DIGESTION AND ABSORPTION 



341 



the fluid food gradually toward the posterior end of the tube. If the 

 chief function of the small intestine is that of absorption, we must look 

 for adaptations which increase the absorbing surface of the tube. This is 

 gained in part by the inner surface of the tube being thrown into transverse 

 folds which not only retard the rapidity with which food passes down the 

 intestine, but also give more absorbing surface. But far more important 

 for absorption are millions of little projections which cover the inner sur- 

 face of the small intestine. So numerous are these projections that the 

 whole surface presents a velvety appearance. Collectively, these structures 

 are called the villi (singular villus) . They form the chief organs of absorp- 

 tion in the intestine, several thousand being 

 distributed over every square inch of surface. 

 Between the villi are found the openings of 

 many small tubelike glands, the intestinal 

 glands. These glands manufacture a digest- 

 ive fluid, the function of which is believed 

 to be somewhat like that of the pancreatic 

 fluid. 



Internal Structure of a Villus. — The in- 

 ternal structure of a villus is best seen in 

 a longitudinal section. We find the outer 

 wall made up of a thin layer of cells, the 

 epithelial layer. It is the duty of these cells 

 to absorb, by osmosis, the semifluid food 

 from within the intestine. Sugars and pep- 

 tones are passed through the cells to a 

 number of tiny capillaries or blood vessels 

 found immediately under the epithelial 

 layer. From here they pass (through what 

 is known as the portal circulation) into the 

 liver, where, as we have seen, sugar is taken 

 from the blood and stored as glycogen. 



Diagram of a longitudinal section 

 through a villus; a, epithelium 

 which takes up food and trans- 

 ports it to the tubes within; 

 &, an artery; c, capillaries; d, a 

 lacteal. 



Course of Food after Absorption. — From the liver, this food 

 within the blood is sent to the heart, from there pumped to the 

 lungs, from there it returns to the heart and is pumped to the 

 tissues of the body, where as nitrogenous food it may build proto- 

 plasm or, if carbohydrate, may be used to furnish energy. A large 

 amount of water and some salts are absorbed through the walls 

 of the stomach and intestine as food passes on its course. The 

 fats, in the form of an emulsion, are also taken up through the 

 cells lining the villi and pass, not into the blood vessels, but into 

 a space which occupies part of the interior of the villus. Each of 



