276 DIGESTION 



tation is going on from decayed teeth or particles of 

 food lying in the recesses of the teeth and mouth, or 

 disorders of digestion. Under the microscope, the 

 saliva will show epithelial cells, salivary corpuscles 

 resembling white corpuscles, particles of food, and 

 various microorganisms (germs). 



The Physiologic Actions of Saliva. The constant 

 presence of saliva in the mouth is essential to the 

 process of digestion. It has a double duty to perform, 

 called physical and chemical. 



Physically the mucin present in the saliva forms 

 the particles of food into a consistent mass after the 

 food has been softened and moistened while mastica- 

 tion has been going on, and renders it in such shape 

 to be swallowed (deglutition). 



The Chemical Action of Saliva. Its main property 

 is the changing or converting of starch into sugar 

 by the means of a starch-changing ferment or enzyme, 

 which is supposed to belong to the group of proteins, 

 called ptyalin, in the saliva. 



Deglutition (the act of swallowing). This is a com- 

 plicated function due to the action of the muscles of 

 the tongue, soft palate, pharynx, and esophagus acting 

 upon the food, called a bolus after it has been chewed 

 or masticated by the teeth, whereupon these structures 

 force it from the mouth into the pharynx and thence 

 through the esophagus into the stomach. 



At the beginning of deglutition the mouth is closed 

 and the tip of the tongue is rested against the incisor 

 teeth. There is a temporary suspension of respiration. 

 The tongue arches from before backward against the 

 roof of the -mouth, due to the action of the intrinsic 

 muscles, those contained therein, and forces the 

 bolus of food through the opening leading from the 

 mouth to the pharynx. These actions are under 

 the control of the will. The food upon reaching the 

 pharynx is seized by the constrictor muscle of this 

 organ and by a sphincter action is forced through the 



