EFFERENT FIBERS OF THE OPTIC NERVE 235 



Leitung in den Sinnesnerven gilt als die eigentliclie Grundlage dieses 

 Abschnittes der Physiologie. 



This statement may have been partially justified in view of 

 the diversity of stunuli reported to be potent in causing; changes 

 in the retinal elements of the frog, but even if the presence of 

 efferent fibers of the kind physiologicall}' demonstrated in Amei- 

 urus were found in all the vertebrate classes, it by no means 

 follows that the fundamental princii^Ies of sensor\' pliysiologA- 

 would be seriously endangered. In tlie normal Ameiurus the 

 efferent function does not in the least interfere witli the moA-e- 

 ments of tlic retinal elements, and stbnulathig agents which do 

 not act directly upon these cells presmnably are ineffectual in 

 producing changes. Since, moreover, there is no e\-idence of 

 any intrusion on the part of these elements upon any of the 

 sensoiy processes, our ideas of retinal physiology- scarcely re(iuire 

 modification and certainly do not demand reorganization. 



From the study of Ameiums exj^erimental jiroof has been 

 advanced sliowing the existence of an hihibitoiy mechanism, not 

 associated with the optic nei-xc. wliicli tends to ]^revent the 

 movements of pigment cells and retinal pigment. Moreo\'ei-, a 

 second mechanism associated with the optic ner\'e was dcjuon- 

 strated, the hitegrity of which is necessaiy for the accom])lish- 

 ment of ty])ical movements on the part of the retinal elements. 



It ma}^ now be fairly asked wlietlier there is any e^•idence 

 indicating the possible modus oi^ei'andi of these two systems. 

 Several schemes are readily suggested by which the facts hitherto 

 presented could be explained. If, however, one conceives of the 

 efferent fibers in the optic nerve as acti\'ely causing moA'cments 

 of the retinal elements, both hi darkness and in light, either two 

 kinds of efferent fibers must be postulated, or one movement of 

 e^ch of the elements is passive — a return to the unstimidated 

 condition — and in some way is interfered with by the mhibitoiy 

 mechanism wlien the optic nerve is cut. Nevertheless^ either 

 one of these ex]ilanations becomes discrepant wlien ai)])lied to the 

 total behavior of the retinal elements. 



Since electrical stimulation of the cut optic nerve in the light 

 induced changes in the retinal elements of an otherwise intact 



