RESPIRATION 



8l 



Look at the lungs of a pig or calf in the butcher's shop, 

 for they are like a man's lung. A frog's lung is a thin 

 bag, about half an inch in diameter. Upon its sides are 

 shallow cups like the 

 pockets of a honey- 

 comb. Each air sac of 

 a man's lung is like a 

 very small frog's lung. 



146. Breathing. 

 The lungs are hung in 

 a box called the chest 

 or thorax. The sides 

 of the chest are the 

 ribs, and its bottom is 

 a leaf of muscle called 

 the diaphragm, which 

 stretches like an arch 

 across the inside of the 

 body. (See page 22.) 

 The ribs and diaphragm 

 can be moved so as to 

 make the chest larger or 

 smaller. When it be- 

 comes larger, the air 

 is sucked into the lungs 

 and makes them larger. 



This is called inspiration. When the chest is made 

 smaller, air is driven out of the lungs. This is called 

 expiration. This takes place about eighteen times a 

 minute, but when you run or work hard you must breathe 

 more often. 



147. Cilia. The lining of the windpipe and bronchi is 

 made of cells whose inner ends are covered with tiny hairs 



ov. PHYSIOL. (INTER.) 6 



The air tubes and lungs. 



a larynx or voice box. 



b trachea or windpipe. 



c bronchi. 



d air sacs, each like a tiny frog's lung. 



