ix ELECTRICAL EXCITATION OF NERVE 213 



tion completely, there is merely a diminution in height of the 

 twitches. In currents of brief duration the inhibition has not 

 time to develop adequately and produce a breach ; unless, at all 

 events, the currents employed are excessively strong. This is 

 doubtless the reason that the breach is not produced as readily 

 with break as with make induction currents (Tigerstedt, 54), at 

 least in cases in which the primary circuit is fully opened. 



If we accept Tick's explanation of the cause of the breach, 

 the reappearance of the twitches, and their rise above the initial 

 maximum (" supramaximal contractions "), demand a special 

 interpretation, more particularly when the effects of ascending 

 excitation with the constant current is compared with that of 

 single induction shocks, from the point of view of Pflliger's law of 

 contraction. In the former, the make twitch never reappears 

 after the third stage, whatever the augmentation of current 

 intensity, the break excitation alone being effective. Presumably 

 the twitches which appear beyond the breach on stimulating with 

 ascending impacts, or with single ascending induction currents, 

 may be viewed as break twitches. These, as we have said, begin to 

 increase again after the breach, and with protracted rise of stimulus 

 may gradually reach the same height as before. In some cases, 

 but not always, the twitches rise with increased strength of 

 current beyond the first maximum, and reach a considerably 

 greater height (i.e. are "supramaximal"); Tigerstedt (I.e. p. 22) 

 has shown that when supramaximal twitches do not appear even 

 with the coils pushed home, it is quite easy to call them out if 

 the nerve is further excited with uniform direction of current at 

 the same rhythm. Whether this is due to a kind of summation 

 of effects, or to other changes in the nerve produced by current, 

 must for the moment be left undetermined. It is easy to see 

 that the appearance of supramaximal twitches with brief 

 descending currents, as described above, can be interpreted 

 on the same principle. On the theory of Pick we have here 

 only summation of the excitations produced by the rise and 

 fall of current. With descending currents we know that ex- 

 citation proceeds from the pole proximal to the muscle. On 

 its passage to the muscle it therefore encounters no inhibition, 

 and arrives with undiminished strength. But when a (break) 

 excitation starts from the positive pole of the induction current, 

 it has a longer course than the make excitation, and reaches 



