Human jaw left view

The jaw is either of the two opposable structures forming, or near the entrance to the mouth. The mouth, buccal cavity, or oral cavity is the first portion of the Alimentary canal that receives food and begins digestion by mechanically breaking up

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. A body plan, or bauplan, is essentially the blueprint for the way the body of an organism is laid out

Arthropods

The mandibles of a Bull ant

In arthropods, the jaws are chitinous and oppose laterally, and may consist of mandibles, chelicerae, or loosely, pedipalps. Arthropods are Animals belonging to the Phylum Arthropoda (from Greek ἄρθρον arthron, " Joint " Chitin ( C 8 H 13 O 5 N)n (ˈkaɪtən is a long-chain Polymer of a N-acetylglucosamine In arthropods the mandible is either of a pair of Arthropod Mouthparts used for biting cutting and holding food The Chelicerae are mouth parts of the Chelicerata, an Arthropod Subphylum that includes Arachnids, Merostomata Pedipalps, are the second pair of Appendages of the Prosoma in the subphylum Chelicerata.

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. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes 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 Cartilage is a type of dense Connective tissue. It is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix

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 mandible (from Latin mandibula, "jawbone" or inferior maxillary bone forms the lower Jaw and holds the lower teeth in place

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. This article is about the Mammal maxilla For Arthropod maxillae see Mouthparts; for Insect maxillae in particular see Insect mouthparts In fields of Anatomy, anatomical terms of location are descriptive terms to help identify relative positions or directions within a species Cleft lip (cheiloschisis and cleft palate (palatoschisis which can also occur together as cleft lip and palate are variations of a type of clefting Congenital

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. Anatomically a nose is a protuberance in Vertebrates that houses the Nostrils or nares which admit and expel air for respiration in conjunction with the In Anatomy, the orbital bone is the cavity or socket of the Skull in which the Eye and its appendages are situated

The jaws typically accommodate the teeth or form the bases for the attachment of a beak. Anatomy Stegosaurus --> Beaks can vary significantly in size and shape from species to species

The jaw in fish and amphibians

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. Moray eels are large cosmopolitan Eels of the family Muraenidae. The Silurian is a geologic period and system that extends from the end of the Ordovician period about 443 The Placodermi were a class of armoured Prehistoric fishes known from Fossils which lived from the late Silurian to the end of the Devonian Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two The Devonian is a geologic period and system of the Paleozoic era spanning from to  million years ago. Jaws are thought to derive from the pharyngeal arches that support the gills in fish. In the development of vertebrate animals, the pharyngeal arches (also called branchial arches or gill arches in fish develop during the fourth A gill is an anatomical structure found in many aquatic organisms 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. Gnathostomata is the group of Vertebrates with Jaws The group is traditionally a superclass, including the familiar classes of Fish, Agnatha ( Greek, "no jaws" is a Paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata

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. Buccal pumping is a method of respiration using the throat muscles Prehistoric amphibian Amphibians (class Amphibia such as Frogs Toads Salamanders Newts Gymnophiona, Sirens and 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. Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers The mandible (from Latin mandibula, "jawbone" or inferior maxillary bone forms the lower Jaw and holds the lower teeth in place In the evolution of mammals, four of these bones were reduced in size and incorporated into the ear. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands In their reduced form, they are known as the malleus and incus; along with the more ancient stapes, they are the ossicles. The malleus or hammer is a hammer-shaped small Bone or ossicle of the Middle ear which connects with the Incus and is attached For the record label see Incus Records. The incus or anvil is the Anvil -shaped small Bone or ossicle The stapes or stirrup is the stirrup-shaped small Bone or ossicle in the Middle ear which attaches the Incus to the Fenestra ovalis The ossicles (also called auditory ossicles) are the three smallest Bones in the human body 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

• Gnathostomata - jawed vertebrates
• Predentary
• Premaxilla
• Rostral bone

External links

• Prehistoric Fish Had Most Powerful Jaws
• MeSH Jaw