HEART 391 



(fig. 165). One rises from the auriculo-ventricular ring 

 above the septum, and hangs down into the ventricle upon 

 the septum. The two larger outer cusps are held down by 

 chordse tendineae rising from the papillary muscles near the 

 apex of the ventricle. When these two sets of papillary 

 muscles contract, the outer cusps are drawn flat against the 

 septal cusp lying upon the bulging septum. 



Fig. 167. — Vertical Mesial Section througli Heart to show Aortic and Mitral 

 Valves. E.V., right ventricle; L.V., left ventricle with papillary 

 muscle; L.A., left auricle with the mitral valve extending into the 

 left ventricle ; Ao., aorta with anterior cusp on top of septum. 



(2) Semilunar Valves. — The valves situated at the opening 

 of the ventricles into the great arteries are also formed as 

 special developments of the endocardium. 



Each is composed of three half-moon-shaped membran- 

 ous pouches attached along their curved margin to the walls 

 of the artery and upper part of the ventricle, and with their 

 concavities directed away from the ventricle. In the centre 

 of the free margin is a fibrous thickened nodule, the corpus 



