55 | 
Conversely, when the stimulus is given much earlier, an extra- 
systole of the auricles is originated, after which a systole of the 
ventricle follows at a normal a —v interval. An instance of this case 
is given in fig. 6 at the third upward deflection of the signal. 
At the first and the second upward deflection of the signal the 
extrastimulus was applied at the peak of the 7 deflection ora short 
time before it, which resulted in an extrapause of the ventricle. At 
the third upward deflection of the signal, however, the extra stimulus 
was applied much earlier, viz. rather more than '/, second before 
the peak of the 7-deflection. It appears that the auricles respond 
already to the stimulus and present a complete extrasystole, but this 
retrograde excitation is not stayed in its course by the periodic 
sinusimpulse in the auricles. After this auricular extrasystole the 
excitation proceeds to the ventricle and induces it to contract. 
Success of the latter experiment depends upon various conditions : 
1. The extrastimulus is to affect the auricles after the refractory 
stage of these chambers. 
2. After the artificial extra-systole the excitation is to reach the 
ventricle after its refractory stage. 
3. The extra-stimulus is to be applied so early that the excitation 
which traverses the auricles after this stimulus in retrograde direction, 
does not encounter the next sinusimpulse in the auricle. 
Finally I wish to advert to the necessity of amplifying ENGEL- 
MANN’S interpretation of the constant deviation of the compensatory 
pause in connection with the present investigation. According to 
ENGELMANN the reason why, instead of the extrasystole normal periodic 
ventricular systole has fallen out, is because the periodic sinusimpulse 
reached the ventricle during the refractory stage of the extrasystole. 
The present research induces me to add that in some cases the 
periodic ventricular systole falls out because after the extra stimulus 
the excitation, which proceeds also in retrograde direction, clashes 
upon the periodic sinusimpulse, so that both excitations are annihilated. 
When we thus amplify the interpretation of the duration of the 
compensatory pause, a fact becomes clear to me that had been known 
to me long since, namely that when an extra stimulus is given to the 
ventricle, we see in some of the experiments, during the extrasystole 
a P-deflection expressed in the electrograms, in others we do not. 
If the P-deflection is absent it is obvious that the periodic sinus- 
impulse has not traversed the whole auricle, but has been stayed in 
its course by the excitation proceeding in retrograde direction, evoked 
by the extra stimulus. 
