SALIVA. 



195 



tor ; 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 ali- 

 mentary bolus so as to subject new portions to trituration. 



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

 sensibility 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 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 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 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 are usually called the 

 salivary glands. The labial and buc- 

 cal glands, the glands of the tongue 

 and general mucous surface and cer- 

 tain glandular structures in the mu- 

 cous membrane of the pharynx also 

 contribute to the production of the 

 saliva. The liquid which becomes 

 more or less incorporated with the 

 food before it descends to the stom- 

 ach, and which must be regarded as 

 the digestive fluid of the mouth, is 

 known as the mixed saliva ; but the 

 study of the composition and prop- 

 erties of this fluid as a whole should 

 be prefaced by a consideration of 

 the different secretions of which it 

 is composed. The salivary glands 

 belong to the variety of glands 

 called racemose. They resemble the other glands belonging to this class, 

 and their structure will be more fully considered in connection with the 

 physiology of secretion. 



Parotid Saliva. The parotid is the largest of the three salivary glands. 

 It is situated below and in front of the ear and opens by the duct of Steno 

 into the mouth, at about the middle of the cheek. The papilla which marks 

 the orifice of the duct is situated opposite the second large molar tooth of the 

 upper jaw. 



The organic matter of the parotid saliva is coagulable by heat (212 Fahr., 

 or 100 C.), alcohol or the strong mineral acids. A compound of sulpho- 

 cyanogen is now generally acknowledged to be a constant constituent of the 

 parotid saliva. This can not be recognized by the ordinary tests in the fresh 

 saliva taken from the duct of Steno, but in the clear, filtered fluid which 



FIG. 54. Salivary glands (Tracy). 



