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HUMAN PHYSIOLOGY 



which are directed well down into the ventricle from the 

 edges of the orifice leading from the auricle. They are 

 never far apart but blood can pass down between them 

 freely, while the first sign of a reflux will bring them 

 together. When closure has been accomplished, added 

 pressure in the ventricle will only press the two flaps 

 more firmly one against the other. The lower edges of 

 these flaps are limited in their movement by a number 

 of fine " tendinous cords " leading to the wall of the 

 ventricle. 



FIG. 546. The principle of the semilunar valves. At (1) the root of 

 the aorta is laid open and spread to show the three pockets. At (2) we 

 look down into the vessel when the passage is open. At (3) the flaps 

 lie in contact closing the channel. 



The valves in the root of the aorta are called semilunar. 

 The same name is applied to those in the beginning of 

 the pulmonary artery. In either case dissection shows 

 three pockets on the inner surface of the vessel. Their 

 free borders are directed away from the heart. The pass- 

 ing of the blood-stream in this the normal direction will 

 lay the flaps back against the wall of the artery and 

 there will be a free channel among them. A reversal of 

 the flow will fill the pockets and throw them together, 

 closing the passage. Each flap has, at the middle point 



