262 MANUAL OF PHYSIOLOGY. 



tion as the ventricle empties itself and the cavity diminishes in 

 size. By this mechanism the valves are prevented from bulging 

 too much into the auricles, or allowing the blood to pass back 

 into them. 



The semilunar valves are mere membranous pockets, and have 

 no tendinous cords attached to them, but on account of the ex- 

 tent of their curved attachment, when their limited free margin 

 is made tense by the pocket being filled from the artery, the valves 

 can only pass a given distance from the wall of the vessel, and 

 are thus held firmly in position. When the force of the blood 

 leaving the ventricle begins to diminish, the semilunar flaps are 

 raised from the distending wall of the artery ; and the moment 

 the current from the ventricle has ceased to flow, the pockets are 

 forcibly distended by the aortic blood pressure, and bulge into 

 the lumen of the vessel, so that the convex surface of the lunated 

 portions of each valve is pressed against corresponding parts of 

 its neighbor. Their union then, which is accomplished by their 

 overlapping to some extent, forms three straight radiating lines, 

 and is a perfectly impervious barrier to any backward flow of 

 blood (Fig. 118, PA. and AO.). 



MOVEMENTS OF THE HEART. 



It is only by means of these valvular arrangements that the 

 heart is enabled to perform its function, namely, to pump the blood 

 in a constant direction onwards, emptying the veins and filling the 

 arteries against great opposition on the part of the latter vessels. 



This pumping is carried on by the successive contractions and 

 relaxations of the muscular walls of the various cavities. 



The blood, flowing from the systemic and pulmonary veins, 

 passes unopposed into the right and left auricles respectively. As 

 soon as the auricles are full their walls suddenly contract and press 

 the blood into the right and left ventricles; immediately the ven- 

 tricles contract, and, pressing upon the blood, force it into the 

 great arteries. 



The contraction of each pair of cavities is followed by their 

 relaxation. 



