THE ESSENTIAL PHENOMENA OP DIGESTION. 39 



exceedingly small particles ; these conditions are indis- 

 pensable that absorption may occur. 



The successive operations of digestion have been clas- 

 sified as a certain number of functions, to which have 

 been given the names mastication, insalivation, degluti- 

 tion, stomachic digestion, and intestinal digestion. 



The mouth is the place of mastication and insalivation. 

 By the first of these operations the food is divided into 

 fragments small enough to pass without difficulty the 

 narrower portions of the digestive tube, and to become 

 saturated with the different liquids it encounters in its 

 course. 



During this operation the tongue constantly carries 

 between the teeth those portions of food that require 

 division, while the lips retain such portions as are forced 

 outside the dental arches. 



Insalivation is the name applied to the action of the 

 saliva on the food while in the mouth. The saliva acts 

 mechanically in rendering sufficiently liquid such ali- 

 ments as are too dry; it aids in the agglomeration of the 

 triturated particles into little masses called boluses, and 

 helps these boluses to glide easily into the oesophagus. 

 It exerts a chemical action on starch and sugar, convert- 

 ing them into a sweet substance, called glucose, and 

 glucose is the soluble form in which such substances are 

 absorbed. 



The saliva is composed almost entirely of water, con- 

 taining two or three per cent, of alkaline salts and two or 

 three thousandths of a peculiar principle, called salivary 

 diastase or ptyalin, which acts in the same manner as the 

 diastase of barley in the manufacture of malt. The pres- 

 ence of food in the mouth causes the saliva to flow, and the 

 flow is greater with food which is rich in taste and flavor. 



