598 A MANUAL OF PHYSIOLOGY 



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. Accord- 

 ingly, 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- 

 mal stimulus. No sensible change of conductivity is caused by 

 weak currents, which suffices to explain (2). 



With a ' medium ' current^ contraction occurs at make and break 

 with both directions. Here the break excitation is effective as well 

 as the make. With anode next the muscle (ascending current), there 

 is of course nothing to prevent the opening excitation, which starts 

 at the anode, from passing down the nerve and causing contraction ; 

 and since there is no block around the anode or in the intrapolar 

 region with ' medium ' currents, there is nothing to keep the closing 

 (kathodic) excitation from reaching the muscle too. With the 

 kathode next the muscle (descending current), the closing excita- 

 tion, which starts from the kathode, has no region of diminished 

 conductivity to pass through, nor has the opening (anodic) excitation, 

 for the kathodic block, caused by moderately strong currents, is 

 removed as soon as the current is broken. 



With ' strong ' currents there are only two cases of contraction 

 out of the four, just as with * weak,' but for very different reasons. 

 There is a break-contraction with ascending, and a make-contraction 

 with descending current. With ascending current the anode is next 

 the muscle, and the break-excitation starting there has nothing to 

 hinder its course. The make-excitation, although as strong or 

 stronger, has to pass through the whole intrapolar region and over 

 the anode, and here the conductivity is depressed and the nerve- 

 impulse blocked. With descending current the kathode is next the 

 muscle, and there is no hindrance to the passage of the make excita- 

 tion. The break-excitation, however, has to traverse the intrapolar 

 region, and the anodic end of this area has a smaller conductivity 

 immediately after opening than during the flow, while the kathodic 

 end does not at once, after a strong current, become passable. The 

 break-excitation, accordingly, cannot get through to the muscle. 



In all these cases of complete or partial block, during or after the 

 flow of a constant current, the progress of the nerve-impulse, its 



