THE ABSORPTION AND USE OF FOODS 443 



alimentary tract, for the absorptive processes. The innumerable 

 projecting villi, each containing a capillary network and a lymph- 

 channel, afford a total absorbing surface many times greater than 

 would the same area if lined with ordinary mucous membrane; 

 they also, by projecting into the intestinal cavity, are brought 

 more readily into intimate contact with the. intestinal contents. 



Nature of the Absorptive Process. There is very good reason 

 to believe that the process of absorption is not a simple physical 

 one, involving only filtration, osmosis, and dialysis, but that it is 

 carried on actively by the living cells which form the innermost 

 intestinal lining, the columnar epithelium (Chap. XXVI). The 

 support for this idea is partly experimental: the observation that 

 blood-serum placed in the intestine is absorbed completely through 

 its walls into the blood, a phenomenon inexplicable on physical 

 grounds; and partly based on clear indications that none of the 

 digested foods, except the single sugars, pass through these cells 

 without undergoing chemical modification during the passage, so 

 that they enter the blood as different compounds than as the^ left 

 the intestine. 



The entire phenomenon of absorption from the small intestine 

 presents so many phases that it will be convenient to consider it in 

 sections, one class of nutrients at a time. 



The Absorption and Temporary Storage of Carbohydrates. 

 Carbohydrate digestion reduces all foods of the class to single 

 sugars. It is in this form, then, that they undergo absorption. 

 However the process may be carried on it results in a flow of single 

 sugars from the intestinal cavity into the blood-capillaries of the 

 villi. These capillaries all drain, as previously stated (Chap. XIX), 

 into the portal vein, which in turn passes to the liver and breaks up 

 therein into the liver-capillaries (Chap. XXVI) ; so that all blood 

 from the intestine, with whatever it may have taken up there, is 

 forced to traverse the liver, and to come into intimate contact with 

 the liver-cells, before it reaches any of the other living tissues of 

 the Body. 



The amount of sugar present in the blood of the portal vein is, of 

 course, variable, there being a higher concentration at times when 

 sugar is being actively absorbed from the intestine than at other 

 times. Curiously, the blood flowing away from the liver, in the 

 hepatic vein, is always found, normally, to contain a certain small 



