VAGUM, GLOSSO-PHARYNGEAL NERVE, AND SPINAL ACCESSORY.
Mouth, (Gr. crropcx; Lat. os; Fr. boucke.)— The mouth is an oval cavity, symmetrical, and situated at the lower part of the face, below the nasal fosste, between the jaws, and in front of the pharynx, with which it communicates by a posterior opening, called the isthmus fau cium, and has also a dilatable aperture ante riorly guarded by the lips : it is liable to con siderable alterations of form and size, from complete closure to a state of extreme exten-. sion, when it represents a quadrangular py ramid with the base in front: the greatest change occurs in its vertical diameter from the movements of the lower jaw ; in it are per formed the various functions of mastication, tasting, partly that of deglutition, and it is subservient also to the production of articulate sounds. The mouth or buccal cavity is bounded both laterally and anteriorly by the alveolar borders of the upper and lower maxillary bones and teeth, the lips completing the boundary in front and the cheeks laterally : above it is roofed in by the arched palate and more pos teriorly by the veluni palati ; inferiorly the tongue forms its floor. In the examination of these boundaries the reader is referred to the articles FACE and TEETH for the description of the maxillary bones and teeth.
The lips (labia) are two moveable curtains placed in front of the mouth, presenting be tween them when applied to each other a transverse slit convertible by their separation into a more or less considerable opening, which constitutes the anterior aperture of the buccal cavity : the lips are united at the lateral limits of this fissure to form the commissures or angles; the anterior surface of the upper lip, which usually projects a little beyond the lower, is covered with hair in the adult male, and exhibits in the median line a vertical groove continued to its free border from the septum of the nose : two ridges bound this furrow on either side, and from thence the upper lip passes off' laterally to the cheek, insensibly in the young and plump face, but otherwise a line of demarcation is produced by an oblique fold of the skin which descends from the side of the nose to near the commissure of the lips on either side of the face : the anterior surface of the lower lip descends more or less abruptly backwards to the chin, divided from it by a transveme groove : it is covered with hair usu ally at the centre only, and slightly bulging near its free border shelves off gradually to the sides of the face : the free borders are the thickest parts of the lips, their large develope ment forming a characteristic feature in the Negro; in their outline they differ, presenting in the upper lip a projection in the centre, from which a curved line, gently arching upwards, proceeds laterally, while, in the lower, the centre exhibits a depression, and the line from it proceeds in a contrary direction, so that when the mouth is closed these borders are applied evenly to each other : they are of a red colour, turned outwardly, and marked from before backwards by slight wrinkles pro duced by the contraction of the orbicular muscle ; their chief interest to the anatomist is in showing the continuity of the skin with the mucous membrane. Besides the tegu
mentary coverings of skin and mucous mem brane, these organs contain within their thick ness the orbicularis muscle, with which are blended the insertions of the greater number of the muscles of the face (see FAcE), whose varied actions render these features so pecu liarly expressive of the passions: numerous glands, vessels, and nerves, and an areolar tissue complete their structure. The labial glands constitute a thick lamina between the muscular and mucous layers, producing slight elevations upon the surface of the latter ; they resemble the salivary glands in appearance, are of small but varying size, placed close to each other but perfectly distinct, each posses sing a separate excretory duct, which opens upon the free surface of the mucous mem brane. The lips are most abundantly supplied with vessels and nerves ; the coronary arteries, from the facial, course along their free borders directly beneath the mucous membrane; they also receive numerous twig-s from the buccal, infra-orbital, and mental bmnches of the in ternal maxillary and subrnental branch of the facial; the veins accompany the arterial bmnches; the lymphatics terminate in the glands at the base of the jaw, as evidenced by the frequent enlargement of the latter from the irritation of cancerous or other sores about the lips: the nerves are derived from the portio dura and fifth pair.