RESPIRATION. 93 



phragm 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 soon 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 ventral ends 

 of the bony parts of the ribs with the breast bone are 

 slightly bent. When the muscles relax, the elasticity of 

 the rib cartilages helps to bring the ribs back to their 

 former position, thus reducing the chest to its former 

 width. 



Expiration Easy. Thus we see why expiration is easy ; 

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

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

 were not for the fact that the descent of the diaphragm 

 meets resistance, and the ribs, in rising, have to overcome 

 resistance in bending the costal cartilages, and in raising 

 the weight of the chest walls and shoulders. 



Potential Energy stored in a Door Spring. When 

 one opens a door that has a spring to shut it, he has to 

 expend more energy to open the door than he would if he 

 did not have to bend (twist or compress) the spring at the 

 same time. But no effort is needed to shut the door. The 

 door was opened and shut at the same time ; i.e. when 

 the door was opened force was stored in the spring (in the 

 form of what is called potential energy), and this stored 

 energy shuts the door while we pass on. We can better 



