132 NUTRITIONAL PHYSIOLOGY 



The statement has been made that the cells lining the 

 intestine do not act in a way that can be imitated by life- 

 less models. The fact is covered by the expression that 

 they exercise selective powers. Too much might easily be 

 inferred from this phrase; there is the same danger which 

 was pointed out in connection with the pyloric sphincter, 

 the inclination to credit the cells with something more like 

 intelligence than it is right to assume for them. It is 

 reasonable to believe that however complex and unex- 

 pected may be the behavior of the cells concerned, a 

 mechanistic explanation would be apparent if our knowl- 

 edge were sufficiently full. What is meant by selective 

 action can be readily illustrated. 



If a comparison is made in the laboratory to determine 

 the relative rates at which glucose and magnesium sul- 

 phate make their way through an ordinary membrane, the 

 sugar will lag behind the salt, although both pass with 

 relative freedom. If a mixture of the two is introduced into 

 the living intestine the impression is totally different. 

 The sugar is absorbed, while the magnesium sulphate is 

 kept back. We say that the mucous membrane is imper- 

 meable to this salt, though we can hardly picture the 

 peculiarity of structure which makes it so. A salt which 

 is refused absorption by the intestinal wall will act as a 

 laxative, for it will hold back from absorption a large quan- 

 tity of water and this will be swept through by the peris- 

 talsis. An investigator has called attention to the fact 

 that all the common precipitants of calcium are denied 

 passage into the blood, and may, therefore, be reckoned 

 as cathartics. These include, beside the sulphates, the 

 phosphates, the citrates, and the tartrates. 



Again and again we find as we pursue the subject that 

 laboratory tests give us little indication of what may be 

 expected of the intestine as an absorbing mechanism. 

 Some substances usually held to be indiffusible pass into 

 the circulation with comparative readiness. Even egg- 

 albumen, a protein of enormous molecule, may enter the 

 blood. This was recently proved by the observation that 



