634 



THE SENSES 



optic nerve is said to be upward of 500,000. The fibers of the optic nerve 

 spread out over the inner surface of the retina as far as the ora serrata. 



The retina itself consists of layers of nerve elements supported by deli- 

 cate connective tissue. The older descriptions recognize some eight or ten 

 layers in the retina, but the newer investigations of Cajal, Golgi, Retzius, 

 and others have shown that the retina is a much simpler structure than hereto- 

 fore described. The retina is formed of essentially three layers of nerve cells. 



Stratum 

 pigment! 



1 Stratum 

 opticurii 



Membrana limitans interna 



FIG. 446. Section of Human Retina. (Cunningham, modified from Schulze.) 



Naming from the center of the eye outward, they are: The ganglionic layer; 

 the layer of bipolar cells; and the layer of rods and cones, figure 447. The 

 cells of these layers have numerous fibrous processes which interlock in such a 

 way that they seem to form different areas when studied in cross-section. If 

 we recognize the strata of interlacing fibers, then the following may be 

 made out: 



The layer of ganglion cells. 

 The layer of bipolar cells . . 

 The layer of visual cells. . . 



. Ganglionic layer, with the fibers of the optic nerve. 



. Internal molecular layer, 



j 3. Internal granular layer. 



I 4. The external molecular layer. 



( 5. The external granular layer. 



| 6. The layer of rods and cones. 



