The two =parietal bones= (Ossa parietalia) form the greater part of the
roof of the cranium; they unite in the median line, forming the
=sagittal suture=. Each is quadrilateral in outline and has two surfaces
and four borders.
The =external surface= (Facies parietalis) is convex, and is marked by a
more or less prominent curved line, the =external sagittal crest=; this
is median in its posterior part, and is continuous with the crest of
like name on the occipital bone; in front it curves outward and is
continuous with the frontal crest. The surface external to the crest
(Planum temporale) enters into the formation of the temporal fossa, and
is roughened for the attachment of the temporal muscle.
The =internal= or =cerebral surface= (Facies cerebralis) is concave. It
presents numerous =digital impressions= (Impressiones digitatæ) which
correspond to the gyri of the cerebrum. There are also furrows (Sulci
vasculosi) for the meningeal arteries. Along the inner border there is a
=sagittal groove= (Sulcus sagittalis) for the superior longitudinal
sinus.
The =anterior border= joins the frontal bone at the =parieto-frontal
suture= (Sutura coronalis).
The =posterior border= meets the occipital bone at the
=parieto-occipital suture= (Sutura lambdoidea). Below this junction it
curves inward and concurs with the temporal bone in the formation of the
=parieto-temporal canal= (Meatus temporalis). A =transverse groove=
(Sulcus transversus) connects this canal with the sagittal sulcus.
The =internal border= is thick and serrated. It joins its fellow at the
=sagittal suture=, and (in the young subject) meets the interparietal at
the =interparietal suture=. The line of junction is marked internally by
the =internal sagittal crest= (Crista sagittalis interna).
The =external border= is beveled and is overlapped by the squamous
temporal bone, forming the parieto-temporal suture (Sutura
parieto-squamosa). The angle of junction of the external and posterior
borders articulates with the posterior angle of the temporal wing of the
sphenoid.
=Development.=—Each parietal bone ossifies in membrane from a single
center. In the young foal the central part of the bone is much more
convex than in the adult and forms a prominence similar to the
pronounced tuber parietale of the young child; the external sagittal
crest is not present, and the external surface is smooth.
The sagittal suture is usually closed at four years, the
parieto-occipital at five years, and the parieto-temporal at twelve to
fifteen years.