THE HEART 117 



In a case of aortic stenosis, for example, in which 0*7 

 of the opening is obstructed, as happens when two of the 

 cusps are completely adherent, the heart does 1,000 kgm. 

 of work at sixty beats per minute, but at seventy beats 

 it does 1,150 kgm. Further, the fluid tension to 

 which the wall of the heart is subjected increases with 

 the radii of curvature of the cavity. Hence a dilated 

 heart must put forth more energy in order to expel its 

 contents than one of normal ventricular capacity, and 

 for that reason it hypertrophies.* 



In addition to this, the mere overfilling of the left 

 ventricle must always lead to an increase of its work 

 during systole, for although the systole is prolonged in 

 such circumstances, yet the prolongation never extends 

 to more than 20 or 30 per cent, of the normal time.t 

 Hence the blood must be expelled more rapidly i.e., 

 more work must be done. 



It has been calculated that if a lesion is so bad as to 

 require 7*7 times as much energy as usual to com- 

 pensate iii.e., a heart 7'7 times heavier than normal 

 the maintenance of the circulation is impossible. 



In normal circumstances the different chambers of 

 the heart transmit equal volumes of blood in the same 

 time. Were this not so, of course, the circulation would 

 come to a standstill, and the usual cause of death from 

 heart failure is inability on the part of one or more 

 chambers of the heart to fulfil its duty in this respect. 

 Even in health, however, the law may be temporarily 



* See Hill in Schafer's ' Physiology,' ii. 40. 

 t See von Frey, Deutsch. Arch. f. Klin. Med. t 1890, 

 xlvi. 398. 



