1000 PHYSIOLOGY 



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

 one of the impulses travelling from the sinus arrives at the ventricle 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. 



FIG. 446. 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 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 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.) 



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

 of artificial stimuli on the sinus venosus. 



On account of the refractory period which ensues on the commencement 

 of the contractile process on heart muscle, it is impossible to throw the 

 muscle into a tetanus, since all the stimuli which fall during systole are 

 entirely ineffective. By using very strong stimuli it is possible to intercalate 

 extra contractions before the heart has returned to the base line, i. e. before 

 diastole is complete. So that in this way one may obtain almost a continuous 

 contraction (presenting however waves on its summit), which differs from 

 the tetanus of skeletal muscle in the fact that its height is no greater than 

 the height of a single contraction. 



