THE HEART BEAT. 555 



authors as to the amount of blood discharged from each ventricle 

 during systole may be tabulated as follows : 



Thomas Youiig 45 gms. 



Volkmann 188 " for weight of 72 kgms. 



Vierordt 180 " " " " " " 



Tick 50-73 " 



Howell and Donaldson 75-90 " " " " 65 *' 



Hoorweg 47 " 



Zuntz 60 " 



Tigerstedt 50-100 " 



Plumier 70 " 



Loewy and v. Schrotter 55 " " " 60-65 kgms. 



Ivrogh and Lindhard 39-103 " 



'The Efficiency and Adaptability of the Heart. — Considered as 

 a machine for pumping blood into the arteries the efficiency of the 

 heart can be expressed by ascertaining what proportion of the 

 total energy expended is utilized in performing mechanical work. 

 The latter factor may be calculated as described in the preceding 

 paragraph, while the total energy liberated may be estimated 

 from the amount of oxygen absorbed during the period of observa- 

 tion. The results obtained in this way indicate a high degree of 

 efficiency, since 20 per cent, or more of the energy of the contrac- 

 tion is converted into mechanical work. A remarkable charac- 

 teristic of the heart is the promptness with which it adjusts itself 

 to conditions calling for a greater or less amount of work. When 

 for any reason the arterial pressure, the resistance against which 

 the heart works, is increased, the heart, if in good condition, re- 

 sponds promptly by an increased contraction, so that an equal 

 amount of blood is discharged against the high resistance. The 

 adaptation is brought about by the dilatation of the heart. StarHng 

 and his co-workers* have shown that in heart muscle as in skeletal 

 muscle the energy of contraction is increased by lengthening the 

 muscle. On this principle the more the heart is dilated by the 

 blood or, to express it in another way, the greater the diastolic 

 volume, within limits, the stronger will be the contraction. If, 

 for example, arterial pressure is increased suddenly the ventricle 

 may not be able to discharge its normal quota against the greater 

 resistance. The result, however, will be that in the following 

 diastole the ventricle will be more distended with blood, since it 

 will receive its usual charge from the auricle and veins, and this 

 will be added to the larger residuum left from the previous con- 

 traction. The succeeding systole will, therefore, be stronger, and 

 in this way after a few beats the ventricle will be delivering its 

 usual volume of blood in spite of the higher resistance. The 

 adaptability of the heart is beautifully exhibited in its response to 



* Patterson, Piper, and Starling, "Journal of Physiology," 4S, 465, 1914. 



