216 HUMAN PHYSIOLOGY 



its cells is endowed with some of the powers of an 

 organism we can say that the epithelium has selective 

 capacity. Energy is probably applied to determine the 

 movement of the various compounds through the cells. 

 Certain bodies which are rated as highly diffusible by 

 ordinary standards are not received through the mucous 

 membrane. 



A comparison between grape sugar and magnesium 

 sulphate (Epsom salts) will make clear what is meant 

 by selective absorption. When a test is made with 

 parchment or any inert membrane the sugar is found 

 to diffuse less readily than the magnesium sulphate. 

 The facts are reversed in the intestine; the sugar is 

 absorbed with ease, while the salt is excluded almost 

 entirely from the cells of the epithelial barrier. The 

 purgative action of the salt is due to its failure to be 

 absorbed and to the fact that it keeps back from ab- 

 sorption a large volume of water required to hold it in 

 solution. 



The compounds ordinarily absorbed are water, certain 

 salts (especially chlorids), simple sugars, glycerin, fatty 

 acids or soaps, and amino-acids. This statement means 

 that these compounds disappear from the intestine, 

 but it does not strictly follow that they arrive in the 

 blood. There is the possibility of changes affecting 

 them in their passage through the cells. A striking 

 comparison has been made between secretion and ab- 

 sorption. Gland cells take various materials from the 

 blood and often manufacture entirely new compounds 

 which they put out from their free border. Absorbing 

 cells receive sundry substances from the cavity of the 

 intestine and work them over before transferring the 

 resulting bodies to the interior of the villi. Such cells 

 may be said to secrete inward instead of outward. 



The best known case of a modifying influence exerted 

 by the lining cells of the intestine is their action upon 

 the products of fat digestion. Glycerin and fatty 

 acids or soaps disappear from the canal but the cells 

 work a change which is the reverse of digestion and that 



