788 



NERVE 



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 dis- 

 tinguished 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. 



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

 traction follows.' 



The explanation generally given is as follows: Wherever there is an 

 increase of excitability sufficiently rapid and sufficiently large, stimula- 

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

 stimulation does not occur. Accordingly, at closure the kathode stimu- 

 lates 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 he 

 current is weak enough the break is less than a minimal 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. Heie 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 excitation, which starts from 

 the kathode, has no reigon 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-excitation. The break-excitation, 

 however, has to traverse the whole intrapolar region, and this does not 



