BREATHING 



409 



The tiniest parts into which the blood vessels are divided, 

 the capillaries, form close networks within the linings of 

 the air sacks. The air and blood are thus separated by an 

 exceedingly thin animal tissue, which allows an exchange 

 of soluble materials. Thus the blood is able to take up the 

 oxygen needed and to rid itself of the carbon dioxide and 

 other waste products which it has accumulated. 



The air-tight thoracic cavity in which the heart and 

 lungs are situated is inclosed and protected by the ribs 

 and at the lower part by 

 a dome-shaped muscle 

 called the diaphragm . 

 Air enters the lungs 

 because the muscles of 

 the chest pull the ribs 

 so that they move up- 

 ward and outward and 

 the muscles of the dome- 

 shaped diaphragm cause 

 it to move downward. 

 These two actions en- 

 large the thoracic cav- 

 ity. The air enters in 



the same way that it enters a hollow rubber ball that has 

 been compressed and then set free. When the ribs move 

 downward and the diaphragm upward, the air is expelled as 

 in the rubber ball when compressed. 



There are then two ways in which air can be made to 

 enter the lungs, the " raising of the chest " and. the move- 

 ment of the diaphragm. In the proper kind of breathing 

 these two movements go on together. The lungs are filled 

 throughout and not simply at either the top or bottom. 



THE LUNGS 



They are here pulled aside to show 

 the heart. 



