Jaw

The jaw is a structure of the skull that allows the mouth to function properly. It is responsible for opening and closing the mouth, which is essential for both chewing and speaking. The jaw is composed of two primary bones: the mandible and the maxilla. Each is subdivided into a number of smaller sections, and both the mandible and the maxilla house teeth. The jaw is controlled by a group of muscles known as the muscles of mastication. Three of these muscles are responsible for closing the jaw, while one is responsible for sideways and forward motion. The muscles of mastication are distinct from other facial muscles, which are responsible for facial expression.

Background

The lower jawbone is called the mandible. The mandible is responsible for providing structure to the bottom half of the face and supporting the bottom row of teeth. Its movement allows for both speech and mastication.

Infants are born with a partially intact lower jaw. Unlike an adult mandible, the lower jaw of an infant is split into two parts. These two pieces, called the hemimandibles, fuse together near the midline of the chin by age two.

The mandible is divided into many parts. The depressed area in the chin is called the mental protuberance. The protrusion immediately below the mental protuberance is called the mental tubercle. The base of the mandible flares out from the mental tubercle on each side, forming the body of the jaw. At the back of the body, the angle of the mandible shifts upward, forming the ramus.

The ramus splits, forming two narrowed ends. The end closest to the front of the face is called the coronoid process. The end closest to the back of the head narrows, forming the neck, pterygoid fossa, and head of the jaw. Collectively, the coronoid process, neck, and head form a flexible joint that attaches to the upper jaw.

The upper jaw is called the maxilla. Unlike the lower jaw, it does not move independently. Instead, it is fused with the rest of the skull. Like the lower jaw, the upper jaw is not fully formed in an infant at birth. Instead, infants are born with several maxillary bones that fuse together over time.

The lowest ridge of the maxilla is composed of the canine fossa, canine eminence, alveolar process, and maxillary tuberosity. These structures are mirrored, presenting symmetrically on each side of the face. The body of the maxilla sits above those structures in front of the cheeks. The zygomatic process of the maxilla and the infraorbital foramen sit around the nose, while the frontal process of the maxilla sits behind the nasal bone.

Overview

The mandible and maxilla interlock, creating the temporomandibular joint on each side of the face. They allow for a wide range of actions, including moving the mandible forward, backward, and side to side. The temporomandibular joints function like a ball and socket, with the head of the mandible sitting inside a specialized cavity in the maxilla.

Both the mandible and the maxilla are responsible for housing teeth. Teeth are essential for biting and chewing food. Incisors, a set of sharp, angular teeth, are located at the front of the jaw. They are flanked by canines, a set of pointed teeth. The premolars, a set of moderately sized flat teeth, follow the canines. Finally, the molars, a set of large flat teeth, are located at the back of the jaw. Each set of teeth has a specific purpose. Incisors are used to bite into food, canines are used to grip, premolars are used to tear, and molars are used to crush and chew.

Teeth are composed of several layers. The outermost layer of the teeth, the enamel, is the hardest material in the human body. It helps protect the teeth. Underneath the enamel is a softer material called dentin, which gives the tooth its structure. It protects the pulp, the soft, nerve-filled tissue on the inside of the tooth. Each tooth is secured to the jaw by a set of dentin roots. These roots extend through the bone of the jaw, providing a secure pathway for dental nerves.

Four muscles, known collectively as the muscles of mastication, are responsible for jaw movement. They are the temporalis, masseter, medial pterygoid, and lateral pterygoid. The masseter is a large, flat, quadrilateral-shaped muscle. When it contracts, the mouth closes. The temporalis is a powerful muscle and tendon that helps close the mouth and pull the jaw backward, toward the spine. The medial pterygoid performs a similar function to the masseter and temporalis, though it also contains the mandibular nerve. The lateral pterygoid pulls the jaw forward or side to side.

The jaw may suffer a number of injuries. One of the most common jaw injuries is a mandibular fracture, which is any fracture in the mandible. Symptoms include pain, difficulty opening the jaw, bleeding, difficulty speaking, and swelling. Treating a broken jaw is similar to treating any other broken bone. The mandible may need to be reset to heal properly. If the fracture is particularly complex, surgery may be required to repair the bone.

Another common injury, a dislocated jaw, occurs when the temporomandibular joint becomes dislocated. Symptoms of a dislocated jaw are similar to those of a broken jaw. To treat a dislocated jaw, the doctor may attempt to reset the jaw. If that fails, surgery may be required to properly position the mandible. The injury may need to be carefully tended for up to six weeks. Opening the jaw wide, such as when sneezing or yawning, can reinjure the jaw.

Bibliography

"Anatomy and Development of the Mouth and Teeth." Stanford Children's Health Clinic, www.stanfordchildrens.org/en/topic/default%3Fid%3Danatomy-and-development-of-the-mouth-and-teeth-90-P01872&rct. Accessed 25 Nov. 2024.

Jahan-Parwar, Babak. "Facial Bone Anatomy." Medscape, 11 Jun. 2013, emedicine.medscape.com/article/835401-overview#a2. Accessed 25 Nov. 2024.

Jones, Oliver. "The Muscles of Mastication." TeachMeAnatomy, 1 Oct. 2024, teachmeanatomy.info/head/muscles/mastication/. Accessed 25 Nov. 2024.

"Mandible." InnerBody, 25 Nov. 2024, www.innerbody.com/image‗dige01/skel48-new.html. Accessed 25 Nov. 2024.