92 PHYSIOLOGY. 



chest expands to fill this greater space. When expanded 

 it exerts less pressure than before, and the air outside, 

 having greater pressure, enters till equilibrium is produced. 

 The air enters through the trachea, presses on the inside 

 of the elastic lungs, and makes their bases extend, follow- 

 ing the diaphragm in its descent. The bases of the lungs 

 remain in contact with the upper surface of the diaphragm 

 all the time. 



Increased Air 



Space 



Inspiration Expiration 



Fig. 43. Diagrammatic Sections of the Body in Inspiration and Expiration. 



Work of the Chest Walls in Inspiration. Certain 

 muscles of the chest wall elevate the ribs and breast bone. 

 This act widens the chest, and the air, as before, presses 

 in through the open trachea, and keeps the sides of the 

 lungs in contact with the inner surfaces of the chest walls. 



Effort required in Depressing the Diaphragm. - 



Inspiration requires considerable effort, because the dia- 



