38 



PHYSIOLOGY 



t 



tract rhythmically and involuntarily. By their con- 

 traction the upper cavity is automatically increased in 



size in every direction. 

 Inspiration. — The 

 lungs are situated in 

 this cavity and their 

 delicate elastic bags re- 

 spond to every change 

 in the size of the cav- 

 ity (Fig. 25). As the 

 cavity becomes larger 

 they become larger and 

 the pressure of the air 

 inside the lungs be- 

 comes less than the 

 pressure outside. Im- 

 mediately the air from 

 the outside rushes 

 through the windpipe 

 into the lungs until an equilibrium is established. This 

 process is called inspiration (Fig. 26, A). 



Expiration.— When the muscles contract the reverse 

 happens. The chest cavity contracts, the lungs become 

 smaller, the air in the lungs becomes denser than the air 

 outside, and passes out. This process is called expira- 

 tion (Fig. 26, B). 



Effect of the Elasticity of the Lungs on the Dia- 

 phragm. — As the air goes out, the lungs, which were 

 stretched by the incoming air, tend to resume the 

 smallest area. They push away from the diaphragm 

 with a force equal to their elasticity so that the pres- 



L~1 

 B- 



M- 



Fig. 25.— Diagram of apparatus to illustrate 

 the effect of the position of the diaphragm 

 on the lungs. B, bottle with bottom re- 

 moved ; M, flexible elastic membrane 

 pulled by string, S; L, an elastic bag 

 representing the lungs. It communicatee 

 with the external air by a glass tube fitted 

 air tight through a stopper. 



