upper jaw



The jaw is either of the two opposable structures forming, or near the entrance to the mouth.



In arthropods, the jaws are chitinous and oppose laterally, and may consist of mandibles, chelicerae, or loosely, pedipalps.

Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing (mastication or chewing).


In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw.

Bones of the jaw

In vertebrates, the lower jaw, dentary 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 jaw in fish and amphibians

The vertebrate 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 directly to 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.


The jaw in reptiles

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. (However, reptiles tend to swallow prey whole because their pace of digestion is different than mammals, so multiple jaw bones may allow flexibility to expand the jaws around prey.)

See also

External links

Search another word or see upper jawon Dictionary | Thesaurus |Spanish
Copyright © 2015, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature