The fibres of the buecinator are wavy, over lapping each other, so that they admit of great distention, which is, however, limited by a buccal fascia, which is given off from the pterygo-maxillary ligament.
Relations.—The buccinator is deeply situated behind, where it is covered by the ramus of the jaw and the edge of the masseter, from which it is separated by a quantity of fat, which projects beyond the mass, fills up the hollow in front of the masseter, and is always found even in thin subjects. In the middle it corresponds to the buccal vessels and nerves and to the transverse facial artery, which runs nearly parallel to its fibres, and to the duct of the parotid gland, which, resting at first upon its fibres, pierces them opposite the second molar tooth of the upper jaw, and opens obliquely into the mouth. A buccal fascia covers the posterior half of the muscle. At the commissure the buccinator is covered by the muscles which are attached to the angle of the mouth, and is crossed at right angles by the external maxillary artery and vein. By its internal surface this muscle covers the mucous membrane of the mouth, from which it is only separated by a layer of buccal glands.
Action.—This muscle, being fixed behind, above, and below, acts principally in front on the commissure of the lips, which it draws horizontally backwards, elongating the aperture of the mouth transversely, and throwing the cheek into the vertical folds which are so re markable in old age. In this respect it is the direct antagonist of the orbicularis oris: if both these muscles act together, thelips are extended and pressed against the teeth: When the cavity of the mouth is distended with air or liquids, this muscle is protruded at the cheeks, and its fibres become separated and curved. If now the muscle acts, the fibres become straightened, and the fluid is expelled from the mouth either abruptly or gradually according to the resistance of the orbicularis. This action of the orhicularis is exemplified either is spirting fluids from the mouth, or in playing on wind instruments. In mastica tion the buccinator presses the food from between the cheek and gums into the cavity of the mouth. It assists also in deglutition
when the mouth is closed, by pressing the food backwards towards the pharynx.
Among the muscles of the face, it is ne cessary to allude to some parts of the platysma, which are not only seen in this region, but which contribute materially to the motion and expression of the face. The platysma ( p, p,p, fig. 138) is a large, broad, membranous layer of fibres, which extend from the upper and an terior part of the chest, where they commence in the subcutaneous tissue, upwards over the anterior and lateral part of the neck, to the jaw and lower part of the face, where they are inserted above. The whole superficial surface of the muscle is subcutaneous, but less firmly attached to the integument just under the jaw than elsewhere. The under surface of its cervical portion is in relation with numerous important parts on the ace: it covers from before backwards the lower part of the chin, the quadratus menti, the triangularis oris, the base of the lower jaw, the facial vessels, and part of the masseter. The arrangement of its facial portion is all that need be described here.
As the fibres of the muscle incline upwards towards the median line, they meet below the symphysis of the chin, and some ascend as high as the levator menti. Externally the fibres seem to split to enclose the depressor anguli oris, and to proceed upwards and for wards with that muscle and the quadratus menti to the lower lip and its angle. The middle fibres are attached to the base of the jaw, and posteriorly they mount over the angle,and are lost on the fascia of the masseter. A curious slip crosses these transversely, de scending a little from the fascia covering the parotid gland towards the angle of the mouth. It is the risorius Santorini, which is, however, often wanting. The platysma draws down the whole of the lower part of the face, or, acting more slightly, depresses the lower lip and the commissure in conjunction with their proper depressors. The slip called risorius, on the contrary, raises the angle of the mouth.