186 ELECTRO-PHYSIOLOGY CHAP. 



If a strong current is kept closed until every trace of 

 persistent shortening has vanished, the muscle will not resume 

 its natural length directly the break twitch has expired, but 

 remains persistently shortened (" persistent opening contraction ") ; 

 the closure of a homodromous current in this case produces 

 not a shortening, but an elongation of the muscle ; it is easy 

 to show that not merely the height of the opening twitch, 

 but the magnitude of the persistent opening contraction also, 

 increase up to a certain limit with the duration of the previous 

 passage of current. The twitch entirely fails to appear, both at 

 closure and opening, with diminished excitability of the muscle, 

 and only the persistent contraction marks the effects of excita- 

 tion. The muscle then shortens when the current is opened, 

 remains contracted for some time, and lengthens instantaneously 



7 10 30 40 SO 100 ISO 1OO 3OO 4OO 



FIG. 78. Series of curves of twitches from the sartorius, fixed by its belly, in the double myograph. A', 

 Kathodic ; A, anodic half. The figures correspond with the rheochord resistance. Effect of 

 increasing strength of current (s, closure ; o, opening). 



upon closure of the homodromous current. Thus, as regards 

 striated muscle, three chief forms of contraction may be dis- 

 tinguished at the opening as at the closing excitation : (i) the 

 simple twitch, (ii) twitch immediately followed by persistent 

 shortening, (iii) persistent contraction without previous twitch. 

 Of these, (i) corresponds with the weakest degree of excitation, 

 (iii) is a fatigue effect. It is evident that Wundt studied only 

 the third form in his experiments on the opening excitation ; i.e. 

 he says : "If the circuit is closed for a long period, contraction 

 will follow upon breaking it ; this occurs much more slowly than 

 contraction in a twitch ; it remains some time at its maximum, 

 and only gradually gives way to elongation " (8, p. 142). Against 

 this it must be observed that even when the current has been 

 passing for hours, a definite twitch will follow on breaking it, 

 provided that excitability and conductivity are preserved as far 

 as possible. 



