jaw(redirected from undershot jaw)
Also found in: Dictionary, Thesaurus, Medical.
(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.
REFERENCESBeklemishev, 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