BOOK VII 



CHAPTER L 

 SPECIAL METABOLISMS 



Absorption. The absorption of the foodstuffs takes place chiefly 

 in the small intestine, and particularly in the middle and lower portions. 

 Considerable discussion has taken place as to the mechanism of this 

 absorption, and at first, when physiological science was young, such 

 comparatively simple processes as filtration, diffusion, and osmosis, 

 were evoked to explain it. It is now generally conceded that it is 

 controlled by unknown forces of the living cells lining this part of 

 the alimentary canal. The chief evidence upon which this conclu- 

 sion is based may be summarized as follows : 



1. If the mucous membrane be removed from a piece of intestine, 

 the absorptive power is abolished. 



2. Poisoning the cells by washing the mucous membrane with 

 a dilute solution of sodium fluoride, or scalding them, destroys the 

 absorptive process. 



3. The absorption of water from the intestine takes place much 

 more quickly than does diffusion through a dead membrane. 



4. The rate of absorption of the products of digestion is too rapid 

 to be explained by simpler physical processes. Peptone is absorbed 

 from the intestine more readily than dextrose; on the other hand, 

 dextrose diffuses through parchment quicker than peptone. Sodium 

 sulphate is not readily absorbed from the intestine, yet it readily 

 diffuses through parchment. 



5. The absorption of water, saline and other salts (magnesium 

 sulphate) is attended by a greatly increased consumption of oxygen 

 by the intestinal cells, showing that absorption, even of water, is an 

 active process (see p. 321). 



6. The animal's own serum, identical in composition and isotonio 

 with the blood, is completely absorbed if introduced within a loop of 

 intestine. 



7. Certain products of digestion, such as those of fat, and probably 

 of protein, are altered during their passage through the cells of the 

 mucous membrane. 



The Metabolism o Protein, In the intestine the protein is broken 

 down into proteoses, peptones, polypeptides, and amino-acids. There 



421 



