THE SENSE OF SIGHT. 



647 



the average about fourteen rods to one cone. In the macula the rods are 

 entirely absent, cones alone being present. 



The layer of visual cells together with the neuroglia constitutes the first 

 of the three layers of the retina proper. The external limiting membrane is 

 formed by the bending of the ends of neuroglia cells. 



The bipolar cells consist of a central portion, found in the inner nuclear 

 layer, from which are given off two processes which pass in opposite direc- 

 tions, one toward the visual cells, the other toward the ganglion cells. The 

 former terminate in tufts which arborize around the tufts and spheric en- 

 largements of the visual cells, and assist in the formation of the outer molec- 

 ular layer; the latter terminate in similar tufts in the inner molecular 

 layer. 



The ganglion cells are arranged in a single layer, as a rule. They 

 are large and nucleated. From the inner side of each cell there is given 

 off a single axon which passes toward the center of the retina (forming the 

 nerve-fiber layer), where it enters and assists in forming the optic nerve 



FIG. 306. HORIZONTAL SECTION THROUGH THE MACULA AND FOVEA OF A MAN SIXTY YEARS 

 OLD. The section is not through the exact center of the fovea, for there are only cone visual cells 

 and no remnants of the confluence of the inner granule and ganglion cell layers are present, i. 

 Cones. 2. External limiting membrane. 3. Outer nuclear layer. 4. Henle's fiber layer. 5. 

 Outer molecular or reticular layer. 6. Inner nuclear layer. 7. Inner molecular or reticular 

 layer. 8. Layer of ganglion cells. 9. Nerve-fiber layer. (After Schaper, Stohr's "Histology") 



From the outer side of the ganglion cell dendrites pass into and assist in 

 forming the inner molecular layer. These dendrites come into physiologic 

 relation with those of the inner processes of the bipolar cells. 



Horizontally disposed nerve-cells are also present in the outer molecular 

 layer in relation with the visual cells. Spongioblasts or amacrine cells are 

 also present at the border of and in the inner molecular layer. 



From the relation of the ganglion cells, in which the optic nerve-fibers 

 take their origin, to the visual cells and the bipolar cells, the former may 

 be regarded as the terminal visual organ, the intermediary between the 

 ether vibrations and the ganglion cell. The visual cells are directed toward 

 the chorioid, away from the entering light, dipping into the pigment cells. 



