545 
I have now detected that artificial extrapauses of the ventricle 
may be evoked in the frog’s heart in quite another manner. Whereas 
in the method described above, the prolongation of the refractory 
stage of the ventricle was the decisive factor, the following method 
is based on a principle unknown as yet in the physiology of the 
heart: When we place the stimulating electrode in the auriculo- 
ventricular groove, we can evoke under certain circumstances (pro- 
longed refractory stage of the ventricle), by the administration of. 
an extra-stimulus towards the close of the diastole of the ventricle, 
an extrapause of the ventricle, which is not preceded by an extra- 
systole of this chamber. | 
In our experiments described above we had to give the extra- 
stimulus at the beginning of the systole to obtain the desired result. 
When the stimulus was given a little later a premature ventricular 
systole succeeded the extrasystole of the auricles. 
It is obvious, then, that when a stimulus at the end of the dias- 
tole of the ventricle produces the same effect, it cannot be explained - 
in the same way. We shall therefore illustrate the latter experiment 
by some curves. In fig. 3 we see a reproduction of the suspension 
curves of a frog’s heart after veratrin poisoning. (The heart was 
left in situ and the circulation of the blood was left intact; some 
drops of 1°/, sol. acetas veratrini, had been injected into the dorsal 
lymphsac about 10 minutes before). At the first upward deflection 
of the signal an opening induction shock was given. After this we 
see an auricular systole represented in the suspension curve, which 
is not followed by a systole of the ventricle. Just as in the experi- 
ments described above, an extrapause of the ventricle follows after 
this auricular systole. At the next upward deflection the same expe- 
riment was repeated in the upper row of curves with the same 
result. Now when measuring the curve we find that the auricular 
systole, which appeared a short time after each of the two stimuli, 
follows after the commencement of the preceding auricular systole 
with an interval of a sinus period. We, therefore applied the extra- 
stimulus in the auriculo-ventricular groove a short time before the 
commencement of anormal periodic auricular systole. At that moment 
the ventricle was apparently still refractory, as there did not appear 
an extrasystole of the ventricle. The auricles, however, respond to 
the stimulus. The excitation now traverses the auricles from the 
auriculo-ventricular boundary in the direction of the sinus venosus. 
But simultaneously the periodic sinusimpulse traverses the auricles 
in an opposite direction. The two excitations meet and rebound. At 
that moment the auricular systole is accomplished under the influence 
