THE MOUTH 387 



(Mentum). The internal surface is covered with mucous membrane which is com- 

 monly more or less pigmented. The sniall papilte on the surface show on their 

 summits the openings of the ducts of the labial glands. Small folds of mucous 

 membrane which pass from the lip to the gum form the fraenula labii (superioris, 

 inferioris). The free border of the lip is dense and bears short, very stiff hairs. 

 The attached border is continuous with the surrounding structures. 



Structtire. — The lips are covered externally by the skin, and are hned by 

 mucous membrane; between these are muscular tissue, glands, vessels, and nerves. 

 The skin hes directly on the muscles, many fibers of which are inserted into it. The 

 muscles have been described (p. 255). The labial glands (Glandular labiales) 

 form a compact mass at the commissures; they are numerous in the upper lip, 

 fewer in the lower. The mucous membrane is often pigmented, and is reflected 

 upon the bones of the jaws to form the gums. 



Vessels and Nerves. — The arteries are derived from the superior and inferior 

 labial and palato-labial arteries. The veins go chiefly to the external maxillary 

 vein. The lymph vessels go to the mandibular lymph glands. The sensory 

 nerves come from the trigeminus, and the motor nerves from the facial nerve. 



The cheeks (Buccse) form the sides of the mouth, and are continuous in front 

 with the Ups. They are attached to the alveolar borders of the bones of the jaws. 



Structure. — This comprises: (1) The skin; (2) the muscular and glandular 

 layer; (3) the mucous membrane. The skin offers nothing in particular to notice. 

 The muscular tissue is formed mainly by the buccinator, but also by parts of the 

 cutaneus, zygomaticus, dilatator naris lateralis, levator nasolabialis, and depressor 

 labii inferioris. The buccal glands (Glandulse buccales) are arranged in two rows. 

 The superior buccal glands lie on the outer surface of the buccinator muscle, near 

 its upper border. The anterior part of the row consists of scattered lobules; the 

 posterior part, which lies under cover of the masseter muscle, is more developed and 

 compact. The inferior buccal glands, less voluminous than the upper, are situated 

 in the submucous tissue at the lower border of the buccinator muscle. The mucous 

 membrane is reflected above and below upon the gums, and is continuous behind 

 with that of the soft palate. It is reddish in color and frequently shows pigmented 

 areas. The parotid duct opens opposite the third upper cheek tooth on a papilla 

 (Papilla salivalis). Linear series of small papillae above and below indicate the 

 orifices of the small ducts of the buccal glands. 



Vessels and Nerves. — The blood-supply is derived from the facial and bucci- 

 nator arteries, and the blood is carried away by veins of the same name. The lymph 

 vessels go to the mandibular lymph glands. The sensory nerves come from the 

 trigeminus and the motor nerves from the facial nerve. 



The gums (Gingivae) are composed of a dense fibrous tissue which is intimately 

 united with the periosteum of the alveolar processes, and blends at the edges of 

 the alveoh with the alveolar periosteum; the latter fixes the teeth in their cavities. 

 They are covered by a smooth mucous membrane, destitute of glands, and of a 

 low degree of sensibility. 



The hard palate (Palatum durum) is bounded in front and on the sides by 

 the alveolar arches, and is continuous with the soft palate behind. Its osseous 

 basis is formed by the premaxilla, maxilla, and palatine bones. The mucous mem- 

 brane is smooth, and is attached to the bones by a submucosa which contains in 

 its anterior part an exceedingly rich venous plexus, constituting an erectile tissue. 

 A central raphe (Raphe palati) divides the surface into two equal portions. Each 

 of these presents about eighteen transverse curved ridges (Rugae palatini) which 

 have their concavity and their free edges directed backward. They are further 

 apart and more prominent anteriorly. The central prominence just behind the 

 first pair of incisors is the papilla incisiva; it is margined by a fissure on each side. 

 There are no glands in- the submucosa. The ductus incisivus (s. ductus nasopa- 



