424 COMPARATIVE ELECTRO-PHYSIOLOGY 



This is the place in which to refer to various anomalous 

 effects observed by previous investigators, and to offer satis- 

 factory explanations of them from the results which I have 

 already demonstrated. I shall therefore give a brief sum- 

 mary of these from the admirable account of Biedermann. 1 



(a) 'In light, frogs i.e. such as have been exposed for 

 hours to the full effect of daylight the positive fore-swing 

 of the negative variation concomitant with the impact of 

 light is entirely wanting, or appears as a trace only.' Since 

 t\\.\s positive variation of the existing current really means, as 

 I have already shown, the ' true excitatory negative/ and the 

 negative variation conversely, the occurrence of positive 

 response, this observation means that a frog's eye, previously 

 exposed for a long time to light, gives positive response. 



This, then, is a simple instance of the reversal of response 

 from negative to positive under fatigue, which I have already 

 dealt with above as case (3). 



(b} ' The fact that the three phases of the retinal action 

 current, due to transitory illumination, appear in sensitive 

 preparations, even when, as with the electric spark, the 

 impact of light is momentary, shows that the medium nega- 

 tive (positive) phase must not be regarded merely as the 

 consequence of permanent illumination, since it is just this 

 phase which alone appears in less excitable preparations with 

 instantaneous light stimuli.' 



With regard to this it may be said that the medium 

 positive phase here referred to, as given by the excitable 

 retina under continuous stimulation, is not the same as that 

 positive response which * alone appears in less excitable pre- 

 parations with instantaneous light stimuli.' The former is 

 due to fatigue-reversal, while the latter is an instance of the 

 abnormal positive response of a sub-tonic tissue to feeble 

 stimulus. This will be understood from the following con- 

 siderations. In the retina, under continuous stimulation, the 

 first phase of response is the true excitatory negative. This 

 gives place to a second, or positive, due to fatigue-decline. 



1 Biedermann, Electro- Physiology (English translation), vol. ii. pp. 474-477. 



