82 



SCIENCE PRIMERS. 



[§v. 



the empty right ventricle, and the full right ventricle is 

 the point at which we began. 



Thus the alternate contractions of auricles 

 and ventricles, thanks to the valves in the 

 heart and in the veins, pump the blood,* 

 stroke by stroke, through the wide system 

 of tubes; and thus in every capillary all 

 over the body we find blood pressed upon 

 behind by over-full arteries, with a way 

 open to it in front, thanks to the auricles, 

 which are, once a second or oftener, empty 

 and ready to take up a fresh supply from the 

 veins. Thus it comes to pass that every little frag- 

 ment of your body is bathed by blood, which a few 

 moments ago was in your heart, and a few moments 

 before that was in some other part of your frame. 

 Thus it is that no part of your body can keep itself 

 to itself; the blood makes all things common as it flies 

 from spot to spot. The red corpuscle that a minute ago 

 was in your brain, is now perhaps in your liver, and in 

 another minute may be in a muscle of your arm or in 

 a bone of your leg : wherever it goes it has something 

 to bring, and something to fetch. A restless heart 

 fc for ever driving a busy blood, which wherever it 

 goes buys and sells, making perhaps an occasional 

 bargain as it shoots along the great arteries and great 

 veins, but busiest of all as it lingers in the narrow 

 pathways of the capillaries. 



35. When you look down upon a great city from a 

 high place, as upon London from St. Paul's, you see 

 stretching below you a network of streets, the meshes 

 of which are filled with blocks of houses. You can 

 watch tlie crowds of men and carts jostling through 



