CHAPTER XIV. 

 THE ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 



i. The Problem of Absorption. The problem of the ab- 

 sorption of foodstuffs from the alimentary tract is the same 

 as the problem of absorption in general. It is self-evident 

 that absorption and secretion are really only different phases 

 of the same thing, for in the one case we are asked to explain 

 why a tissue takes up a certain chemical substance, while 

 in the other we are called upon to tell how a tissue rids itself 

 of this same chemical substance. In the last anlaysis we 

 have to answer these questions for every individual *cell, for 

 each absorbs certain substances and secretes others. Very 

 often one and the same cell absorbs and secretes the same 

 substance. As will become apparent later the intestinal 

 epithelium, for instance, absorbs fat from the lumen of the 

 gut and secretes it into the lymph stream. The lymph may 

 therefore be looked upon as an absorptive (fluid) tissue which 

 in turn becomes a secretory tissue when the body cells begin 

 to take the fat away from it. 



Put briefly, therefore, we can say that in considering the 

 problem of absorption we have to answer the question, How 

 do the various chemical substances pass from one cell into 

 another, or from one cell into a liquid (such as the lymph or 

 blood), or finally from such a liquid into a cell? Under the 

 last heading comes, for example, the passage of the chemical 

 substances contained in the lumen of the alimentary tract 

 into the cells lining this tract, while the passage of these same 



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