553 
When considering this fraction more carefully, we can say before- 
hand in what way the normal rhythm can be changed into a halved 
rhythm and the reverse, for if we take the relative duration of the 
refractory stage larger than J, the ventricle will pulsate with half 
the rhythm. If, on the contrary, we take it smaller than 1 the 
ventricle will beat in the normal rhythm, in which every sinus- 
impulse is followed by a systole of the ventricle. We can make the 
fraction greater than 1 by increasing the numerator or also by 
lessening the denominator. Now in the case of a heart of which 
the total refractory stage is lengthened and which still beats in the 
normal rhythm, we can indeed prolong the total refractory stage so 
much as to make it outlast the sinusperiod. So we can make the 
fraction greater than 1, as we have only to evoke an enlarged 
systole, whose refractory stage has been prolonged. 
Now such an enlarged systole is the postcompensatory systole. 
When, therefore, we have lengthened the refractory stage of a ven- 
tricle (through poisoning or through bleeding), we evoke an extra- 
systole or extrapause of the ventricle. After the compensatory pause 
or extrapause the next ventricular systole is enlarged, while its 
refractory stage has been lengthened. Therefore, the subsequent 
sinusimpulse will be checked by this prolonged refractory stage; 
again a prolonged pause ensues, and after this the next ventricular 
systole is again enlarged and has a prolonged refractory stage with 
all its consequences. Thus the ventricle is caught in the halved 
rhythm by the enlarged and broadened postcompensatory systole '). 
An increase of the duration of the refractory stage, i.e. an increase 
of the numerator of the above-mentioned fraction sufficed to bring 
about the ventricular halved rhythm. 
Another method producing the same result, is heating the sinus 
venosus, which will increase the frequency of the sinusimpulses and 
consequently decrease the duration of the sinusperiod. The denomi- 
nator of the fraction is diminished. When the ventricle pulsates in 
the halved rhythm, the relative duration of the refractory stage is 
greater than 1. The fraction may then be made smaller by decreas- 
ing the numerator or by increasing the denominator. 
The first may be effected by administering an extrastimulus to 
the ventricle during the diastole. Then an extrasystole of the ventricle 
originates, which lasts much shorter than the ventricular systole 
from the ventricular halved rhythm. Therefore, the duration of its 
1) Not every post-compensatory systole is followed by a ventricular halved-rhythm. 
This happens only when the refractory stage has been lengthened before by a 
disturbance of the metabolic equipoise. 
