EFFERENT FIBERS OF THE OPTIC NERVE 237 



nents of the optic nerve block, i.e., counteract, the tonic inhi- 

 bition produced by the second nervous mechanism, thereby allow- 

 ing conditions of light and darkness to act directly upon the 

 retinal elements.^ This would explain why electrical stunula- 

 tion of the cut optic nerves of dark-adapted fish, in the light only, 

 led to the usual changes in the retina. Although no determina- 

 tions were made, it follows that if this hypothesis is tnie, the 

 converse experiment with light-adapted fish in the dark should 

 I'esult in the assum])ti()n of the dark phase on the part of the 

 retinal elements. 



Assuming that the balanced action of a system lik(> the one 

 suggested is indeed a reality, it is eWdent, nevertheless, that 

 questions relative to its adaptive significance are not easily 

 answered. It is certainh' diflicidt to ex]:>lain the rationale of a 

 situation whereby an animal possesses a mechanism the com- 

 ponents of which act antagonistically, thus allowing photome- 

 chanical influence to be exerted un(hsturbed. 



Since stnicture and function go hand in hand, the value of 

 i:)hysiol()gical evidence always liecojues much enhanced by the 

 coexistence of a correlated structiu'al basis. The possibility of 

 double conduction in one set of nerve fibers is hardly to be con- 

 sidered, hence there arises a pertinent quer>' relative to the ana- 

 tomical proof for the existence of efferent components of the optic 

 nerve. 



Englemann ('85) first postulated 'retino-motor' nerve fibers 

 to explain certahi conditions which he asserted occurred in the 

 frog, and fiu-ther suggested that the anterior arcuate conunissure 

 of Hannover in the chiasma, tlu^ ])hysiological significance of 

 which had pre\'iously been unknown, served as an association 

 tract through which the movements of the retinal elements of 

 the two e\'es were interrelated. 



" It w ill lie remembered in the experiments on Abramis and Fundulus (p. 233) 

 that after severing the optic nerves the usual responses to light and darkness 

 were maintained. Hence it seems probable (since the retinal elements undergo 

 no changes in excised eyes) that the direct action of darkness as well as light is 

 effective, provided normal circulatory conditions are maintained. 



