688 ELECTKICAL STIMULATION OF THE FROG'S HEART. 



ventricular systole as it advances towards its maximum, though 

 the effect is never produced at the maximum. 



This omission of the ventricular beat is most usually associated 

 with a reduplication of the auricular beat, the second auricular 

 contraction occurring within that ventricular systole at the 

 commencement of which the shock was communicated 

 (Fig. 174, h). 



Eeduplication occurs in all phases except at the period when 

 stimulation causes omission. The latent period of this redupli- 

 cation is usually short, as in the case of a ventricle stimulated 

 directly, inasmuch as the induced auricular contraction does not 

 precede the induced ventricular, except when stimulation falls 

 before the maximum of ventricular systole, in which case there 

 is usually a regular sequence of auricular and ventricular con- 

 traction (Fig. 174, c). 



Usually after the maximum ^of ventricular systole stimula- 

 tion causes a reduplicated beat with short latency, inside of 

 which curve falls that of the induced auricular contraction ; 

 however, genuine sequential reduplication of auricle and ven- 

 tricle with long latency is not uncommon. Not unfrequently, 

 after repeated stimulation of the sinus, the heart assumes a new 

 rhythm, which may be twice as rapid as it was originally, and 

 though omission of the alternate beat may still be produced by 

 stimulation at the time already indicated, the organ returns 

 again to its accelerated pace. In time, if stimulation be with- 

 held, the rhythm returns again to the normal. The auricle 

 shares in the ventricular excitement (Fig. 175). 



Fig. 175. 



Rhythm which has been changed by repeated stimulation of Sinus returning 



to normaL 



