THE CIRCULATION. 



body back to the heart, open into the right auricle. Be- 

 tween the right auricle and the right ventricle, there is 

 an opening about an inch in diameter. This opening is 

 closed by the tricuspid (three-pointed) valve. This con- 

 sists of three thin flaps, whose edges meet when the valve 

 is shut. This valve opens from the auricle, and shuts so as 

 to prevent the blood from passing back from the ventricle 

 to the auricle. There are fine cords attached to the edges 

 of these flaps, and to the wall of the cavity below, to pre- 

 vent their shutting back too far. 



8. Opening out of the right ventricle is the pulmonary 

 artery. This soon divides into two, one of which goes to 

 the right, the other to the left, lung. They there divide 

 into capillaries, and 



the capillaries pass 

 into the pulmonary 

 veins. These are 

 two in number for 

 each lung. They 

 empty into the left; 

 auricle. 



9. At the begin- 

 ning of the pulmo- 

 nary artery is a set of valves which prevent the blood 

 from flowing back into the heart. These valves, called 

 semilunar valves, are three pockets hung on the wall of 

 the artery, their edges meeting all around. They open 

 away from the heart; and, when the blood is going in 

 that direction, they flatten up against the wall, just as a 

 pair of doors will when a crowd is pushing through them. 

 When the blood sets back toward the heart, it fills the 

 pockets; and they bulge out, and together fill the tube, 



Fig. 30. 

 BBMILUNAR VALVES OPEN. 



