224 LESLIE B. AREY 



decussation of fibers distal to the cut, and that these fibers, 

 although few in number, since they come from the intact portion 

 of the nerve and distribute themselves through the retina of the 

 opposite side are capable of allowing the pigment to migrate when 

 the eye is subjected to the action of light. In producing this re- 

 sult, these fibers are doubtless servient to the direct action of light 

 on the pigment cells themselves. In darkness, without the aid of 

 some independent and efficient factor, such as the direct stimulus 

 of light, it may well be that these straj^ fibers are not sufficiently 

 potent to overcome the inertia of the pigment. 



If the cause of the apparently inconsistent behavior described 

 in these two types of experiment is something of the nature of 

 that just outlined, then the evidence for the control of pigment 

 migration through the optic nerve thereby receives additional 

 support. 



It was thought that if a small cut were made through the 

 eyeball and retina, near the entrance of the optic nerve, the por- 

 tion of the retina peripheral to the cut would be freed from all 

 connections with the optic nerve, and consequently some decision 

 could be reached concerning the role of the decussating fibers 

 just considered. This method, if successful, would also corrobo- 

 rate the general conclusions which were drawn as to the control 

 of pigment migration by the optic nerve fibers. 



On the whole, the results showed quite conclusively that in 

 the earlier experiments the presence of decussating fibers had 

 caused the pigment expansion on the side of the retina adjacent 

 to the cut optic nerve, when the eye was stimulated with light. 

 Cuts about 1.0 mm. in length were made with a cataract scalpel. 

 Although the blade of the scalpel was very thin, tapering to a 

 needle-like point and the edge was of great keenness, nevertheless, 

 in many cases the retina was torn away from the contracted 

 pigment layer. The ease with which a* separation of the retina 

 and the pigment layer is effected, especially when the pigment 

 is not expanded to strengthen the union, is well known. 



In successful operations (fig. 5), the pigment lying peripheral 

 to the cut showed but little expansion, although it was often the 

 case that toward the extreme periphery of the retina, pigment 



