138 ABSORPTION 



relation of the crystalloid solution to the rate of exchange 

 is spoken of as the osmotic pressure of the crystalloid. To 

 say that potassium bichromate has a greater osmotic pres- 

 sure than peptone means simply that the rate of flow of 

 watqr through a membrane toward the potassium bichro- 

 mate solution is greater than it is toward peptone solution. 

 This process of osmosis enables us to separate a crystalloid 

 from a colloid by putting the mixture in a membrane bag 

 and suspending the bag in water. Under those conditions 

 the crystalloid solution will pass through the membrane 

 while the colloid will not. Separation of a crystalloid 

 from a colloid by osmosis is called dialysis. 



Relation of osmosis to absorption in the body. — If, now, 

 we refer to the conditions in the intestine we shall note that 

 the relation of the digested food to the fluid in the lymph 

 capillaries and blood capillaries fulfils exactly the condi- 

 tions for osmosis and dialysis. In other words, the mucous 

 membrane and capillary walls furnish the thin membrane 

 separating two fluids, and by dialysis and osmosis the solu- 

 ble crystalloid substances in the intestine are separated 

 from the colloids and pass into the blood. 



In this explanation of the relation of digestion to dial- 

 ysis is seen a new reason for the action of the various 

 enzymes upon foods. In other words, they not only 

 render them soluble but at the same time transform them 

 into crystalloids in order to make possible this action of 

 dialysis. 



While the ordinary process of osmosis accounts for 

 the passing of crystalloid material from the alimentary 

 tract to the lymph and blood, it must be admitted that 

 it does not fully explain the disposal of proteid matter. 

 For example, if peptone were a simple crystalloid sub- 



