174 RESPIRATION 



allows the abdominal contents to push upward against 

 the base of the lungs. 



In forced expiration, as in shouting or blowing upon 

 a horn, the abdominal muscles also contract. They pull 

 down on the ribs and push the abdominal contents with 

 the diaphragm up into the chest cavity, thus strongly 

 pushing the air out of the lungs. 



Amount of air breathed. The difference in the size 

 of the lungs between an extreme inspiration and an ex- 

 treme expiration is measured by the amount of air 

 which can be blown out after the fullest inspiration. 1 

 It varies much with the size of the individual and the 

 flexibility of his chest wall. It may be as much as two 

 quarts, and as little as two pints if the chest walls are 

 extremely rigid. 



Lungs filled by air pressure. The entrance of the 

 outside air into the lungs and its exit from them is in 

 accordance with the physical laws governing the move- 

 ment of all gases. It must be remembered that, since 

 we are living at the bottom of a sea of air, we are ex- 

 posed to a constant atmospheric pressure due to the 

 weight of this air of nearly fifteen pounds to the square 

 inch. When the air presses equally upon all side of 

 an object, as upon a thin sheet of rubber, the pressures 

 completely neutralize each other. As a result, the rubber 

 is flaccid and apparently without pressure. If, however, 

 the rubber is held tightly over the open mouth and in- 

 spiration is attempted, the rubber is seen to be pushed 

 inward. This is due to the pushing in of the outside 

 air to fill the increased space made by the enlargement 



1 Of course it must be borne in mind that the amount blown out 

 does not represent the total actual capacity of the lungs, since, 

 even after they have been emptied as completely as possible, they 

 still contain two or three pints of air. They are never normally 

 collapsed during life. 



