The depressor labii inferioris (or quadratus labii inferioris) is a facial muscle. It helps to lower the bottom lip.

Structure

The depressor labii inferioris muscle arises from the lateral surface of the

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

. This is below the

mental foramen The mental foramen is one of two foramina (openings) located on the anterior surface of the mandible. It is part of the mandibular canal. It transmits the terminal branches of the inferior alveolar nerve and the mental vessels. Structure Th ...

, and the origin may be around 3 cm wide. It inserts on the

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

of the lower lip, blending in with the orbicularis oris muscle around 2 cm wide. At its origin, depressor labii is continuous with the fibers of the platysma muscle. Some yellow fat is intermingled with the fibers.

Nerve supply

The depressor labii inferioris muscle is supplied by the marginal mandibular branch of the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...

Function

The depressor labii inferioris muscle helps to depress and everts the lower lip. It is the most important of the muscles of the lower lip for this function. It is an antagonist of the orbicularis oris muscle. It is needed to expose the mandibular (lower) teeth during smiling.

Clinical significance

Resection

The depressor labii inferioris muscle may be resected (cut and removed) using

surgery Surgery is a medical specialty that uses manual and instrumental techniques to diagnose or treat pathological conditions (e.g., trauma, disease, injury, malignancy), to alter bodily functions (e.g., malabsorption created by bariatric surgery s ...

to correct an asymmetry of the lower lip when smiling. This asymmetry can be caused by paralysis of the marginal mandibular branch of the

on one side, so the healthy side may be cut to create symmetry. Local anaesthesia may be used, such as by blocking the mental nerve. This operation tends to be successful.

History

The depressor labii inferioris muscle has also (mainly historically) been called the quadratus labii inferioris muscle. Note that in the 1918 edition of Gray's Anatomy illustration used here that its abbreviation is "Quad. labii inf." (sic).

Additional images

File:Depressor_labii_inferioris_muscle_animation small.gif, Position of depressor labii inferioris muscle (red). File:Gray176.png, Mandible. Outer surface. Side view.

References

{{Authority control Muscles of the head and neck