THE HEART BEAT. 489 



momentarily blocking or retarding the flow from the large veins, 

 whence the normal venous pulse in the jugular veins. The ven- 

 tricular systole follows at once upon the auricular systole, the 

 exact relations in this case depending somewhat upon the pulse 

 rate. As the ventricle enters into contraction the auriculo- ventric- 

 ular valves are tightly closed, the first sound is heard, and for a 

 short interval the ventricular cavity is again shut off on both 

 sides. Soon the rising pressure in the interior forces open the 

 semilunar valves, and then a column of blood is discharged 

 into aorta and pulmonary artery as long as the contraction lasts. 

 During this interval the flow at the venous end of the heart 

 continues, the blood being received into the yielding auricles. 

 Indeed, this capacity for receiving the venous inflow dur- 

 ing the comparatively long -lasting ventricular systole may be 

 considered as one valuable mechanical function fulfilled by the 

 auricles. The venous flow is never completely blocked and at the 

 most suffers only a slight retardation during the very brief auricular 

 systole. At the end of the ventricular systole the excess of pressure 

 in the aorta and the pulmonary artery closes the semilunar valves 

 and completes the cycle. 



Time Relations of Systole and Diastole. The duration of the 

 separate phases of the heart beat depends naturally on the rate 

 of beat. Assuming a low pulse rate of 70 per minute, the average 

 duration of the different phases may be estimated as follows: 



Ventricular systole = 0.379 sec. 



Ventricular diastole and pause = 0.483 " 



Auricular systole =0.1 to 0.17 " 



Auricular diastole and pause = 0.762 to 0.692 " 



Einthoven and Geluk, in the investigation referred to above, 

 measured the time intervals of systole and diastole during fifteen 

 heart periods of a healthy man, and found that the time for the systole 

 varied between 0.312 and 0.346 sec., while that for the diastole 

 varied from 0.385 to 0.518 sec. Experiments by a number of ob- 

 servers indicate that in the great changes of rate which the heart 

 may undergo under normal conditions the diastolic phase is affected 

 relatively much more than the systolic, as we should expect. 



The Normal Capacity of the Ventricles and the Work 

 Done by the Heart. Various efforts have been made to measure 

 the normal capacity of the ventricles, but the determination has 

 encountered many difficulties. Experiments and observations made 

 upon the excised heart are of little value, since the distensible 

 walls of the ventricles yield readily to pressure, and it is difficult 

 or impossible to imitate exactly the conditions of pressure that 

 prevail during life. Nor is it certain whether normally the ventricles 

 empty themselves completely during systole. The older observers 



