ELECTROMOTIVE ACTION IN THE EYE 



477 



f r0 g { t6f greatest in the uninjured eyeball, least, and often 

 reversed, in the isolated retina the photo-electrical variations are 

 essentially different in character and distribution (Fig. 285). 

 In the eyeball there is at the commencement of illumination a 

 positive and very slowly increasing variation, that only rises 

 abruptly at the end to its maximum. This is succeeded by an 

 increase of the current of rest which persists during the impact 

 of the light, and at the close of illumination is followed by an- 

 other, but much weaker, posi- 

 tive variation. On the other 

 hand, the posterior half of the 

 eye, as also the isolated retina, 

 yields an initial negative varia- 

 tion, followed immediately by 

 a positive phase, in which the 

 current of rest is more or less 

 augmented beyond its original 

 proportions. At cessation of 

 light there is, as in the frog, 

 another positive variation of 

 considerable magnitude. If 

 any alteration occurs in the 

 retina, the current of rest will 

 not, at the close of the first 

 negative phase, regain its ori- 

 ginal base - line during the 

 illumination while finally, in 

 fatigued or dying preparations, 

 a decrement of the negative 

 variation fails altogether, so that this phase remains as the sole 

 effect of excitation. 



Van Genderen-Stort (5) observed that the movements 

 (change of position) in the cones of the retina, as well as the 

 displacement of pigment in the retinal epithelium, were pro- 

 duced not merely by the direct illumination of that eye, but 

 also by illumination of the other ; whence it might be concluded 

 that the optic nerve contains not merely sensory but also centri- 

 fugal (retino-motor) fibres. Engelmann has shown (5) that this 

 might incite reflex alteration in the electromotivity of the eyeball. 

 Obvious variations were obtained in every instance, on leading off 



Bulbus. 



HoU- 

 schaale. 



Netzhaut. 



Netzhaut. 



