586 



ELECTRICAL STIMULATION OF THE FROG S HEART. 



Maximal stimulation usually induces a ventricular beat when- 

 ever it is applied (Fig. 172, 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. 172, I 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 ventricular one, is followed, on the contrary, 

 by an omission of the ventricular beat (Fig. 172, a 1). 



At this point the latent period may be looked upon as indefi- 

 nitely 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 an effect, although they are much slighter than the 

 minimal stimuli of either auricle or ventricle. 



Fig. 173. 



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

 shock effective. 



