Depressor anguli oris

At a glance

English

Depressor anguli oris

Latin (TA2)

Depressor anguli oris

System

Muscle

Region

Head

Side

Paired, left and right

About the depressor anguli oris

The depressor anguli oris, once called the triangularis, is a superficial muscle of facial expression on the lower face. It is shaped like a triangle, with a broad base along the jaw and its apex at the corner of the mouth, and it lies over the depressor labii inferioris, just behind and lateral to it. Lowering the corners of the lips, as in sadness, disapproval or a sulky look, is its characteristic work.

Its broad origin is a line on the outer surface of the mandible, along the oblique line below the canine, premolar and first molar teeth. At this lower edge its fibres merge with the platysma coming up from the neck. The fibres then gather together as they rise and end in a narrow bundle at the modiolus, the meeting point of muscles at the angle of the mouth, where they mingle with orbicularis oris and risorius. Some fibres run on into the levator anguli oris, and a few may cross below the chin to the muscle of the other side, forming a small transverse slip called the transversus menti.

When it contracts, it pulls the angle of the mouth downwards and outwards, opposing the zygomaticus major and levator anguli oris, which lift it in a smile. With the platysma it also helps to lower the lip line and tense the skin of the lower face, for instance in an expression of horror or strain. It draws its blood from the inferior labial branch of the facial artery, and its motor nerve is the facial nerve's marginal mandibular branch, which runs close to the lower border of the jaw.

That branch is easily injured during surgery in the upper neck, such as removal of the submandibular gland, and also in some facial palsies. Loss of the depressor anguli oris then gives a lopsided smile, with the lower lip on the weak side failing to drop. One way of restoring balance is to weaken or remove the depressor labii inferioris on the healthy side. The muscle is also a common target for botulinum toxin injections to soften a downturned mouth.

Attachments, action and supply

Origin

Oblique line on the outer surface of the mandible, below the canine and premolar teeth, continuous with the platysma.

Insertion

Modiolus at the angle of the mouth, blending with orbicularis oris and risorius.

Action

Draws the corner of the mouth downwards and laterally, as in frowning or sadness.

Nerve supply

Facial nerve (VII), marginal mandibular branch.

Blood supply

Inferior labial branch of the facial artery, with the mental artery.