Use.—The lips are of great importance,more particularly the lower, in retaining the saliva within the cavity of the mouth, and are actively engaged in the acts of sucking and blowing; the uttemnce of many articulate sounds de pends chiefly upon their action, and when viewed as organs of expression they are parti cularly adapted by their extreme mobility to indicate the passing thought.
The cheeks (buccie) form the lateral exten sible walls of the buccal ca.vity ; examined from the interior of the mouth they will be found limited above and below by the reflexion of their investing mucous membrane upon the external surfaces of the superior and inferior maxillary bones : their superficial surface is bounded behind by the external ear and the posterior border of the lower jaw, below by the horizontal mmus of the same bone ; supe riorly they may be arbitrarily separated from the temple by the zygoma, and from the orbit by the lower margin of its cavity, and are con tinued anteriorly into the sides of the nose and lips ; they therefore present somewhat of a quadrilateral outline, and in the young and healthy form a rounded projection outwards, but in the emaciated fall in towards the mouth. The skin of the cheek is smooth, thin, and delicate in front and above, and remark able for its extreme vascularity, as seen in the act of blushing ; it is covered behind and below in the adult male with hair, and in the aged its surface is more or less furrowed with wrinkles : the subcutaneous cellular tissue is dense and loaded with a variable quantity of fat, a particularly abundant mass of which is lodged between the buccinator and masseter muscles. The muscular structure of the cheeks has been already described in the article FACE. Between the muscles and mucous membrane are irregularly dispersed a considemble num ber of buccal glands; they are of small size, similar to those of the lips, and like them open upon the mucous surface by separate ducts: these openings are not likely to be mistaken for that of the parotid duct, which is marked by a very distinct prominence, is of larger size and situated opposite the interval between the second and third molar teeth in the upper jaw : there is an aggregation of several of these buccal glands, imbedded in the fat between the buccinator and masseter muscles, form ing a larger glandular mass which opens into the mouth opposite the last molar tooth, and has been called the molar gland. The cheeks receive a rich supply of vessels from the facial, transverse facial, and internal maxillary arteries; the veins correspond to these branches and empty themselves into the internal and external jugular veins. The lymphatics probably ter
minate in the glands of the neck. As with other parts of the face, the cheeks derive their nervous filaments from the portio dura and fifth pair.
Use.—While the tongue guides the food outwardly to the teeth, the cheeks act in re taining it between them during mastication; they are employed during the act of :.ticking, and when distended by air or fluids they are actively engaged in forcibly expelling them, as exemplified in playing upon wind instruments or in squirting liquids from the mouth.
The palatine arch and gums.—The palatine arch or hard palate forms the greater part of the superior boundary of the buccal cavity : it has a parabolic figure, bounded laterally and in front by the teeth, and is continued poste riorly into the velum palati without exhibiting any line of demarcation it presents mesially a whitish ridge, more prominent before than behind, which commences from a small emi nence situated immediately behind the incisor teeth and corresponding with the lower orifice of the anterior palatine canal; the ridge then extends backwards and is traceable as far as the uvula; from it are passing laterally a vari able number of transverse rugte, apparent only at the anterior part of the palate, its buccal surface being, in the greater part of its extent, perfectly smooth. The palatine arch is framed by the palate processes of the superior maxil lary and palate bones,* and invested on their under surface by a dense and • thick mucous membrane : numerous glands with vessels and nerves also enter into'its structure. The mu cous membrane covering the bony palate with that forming the gums is of a paler colour than elsewhere in the interior of the mouth, and is united by a remarkably condensed and thick submucous areolar tissue to the periosteum, especially in the mesial line, where the two structures appear blended : on either side the union occurs by fibrous prolongations, allowing a thick layer of glands and the vessels and nerves of the palate to intervene : the mem brane has a thick investment of epithelial scales, and is capable of resisting considerable pressure; of greater thickness before than be hind, indifferently sensible, and in structure bears some analogy to that of the skin : the glands in every way resemble those of the cheeks and lips, and open in like manner upon the mucous surface ; two larger openings than the rest may often be seen on either side the median line towards the back part of the palate.