jaw


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jaw

1. the part of the skull of a vertebrate that frames the mouth and holds the teeth. In higher vertebrates it consists of the upper jaw (maxilla) fused to the cranium and the lower jaw (mandible)
2. the corresponding part of an invertebrate, esp an insect
3. a pair or either of a pair of hinged or sliding components of a machine or tool designed to grip an object

Jaw

 

(1) In animals, any of various organs of different origin used to capture and break up food. Jaws vary in structure among different taxonomic groups and are formed in the course of individual development from different rudiments; that is, they are analogous organs. Jaws exist in several invertebrates, including some worms, mollusks, and arthropods.

In arthropods, modified appendages (extremities) of the head function as jaws. In organisms whose head is fused with the thorax into a cephalothorax, the extremities of the thoracic segments usually serve to capture food, and they are called maxillipeds (crustaceans, chilopods). Particularly characteristic of arthropods is the presence of a pair of upper jaws (mandibles) and two pairs (less commonly one pair) of lower jaws (maxillae). In insects, a second pair of lower jaws fuse together to form the lower lip. Chelicerata have two pairs of head appendages that perform in part the function of jaws. The chelicerae are used to capture food, while the pedipalpi, located behind them, crush it. Among the echinoderms, sea urchins possess a complex maxillary apparatus called Aristotle’s lantern.

All vertebrates are divided into two large groups: those without jaws—the Agnatha, represented by the class Cyclostomata— and those with developed jaws—the Gnathostomata, which constitute all the other classes.

In gnathostomatous vertebrates, the jaws are situated on the facial (visceral) part of the skull. In the course of evolution, they developed first in fish as a result of the transformation of one of the anterior (third) pairs of gill arches. The upper and lower elements became reduced, while the middle portions enlarged to form the primary upper jaw, or palatoquadrate cartilage, and the primary lower jaw, or Meckel’s cartilage. Cartilaginous fishes (Chondrichthyes) have only primary cartilaginous jaws, equipped with teeth derived from the placoid scales. In bony fishes, amphibians, reptiles, birds, and mammals, substituting ossifications covered with tectorial bones developed in the posterior portions of the upper and lower jaws, from which secondary jaws then developed. Beginning with the bony fishes, a secondary jaw consisting of the premaxillary and maxillary bones forms the upper jaw. With the development of the secondary jaw, whose bones are situated along the margin of the mouth, the primary upper jaw is pushed back still farther into the palatine region of the skull and in reptiles, birds, and mammals loses its teeth. In some forms, the jaws have no teeth, and they acquire a horny structure (bill).

The development of jaws was a very important stage in the evolution of vertebrates, since it enabled vertebrates to shift from passive feeding to the active seizure of prey.

(2) In humans, the jaws are the largest bones of the visceral cranium. The upper jaw (maxilla) is a paired bone and consists of two superior maxillary bones. It occupies half of the upper part of the face, and its size and configuration greatly influence the shape of the face. Each superior maxillary bone consists of the body, with external surfaces (directed toward the nasal cavity) and posterior surfaces, and four processes—nasal, alveolar, zygomatic, and palate processes. The upper part of the bone, which is in the shape of a bent plate, forms the floor of the orbit. The interior of the body forms a cavity—the maxillary sinus, or antrum of High-more—which communicates with nasal fossae through an aperture on the internal, or nasal, surface. The alveolar process, the lower margin of the upper jaw, contains eight cavities for teeth. Together with the process of the other supermaxillary bone, it forms the dental alveolar arch.

The lower jaw (mandible) consists of the unpaired inferior maxillary bone of the facial skeleton. This only mobile cranial bone develops from symmetrical left and right halves that fuse in the center and occupies the lower part of the face. It is characterized by a V shape. Two rami project upward vertically or diagonally from the body, or horizontal portion, of the lower jaw. The superior border of the body consists of the alveolar arch, hollowed into 16 cavities for teeth. The end of each ramus breaks up into two processes: the coronoid process, to which the temporal muscle is attached, and the condyloid process, which articulates with the temporal bone. The angle between the ramus and the body varies from 90° to 140°. Nerves and blood vessels pass through the upper and lower jaws to innervate and supply the teeth with blood.

REFERENCES

Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1–2. Moscow, 1964.
Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Prives, M. G., N. K. Lysenkov, and V. I. Butkovich. Anatomiia cheloveka, 8th ed. Leningrad, 1974.

A. V. IVANOV, N. S. LEBEDKINA, and V. V. KUPRIIANOV

jaw

[]
(anatomy)
Either of two bones forming the skeleton of the mouth of vertebrates: the upper jaw or maxilla, and the lower jaw or mandible.
(engineering)
A notched part that permits a railroad-car axle box to move vertically.
(geology)
The side of a narrow passage such as a gorge.
References in classic literature ?
This whale is not dead; he is only dispirited; out of sorts, perhaps; hypochondriac; and so supine, that the hinges of his jaw have relaxed, leaving him there in that ungainly sort of plight, a reproach to all his tribe, who must, no doubt, imprecate lock-jaws upon him.
After a few drinks Goldberger began, with some hesitation, to narrate how he had had a quarrel over his best girl with a professional "cardsharp," who had hit him in the jaw.
At the daily dental hour there would always be about five hundred soldiers gathered together in the neighborhood of that dental chair waiting to see the performance--and help; and the moment the surgeon took a grip on the candidate's tooth and began to lift, every one of those five hundred rascals would clap his hand to his jaw and begin to hop around on one leg and howl with all the lungs he had
Environed by them, while the Woodman and the Farmer worked unheeded, those two of the large jaws, and those other two of the plain and the fair faces, trod with stir enough, and carried their divine rights with a high hand.
I looked, or seemed to look, and then I thought that the hanging jaw moved, and from it came a voice that was harsh and hollow as of one who speaks from an empty belly, through a withered throat.
And then, all of a sudden, he stopped, and his jaw dropped as though he had remembered something.
The youth trembled with horror, and his blood ran cold, yet he did not lose his courage; but, holding the iron spear upright in his hand, he brought it down with all his might right through the monster's lower jaw.
As he banged me down upon my feet his face was bent close to mine and I did the only thing a gentleman might do under the circumstances of brutality, boorishness, and lack of consideration for a stranger's rights; I swung my fist squarely to his jaw and he went down like a felled ox.
These tears ran slowly down his face and over the hinges of his jaw, and there they rusted.
But its most ferocious and repulsive feature is its jaws, the entire bony structure of which protrudes several inches beyond the flesh, revealing five sharp, spadelike teeth in the upper jaw and the same number of similar teeth in the lower, the whole suggesting the appearance of a rotting face from which much of the flesh has sloughed away.
But to his great surprise he could say no more, for tears came to his eyes and his lower jaw began to quiver rapidly.
And Big-Fat and Pig- Jaw and Tiger-Face and all the rest said it was true.