PHYSIOLOGICAL RETINAL PROCESSES. 1049 



of pigment, while in those killed immediately after the minute's exposure 

 there was little change from the dark position. The forward position 

 was found in frogs when respiration had been interfered with, either 

 by keeping them in closed vessels or under water, or by mixing CO 2 

 with the air breathed. In nearly all cases the lower half of the retina 

 showed a more forward position of the pigment than the upper half, 

 and Tick believes that this is due not only to the fact that the lower part 

 of the retina is usually more exposed to light, but also to a greater ten- 

 dency to forward movement. 



The influence of the nervous system on the movement of the pigment 

 will be considered in connection with the movements of the cones. 



The cones. The movement of the cones under the action of light 

 was observed later by Engelmann and van Genderen Stort. 1 While 

 investigating the recession of the pigment in frogs in the dark, it 

 was noticed that the cones were situated between the outer limbs of 

 the rods, and not close to the external limiting membrane, as had been 

 previously described. They were found to occupy the latter position in 

 frogs exposed to the light. The change was found to be constant and 

 to depend on contraction and relaxation of the inner part of the inner 

 limb of the cone, which Engelmann named the cone-myoid. In the 

 dark position, the ellipsoidal body of the cone is situated between the 

 outer limbs as far back as the retracted pigment, and connected with the 

 nucleus of the cone in the external nuclear layer by a narrow process, the 

 elongated cone-myoid. In the light position, the body of the internal limb 

 was directly continuous with the nuclear portion. The degree of lengthen- 

 ing observed was greater the more speedy the process of hardening for 

 examination, the cone-myoid having a tendency to contract on the action 

 of the hardening reagent. The greatest difference in length was observed 

 in frogs and fishes ; it was less marked, though distinctly present, in 

 birds and mammals. The contraction with light took place rapidly, the 

 maximal contraction occurring in a few minutes ; extension in the dark 

 was more gradual. In the twin cones observed in the frog's retina, only 

 one of the two showed differences of length. The degree of action of 

 the different parts of the spectrum was not very fully investigated, but 

 the violet end appears to be the most active. The influence of the 

 coloured globules found in the outer ends of the inner limbs of the cones 

 of birds and some reptiles was investigated. The degree of contraction 

 did not seem to depend on the globules, for in the tortoise, with 

 intensely coloured globules, very little, if any, change in length was 

 observed. It was found that cones with red globules were affected 

 most by green light, but, on the other hand, in the red area of the 

 dove's retina, the cones with small red globules scattered through the 

 whole length of the internal limb, shortened less actively than others in 

 green light. The cones with orange, yellow, or green globules showed in 

 general no distinctive action. 



The Rontgen rays have no influence on the movements of cones or 

 of pigment, both being found in the dark position when the retina has 

 been exposed in a dark box to their influence (Pergens, Gatti), 



Influence of nervous system, on movements. It has already 

 been stated that the bleaching of visual purple is confined to the eye 

 that is acted on by light. This is not the case with the movement 



1 Arch.f. d. gcs. PhysioL, Bonn, 1885, Bd. xxxv. S. 498; and Arch. /. Ophlh., 1888, 

 Bd. xxxiii. Abth. 3, S. 229. 



