in ELECTRICAL EXCITATION OF MUSCLE 185 



till much later that this effect also vanishes. In every such 

 case striated muscle then reacts from the beginning exactly 

 like smooth molluscan muscle ; there is, as a rule, no twitch at 

 closure, only a more or less considerable sustained contraction, 

 so that in this particular also there is agreement between _ 

 fatigued striated and normal smooth muscle. Taken in con- 

 junction with previous evidence, the persistent closure contraction 

 shows indisputably that the electrical current sets up a process of 

 excitation in striated, as in smooth, muscle, throughout the duration 

 of its passage. 



The effect of duration of current is even more striking in the 

 opening excitation than at closure, so that the influence of cur- 

 rent intensity is relatively at a discount. With low current 

 intensity, and short duration of closure, there will be no opening 

 excitation ; currents to the closure of which a curarised muscle 

 responds with maximal twitches and strong sustained contraction, 

 often provoke no trace of visible excitation when they are broken, 

 or in the most favourable cases a weak opening twitch may 

 occur after prolonged closure only. Although, on the other hand, 

 strong currents will often effect an obvious break excitation 

 after even a short closure, it is not primarily the intensity of the 

 current which causes the break effect, but the duration of its 

 passage. The same alterations as may be observed in the curve of 

 the closure contraction with increasing intensity of current, appear 

 again in the curve of the opening contraction, if the passage of 

 current which precedes it has been of long enough duration (24). 



The simplest change of form with which a striated muscle 

 reacts to the opening excitation is again a (break) twitch; contrac- 

 tion occurs quickly at the moment the circuit is opened, and the 

 muscle almost as quickly returns to its normal resting pro- 

 portions, so that curves are produced analogous to those of the 

 closure of weaker currents. But the opening twitch only occurs 

 in this simple form when the muscle is highly excitable, the 

 current not too strong, and the duration of closure not unduly 

 lengthened. Strong currents almost regularly produce more or 

 less extended (tetanic) opening twitches, which always appear to 

 be antagonistic to the previous persistent closure contraction, 

 since the ascending shoulder of the curve rises from the line 

 of the persistent contraction as its abscissa, while the descending 

 portion drops to the original abscissa line (Fig. 78). 



