Jaw

The jaw (also called jaqal) is the bony part of the lower face in vertebrate animals. It is divided into the upper jaw, which is fixed to the bones of the face, and the lower jaw, which is movable. The jaw holds the teeth and enables food to be grasped, crushed, and moved.

In the process of eating, the jaw plays a role in human speech, breathing, and facial appearance. Muscles that move the jaw move the lower jaw up and down, working through a pair of joints that connect the jaw to the skull.

Structure in humans

In humans, the lower jaw is a single large bone with a distinctive shape. The upper jaw is not a single bone; it is part of the skull bones that does not move independently. Both jaws have sockets in which the teeth are set. The joint between the lower jaw and the bone near the ear allows for upward and downward movement and side-to-side movement, which aids in chewing.

Variation among animals

Jaws vary from species to species depending on their diet. Carnivorous mammals typically have powerful chewing muscles and sharp teeth. Herbivorous animals have jaws that move side to side to grind vegetation. Birds with teeth do not exist; they have a beak (terminal mouth) made of hard bone covered with a horny sheath.

Evolutionary origin

The standard biological view is that the jaw evolved from part of the gill arch support structures of early fish. Fish without jaws, such as lampreys, still exist and show an earlier stage. This transformation is considered important to the development of feeding methods in vertebrates.

Health and injury

The jaw can suffer fractures, joint dislocation, and pain in the chewing muscles. Incorrect alignment of the teeth (malocclusion) is usually treated with braces or jaw surgery. Jaw joint disorder is marked by clicking, limited mouth opening, and ear pain.