VII 



EETINAL EXCITATION 



345 



retina due to the protoplasmic movements observed under the 

 action of light. 



The contraction of the outer limbs of the rods, described by 

 Angelucci (1884) and his pupils, and the contraction of the inner 

 segments, as observed by Gradenigo (1885), were subsequently 

 disputed by van Genderen-Stort, by Greeff, and by Chiarini from 

 exact measurement and comparison. According to van Genderen- 



Fio. 168. Vertical section- of temporal half of retina of Leuciscus aula, fixed in Flemming's fluid. 

 (P. Chiarini.) A, after keeping the animal 24 hours in the dark ; B, after exposing it for 

 6 hours in direct sunlight ; C, after exposing it for 5 hours in direct sunlight, and subse- 

 quently for 1 hour in total darkness. 



Stort, Greeff, and Chiarini, when a frog is exposed to light the 

 inner limbs of the rods merely suffer a passive change of form, 

 due to pressure from the ellipsoidal bodies of the cones which 

 approach the external limiting membrane (Fig. 169). 



Another important fact was discovered by Engelmann (1885), 

 who found that the movements of the cones and pigment-cells of 

 the retina under light are directly dependent on the nervous 

 system. He saw that when one eye only of the frog was exposed 



