CHAPTER XV. 



ABSORPTION AND THE ROUTES OF FOOD. 



In the preceding chapter the various changes were fol- 

 lowed by which the undigested foods were transformed into 

 substances which were able to dialyze through the alimen- 

 tary wall into the blood. It was until recently believed 

 that the absorption of all of the foods, with the possible ex- 

 ception of fat, was a mere physical process, and therefore 

 animal membranes were taken to establish experimentally 

 the details of the process. More recent work in this field 

 proves conclusively that we have to do here with something 

 more than mere physical osmosis; something more than 

 mere filtration. We have to do here with living epithelium 

 cells, which in a very active way, and according to physio- 

 logical laws of their own, materially modify the simple 

 physical process. 



No doubt much of the food employed does pass into the 

 blood by simple physical osmosis, but there is reason to be- 

 lieve that by far the largest proportion of the food absorbed 

 is transferred into the system in consequence of the active 

 participation of the living epithelium cells. This will ex- 

 plain why a dead piece of intestine has lost to so great an 

 extent its power to absorb. 



The exact way in which these living cells participate in 

 this absorption we do not at present understand. Simple 

 physical osmosis, however, may be easily studied on dead 

 animal membranes. If, for instance, such a membrane be 

 placed between two liquids of different composition, currents 

 are at once set up through it tending to equalize the com- 

 position of the two fluids, the strength of the currents de- 

 pending upon the dialyzing power of the substances dis- 

 solved. Thus, if on one side of such a membrame a solu- 

 tion of salt be placed,- and on the other side a solution of 



