Jaw
This article is about the anatomical part. For other uses of the term, see Jaws (disambiguation).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.
The vertabrate jaw probably originally evolved in the
Silurian period and appeared in the
Placoderm fish which further diversified in the
Devonian. Jaws are thought to derive from the pharyngeal arches that support the
gills in fish. The two most anterior of these arches are thought to have become the jaw itself and the hyoid arch, which braces the jaw against the braincase and increases mechanical efficiency. While there is no fossil evidence to directly support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed (the
Gnathostomes), which have seven arches, and primitive jawless vertebrates (the
Agnatha), which have nine.
It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increased respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and
amphibians) that pumps water across the gills, or air into the lungs in the case of amphibians. Over evolutionary time the more familiar use of jaws (to humans), in feeding, was selected for and became a very important function in vertebrates.
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 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
Gnathostomata - jawed vertebrates
