EFFERENT FIBERS OF THE OPTIC NERVE 225 



expansion was again found. Since it is probable that the fibers 

 of the nerve-fiber layer do not have a precise radial distribution, 

 the latter condition is readily explained by assuming the presence 

 in this peripheral expanded region of intact nerve fibers, which, 

 near the fundus, bordered on the incision. 



The decrease in intraocular pressure necessitated by cutting 

 the eyeball has no effect on the activity of the pigment, for nor- 

 mal animals whose corneas have been punctured show typical 

 responses. 



One is driven to the conclusion, by all the experiments hereto- 

 fore described, that there must be some mechanism, either in the 

 eye muscles or in the blood vessels of the eye, that exerts an inhi- 

 bition on the movements of the retinal pigment, for proper con- 

 trol experiments have eliminated the skin as a possible factor. 



If this supposition is true, the retinal pigment should undergo 

 expansion when all the eye muscles and blood vessels of a dark- 

 adapted fish are cut, and the eye, connected to the bodj^ by the 

 optic nerve only, is subjected to light. This condition was indeed 

 realized, the pigment distribution being essentially like that in 

 totally excised eyes. The same experiment under reversed light 

 conditions did not result in a contraction of the pigment. As in 

 the similar failure of excised eyes to show contracted pigment 

 when subjected to darkness, I believe there is a strong proba- 

 bility of an anaesthetic action on the pigment cells due to the 

 accumulation of catabolic waste. If the vascular circulation 

 could be preserved, it is probable that the pigment cells would 

 contract in an experiment of this kind. Possibly with artificial 

 circulation the pigment of an excised eye would also contract in 

 the dark. 



The next step was to discover whether the presence of certain 

 eye muscles could be correlated with the inhibition of the pigment 

 response. \Mien dark-adajDted Ameiurus, having the optic nerves 

 cut, were brought into the light and the dorsal oblique and pos- 

 terior rectus muscles (those innervated by trochlear and abducens 

 nerves respectively) were severed, no pigment migration occurred. 

 It is evident, therefore, that the inhibiting mechanism does not 

 involve these muscles alone. Reciprocally, the dorsal, ventral 



