332 THE DIGESTIVE APPARATUS IN MAMMALIA. 



contraction; the sensitive nerves, which are furnished by the maxillary 

 branches of the fifth encephalic pair, are distinguished by their nuniber and 

 considerable volume, and are nearly all buried in the cutaneous integument, 

 fco which they communicate an exquisite sensibility. 



FUNCTIONS. The lips serve for the prehension of solid and liquid food ; 

 they retain it in the mouth after its introduction thereto, and likewise pre- 

 vent the escape of the saliva. They ought also to be regarded, especially 

 the upper lip, as very delicate organs of touch. 



2. The Cheeks. (Fig. 110.) 



These are two membranous walls, which inclose the mouth laterally. 

 In the interior of the buccal cavity they are limited: behind, by the 

 posterior pillars of the tongue ; in front, by the lips, with which they are 

 confounded around the commissures ; above and below, by the groove 

 formed by the gingival mucous membrane, where it is reflected from the 

 molar arches on to the cheeks. 



The greatest diameter of the cheeks is antero-posterior, like that of the 

 cavity it incloses. Their vertical diameter is very narrow, especially 

 behind ; in the anterior region, however, it can assume a certain amplitude 

 by the separation of the jaws. 



STRUCTURE. The cheeks are formed by the buccal mucous membrane, 

 external to which we find muscular tissue and glands. Vessels and nerves 

 pass through these parts for the conveyance of nutritive fluids, sensibility, or 

 the stimulus to contractility. 



1. Mucous membrane. The external face of this membrane is united in an 

 intimate manner to the buccinator muscle, and to the inferior molar glands. 

 Its free face presents, at the level of the third superior molar tooth, the 

 buccal opening of the parotid duct, pierced at the summit of a variable- 

 sized tubercle. On the face of each dental arcade there is also remarked a 

 linear series of little salient points, analogous in their constitution to the 

 large parotideal tubercle ; these are the excretory orifices of the molar glands. 

 Its structure is the same as the mucous membrane of the lips. (It is of a 

 pale colour, and sometimes stained in patches with pigment.) 



2. Muscular tissue. This is the buccinator or alveolo-labialis muscle 

 already described. It may be remembered that the external face of this muscle 

 is covered by the masseter, the superior molar glands, and the skin ; while the 

 internal responds to the mucous membrane and the inferior molar glands. 



3. Glands. These are two masses of glandular lobules, known as the 

 molar glands. They will be described with the salivary glands. 



Vessels and nerves. The external maxillary, coronary, and buccal arteries 

 carry blood to the cheeks. The veins empty themselves into the satellite 

 branches of these arteries. 



The lymphatics proceed to the submaxillary glands. The nerves are of 

 the same kind, and proceed from the same source, as those supplying the lips : 

 being the seventh pair of encephalic or facial nerves for the muscular layer, 

 and the fifth pair for the integuments (with filaments of the sympathetic for 

 the circulation and the labial glandules). 



FUNCTIONS. The cheeks are very active agents in mastication, by con- 

 stantly pushing the aliment, through the action of the buccinator, between the 

 dental grinding surfaces. 



3. The Palate. (Fig. 148.) 

 Preparation. Separate the head from the trunk ; saw through the branches of the 



