44 PHYSIOLOGY 



with the membrane. By a contraction of the diaphragm 

 and of the muscles that lift the ribs, the chest cavity 

 is enlarged. Inspiration occurs and fresh air is brought 

 in contact with the lung membrane. When the muscles 

 relax the cavity becomes smaller, expiration occurs, and 

 the impure air is removed from contact with the mem- 

 brane. 



These processes do not happen by accident. Neither 

 do they happen because we need them to happen. Every 

 phase of the process of respiration is governed by a defi- 

 nite law. A gas always passes in the direction of the 

 least pressure. In accordance with this law we will get 

 the oxygen that we need if we do our part by seeing to it 

 that the air about us is reasonably pure and that our 

 muscles are strong enough to keep a good supply in con- 

 tact with the lung membrane. But if we do not do our 

 part, if, for example, we sit in an ill-ventilated room, 

 we will not get the oxygen that we need. The law will 

 act whenever gases are in contact with the membrane, 

 but from vitiated air not oxygen but poisons enter the 

 body. 



The health of the entire body is dependent upon respi- 

 ration. Give the healthy body plenty of oxygen and if it 

 is not abused in any way it will probably remain healthy. 

 Deprive the body of oxygen and it cannot remain 

 healthy. 



