240 THE HUMAN BODY. 



the more will the bag expand. Finally, if the rubber bag 

 is distensible enough, when all the air in the bottle is 

 sucked out d will be distended by the push of the air inside 

 it until it completely fills the bottle, whose-walls prevent 

 it from going any farther. If now we open c and let in 

 the outside air, the bag will again collapse to its original 

 shrunken dimensions. 



Application to the Lungs. The above experiment illus- 

 trates very perfectly how the lungs are kept distended dur- 

 ing life. The chest is an air-tight chamber containing, 

 among other things, the elastic hollow lungs. On the 

 interior of the lungs the weight of the atmosphere presses 

 through the air-passages which lead to them, and answer 

 to the tube b in Fig. 67. Outside the lungs in the thorax 

 there is no air at all, and the uncounterpoised aerial pres- 

 sure inside them overcomes their tendency to shrink, and 

 expands them so as to completely fill all holes and corners 

 of the thoracic chamber not occupied by other organs. If, 

 however, we make a hole in the chest wall and let the air 

 press on the outside of the lungs they collapse at once. 



How the Air is renewed in the Lungs. Suppose in Fig. 

 67 that the bottle A has a movable bottom, by pulling down 

 which its capacity can be increased, and that all air has 

 been sucked out of the bottle through c, which is then 

 closed. The elastic bag will then be distended by the weight 

 of the atmosphere acting upon its interior so as to fill the 

 bottle and press against its sides. If now the movable 

 bottom of A be pulled down so as to make the cavity of the 



Apply to the lungs the facts illustrated by the model represented 

 in figure 67. 



Suppose we have a bottle like that in Fig. 67, but with a movable 

 bottom, what would happen when the bottom was pulled down ? 



