Mandible. The mandible, or lower jaw, is the bone that forms the lower part of the skull, and along with the maxilla (upper jaw), forms the mouth structure. Movement of the lower jaw opens and closes the mouth and also allows for the chewing of food.The lower set of teeth in the mouth is rooted in the lower jaw.he mandible, or lower jaw, is the bone that forms the lower part of the skull, and along with the maxilla (upper jaw), forms the mouth structure.