EFFERENT FIBERS OF THE OPTIC NERVE 239 



entiated portion of the brain. Johnston ('06) and Herrick ('15) 

 have assigned the optic nerve proper to the central system of 

 tracts of the somatic afferent division. From the conchision 

 previously drawn concerning the function of the efferent o])tic 

 nerv-e fibers, it is evident that they may, at least with as much 

 propriety, tentatively be called visceral efferent elements. 



SUMMARY 



When the optic ner\'e only of Ameiurus is severed, the rods, 

 cones and retinal pigment fail to execute their characteri^tic 

 photf)mechanical responses. In other words, the movements of 

 the retbial elements dojiend upon the maintenance of the integ- 

 rity of this nen-e. After hemisection of the nen-e, movements 

 of the elements occur only in tliiit region of the retina adjacent 

 to its intact side. It can not only be shown (since essentially 

 normal responses occur m excised eyes and in eyes attached to 

 the body by the o])tic nerve alone) that a second mech;inism 

 exists in association with the muscles innen'ated by the oculo- 

 motor nerve whidi inhil)its these movements when the o]Mic 

 nerve is cut, but also that electrical stimulation of the ])(Mi])h(Mai 

 stump of the optic nerve can o\'ercome this inhibition. 



Hence experunental evidence has been advanced which offers 

 physiological ])roof for the existence of functional efferent nerve 

 fibers in the optic nerve of Ameiurus. Only by the balanced 

 interaction of these elements with a second extrinsic set of nerve 

 fibers (possibly the ciliaiy ner\'es), which independently exert 

 an inhibitory effect upon the retinal elements, are normal photo- 

 mechanical movements of the rods, cones, and retinal pigment 

 accomplished. Although light is ineffectual after section of the 

 optic ner\'e, temi^eratui-e ])roduces essentially nonnal res]ionses 

 in both the pigment and ^'isual cells. It is probable that efferent 

 ijui)ulses in the optic nerve fibers do not directly stimulate the 

 retinal elements to motion, but rather such impulses have an 

 indirect action, possibly by counteracting, that is, blocking, the 

 tonic inhibition exerted by the second system of nerve fibers. 

 If these efferent optic nerve fibers fit at all into the scheme of 



