126 Dr. J. A. McWilliam. The Structure and Rhythm [Jan. 29, 



sion of the auricular excitability without any absolute suspension of 

 that excitability. Very weak vagal stimulation is often able to cause 

 the auricle to beat more slowly than the rest of the heart, or to stop 

 action for a time while the other parts of the organ go on beating. 



At high temperatures (29 — 33° C, &c.) the auricular excitability to 

 direct stimulation does not seem to be depressed during cardiac stand- 

 still brought about by vagus stimulation. 





+ +. 



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Tracing of auricle showing vagal inhibition. The beats marked + were elicited by 

 direct stimulation (single induction shocks) applied to the aimcle before its 

 action had recommenced. It will be seen that the auricular beats continue to 

 be of diminished size for some time after the spontaneous action has recom- 

 menced. 



Vagal stimulation is also able to depress very markedly the contrac- 

 tion force of the auricular muscle. Any beats that can be elicited by 

 strong direct stimulation (when the inhibitory phase is passing away) 

 are much diminished in size, and the beats with which the auricular 

 action recommences are as a rule very small ones ; they gradually 

 increase till they regain the normal size. The depressing effect on the 

 contraction force accompanying vagal inhibition occurs both in the 

 intact heart and in the bloodless auricle ; it is not due to over-disten- 

 sion during the inhibitory period. 



The conduction power of the auricle is, during inhibition, tempo- 

 rarily abolished. 



Tracing showing the contractions of both auricle and ventricle. The large beats 

 are those of the ventricle ; the smaller ones between the large beats are those 

 of the auricle. The effects of vagal inhibition are shown. The recommenc- 

 ing auricidar beats are much diminished in size ; the recommencing ventricular 

 beats are large. 



These primary depressing effects of vagal stimulation on the auri- 

 cular tissue are by far the most constant and powerful ones ; the 

 secondary accelerating effects are slight and variable. The increase 



