220 THE HOUSE I LIVE IN. 



the veins so long as the blood in them is run- 

 ning towards the auricles. But as soon as the 

 auricles contract and the blood attempts to get 

 back by the way it came, the valves spread out 

 and form a kind of floor or partition which 

 obstructs it. 



These valves, by the way, are found in the 

 larger veins all over the body ; and now comes 

 the reason why the blood can run up hill. 

 The pressure in the veins is all the while 

 diminishing, as you may easily see, on the side 

 towards the heart, even though it is the up hill 

 side ; and as the arteries, at their extremities, 

 are all the while pouring their blood into them, 

 the pressure must be as constantly and cer- 

 tainly increasing on the other side. Besides 

 this general pressure, there is also local pres- 

 sure. The veins lie, most of them, in the 

 skin, or among the muscles, or among parts 

 that are performing some sort of motion. This 

 motion must push the blood in one direction or 

 another. But as the valves prevent its going 

 back, the pressure is hard enough to make it 

 go slowly up hill ; and thus it moves on and 

 on, till it finds its way to the heart. 



