230 LESLIE U. AREY 



2. ]'isu(tl cells, 'llw luoro important oxpei'huonts, which show 

 the rohition of the optic nerve to the pigment migration of 

 Ameiiiriis, were repeated in order to discover whether a similar 

 set of relations exists between the optic nerve and the visual 

 cells. 



It will be remembered that the myoid of the cone cells of fishes 

 elongates in the dark and shortens in the light (figs. 10 and 11), 

 whereas the behavior of the rod myoid is the converse of this. 

 Moreover, in excised eyes both types of cells are stimulated to 

 movement through the direct action of light, whereas only the 

 rods show a response in darkness. 



In general it may be said that these experiments, which ex- 

 tended discontinuously throughout a period of two years, showed 

 that whenever the migration of pigment was inhibited the move- 

 ments of the rods and cones likewise failed to occur. In those 

 cases, discussed previously (p. 220), where cutting of the optic 

 nerve only partially inhibited the pigment response, the move- 

 ments of the visual cells usually were not prevented. Practically 

 all of the experiments to be described were performed in the 

 spring and autumn of 1913 before cases of incomplete control 

 were discovered. 



When the optic nerve of a dark-adapted fish was severed, and 

 the animal was subjected to light (fig. 12), the rods remained as 

 in the condition of dark adaption. The cones, on the contrary, 

 usually became more or less shortened, although they did not 

 approach the external limiting membrane as closely as when 

 exposed to light under normal conditions. 



In the converse experiment, in which operated fish were trans- 

 ferred from light to darkness, the cones showed no tendency to 

 elongate, but in some cases remained maximally shortened for 

 several hours. The rods, however, did not always retain the 

 highly elongated position characteristic of darkness, for they 

 commonly exhibited partial retraction, although the final position 

 was unmistakably that of semi-elongation. 



From these two types of experiment one striking correlation 

 is evident. The cones have a greater tendency to undergo posi- 

 tional changes in the light, while the rods behave similarly in 



