RESPONSE OF RETINA TO STIMULUS OF LIGHT 425 



And this is again succeeded by a transitory negative effect 

 on the sudden cessation of light. These constitute the three 

 phases of retinal action-current just referred to, under con- 

 tinuous stimulus. This sequence is wrongly represented in 

 symbols as ( H h ), since the actual changes concerned are 



(- + -) 



Now in highly excitable tissues, under instantaneous 

 stimulation, we observe a sequence of response apparently 

 similar, the first being normal negative, the second positive, 

 owing partly to recovery and partly to the positive after- 

 effect ; and the last phase representing a return from this 

 positive. These three phases, therefore, though apparently 

 similar, are not really the same as those just referred to 

 under continuous stimulation, where the * medium negative 

 (positive] phase ' was the result of fatigue-reversal. The so- 

 called negative {positive) effect which alone appears on the 

 instantaneous stimulation of relatively inexcitable tissues is, 

 again, the positive response of a sub-tonic tissue to a stimulus 

 deficient in true excitatory value, already described (p. 422) 

 as case (i). Similar positive responses of vegetable tissues 

 in a sub-tonic condition under the action of light were seen 

 in fig. 240. 



We come next to the question whether or not we may 

 discover multiple responses in the retina analogous to those 

 which have already been demonstrated in the case of vege- 

 table tissues. The occurrence of such an effect has not 

 hitherto been suspected. We have seen that, under the 

 continued action of light, vegetable tissues exhibit multiple 

 responses ; and since we have found a general close analogy 

 to exist between the responses to light of the retina and of 

 these, we should expect that similar multiple responses would 

 be found in the eye also. The reason why these were not 

 hitherto detected lay in the inevitable depression of excit- 

 ability in the isolated retina or eyeball. In the retina of certain 

 fishes, where the excitability does not appear to decline so 

 rapidly, I have often obtained records of multiple responses 

 For example, in the retina of Wallago attu fish the stimulus of 



