SALIVA. 



205 



easily described. After solid food is taken into the mouth, the tongue prevents its escape 

 from between the teeth, and, by its constant movements, rolls the alimentary bolus over 

 and over and passes it at times from one side to the other, so that the food may undergo 

 thorough trituration. Aside from the functions of the tongue as an organ of taste, its sur- 

 face is endowed with peculiar sensibility as regards the consistence, size, and form of dif- 

 ferent articles ; and this property is undoubtedly important in determining when mastica- 

 tion is completed, although the thoroughness with which mastication is accomplished is 

 very much influenced by habit. 



Tonic contraction of the orbicularis oris is necessary to keep the fluids within the mouth 

 during repose ; and this muscle is sometimes brought into action when the mouth is very 

 full, to assist in keeping the food between the teeth. This latter function, however, is 

 mainly performed by the buccinator; the action of which is to press the food between 

 the teeth and keep it in place during mastication, assisting, from time to time, in turning 

 the alimentary bolus so as to subject new portions to trituration. 



The process of mastication is regulated to a very great extent by the exquisite sensi- 

 bility of the teeth to the impressions of hard and soft substances. It is only necessary 

 to call attention to the ease and certainty with which we recognize the presence and the 

 consistence of the smallest substance between the teeth, in order to appreciate the 

 advantages of this tactile sense in mastication. It is in this way, mainly, that we be- 

 come aware that the process of mastication is completed ; and it is this sense which ad- 

 monishes us instantly of the presence of bodies too hard for mastication, which, if allowed 

 to remain in the mouth, might seriously injure the teeth. 



One of the most important of the digestive processes which take place in the mouth 

 is the incorporation of the saliva with the food, or insalivation. Not only has the saliva 

 a mechanical function, assisting to reduce the food to the proper form and consistence to 

 be easily swallowed, but it seems to be 

 necessary to the proper performance of 

 the subsequent processes of digestion 

 and is concerned to a certain extent in 

 the transformation of starch into sugar. 

 That the saliva is necessary to digestion 

 is proven by the grave effects upon the 

 general function of nutrition which fol- 

 low its loss in any considerable quan- 

 tity. This occasionally occurs from the 

 habit of excessive spitting or as the re- 

 sult of salivary fistula. It becomes im- 

 portant, therefore, to study the physical 

 and chemical properties of the saliva, 

 the sources from which it is derived, 

 and its mechanical and chemical func- 

 tions in digestion. 



Saliva. 



The fluid which is mixed with the 

 food in mastication, which moistens the 

 mucous membrane of the mouth, and 

 which may be collected at any time in 

 small quantity by the simple act of spu- 

 tation, is composed of the secretions of 

 a considerable number and variety of 

 glands. The most important of these 

 which are usually called the salivary 



FIG. 51. Salivary glands. (Le Bon.) 

 1, 2, parotid; 8, duct of Steno; 4, fnibmaarill(ir>/ ; 5. 



gual; 6, mylo-hyoid muscle; 7, lingual branch of the fifth 

 nerve; 8, duct of Wharton; 9, digastric muede; 10 ? 

 Bterno-mastoid muscle; 11, external jnpular vein; 1'2. facial 

 vein; 13. temporal vein; 14, 15. internal juiriilar vein; 16, 

 branch of the cervical plexus ; 17, sublingual nerve. 



are the parotid, submaxillary, and sublingual, 

 In addition, we have the labial and buccal 



