THE STORY OF THE HEART 69 



duly prevented might return into the auricle. This 

 reflux is prevented by the valves of the heart (Figs. 

 20 and 21) which constitute one of the most in- 

 teresting points connected with the structure of that 

 remarkable organ. There are two valves on each 



LV. 



Fig. 21. LEFT SIDE OF HEART OF MAN. 



(la) left auricle ; (Iv) thick wall of left ventricle : (mv) 



mitral valve ; (pa) pulmonary artery, and (sv 1 ) its valve ; 



(sy) semilunar valve of aorta (ao). 



side of the heart, each valve being more or less a 

 replica of its neighbour on the other side. On the left 

 side we find a valve between the auricle and ventricle 

 called the mitral or bicuspid valve (Fig. 21 mv), the 

 neighbour valve on the right side being called the tri- 

 cuspid valve (Fig. 20, 9). Each of these valves may 



