According to recent physiologic investigations (Wiggers1), ventricular systole may be divided into a period of isometric contraction and a period of ejection, the latter being again divided into a phase of maximum and reduced ejection. Recent studies on animals2have, moreover, shown how the duration of these phases as well as the duration of the entire systole is modified by a number of dynamic factors.3In brief, while the duration of the average systole is determined by the heart rate or average cardiac cycle—or more precisely the duration of any systole is determined by the duration of the previous diastole—it is also affected by other experimentally produced dynamic changes, namely, the specific effect of the cardiac nerves; the action of chemicals, drugs and hormones; changes in arterial resistance; and alterations in venous pressure. The isometric contraction period, for instance, is shortened by an increase in venous pressure and by