THE LOWER JAW (MANDIBLE)
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.