154 STUDIES IN ADVANCED PHYSIOLOGY. 



valve between the left auricle and left ventricle consists of 

 but two flaps, and is therefore called the bicuspid, or, from 

 its resemblance to a mitre, the mitral valve. A rather rough 

 illustration of these valves may be made by cutting say the 

 top of an ordinary tomato can in such a way that it will be 

 divided into three triangular flaps, meeting at a point in the 

 middle of the surface, somewhat like a pie cut into equal 

 thirds. If these parts of the can were then bent downward 

 into the can one might easily see how a liquid could be 

 poured into the can. But if these flaps should by the liquid 

 poured in them be gradually lifted up, until finally they 

 should all meet again, and joining with their edges fit per- 

 fectly, one may see how the return of such water out through 

 this opening would be prevented. Such is in a general way 

 the condition of things in the heart. To prevent these 

 valves in the heart from turning upwards into the auricles, 

 they are fastened to the walls of the ventricles by means of 

 strong cords called the chordae tendinae. Such cords do 

 not end, however, abruptly in the wall of the ventricle. If 

 such were the case, these cords on the contraction of the 

 ventricles would become slack, and so allow the valves to 

 turn back into the auricles. To prevent the production of 

 such slack, the cords run into special muscles attached to 

 the ventricular walls, which by their contraction pull in the 

 slack on the cords as rapidly as it is produced by the ap- 

 proximation of the ventricular walls in the heart beat. These 

 muscles are called the papillary muscles. 



At the orifice of the pulmonary artery and of the aorta 

 are the semilunar valves. These consist in each case of 

 three sack-like flaps attached to the wall of the issuing ves- 

 sel in such a way that when they are distended with blood 

 they meet in the center and prevent the flow of blood past 

 them. These semilunar valves do not differ materially from 

 the valves which occur regularly in veins. These valves 

 permit the blood to push the flaps apart in entering the 

 vessel, but do not permit the flow in the opposite direction. 

 On the middle of the margin of each of these flaps there is 



