728 COMPARATIVE ELECTRO-PHYSIOLOGY 



to the greater excitability induced by the action of the anode, 

 or to both. 



This summary of results will conclude with a brief 

 account of the demonstration of the physico-physiological 

 nature of the basis of sensation. The effect of a single 

 stimulus has been shown to consist of two different waves 

 sent out from the point stimulated, of which the hydro- 

 positive travels with a greater velocity than the true excita- 

 tory or negative. If the stimulus applied, moreover, be 

 feeble, the positive wave alone will be transmitted. If the 

 stimulus, again, be very strong and the path of conduction 

 short, one wave will be superposed over the other, the nega- 

 tive masking the positive. The two waves, however, may 

 be separated from each other by inducing a depression of the 

 conductivity of the nerve, when the negative will be made to 

 lag behind the positive. By the suppression of the negative, 

 owing to sufficient reduction of conductivity, the positive 

 may be made to arrive at the responding point alone. 



Nervous impulses have thus been shown to be of two 

 different kinds, positive and negative, and contrary to the 

 universal assumption that the nerve gives no visible indication 

 of its state of excitation, it has been shown that these are 

 accompanied by waves of expansion and contraction respec- 

 tively. In addition to these visible mechanical expressions, 

 we have also the concomitant electrical expressions of 

 galvanometric positivity and negativity. I have been able, 

 moreover, to identify the wave of expansion as the vehicle 

 of that change which gives rise to the positive tone of 

 sensation, which may be described as pleasurable or at 

 least riot unpleasurable. The negative or contractile wave, 

 similarly, has been shown to be doloriferous. These two 

 waves we saw to be separable from each other, whenever the 

 conducting nerve was sufficiently long. Thus, when the sole 

 of the foot receives a smart stroke from a rod, two different 

 impulses are sent out, first the positive or sensation of con- 

 tact, which is not unpleasurable, followed by the negative, 

 with its different and painful tone of sensation. In various 



