EXCITATION OF MUSCLE 215 



contraction limited to the immediate neighbourhood of the cathode, 

 which lasts as long as the current is passing through the muscle, and 

 is not propagated to the rest of the muscle. If the current be now 

 broken, the muscle may remain quiescent. If however the current 

 is above a certain strength, the muscle responds to the break of the 

 current with another single rapid contraction. With a current of 

 moderate strength we may get a contraction both at make and break 

 of the current, but the make- contraction may be stronger than the 

 break- contraction. Thus stimulation is caused by the make and 

 break of a constant current, the make-stimulus being more effective 

 than the break-stimulus. If the duration of the passage of the current 

 is sufficiently short, no contraction is produced at the break of the 

 current, however strong this may be. The same phenomenon of a 

 single twitch may be evoked by the passage of an induction shock. 



This is the current of momentary duration produced in the second 

 circuit of an induction-coil by the make or break of a constant current 

 in the primary. Using this mode of stimulus, it is found that the 

 contraction on break of the constant current is much stronger than 

 that on make. It must not be imagined, however, that there is 

 any contradiction between this and the fact [that [the make of a 

 constant current is a stronger stimulus than the break. 



When we put a muscle in the secondary circuit and make a current 

 in the primary, there is a current of momentary duration induced in 

 the secondary , so that there is a current made and broken through the 

 muscle ; and the same thing takes place again when the primary circuit 

 is broken. It has been shown that, when we use currents of such 

 short duration, the break stimulus is ineffective ; so in both cases, 

 whether we make or break the current in the primary circuit, we are 

 dealing with a make stimulus in the muscle. The difference in the 

 efficacy of make and break induction shocks is purely physical, and 

 depends on the fact that the current induced in the secondary coil 

 on make is of slower rise and smaller potential than that produced 

 at break. 



