PHYSIOLOGICAL RETINAL PROCESSES. 1051 



inner nerve fibre side of the retina is connected, the retina is positive, 

 the nerve negative. If the inner and outer side of the retina are con- 

 nected, the former is positive, the latter negative. 



If the optic nerve is connected with the eyeball, the direction of the 

 current varies with the part of the eyeball ; if with the cornea, the 

 nerve is negative ; if with the sclerotic, the nerve is positive. This is 

 probably due to the fact that when connection is made with the 

 sclerotic, the effect is the same as if it were connected with the rods ; 

 while in the case of the cornea, conduction takes place through the 

 ocular media to the inner surface of the retina. 



When light falls on the retina, this current undergoes a more or less 

 complicated variation. When the isolated retina is employed, the 

 changes are as shown in Fig. 381 ; on the action of light, a positive suc- 

 ceeded by a negative variation ; on cutting off light, a positive variation. 

 The relative amount of the variations depends on the condition of the 

 retina; in some cases the negative variation is considerable, while the 

 preceding positive phase is very slightly marked ; in other cases the 

 negative phase is not sufficient to bring the current back to the condi- 



FIG. 381. SteinerandKiihne. FIG. 382. SteinerandKuhne. FIG. 383. Steiner and Kiihne. 



tion at the time of the stimulus (Fig. 382). According to Kiihne and 

 Steiner, 1 this occurs when the retina is especially fresh, and they 

 regarded it as probably the nearest approach to the normal condition ; 

 and this seems more probable from the fact that when the whole eye- 

 ball is used the negative phase is entirely absent (Fig. 383) ; and this 

 phase is probably due to injury in removal of the retina. The retina is 

 extremely sensitive to light, and shows a change with light of an 

 intensity only just sufficient to produce sensation in the living eye. 

 The duration of the light stimulus is of influence on the variation ; the 

 three changes described also occur with instantaneous illumination, and 

 according to Fuchs 2 the positive phase is much shorter with instantaneous 

 than with longer illumination. 



Holmgren 3 found that the retinal current did not depend on visual 

 purple, and that the changes occurred in bleached eyes ; but, according to 

 Kiihne and Steiner, 4 the reaction is more intense when visual purple is 

 present, while the bleached eye showed a marked negative phase, and 

 sometimes even absence of the positive phase. These changes may, 

 however, have been due to fatigue changes other than those of visual 



1 Untersuch. a. d. physiol. Inst. d. Univ. Heidelberg, Bd. iv. S. 72. 



2 Arch.f. d. ges. Physiol., Bonn, 1894, Bd. Ivi. S. 408. 



3 Untersuch. a. d. physiol. Inst. d. Univ. Heidelberg, Bd. ii. S. 81. 



4 Ibid., Bd. iii. S. 354. 



