cell layer 

 Nerve fiber layer. - 



Membrana limitans 

 interna. 



Vessels of the 



S chqriocapil- 



laris. 

 Lamina basalis. 



Rods 1 Outer. 



- Cones J segment. 



Cones 



s Rods 



Inner 

 segment. 



1 ,. Base of a cone fiber. 



Nucleus of a radial 

 fiber. 



Nucleus of an 

 amakrine cell. 



Pyramidal base of 

 a radial fiber. 



Blood vessels. 



FIG. 453. VERTICAL SECTION OF A HUMAN RETINA. X 36. 



the pigment " fuscin." In albinos the pigment is lacking. From the inner 

 surface of the pigmented epithelium, numerous processes extend between 

 the rods and cones. 



The visual cells, which are found along the outer surface of the inner 

 retinal layer, are of two sorts, rod cells and cone cells. In both, the nucleus 

 is found in the inner half of the cell, and the outer non-nucleated half 

 projects through a membrane, the membrana limitans externa. This 

 causes the visual cells to appear divided into layers, their nucleated 

 parts beneath the limiting membrane constituting the outer nuclear layer 

 (or outer granular layer), and the non-nucleated parts outside of the 

 membrane forming the layer of rods and cones. 



The rods are four times as numerous as the cones. They are regularly 

 placed so that three or four rods are found between every two cones (Fig. 



