THE LUNGS 1 97 



deeper. Thus the size of the chest can be increased in 

 all directions. When the chest expands, air rushes in 

 to distend the lungs. The entrance of air into the lungs 

 is called inspiration. At the end of inspiration the 

 muscles relax. Then the weight of the parts and the 

 elasticity of the distended lung forces out the air. In 

 addition, the muscles of the abdomen and arms can be 

 made to contract so as to expel the air with greater force. 

 Driving out the air from the lungs is called expiration. 



341. Amount of expansion. In an ordinary inspiration the 

 chest becomes from one half an inch to one inch larger around. By 

 taking a very deep breath most people can expand the chest two or 

 three inches. An expansion of four or five inches is exceptional. By 

 breathing exercises the expansion can be increased. 



342. Amount of air used in each breath. After the . 

 fullest possible inspiration, the lungs contain about 330 

 cubic inches of air. After the fullest possible expiration, 

 the lungs still contain about 100 cubic inches of air. So it 



is possible, by strong effort, to inhale and exhale about 

 230 cubic inches of air. This is called the vital capacity 

 of the lungs, and is the breathing power which can be 

 used in violent exercise. But in quiet breathing only 

 about 30 cubic inches of air are inhaled. This is called 

 tidal live. By an effort about 100 cubic inches of air can^ 

 be inhaled in addition to the tidal air. This is called the 

 complemental air. By a forced expiration, the lungs can 

 expel about 100 cubic inches of air more than in quiet 

 breathing. This is called the reserve or supplemental air. 

 There still will be left 100 cubic inches of air, called 

 residual air. 



343. Action of the cilia. The motion of the cilia creates 

 an air current in the smaller bronchi, which mixes the in- 

 coming fresh air with that already in the lungs, so that 



