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

Structure

This muscle arises from the oblique line of the mandible, and inserts on the skin of the lower lip, blending in with the orbicularis oris muscle.
At its origin, depressor labii is continuous with the fibers of the platysma muscle

Platysma muscle

The platysma is a superficial muscle that overlaps the sternocleidomastoid.It is a broad sheet arising from the fascia covering the upper parts of the pectoralis major and deltoid; its fibers cross the clavicle, and proceed obliquely upward and medially along the side of the neck.The anterior...

. Much yellow fat is intermingled with the fibers of this muscle.

Innervation

The depressor labii inferioris is innervated by the mandibular division of the facial nerve

Facial nerve

The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...

.