The parietal bones are bilateral skull bones that form the superior and lateral walls of the cranium. They overlie the parietal lobes of the brain and are covered superficially by the epicranial aponeurosis. The parietal bones are part of the neurocranium, together with the frontal, ethmoid, sphenoid, temporal and occipital bones. The bulk of each parietal bone forms the calvaria (skull cap), while the remaining smaller part is a component of the cranial base (basicranium). The function of the cranium, and hence the parietal bones, is to protect the underlying fragile brain.
The parietal bone is slightly curved and has a quadrilateral shape. It has two surfaces, four borders and four angles. The borders articulate with the neighbouring skull bones to form various cranial sutures. The surfaces contain numerous bony features and indentations that correspond to the anatomical structures lying against the bone, such as superficial blood vessels of the brain.
This article will describe the anatomy of the two parietal bones.
|Borders||Sagittal (superiorly), squamosal (inferiorly), frontal (anteriorly), occipital (posteriorly)|
|Angles||Frontal (anterosuperior), sphenoidal (anteroinferior), occipital (posterosuperior), mastoid (posteroinferior)|
|Cranial sutures||Sagittal, sphenoparietal, parietomastoid, coronal, lambdoid|
|External surface||Superior and inferior temporal lines, parietal eminence, temporal fossa, parietal foramen|
|Internal surface||Grooves for middle meningeal artery, superior sagittal sinus and sigmoid sinus|
The parietal bone has four borders which articulate with the surrounding skull bones via various sutures. These borders include;
- Sagittal border, located superiorly. It is the thickest and longest out of all of them. The two parietal bones articulate at the sagittal borders with each other to form the sagittal suture.
- Squamosal border, situated inferiorly. The border starts thin and straight anteriorly, arches in the middle and then thickens posteriorly. The squamosal border comes in contact with three bony structures. From anterior to posterior these are the greater wing of the sphenoid bone and the squamous and petrous parts of temporal bone. These articulations form the sphenoparietal and parietomastoid sutures.
- Frontal border, located anteriorly. It is the most serrated margin of the parietal bone. It comes in contact with the frontal bone to form the superolateral half of the coronal suture.
- Occipital border, located posteriorly. It is also highly irregular and forms the inferolateral half of the lambdoid suture by articulating with the occipital bone.
The parietal bone has four angles;
- Frontal angle pointing anterosuperiorly. It is formed by the intersection of the sagittal and frontal borders. The frontal angle is located at the bregma, which represents the intersection of the sagittal and coronal sutures.
- Sphenoidal angle facing anteroinferiorly. It is created by the union of the frontal and squamosal borders. The sphenoidal angle is situated at the pterion, which represents the intersection of the coronal, sphenoparietal and sphenofrontal sutures.
- Occipital angle which points posterosuperiorly, being more rounded compared to the rest. It is formed by the intersection of the sagittal and occipital borders. The angle is situated at the lambda, which represents the union of the lambdoid and sagittal sutures.
- Mastoid angle which faces posteroinferiorly. It is created by the intersection of the occipital and squamosal borders. The angle is found at the asterion where the parietomastoid and lambdoid sutures meet.
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The parietal bone has an external and internal surface. The external surface is smooth and convex. It has several important features:
- Superior temporal line which forms an arch that travels between the frontal and occipital borders of the parietal bone. The superior temporal line represents the attachment point of the temporal fascia. Superior to this line is the epicranial aponeurosis, while inferior to it is the temporal fossa.
- Inferior temporal line which forms an identical arch to the previous one but located more inferiorly. It represents the origin of the temporal muscle.
- Parietal eminence which is located centrally on the external surface of the parietal bone. It marks the origin of ossification of the parietal bone.
The internal surface is concave and highly irregular compared to its external counterpart. It contains several grooves that house various blood vessels;
- A thin and highly branched groove for middle meningeal artery. It travels posterosuperiorly along the internal surface of parietal bone starting from its sphenoidal angle and squamosal border. The frontal branches of the middle meningeal artery travel within a groove located on the internal surface of the sphenoidal angle.
- A broader groove for superior sagittal sinus. It travels anterosuperiorly along the sagittal border. The groove is surrounded by granular foveolae which contain arachnoid granulations.
- A small portion of the groove for sigmoid sinus overlies the occipital angle.
The parietal foramen is located on the posterosuperior aspect of the parietal bone, alongside the sagittal border. When present, it is a common feature of both surfaces. The role of the parietal foramen is to transmit branches of the superior sagittal sinus and occipital artery.
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For more information about the skull, including the parietal bone, take a look at the videos, articles, illustrations and quizzes included in the following study unit:
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