114 THE DIGESTIVE SYSTEM. 



portion of the molars the gums are situated, which, in common 

 with the other parts of the mouth, are covered by mucous mem- 

 brane, in some parts a firm dense material being interposed 

 between this membrane and the bone. 



The sides of the mouth cavity are formed by the cheeks, 

 made up of powerful muscles, covered internally by mucous 

 membrane and externally by the skin, they form the greater part 

 of the bulk of the face and are the agents Avhich control the 

 grinding motion of the lower jaw. The lips serve for the pre- 

 hension or gathering of both the solid and liquid food, retaining- 

 it in the mouth after its introduction therein, and preventing the 

 escape of the saliva; they are furnished with nerves of feeling,, 

 and are composed of muscles externally covered by the skin, 

 the inside surface being covered by mucous membrane, they 

 possess the powers of motion and sensation to a considerable 

 degree. The mouth is plentifully supplied with a watery,, 

 slightly alkaline fluid termed the saliva. This is secreted by 

 three pairs of glands, collectively termed the salivary glands. 

 There is a slight flow of saliva from these glands into the mouth 

 at all times, which is considerably increased during mastication. 



The largest pair of these glands is known as the parotid, 

 situated at the base of each ear, extending downward in a line 

 with the jugular vein, they connect with the mouth cavity by 

 means of a duct, which opens into the mouth about the level of 

 the fourth molar. This is known as Steno's duct, and some- 

 times becomes enlarged, forming a fluctuating tumor at the side 

 of the face, due to obstruction from inflammation and obliter- 

 ation of it, from injury, where it lies close to the surface in the 

 meshes of the masseter muscle, passing upwards over the lower 

 jaw. The next largest pair is known as the submaxillary. They 

 are situated under the jaws, their ducts opening into the mouth 

 by projecting elevated points or barbs placed near the fraenum. 



