EFFERENT FIBERS OF THE OPTIC NERVE 215 



was performed by Hamburger ('89), whose results were corrobo- 

 rated by Fick ('91). After the optic chiasma of the frog was 

 severed, light- and dark-adaption occurred as in normal animals, 

 hence the pigment and cone cells were viewed as independent 

 structures, responding to direct stimulation, the movements of 

 which are not dependent upon the mtegrity of the optic ner\'e. 

 Arcoleo ('90) likewise foiuid that the retinal elements of the 

 pithed toad exhibited ]ihotomechanical changes. On the other 

 hand,* Nahnunacher ('93) showed that stimulation of the frog's 

 optic chiasma with salt crystals induced clianges in the cones, 

 only if the optic nerve was intact. 



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

 are capable of more or less intle])endent n\ovemeiit, but over 

 this is superhnposed a nervous (efferent) control, the nature of 

 which is not altogether evident. 



Apart from the work of Pergens ('96), who l)elieved that the 

 illumination of one eye of a fish induced cone contraction m the 

 other, practically no attempt has been made to detennine the 

 conditions under which movements of the retinal elements of 

 fishes are accomj^lished. The negative results of Ciertz ('11) 

 concerning the etfeet of electrical stuuulation of the eyes of 

 Abramis brama differ only sliglitly from those of Fujita ('11), 

 who, however, maintained that intluction shocks caused an insig- 

 nificant pigment contraction in the light-adapted eye of the 

 'Weissfisch.' 



STATEMENT OF THE PROBLEM 



It has been shown that the retinal pigment of most fishes 

 expands to a marked degree when the normal animal is brought 

 from a situation oi darkness to one of light, and that tlie pig- 

 ment again contracts when the procedure is reversed (figs. 1 and 

 4). 



The writer (Arey, '16) has also found that the pigment in the 

 excised eyes of Ameiurus, at least, is capable of expanding when 

 subjected to light, which, in this case, presumably acts as a direct 

 stunulus on the pigment cells; a contraction in darkness, how- 



