On Electrical Stimulation of the Frog's Heart. 463 



Maximal stimulation usually induces a ventricular beat whenever it 

 is applied (fig. 6, a), excepting when it falls just after the summit of 

 the auricular contraction. 



Stimulation at this point may cause no auricular contraction, but 

 on the contrary may induce omission of the subsequent auricular 

 and ventricular beat (fig. 6, & 2). 



When an auricular beat has been induced by stimulation, it is 

 followed in the ordinary way by a beat of the ventricle, excepting 

 when the stimulus is applied to the auricle just at the commencement 

 of the ventricular systole. In this case an auricular beat may be 

 induced, which instead of being followed by a corresponding ventri- 

 cular one, is followed, on the contrary, by an omission of the ventri- 

 cular beat (fig. 6, a 1 ) . 



At this point the latent period may be looked upon as indefinitely 

 long, as stimulation produces no contraction at all. 



The more closely after this point stimulation is applied the longer is 

 the ventricular latency. 



Stimulation of Venous Sinus — Minimal. 



The venous sinus appears to be more sensitive to stimulation than 

 either auricle or ventricle, so that stimuli applied to it produce air 

 effect, although they are much slighter than the minimal stimuli of 

 either auricle or ventricle. 



Stimulation of the venous sinus by a minimal shock is usually 

 potent to produce some effect or other at every stage of ventricular 

 activity (fig. 7, &). 



Fig. 7. 



b. 



Stimulation of Venous Sinus (minimal) . In neither a nor b is the closing shock 



effective. 



Stimulation at the instant of commencement of ventricular systole 



