Depressor labii inferioris

At a glance

English

Depressor labii inferioris

Latin (TA2)

Depressor labii inferioris

System

Muscle

Region

Head

Side

Paired, left and right

About the depressor labii inferioris

The depressor labii inferioris, in older texts the quadratus labii inferioris, is a flat, roughly square muscle of facial expression between the chin and the lower lip. It lies deep and medial to the depressor anguli oris, which partly covers it, and lateral to the mentalis. It is the principal muscle for drawing the lower lip down, and it is busy whenever the lower teeth are shown, as in a broad smile or an expression of irony or distaste.

It arises from a band about three centimetres wide on the outer face of the mandible, along the oblique line between the mental symphysis and the mental foramen, so the foramen lies just above and lateral to its origin. At this lower edge it is continuous with fibres of the platysma. It runs upwards and medially and spreads out in the skin and mucosa of the lower lip, mingling with orbicularis oris and with the muscle of the other side. Strands of yellow fat are interwoven among its fibres, giving it a slightly mottled look.

Its action is to pull the lower lip down and somewhat laterally and to evert it, rolling the red margin outwards. In this it works against orbicularis oris, which closes and tightens the lips. Its nerve is the facial nerve's marginal mandibular branch, and it receives blood from the inferior labial branch of the facial artery and the mental artery.

When the marginal mandibular branch is damaged on one side, the lower lip on that side can no longer be pulled down, and the smile looks uneven. Surgeons may then restore symmetry by cutting out part of this muscle on the healthy side. The operation can be done under local anaesthetic, using a block of the mental nerve as it leaves the mental foramen, and it usually gives good results. The muscle can also be injected with botulinum toxin for the same purpose.

Attachments, action and supply

Origin

Outer surface of the mandible along the oblique line, between the symphysis and the mental foramen.

Insertion

Skin and mucosa of the lower lip, merging with orbicularis oris and the opposite muscle.

Action

Draws the lower lip downwards and laterally and everts it, exposing the lower teeth.

Nerve supply

Facial nerve (VII), marginal mandibular branch.

Blood supply

Inferior labial branch of the facial artery and the mental artery.