342 PECULIARITIES OF THE CARDIAC MUSCLE TISSUE 



extra contractions may then be incited without giving rise to compen- 

 satory pauses, nor do we then obtain a significant disturbance of the 

 rhythm. It seems, therefore, that this phenomenon can only develop 

 in the spontaneously beating heart, the activity of which, as we have 

 seen above, is dependent upon rhythmic discharges from the "pace- 

 maker" situated at the venous vestibule. Under normal conditions, 

 these stimuli are generated at regular intervals and activate the auricles 

 and ventricles in quick succession. The latter in particular are well 



Fig. 179.— 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 prepa- 

 ration was already considerable, and the latent period inappreciable. The " compensa- 

 tory pause " after the extra beat is also well shown in 4, 5, 6, 7, 8. (Marey.) 



supplied with contractile substances, and are therefore very irritable 

 and responsive. If they are now made to give an extrasystole, the 

 subsequent normal wave of excitation must arrive in them at a time 

 when they are just engaged in producing this contraction. Conse- 

 quently, they are impermeable to this stimulus and refractory. Inas- 

 much as this excitation remains without results, the ventricles continue 

 inactive during the period ordinarily occupied by the next normal 

 contraction. The succeeding normal wave of irritability, however, 



