The jaw is either of the two opposable structures forming, or near the entrance to, the mouth. In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. In arthropods, the jaws are chitinous and oppose laterally, and may consist in mandibles, chelicerae, or, loosely, pedipalps. Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing (mastication or chewing). The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it.
In vertebrates, the lower jaw or mandible is the mobile component that articulates at its posterior processes, or rami (singular ramus), with the temporal bones of the skull on either side; the word jaw used in the singular typically refers to the lower jaw. The upper jaw or maxilla is more or less fixed with the skull and is composed of two bones, the maxillae, fused intimately at the median line by a suture; incomplete closure of this suture and surrounding structures may be involved in the malformation known as cleft palate. The maxillary bones form parts of the roof of the mouth, the floor and sides of the nasal cavity, and the floor of the orbit or eye socket. The jaws typically accommodate the teeth or form the bases for the attachment of a beak.
In reptiles, the mandible is made up of five bones. In the evolution of mammals, four of these bones were reduced in size and incorporated into the ear. In their reduced form, they are known as the malleus and incus; along with the more ancient stapes, they are the ossicles. This adaptation is advantageous, not only because a one-bone jaw is stronger, but also because the malleus and incus improve hearing.
The term jaws is also used for articles resembling jaws in appearance or function, for example:
- the pincer-like grasping or crushing parts of a hand tool, as of pliers, a clamp, or a wrench
- the sides of a topographical pass or channel
- the space bounded by a croquet hoop, or the angled pair of cushions marking the entrance to the pocket of a billiards table
- metaphorically, the threshold of a usually perilous situation