43 8 ABSORPTION 



absorption by the intestine, but without much success. The very 

 fact that the theory is held to apply to practically any cell greatly 

 circumscribes at the outset its power of dealing with cells like those 

 lining the intestine, which are adapted to absorb nutrient materials 

 for all the body, and must necessarily differ in their permeability 

 from cells adapted for some limited function involving only a 

 limited and specialized nutritive exchange. Thus sodium chloride 

 practically does not penetrate the red blood-corpuscles, the muscle 

 fibres, and many other tissue elements. It is a lipoid-insoluble sub- 

 stance, and the lipoid theory says that this is the reason why it 

 penetrates these cells with such difficulty. But sodium chloride 

 must and does penetrate the intestinal mucosa, and with consider- 

 able ease, in order that the body, especially its extracellular liquids, 

 may obtain a sufficient supply of this indispensable material. It is 

 still a lipoid-insoluble substance, and pays no heed to the lipoid 

 theory at all. It is perfectly true that some substances e.g., ethyl 

 alcohol which are much more soluble in lipoids than sodium 

 chloride, are also even more readily absorbed from the intestine. 

 It has been stated also that, as regards the velocity of their absorp- 

 tion, the three alcohols, glycerin, erythrite, and mannite, are related 

 to each other in the same way as in regard to their lipoid-solubility. 

 There is, of course, some reason for this, and also some reason why 

 ethyl alcohol is taken up more easily than salt, but we do not know 

 that it has anything to do with lipoid-solubility. If there is a 

 lipoid layer at the free ends of the cells covering the villi, it is very 

 possible that a substance soluble in lipoids may be able to enter cells 

 which would otherwise have denied it entrance. It may even inflict 

 temporary or permanent injury on the cells in doing so, and may 

 thus be taken up in greater amount than by normal cells, and this 

 possibility has to be reckoned with in giving a physiological value 

 to experiments with materials essentially foreign to the intestine, 

 and to which it cannot have developed any adequate adaptation. 

 For the essential food materials it is quite certain that, apart from 

 any general relations of cell envelope and environing liquids which 

 are common to the intestinal and to other cells, special relations of 

 an adaptive nature have been developed between the intestinal cells 

 and the very special liquids, elsewhere unknown in the body, with 

 which they come in contact in the lumen of the gut. It is unlikely 

 that the mucosa has developed a special adaptation for lipoid- 

 soluble food materials; it must have developed an adaptation for 

 such food materials, lipoid-soluble or not, as have been offered to 

 it through countless ages, and as are necessary for the nutrition of 

 the organism. 



But if it be true that the action of the columnar epithelium of the 

 intestinal mucous membrane in the absorption of the food is in the 

 main a process of selective secretion such as is found in glandular 



