THE CAUSATION OF THE HEART-BEAT 957 



responds with a contraction to a minimal stimulus. These facts are well 

 shown in Fig. 437. 



When a tracing is being taken from part of the heart, e.g. the ventricle, 

 which is beating rhythmically in consequence of a stimulus communicated 

 to it from some other part, such as the sinus venosus, an extra contraction is 



FIG. 437. Tracings of spontaneous contractions of frog's ventricle, to show refractory 

 period. In each series the surface of the ventricle was stimulated by an induction 

 shock at E, as indicated by the tracing of thesignal. In 1,2 and 3, this stimulus 

 had absolutely no effect, since it fell during the refractory period. In 4, 5, 6, 7 

 the effect of the shock was to interpolate an extra contraction in the series, the 

 latent period (shaded part) gradually diminishing from 4 to 7 (diastolic rise of 

 irritability). In 8 the irritability of the preparation was already considerable, 

 and the latent period inappreciable. The ' compensatory pause ' after the 

 extra beat is also well shown in 4, 5, 6, 7, 8. (MABEY.) 



followed by a ' compensatory pause,' and in certain cases the first contraction 

 following the pause is considerably augmented. This is due to the fact that 

 one of the impulses arriving from the sinus arrives at the ventricles during 

 the refractory period ensuing on the application of the artificial stimulus ; 

 hence it produces no effect and the ventricle has to wait- for the arrival of the 

 next succeeding excitatory wave from the sinus before it gives its next beat. 

 Hence the compensatory pause does not occur when we are testing the effects 

 of artificial stimuli on the sinus venosus. 



