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Jai Ganesh

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Mandible

Mandible

Gist

In jawed vertebrates, the mandible (from the Latin mandibula, 'for chewing'), lower jaw, or jawbone is a bone that makes up the lower – and typically more mobile – component of the mouth (the upper jaw being known as the maxilla).

Upper jaw (maxilla): Fixed bone that holds your upper teeth and shapes your face. Lower jaw (mandible): The only movable bone in your skull and the strongest facial bone.

The mandible is the largest, strongest, and only movable bone in the human skull, forming the lower jaw, holding the lower teeth, and enabling mastication and speech. It is a U-shaped bone consisting of a horizontal body and two vertical rami that articulate with the temporal bone. It fuses into one bone in early life.

Summary

In jawed vertebrates, the mandible (from the Latin mandibula, 'for chewing'), lower jaw, or jawbone is a bone that makes up the lower – and typically more mobile – component of the mouth (the upper jaw being known as the maxilla).

The jawbone is the skull's only movable, posable bone, sharing joints with the cranium's temporal bones. The mandible hosts the lower teeth (their depth delineated by the alveolar process). Many muscles attach to the bone, which also hosts nerves (some connecting to the teeth) and blood vessels. Amongst other functions, the jawbone is essential for chewing food.

Owing to the Neolithic advent of agriculture (c. 10,000 BCE), human jaws evolved to be smaller. Although it is the strongest bone of the facial skeleton, the mandible tends to deform in old age; it is also subject to fracturing. Surgery allows for the removal of jawbone fragments (or its entirety) as well as regenerative methods. Additionally, the bone is of great forensic significance.

Details:

Introduction

The mandible is the largest and strongest bone of the human skull. It is commonly known as the lower jaw and is located inferior to the maxilla. It is composed of a horseshoe-shaped body which lodges the teeth, and a pair of rami which projects upwards to form a temporomandibular joint.

Structure

The mandible is formed by a body and a pair of rami along with condyloid and coronoid processes.

Body

Body is the anterior portion of the mandible. Body has two surfaces: outer and inner and two borders: upper and lower border. The body ends and the rami begin on either side at the angle of the mandible, also known as the gonial angle.

1. Outer surface is also known as external surface and has following characteristics:

* Mandibular symphysis/ Symphysis menti at midline which joins left and right half of the bone, detected as a subtle ridge in the adult.

* The inferior portion of the ridge divides and encloses a midline depression called the mental protuberance, also known as chin. The edges of the mental protuberance are elevated, forming the mental tubercle.

* Laterally to the ridge and below the incisive teeth is a depression known as the incisive fossa.

* Below the second premolar is the mental foramen, in which the mental nerve and vessels exit.

* The oblique line courses posteriorly from the mental tubercle to the anterior border of the ramus.

2. Inner surface is also known as internal surface and has following features:

* The mylohyoid line is a prominent ridge that runs obliquely downwards and forwards from below the third molar tooth to the median area below the genial tubercles.

* Below the mylohyoid line, the surface is slightly hollowed out to form the sub-mandibular fossa, which lodges the submandibular gland.

* Above the mylohyoid line, there is the sublingual fossa in which the sublingual gland lies.

* The posterior surface of the symphysis menti is marked by four small elevations called the superior and inferior genial tubercles.

3. Upper border (Alveolar border)

It consist of sockets for the teeth.

4. Lower border (Inferior border)

It is also known as base. There is a fossa present at the side of midline known as digastric fossa.

Ramus

The ramus is lateral continuation of the body and is quadrilateral in shape. The coronoid process is the anterosuperior projection of the ramus which is triangular in shape. Whereas posterosuperior projection of ramus is known as condyloid process whose head is covered with fibrocartilage and form a temporomandibular joint. The constricted part below condyloid process is neck. Condyloid and coronoid process are separated by a mandibular notch. It has two surfaces: medial and lateral and four borders: superior, inferior, anterior and posterior.

Ossification

Mandible is the second bone to ossify after clavicle. Each half of the mandible ossifies from only one centre at the sixth week of intrauterine life in the mesenchymal sheath of Meckel's cartilage near the future mental foramen. The first pharyngeal arch, known as the mandibular arch, gives rise to the Meckel cartilage. A fibrous membrane covers the left and right Meckel cartilage at their ventral ends. These two halves eventually fuse via fibrocartilage at the mandibular symphysis. Thus, at birth, the mandible is still composed of two separate bones. Ossification and fusion of the mandibular symphysis occur during the first year of life, resulting in a single bone. The remnant of the mandibular symphysis is a subtle ridge at the midline of the mandible.

The mandible changes throughout the life. In infant and children, the angle of mandible is obtuse with 140 degrees or more making head in line with the body of the mandible. Whereas in adult the angle decreases to about 110-120 degree making ramus almost vertical.

Additional Information

Mandible, in anatomy, is the movable lower jaw, consisting of a single bone or of completely fused bones in humans and other mammals. In birds, the mandible constitutes either the upper or the lower segment of the bill, and in invertebrates it is any of the various mouthparts that holds or bites food materials, including either of the paired mouth appendages of an arthropod that form the biting jaws.

In humans, the mandible is the only mobile bone of the skull (other than the tiny bones of the middle ear). It is attached to muscles involved in chewing and other mouth movements and functions by moving in opposition to the maxilla (upper jaw); together, the two parts are used for biting, chewing, and handling food. The structure of the human mandible resembles a more or less horizontal arch, which holds the teeth and contains blood vessels and nerves. At the rear of the mandible, two more or less vertical portions (rami) form movable hinge joints, one on each side of the head, articulating with the glenoid cavity of the temporal bone of the skull to form the temporomandibular joints. The rami also provide attachment for muscles important in chewing. The centre front of the arch is thickened and buttressed to form the chin, a development unique to humans and some of their recent ancestors; the great apes and other animals lack chins. In the human fetus and infant, the maxilla and the mandible are each separated at the midline; the halves fuse a few months after birth.

Invertebrate jaw and mouthpart structures vary markedly. For example, in the primitive bloodsucking flies (e.g., the horse fly [Tabanus]), the mandibles and maxillae form serrated blades that cut through the skin and blood vessels of the host animal. In the mosquito (Culicidae), the mandibles and associated structures have become exceedingly slender stylets that form a fine bundle used for piercing skin and entering blood vessels. In the housefly (Musca domestica), the mandibles and maxillae have been lost; the tonguelike labium alone remains and serves for feeding on exposed surfaces. Among crustaceans, processes at the base of the antennae may help the mandibles push food into the mouth. The paired mandibles of a nauplius (the most widespread and typical crustacean larva to emerge from the egg) each have two branches, one with a chewing lobe and the other with a compressing lobe at the base; the mandibles may also be used for swimming. In the adult crustacean, each mandible loses one of the branches, sometimes retaining the other as a palp, and the base may develop into a powerful jaw. An alternative development is found in some of the blood-sucking parasites, in which the mandibles form needlelike stylets for piercing a host.

In humans, the most common conditions that affect the function of the mandible are temporomandibular joint disorders (TMDs), of which there are about 30 different types. TMDs may have an impact on the function of the jaw muscles and the temporomandibular joints and may irritate associated nerves. The cause of a TMD is often unclear; factors that may play a role include osteoarthritis and physical trauma. Symptoms vary but may include dizziness, earache, facial pain, headache, jaw tenderness, and reduced jaw mobility. Many TMDs resolve on their own; otherwise, treatment ranges from simple dietary changes (e.g., eating only soft foods) to physical therapy or medication to the use of intraoral appliances (devices fitted over the teeth) to complex surgical or dental procedures.

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