CHAPTER IV. 



DIGESTION (Cont'd). 



The Chemistry of Saliva and the Relationship of Saliva to 



Dental Caries. 



A knowledge of the composition and chemical properties of 

 saliva is of great importance because of the undoubted etiologi- 

 cal relationship which exists between this secretion and dental 

 caries. Mixed saliva when freshly secreted is a watery, more or 

 less opalescent and sticky fluid, often containing small masses 

 of mucin, but on standing it becomes cloudy because of precipi- 

 tation of calcium carbonate. Its specific gravity is 1002-1006, 

 and it contains about 0.05 per cent of solids. The saliva from 

 the sublingual and submaxillary glands is very much richer in 

 solids than that from the parotid. The parotid saliva also differs 

 from that of the other glands in containing no mucin, although 

 it is often rich in ferment. The solid constituents, with some of 

 their properties, are as follows : 



Glycoprotein (mucin) : precipitated by acid. 

 Other proteins: coagulated by heat. 

 Organic. . .-\ Ptyalin: a starch-splitting enzyme. 



Potassium sulphocyanide : gives a red color with 

 ferric chloride. 



Sodium chloride : ) give a precipitate with sil- 



Potassium chloride : | ver nitrate. 



Calcium bicarbonate : in fresh saliva. 



Calcium carbonate : precipitated in saliva after 

 Inorganic.^ standing. 



Calcium and magnesium 1 Have an important re- 

 phosphates: lationship to the neu- 



Sodium and potassium | tralizing properties 

 phosphates: J of saliva. 



Organic Constituents. Mucin is the substance to which 

 saliva owes its stickiness. Being a glycoprotein, it yields reduc- 



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