64 



SCIENCE PRIMERS. 



[§v. 



one heart receiving all the veins from the body and 

 sending its arteries (branches of the pulmonary artery) 

 all to the lungs, and another heart receiving all the 

 veins from the lungs and sending its arteries (branches 

 ci the aorta) all over the body. And you would have 

 wo circulations, one through the lungs, and another 

 through the rest of the body, both joining each other. 

 Very often two circulations are spoken of, and because 

 the lungs are so much smaller than the rest of the 

 body, the circulation through the lungs is called the 

 lesser circulation, that through the rest of the body 

 the greater circulation. ' 



29. I have described the circulation as if the blood 

 always went in one direction from the right side of 

 the heart to the left, from arteries to veins, the way 

 the arrows point in the diagram. And so it does. 

 It cannot go the other way round. Why does it 

 go that way ? Why cannot it go the other 

 way round ? 



The reasons are to be found partly in the heart, 

 partly in the veins. 



In the veins the blood will only pass from 

 the capillaries to the heart. Why not from the 

 heart to the capillaries ? You remember the little 

 watch-pocket-like valves, here and there, sometimes 

 singly, sometimes two or three abreast. You re- 

 member that the mouths of the watch- 

 pockets were always turned towards the 

 heart. Now suppose a crowd of little corpuscles 

 hurrying along a vein towards the heart. When 

 they came to one of thesj watch-pocket valves 

 they would simply trample it down flat, and so 

 pass over it without hardly knowing it was there, 



