216 LESLIE B. AREY 



ever, does not occur. In several other fishes (Abramis ; Fundulus ; 

 Carasshis) a general pigment migration could not be demon- 

 strated under these conditions even in the light, and such a lack 

 of response was accounted for by postulating the occurrence of 

 an autoanaesthetization of the pigment cells, presumably through 

 the accumulation of catabolic wastes. Furthermore, the rods of 

 the excised eyes of Ameiurus undergo movements both in the 

 Hght and in the dark, whereas the cones move in the hght only; 

 yet neither darkness nor light induces positional changes of the 

 cones in the excised eyes of certain other fishes (Abramis and 

 Fundulus). As will be seen farther on, the total absence of 

 .response in the excised eyes of these fishes throws important 

 light upon the question of a probable autoanaesthetization of the 

 retinal elements. 



If the optic nerve transmits afferent impulses exclusively, as 

 hitherto has been believed, no disturbance in the movement of 

 the retinal pigment should be introduced when the optic nerve 

 only is severed, and the retina is thereby freed from this source 

 of cranial innervation. The present paper will be devoted to 

 the description of experiments devised to test the validity of 

 this hypothesis. 



In order that the reader may better interpret the various 

 experiments about to be described, as well as the significance of 

 their progressive sequence, it is advisable to state in advance 

 the thesis which this paper aims to establish. Experimental 

 evidence will be presented which offers physiological proof for 

 the existence in Ameiurus of two distinct components of a 

 mechanism, through the balanced action of which are move- 

 ments of the visual cells and retinal pigment alone possible. One 

 component involves the efferent nerve fibers of the optic nerve, 

 whereas the second component (possibly the ciliary, autonomic, 

 nerves) is closely associated with the eye muscles. This latter 

 set of nerve fibers exerts a constant inhibition upon the move- 

 ments of the retinal elements, while the impulses in the efferent 

 optic nerve fibers, on the other hand, serve only as a block to 

 this tonic inhibition, thus allowing photomechanical responses 

 to occur. 



