DIGESTION AND ABSORPTION 1 83 



processes begun in the small intestine may take place for some 

 time in the upper portion of the large intestine. The greater 

 part of the digestion of food material is, however, completed 

 by the time the food mass enters the large intestine. 



ABSORPTION OF FOOD 



The absorption of food, or more exactly of the end products 

 of food digestion, takes place more or less continually from the 

 time the food mass reaches the stomach until it has passed 

 through the small intestine and into the large intestine. This 

 absorption process is one of diffusion through the cell mem- 

 branes lining the digestive tract. It has been found, however, 

 that the process is not one of simple diffusion through mem- 

 branes in accordance with physico-chemical laws, but that 

 it takes place much more rapidly than can be explained by such 

 laws. On this account the process has been termed resorption 

 rather than absorption. We shall, however, use the more 

 common term, viz. absorption. 



Carbohydrates. — The carbohydrate food converted by the 

 process of digestion into the three monosaccharoses, glucose, 

 fructose and galactose, is not absorbed through the cell mem- 

 branes of the digestive tract until it reaches the small intestine. 

 At this point, when the digestion is practically complete, absorp- 

 tion occurs and the monosaccharoses pass through the cell 

 walls of the intestine and enter the capillary blood vessels in 

 the wall of the intestine and then pass into the portal vein 

 leading to the liver. The blood present in the portal vein 

 always contains a large amount of these monosaccharoses, 

 principally glucose, though both fructose and galactose may 

 be present. After the food mass leaves the small intestine, any 

 digested carbohydrate material remaining is absorbed through 

 the walls of the large intestine and reaches the same destina- 

 tion. It has been found, however, that less than 10 per cent of 

 digested carbohydrate food remains unabsorbed when the food 

 leaves the small intestine. Carbohydrate food, therefore, after 

 digestion reaches the circulation in the portal vein leading to 



