Mandible

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This article is about the vertebrate jawbone. For the human jawbone, see human mandible. For other uses, see Mandible (disambiguation).
"Mandibular" redirects here. For other uses, see Mandibular (disambiguation).

In vertebrates, the mandible, lower jaw or jawbone[1] is a bone forming the skull with the cranium.

In humans[edit]

Lower jaw in a human
Main article: Human mandible

In humans, the mandible is the largest, strongest and lowest bone in the face.[2] It forms the lower jaw and holds the lower teeth in place. In the midline on the anterior surface of the mandible is a faint ridge, an indication of the mandibular symphysis, where the bone is formed by the fusion of right and left processes during mandibular development. Like other symphysis in the body, this is a midline articulation where the bones are joined by fibrocartilage, but this articulation fuses together in early childhood.[3]

In other vertebrates[edit]

Sperm whale mandible

In lobe-finned fishes and the early fossil tetrapods, the bone homologous to the mandible of mammals is merely the largest of several bones in the lower jaw. In such animals, it is referred to as the dentary bone, and forms the body of the outer surface of the jaw. It is bordered below by a number of splenial bones, while the angle of the jaw is formed by a lower angular bone and a suprangular bone just above it. The inner surface of the jaw is lined by a prearticular bone, while the articular bone forms the articulation with the skull proper. Finally a set of three narrow coronoid bones lie above the prearticular bone. As the name implies, the majority of the teeth are attached to the dentary, but there are commonly also teeth on the coronoid bones, and sometimes on the prearticular as well.[4]

This complex primitive pattern has, however, been simplified to various degrees in the great majority of vertebrates, as bones have either fused or vanished entirely. In teleosts, only the dentary, articular, and angular bones remain, while in living amphibians, the dentary is accompanied only by the prearticular, and, in salamanders, one of the coronoids. The lower jaw of reptiles has only a single coronoid and splenial, but retains all the other primitive bones except the prearticular and the periosteum.[4]

While, in birds, these various bones have fused into a single structure, in mammals most of them have disappeared, leaving an enlarged dentary as the only remaining bone in the lower jaw - the mandible. As a result of this, the primitive jaw articulation, between the articular and quadrate bones, has been lost, and replaced with an entirely new articulation between the mandible and the temporal bone. An intermediate stage can be seen in some therapsids, in which both points of articulation are present. Aside from the dentary, only few other bones of the primitive lower jaw remain in mammals; the former articular and quadrate bones survive as the malleus and the incus of the middle ear.[4]

Finally, the cartilagenous fish, such as sharks, do not have any of the bones found in the lower jaw of other vertebrates. Instead, their lower jaw is composed of a cartilagenous structure homologous with the Meckel's cartilage of other groups. This also remains a significant element of the jaw in some primitive bony fish, such as sturgeons.[4]

References[edit]

  1. ^ The mandible is also in some sources still referred to as the inferior maxillary bone, though this is an outdated term which goes back to at least the 1858 first edition of Gray's Anatomy, if not earlier.
  2. ^ Gray's Anatomy - The Anatomical Basis of Clinical Practice, 40th Edition, page: 530
  3. ^ Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, page 59
  4. ^ a b c d Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 244–247. ISBN 0-03-910284-X. 

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