THE POSITIVE RESPONSE 193 



* 



the excitatory response is independent of the positive 

 impulse. The physiological character of the negative 

 impulse has, moreover, been fully demonstrated by various 

 crucial experiments described in the last chapter. 



For deciding the question as to the physical or physio- 

 logical character of the positive impulse, I employed the 

 test of local application of cold on the velocity of trans- 

 mission. It was found that while moderate application 

 of cold delayed the transmission of the negative impulse, 

 it had little effect in retarding the transmission of the 

 positive. Further test by the physiological block induced 

 by excessive cold was, however, less satisfactory. This 

 block not only arrested the negative impulse, but sometimes 

 brought about an apparent abolition of the positive also. 

 This latter result cannot, however, be taken as decisive. 

 The amplitude of positive response is naturally small ; 

 under unfavourable circumstances it may have become 

 so diminished as to pass unnoticed. In these circumstances 

 the question of the character of the positive impulse may 

 for the present be left an open one. 



Masking and Unmasking of the Positive Effect 



It has been shown that as the distance of the point of 

 application of stimulus is reduced, the transmission period 

 of the negative impulse is reduced at a greater rate than 

 that of the positive. When the point of application of 

 stimulus is at some distance, the negative impulse lags con- 

 siderably behind the positive ; but as the distance is reduced, 

 the lag tends to disappear. It thus happens that when the 

 distance of application is sufficiently reduced, the positive 

 effect is masked by the predominant negative. 



In an experiment with Biophytum, electric stimulus was 

 applied at a distance of 30 mm. from the responding leaflet. 

 The positive response was initiated 2 seconds after the 

 appHcation of the stimulus and persisted for a further period 



