354 FOOD AND DIGESTION 



Saliva. Saliva, as it commonly flows from the mouth, is the mixed 

 secretion of the salivary glands proper and of the glands of the buccal mucous 

 membrane and tongue. When obtained from parotid ducts, and free from 

 mucus, saliva is a transparent watery fluid, the specific gravity of which 

 varies from i . 004 to i . 008 and in which, when examined with the micro- 

 scope, are found floating a number of minute particles, derived from the 

 secreting ducts and vesicles of the glands. In the impure or mixed saliva 

 are found, besides these particles, numerous epithelial scales separated from 

 the surface of the mucous membrane of the mouth and tongue, and the so- 

 called salivary corpuscles, discharged probably from the mucous glands of the 

 mouth and the tonsils. These subside when the saliva is collected in a deep 

 vessel and left at rest. They form a white opaque sediment leaving the 

 supernatant fluid transparent and colorless, or with a pale bluish-gray tint. 

 Saliva also contains various kinds of micro-organisms (bacteria). The 

 saliva, when first secreted, appears to be always alkaline in reaction; the 

 alkalinity is about equal to o . 08 per cent, of sodium carbonate, and is due 

 to the presence of disodium phosphate, Na 2 HPO 4 . 



The presence of potassium sulpho cyanide, 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, HgCl 2 , but not by hydrochloric acid. 



CHEMICAL COMPOSITION OF HUMAN SALIVA. (HAMMERBACHER.) 



In 1,000 Parts. 



Water 994 . 2 



Solids 5.8 



Mucus and epithelium 2.2 



Soluble organic matter (ptyalin) 1.4 



Potassium sulphocyanide o . 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 submaxil- 

 lary. 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. 



