The depressor labii inferiois muscle is a quadrilateral facial muscle located in the jaw area that draws the lower lip down and to the side as in an expression of irony. Originating on the oblique line of the mandible, it inserts on the skin of the lower lip and blends with the orbicularis oris muscle. It extends the fibers of the platysma muscle while its own fibers are combined with yellow fat. Also known as the quadratus labii inferioris and the quadratus menti, the depressor labii inferioris allows for facial expressions, the playing of the trumpet, and kissing. It is aided by the other muscles that lower the lip -- the risorius, the depressor anguli oris and the mentalis. Innervated by the facial nerve, ailments affecting the depressor labii inferioris include myalga, stroke, myopathy, strains, Bell's palsy, lacerations, contusions, infectious myositis, tears, atrophy and neuromuscular diseases. Symptoms related to these ailments include decreased movement of the lip, loss of muscle control, paralysis, and muscle spasms.