CIRCULATION OF THE BLOOD. 



47 



are forced open, and nearly all the blood is driven out of 

 the ventricles ; from the right ventricle into the pulmonary 

 artery, and from the left ventricle into the aorta. 



While the ventricles are contracting and forcing their 

 blood out, the auricles are slowly filling by the steady 

 inflow through the veins. 



Systole and Diastole. The contraction of the heart is 

 called the systole, and its dilation the diastole. 



Dilation of the Ventricle. As soon as the ventricle 

 has completed its contraction it dilates, and most of the 

 blood that has accumulated in the auricle simply falls into 

 the ventricle. The dilating ventricle exerts a slight suc- 

 tion, so the blood is in part drawn into the ventricle. Dur- 

 ing the remainder of the pause the blood accumulates in 



Auricle 



Fig. 1 8. Diagram of the Heart, showing the Action of the Valves. 



the auricle and ventricle till the auricle again contracts and 

 the cycle is repeated. This is true of both halves of the 

 heart, which work simultaneously, the right heart pumping 

 dark blood while the left heart pumps bright blood. The 

 left ventricle is thicker walled and stronger than the right. 



