930 PHYSIOLOGY 



cavities and therefore on the pressure in the great veins. Increased fre- 

 quency of heart beat need not therefore necessarily increase the total 

 output of the heart into the arterial system. If the heart is beating with 

 optimum rate and force, it will keep the venous system, at any rate that 

 part nearest the heart, practically empty, and it is not possible for it to 

 obtain more blood to put into the arterial side, however frequently it may 

 beat. There will be an optimum frequency of the heart beat which will 

 depend on the state of filling of the great veins. The fuller these are the 

 more rapidly the heart may beat and increase the total output. On the 

 other hand, in a normal animal with the heart beating at its optimum rate 

 and with effective contraction of its muscular walls, while slowing the 

 heart rate will diminish the total output and therefore the arterial pressure 

 increase in the frequency of the beat cannot raise the arterial pressure to 

 any appreciable extent, though the heart may tend to wear itself out by 

 beating at a greater rate than the optimum. 



