Jaw

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 and is part of the body plan of most animals.

Arthropods
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).

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.)

The human jaw
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.

References & Bibliography


Books

 * Dolan, E. A. (1987). Temporomandibular joint disorders: Diagnosis and treatment. New York, NY: Human Sciences Press.
 * Dworkin, S. F. (1999). Temporomandibular disorders: A problem in dental health. New York, NY: Guilford Press.
 * Hollingworth, H. L. (1939). Psycho-dynamics of chewing: (1939) Psycho-dynamics of chewing 90 pp Unknown Publisher.
 * Jacobs, B. L., Stafford, I. L., & Ribeiro do Valle, L. E. (1989). The masseteric (jaw closure) reflex: A simple mammalian brainstem system for studying neurochemical modulation. New York, NY: New York Academy of Sciences.
 * Ostry, D. J., Flanagan, J. R., Feldman, A. G., & Munhall, K. G. (1991). Human jaw motion control in mastication and speech. New York, NY: Kluwer Academic/Plenum Publishers.
 * Rollman, G. B., & Gillespie, J. M. (2004). Disturbances of pain perception in temporomandibular pain syndrome. New York, NY: Kluwer Academic/Plenum Publishers.

Books

 * Scavio, M. J., Jr. (1987). Appetitive-aversive interactions in rabbit conditioning preparations. Hillsdale, NJ, England: Lawrence Erlbaum Associates, Inc.

Papers

 * Google Scholar

Dissertations

 * Bermejo, R. (1987). Descriptive and kinematic analysis of jaw movements during eating behavior in the pigeon: Dissertation Abstracts International.
 * Brill, R. L. (1989). The acoustical function of the lower jaw of the bottlenose dolphin, Tursiops truncatus (Montagu), during echolocation: Dissertation Abstracts International.
 * Brown, C. M. (1998). Research Diagnostic Criteria for assessment of temporomandibular joint pain and dysfunction in children with juvenile rheumatoid arthritis. Dissertation Abstracts International: Section B: The Sciences and Engineering.
 * Diddel, R. M. (1989). The relationship between childhood experience and response to chronic pain in women with temporomandibular joint dysfunction: Dissertation Abstracts International.
 * Erlandson, P. M. (1983). Electromyograph biofeedback rest position training of masticatory muscles in myofascial pain-dysfunction patients: Dissertation Abstracts International
 * Harness, D. M. (1989). Comparison of clinical characteristics in myogenic, TMJ internal derangement and atypical facial pain patients: Dissertation Abstracts International.
 * Klein, A. T. (2004). Adjunctive behavioral therapy for the treatment of temporomandibular joint pain syndrome: A handbook and guide for the professional practitioner. Dissertation Abstracts International: Section B: The Sciences and Engineering.
 * Linville, R. N. (1982). Temporal aspects of articulation: Some implications for speech motor control of stereotyped productions: Dissertation Abstracts International.
 * Moss, R. A. (1982). Assessment of temporomandibular joint dysfunction syndrome: Dissertation Abstracts International.
 * Porrazzo, J. G. (2000). Prosodic factors in speechreading: Visual disambiguation of syntactic ambiguity. Dissertation Abstracts International: Section B: The Sciences and Engineering.
 * Reeves-Hoche, M. K. (1997). The association of mandibular molar loss and obstructive sleep apnea. Dissertation Abstracts International: Section B: The Sciences and Engineering.
 * Schnurr, R. F. (1989). Psychological factors in the development, maintenance, and treatment outcome of temporomandibular joint pain and dysfunction: Dissertation Abstracts International.
 * Shiller, D. M. (2004). Understanding speech motor control in the context of orofacial biomechanics. Dissertation Abstracts International: Section B: The Sciences and Engineering.
 * Sirisko, M. A. (1985). The sensorimotor cortical control of face, jaw and tongue movements in the Macaca fascicularis: Dissertation Abstracts International.
 * Souris, M. J. (1993). The relationship between the Type A pattern of behavior and temporomandibular joint dysfunction syndrome: Dissertation Abstracts International.
 * Weinhold, P. M. (1977). The effects of caudate nucleus stimulation upon jaw movements: Dissertation Abstracts International.