CHAP. IX.] 



THE VASCULAR SYSTEM. 



107 



ings, the auriculo-ventricular orifices, which are strengthened 

 by iibrous rings, and protected and guarded by valves. The 

 valve guarding the right auriculo-ventricular opening is com- 

 posed of three triangular flaps, and is hence named tricuspid. 

 The flaps are mainly formed of fibrous tissue covered by endo- 

 cardium. At their bases they are continuous with one another, 

 and form a -ring-shaped membrane around the margin of the 

 auricular opening: their pointed ends are directed downwards, 

 and are attached by cords, the chordce tendi- 

 nece, to little muscular pillars, the colum- * 

 ncs carnece, provided in the interior 

 of the ventricles for this purpose. 

 The valve guarding the left 

 auricular opening consists 

 of only two flaps, and is 

 named the bicuspid, or 

 mitral valve. It is 

 attached in the 

 same manner as 

 the tricuspid 

 valve, which it 

 closely resem- 

 bles in struc- 

 ture, except 

 that it is much 

 stronger and 

 thicker in all 

 its parts. 



FIG. 82. 

 LEFT SIDE OP 

 HEART. 1, cav- 

 ity of left auricle ; 

 3, opening of right 

 pulmonary veins ; 5, left 

 pulmonary veins ; 6, auri- 

 culo-ventricular opening ; 8, wall 

 of left ventricle ; 9, cavity of left ven- 

 tricle ; a, mitral valve (its flaps are attached by the chordae tendineaa to the mus- 

 cular pillars (6, &) ; d, arch of aorta) ; e, pulmonary artery. 



These valves oppose no obstacle to the passage of the blood 

 i'nmi the auricles into the ventricles; but any flow forced back- 

 wards gets behind the flaps of the valve (between the flap and 

 the- wall of the ventricle) and drives the flaps backwards and 

 upwards, until, meeting at their edges, they unite and form 

 a complete transverse partition between the ventricle and auri- 

 cle. Being retained by the chordae tendinese, the expanded 

 flaps of the valve resist any pressure of the blood which might 

 otherwise force them back to open into the auricle; the mus- 

 cular pillars, also, to which the chordae tendinese are attached, 



