The depressor DAO is a paired bilateral muscle that originates from the soft tissue overlying the oblique line and the inferior border of the anterior body and parasymphysis of the mandible. DAO inserts onto the lateral aspect of the lower lip and oral modiolus (FIG 1).

The DAO is innervated by the marginal mandibular branch of the facial nerve, and its activation moves the oral commissure inferolaterally.

The shape of the DAO is roughly a triangle with its base inferiorly at its origin and its apex superiorly near the modiolus of the oral commissure.

At its origin, the DAO muscular body is continuous with the platysma muscle inferiorly within the superficial musculoaponeurotic system.

The DLI is a paired bilateral muscle that is directly medial to the DAO, which also originates from the inferior border of the parasymphysis of the mandible but inserts mostly on the body of the lower lip (see FIG 1). Its action, as its name implies, depresses one side of the lower lip.

In the areas where the DAO and DLI overlap, the DLI lies deep to the DAO (see FIG 1).

The facial artery runs superomedial from the mandibular notch, and the inferior labial artery arises inferior to the modiolus. The inferior labial artery, a branch of the facial artery, runs deep to the DAO fibers (FIG 2).