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A MANUAL OF PHYSIOLOGY 



alterations of conductivity i.e., of the power of a portion of the 

 nerve to conduct an impulse set up elsewhere are also produced by 

 the constant current, which even outlast its flow. For all currents 

 except the weakest the conductivity at the kathode and in its 

 neighbourhood is diminished, and with currents still only moderately 

 strong the block deepens into utter impassability. The conductivity 

 at the anode is, during all this stage, but little affected, and is at 

 any rate much higher than at the kathode, so that at the time of 

 full kathodic block the nerve-impulse still freely passes through the 

 region around the positive pole. With still stronger currents the 

 conductivity here, too, begins to diminish, until at last the anode 

 is also blocked ; but this is to be looked upon as merely an extension 

 of the defect of conductivity which has been creeping along the 

 intrapolar area from the kathode. After the opening of the current, 

 the relation between kathodic and anodic conductivity is reversed, 

 for now the post-kathodic region conducts the nerve-impulse rela- 

 tively better than the post-anodic. 



The above facts serve to explain the manner in which the effects 

 of stimulation of a nerve with the constant current vary with the 

 strength and direction of the stream. These effects, so far as the 

 contraction of the muscles supplied by the nerve is concerned, have 

 been formulated in what has been somewhat loosely termed the 

 law of contraction. In this formula the direction of the current 

 in the nerve is commonly distinguished by a thoroughly bad but 

 now ingrained phraseology, as ascending when the anode is next the 

 muscle, and descending when the kathode is next the muscle. 



Law of Contraction. 



Here M means ' make,' B, ' break,' of the current ; C means ' con- 

 traction follows.' 



The explanation generally given of the facts summed up in the 

 ' law of contraction ' is as follows : Wherever there is an increase 

 of excitability sufficiently rapid and sufficiently large, stimulation 

 is supposed to take place ; where there is a fall of excitability, 

 stimulation does not occur. Accordingly, at closure the kathode 

 stimulates the anode does not ; while at opening, the anode, at 

 which the depressed excitability jumps up to normal or more, is 

 the stimulating pole ; the kathode, at which it declines to normal 

 or under it, is inactive. 



With a weak current, (i) contraction only occurs at make, and 

 (2) the direction of the current is indifferent. The explanation of 

 the first fact is that the make is a stronger stimulus than the break, 

 and when the current is weak enough the break is less than a mini- 



