566 



PHYSIOLOGY 



When light falls upon the retina the cones are retracted, and lie close upon 

 the external limiting membrane ; whereas in an eye that has been kept in the 

 dark they extend down between the rods as far as the pigmented layer. 



l. 







II. 



III. 



FIG. 293. Electrical variation in frog's eye as recorded by the string galvanometer. 



(EINTHOVEN and JOLLY.) 



I, on exposure to a single flash ; II, on exposure to light of moderate 

 duration ; III, effect on a light eye of momentary darkening. 



The falling of light on the retina is also accompanied by an electrical change, which 

 may be regarded as analogous to the current of action in nerve. It is, however, much 

 more complicated than the latter. The eyeball of the frog led off from its anterior and 

 posterior surfaces shows a current directed in the eyeball from behind forwards (the 

 resting or demarcation current). It was first shown by Holmgren that this resting 

 current undergoes modification when light is allowed to fall on the eyeball. Of late 

 years the nature of this modification has been studied especially by Waller with the 

 galvanometer, by Gotch with the capillary electrometer, and by Einthoven and Jolly 

 with the string galvanometer. The nature of the response varies according to the 



