554 
refractory stage is shortened and consequently the subsequent sinus- 
impulse can elicit a ventricular systole. Owing to the short duration 
of the preceding ventricular pause this systole will also be short and 
accordingly will have only a short refractory stage. Therefore, here 
also the next sinusimpulse is followed by a systole of the ventricle. 
Thus the ventricular halved-rhythm is changed into the normal 
rhythm of double velocity. The extra stimulus during the halved- 
rhythm may, however, be administered towards the end of the pause 
instead of during the diastole. Then the next sinusimpulse reaches 
the ventricle during the diastole of the extrasystole and elicits a 
small ventricular systole. Whereas in the first case the normal ven- 
tricular rhythm was initiated by a small extrasystole, there now 
appears the normal rhythm under the influence of a sinusimpulse, which 
reaches the ventricle in the diastole of an extrasystole and, therefore, 
yields a small systole. In both cases it was a small ventricular systole 
with a short refractory stage, that made the normal rhythm possible. 
In the second place we can change the halved rhythm into the 
normal rhythm of twice its velocity by cooling the sinus venosus. 
Then the tempo of the sinusimpulse is slackened by which the 
sinusperiods are lengthened. We will elucidate some of the above 
artificial changes of rhythm by some results obtained in experiments 
with the bled frog’s heart. *) 
Let us first look at Fig. 5 of the previous publication.?) The 
stimulating electrode is applied in the auriculoventricular groove. 
At the downward deflection of the signal the ventricle receives a 
closing inductionshock, which engenders an extrasystole. At the end 
of the diastole of the posteompensatory systole which has been 
enlarged, an opening inductionshock is administered, which results 
in an extrapause of the ventricle. *) 
After the extrapause the first ventricular systole has increased 
still more in magnitude and in breadth, so that now the next sinus- 
impulse rebounds on the refractory stage. The subsequent prolonged 
ventricular pause again causes an enlarged ventricular systole with 
a prolonged refractory stage. 
Again the next sinusimpulse does not result in a ventricular 
systole. Thus the ventricle, pulsating in the halved rhythm through 
the prolonged refractory stage is, so to speak, caught in its own 
1) In all the figures of this publication the upper row represents the suspension- 
curves of the ventricle, the lower row the suspension curves of the auricles. 
4) S. pe Boer. On the artificial extrapause of the ventricle in the frog’s heart. 
These Proceedings p. 542. 
5) For the causes of this extrapause I refer to the previous publication. 
