THE RETINA 845 



This variation is the ordinary current of injury caused by the cutting 

 of the optic nerve. At this time, however, this nerve is galvanometri- 

 cally positive to the lateral zones of the fundus of the eyeball.^ Like 

 all living tissues, the retina also becomes the seat of electrical variations 

 when stimulated. Thus, the falling of light into a dark-adapted eye 

 gives rise to an electrical change which may be regarded as analogous 

 to the current of action of any motor or sensory nerve.^ While the 

 nature of this response is greatly dependent upon the strength and 

 duration of the stimulus, and the condition of the eye, it generally 

 results after a latent period of not more than 0.01 second.^ Its 

 direction is the same as that of the preexisting current of injury, 

 provided the electrodes have been applied in the same manner as 

 before. Consequently, since it passes from the fundus to the cornea 

 and thus merely intensifies the current of injury, it forms a positive 

 variation. This is succeeded by a gradual diminution and later on 

 by a second prolonged increase. Einthoven and Jolly^ who have 

 analyzed this current with the aid of the string galvanometer, 

 endeavor to explain its unusual complexity by assuming the occur- 

 rence in the retina of three distinct processes, called A, B and C. 

 The first develops more rapidly than the other two and is especially 

 marked in a light-adapted eye. When this eye is stimulated, it gives 

 rise to a negative and when darkened, to a positive potential difference. 

 The second process is less speedily initiated, and leads to a positive 

 variation on stimulation and a negative difference on darkening. 

 This process is brought into play with greatest intensity in a dark- 

 adapted eye, when it is illuminated with a moderate light. The third 

 process yields the same results as the second, but its speed of develop- 

 ment is much slower. It is not initiated in a light-adapted eye. 



When the non-polarizable electrodes are adjusted to the longitu- 

 dinal and cut surfaces of the optic nerve itself, a simple negative varia- 

 tion is obtained, presenting the same characteristics as the ordinary 

 action current of nerve. Peculiarly enough, however, this variation is 

 evoked not only when the light is flashed into the eye, but also when 

 it is withdrawn. Photo-electrical phenomena have also been observed 

 in plants when alternately darkened and lightened.^ 



1 Holmgren, Zentralbl. fiir Physiol., xi, 1897. 



2 Gotch, Jour, of Physiol., xxxi, 1904, 31. 



3 Nagel, Handb. der Physiol., 1905, iii, 103. 



4 Quart. Jour, of Exp. Physiol., i, 1908, 373. 



^ Waller, Proc. Royal Soc, London, Ixvii, 1900. 



