106 THE WONDERFUL HOUSE THAT JACK HAS 



truly remarkable. Nor can the arrangement of the 

 cells and the wonderful network of blood-vessels be 

 easily imagined until experiments have been made 

 with a pair of lungs. 



The story thus far is not hard to understand, for it 

 is as easy to comprehend how oxygen gets into the 

 lungs as it is to see how air goes through the draughts 

 into a furnace. But how does this useful gas get to 

 the tissues in other parts of our bodies? The answer 

 to this question is so curious that it almost seems to 

 take us into Fairyland. We remember how the villi 

 in the intestines take up the digested food and pass 

 it on to the tiny blood-vessels. In tracing these 

 blood-vessels, we find that they gradually lead into 

 larger and larger tubes which finally join cordlike 

 canals. 



By examining further, we discover that these canals, 

 or tiny pipes, extend into every part of the body, 

 but grow larger as they approach and join the heart. 

 Opening some of these pipes, we find they contain bright 

 red blood. Others appear to be of a bluish color and 

 carry darker blood. The former are called arteries, 

 the latter veins. The minute blood-vessels that con- 

 nect the arteries and veins are called the capillaries. 



But what makes the blood run through these tube- 

 like canals? Some of them seem to go straight up- 

 hill. We know that when city water runs up a hill 

 higher than its source, there are great pumps that force 

 it to make the ascent. Can it be possible that our 



