The Synarthroses of the Skull

Most of the bones of the skull are united with the adjacent bones by sutures; a few are united by cartilage. The difference in the uniting medium depends on the fact that most of these bones are developed in membrane, but some are preformed in cartilage. Most of these joints are temporary, and are obliterated at various periods during development and growth. Their importance rests on the fact that so long as they persist, continuous growth is possible. They are usually designated according to the bones which enter into their formation, e. g., spheno-squamous, naso-frontal, internasal, etc. Special names (borrowed from human anatomy) are sometimes used; thus the interparietal, the parieto-occipital, and the parieto-frontal sutures are often called the sagittal, lambdoid, and coronal respectively.

Detailed description of the sutures has not sufficient clinical value to justify much addition to the statements made in the osteology in this connection. The obliteration or closure of the sutures is, however, worthy of brief mention. The cranial sutures are usually all closed at seven years, but the apex only of the petrous temporal is fused with the occipital and squamous temporal. Most of the facial sutures are practically closed at ten years, although complete synostosis may in some be delayed for years or may not occur at all; the nasal suture, for example, often persists even in advanced age, so far as its anterior part is concerned.

The principal synchondroses are: (1) That between the basioccipital and the body of the sphenoid (Synchondrosis sphenooccipitalis); (2) that between the presphenoid and postsphenoid (Synchondrosis intersphenoidalis); (3) those between the parts of the occipital bone (Synchondroses intraoccipitales). The first is usually ossified at four years, the second at three years, and the occipital bone is consolidated at two years.

The symphysis of the lower jaw (Symphysis mandibulæ) ossifies at one to six months.