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BONES. 71

the opposite side, forms the groove for the longitudinal sinus. The
elevated edges of this groove give attachment to the falx cerebri. Below
this groove, especially in bones of old subjects, are seen several depres-
sions, which lodge the Pacchionian bodies. Extending across the poste-
rior inferior angle of the bone is a depression which forms part of the
groove for the lateral sinus.
Borders.— The Anterior Border is deeply serrated, and above is
slightly bevelled by the prolongation of the inner table, but toward the
lower angle it is bevelled bv the extensions of the external table. This
anterior border articulates with the frontal bone, forming the parieto-
frontal part of the coronal suture.
The Posterior Border is irregular in outline, deeply serrated, and
articulates with the occipital bone, forming the parieto-occipital part of
the lambdoid suture. Xumbers of what are known as Wormian bones
are often found attached to this border.
The Superior Border is the longest and thickest of the four. It is
deeply serrated, and articulates with its fellow on the opposite side,
forming the interparietal or posterior portion of the sagittal suture,
being all that remains after the ossiiic union of the two halves of the
frontal bone. Wormian bones are less frequently found in this border.
The Inferior Border is divided into three portions. The anterior
portion, about half an incli in extent, is thin and fluted, bevelled on its
outer surface for articulation with the great wing of the sphenoid bone,
which overlaps it and forms the spheno-parietal suture. The posterior
portion is thick, serrated, and articulates with the mastoid portion of
the temporal bone, forming the masto-parietal suture.
Angles.—The anterior superior angle is nearly a right angle. It
is completely ossified in adult life, but in infancy it is membranous,
and in conjunction with the membranous portions of the bone of the
opposite side and the adjoining jjortion of the frontal bone forms the
anterior or great fontanelle.
The posterior superior angle is an obtuse angle. In the articulated
skull it is situated at the junction of the interparietal (sagittal) and
parieto-occipital (lamljdoid) sutures. In infancy this space is occupied
by the posterior superior fontanelle.
The Anterior Inferior Angle is the most prominent of the four.
It is thin and elongated, filling up the space between the frf)ntal
bone and the squamous portion of the temporal bone. It articulates
with the great wing of the sphenoid bone. In infancy this space is
occupied by the anterior inferior fontanelle.
The Posterior Inferior Angle is thick, broader, and more rounded
than the others, and articulates with the mastoid portion of the tem-
poral and the lateral angle of the occipital bone ; this is the location
of the posterior inferior fontanelle in infancy.
Structure.—The parietal bone is made up of an outer plate of
compact tissue and an inner vitreous table, enclosing between them a
mass of cancellated fibrous bone.
Development.—The parietal bone is developed within a membran-
ous matrix from one point of ossification, which appears about the
sixth week of embryonal life.