BLOOD SYSTEM. 



381 



FIG. 262. — Aorta cut open 

 and spread out to show 

 the semilunar valves. 



into the auricle they flap back, pressing against one an- 

 other, closing the way, and are held in place by the 

 cords* (Fig. 261, B). The valve between the right 

 auricle and ventricle is called the tricuspid, because it 

 has three cusps. That in the left 

 heart is called the bicuspid, as hav- 

 ing only two cusps. 



The semilunar, or pocket valves 

 are at the base of the aorta and 

 of the pulmonary artery, three in 

 each, of crescentic shape like shal- 

 low pockets, which, when filled, 

 press against each other from 

 three sides and completely close 

 the artery. When the ventricle 

 contracts, the blood rushes into 

 the artery, the valves pressing 

 close against the wall ; but as 



soon as the ventricle relaxes, the pockets fill, press 

 against each other, and close the gate (Fig. 262). 



We now follow again the course of the blood, show- 

 ing how the valves work. Blood, dark from the tissues, 

 enters the right auricle. The auricle contracts, the cur- 

 tains flap wide open into the ventricle, and the blood 

 enters freely. The ventricle contracts, the blood shuts 

 the curtains, they are held in place by the cords, and 

 the blood is forced into the pulmonic artery. When the 

 ventricle relaxes, in order to fill itself again, the pocket 

 valves fill and close the way. The action on the other 

 side of the heart is the same. The blood coming from 

 the lungs into the auricle is thrown into the ventricle and 

 the door closed behind. It is therefore forced upward 

 into the aorta and again the door closed behind. It is 



* To appreciate the action of these they must be examined on 

 a heart or a good model. 



