THE HEART BEAT. 553 



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 Average Capacity of the Ventricles and the Work Done 

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

 average capacity of the ventricles in man, but the determination 

 has encountered many difficulties. Experiments and observations 

 made upon the excised heart are of little value, since the dis- 

 tensible 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. Moreover, the evidence from experiments 

 on the lower animals indicates that, contrary to the opinion which 

 formerly prevailed, the ventricles throw out only a portion of 

 their blood at each beat. The older observers (Volkmann, Vier- 

 ordt) 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 generalization 

 that at each systole the amount of blood ejected from the ven- 

 tricles is equal to about -^^ of the body weight. For a man 

 weighing, say, 72 kilograms (158 lbs.) this ratio would give an 

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

 ers, however, have found this estimate too high. Howell and 

 Donaldson* measured the output directly for the heart of the 

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

 by an artificial circulation. The ratio of the output varied with 

 the rate of beat; for a rate of 180 beats per minute it was equal to 

 0.00117 (sis) of the body weight; for a rate of 120 beats per 

 minute it was equal to 0.0014 (y^). This ratio is therefore about 

 one-half of that proposed by Volkmann. Tigerstedt, from ob- 

 servations upon rabbits, obtained a lower ratio still (0.00042); 

 but from his own results and those obtained by other workers he 

 concludes t that an average valuation for the volume of blood dis- 

 charged by each ventricle of the human heart is from 50 to 100 

 c.c. As was stated in a preceding paragraph this volume may be 

 greatly increased by conditions, such as muscular work, which 



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

 don, 1884. 



t Tigerstedt, "Lehrbuch der Physiologie des Kreislaufes," p. 152, 1893. 



