422 JAMES ROLLIN SLONAKER 



a few of these tangential sections through the fovea at different 

 levels are shown in plate 7, figures 35, 36, 37, and 38. 



Figures 29 and 30, plate 6 or figure 48, plate 9, show that this 

 depression is due to a thnning out of all the layers of the retina 

 except the rod and cone layer and the pigment layer. The rod 

 and cone layer is much increased in thickness in the center of 

 the fovea. This is largely due to the lengthening of the connec- 

 tions between the cones and their nuclei. 



The thicknesses of the different layers in the center of the 

 fovea and at its margin are tabulated in table 3, (p. 424). This 

 shows that there has been a reduction from .3306 mm., the total 

 thickness of the retina at the edge of the fovea, to .1555 mm., 

 the thickness at the center of the fovea. This reduction in 

 thickness is apparently due to a pushing to the side or a radial 

 migration of the cells from the center of the fovea into the area. 

 This assumption is based on a comparison of the number of 

 cells found in equal areas in the center of the fovea and in the 

 area centrahs. This is also 'ndicated bj^ the slanting arrange- 

 ment of the cells and their processes from the periphery of the 

 fovea toward the center (fig. 48, plate 9). This arrangement 

 conforms in general to that of the foveal depression. 



Table 4 (p. 425) represents the number of cells of the different 

 layers in equal areas at various regions. Owing to the fact that 

 the dimensions of the center of the fovea are so small, it is very 

 difficult to get a section which does not include some cells from 

 the sloping walls. In the above table the count was made from 

 sections of the whole eye. These were necessarily thicker 

 than the diameter of the center of the fovea. The count of the 

 cells in the fovea, therefore, must necessarily mclude some cells 

 from the sloping walls and is somewhat larger than it should be. 



According to Cajal ('94), a one to one relationship exists in 

 the fovea between the ganglion cells, bipolar cells, and the 

 cones. This makes possible the sharp and distinct vision char- 

 acteristic of the fovea. Clearness of vision in the fovea is also 

 assisted by the thinning of the retina in this region. This allows 

 the rays of light to reach the cones with less obstruction than in 

 other portions of the retina. 



