THE HEART BEAT. 557 



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 t = 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 (period of diastasis) 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 in man, but the deter- 

 mination 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; in fact, the evidence from experiments on the lower 

 animals indicates that, contrary to the opinion, which for- 

 merly prevailed, the ventricles throw out only a portion of their 

 blood at each beat. The older observers (Volkmann, Vierordt) 

 attempted to arrive at a determination of the normal output 

 of the ventricles by calculations based upon the velocity of 

 the blood in the carotid and the width of the stream bed. from 

 observations on many animals they arrived at the general- 

 ization that at each systole the amount of blood ejected 

 from the ventricles is equal to about ^ of the body weight. For 

 a man weighing, say, 72 kilograms (158 Ibs.) this ratio would give 

 an output for each systole of 180 gms. (6 ozs.). More recent 

 observers, however, have found this estimate too high. Ho well 

 and Donaldson* measured the output directly for the heart of the 

 dog, making use of a heart isolated from the body and kept beating 



* Howell and Donaldson, "Philosophical Transactions," Royal Soc., Lon- 

 don, 1884. 



