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A TEXTBOOK OF PHYSIOLOGY 



The secretion of the buccal glands consists chiefly of thick mucus. 



The mixed saliva obtained from the mouth has therefore a com- 

 bination of 'the properties of the secretions forming it. Its quanti- 

 tative composition may be seen by the following analysis: 



Water 



Solids 



Mucin 



Soluble organic bod es 



Sulphocyanide 



Salts 



It is usually somewhat turbid, owing to the presence of epithelial 

 cells and of food particles. Upon standing, it becomes more so, owing 

 to the deposition of calcium carbonate and organic matter, which forms 

 the tartar deposited on teeth. 



In some animals, saliva contains an oxidase and maltase. The 

 exact significance of the potassium sulphocyanide is not known. 

 Smokers are said to have more than non-smokers. This is very doubt- 

 ful. Its presence can readily be detected by the red colour obtained 

 upon adding a little HC1 and some ferric chloride to the saliva. 



The functions of the saliva are 



1. To act as a solvent by virtue of the large amount of water it 

 contains, and also in part as a solvent for the digested soluble nutritive 

 substances. Probably, saliva plays a larger part in this way than is 

 usually believed. 



2. To act as a lubricant by virtue of its mucus content, and thus 

 facilitate the act of swallowing. 



3. To act upon boiled starch as a digestive agent. 



4. To aid taste dry substances cannot be tasted and indirectly, 

 through taste, to stimulate the flow of saliva and gastric juice. 



5. To moisten the mouth, and thereby aid articulation; to wash 

 out the mouth, and thereby get rid of noxious, evil-tasting, or poisonous 

 substances ; to clean and protect the teeth from decay. 



6. Possibly, when swallowed, to act as an excitant to the flow of 

 gastric juice. 



The manifold nature of its functions accounts for the variation 

 found in the composition of saliva. The nature of the saliva secreted 

 is adapted in the main to that function which is in most demand. 

 This adaptation depends upon three factors chemical, physical, and 

 psychic. Acids and evil-smelling substances cause a great flow of 

 thin parotid saliva; dry bread, cooked potatoes, hard-boiled eggs, 

 cause a flow of saliva rich in ptyalin, the mucus and water content 

 varying according to the dryness; sugar evokes a saliva poor in mucin ; 

 a pebble put in the mouth evokes no secretion, while sand calls forth 

 a large flow. 



The movements of chewing and speaking excite the flow; the hold- 

 ing of food in the mouth evokes little or no secretion. The sight of 

 food makes an animal's mouth " water "; the sight of a colour asso- 

 ciated with a given food may do this also. A stone painted like a 



