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Human jaw left view

The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. 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 and is part of the body plan of most animals.

Contents

Arthropods

The mandibles of a Bull ant

In arthropods, the jaws are chitinous and oppose laterally, and may consist of mandibles or chelicerae. These jaws are often composed of numerous mouthparts. Their function is fundamentally for food acquisition, conveyance to the mouth, and/or initial processing (mastication or chewing). Many mouthparts and associate structures (such as pedipalps) are modified legs.

Vertebrates

In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. The vertebrate jaw is derived from the most anterior two pharyngeal arches supporting the gills, and usually bears numerous teeth.

The jaw in fish

Moray eels have two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to the esophagus for swallowing

The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian. The two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively. The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws. 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.

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 of fish 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. Many teleost fish have substantially modified their jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved.

The jaw in amphibians, reptiles, and birds

The jaw in tetrapods is substantially simplified compare to fish. Most of the upper jaw bones (premaxilla, maxilla, jugal, quadratojugal, and quadrate) have been fused to the braincase, while the lower jaw bones (dentary, splenial, angular, surangular, and articular) have been fused together into a unit called the mandible. The jaw articulates via a hinge joint between the quadrate and articular. The jaws of tetrapods exhibit varying degrees of mobility between jaw bones. Some species have jaw bones completely fused, while others may have joints allowing for mobility of the dentary, quadrate, or maxilla. The snake skull shows the greatest degree of cranial kinesis, in order to allow the snake to swallow large prey items.

The jaw in mammals

In the evolution of the mammalian jaw, two of these bones were reduced in size and incorporated into the ear, while many others have been fused together. As a result, mammals show little or no cranial kinesis, and the mandible is attached to the temporal bone by the temporomandibular joint. A common disorder of this joint is temporomandibular joint disorder which can cause pain and loss of mobility.

Sea urchin jaws

Sea urchins possess unique jaws which display 5-part symmetry, termed the "Aristotle's lantern". Each unit of the jaw holds a single, perpetually-growing tooth composed of crystalline calcium carbonate.

See also

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

JAW (Mid. Eng. jawe, jowe and geowe, O. Eng. cheowan, connected with "chaw" and "chew," and in form with "jowl"), in anatomy, the term for the upper maxillary bone, and the mandible or lower maxillary bone of the skull; it is sometimes loosely applied to all the lower front parts of the skull (q.v.).

ABU MANSUR MAUHUB UL-JAWALIQI (1073-1145), Arabian grammarian, was born at Bagdad, where he studied philology under Tibrizi and became famous for his handwriting. In his later years he acted as imam to the caliph Moqtafi. His chief work is the Kitab ul-Mu`arrab, or "Explanation of Foreign Words used in Arabic." The text was edited from an incomplete manuscript by E. Sachau (Leipzig, 1867). Many of the lacunae in this have been supplied from another manuscript by W. Spitta in the Journal of the German Oriental Society, xxxiii. 208 sqq. Another work, written as a supplement to the Durrat ul-Ghawwas of Hariri (q.v.), has been published as "Le Livre des locutions vicieuses," by H. Derenbourg in Morgenlandische Forschungen (Leipzig, 1875), pp. 107-166. (G. W. T.)


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Bible wiki

Up to date as of January 23, 2010
(Redirected to Jaw-bone article)

From BibleWiki

The jaw-bone of an ass afforded Samson a weapon for the great slaughter of the Philistines (Jdg 15:15), in which he slew a thousand men. In verse 19 the Authorized Version reads, "God clave a hollow place that was in the jaw, and there came water thereout." This is a mis-translation of the words. The rendering should be as in the Revised Version, "God clave the hollow place that is in Lehi," etc., Lehi being the name of the hill where this conflict was waged, possibly so called because it was in shape like a jaw-bone.

This entry includes text from Easton's Bible Dictionary, 1897.

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Simple English

For the film about a shark, see Jaws (film).
File:Human jawbone
The left side of a human jaw

The jaw is the bone in the mouth that, in humans, lets the mouth move up and down in order to chew. It is joined onto to the skull.

However, in insects, the jaw may open left and right instead.bjn:Wihang








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