THE BLOOD AND CIRCULATION 



325 



commodate whatever blood fails 

 to pass through the capillaries 

 during the heart beat. The 

 elasticity of the walls of the 

 arteries exerts sufficient con- 

 stant pressure to force the blood 

 through the capillaries between 

 heart beats. Jn this way the 

 amount of blood passing 

 through the capillaries between 

 heart beats equals the whole 

 amount that leaves, the ven- 

 tricle at each succeeding beat, 

 and a constant flow is kept up. 



The blood rushes very rap- 

 idly through the arteries, slowly 

 but steadily through the capil- 

 laries, and moderately slowly 

 through the veins. 



During diastole the chambers 

 of the heart enlarge and fill 

 up with blood from the veins. 

 During systole this blood is 

 forced into the arteries. 



The pulse. — During systole, 

 a wave of muscular contraction 

 begins in the auricles and eiids 

 in the ventricles. This sudden 

 and forcible contraction of the 

 heart causes a wave of contrac- 

 tion to run throughout the 

 whole length of the arterial 



Fig. 165. Diagram of circulation. 1, 

 heart; 2, lungs; 3, head and upper 



extremities; 4, spleen; 5, intestine; 6, kidney; 7, lower extremities; 8, liver. 



(After Dalton.) 



