Muscle · Head

Depressor labii inferioris

Musculus depressor labii inferioris

A small square of muscle lying just in front of and deeper than the triangular muscle of the chin, running upward and inward from the jawbone into the skin and muscle of the lower lip. It pulls the lower lip down and slightly sideways — the movement that bares the lower teeth, and part of the expression of irony or dismay. Its fibres blend with those of the same muscle from the other side, so the two behave as one sheet across the midline. Because it inserts into lip tissue rather than bone, its weakness shows as a lip that will not come down evenly, not as a jaw problem.