DIGESTION AND RESORPTION 107 



tion of that substance as well as of salts. There are other facts, 

 however, for which it is difficult at present to offer any physico- 

 chemical explanation. Notable among these is the predominant per- 

 meability of the intestinal epithelium in one direction, viz., from the 

 intestinal lumen towards the blood and lymph vessels. 



For the present, it is necessary to be content with the state- 

 ment that resorption is a function of the living epithelial cells, 

 although such a statement, of course, explains nothing but 

 simply means that it is impossible at present to form an adequate 

 conception of the intimate mechanism of the process. 



Resorption might be characterized briefly as a reverse se- 

 cretion. In secretion the active cells of a gland take materials 

 from the blood or lymph, transform them into the specific 

 substances characteristic of the cells, and then eject the latter 

 into the duct of the gland. The epithelial cells of the digestive 

 tract, on the other hand, take up digested materials from the 

 contents of the alimentary canal, modify them more or less 

 chemically and transmit the products to the blood or lymph. 



151. Paths of resorption. Most of the resorbed substances 

 seem to pass from the epithelial cells to the blood in the capil- 

 laries and thus finally through the portal vein (182) to the liver. 

 This is true of the cleavage products of the proteins, of carbo- 

 hydrates, organic acids, salts and water. Fats, on the other 

 hand, enter the circulation chiefly or wholly through the lymph 

 in the form of minute droplets which are secreted by the epi- 

 thelium into the lacteals of the villi, whence they pass through 

 the lymphatics to the thoracic duct (186). 



152. Chemical changes in resorption. It is somewhat 

 generally believed that the products of digestion, especially of 

 the proteins and fats, undergo rather extensive chemical changes 

 in the epithelial cells during the process of resorption. This 

 question is considered more particularly in Chapter V but may 

 be briefly referred to here for the sake of completeness. 



Proteins. In digestion the proteins yield comparatively 

 simple cleavage products. It has been maintained, especially 

 by Abderhalden and his school, that these cleavage products 

 are resynthesized in the epithelial cells into serum albumin, 

 which is regarded as the common source of all the body proteins. 

 This view has been based chiefly on the failure to detect amino 

 acids or other protein cleavage products in the blood coming 



