PHOTOMECHANICAL RESPONSES 81 



excised eyes of fishes, whether in hght or in darkness. 

 Chloretone and urethane on the other hand had no effect. 



Arey (19166) also investigated the role of the integrity 

 of the central nervous system upon the photomechanical 

 responses with interesting results. Observations are recorded 

 in the literature which ascribe a central control over retinal 

 pigment and visual cell movements — particularly in the 

 frog. It has been claimed also that stimulating agents 

 applied to one eye will induce changes in the other ; and some 

 workers even claimed that this interrelation was independent 

 of the brain, and persisted after optic nerve section distal 

 to the chiasm. This effect upon the non-stimulated eye was 

 supposed to be brought about by what Englemann (1885) 

 called retino-motor fibers. He made the statement that 

 illuminating the skin of dark-adapted frogs would bring 

 about changes in the pigment and the cones. This observa- 

 tion, however, was not borne out by the experiments of 

 Fujita (1911) and Arey (1916a). Hamburger (1889) and 

 Fick (1891) showed that after section of the optic chiasm 

 of the frog, light and dark adaption proceeded as in animals 

 with an intact optic nerve. This suggested that photome- 

 chanical changes were responses to direct light stimulation 

 and in no way dependent upon the integrity of the optic 

 nerve. Nahmacher (1893) showed, however, that stimula- 

 tion of the optic chiasm of the frog by salt crystals caused 

 changes in the cones only if the optic nerve was intact. 



In discussing the various observations, Arey says, ''Hence 

 it seems probable that, in the frog, the retinal elements are 

 capable of more or less independent movement, but over 

 this is superimposed a nervous (efferent) control, the nature 

 of which is not altogether evident." 



Working upon three teleost fishes (Abramis, Fundulus, 

 and Ameiurus), Arey (19166) showed that in Ameiurus pig- 

 ment migration and visual cell movements failed after section 

 of the optic nerve, whereas in Abramis and Fundulus the 

 retinal photomechanical changes were not effected by similar 

 surgical interruption. Since in Ameiurus the typical photo- 



