MOVEMENTS IN THE VISUAL CELLS 163 



ture (3° to 28°C.) were not observed. Individual cells vary more 

 or less in the height at which they are situated above the exter- 

 nal limiting membrane, yet no constant differences of significant 

 amount could be correlated with definite temperatures. With 

 these results should be compared Garten's ('07) denial of a 

 change in the position of the cones of the 'salamander' through 

 the influence of light, such as Angelucci ('90) had previously 

 claimed. Stort ('87), however, described movements in both the 

 rods and the cones of Triton. 



C. EXPERIMENTATION UPON EXCISED EYES 



a. Effect of light and darkness 



The results of a number of investigators since the first work 

 of Englemann ('85) have indicated that the retinal elements of 

 some vertebrates, and especially the frog, are subject to a nervous 

 control, the action of which is not well understood. 



The role which the nervous system plays either in producing 

 or in assisting the movements of the various retinal elements 

 is hard to demonstrate. Experimentation involving the direct 

 action of light on excised eyes can not be expected to solve the 

 problem decisively, for if no movements result, autoanaestheti- 

 zation, or some similar disturbance due to the interrupted blood 

 supply, may be the real cause. If, on the other hand, responses 

 are called forth by direct stimulation, it by no means follows 

 that a similar phenomenon necessarily occurs in the living animal, 

 any more than a demonstration of the direct stimulation of 

 muscle fibers proves that this rather than a nervous impulse is 

 the normal method of muscle stimulation. The limitations which 

 restrict a wide interpretation of results, however, do not lessen 

 the interest involved in determining the extent to which the 

 retinal elements can be direct!}' stimulated. 



Hamburger ('89) maintained that the cones and retinal pig- 

 ment in excised eyes of the frog assumed the positions characteris- 

 tic of light or darkness according to the conditions of the experi- 

 ment. Dittler ('07) working on isolated frog's retinas obtained 



