492 THE HUMAN BODY 



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 chiefly experimental: the observation that 

 blood-serum placed in the intestine is absorbed completely through 

 its walls into the blood so long as the mucous lining is alive and 

 functioning, but fails to be absorbed if the cells are injured, as by 

 sodium fluorid, or some similar poison. Since the blood-serum 

 placed in the intestine has presumably precisely the same osmotic 

 pressure and percentage composition as the animal's own it is 

 difficult to see how purely physical factors could bring about the 

 absorption. 



Channels of Absorption. We noted above that each villus 

 contains a capillary network and a lymph-channel. The absorbed 

 food stuffs might pass, therefore, either to the blood-stream directly 

 or by the lymph-channels be conveyed to the receptaculum chyli 

 (p. 382), and thence by way of the thoracic duct enter the blood- 

 stream at the great vein of the shoulder. The essential difference 

 between these two pathways is that the intestinal blood-stream 

 drains into the portal vein, and must pass, therefore, through the 

 capillaries of the liver before reaching the general circulation, while 

 the lymph-stream reaches the general circulation without first 

 traversing the liver. The significance of these two pathways will 

 appear presently. 



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 



