72 SCIENCE PRIMERS. [§ v. 



to prevent the blood going back from the 

 artery into the ventricle. 



On the right side of the heart we have, then, two 

 great valves, the triscuspid valve between the auricle 

 and the ventricle, and the semilunar valve between 

 the ventricle and the pulmonary artery. These let 

 the blood flow easily one way, but not the other. If you 

 doubt this, try it. Put a tube into either the superior 

 or inferior vena cava of a fresh heart, tying the other 

 vena cava and another tube into the pulmonary artery. 

 If with a funnel you pour water into the tube in the 

 vein, it will run through auricle and ventricle and out 

 through the tube of the pulmonary artery as easily as 

 possible ; but if you try to pour water the other way 

 down the pulmonary artery, you will find you cannot 

 do it ; the tube gets blocked directly, and only a few 

 drops come back through the heart into the vein. 



Now slit up the pulmonary artery as far as you can, , 

 and note when you cut it how stout and firm are its 

 walls. You will find that it soon divides into two 

 branches, one for the right lung, one for the left. 

 Each of these, when it gets to the lung, divides into 

 branches, and these again into others, as far as you 

 can follow them. You know from what you have 

 learnt already that these branches end in capillaries 

 all over the lungs. _ ^^ 



32. Not far from the two main branches of the pul- 

 monary arterj' you will find, covered up perhaps with fat 

 and other matters, some tubes which you will at once 

 recognize as veins, and if you open any one of these 

 you will find that you can put a thin rod into it, and 

 that it leads in one direction to the lungs, and in the 

 other into the left side of the heart. These are the 



