The lower jaw (mandible) is the largest bone in the face and forms the lower face. It is the only freely movable bone of the face and is the movable portion of the temporomandibular joint. The right and left mandibles are joined at the chin by an invisible suture. These two bones appear to be one bone and are referred to as one bone. See figure 2-2. The mandible consists of a body with two vertical extensions called the rami (one ramus on each side). On the body of the mandible, as with the maxilla, is the mandibular alveolar process (a projection from the bone) which contains the roots and the alveoli (bony sockets) of the mandibular teeth.
a. Body. The anterior part of the mandible (the body) lies horizontally at the base of the face. It is composed of strong, hard bone. It is shaped like a horseshoe, corresponding to the maxillary alveolar process. The lower anterior part of the mandible forms a triangular prominence called the mental process. This is the bony chin, a feature unique to humans. The body has an external (outer) and internal (inner) surface (figures 2-7 and 2-8) and an inferior (lower) and superior (upper) border.
(1) Mental foramina. On both the right and left external surfaces of the mandible is a mental foramen which is located midway between the inferior and superior border. The mental foramina are usually between the apical portions of the first and second bicuspids, occasionally below the second bicuspid, and rarely below the first bicuspid. The mental foramina are small openings for the passage of blood vessels and nerves. These blood vessels and nerves supply a part of the lower lip and the skin of the chin.
(2) Mandibular alveolar process. This process is the upper part of the body of the mandible. The mandibular alveolar process is in the form of a ridge in which the teeth are embedded in the alveoli. When teeth are extracted, the alveoli disappear through a combination of filling in and resorption, as in the maxilla.
b. Rami. The posterior part of the mandible consists of two vertical extensions called rami. They form the upper part of the jawbone. Each of the two rami is broad, flat, and roughly quadrangular in shape. The upper border of each ramus ends in two projections, one anterior and one posterior. The base of each ramus forms the angle of the jaw below the ear.
(1) Coronoid process. The anterior projection on the ramus is the coronoid process. This process serves as an attachment for the temporalis muscle.
(2) Condyloid process. The posterior projection is the condyloid process. This process ends in a semi-cylindrical head called the condyle. The condyle, with its articular disk, approximates a depression. This depression is called the mandibular fossa. The mandibular fossa is located on the lower surface of the temporal bone. The condyle, articular disk, and fossa together form the temporomandibular joint.
(3) Mandibular (sigmoid) notch. Between the coronoid process and the condyloid process is a U-shaped space called the mandibular notch.
(4) Mandibular foramina. Near the center, on the inner surface of each ramus, is a mandibular foramen. These two foramina are openings of the mandibular canals. The canals pass through the mandible near the apices of the mandibular teeth They carry the blood vessels and nerves that supply the teeth. They also supply most of the soft tissues supporting the teeth as well as the lower lip and chin. The foramina are important anatomical landmarks when administering local anesthesia on the mandibular nerve.
c. Fractures. The mandible is frequently fractured in automobile accidents. Fractures that occur in the region of the angle of the mandible are not common because of heavy muscular attachment. The most common fractures of the mandible occur at or behind the mental foramen. Note in figure 2-8 that the assisting muscles of mastication that pull the jaw downward are attached anterior to this fracture site.
Figure 2-9. Masseter and temporalis muscles.
Figure 2-9. Masseter and temporalis muscles.
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