310 FOOD AND DIGESTION 



action of a dilute mineral acid into globulin and a carbohydrate whose exact 

 character has not yet been established, though it resembles a sugar in reducing 

 copper-sulphate solutions. The presence of potassium sulphocyanide, KCNS, 

 in saliva may be shown by the blood-red coloration which the fluid gives 

 with a solution of ferric chloride, Fe 2 Cl 6 , and which is bleached on the addition 

 of a solution of mercuric chloride, HgQ 2 , but not by hydrochloric acid. 



CHEMICAL COMPOSITION OF HUMAN SALIVA. (HAMMERS ACHER.) 



In i.ooo Parts. 



Water 994-2 



Solids 5-8 



Mucus and epithelium 2.2 



Soluble organic matter (ptyalin) 1.4 



Potassium sulphocyanide 0.04 



Salts 2.20 



Saliva from the parotid is less viscid; less alkaline, the first few drops 

 discharged in secretion being even acid in reaction; clearer, although it may 

 become cloudy on standing from the precipitation of calcium carbonate by the 

 escape of carbon dioxide; and more watery than that from the submaxillary. 

 It has moreover a less powerful action on starch. Sublingual saliva is the 

 most viscid, and contains more solids than either of the other two, but has 

 little diastasic action. 



Rate of Secretion and Quantity of Saliva. The rate at which saliva 

 is secreted is subject to considerable variation. When the tongue and muscles 

 concerned in mastication are at rest, and the nerves of the mouth are subject 

 to no unusual stimulus, the quantity secreted is not more than sufficient with 

 the mucus to keep the mouth moist. During actual secretion the flow is much 

 accelerated. 



The quantity secreted in twenty-four hours varies greatly, but is at least 

 i liter. 



Function of Saliva. The purposes served by saliva are mechanical 

 and chemical. 



Mechanical, (i) It keeps the mouth in a due condition of moisture, 

 facilitating the movements of the tongue in speaking, and the mastication 

 of food. (2) It serves also in dissolving sapid substances, and renders them 

 capable of exciting the nerves of taste. (3) But the principal mechanical 

 purpose of the saliva is that, by mixing with the food during mastication, it 

 makes a soft pulpy mass such as may be easily swallowed. To this purpose 

 the saliva is adapted both by quantity and quality. For, speaking generally, 

 the quantity secreted during feeding is in direct proportion to the dryness 

 and hardness of the food. 



Chemical. The chemical action which the saliva exerts upon the food in 

 the mouth is to convert the starchy materials which it contains into soluble 

 starch and then into sugar. This power the saliva owes to the enzyme ptyalin. 



