FURNITURE, AND ITS USES. 219 



suppose the heart would do. As soon as it 

 should cease to contract, and begins to relax so 

 that its cavities or chambers would hold just as 

 much as they did before, the blood would run 

 back into it. Why should it not ? No mo- 

 tion like that in our bodies would ever, in this 

 way, be produced. 



I have told you what one might naturally 

 think who knew nothing about the circulation. 

 But let us see for a moment what the facts are. 



When the two auricles, one on each side of 

 the heart, are full of blood, they contract at 

 the same time and push the blood into the two 

 ventricles. If you ask why this blood is not 

 just as likely to go back into the veins again, 

 when the auricles contract, as to go into the 

 ventricles, I will give you two reasons. First, 

 the veins are already full, and the mass of 

 blood in them is flowing onward and pressing 

 towards the auricles ; and to force the blood 

 back into them would be somewhat like push- 

 ing it up hill. But secondly, there are little 

 clappers or valves, as they are called, in the 

 sides of the veins, which, like so many small 

 swinging doors, hang down against the sides of 



