548 
same time by an excitation in a retrograde direction. This excitation, 
which traverses the auricle after the extra stimulus, encounters in 
the auricles the periodic sinusimpulse, which was already on its 
way from the opposite side at the moment when the extra stimulus 
was given. Both excitations are then annihilated, so that no prema- 
ture ventricular systole can follow and an extrapause of the ventricle 
manifests itself. Hereafter the normal ventricular rhythm is trans- 
posed into the halved rhythm’). 
It is beyond doubt that in this case the greater part of the auri- 
cular systole is owing to the periodic sinusimpulse, because this 
impulse was already traversing the auricles at the moment when 
the extrastimulus was being administered. We have seen heretofore 
that at the moment when the extrastimulus in the auriculoventri- 
cular groove is administered, the ventricle must be refractory. To 
ensure success of this experiment it will be well to lengthen the 
refractory stage of the ventricle. 
In the two preceding experiments we have effected this lengthen- 
ing by veratrin-, or by antiarin-poisoning. We can now avail our- 
selves also of the fact that the refractory stage of the ventricle is 
lengthened by the postcompensatory systole. This is instanced in Fig. 5. 
It represents the suspension curves of the auricles (lower curves) 
and of the ventricles (upper curves) of a frog’s heart after bleeding. 
The stimulating electrode is applied in the auriculoventricular groove. 
At the downward deflection of the signal a closing shock is admi- 
nistered *). This gives rise to an extrasystole of the ventricle, which 
is followed by a compensatory pause. During the postcompensatory 
systole an opening shock is applied. Although this shock was admin- 
istered at the commencement of an auricular systole just as the 
previous shock, the result is quite different. The refractory stage of 
contracted. We are safe to conclude that the sinus impulse cannot rebound on 
this extremely small partial contraction, since, indeed, in the frog’s heart the 
auricles are interconnected with the ventricle all along the auriculo-ventricular 
groove (auriculo-ventricular funnel.) Similarly we see in fig. 3 a slight difference 
in the magnitude of the deflections of the suspension curve, after the four stimuli 
which initiate the extrapauses of the ventricle. Very likely also here an extremely 
small portion of the ventricle has been made to contract once or twice. 
') | need not enlarge upon these transpositions of rhythm and the changes they 
involve for the ventricle-electrograms. They were discussed by me in Koninklijke 
Akademie van Wetenschappen te Amsterdam Proceedings Vol. XX p. 696, 
Vol. | (1917) p. 271 and 502. Archives Neéerl. de Physiologie tome Ill (1918) 
p. 7 and 90. Pfliiger’s Archiv. Bd. 173, S. 78. 
%) In this figure the closing induction shocks are not shut off and are announced 
by a downward deflection of the signal 
