IX 



IIII. IIKART 



89 



r.ipillarii's of the lungs, ami along the veins from the lungs 

 lurk to the left auricle. It is the force of the right ventricle 

 alone which drives the blood round the pulmonary circulation 

 back to the left auricle. 



The blood flows from the pulmonary veins into the left 

 auricle, and when the left auricle is full it contracts in the same 

 manner and at the same time as the right auricle, driving the 

 blood into the left ventricle. The left ventricle then contracts 

 in the same manner and at the same time as the right ventricle. 



... v 



V- 



aur 



m- 



vent .2 



A B 



FIG. 43. Diagram to illustrate the action of the heart, 

 ricle ; vent, ventricle; v, veins; a, aorta; in, mitral valve; 



Mimlunar 



WVMi 



In A, auricle contracting, ventricle dilated, mitral valve open, scmilunar valves closed. 

 In H, auricle dilated, ventricle contracting, mitral valve cloScd, scmilunar valves 

 open. 



Directly its contraction begins the blood is forced behind the 

 flaps of the mitral valve, pressing the flaps together and so 

 blocking the way back to the left auricle. The papillary 

 muscles contract also, tightening the chordae tendiiira- in the 

 same way as in the ri^ht \cntricle. Soon the contraction of 

 the left vi-ntrirle ha's got up enough pressure to open the semi- 

 lunar valves of the aorta and then it forces the blood into the 

 .101 ta. \Vhcn it has nearly or quite emptied itself it relaxes, 



