228 LESLIE B. AREY 



An ;itt(»inj)t was made to cut the oculomotor nerve and thus to 

 separate the sym])athetic fibers which arise with it from the brain. 

 To make a large opening through the cranium leads to operative 

 shock from which Ameiurus does not recover. Accordingly, a 

 relatively small aperture was made, through which a cataract 

 scalpel was introduced, all the nerves presumably being cut on 

 one side of the brain between the optic and the trigeminal. A 

 dark-adapted Ameiurus with severed optic nerve, when exposed 

 to the light after this treatment, showed pigment expansion. 

 This experiment, although properly controlled, was not of a 

 refined type. It is possible that disturbances other than the 

 mere section of the oculomotor nerve may have led to the ob- 

 served results. 



It is not intended that any of the experimentation in which 

 an attempt was made to locate the fibers of the inhibiting mecha- 

 nism, should be received as conclusive. The results obtained 

 from cutting muscles and from the severance of the oculomotor 

 nerve suggest that autonomic fibers are involved, and that these 

 enter with the oculomotor nerve. A strong suspicion is there- 

 fore cast upon the sympathetic fibers in connection with the 

 ciliary ganglion as the causal agents in preventing movement of 

 the retinal pigment when the optic nerve is cut. . 



No statement has previously been made concerning the nature 

 of the mechanism in the optic nerve, the integrity of which is 

 necessary for allowing positional changes of the pigment to occur. 

 If the inhibitory fibers associated with the eye muscles are con- 

 ceived as acting after the manner of a brake, it follows that the 

 optic nerve must contain active components, which in some way, 

 directly or indirectly, permit expansion and contraction of the 

 pigment. 



If the optic nerve contains fibers of an efferent nature, it is of 

 interest to discover whether these components can be made to 

 function by electrical stimulation. The description of a typical 

 experiment will best illustrate this point. 



Experiment 8.1 .23. A previously dark-adapted Ameiurus, in which 

 both optic nerves had been cut, was retained in the air, respiratory 

 water being supplied through a tube. The peripheral end of the cut 



