584 ELECTRICAL STIMULATION OF THE FROG'S HEART. 



These results may be possibly due, in part at least, to direct 

 stimulation of the auricle itself by the strong currenc used to 

 stimulate the ventricle. 



Stimulation of the Auricle — Minimal. 



When minimal stimuli are applied to the anricle, there is 

 occasionally a refractory period, extending from the beginning 

 to the maximum of auricular systole. When the stimuhis is 

 applied at the maximum of auricular systole, or just after it, it 

 sometimes produces an omission, or, as we may term it, an 

 apparent inhibition of the next auricular and ventricular 

 systoles (Fig. 171, h). Stimulation falling after this point and 

 occasionally on it, will cause a reduplication of auricular and 

 ventricular contractions to occur which may have a latency of 

 as much as 1*25 seconds. 



Fm. 171. 



b. 

 Stimulation of Auricle (minimal). 



No secondary contraction can usually be produced in the 

 ventricle till an induced auricular contraction has occurred ; and 

 as the auricular latency is considerable, the ventricular latency 

 is also very long. Thus, should the stimulus producing con- 

 traction fall at the commencement of ventricular systole, the 

 auricle may have a latency of one second and the ventricle of 1'4 

 seconds. 



The sensibility of the auricle to minimal stimulation may 

 generally be divided into three phases : — 



