THE BLOOD-VESSELS AND CIRCULATION 



133 



auricles respectively, and thence into the ventricles. But as the 

 auricles receive more than passes at once 'into the ventricles, their 

 walls become fully distended. They then contract simultaneously, 

 each one forcing its contents into the ventricle below it, the force 

 of this contraction not being sufficient to send the blood back 

 into the great veins, where it would be resisted by the pressure of 

 the fluid they already contain. As the ventricles fill, the cusps of 

 the tricuspid and mitral valves float up on the surface of the 

 blood. Now follows the simultaneous contraction of the ventricles, 

 the effect of which is to close the valves between them and the 

 auricles, and also to open the semilunars at the entrances of the 



Fig. 126. Diagram showing the 

 position of the Valves of the 

 Heart while the walls are re- 

 lazed. 



Fig. 127. Diagram showing the 

 position of the Valves of the 

 Heart during the 1 contraction of 

 the ventricles. 



great arteries. The cusps of the tricuspid and mitral valves are 

 raised with such force that the chordae tendineae are tightly stretched, 

 and the regurgitation of bjood into the auricles is rendered im- 

 possible by the meeting of the edges of the cusps. The chief use 

 of the tendinous cords is to prevent the valves from going too far 

 into the auricles. It will thus be seen that when the ventricles 

 contract there is but one course on each side open to the blood, 

 namely, that furnished by the great arteries the pulmonary 

 arteries and the aorta, on the right and left respectively. These 

 vessels are not only filled with blood, and that almost suddenly, 

 but their walls are distended, since the blood is forced into them 



