CHAPTER XVIII 



THE MECHANISM OF ABSORPTION FKOM THE 

 SMALL INTESTINE 



WATER and material in solution pass from the lumen of the 

 small intestine through its epithelial lining into the subepi- 

 thelial lymphatics and blood capillaries. In this, as in every 

 other instance of a similar phenomenon in the body, two opposing 

 explanations of the means by which it is brought about have 

 been put forward. On the one hand, there are those who 

 attempt to explain the result by diffusion, osmosis, filtration 

 and other physical processes, and who look upon the epithelium 

 as an inert membrane with a permeability which is peculiar to 

 itself, and which is constant so long as the physical conditions 

 of the cells remain unaltered. On the other hand, there are 

 those who consider that while the epithelium makes use of these 

 physical processes, it can always control them, and may bring 

 about results in direct defiance of physical laws. The point 

 to be discussed is which of these two views is the nearer to the 

 truth in the case of the small intestine. 



In considering experiments on intestinal absorption it is 

 necessary to bear in mind continually certain obvious facts. 

 Normally the fluid of the two sides of the gut epithelium is of 

 very different composition. We have therefore to try and 

 estimate how far the end result of an experiment is due to this 

 difference in the fluid on the two sides of the membrane and 

 how far it may be due to a specific action of the membrane 

 itself. Again, it is necessary to remember that the epithelium 

 lining the intestines, no matter whether it acts as a physical 

 membrane or not, is composed after all of living cells, and 

 although they must be adapted to having on their gut side 

 solutions of very various composition, there must be limits 

 within which alone the epithelium can function in a strictly 

 normal way. When, therefore, we introduce into the gut a 



