140 



OUR BODIES AND HOW WE LIVE 



As through a city the great water mains branch and 

 subdivide into smaller pipes for the supply of districts, 



streets, houses, and rooms, so 

 in the body the blood vessels 

 divide over and over again to 

 furnish blood to the smallest 

 organs and the most remote 

 parts of the tissues. The par- 

 allel ends here. The water 

 supplied to the city does not 

 return to the pumping station, 

 whereas the blood returns to 

 the heart. 



When the blood has been 

 pumped through every part of 

 the body, and has given to it 

 the food which it needs, it re- 

 ceives from the tissues certain 

 waste matters, the result of 



wear and tear. The blood is 



, JL . , 



now no longer fit for nourish- 



A. B. right pulmonary veins; S, , , . ,., i i 



openings of the left pulmonary^ ment, but IS more like a kind 



veins ;, D, C, aortic valves ; R, o f sewer Stream laden With 



aorta ; P. pulmonary artery ; O, _,, . , 



pulmonic valves ; H, mitral valve ; Waste matters. These, in dUC 



K, columns cames; M, right ven- time, are brought to certain 



tricular cavity. , , . 



organs, as the lungs, the skin, 

 and the kidneys, and cast out of the body. 



197. The Heart. The heart is hollow and muscular, 

 somewhat like a pear in shape. It is hung almost in the 

 center of the chest, above the diaphragm, and is partly 

 overlapped by the lungs. It is about the size of the 

 closed fist of the person to whom it belongs. 



FIG. 86. Cavities of the Heart. 



