CAUSES OF INHALING AND EXHALING 323 



well, the jar is now air-tight, and the only connection 

 of the interior "of the balloon with the outer air is through 

 the glass tube. This apparatus represents the relation 

 of the lungs to the chest cavity, since the sides of the 

 balloon are not in any way connected with the rubber 

 sheet (diaphragm) or with the sides of the bell jar (the 

 rib walls). Now pull the rubber sheet downward. The 

 balloon becomes inflated as air enters through the tube 

 (trachea). Let the sheet return to place, and the balloon 

 collapses, expelling the air through the glass tube. "\^Tiat 

 is the explanation? The answer is found in what the 

 physicists call the laws of air pressure. They tell us that 

 every square inch of surface exposed to the air is under a 

 pressure of fifteen pounds. At the start of our experiment, 

 therefore, both the inside and outside of the balloon are 

 under equal pressure. Since no air can enter the bell jar, 

 when we lower .the diaphragm the air inside it must occupy 

 a greater space, and its pressure on each square inch of the 

 balloon surface becomes less. The only way in which 

 the pressure can be equalized is for air to rush in from 

 the outside and distend the balloon until the volume of 

 air in the jar is the same as before. Then the inflow 

 stops, since both surfaces of the balloon are again at the 

 same pressure. When the diaphragm returns to position 

 the air in the jar is compressed, and the surface of the 

 balloon exposed to this compressed air is now under a 

 greater pressure than the outside air can exert upon the 

 inside through the glass tube, and the walls are gradually 

 forced together with the outpouring of the contents of 

 the balloon, until again the pressure on the outside of the 

 balloon is the same as on the inside. Meanwhile, the 

 balloon has performed all the movements of the lungs in 



