CHAPTER XII. 



DIGESTION. 



The Object of Digestion. Some of the foodstuffs which 

 we eat are already in solution and ready to soak at once 

 into the lymphatics and blood-vessels of the alimentary 

 canal; others, such as a lump of sugar, though not dis- 

 solved when put into the mouth, are readily soluble in the 

 liquids found in the alimentary canal and need no further 

 digestion. In the case of many most important foodstuffs, 

 however, special chemical changes hare "to be brought 

 about to make them soluble and capable of absorption. 

 The different secretions poured into the alimentary tube 

 act in various ways upon different foodstuffs, simply dis- 

 solving some and chemically changing others, until at last 

 all are got into a condition in which they can be taken up 

 into the lymph and blood-vessels for transference to distant 

 parts of the body. 



The Saliva. The first solvent poured upon the food is 

 the saliva, which, when it meets the food, is a mixture 

 of pure saliva with the mucus secreted by the membrane 

 lining the mouth. This mixed saliva is a colorless, cloudy, 

 feebly alkaline liquid. 



The Uses of Saliva are mainly physical and mechanical. 

 It keeps the mouth moist and allows us to speak with corn- 

 Explain the object of digestion. 



What is the first digestive liquid which the food meets with? 

 How does it differ from pure saliva? Describe mixed saliva. 



[152] 



