FOOD AND DIGESTION. 337 



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 ; it is often mixed with air, which, being retained 

 by its viscidity, makes it frothy. When obtained from the parotid ducts, 

 and free from mucus, saliva is a transparent watery fluid, the specific 

 gravity of which varies from IQ&Lto. 1008, and in which, when examined 

 with the microscope, are found floating a number of minute particles, 

 derived from the secreting ducts and vesicles of the glands. In the im- 

 pure or mixed saliva are found, besides these particles, numerous epithe- 

 lial 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, which, 

 when the saliva is collected in a deep vessel, and left at rest, subside 

 in the form of a white opaque matter, leaving the supernatant salivary 

 fluid transparent and colorless, or with a pale bluish-ray tint. It also 

 contains various kinds of micro-organisms (bacteria). In reaction, the 

 saliva, when first secreted, appears to be always alkaline : the alkalinity 

 is about equal to .08 or .10 percent of sodium carbonate and is due to the 

 presence of disodium hydrogen phosphate Na 2 HP0 4 . During fasting, the 

 saliva, although secreted alkaline, shortly becomes neutral ; especially 

 when it is secreted slowly and is allowed to mix with the acid mucus of 

 the mouth, by which its alkaline reaction is neutralized. 



CHEMICAL COMPOSITION OP HUMAN SALIVA (HAMMERS ACHER). 



In 1,000 parts. 



Water . . . . . .,..'., . '.'..',-- . 994.2 



Solids *..; 5.8 



Mucus and epithelium . . . . . ' " ^ v '.' ' . 2.2 

 Soluble organic matter (ptyalin) ...'..' , . . 1.4 



Potassium sulpho-cyanide . 0.04 



Salts .... -. ": J " ' '. ' ' '- . ''"'.' *"'''-. 2.20 



The mucin is the largest representative of the organic nitrogenous 

 class of bodies in the saliva ; it may be thrown down by addition of ace- 

 tic acid, if sodium chloride be absent. It gives the three chief proteid 

 reactions, and may easily be split up by the 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 solu- 

 tions. 



The presence q| potassium sulphocyanide (CNKS) in saliva, may be 

 shown by the blood-red coloration which the fluM 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.,), but not by hydrochloric acid. 



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