24 PHYSIOLOGY 



body surface covered with a moist membrane, gases can 

 get into, or out of, the body only by passing through the 

 membrane. This fact is important because, (1) All 

 animals have more or less of the body surface covered 

 with a moist membrane; (2) Any gas coming in contact 

 with a moist membrane passes through. 



The Law of Gases. — Why should a gas pass through 

 a membrane ? When we explain this we shall have ex- 

 plained the underlying principle which governs respira- 

 tion. A gas always passes in the direction of the least 

 pressure. It does this even if it has to pass through a 

 membrane. If the pressure of a gas on the outside of a 

 membrane is greater than the pressure of the same gas 

 on the inside, then that gas must pass from without in ; 

 conversely, if the pressure is greater on the inside than 

 on the outside, it must pass from within out. The pres- 

 sure of carbon dioxide is continually greater on the in- 

 side of the body than on the outside ; it must, therefore, 

 pass out of the body. The pressure of oxygen is con- 

 tinually greater on the outside of the body than on the 

 inside ; it must therefore pass into the body. 



Passage of a Gas Through a Membrane. — If a gas 

 is in contact with a moist membrane, its particles, mov- 

 ing in every direction quite unrestrainedly, hit against 

 the membrane. When they hit another particle they re- 

 bound, but when they come to a space they pass through 

 if the space is large enough. The ease with which a sub- 

 stance passes through a membrane, or whether it passes 

 through at all, depends on the relative size of its par- 

 ticles and of the spaces between the particles in the mem- 

 brane. In a membrane the spaces between the particles 



