192 ANATOMY FOR NUKSES. [CHAP. XVI. 



Changes the food undergoes in the mouth; mastication and deg- 

 lutition. When solid food is taken into the mouth it is cut 

 and ground by the teeth, being pushed between them again and 

 again by the muscular contractions of the cheeks and the move- 

 ments of the tongue until the whole is thoroughly crushed and 

 ground down. During this process of mastication the salivary 

 glands are excited to very active secretion, the saliva is poured 

 in large quantities into the mouth, and mixing with the food 

 moistens it and reduces it to a soft pulpy condition. A certain 

 amount of air caught in the bubbles of the saliva also becomes 

 entangled in the food. 



The food thus softened and moistened is collected from every 

 part of the mouth by the movements of the tongue, brought 

 together upon its upper surface, and then pressed backwards 

 through the fauces into the pharynx. The elevation of the 

 soft palate prevents the entrance of food into the nasal cham- 

 bers, while the epiglottis bars its entrance into the air passages, 

 and it is guided safely and rapidly through the pharynx into the 

 O3sophagus. Here it passes beyond the control of the will; it 

 is grasped by the oesophageal muscles and by a continuous and 

 rapid peristaltic action is carried onwards and downwards into 

 the stomach. 



Saliva. Mixed saliva (spittle) as it appears in the mouth 

 is a glairy, frothy, cloudy fluid, the glairiness or ropiness being 

 due to mucus ; micro-organisms are also present in it to some 

 extent, and other foreign matters derived from the food. 



Saliva is mainly water containing but little solid matter, its 

 specific gravity varying from 1002 to 1006. It depends for its 

 special action, as a digestive solvent, upon an enzyme or fer- 

 ment which it contains called ptyalin. 



The action of saliva upon the food. r The chief function of 

 saliva is to soften and moisten the food and to assist in masti- 

 cation and deglutition. It has, however, a certain digestive 

 action upon food-stuffs, especially starch. Upon the fats and 

 proteids it has very little effect except to render them softer 

 and better prepared for the action of the other digestive 

 juices. 



By the ptyalin-ferment present in saliva, starch, which is an 

 insoluble substance, is changed into malt sugar or maltose, a 

 highly soluble and absorbable product. This change is best 



