ABSORPTION IN GENERAL 123 



The only explanation at present to be offered of anomalous 

 action is to refer it to some peculiar property inherent in the 

 cells themselves the epithelium in case of the alimentary 

 canal. So profoundly important in connnection with physio- 

 logical 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 intelligent conception of many vital 

 phenomena. 



Osmosis. When two different kinds of gases are brought 

 in contact they mingle with each other, making a homogen- 

 eous 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 rea- 

 son 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 sub- 

 stances the molecules of which can penetrate the interposed 

 membrane, there will likewise be an interchange of these sub- 

 stances, and the fluids on both sides will come ultimately to 

 have the same composition. This passage of liquids and dis- 

 solved matters through an animal membrane is known as 

 osmosis. 



It must be remembered that in the body particularly the 

 interposed 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 inter- 

 change continues because of incessant molecular motion ; but 

 the currents equalize each other, and no alteration in volume 

 or composition becomes apparent. But if to the water on 

 one side there be added a solution of some crystalloid, as 

 sugar, the excess of water will pass to that side. The crys- 

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



