Depressor avail oris (triangularis oris) (t, fig. 134) is a thin, triangular, subcutaneous muscle, situated at the lower part of the face. It arises by a broad base from the lower border of the inferior maxilla, and from the surface of the bone between this border and the external oblique line, extending from the chin to within half an inch of the masseter. The fibres con verge and ascend towards the commissure of the lips, the posterior fibres taking a direction upwards and forwards, the middle nearly ver tical, and the anterior describing a curve up wards and backwards: they all terminate at the commissure of the lips, where they become united with those of the orbicularis and of the buccinator, and more superficially with the great zygomatie and levator anguli oris.
Relations.—Its superficial surface is covered by the skin and by the fibres of the platysma, with which it is mingled. Its deep surface rests upon part of the depressor labii inferioris and buccinator: above it is connected with all the muscles of the commissure and with the skin.
Action.—This muscle draws down the angle of the mouth, and in this respect is the anta gonist of the great zygomatic and levator an guli oris.
Depressor labii inferioris (quodrulus menti), (d, fig. 136, 137) fiat and of a square form, is placed internal to the preceding, which partly conceals it. It arises from the inner half of the external oblique line of the lower jaw, and also from the platysma, with whose fibres it is continuous. Its fibres, which are parallel, pro ceed upwards and inwards to be attached to the lip; the deep fibres mingle with those of the orbicularis; the superficial pass in front of that muscle, and are fixed in the skin of the lip. The inner fibres decussate above with those of the muscle on the opposite side; below, with those of the levator menti.
Relations.—At its origin this muscle is co me(' by the triangularis, and elsewhere by the skin, to which it adheres intimately above. Its deep surface covers part of the lower jaw, the mental vessels and nerves, part of the orbicu laris oris and levator menti. Through the an gular interval between the two depressors of the lower lip, the levatores menti pass to their insertion.
Its action is to draw downwards and out wards one side of the lower lip; if the muscles on both sides act, the lip is drawn downwards and extended transversely. The stronger ac
tions of this muscle are usually accompanied by those of the platysma, with whose fibres, as we have seen, it is continuous.
Levatur menti (houppe du menton) (e, fig. 136,137) may be exposed by everting the lip and dividing the mucous membrane : it is a small round muscle, situated at the lower part of the face, and forming on each side a great part of the prominence of the chin. It arises in the incisive fossa below the incisor teeth of the lower jaw, external to the symphysis, and pro ceeds downwards and forwards: it passes under the lower border of the orbicularis oris, and emerging between the depressor labii inferioris, expands a little to be inserted into the skin of the chin. Its fibres below are mingled with fat ; internally they are confounded with those of the fellow muscle, and externally with the fibres of the quadratus menti.
In its action this muscle raises and corru gates the chin, and by so doing raises also the lower lip and throws it forward.
Baccinator (b,fig. 136, 137). This muscle is situated on the side of the cheek, and to ex pose it completely it is necessary to divide the muscles attached to the angle of the mouth, and to remove the ramus of the jaw and the muscle attached to it. The buccinator is a broad flat muscle, and arises, 1. behind and in the middle from an aponeurotic line, the pterygo-maxillary ligament or inter-maxillary ligament, which is common to it and the su perior constrictor of the pharynx, and which is extended between the lower extremity of the internal pterygoid plate of the sphenoid hone and the posterior extremity of the internal ob lique line of the lower. Above, the buccinator arises, 2. from the outer surface of the upper alveolar process, between the first molar tooth and the tuberosity; 3. below from the outer side of the alveolar border opposite the three last molar teeth. From these three origins the fibres proceed forwards, the superior curving a little downwards, the inferior upwards, and the middle passing horizontally towards the angle of the mouth, where they mingle with the fibres of the orbicularis and the elevators and depressors of the commissure. The infe rior and superior fibres become shorter as we trace them forwards, and some of them.decus sate at the angle of the mouth to unite with the opposite labial half of the orbicularis.