108 THE WONDERFUL HOUSE THAT JACK HAS 



blood changes from a dark red to a bright red color, 

 finally passing through the large pulmonary veins into 

 the left auricle of the heart. Thence it is forced by 

 contraction into the left ventricle. This chamber 

 then contracts, forcing the blood into a large artery 

 called the aorta, through its numerous branches, to 



the capillaries in the re- 

 motest part of the body, 

 and then back through 

 the veins into the right 

 auricle of the heart. 



How remarkable all 

 this is ! A network of 

 tubes of many sizes car- 

 rying blood to every 

 part of our bodies ! The 

 water system of a great 

 city with its numerous 

 pipes running in and out 

 of the various houses 

 may be compared to it. 



Diagram showing the course of the blood ^ . . 



through the heart. " large mam pipes 



The vessels containing impure blood are drawn running f rom the 

 darker than the others. 



ing stations and along 



the principal streets correspond to the great arteries that 

 run from the heart. The pipes leading from the streets 

 to the houses suggest the branching arteries, while those 

 which carry the waste water to the sewer and the 

 sewer pipes themselves serve a purpose similar to that 



