Circulation of the Blood. 



57 



suddenly contracts, thus forcing more blood into the ven- 

 tricle, and distending it. At the same time the aur-vent 

 valves, whicli were already nearly closed, are tightly closed 

 by the pressure of the blood which is forced up behind 

 them. The flaps of the valves are kept from going up too 

 far by the tendinous cords and by the muscles to which 

 the cords are attached. 



Fig- 33. Diagram of the Heart, showing the Action of the Valves. 



The Contraction of the Ventricle. Next comes the con- 

 traction of the ventricle, slower, but more powerful than 

 that of the auricle. As the walls of fhe ventricle are 

 drawn together, they press upon the blood. It cannot go 

 back into the auricles, for the more- it presses against the 

 aur-vent valves, the more tightly they are closed. The 

 vent-art (semilunar) valves are closed by back pressure in 

 the aorta and pulmonary artery. But the pressure of the 

 blood in the ventricles is so much greater that the vent-art 

 valves are forced open, and 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 



