232 LESLIE B. AREY 



changes of tlio cones occur. The .muscles innervated by the 

 trochlear and abducens nerves evidently are not essential in 

 causing an inhibition of movement. 



A few reciprocal experiments were performed by cutting all 

 the eye muscles except the dorsal oblique and the posterior 

 rectus. Although the extent of positional change was not as 

 striking as in excised eyes, it seems probable that the same 

 mechanism that controls the migration of pigment, and is pre- 

 sumably associated with the muscles innervated by the oculo- 

 motor nerve, also acts on the visual cells. 



The evidence, therefore, suggests that normally there are im- 

 pulses which travel in the optic nerve and render ineffective the 

 inhibition produced by a second set of fibers. Hence it becomes 

 a matter of interest to discover whether it is possible, by arti- 

 ficial stimulation of the severed optic nerve, to cause the visual 

 cells to move more freely than they would otherwise do. The 

 details of these experiments were similar to those previously 

 described when the migration of pigment was tested. , The cut 

 optic nerves of dark-adapted fish were stimulated in the light by 

 a weak faradic current. Both rod and cone cells assumed their 

 characteristic light positions, while in control eyes of the same 

 animals, the optic nerves of which had been cut but not stimu- 

 lated, the visual cells remained for the most part as in darkness. 



These results on the visual cells of Ameiurus, taken as a whole, 

 closely agree with those obtained from the study of retinal pig- 

 ment, and the general conclusion concerning their significance 

 is so identical with one previously stated (p. 229) that it hardly 

 needs to be repeated here. 



It was shown in a former paper (Arey, '16) that in fishes an 

 elevated temperature (25°C. ±) causes an elongation of the my- 

 oids of dark-adapted cone-visual cells, whereas a low tempera- 

 ture (0°C. ±) induces a shortening of the myoids. The rod 

 myoid exhibits a similar behavior but in a less marked degree. 



The effect of temperature was tested on the visual cells of dark- 

 adapted fish the optic nerves of which had been cut. At 5°C. 

 in the dark the cone myoid measured 18 m, the rod myoid 8 /x ± 

 (fig. 8). At 25°C. in the dark the cone myoid elongated to 25 fi, 

 the rod to 10 ^u ± (fig. 9). These results are in agreement with 



