CHAP. VIII.] THE VASCULAR SYSTEM. 



85 



FIG. 66. LEFT SIDE OF HEART. 1, cavity 



drives the flaps backwards of left auricle; 3 ' opening of right 



two flaps, and is named the bicuspid, or mitral valve. It is 

 attached in the same manner as the tricuspid valve, which it close- 

 ly resembles in structure, 

 except that it is much 

 stronger and thicker in 

 all its parts. 



These valves oppose no 

 obstacle to the passage of 

 the blood from the auri- 

 cles into the ventricles ; 

 but any flow forced back- 

 wards gets behind the 

 flaps of the valve (be- 

 tween the flap and the 

 wall of the ventricle) and 



veins ; 5, left pulmonary veins ; 6, auriculo-ven- 

 and Upwards, Until, meet- tricular opening; 8, left ventricle; 9,9, cavity of 

 , , left ventricle; a, mitral valve, its flaps are at- 



ing at their edges, tney tached by the chorda? tendinese to b, b, the mus- 

 Ullite and form a com- cu ^ ar pillars, or colunmae carnse ; d, arch of aorta; 



e, pulmonary artery. 



plete transverse partition 



between the ventricle and auricle. 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 tendi- 

 neae are attached, contract 

 and shorten at the same 

 time, and thus help to 

 keep them taut. 



Beside the openings be- 

 tween the auricles and 

 ventricles, each auricle 

 has two or more veins 

 opening into it, and each 

 ventricle has a large ar- 

 tery opening out of it. 



The openings of the veins do not require valves, but both the 



FIG. 67. SECTION OF HEART AT LEVEL OF 

 VALVES. P, pulmonary artery; A, aorta; M, 

 mitral valve ; T, tricuspid valve. 



