158 ANATOMY AND PHYSIOLOGY 



intestines or saliva, gastric juice, and intestinal fluids; it is 

 greatly facilitated by the presence of enzymes in the fluids. En- 

 zymes are organic substances which are the result of cell ac- 

 tivity. Their composition is undetermined; we know them 

 only by their works. The characteristic which makes them valu- 

 able is their power to stimulate rapid changes in certain other 

 substances by their presence alone, while they themselves remain 

 unchanged. In this way, the smallest quantity of an enzyme 

 may effect changes in a large amount of material. (See p. 165.) 



In the mouth the mechanical process includes mastication and 

 insalivation. By the teeth the food is divided, then crushed and 

 ground; at the same time it is softened by saliva. The parotid 

 saliva does most of this, being the most abundant; it is poured 

 into the mouth just outside the upper second molar and thus it 

 mixes at once with the mass as it is crushed and ground. Sub- 

 maxillary and sublingual saliva contain much more mucin and 

 lubricate as well as soften the food. The saliva also dissolves 

 the sapid substances, in order that the nerves of taste in the tongue 

 may appreciate them. One can neither taste nor swallow a per- 

 fectly dry substance. 



In the mouth the chemical process is the conversion of starch 

 into sugar. The digestive fluid is saliva; the two enzymes are 

 ptyalin (salivary diastase) and maltase. Ptyalin does most of the 

 work changing the starch molecule first into dextrin and then 

 into maltose (and a little dextrose) . Not all the starch taken at 

 one time is digested in the mouth for the reason that it leaves 

 the mouth too soon. (If it is retained in the mouth for some 

 time, especially if mastication be continued, the presence of the 

 sugar thus formed will be evident to the taste.) 



The digestion of starch requires an alkaline medium; ptyalin 

 cannot act in acid fluids. 



Saliva is alkaline. 



Being masticated, insalivated and hydrolyzed (see p. 166), the 

 food is now prepared for deglutition or swallowing, by which it is 

 passed through the pharynx and esophagus into the stomach. 

 The tongue presses against the hard palate, thus giving the bolus 

 (as the prepared mass is called) an impulse toward the isthmus 

 of the fauces; as it passes through this space the upper pharyngeal 

 constrictor muscle grasps it and passes it on then the middle and 



