CIRCULATION 



25 



blood it will hold and presses the valves tightly across 

 the opening- into which it would project if the papillary 

 muscles did not contract and hold the edges of the valve 

 at just the right angle. This action of the auricles is 

 called auricular systole (from a Greek word meaning 

 to contract). With a barely perceptible pause ventricular 

 systole begins, the blood is forced into the aorta and pul- 

 monary artery, whose openings are guarded by three 

 cuplike folds called semilunar valves. As the blood 

 rushes between these cups some of it gets into three lit- 



Fig. 4. Diagram of valves of the heart. The valves are supposed to be 

 viewed from above, the auricles having been partially removed. A, aorta 

 with semilunar valve; B, pulmonary artery and valve; C, tricuspid, and D, 

 mitral valve; E, right, and F, left coronary artery; G, wall of right, and H, 

 of left auricle; /, wall of right, and /, of left ventricle. (Stewart's Physi- 

 ology.) 



tic pockets, sinuses of Valsalva, between the valve folds 

 and the arterial Avails, so that when the force from be- 

 hind ceases the weight of the blood and the elastic re- 

 coil of the vessels force each flap towards the opening, 

 and, the three flaps coming together, the return of blood 

 to the heart is prevented. This completes the contrac- 

 tile or systolic period of the heart cycle and the period of 

 rest, diastole, begins. A cardiac cycle, therefore, con- 



