The depressor anguli oris muscle is attached to the lower jaw (mandible). It ends at the orbicularis oris, which is the sphincter muscle that encompasses the lips. It is attached at the edge of the lips. The depressor anguli oris muscle is associated with frowning, as it pulls down the edges of the lips. There are two of these muscles, one on either side of the mouth.
The depressor anguli oris muscle sometimes does not develop during pregnancy and the child is born without the muscle. This absence is not noticeable when the baby is quiet and awake. However, when awake and crying, a facial asymmetry is visible. The corner of the mouth on the side where the muscle is missing will be displaced towards the normal side. The condition is usually not treated with surgery because when the child is older, the asymmetry will not be noticeable.
The depressor anguli oris muscle is the muscle that produces what are commonly known as the jowls of the face. A small amount of Botox can be injected into this muscle to remove what may appear in some people to be a perpetual frown.