DIGESTION 395 



juices added to the foods, but those already mixed 

 with the food continue their action, diffusion goes on, 

 and finally there are left in the large intestine only 

 such substances as can not be digested. These sub- 

 stances, known as the feces, consist chiefly of woody 

 parts of cell walls of vegetables, hard tough parts of 

 meats, and brown colored Avaste from the bile. 



It is very necessary that these waste products be 

 regularly expelled from the body. In them the con- 

 ditions are excellent for the growth of bacteria, which 

 result in the formation of poisons. These poisons, if 

 allowed to accumulate, may become absorbed and cause 

 headaches followed by more serious effects. Proper 

 exercise and a suitable diet, including fruits, large 

 amounts of water, and much coarse food will aid greatly 

 in preventing clogging of the intestines. 



Fig. 155. A small portion of the wall of the small intestine 

 magnified to show the villi. 



Absorption. As we have stated before, the chief 

 purpose of the process of digestion is to prepare the 

 food so that it will diffuse. This process of diffusion 



