Orbicularis oris muscle

At a glance

English

Orbicularis oris muscle

Latin (TA2)

Musculus orbicularis oris

System

Muscle

Region

Head

Side

Paired, left and right

About the orbicularis oris muscle

The orbicularis oris forms the muscular core of both lips and surrounds the opening of the mouth. It behaves like a sphincter, but it is not a single continuous ring like the orbicularis oculi around the eye. Instead it is built from several layers of fibres running in different directions, arranged as four interlocking quadrants, upper and lower on each side, that together only look circular. It is the muscle that shuts the mouth, purses the lips for kissing or whistling, and shapes them during speech; players of brass and many woodwind instruments rely on it to form the embouchure.

Much of the muscle is borrowed from its neighbours. The deepest layer comes from the buccinator: its middle fibres cross at the corner of the mouth, so that maxillary fibres enter the lower lip and mandibular fibres the upper lip, while its highest and lowest fibres run straight across their own lip. Over this lies a layer formed by the levator anguli oris and depressor anguli oris, which also cross at the angle and run along the opposite lip to the skin near the midline. Added to these are fibres proper to the lips, which run obliquely from the deep surface of the skin through the lip to the mucous membrane.

Small accessory bands tie the muscle to bone. In the upper lip a lateral band, the incisivus labii superioris, springs from the maxilla level with the second incisor and arches out to the angle of the mouth, and a medial band links the lip to the nasal septum; the groove of the philtrum lies between the two medial bands. In the lower lip a matching incisivus labii inferioris comes from the mandible beside the mentalis. All these strands converge on the modiolus at each corner.

The muscle is supplied by the buccal and marginal mandibular branches of the facial nerve, and its blood comes from the superior and inferior labial branches of the facial artery, which run in the lips just behind it. Weakness in facial palsy makes it hard to keep food and fluid in the mouth or to whistle. In cleft lip repair the muscle must be carefully rebuilt.

Attachments, action and supply

Origin

Mainly from fibres of the surrounding facial muscles (buccinator, levator and depressor anguli oris and others), with accessory bands from the maxilla, mandible and nasal septum.

Insertion

Skin and mucous membrane of the lips and the modiolus at each corner of the mouth.

Action

Closes the mouth, presses the lips against the teeth, purses and protrudes them, and shapes them for speech.

Nerve supply

Buccal and marginal mandibular branches of the facial nerve (cranial nerve VII).

Blood supply

Superior and inferior labial branches of the facial artery.