It is not a simple muscle composed of bundles arranged in concentric curves, like the orbicularis oculi. Besides some intrinsic fibers of its own, it is composed of the fibers of all the other muscles that converge on the mouth. (1) The intrinsic bundles pass obliquely between the skin and the mucus membrane. (2) Incisive slips arise from the jaws opposite the roots of incisor teeth and pass laterally into the lips. (3) The fibers of the other muscles converging upon the mouth mingle and sweep in curves through the lips – some obliquely and some transversely.
Owing to this complex arrangement, the movements of the lips are very varied – closing, pressing against the teeth, pursing, protruding, and twisting.
The levator labii superioris alaeque nasi is a small muscle that lies alongside the attachment of the nose, and divides to be inserted into the ala and into the upper lip. The levator labii superioris – thin and wide – descends from the infraorbital margin into the upper lip. It is overlapped by the orbicularis oculi.
The zygomaticus minor is lateral to the levator; it is a mete slip. and is often absent. The zygomaticus major is both longer and thicker; it runs obliquely from the zygomatic bone to the angle of the mouth.
The platysma is a long, thin, wide sheet of muscle. It arises from the skin and fascia below the clavicle, ascends in the superficial fascia of the neck, and enters the face. Its anterior fibers are inserted into the anterior part of the lower border of the mandible; its posterior fibers ascend over the posterior part of the border, and curve forwards into the lips. The risorius lies horizontally opposite the angle of the mouth. It consists chiefly of the uppermost (or most posterior) bundles of the platysma.
The fibers of the depressor anguli oris arise from the part of the mandible situated below the angle of the mouth, and converge upon the angle. The depressor labii inferioris, short and wide, is in front of the depressor anguli and is overlapped by it.
The buccinator and levator anguli oris belong to the deep group of muscles.
Related category ANATOMY AND PHYSIOLOGY
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