EFFECT OF LIGHT ON THE RETINA 185 



tains only cones and no rods. This has been shown, not only 

 morphologically but physiologically as well, in that it is impossi- 

 ble to demonstrate the presence of visual purple (see Garten 

 '07 b, p. 148 and Hess '10, p. 281). Nevertheless, pigment 

 migration and cone contraction do take place, however slight in 

 amount. It is perfectly true that the outer segments and the 

 oil drops are always covered with pigment, but when bright 

 light (sun-light) is thrown on the retina the cones contract and 

 the pigment migrates forward. 



It cannot be argued, if we accept Garten's theory, that there 

 is any physiological need for a moving backward of the pigment 

 and a stretching of the cones in faint light since there is onty 

 one kind of visual cell. But, on the other hand, it must be 

 assumed that the pigment and the cones are sensitive to light 

 and respond to its stimulus, the one moving forward, the other 

 contracting. Dittler ('07 a and '07 b) thinks that the cone 

 myoid of the frog is not itself sensitive to light but that its con- 

 traction is brought about by a metabolic product which is set 

 free by the activity of the retina under the influence of light. 

 Garten ('07, p. 96) supports this theory and thinks that the 

 migration of pigment may also be bound up in some way with 

 this chemical stimulus effect. Now, if this be true for the retina 

 of the frog, there is no reason, simply because the tortoise and 

 lizard retina have only cones, to assume that it is not true for 

 the retina of these animals. 



Gaupp ('04, p. 815) is of the opinion that in the migration of 

 the pigment in the protoplasmic processes between the visual 

 elements we have a process very similar to that seen in the 

 melanophores of the skin, where the pigment streams back and 

 forth in the processes of the cells. We have seen in the tortoise 

 eye where the optic nerve has been cut that migration of the 

 pigment, as well as contraction of the cones, still takes place, 

 which is evidence that both the pigment cell and the cone are 

 also directly sensitive to light. Garten's objection that there 

 has been no proof that pigment migration in the retina may be 

 brought about by the direct effect of light because chemical 

 stimuli could possibly play a part has, it seems to me, no weight. 



