THE PHYSIOLOGY OF THE HEARTBEAT 179 



period to expend itself, then obviously a slight relaxation will have oc- 

 curred before the effect of the second stimulus develops itself, and tet- 

 anus will be incomplete. These facts will be evident from the accom- 

 panying tracings (Fig. 47). 



Skeletal muscle Stannius' heart 



Fig. 47. The effects of successive stimuli and of tetanizing stimuli on skeletal muscle and 

 cardiac muscle. The small vertical marks show when the stimuli were introduced. (Compiled 

 from tracings published by T. G. Brodie and Leonard Hill.) 



In the case of cardiac muscle the above described properties are quite 

 different, for the refractory phase extends throughout the whole period of 

 contraction; that is, a second stimulus applied during the contraction 

 produced by a previous stimulus has no effect whatsoever; it does not 



