DIGESTION 157 



small pieces by means of the teeth. This is mastication. At the 

 same time it is mixed with saliva; this is hydration and insalivation. 



By the act of swallowing, the softened mass is passed into the 

 pharynx and down through the esophagus to the stomach. This 

 is deglutition. (The soft palate prevents it from going upward to 

 the nose, and the epiglottis prevents it from entering the larynx.) 



The stomach now takes charge. The mass is compressed and 

 moved about by the layers of the muscular coat until it is thor- 

 oughly saturated with gastric juice, and becomes a pale yellowish 

 fluid called chyme. As fast as this is accomplished, the pylorus, 

 or gate-keeper, allows it to go through into the duodenum, where 

 it meets the intestinal and pancreatic juices, and bile. 



Continuing through the small intestine it loses in increasing 

 measure its fluid and nutritious portions, and in the large intes- 

 tine it is still further reduced to waste alone, which is expelled 

 from the body. 



Mechanical Processes of Digestion 



The passage of food through the several organs, as above 

 outlined, represents the sum of the mechanical processes resulting 

 from the peristaltic action of the muscles of the tract, which are al- 

 ready described as consisting of layers of circular and longitudinal 

 fibers surrounding the tube of mucous membrane. In addition 

 to these, there is an entirely different set the muscles of masti- 

 cation, which move the mandible or lower jaw, and keep the food 

 between the upper and lower teeth. Their action constitutes 

 the first mechanical process of digestion; this is of great importance, 

 because only when the food is in small fragments (or masticated) 

 can the digestive juices have access to the whole mass. 



Chemical Processes of Digestion 



The first occurrence which follows the introduction of food is 

 an increased flow of blood to the part and activity of the secreting 

 cells as the food arrives, beginning with the secretion of saliva. 

 In fact, the cells may begin to work beforehand, being stimulated 

 by the thought of food. This is true of both saliva and gastric 

 juice. 



The chemical process of digestion is brought about by 

 the action of digestive fluids in the mouth, stomach and 



