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 



COSP'JS ARANTII 



FIG. 62. Semilunar valves. A, in longitudinal-mesial section, li, 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 th 

 arteries arc similar in shape and in action to the pocket valves 

 the veins (Fig. 02). Each is composed of three pockets or hal 

 cups attached along their curved margins to the walls of th( 

 artery and upper part of the ventricle and with their opening 

 set away from the ventricle. 



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

 cusp lies somewhat deeper in the heart than the others. Thi? 

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



