The depressor labii inferioris is a quadrilateral-shaped facial muscle of the oral group that is found in the chin area. As its name suggests, it primarily depresses the lower lip. Overall, this muscle controls the movements, position, and shape of the lower lip.

Origin and insertion of depressor labii inferioris

The depressor labii inferioris originates from the oblique line of the mandible and extends superomedially. It inserts into the medial portion of the half of the lower lip. At the insertion site, the fibers of this muscle also blend with the fibers of the orbicularis oris muscle. Additionally, fibers from both depressor labii inferioris muscles intertwine at the midline beneath the lower lip.

The inferolateral aspect of the depressor labii inferioris lies deep to the depressor anguli oris, while its superolateral portion partially covers the origin site of the mentalis muscle. It is important to note that this muscle is closely related to the marginal mandibular branch of the facial nerve (CN VII).

Origin: oblique line located on the external surface of the body of the mandible (just superior to the origin of the depressor anguli oris muscle).

Insertion: skin of the medial portion of the lower lip half (almost up to the vermillion border); blends with the fibers of the orbicularis oris muscle. Fibers of both depressor labii inferioris muscles meet and blend at the midline below the lower lip.

Action: pulls the lower lip downward and laterally. Contractions of the depressor labii inferioris participate in producing facial expressions associated with sadness, doubt, and contempt.

Innervation: marginal mandibular branch of the facial nerve (CN VII).

Arterial blood supply: inferior labial and labiomental arteries of the facial artery and the mental branch of the inferior alveolar artery (a branch of the maxillary artery).

Venous drainage: inferior labial vein and submental vein, which drain into the facial vein.

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Depressor labii inferioris