THE RETINA 



841 



receptacles along definite channels. It is true, however, that the 

 latter is chiefly associated with the rods and is absent in the fovea 

 centraUs which is wholly composed of cones. Since we shall have 

 occasion to refer to this point again later on, it suffices at this time to 

 note that a dark adapted eye presents a sharply defined basement 

 layer of pigment, while a light adapted eye shows a dissemination of 

 this pigment in between the rods so that their outer poles are thor- 

 oughly invested by it ^ (Fig. 443). Secondly, it has been observed 

 by Stort^ that the cones are contractile and move outward under the 

 influence of light. Thus, the dark adapted eye contains these elements 



Fig. 443. — Section of Frog's Retina Showing the Action of Light upon the Pig- 

 ment-cells, AND upon the Rods and Cones. Highly Magnified, (v. Genderen-Stort.) 

 A, From a frog which had been kept in the dark for some hours before death, B, 

 from a frog which had been exposed to light just before being killed. Three pigment- 

 cells are shown in each section. In A the pigment is collected towards the body of the 

 cell; in B it extends nearly to the bases of the rods. In A the rods, outer segments, 

 were colored red (the detached one green) ; in B they had become bleached. In A the 

 cones, which in the frog are much smaller than the rods, are mostly elongated; in B 

 they are all contracted. 



in a position next to the pigment layer and retracted in between the 

 neighboring rods, while the light adapted eye shows them in close 

 relation with the membrana limitans externa. 



These changes may be demonstrated very easily in the eyes of 

 frogs which have been kept for some time in the dark or have been 

 exposed to strong daylight. After its removal the eye is quickly 

 bisected equatorially and placed in a fixing solution and subjected to 

 the ordinary histological processes. In the normal eye, the visual 

 purple can only be seen in fish, because the layer of the rods and cones 

 is here situated upon a white tapetum. In man, on the other hand, the 



iKiihne, Untersuchungen aus dem physiol. Institut zu Heidelberg, 1878. 

 2 Onderzoek, Physiol. Labor., Utrecht, ix, 145. 



