330 ESSENTIALS OF PHYSIOLOGY. 



own blood serum, so that diffusion and osmosis do not provide a com- 

 plete explanation. 



Although these physical processes do not account for absorption, 

 they may nevertheless occur in the intestine. Thus, if the bowel con- 

 tain a hypertonic solution of salt, water passes from the mucous 

 membrane into the solution until the latter becomes isotonic with the 

 blood, after which it is steadily absorbed. Osmosis may therefore 

 retard, or on the other hand it may assist, absorption, but the passage 

 of material from the lumen of the intestine into the villi must be 

 regarded as due to the activity of the epithelial cells. If, however, the 

 epithelial cells are injured by means of sodium fluoride, absorption is 

 entirely regulated by the processes of diffusion and osmosis, and is 

 therefore incomplete. Histological evidence as regards the absorption 

 of fats shows that these substances pass through the epithelial cells, and 

 experiments with dyes soluble in lipoids show that these also enter the 

 cells themselves. The absorption of the products of protein digestion 

 is also said to be accompanied by structural changes in the cells, and 

 probably both amino-acids and dextrose pass through the cell substance. 

 On the other hand, dyes insoluble in lipoids have been shown to pass 

 between the cells, and the possibility of the intercellular cement forming 

 a route for absorption cannot be considered to be absolutely excluded. 



Generally speaking, therefore, absorption is an active or vital 

 process, even in the case of water and salts, but it may be assisted or 

 retarded by the physical processes of diffusion and osmosis. 



The Absorption of the Products of the Digestion of Proteins. The 

 final stages of the digestive hydrolysis of the food proteins may take 

 place in the mucous membrane of the intestine after absorption has 

 begun. It has been shown that during the absorption of a protein 

 meal the presence of peptone can be demonstrated in the wall of the 

 bowel. If, however, the mucous membrane be kept at the body 

 temperature for half an hour before it is analysed, no peptone will be 

 found in it. Proteose and peptone may be taken up by the epithelial 

 cells, and may be further hydrolysed by erepsin in the cells themselves 

 with the formation of amino-acids. Even coagulable proteins may be 

 taken up by the epithelial cells. It has already been pointed out that 

 an animal can absorb its own serum, and this will take place when the 

 intestine has been washed free of enzymes. Similarly egg-albumin 

 may be absorbed, the amount introduced into the bowel being reduced 

 by one-fifth in three hours. It is probable that, in these circum- 

 stances, complete digestive hydrolysis takes place in the mucous 

 membrane, and that all food proteins are reduced to the amino-acid 

 condition before being utilised in the body. 



