The parietal bone (latin: os parietale) is located on each side of the skull right behind the frontal bone. Both parietal bones together form most of the cranial roof and sides of the skull. Each parietal bone takes an irregular quadrilateral shape and has four angles, four margins, and two surfaces.
Angles of parietal bone
The four angles of the parietal bone are called:
- frontal angle,
- sphenoidal angle,
- occipital angle,
- mastoid angle,
The frontal angle of the parietal bone is the anterior superior angle. The sphenoidal angle is the anterior inferior angle, and the most sharpest angle of the parietal bone. The occipital angle is the posterior superior angle. And the mastoid angle is the posterior inferior angle of the parietal bone.
Margins of parietal bone
The four margins or borders of the parietal bone are the following:
- frontal margin,
- occipital margin,
- squamous margin,
- sagittal margin.
The frontal margin of the parietal bone forms the anterior border. The occipital margin forms the posterior one. The squamous margin is the inferior margin of the parietal bone. And the sagittal margin is the upper, medial margin of the parietal bone.
Surfaces of parietal bone
The parietal bone has an external and an internal surface.
The external surface of the parietal bone features the following landmarks:
- parietal tuber,
- superior temporal line,
- inferior temporal line,
- parietal foramen.
The parietal tuber (or parietal eminence) is a prominence located near the middle of the external surface of the parietal bone.
The superior temporal line is a curved line seen on the external surface of the parietal bone for attachment of the temporal fascia. The superior temporal line also represents the upper margin of the temporal plane.
The inferior temporal line is a curved line on the external surface of the parietal bone for attachment of the temporalis muscle.
The parietal foramen is an opening occasionally present at the back part of the parietal bone, that serves as a passage for the parietal emissary vein, which drains into the superior sagittal sinus.
The internal surface of the parietal bone features:
- Groove for superior sagittal sinus
- Granular foveolae
- Groove for sigmoid sinus
- Impressions of cerebral gyri
- Arterial grooves
The granular foveolae are small pits located on the internal surface of the parietal bone along the superior sagittal sinus. The granular foveolae are formed by the penetration of the arachnoid sheath in the bone.
The groove for the superior sagittal sinus is a shallow depression on the frontal, parietal, and occipital bones forming a channel for the sagittal superior sinus; its margins come together as it passes downward and become continuous with the frontal crest.
The groove for the sigmoid sinus is located in the posterior cranial fossa, found on the lateral part of the occipital bone, then curving around the jugular process on the mastoid part of the temporal bone, finally, turning sharply on the inner surface of the parietal bone, continuing as the transverse groove.
The impressions of cerebral gyri (also called the digital impressions) are flat indentations on the inner surface of the skull corresponding to the cerebral gyri which produce them.
The arterial grooves are located on the inner surface of the skull produced by pressure from arteries, primarily the middle meningeal artery and its branches.