Muscular system · Head and neck
Depressor labii inferioris !
The depressor labii inferioris is a small muscle from the lower jaw to the lower lip; it pulls the lower lip down.
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What it is
A small muscle from the jaw to the lower lip.
What it does
Pulls the lower lip down.
Key facts
- Origin
- Oblique line of the mandible
- Insertion
- Lower lip
- Action
- Pulls the lower lip down
- Nerve
- Facial nerve (CN VII): marginal mandibular branch
- Blood supply
- Facial artery branches
Relations
- Superficial
- Depressor anguli oris laterally and the skin of the chin; fibres of the platysma blend into it
- Deep
- Mandible and the mental foramen with the mental nerve and vessels; the lower lip mucosa above
- Medial
- The opposite muscle and the mentalis, which lies deeper in the midline
- Above
- Orbicularis oris in the lower lip
How to find it
Asked to show the lower teeth, a small sheet of muscle draws the lower lip down and out on each side of the chin.
Examination
- Showing the lower teeth: on the weak side the lower lip stays up and the smile is asymmetric
- Lip eversion test: if the person can still evert the lower lip (mentalis intact) but cannot draw it down, a cervical branch or platysma problem is more likely than a marginal mandibular one
On imaging
Rarely imaged on its own: on CT or MRI it is only a faint layer in the chin soft tissue, lateral to the mentalis, over the mental foramen. When the lower lip droops on one side, imaging looks at the facial nerve instead: ultrasound or MRI of the parotid and of the angle of the mandible for a mass along the marginal mandibular branch.
Clinical notes
- The marginal mandibular branch runs deep to the platysma near the lower border of the mandible and can dip below it, so neck incisions for the submandibular gland are placed a couple of fingerbreadths below the jaw
- Selective weakening of the healthy side's depressor (surgery or toxin injection) is used to balance a lopsided smile after facial palsy
Development
Second pharyngeal arch muscle, supplied by the facial nerve.
Test yourself
Think of the answer, then tap the question to see it.
How is the depressor labii inferioris tested, and which nerve branch does it check?
By asking the person to show the lower teeth; it fails when the marginal mandibular branch of the facial nerve is cut.
How does the lip eversion test separate a cervical branch from a marginal mandibular branch injury?
If the person can evert the lower lip (mentalis intact) but cannot draw it down, a cervical branch or platysma problem is more likely.
Which nerve emerges beneath the depressor labii inferioris?
The mental nerve, from the mental foramen beneath the depressor labii inferioris and depressor anguli oris.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- depressor labii inferioris TA2 2083
- Official Spanish (DeCS)
- Músculo depresor del labio inferior
- English (TA2)
- Depressor labii inferioris
- Also called
- depressor labii inferioris muscle · quadratus labii inferioris muscle · muscle of Aeby · musculus quadratus labii inferioris · musculus labii mandibularis
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Head and Neck: Facial Muscles; Anatomy, Head and Neck, Lips.
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Representative model, for educational purposes only.
