64 



THE HUMAN VODY. 



Fig. 31. 



SEMILUNAR VALVES 

 PARTLY CLOSED. 



and stop it up. These are very perfect valves. They are 

 flexible, and float open like thin veils when the blood 

 wishes to pass in the right direction; but they are strong; 

 and, the harder it presses back, the tighter they fit. 

 10, The left auricle is similar to the right. The opening 

 between it and the left ventricle is an 

 inch in diameter, and is closed by the 

 mitral valve. 



The mitral valve differs from the tri- 

 cuspid in having two flaps instead of 

 three. When closed, it resembles a 

 miter, or bishop's cap. 



11, The left ventricle differs from the 

 right only in having thicker walls. 



12, Out of the left ventricle opens 

 the great artery of the body, the aorta. 



Its entrance is guarded by semilunar valves precisely like 

 those of the pulmonary artery described above. It gives 

 off numerous branches, which go to all parts of the body, 

 finally dividing into capillaries. From these capillaries 

 the blood is returned by the veins to the right auricle. 



13, The muscle of which the heart is composed is pe- 

 culiar. It is striped, and yet not voluntary. As it must 

 act during sleep, it is made independent of will or thought. 



The heart and the blood-vessels are lined by a mem- 

 brane as smooth as satin. 



jf 



ACTION OF THE HEART. 



SECTION III. 1, The heart is, in the body, what the 

 mainspring is, in a watch. *Like all other muscle, it has 

 the power of contracting. By constant and regular con- 

 tractions it keeps up the circulation, and thus sustains life. 



