Ossification of the mandible

The mandible is ossified in the fibrous membrane covering the outer surfaces of Meckel's cartilages.

These cartilages form the cartilaginous bar of the mandibular arch (see p. 66), and are two in number, a right and a left.

Their proximal or cranial ends are connected with the ear capsules, and their distal extremities are joined to one another at the symphysis by mesodermal tissue.

They run forward immediately below the condyles and then, bending downward, lie in a groove near the lower border of the bone; in front of the canine tooth they incline upward to the symphysis.

From the proximal end of each cartilage the malleus and incus, two of the bones of the middle ear, are developed; the next succeeding portion, as far as the lingula, is replaced by fibrous tissue, which persists to form the sphenomandibular ligament.

Between the lingula and the canine tooth the cartilage disappears, while the portion of it below and behind the incisor teeth becomes ossified and incorporated with this part of the mandible.

Ossification takes place in the membrane covering the outer surface of the ventral end of Meckel's cartilage (Figs. 178 to 181), and each half of the bone is formed from a single center which appears, near the mental foramen, about the sixth week of fetal life.

By the tenth week the portion of Meckel's cartilage which lies below and behind the incisor teeth is surrounded and invaded by the membrane bone.

Somewhat later, accessory nuclei of cartilage make their appearance:
 * a wedge-shaped nucleus in the condyloid process and extending downward through the ramus;
 * a small strip along the anterior border of the coronoid process;
 * smaller nuclei in the front part of both alveolar walls and along the front of the lower border of the bone.

These accessory nuclei possess no separate ossific centers, but are invaded by the surrounding membrane bone and undergo absorption.

The inner alveolar border, usually described as arising from a separate ossific center (splenial center), is formed in the human mandible by an ingrowth from the main mass of the bone.

At birth the bone consists of two parts, united by a fibrous symphysis, in which ossification takes place during the first year.

The foregoing description of the ossification of the mandible is based on the researches of Low 44 and Fawcett, 45 and differs somewhat from that usually given.