The muscles of facial expression are characterized by:
• their superficial arrangement in the face
• their activities on the skin (brought about directly by their attachment to the facial integument)
• by their common motor innervation, the facial nerve. Functionally, the muscles of facial expression are grouped around the orifices of the face (the orbit, nose, ear and mouth) and should be considered primarily as muscles controlling the degree of opening and closing of these apertures; the expressive functions of the muscles have developed secondarily. The muscles of facial expression vary considerably between individuals in terms of size, shape and strength. The superficial muscles around the lips and cheeks may be subdivided into two groups:
• The various parts of the orbicularis oris muscle
• Muscles that are radially arranged from the orbicularis oris muscle. The fibres of orbicularis oris pass around the lips. The muscle is divided into four parts, each part corresponding to a quadrant of the lips. Its muscle fibres do not gain attachment directly to bone but occupy a central part of the lip. Muscle fibres in the philtrum insert onto the nasal septum. The range of movement produced by orbicularis oris includes lip closure, protrusion and pursing. The radial muscles can be divided into superficial and deep muscles of the upper and lower lips:
• The levator labii superioris, levator labii superioris alaeque nasi and zygomaticus major and minor are superficial muscles of the upper lip.
• The levator anguli oris is a deep muscle of the upper lip.
• The depressor anguli oris is a superficial muscle of the lower lip.
• The depressor labii inferioris and mentalis muscle are deep muscles of the lower lip. As their names suggest, the levator labii superioris elevates the upper lip, the depressor labii inferioris depresses the lower lip, and the corners of the mouth are raised and lowered by the levator and depressor anguli oris muscles. Two muscles extend to the corner of the mouth: the risorius and buccinator muscles, risorius lying superficial to the buccinator:
• The risorius muscle stretches the angles of the mouth laterally.
• The buccinator muscle arises from the pterygomandibular raphe and from the buccal side of the maxillary and mandibular alveoli above the molar teeth. Most of its fibres insert into mucous membrane covering the cheek; other fibres intercalate with orbicularis oris in the lips. As the fibres of the buccinator converge towards the angle of the mouth, the central fibres decussate. The main function of the buccinator muscle is to maintain the tension of the cheek against the teeth during mastication.
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