CIRCULATORY SYSTEM 



341 



tricles; and then there is a pause. This is followed 

 immediately by another contraction of the auricles. 

 The heart beats in an adult about seventy two times a 

 minute, but its rate varies in different individuals. 



Capillaries 

 Lungs 



Pulmona 

 Artery 



Vena Ca 



Capillaries 

 of Body 



Pulmonary 

 Vein 



\-Aorta 



Fig. 160. 



Diagrammatic representation of the circulation of the 

 blood. 



The structure of the heart is such that the blood 

 passes into the ventricles. The thick walls of the 

 ventricles then contract and force the blood out into 

 the arteries. While the ventricles are contracting 



