424 



ELEMENTARY PHYSIOLOGY 



LESS. 



characteristic appearance from which it derives its name, 

 other cells also occur in this layer. These are shown at h. c. 

 and s. ; but their relationships to the other structural 

 elements of the retina are so uncertain that we must 

 content ourselves with merely drawing attention to their 

 existence. 



At the entrance of the optic nerve itself, the nervous 

 fibres predominate, and the rods and cones are absent. 

 In the yellow spot, on the contx'ary, the cones are abun- 

 dant and close set, becoming at the same time longer and 

 more slender, while rods are scanty, and are found only 





Fio. 139.— Pigmented Ehithelium of the Hi'man Retina. (Max 

 ScHULTZE.) Highly magnified. 



a, cells seen from the outer (choroidal) surface ; 6, two cells seen side- 

 ways, with fine processes on their inner side ; c, a cell still in connection 

 with the layer of rods of the retina. 



towards its margin. In the centre of the macula lutea 

 (Fig. 138) the layer of fibres of the optic nerve 

 disappears, and all the other layers, except that of the 

 cones, become extremely thin. 



The outer ends of the rods and cones lie buried among 

 certain fine processes of those pigment cells, adjacent to 

 the choroid coat, to whose existence we have previously 

 alluded (pp. 401, 419). When seen from the surface by 

 which they are in contact with the choroid, these cells 

 present the appearance of small black hexagons arranged 

 in a sort of mosaic (Fig. 139, a.). 



