in ELECTRICAL EXCITATION OF MUSCLE 187 



Since the sluggish, smooth muscles do not yield any twitch, 

 it is self-evident that at break, as at make, of a constant current, 

 the change of form will always correspond in character with 

 the more or less pronounced persistent contraction only. If 

 experiments are tried with the adductor muscle of anodonta^ 

 when free of tonus, and as relaxed as possible, somewhat strong 

 currents, and a long closure, will be required to produce a distinct 

 opening contraction, the curve of which then appears superposed 

 upon the curve of the closure contraction near its summit 

 in consequence of the slow relaxation of the muscle (Fig. 

 75, 0). On the ureter of the rabbit also, Engelmann ascertained 

 that in order to produce a break contraction, the closure must 

 exceed a certain duration. It is arrived at earlier with strong 

 than with weak currents, in proportion with the increase of 

 excitability. " With greater strength of current, and higher 

 excitability, an opening contraction may occur even after a 

 closure of less than ^ sec. ; with currents of lower inten- 

 sity, and with diminished excitability, a closure of 30 60 

 sees, is not seldom required." For the rest given the same 

 current, and a certain degree of excitability the total duration 

 of the break contraction increases up to a certain limit, with 

 increasing duration of closure. Accordingly, both on opening and 

 on closing the constant current, a persistent excitation will be 

 produced, not merely in smooth muscle, but in striated muscle 

 also, the magnitude of which depends, in the first case, mainly on 

 current intensity, while in the second it is also to a considerable 

 degree dependent upon the duration of passage of the current. 



The reaction of smooth molluscan muscle that has shortened 

 at a certain degree of tonus, is quite characteristic with regard to 

 the appearance of the break excitation. We have already seen 

 that in each such case the closure of a battery current, if effective at 

 all, produces only a very weak excitation. As the break stimulus, 

 both in striated and in smooth muscle (free of tonus), always 

 produces a much smaller effect than the make stimulus under 

 the same conditions, it is very striking that the first visible 

 effect of excitation upon a fresh, highly " tonic," preparation of 

 molluscan muscle should occur without exception on opening the 

 circuit only, while its closure either produces no effect, or a 

 shortening that is minimal in comparison with the opening con- 

 traction (Fig. 79, a). Even when the intensity of a just effective 



