THE MOUTH 387 
(Mentum). The internal surface is covered with mucous membrane which is com- 
monly more or less pigmented. The small papillee 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 frenula 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. 
Structure.—The lips are covered externally by the skin, and are lined by 
mucous membrane; between these are muscular tissue, glands, vessels, and nerves. 
The skin lies directly on the muscles, many fibers of which are inserted into it. The 
muscles have been described (p. 255). The labial glands (Glandule 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 (Buccz) form the sides of the mouth, and are continuous in front 
with the lips. 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 (Glandulz 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 papille above and below indicate the 
orifices of the small ducts of the buccal giands. 
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 (Gingivze) 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 alveoli 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 raphé (Raphé palati) divides the surface into two equal portions. Each 
of these presents about eighteen transverse curved ridges (Ruge 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- 
