ATRIO-VENTRICULAR VALVES 



515 



guarded by the mitral valve leads from the anterior part of the atrium forward and 

 shghtly downward and to the left. The interior of the left atrium is smooth ex- 

 cept in the auricle, in which musculi pectinati are well marked. 



Fig. 429. — Left Ventricle and Part of the Atrium. 



The aorta is opened through the anterior cusp of the mitral valve. The plainly visible left 



limb of the atrio-ventricular bundle has been accentuated. 



Pulmonary artery 



Right auricle 



Right aortic 

 valve 



Anterior mi- / 

 tral CUSP, , / . 

 longitudin- / 

 allydivided | ■; 



A tr i o -ven- : 

 tr ic u 1 a r 

 bundle 



Anterior •' 

 pa pi llar\— 

 muscle 



Cut wail of left atrium 



Membranous ven- 

 tricular septum 



Left atrium 



Apex of the left ventricle 



ATRIO-VENTRICULAR VALVES 



The atrio-ventricular valves (figs. 427, 428, 429, 431) are attached around the 

 venous ostia of the ventricles in such a way as to open freely into the ventricles, 

 but to prevent regurgitation of the blood into the atria during ventricular systole. 

 Each valve is continuous along its line of attachment, but its free edge is notched 

 so as to produce an irregular margin; some of the notches are so deep as to 

 partially divide the valve into cusps. The right atrio-ventricular valve is 

 commonly divided by three deep notches into three cusps; this valve is therefore 

 called the tricuspid [valvula tricuspidalis]. The left is similarly divided into two 

 cusps and is called the bicuspid [v. bicuspidalis] or mitral. The depth of the 

 notches, however, is very variable and there may be an increase or (more rarely) 

 a diminution in the number of cusps; the addition of small subsidiary cusps is 

 quite common. Each valve cusp is tied down to the papillary muscles [mm. 

 papillares] of the ventricle by chordae tendineae. The latter are fibrous cords, 

 generally branched, of varying thickness. The thinnest cords are attached to the 

 free margin of the cusp; those of intermediate thickness to the ventricular surface 

 a few millimetres from the margin, and the thickest to the ventricular surface 



