SALIVA 169 



Movements in a forward direction, so as to make the lower teeth project 

 beyond the upper, are effected by the pterygoids, the oblique fibres of 

 the masseter and the anterior fibres of the temporal. By the combined 

 action of the posterior fibres of the temporal, the digastric, mylo-hyoid 

 and genio-hyoid, the jaw is brought back to its position. By the same 

 action it may also be drawn back slightly from its normal position while 

 at rest. 



Action of the Tongue, Lips and Cheeks, in Mastication. The varied 

 and complex movements of the tongue during mastication are not 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 uses of the tongue as an organ of taste, its surface is endowed 

 with peculiar sensibility as regards the consistence, size and form of 

 different articles ; and this is important in determining when masti- 

 cation is completed, although the thoroughness of mastication is much 

 influenced by habit. 



Tonic contraction of the orbicularis oris is necessary to keep the 

 liquids within the mouth during repose ; and this muscle is sometimes 

 brought into action when the mouth is full, to assist in keeping the food 

 between the teeth. This latter office, however, is performed mainly 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. 



Mastication is regulated to a considerable extent by the sensibility 

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

 only to call attention to the ease and certainty with which the presence 

 and the consistence of the smallest substance between the teeth are 

 recognized, to show the importance of this tactile sense in mastication. 



SALIVA 



The liquid 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 sputation, is composed 

 of the secretions of a considerable number and variety of glands. The 

 most important of these are the parotid, submaxillary and sublingual, 

 which usually are called the salivary glands. The labial and buccal 

 glands, the glands of the tongue and general mucous surface and certain 

 glandular structures in the mucous membrane of the pharynx also con- 



