THE STORY OF THE HEART 71 



of three pockets placed in a circle around the blood- 

 vessel. The edges of these pockets being crescentic 



Fig. 22. DIAGRAM OF THE HEART'S ACTION. 



The tricuspid valve (a) is shut, and the semilunar valve 

 (&) is open to allow blood to pass to lung. The valve (a) 

 keeps blood from passing back into the auricle (ra). 

 From the lung purified blood returns to the left auricle 

 (la) by the pulmonary veins (pv) passes into the left 

 ventricle (Iv) and out to the body through the aorta (ad). 

 (Note the cords which in the valve (a) prevent the flaps 

 from floating up into the auricle.) 



in shape have given rise to the term "semi-lunar" or 

 "half-moon" shaped. These valves are somewhat 



