INSPIRATION AND EXPIRATION. 115 



Thus we see why expiration is easy ; in fact, " does itself " 

 (in ordinary respiration), by the elastic reactions mentioned. 

 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. 



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 afford to 

 employ more energy while opening the door, than to take the 

 extra time to shut it. If, then, a door with such spring were 

 fastened open, it might remain open for a long time. When 

 released it flies shut. If one, in this case, asks, "Who shut 

 the door ? " the answer is, " The person who opened it." 



So in the act of inspiration we perform a double work in 

 storing energy by which the expiration is performed without 

 active muscular effort. 



Reviewing these points we have : 



FORCES OF INSPIRATION. 



1. Depression of the diaphragm. 



2. Muscles elevating the ribs. 



3. Pressure of the external air. 



RESISTANCES TO INSPIRATION. 



1. Compression of the abdominal organs, and stretching 

 abdominal walls. 



