1076 PHYSIOLOGY 



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

 well shown in Fig. 439. 



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, 



FIG. 439. Tracings of spontaneous contractions of frog's ventricle, to show 

 refractory period. In each series the surface of the ventricle was stimu- 

 lated by an induction shock at E, as indicated by the tracing of the signal. 

 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 inter- 

 polate 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.) 



an extra contraction is followed by a ; compensatory pause/ and 

 in certain cases the first contraction following the pause is con- 

 siderably 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 



