CIRCULATION.] * PHYSIOLOGY, jt 



stretched the closer the flaps fit together, and the 

 more completely do they block the way from the 

 ventricle back into the auricle. 



The tricuspid valve, then, lets blood flow 

 easily from the right auricle Into the right 

 ventricle, but prevents it flowing from the 

 ventricle into the auricle. 



31. Now look at the cavity of the ventricle. Its 

 walls are fleshy, that is muscular, and you will notice 

 that they are much stouter and thicker than those of 

 the auricle. Besides the opening from the auricle 

 there is but one other, which is at the top of the ven- 

 tricle, side by side with the former. If you put a 

 penholder or your finger through this second opening, 

 you will find that it leads into the large vessel which 

 you have already learnt to recognize as the pulmonary 

 artery (Fig. 5, P. A.) 



Slit up the pulmonary artery from the ventricle with 

 a pair of scissois, as has been done in Fig. 8, P.A. 

 You will notice at once the line where the red soft flesh 

 of the muscular ventricle leaves off, and the yellow 

 firmer material of which the artery is made begins. 

 Just at that line you will see. a row of three (perhaps 

 you may have cut one of the three with your scissors) 

 most beautiful, watch-pocket valves, made on just the 

 same principle as those in the veins, only larger, and 

 more exquisitely finished. These are called semi- 

 lunar valves, because each pocket is of the shape 

 of a half-moon. Lift them up carefully f.nd see how 

 tender and yet how strong they are. There is no 

 need to tell you the use of these. You know it at 

 once. They are to let the blood flow from 

 the ventricle into the pulmonary artery, and 



