96 THE DIGESTION OF FOOD 



to the ferments, and tends to prevent the decay of food during 

 digestion. 



The digestion of starch begins in the mouth, the digestion 

 of protein begins in the stomach, but the change of fats into 

 substances which can be used by the body takes place only in 

 the intestine, because fat is chemically unaffected by ptyalin 

 and pepsin. Food remains in the small intestine from five to 

 fifteen hours and during that time all of it which can be used by 

 the body is digested and absorbed through the thin-walled 

 vessels thickly scattered over its surface. But not every parti- 

 cle of food that we eat is digested ; cellulose, for example, is 

 chemically unaffected by all the digestive juices and cannot be 

 changed by them into substances usable by the body. Such 

 undigested matter passes from the small intestine into the large 

 intestine, from which it is expelled by the daily discharge of the 

 bowels. If large quantities of food are eaten or the digestive 

 system is out of order, some digestible matter may also escape 

 digestion and pass off with the undigestible matter. 



