External Respiration. 95 



space in the chest. Air, from the outside, enters through 

 the trachea, presses on the inside of the elastic lungs, and 

 makes their bases extend, following the diaphragm in its 

 descent. The bases of the lungs remain in contact with 

 the upper surface of the diaphragm all the time. 



Work of the Chest Walls in Inspiration. Certain 

 muscles of the chest wall raise the ribs and breastbone. 

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

 through the open windpipe, and keeps the sides of the 

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



Effort required in Depressing the Diaphragm. Inspira- 

 tion requires considerable effort, because the diaphragm 

 in its descent presses upon the elastic organs of the 

 abdomen (stomach, liver, etc.), and these organs, in turn, 

 are pressed against the elastic walls of the abdomen. It is 

 somewhat like pressing a pillow down into a rubber bag; 

 the pillow springs up as sooa as the pressure is stopped, 

 because of its own elasticity as well as that of the bag. 

 Therefore, as soon as the diaphragm relaxes, the elastic 

 walls of the abdomen retreat, and the abdominal organs 

 rise to their former place. 



Effort required in Raising the Ribs. When the ribs are 

 elevated, the cartilages which connect the front ends of the 

 bony parts of the ribs with the breastbone (see Fig. 6) are 

 slightly bent. When the muscles relax, the elasticity of 

 the rib cartilages makes the ribs spring back to their 

 former position, thus reducing the chest to its former 

 width. 



Expiration Easy. Thus we see why expiration- jjs easy ; 

 in fact, "does itself" (in ordinary respiration) by elastic 

 reactions. But inspiration is harder than it would be if it 



