RESPIRATION 



49 



are compressed, and air escapes freely through the' 

 air passages, and we exhale. We can easily under- 

 stand how we exhale because, when the chest grows 

 smaller, air is pushed out through the air passages just 

 as water may be squeezed 

 out of a sponge. But how 

 do we inhale ? What pushes 

 air into the lungs through 

 the air passages ? This is 

 not quite as easy to under- 

 stand. The atmosphere is 

 exerting a pressure of about 

 fifteen pounds on every 

 square inch of surface. 

 Since the air in the lungs 

 and the atmosphere outside 

 are connected by means of 

 open tubes, the air passages, the atmospheric pressure in 

 the lungs must remain about fifteen pounds to the square 

 inch. Now, when the lungs enlarge, air must be forced 

 into them to keep the pressure the same as it is outside. 

 We may say that the atmospheric pressure forces air 

 into the lungs when the chest enlarges, and we inhale. 



This constant rhythmic change in the volume of the 

 air-tight chest makes inspiration and expiration alter- 

 nate all the time from birth to death, with no attention 

 whatever on our part. We never need to think about 

 breathing, because the nervous system is so beautifully 

 and perfectly adjusted that it keeps up this important 

 process with never failing accuracy and faithfulness. 



FIG. 22. Showing positions of the 

 diaphragm, sternum, and walls of 

 the abdomen in inspiration and 

 expiration (Zuppke). 



