jaw

Information about jaw

This article is about the anatomical part. For other uses of the term, see Jaws.
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Human jaw front view
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Human jaw left view
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Human jaw top view
The jaw is either of the two opposable structures forming, or near the entrance to, the mouth.

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.

Arthropods

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).

Vertebrates

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.

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.

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

Anatomy (from the Greek ἀνατομία anatomia, from ἀνατέμνειν
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Jaws may refer to the following:
  • Jaws are the two opposable structures of the mouth
  • Jaws, a novel by Peter Benchley
  • Jaws (film), the 1975 film based on this novel

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mouth, also known as the buccal cavity or the oral cavity, is the orifice through which an organism takes in food and water.

Location

In all mammals, the mouth is forward-facing in the face. Non-mammals have mouths in other locations (e.g.
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Arthropoda
Latreille, 1829

Subphyla and Classes
  • Subphylum Trilobitomorpha
  • Trilobita - trilobites (extinct)
  • Subphylum Chelicerata

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Chitin (C8H13O5N)n (IPA: [ˈkaɪtn̩]) is a long-chain polymer of beta-glucose that forms a hard, semitransparent material found throughout the natural world.
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Chelicerae are mouth parts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata (horseshoe crabs), and Pycnogonida (sea spiders). Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other
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Pedipalps, the second pair of appendages of the cephalothorax in Arachnida, is homologous with mandibles in Crustacea, and corresponding to the mandibles of insects. The pedipalps are appendages of six segments: the coxae, a single trochanter, the femur, a short patella, the tibia,
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Vertebrata
Cuvier, 1812

Classes and Clades

See below
Vertebrates are members of the subphylum Vertebrata (within the phylum Chordata), specifically, those chordates with backbones or spinal columns.
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Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals.
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Cartilage is a type of dense connective tissue. It is composed of collagen fibers and/or elastin fibers, and can supply smooth surfaces for the movement of articulating bones.
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mandible (from Latin mandibūla, "jawbone") or inferior maxillary bone is, together with the maxilla, the largest and strongest bone of the face . It forms the lower jaw and holds the lower teeth in place.
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skull is a bony structure found in many animals which serves as the general framework for the head. The skull supports the structures of the face and protects the head against injury.

The skull can be subdivided into two parts: the cranium and the mandible.
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The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible, which is also a fusion of two halves at the mental symphysis.
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Sutures are the stitches that doctors, and especially surgeons, use to hold skin, internal organs, blood vessels and all other tissues of the human body together, after they have been severed by injury or surgery.
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Cleft lip and cleft palate, which can also occur together as cleft lip and palate are variations of a type of clefting congenital deformity caused by abnormal facial development during gestation. This type of deformity is sometimes referred to as a cleft.
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nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth.

In most humans, it also houses the nosehairs, which catch airborne particles and prevent them from reaching the lungs.
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In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated.

It can also mean the skin which surrounds the eye of a bird.

In the adult human, the volume of the orbit is 30 ml, of which the eye occupies 6.5 ml.
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Teeth (singular, tooth) are structures found in the jaws (or mouths) of many vertebrates that are used to tear, scrape, and chew food. Some animals, particularly carnivores, also use teeth for hunting or defense. The roots of teeth are covered by gums.
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The beak, bill or rostrum is an external anatomical structure of birds which, in addition to eating, is used for grooming, manipulating objects, killing prey, probing for food, courtship, and feeding their young.
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The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma (million years ago), to the beginning of the Devonian period, about 416.0 ± 2.8 Ma (ICS 2004).
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Placodermi
McCoy, 1848

Orders

Antiarchi †
Arthrodira †
Brindabellaspida †
Petalichthyida †
Phyllolepida †
Ptyctodontida †
Rhenanida †
Acanthothoraci †
?Pseudopetalichthyida †
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Devonian is a geologic period of the Paleozoic era spanning from roughly 416 to 359 million years ago. It is named after Devon, England, where rocks from this period were first studied.
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A gill is a respiration organ that functions for the extraction of oxygen from water and the excretion of carbon dioxide. Unlike many small aquatic animals, which can absorb oxygen through the entire surface of their bodies, more complex aquatic organisms have gills specially
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Gnathostomata

Subgroups
  • Class Placodermi
  • Superclass Chondrichthyes
  • Microphylum Teleostomi
  • Class Acanthodii
  • Class Actinopterygii

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Agnatha (Greek, "no jaws") is a paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. Agnatha have existed since the Cambrian, and continue to live in modern times.
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Buccal pumping is a method of respiration using the throat muscles. Animals using this method will typically move the floor of the mouth or throat in a rhythmic manner that is externally apparent.

This method has several stages.
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Amphibia
Linnaeus, 1758

Subclasses and Orders

   Order Temnospondyli - extinct
Subclass Lepospondyli - extinct
Subclass Lissamphibia
   Order Anura
   Order Caudata
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Sauropsida*
Goodrich, 1916

Subclasses
  • Anapsida
  • Diapsida
Synonyms
  • Reptilia Laurenti, 1768
Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane, and members of the class
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mandible (from Latin mandibūla, "jawbone") or inferior maxillary bone is, together with the maxilla, the largest and strongest bone of the face . It forms the lower jaw and holds the lower teeth in place.
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