218 THE HOUSE I LIVE IN. 



hearts placed side by side, and closely pressed 

 together ; and in some animals, I believe 

 there really are two. 



Both the hearts however fill, as we have 

 already seen, in the same instant. Now let 

 us suppose them filled. What is next to be 

 done ? The heart contracts shrinks and 

 compresses the blood with as much force as a 

 strong man could compress it with his hand. 

 But suppose you held in your hand a fleshy 

 sack of blood that contained two or three 

 ounces, with little hollow branches, that parted 

 into ten thousand more into which the blood 

 could flow, but could not get out of their sides 

 or extremities. Suppose them now all full, 

 and the sack full, too. If you press the sack 

 hard with your hand, what will happen ? Why, 

 the blood, you will tell me, will go out of it 

 into the branches. It will ; but it will be as 

 likely to go into one as another, provided it is 

 equally large. 



But there is another difficulty. As soon as 

 I cease to press the sack, and the blood has an 

 opportunity to do so, it will run back into it 

 again. So you may perhaps, at first view, 



