THE ABSORPTION OF THE FOOD-STUFFS 829 



increase in the rate of lymph flow from the thoracic duct. On the 

 other hand, the absorption of large amounts of fluid may cause 

 an actual diminution of the solids of the plasma, so that we are 

 justified in regarding the capillary network of blood-vessels at the 

 surface of the villi as solely responsible for the absorption. 



What are the forces which cause this transference of fluid and 

 dissolved substances from one side to the other of the membrane, 

 composed of epithelial cells plus capillary endothelium ? Like other 

 cells, those of the intestinal epithelium are probably bounded on their 

 free surface by a ' lipoid ' membrane, i.e. one containing some complex 

 of lecithin and cholesterin and permeable only by such substances as 

 are soluble in lipoids. On the other hand, the cement substance between 

 the cells may be of a different character and possibly permeable to 

 water-soluble substances. The question has been propounded whether 

 the greater part of the substances which enter the blood serum from 

 the gut pass between the cells or through the cells. Water could, of 

 course, pass in either way. Most of the inorganic salts, such as sodium 

 chloride, as well as the very important constituents of the food, the 

 sugars, are insoluble in lipoids, and would have to pass between the 

 cells. When the question is investigated by the use of dyestuffs, soluble 

 or insoluble in lipoids, it is found that the lipoid-soluble dyestuffs, such 

 as neutral red or toluidin blue, pass into the cells, whereas the dyestuffs 

 which are insoluble in such substances pass into the intercellular 

 spaces. Too much stress, however, must not be laid on these experi- 

 ments. All these dyestuffs are abnormal so far as the body is concerned. 

 We cannot imagine that at any time in the course of evolution of the 

 properties of the intestinal epithelium the cells were ever presented 

 with or had to discriminate between different dyestuffs. The fact 

 that absorption of these dyestuffs is determined by the physical 

 conditions of the ceU membrane is no proof that the absorption of 

 the normal food constituents is determined in the same way. In 

 fact, it is quite legitimate to assume that the lipoid membrane or 

 limiting layer round every cell has as its main office, not the regula- 

 tion of the access of food -stuffs to the cell, but its protection from 

 any of the food -stuffs which it does not require for its metabolism. 

 If it were not for such a membrane the assimilation of a salt would 

 be determined entirely by its concentration in the immediate sur- 

 roundings of the cell, whereas we know that the assimilation of 

 any living organism, whether uni- or multi-cellular, is regulated 

 in the first place by the activity of the organism itself. According 

 to this activity and the needs thereby induced the uptake of food 

 material may be large or small whatever its concentration in the 

 surrounding medium. It would indeed be strange that the whole 

 absorbing surface of the intestine should be covered by a membrane, 



