AUR1CULO-VENTRICULAR VALVES. 201 



The Auriculo-Ventricular Valves. Between eacL auricle 

 of the heart and the ventricle of the same side are found 

 valves which allow blood to pass from the auricle to the 

 ventricle, but prevent any flow in the opposite direction. 

 These valves are known as the tricuspid and mitral valves. 

 The mitral valve (Fig. 59) consists of two flaps fixed by 

 their bases to the margins of the opening between the left 

 auricle and the left ventricle ; their edges hang down into 

 the ventricle when the heart is empty. These edges are not 

 free, but have fixed to them a number of stout connective- 

 tissue cords, the chorda tendinece, which are fixed below to 

 muscular elevations, the papillary muscles, Mpm and Mpl, 

 on the interior of the ventricle. The cords are long enough 

 to let the valve flaps rise into a horizontal position and so 

 to close the opening between auricle and ventricle, which 

 lies behind the opened aorta, Sp, represented in the figure. 

 The tricuspid valve is like the mitral, but with three flaps 

 instead of two, 



Semilunar Valves. These are six in number ; three at 

 the mouth of the aorta, Fig. 59, and three, quite like them, 

 at the mouth of the pulmonary artery. Each is a strong 

 crescentic pouch fixed by its more curved border, and with 

 its free edge turned away from the heart. When the valves 

 are in action their free edges meet across the vessel and 

 prevent blood from flowing back into the ventricle. 



The Course of the Main Arteries of the Body (Fig. 60). 

 The aorta after leaving the left ventricle makes an arch 



What is found between each auricle and ventricle? What are 

 they called? Describe the mitral valve. Where is it placed? What 

 are the chordae tendineae? The papillary muscles? How far will 

 the cords allow the valve flaps to rise? How does the tricuspid valve 

 differ from the mitral? 



How many semilunar valves are there? Where are they placed? 

 Describe them. What is their use? 



