332 



PHYSIOLOGY 



CHAP. 



fringes, which penetrate between the outer segments of the rods 

 and cones (Fig. 155). 



Fi. . 1. ".."..- I 'i-iiu'iit'-.l ([ pit ln-11 inn of human n-tina. Highly m.-i^nilii-il. (Max Srlmll/r.) a, i-rlls s 

 from rface, with clear lines of intercellular subatance between ; '>. t" n-lls in j. 



witli In M- processes .-\lcii. lin^ iiiwanl- ; -. i-''IU still in n-lation with tin- mil n rii.|-< ..I tlir 



The latter are the visual cells, if. 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. \\lncli 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 

 j n ^ o ^g outer nuclear layer, as fine varicose 



, . i i T , i i rrn i 



fibres in which lie their nuclei. The chro- 

 matic substance of the nuclei of the rods, 

 unlike those f ^e cones, has a stratified 

 fi, and longitudinally arrangement (rig. 158). 

 '" 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. 



i.i. i.v.. Hod ami cone ( ,r 



human retina. Highly 



ma-Lin.-.!. (Max s, .hiiit/.-.) 



