EFFERENT FIBERS OF THE OPTIC NERVE 223 



This relation should stand the physiological test imposed by 

 partial section of the optic nerve. In view of the results already 

 established, one would expect to find the pigment situated in 

 a sector adjacent to the cut to be unaffected by varying condi- 

 tions of light and darkness, while the pigment adjacent to the 

 intact portion of the nerve should expand or contract in a normal 

 manner. The description of an actual experiment will make this 

 clear. 



Experiment S.1.3S. The (Ji)tic nerve of a lij^ht-iulapted Aineiurus 

 was two-thirds severed close to the eyeball, and the fish was trans- 

 ferred to darkness. After 2 hours the eye was excised, a pointed Hap 

 of skin bein}< left attached to the eyeball to insure proper orientation. 

 In a preparation, sectioned to include portions of the retina adjacent to 

 both the cut and the uncut fibers, a striking contrast was evident 

 (fig. 7). On the uncut side the pigment was contracted maximally, 

 while on the cut side it still remained in the expanded position charac- 

 teristic of light adaption. 



I can think of no experiment that could be devised to support 

 better the view concerning the role which the optic nerve plays 

 in the movement of retinal pigment, than the one just described. 



The similar treatment of dark-adapted fi.sh which were sub- 

 jected to light did not give as decisive results as those of the 

 reciprocal set. The pigment on the 'cut' side of the eye was 

 not retained in a contracted state, but migrated to a considerable 

 extent (fig. 6). In no case, liowever, was the expansion maximal, 

 that is, involving an accunudation near the external limiting 

 membrane, but at most only a broad evenly pigmented zone was 

 formed, which was markedly in contrast with the maximal expan- 

 sion of pigment in that half of the retina adjacent to the intact 

 portion of the nerve. 



By the evidence of previous experimentation, a reason for this 

 difference in behavior is suggested. It has already been shown 

 that light is an efficient stimulus in producing expansion of the 

 pigment in excised eyes, whereas darkness does not contract 

 expanded pigment. Furthermore, after complete section of the 

 optic nerve, the pigment in certain cases still tended to migrate 

 when the animal was exposed to light. In the experiment last 

 described, it is probable that there was a more or less extensive 



