136 



DIGESTION AND ABSORPTION 



acids, glycerol, and soaps from fat, they are in a diffusible form 

 and may enter the blood where they will be utilized. The process 

 of food passing through the lining of the alimentary canal and 

 through the walls of the blood vessels is called absorption. Ab- 

 sorption is a process of osmosis. Very little absorption takes place 

 in the mouth, throat, gullet, and stomach because the food is in 



constant motion, and because very 

 little of it has reached the end 

 point of digestion. 



Most absorption takes place in 

 the small intestine for the follow- 

 ing reasons : (1) The digestion of 

 most of the food has been com- 

 pleted. Many of the end products 

 of the digestive process are formed 

 in the small intestine. (2) The 

 great length of the small intestine 

 gives a larger absorbing surface. 

 (3) The small intestine is narrow 

 and the food is pressed against all 

 its surfaces. Consequently, not only the lower surface but all its 

 surfaces are used in the absorbing process. (4) Muscular activity 

 helps the food to mix with the digestive juices so that complete 

 digestion takes place. At the same time, the food is pressed against 

 the absorbing surface. (5) There are special adaptations, folds, 

 and mill, for increasing the absorbing surface. The length of the 

 small intestine is at least twenty feet in an adult. The number 

 of epithelial cells used in absorption is tremendously increased by 

 the presence of circular folds or ridges around the circumference of 

 the lining of the intestine. On these ridges are little structures 

 called villi, so numerous and so close together that they resemble 

 the nap on carpet. Each villus consists of an outer layer of epi- 

 thelial cells inclosing a network of capillaries and a central lymph- 



Hairlike microscopic structures known 

 as villi are found in great numbers on the 

 lining of the small intestine. They in- 

 crease the absorbing surface. The mouths 

 of intestinal glands show among them. 



