DIGESTION AND ABSORPTION OF NUTRIENTS 99 



have seen that the foods we eat are ground up in the mouth 

 cavity by the teeth and thus made ready for the action of the 

 various digestive juices. We have also demonstrated that 

 sugars and soluble salts are dissolved in the mouth; that 

 insoluble mineral matters are made soluble in the stomach ; 

 that starch is changed to sugar by the saliva and pancreatic 

 juice; that proteins are converted into peptones by the 

 pancreatic and gastric juices ; and that fats are digested in 

 the intestines by the combined action of bile and pancreatic 

 juice. Were the food to remain within the alimentary 

 canal, however, even though it had been thoroughly digested, 

 it would still be, in a certain sense, outside the body, since 

 this canal is a continuous tube opening to the exterior at 

 either end. In order to furnish material for building and 

 repairing the various tissues, the liquid nutrients must be 

 distributed to the tissues wherever needed. This is accom- 

 plished through the agency of the blood system. We have 

 now to consider the process of absorption, which includes the 

 final steps whereby foods become a part of blood. By absorp- 

 tion is meant the passage of the digested food through the lining 

 of the alimentary canal, and through the thin walls of the count- 

 less blood vessels that lie close at hand. 



139. Absorption in the mouth, throat, gullet, and stomach. 



While the mouth, throat, and gullet all have a moist lining, gener- 

 ously supplied with thin-walled blood vessels, relatively little ab- 

 sorption takes place in these regions; first, because only a small 

 amount of the food has been digested, and secondly, because the food 

 does not remain long enough in these organs for absorption to take 

 place. 



The food usually remains in the stomach for several hours, and 

 one would naturally expect that a good deal of absorption would 

 take place during this time. But we must remember that the con- 

 traction of the stomach muscles keeps the food in constant motion, 



