332 



PHYSIOLOGY 



CHAP. 



fringes, which penetrate between the outer segments of the rods 

 and cones (Fig. 155). 



Fir;. 155. Pigmented epithelium of human retina. Highly magnified. (Max Schultze.) a, cells seen 

 from outer surface, with clear lines of intercellular substance between ; b, two cells in profile, 

 with line processes extending inwards ; c, cells still in relation with the outer ends of the rods. 



The latter are the visual cells, i.e. the receptors of the 

 luminous stimuli. In both cones and rods a 

 thinner outer segment can be distinguished 

 from a thicker inner limb : the differences in 

 form, length, and structure are clearly shown 

 in Fig. 156. 



The outer segment of both rods and cones 

 consists of a shining, doubly refractive sub- 

 stance, which splits up with certain reagents 

 into a series of transverse discs. The outer 

 segments stain only with osmic acid, and 

 become greenish brown ; the inner, on the 

 contrary, can be stained with carmine, iodine, 

 and other dyes. 



The rods are more numerous than the 

 cones, except in the macula lutea, which con- 

 sists exclusively of cones. In the immediate 

 neighbourhood of the macula lutea each cone 

 is surrounded by a single row of rods; in 

 the peripheral portions, on the contrary, each 

 cone is surrounded by several rows of rods ; 

 so that the cones become proportionately 

 less frequent with the distance from the 

 macula lutea (Fig. 157). 



Both rods and cones are prolonged inter- 

 nally, through the membrana limitans externa 

 Fi. LOG. Rod and com- ot [^Q the outer nuclear layer, as fine varicose 



human retina. Highly . . . . , .. ,, . J -, . -T,, , 



magnified. (Max Schuitxe.) hbres in which lie their nuclei. Ine chro- 

 matic substance of the nuclei of the rods, 

 unlike those of the cones, has a stratified 



outer, "and' longitudinally arrangement (Fig. 158). 



marked in the inner se-j- mi < ~' , v ,, " , , . . ,.. 



ment; /., membrana Hmi- The fibres of the rods terminate in small 

 rounded swellings in the outer molecular 

 layer. The cone-fibres also terminate in this layer, as conical 

 dilatations from which short fibres are given off. 



