EFFERENT FIBERS OF THE OPTIC NERVE 233 



those obtained with normal animals. The effective action of 

 high temperature in causing an elongation of the cone cells is 

 noteworthy, since without the aid of temperature no change in 

 the position of the cones occurred under these conditions. 



b. Experimentation upon Abratnis and Fundulus 



Having presented evidence which indicates that the optic nerve 

 fibers of .\meiurus are not all afferent in function, the question 

 arises as to the occurrence of this condition among other fishes 

 as well as among other classes of vertebrates. A series of exiieri- 

 ments was carried out upon Abramis and Fundulus in which the 

 effect produced on the retinal oh^nents liy the severance of the 

 optic nerve was ti-ied. 



Both in light and in darkness the retinal ])igment and the cone 

 cells of Abramis underwent movements which were essentially 

 normal, and hence independent of the cut nerve. Both in the 

 cone myoid, which is capable of a 90 per cent retraction, and in 

 the pigment, which exhil)its extreme conditions of contraction 

 and expansion, was an inde])endent movement strikingly (ex- 

 hibited. The stain used in' niiiknig these preparations did not 

 demonstrate the rods to advantage, except in one instance where 

 a light-adapted fish had been subjected to darkness. In this 

 case the rods were shortened, occui>ying the characteristic dark 

 position. 



A\'hen all the eye muscles and blood vessels of dark-adapted 

 Abramis were cut, the eye remaining attached to the body by 

 the optic nerve only, no marked change in the position of the 

 pigment or cones accompanied a removal into the light. Since 

 the pigment and cones show no mo\'ements when the eyes are 

 com])letely excised and exposed to light or darkness, these experi- 

 ments fa\'or the view that the absence of mo^'ement in such cases 

 is not due to an inhil)ition tlirough the optic ner\'e but to an 

 int(M-i-u])tion in tlie vascular supply." 



' 111 several instances a large blood vessel that lies near the optic nerve was 

 accidentally cut in experiments where the optic nerve only was being severed, 

 yet the changes in the retinal elements occurred as before. This further sup- 

 ports the view that the blood supply to the retina by means of vessels in con- 

 nection with the eye muscles is important in allowing the positional changes 

 to occur. 



