290 



CIRCULATION 



During systole, the strong anterior cusp of the mitral valve 

 does not materially shift its position. The other cusp is pulled 

 forward against it. 



On the right side, one of the cusps hangs down on the septum 

 and is practically immovable. The other two cusps by the action 

 of their papillary muscles are pulled over towards the septal cusp. 

 The mass of blood pressing on the sides of the cusps completes 

 the closing of the orifice. 



When this mass of blood, under the pressure induced by the 

 contraction of the ventricles, stretches the auriculo-ventricular 

 valves it causes them to emit a sound which is a component of 



CCSP'JS ARANTII 



FIG. 62. Semilunar valves. A, in longitudinal-mesial section. B, Artery laid 

 open and exposed, and C, closed valves from the arterial aspect. 



the first sound of the heart. The other component is the sound 

 produced at the same time by the contraction of the ventricular 

 walls. It is said that a trained ear can pick out the notes due to 

 closure of the valves from those due to stretching of the muscular 

 walls. 



(2) The valves situated at the openings of the ventricles into the 

 arteries are similar in shape and in action to the pocket valves of 

 the veins (Fig. 62). Each is composed of three pockets or half 

 cups attached along their curved margins to the walls of the 

 artery and upper part of the ventricle and with their openings 

 set away from the ventricle. 



The cusps are not placed all exactly on the same plane. One 

 cusp lies somewhat deeper in the heart than the others. This 

 cusp is mounted on a muscular septum which acts as a cushion, 



