142 VALVES OF THE HEART. 



18. The Valves of the Heart only permit blood to flow 

 through it in one direction. When the heart -is expand- 

 ing and receiving blood, none flows back into the ven- 

 tricles from the arteries, because the semilunar (half- 

 moon-shaped} valves , at the mouths of the pulmonary 

 artery and aorta, block the road. They will open out- 

 wards from the heart, and let blood flow from the ventri- 

 cle, but they will not open the other way and let blood 

 flow back from the artery into the heart. 



When the heart dilates, it fills with blood from the veins. 

 Then a ring of muscle round the mouth of each vein close 

 to the heart, contracts and narrows the opening. Next 

 the auricles contract, and the only way each can drive 

 the blood collected in it, is into the ventricle of the same 

 side. Immediately afterwards the ventricles contract 

 and squeeze on the blood which has collected in them. 

 This blood would go back into the auricles but for the 

 valves which lie in the openings between each auricle 

 and its ventricle, and only open towards the ventricle. 

 As soon as any blood tries to flow back, the valves close 

 and block the road, so the only way the contracting ven- 

 tricle can send its blood is on into the arteries. 



The valve between the right auricle and ventricle is 

 named the tricuspid or three-pointed valve. That between 

 the left auricle and ventricle is the bicuspid or two- 

 pointed valve. It is sometimes named the mitral valve, 

 from being shaped like the two points of a bishop's mitre. 



18. Use of the valves of the heart ? Position of semilunar valves ? 

 Action ? How are the mouths of the veins narrowed before the auri- 

 cles contract ? Where does each auricle pump its blood ? What hap- 

 pens after the auricles have contracted ? Why does not blood flow 

 back in the auricles when the ventricles contract ? Where do the 

 ventricles pump blood ? Where is the tricuspid valve ? The mitral ? 



