292 ANATOMY FOR NURSES [Chap. XV 



The functions of saliva. — Saliva has four distinct functions. 

 (1) It assists in speech by moistening the mucous membrane of 

 the mouth and tongue. (2) It assists in mastication and degkiti- 

 tion by softening and moistening the food. SaHva causes the 

 masticated portions to stick together and thus helps to form a 

 bolus which is coated with mucus and easily swallowed. 



(3) It renders soluble substances capable of being tasted. Salts, 

 sugar, acids, and bitters are dissolved in the saliva. 



(4) It acts upon starch. 



Ptyalin. — By the ptyalin-ferment present in saliva, starch, 

 which is an insoluble substance, is partially changed to dextrin 

 and sugar (maltose). This process is a complicated one and it is 

 probable that a number of intermediate compounds exist between 

 the huge starch molecule and the dextrin and maltose. This 

 change is best effected at the temperature of the body,^ in a slightly 

 alkaline solution, saliva that is distinctly acid hindering or ar- 

 resting the process. Boiled starch is changed more rapidly and 

 completely than raw, but the food is never retained in the mouth 

 long enough for the saliva to more than begin the transformation 

 of starchy matters.- 



Deglutition, or swallowing. — The food thus softened and moist- 

 ened is collected from every part of the mouth by the movements 

 of the tongue, brought together upon its upper surface, and then 

 pressed backward through the fauces into the pharynx. The 

 elevation of the soft palate prevents the entrance of food into the 

 nasal chambers, while the epiglottis bars its entrance into the 

 air-passages, and it is guided safely and rapidly through the 

 pharynx into the oesophagus. Here it passes beyond the control 

 of the will ; it is grasped by the oesophageal muscles and by a 

 continuous peristaltic action is carried downward. The cardiac 

 orifice of the stomach is guarded by a sphincter muscle which is 

 normally in a state of contraction. The peristaltic wave which 

 passes slowly down the oesophagus inhibits this contraction and 

 forces the food into the stomach. 



During the process of mastication, insalivation, and deglutition 

 the food is first reduced to a soft, pulpy condition ; second, 



1 A temperature of 100° F. in the alimentary canal is necessary for digestion, 

 hence iced drinks or iced foods that lower this temperature delay digestion, 



* The salivary glands do not become active until the subject is from four to six 

 months old ; hence the reason for avoiding starchy food for young infants. 



