EXPLANATION OF THE LAW OF PULSATIONS. 143 



fore, unable to reach tlie muscle, so that pulsation is 

 necessarily absent on the closing of the current. 



In order to apply the corresponding case to the 

 opening of a descending current, the help of another 

 hypothesis is required, according to which the great 

 modification which follows the disappearance of katelec- 

 trotonus, and which so greatly decreases the excitability, 

 also involves a hindrance to transmission. This assump- 

 tion has not yet been experimentally proved ; proof is 

 indeed difficult, on account of the ephemeral charac- 

 ter of the modifications. The similarity of negative 

 modification to anelectrotonus, both decreasing the 

 excitability, favours the hypothesis that in negative 

 modification also an obstacle is afforded to transmission. 

 According to this view, the case is the same on the 

 opening of a descending current as on the closing of an 

 ascending current. According to Pfliiger's hypothesis 

 excitement occurs on the opening of a current only in 

 that portion of the nerve at which anelectrotonus dis- 

 appears. This, in the case of a descending current, 

 is the upper portion of the nerve. In order to reach 

 the muscle thence, the excitement would have to tra- 

 verse the lower portion, which is at the same time taken 

 possession of by a strong negative modification, and this 

 prevents propagation of the excitement; no opening 

 pulsations, therefore, occur in the case of the descend- 

 ing current. 



Pfliiger supported his hypothesis by the following 

 experiment. Mention has already been made of the 

 so-called Eitter's tetanus, which intervenes when a 

 current which has traversed a nerve for some time is 

 interrupted. According to Pfliiger's hypothesis, this 

 excitement should also be located on the side of the 



