THE BLOOD -PRESSURE 885 



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. 



