THE HEART BEAT. 507 



momentarily blocking or retarding the flow from the large veins, 

 and causing one of the waves seen in the normal venous pulse as 

 recorded in the jugular veins. The ventricular 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- ventricular valves are tightly closed, 

 the first sound is heard, a/id 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 the 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 during 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 sys- 

 tole 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 

 ventricular 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 observers 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 



