ABSORPTION IN GENERAL 123 



are indeed, in some cases, at variance with them. The only 

 explanation at present to be offered of anomalous action is to 

 refer it to some peculiar property inherent in the cells them- 

 selves the epithelium in case of the alimentary canal. So 

 profoundly important in connection with physiological activity 

 are the laws of osmosis outside of the body, and what is known 

 concerning the mutability of those laws inside the body, that a 

 brief consideration of the subject seems necessary to an intelli- 

 gent conception of many vital phenomena. 



Osmosis. When two different kinds of gases are brought in 

 contact they mingle with each other, making a homogeneous 

 mixture. This is due to the continual motion of their molecules. 

 When two different kinds of liquids are brought in contact, a 

 homogeneous mixture results for the same reason unless the 

 liquids be non-miscible, as oil and water. If now the liquids 

 happen to be separated by a membrane permeable by both, the 

 result, while it may be delayed, will be the same. If, further, 

 these liquids hold in solution substances the molecules of which 

 can penetrate the interposed membrane, there will likewise be 

 an interchange of these substances, and the fluids on both sides 

 will come ultimately to have the same composition. This pas- 

 sage of liquids and dissolved matters through an animal mem- 

 brane is known as osmosis. 



It must be remembered that in the body particularly the inter- 

 posed membrane may be permeable to the solvent, water, and less 

 so, or not at all, to the dissolved substances. Materials which 

 will in solution pass through a membrane are called crystalloids; 

 those which will not, colloids. If simple water be on both sides 

 of the membrane, the interchange continues because of incessant 

 molecular motion; but the currents equalize each other, and no 

 alteration in volume or composition becomes apparent. B ut if to 

 the water on one side there be added a solution of some crystal- 

 loid, as sugar, the excess of water will pass to that side. The 

 crystalloid in solution is said to exert an osmotic pressure, and 



