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.