EFFERENT FIBERS OF THE OPTIC NERVE 231 



darkness. The end result of this tendency towards movement, 

 nevertheless, is identical in both kinds of \'isual cells, for a shorten- 

 ing of the myoid results in either case. This would suggest that 

 the process of retraction, as in simple contractile tissue in general, 

 is more vigorous than is elongation. This behavior of the cones 

 is comparable to that observed in excised ej'esof Ameiurus, 

 where these cells shortened in the light but remained unchanged 

 in darkness; the rods of excised eyes, on the contrarj^ moved both 

 in light and in darkness. The posulation of a more vigorous 

 I'esponse in retraction offers an explanation for the obser\'ed facts, 

 for under such conditions the inhibitoiy mechanism would be 

 less likely to check com])lotely the positional changes of these 

 elements. 



Reference has been made (p. 223) to experiiuonts in wliicli the 

 optic nerves of light -ada])tod fish were ])artly severed. \\'hen 

 these annuals were transferred into the dark, the retinal ]:)ig- 

 ment showed distinct areas of expansion and contraction, corre- 

 sponding respecti\'ely to the cut and uncut sides of the ojitic 

 nerve. In one such jireparation which had been stained, the 

 position of the cone cells was also obser\(Hl to \'ar>' with respect 

 to the same areas. Where the pigment lia<l undergone move- 

 ment, the cones were elongated, whereas in th(> half of the retina 

 adjacent to the cut fibers the cones remained inaxunally shortened. 



A control experunent, in which the light-ada])ted eye, con- 

 nected to the bod}' by the optic ner\'e only, was subjected to 

 darkness, showed that the rod cells had shortened although not 

 always completely. The cones, as in excised eyes, did not change 

 their positions in the least. There is no evidence that the optic 

 nerve of itself ]irohil)its the inoveinents of the rod and cone 

 cells. 



The results which ha\'e thus far been outlined indicate that, 

 just as with the retinal pigment, there is a mechanism asso- 

 ciated with the muscles or blood vessels of the eye which tends 

 to inhibit mo\-ements of the visual cells, ^^'^len the dorsal 

 oblique and posterior rectus muscles are cut, and after section 

 of the optic nerve the dark-adapted Aineiurus is exposed to 

 light, no changes in the position of the rods and only incomplete 



