NERVE 685 



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 con- 

 ductivity 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 

 gradual weakening, and final extinction can be very well shown by 

 means of the action stream (p. 719). 



The above formula can only be verified upon isolated nerves, and, 

 even for these, exceptional results are apt to be obtained as soon as 

 the nerves begin to die. 



A formula similar to the law of contraction has been shown to 

 hold for the inhibitory fibres of the vagus (Bonders), ' inhibition ' 

 being substituted for ' contraction.' There is also some evidence 

 that a similar law obtains for sensory nerves. 



It is not difficult to see that with currents of brief duration the 

 break follows so quickly on the make that interference of their 

 opposed effects may occur. This is the reason or, at least, one 

 reason why, above a certain frequency, a muscle or nerve ceases 

 to respond to all of a series of rapidly recurring electrical stimuli 

 (p. 658). It is also the reason why, with single very brief stimuli, 

 a greater current intensity must be employed in order to cause 

 excitation than when the duration of the stimulating current is 

 greater (Wood worth, Lucas). Not enough weight has been given 

 to this circumstance by some of the writers, who, in attacking 

 du Bois - Reymond's law of the dependence of excitation upon 

 variation in current density (p. 636), have sought to establish a 

 relation between the excitatory effect and some such factor as 

 current strength, the quantity of electricity passed, or the electrical 

 energy expended. 



