ACTION OF THE VALVES. 



267 



ventricle becomes fuller the tendinous cords coming from the 

 elastic papillary muscles draw the valve curtains away from the 

 wall of the ventricle into the midst of the fluid, and the back 

 eddies, following the rapid central influx of blood, get behind the 

 curtains of the valves and push them toward one another, so 

 that they come together at the end of the auricular systole. 

 When the ventricle contracts upon its contained blood, the pres- 

 sure makes tense and bellies out the sail-like valves toward the 

 auricles, so that their convex sides come into still closer apposi- 



FIG. 118. 



LA 



The Orifices of the Heart seen from above, both the auricles and the great vessels 

 being removed. (Huxley.) PA. Pulmonary artery and its semilunar valves. Ao. 

 Aorta and its valves. RA V. Tricuspid, and LA V. Bicuspid valves. 



tion with one another. Their free margins are held in position 

 by the papillary muscles contracting and tightening the cords. 

 The flaps are kept at much the same tension by the papillary 

 muscles shortening in proportion 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 allow- 

 ing the blood to pass back into them. 



The semilunar valves are mere membranous pockets, and have 



