jaw

One of two bony structures that form the framework of the mouth in all vertebrates except lampreys and hagfishes (the agnathous or jawless vertebrates). They consist of the upper jawbone (maxilla), which is fused to the skull, and the lower jawbone (mandible), which is hinged at each side to the bones of the temple by ligaments.

The lower jawbone (mandible), the largest bone of the face, consists of a horizontal portion and two upright portions (the rami).

Both jaws bear teeth. The closing and opening of the jaws is effected by four pairs of muscles, two attached to the outer, and two to the inner side of the rami of the lower jaw. The bones were derived originally from the anterior gill arches in primitive vertebrates.