372 LESLIE B. AREY 



or whether it is accompHshed through some nervous mechanism 

 related to the cerebral ganglion. It is true that the probability 

 is against the existence of nervous connections with the pigment 

 cells, since all previous researches have failed to demonstrate 

 structures of this kind, yet it is entirely conceivable that such 

 neuro-fibrils may exist and have hitherto escaped detection; 

 indeed, the extreme difficulty with which the neuro-fibrils of 

 the sensory cells are demonstrated, makes this possibility all 

 the more real. Even if a direct influence of light can be proved, 

 the existence of such neuro-fibrils is not precluded, nor yet 

 their cooperation in effecting the migration of pigment; but it 

 does render this event less likely, and definitely shows, more- 

 over, that whatever may happen in the normal animal, a direct 

 action of light on the pigment cell is a demonstrable phenomenon. 

 If the direct action of light can not be shown to induce migra- 

 tory movements of the pigment in excised eyes, three possibilities 

 exist: (1) fibers in connection with the pigment cell perform 

 double conduction, transferring afferent impulses to the cere- 

 bral ganglion and efferent impulses back to the retina; (2) affer- 

 ent impulses travel in the optic nerve to the cerebral ganglion 

 while efferent impulses return by hypothetical nerves to the 

 pigment cells, or (3) the optic nerve is of a mixed nature, possess- 

 ing both afferent and efferent components. Since, however, 

 the existence of double conduction in nerves is a mere postula- 

 tion, the first of the three possibilities may be safely eliminated. 

 There is, nevertheless, still another way of viewing the situa- 

 tion, and one that demands serious consideration. The metabo- 

 lism of an organ isolated from the body to which it belongs is, 

 of necessity, fundamentally altered. Not only is its supply 

 of nutriment cut off, but what is more serious in a short experi- 

 ment, the elimination of waste is not adequately provided for. 

 If isolated pigment cells do not respond to the direct action 

 of light, the reason may easily be ascribable to an autoanaes- 

 thetization caused by the accumulation of the cell's own catabolic 

 products. The probability of this course of events was suggested 

 in some experiments upon the eyes of fishes (Arey, 16^; IG*"). 

 Moreover, it will be shown in another part of this paper (p. 378) 



