| Levator labii superioris alaeque nasi | Mentalis |
| Levator labii superioris | Depressor labii inferioris |
| Zygomaticus minor | Depressor anguli oris |
| Levator anguli oris | Buccinator |
| Zygomaticus major. | Orbicularis oris |
| Risorius |
Nerve supply.- The Levator labii superioris alaeque nasi supplied by the buccal branches of the facial nerve.
Actions.-The lateral slip raises and everts the upper lip; the medial slip acts as a dilator of the nostril.
The Levator labii superioris arises from the lower margin of the orbital opening immediately above the infra-orbital foramen, some of its fibers arising from the maxilla. and others from the zygomatic bone. Its fibers converge to be inserted into the muscular substance of the upper lip between the lateral slip of the Levator labii superioris alaque nasi and the Levator anguli oris.
Nerve-supply.-The Levator labii superioris is supplied by the buccal branches of the facial nerve.
Actions.-The Levator labii superioris raises and everts the upper lip. It assists the Zygomaticus minor to form the nasolabial furrow, which passes from. the side of the nose to the upper lip and gives to the face an expression of sadness.
The Zygomaticus minor arises from the lateral surface of the zygomatic bone immediately behind the zygomaticomaxillary suture, and passes downwards and medially to be inserted into the muscular substance of the upper lip. It is separated from the Levator labii ,superioris by a narrow interval (fig. 579).
Nerve-supply.-The Zygomaticus minor is supplied by the buccal branches of the facial nerve.
Actions.—-The Zygomaticus minor assists in elevating the upper lip and in the production of the nasolabial furrow. When the Levator labii superioris alaeque nasi, the Levator labii superioris and the Zygomaticus minor are in action together they give to the countenance the expression of contempt and disdain.
The Levator anguli oris (Caninus) arises from the canine fossa, just below the infra.-orbital foramen, and is inserted into the angle of the mouth, intermingling with the fibers of the Zygomaticus major, Depressor anguli oris (Triangularis), and Orbicularis oris. Between the Levator anguli oris and the Levator labii superioris are the infra-orbital vessels and plexus of nerves.
Nerve-supply.-The Levator anguli oris is supplied by the buccal branches of the facial nerve.
Actions.–The Levator anguli oris raises the angle of the mouth and assists in producing the nasolabial furrow.
The Zygomaticus major (Zygomaticus) arises from the zygomatic bone, in front of the zygomatico-temporal suture, and is inserted into the angle of the mouth, where it blends with the fibers of the Levator anguli oris (Caninus). Orbicularis oris, and Depressor anguli oris (Triangularis).
Nerve-supply.–The Zygomaticus major is supplied by the buccal branches of the facial nerve.
Actions.-The Zygomaticus major draws the angle of the mouth upwards and laterally as in laughing.
The Mentalis is a conical fasciculus, situated at the side of the frenulum of the lower lip. It arises from the incisive fossa of the mandible and descends to be inserted into the skin of the chin.
Nerve supply – The Mentalis is supplied by the mandibular branch of the facial nerve.
Actions.–The Mentalis raises and protrudes the lower lip, and at the same time wrinkles the skin of the chin, expressing doubt or disdain.
The Depressor labii inferioris (Quadratus labii inferioris) is a quadrilateral muscle. It arises from the oblique line of the mandible, between the symphysis menti and the mental foramen, and passes upwards and medially, to be inserted into the skin of the lower lip, its fibers blending with those of its fellow of the opposite side and with the Orbicularis oris. At its origin it is continuous with the fibers of the Platysma. Much yellow fat is intermingled with the superficial fibers of this muscle.
The Zygomaticus major and minor and the Levator labii superioris are sometimes more or loss concealed by a thin, sheet of muscle, named the Miaculus malaris, and continuous with the Orbicularis oculi. (Consult an article on the facial muscles, etc., by G. H. S.. Lightoller, Journal of Anatomy, vol. Ix. 1925.)
Nerve-supply.-The Depressor labii inferioris is supplied by the mandibular branch of the facial nerve.
Actions.-The Depressor labii inferioris draws the lower lip downwards and a. little laterally, as in the expression of irony.
The Depressor anguli oris (Triangularis) arises from the oblique line of the mandible below and lateral to the Depressor labii inferioris; its fibers converge and are inserted by a narrow fasciculus into the angle of the mouth. At its origin it is continuous with the Platysma, and at its insertion with the Orbicularis oris and Risorius; some of its fibers are directly continuous with those of the Levator anguli oris (Caninus), and others are occasionally found crossing from the muscle of one side to that of the other; these latter fibers constitute the Transversus menti.
Nerve-supply – The Depressor anguli oris is supplied by the mandibular branch of the facial nerve.
Actions.-The Depressor anguli oris draws the angle of the mouth downwards and laterally.
The Buccinator (fig. 583) is a thin quadrilateral muscle, occupying the interval between the maxilla and the mandible, at the side of the face. It arises from the outer surfaces of the alveolar processes of the maxilla and mandible, opposite to the three molar teeth; and, behind, from the anterior border of the pterygomandibular ligament (pterygomandibular raphe), which separates the muscle from the Superior constrictor of the pharynx. Between the tuberosity of the maxilla and the upper end of the pterygomandibular ligament, a few fibers arise from a fine tendinous band which bridges the interval between the maxilla and the pterygoid hamulus. The tendon of the Tensor palati on its way to the soft palate pierces the pharyngeal wall in the small gap which lies behind this tendinous band. The fibers of the Buccinator converge towards the angle of the mouth, where the central fibers intersect each other, those from below being continuous with the upper segment of the Orbicularis oris, and those from above with the lower segment; the highest and lowest fibers are continued forward into the corresponding lip without decussation.
Nerve-supply. The Buccinator is supplied by the lower buccal branches of the facial nerve.
Actions.—-The Buccinator muscles compress the cheeks against the teeth, so that during the process of mastication the food is kept under the immediate pressure of the teeth. When the cheeks have been previously distended with air, the Buccinators expel it between the lips, as in blowing a trumpet; hence the name (buccina, a trumpet).
The pterygomandibular ligament (pterygomandibular raphe) is a tendinous band which is attached by one extremity to the hamulus of the medial pterygoid plate, and by the other to the posterior end of the mylohyoid line of the mandible. Medially it is covered by the mucous membrane of the mouth. Laterally it is separated from the ramus of the mandible by a quantity of adipose tissue. Posteriorly it gives attachment to the Superior constrictor of the pharynx, and anteriorly to a part of the Buccinator (fig. 583).
The Orbicularis oris (figs. 579, 584) is not a simple sphincter muscle like the Orbicularis oculi; it is made up of several strata of fibers; which surround the orifice of the mouth but have different directions. It consists partly of fibers derived from the other facial muscles which are inserted into the lips, and partly of fibers proper to the lips. Of the former, a considerable number are derived from the Buccinator, and form the deeper stratum of the Orbicularis. Some of the Buccinator fibers namely, those near the middle of the muscle—decussate at the angle of the mouth; the uppermost and lowermost fibers pass across the lips from side to side without decussation. Superficial to this is a second stratum, formed by the Levator and Depressor anguli oris (Caninus and Triangularis), which cross each other at the angle of the mouth; the fibers from the Levator pass to the lower lip, and those from the Depressor to the upper lip, along which they run, to be inserted into the skin near the anterior median line. Fibers are also derived from the Levator labii superioris, the Zygomaticus major and minor, and the Depressor labii inferioris (Quadratus labii inferioris); these intermingle with the transverse fibers described above, and have principally an oblique direction. The proper fibers of the lips are oblique, and pass from the deep surface of the skin to the mucous membrane, through the thickness of the lip. Finally there are fibers by which the muscle is connected with the maxillae and the septum of the nose above and with the mandible below. In the upper lip these consist of two bands, lateral and medial, on each side; the lateral band (m. incisivus labii superioris) arises from the alveolar border of the maxilla, opposite the lateral incisor tooth, and arching laterally is continuous with the other muscles at the angle of the mouth; the medial band (m. nasolabialis) connects the upper lip to the back of the septum of the nose. The interval between the two medial bands corresponds with the depression, called the philtrum, seen on the upper lip beneath the septum of the nose. The additional fibers for the lower lip constitute a slip (m. incisivus labii inferioris) on each side; this slip arises from the mandible, lateral to the Mentalis, and mingles with the other muscles at the angle of the mouth.
Actions.—-The Orbicularis oris in its ordinary action effects the direct closure of the lips; by its deep, assisted by its oblique, fibers, it compresses the lips against the teeth. The superficial part, consisting principally of the decussating fibers, brings the lips together and protrudes them.
The Risorius arises from the parotid fascia and is inserted into the skin at the angle of the mouth (fig. 579). It is a narrow bundle of fibers, broadest at its origin, but varying much in its size and form.
Nerve-supply. -The Risorius is supplied by the buccal branches of the facial nerve.
Actions.-The Risorius retracts the angle of the mouth, and produces an unpleasant grinning expression.
Lightoller gives a detailed description of a knot or localized thickening, termed the modiolus, where the fibers of the muscles surrounding or running to the oral fissure meet and intermingle. This knot is placed about 1 cm. lateral to the angle of the mouth, and has the form of a flattened cone with its base on the mucous membrane of the mouth; the base, has a vertical measurement of about 4 cm, and curves forward for a short distance into the lips.
The modiolus is a chiasma of facial muscles held together by fibrous tissue, located lateral and slightly superior to each angle of the mouth. It is important in moving the mouth, facial expression and in dentistry. It derives its motor nerve supply from the facial nerve, and its blood supply from labial branches of the facial artery. It is contributed to by the following muscles: orbicularis oris, buccinator, levator anguli oris, depressor anguli oris, zygomaticus major, zygomaticus minor, risorius.
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