THE BLOOD AND CIRCULATION 319 



The opening between right auricle and right ventricle is 

 guarded by the tricuspid valve. This is similar in structure 



to the mitral except 

 that there are three 

 flaps instead of two. 

 The position of 

 these valves in action 

 is shown in figs. 163 

 and 164. 



Blood-vessels direct- 

 ly connected with the 

 heart. — The large 

 veins open into the 

 auricles. On the 

 right side of the heart 

 opening into the right 

 auricle there are three 

 veins, the superior 



Fig. 162. Left auricle and 

 ventricle opened and a 

 part of anterior and left 

 wall removed. 1, two 

 right pulmonary veins 

 cut short; 1', within cavity 

 of left auricle; 2, wall 

 between auricle and ven- 

 tricle; 3, 3', 3", cut walls 

 of ventricle; 4, portion of 

 wall of ventricle with 

 muscular pillar to which 

 tendons of mitral valve are attached; S, S, muscular pillars on inner wall 

 of left ventricle; S', within cavity of left ventricle; 6, 6' mitral valve; 7, 

 commencement of aorta (with semilunar valve below) ; 7', aorta; 8, root 

 of pulinonary artery and its semilunar valve; 8', continuation of the pul- 

 monary artery; 9, artery connecting pulmonary artery and aorta; 10, arteries 

 arising from summit of aortic arch. (After Allen Thomson.) 



vena cava (descending from the parts of the body above 

 the heart) the inferior vena cava (ascending from the parts 



