Chap. 11 FOODS AND NUTRITION 193 



that act upon cane sugar, malt sugar, and milk sugar, respectively, turning 

 them into simpler sugars, such as glucose, which can be absorbed by ceils 

 of the body. The intestinal glands also secrete enterokinase which changes 

 inactive trypsinogen into active trypsin that simplifies proteins. An intestinal 

 lipase acts on fats. Glands in the intestinal wall also secrete a large amount 

 of mucus that lubricates the passage of the food. 



Protein 



apillary 



Lymph vessel 



sulphate 



Fig. 11.20. The surroundings of a villus, one among millions, and names of 

 some substances absorbed. Large particles cannot penetrate the wall of the villus; 

 most others enter it and the blood capillary along with certain minerals. Fats 

 enter the lymph vessel and later pass into the blood. 



Absorption of Food 



Absorption of food occurs almost entirely in the walls of the small in- 

 testine. Only then is it actually inside the body. This wall has in superlative 

 degree three essentials for the absorption of food: means for keeping the 

 food in constant agitation, a great area of semipermeable membrane, and 

 blood and lymph to pick up and transport the absorbed food (Figs. 11.19, 

 11.20). The walls of the small intestine contract peristaltically, pushing the 

 content of the food through the lumen. They also constrict rhythmically, 

 sharply squeezing the tube into segments hke a chain of sausages and repeating 

 the process again and again, twenty times per minute or more (visible by 

 x-ray), each time pressing the food against the absorptive wall. 



Especially in the duodenum the lining is thrown into circular folds and its 

 entire surface, folds and all, is covered with millions of villi all in more or 

 less continual, slightly tremulous motion. The shortening, lengthening, and 

 bending of the villi keep the digested food agitated and moving against the 



