The jaw/الفك/颚(È)

The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. 

Human jaws - Upper & Lower Jaw Model

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.

Vertebrates
 In most vertebrates, the jaws are bony or cartilaginous and oppose vertically, comprising an upper jaw and a lower jaw. The vertebrate jaw is derived from the most anterior two pharyngeal arches supporting the gills, and usually bears numerous teeth. Jaws of great white shark

The jaw in fish 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.

 The two most anterior pharyngeal arches are thought to have become the jaw itself and the hyoid arch, respectively. The hyoid system suspends the jaw from the braincase of the skull, permitting great mobility of the jaws.

 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 vertebrates (the Gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine.

The Alligator Gar

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.

 Many teleost fish have substantially modified their jaws for suction feeding and jaw protrusion, resulting in highly complex jaws with dozens of bones involved.

The jaw in amphibians, reptiles, and birds The jaw in tetrapods is substantially simplified compared to fish. Most of the upper jaw bones (premaxilla, maxilla, jugal, quadratojugal, and quadrate) have been fused to the braincase, while the lower jaw bones (dentary, splenial, angular, surangular, and articular) have been fused together into a unit called the mandible.

The jaw articulates via a hinge joint between the quadrate and articular.

The jaws of tetrapods exhibit varying degrees of mobility between jaw bones. Some species have jaw bones completely fused, while others may have joints allowing for mobility of the dentary, quadrate, or maxilla.

The snake skull shows the greatest degree of cranial kinesis, in order to allow the snake to swallow large prey items.

The jaw in mammals In the evolution of the mammalian jaw, two of the bones of the jaw structure (the articular bone of the lower jaw, and quadrate) were reduced in size and incorporated into the ear, while many others have been fused together.

As a result, mammals show little or no cranial kinesis, and the mandible is attached to the temporal bone by the temporomandibular joint.

 Temporomandibular joint disorder is a common disorder of this joint, and can cause pain and loss of joint mobility.