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Parietal Bone Definition
The parietal bone, also known as the os parietale, is a flat, paired cranial bone that covers the middle of the skull. The left and right parietal lobes of the brain are covered by both bones.
The parietal bone, which is part of the neurocranium, aids in the formation of the head’s shape and protects the brain. Both bones are part of the calvaria (skull cap) and skull base, respectively (basicranium).
Parietal Bone Location
So, where exactly is the parietal bone?
Sutures with the following names are attached to each bone:
- Sphenoid bone’s greater wing
- Temporal skeleton
- The front bone
- The occipital bone is a bone on the back of the head.
This makes imagining the exact position of the parietal bone very simple. The top of the skull is mostly made up of parietal bones.
Parietal Bone Anatomy
The boundaries, angles, and surfaces of the parietal bone are all important aspects of its anatomy. It’s a flat bone made up of two layers of compact bone sandwiched between two layers of spongy bone (cancellous bone).
The honeycomb-like structure of cancellous bone has a network of gaps that contain red bone marrow. Blood cells are generated by red bone marrow, which generates far less in the skull than in the long bones.
Cancellous bone also functions as a shock absorber, which is critical for the preservation of the brain’s fragile and delicate tissue.
Parietal Bone Borders (Sutures)
Sutures are the articulations of the parietal bone with the sphenoid, temporal, frontal, and occipital bones. Sutures are immovable joints with a small degree of give that gradually fuse during infancy.
We all know how fragile a baby’s skull is, which is due to the fact that many sutures have yet to fuse. Soft patches, also known as fontanelles, are areas of the skull that have not connected correctly.
The anterior fontanelle, where the frontal bone, left parietal bone, and right parietal bone have not yet joined, is the biggest. The sphenoid, mastoid, and posterior fontanelles are also soft areas.
The bones of the skull are considered to be sutured once they have fused.
There are five sutures in the parietal bones:
- Sagittal: connects the parietal bones on the left and right sides.
- Lambdoidal: connects the posterior parietal to the occipital bone’s top.
- Sphenoparietal: the smallest suture between the parietal bone and the sphenoid bone’s larger wing.
- Coronal: front and rear of parietal and frontal bones.
- Squamosal: parietal bone’s bottom and temporal bone’s top.
Parietal Bone Surfaces
Any skull bone’s exterior surface is the one we can feel. The rounded casing of the last two-thirds of the top of the skull is formed by the convex exterior surfaces of the parietal bones.
The exterior surface serves as more than just a protective shell. It also serves as a site of attachment for muscles. The temporal fascia is attached to the superior temporal line, which is the top curve that surrounds the temporal fossa.
The temporal fascia is a mastication muscle that covers the temporalis muscle. The temporalis muscle arises from the inferior temporal line, which is located beneath the posterior line.
The interior surface is a little more difficult to work with. Three grooves may be seen from the inside:
- Sulcus arteriae meningeae mediae (grooves for the middle meningeal artery)
- Sulcus sulcus sagittalis superioris (midline groove for the superior sagittal sinus)
- Sulcus sinus sigmoidei (part of the groove for the sigmoid sinus)
A cranial sinus, also known as a dural venous sinus, is a region that absorbs blood and CSF. The sinuses drain into bigger veins, which return the fluids to the venous system.
A set of two holes is another characteristic of this paired bone. The foramina in the parietal lobe are called parietal foramina.
Blood vessels can flow through the bone plates because of them. Each parietal foramen houses the parietal emissary vein (and occasionally the occipital artery).
The lambda — a midline skull feature where the lambdoid and sagittal sutures meet—is home to the parietal foramen. These “inconsistent” foramina are not seen in everyone.
Alternatively, they can shrink over time or become too big, resulting in a single huge hole. Enlarged parietal foramina are a symptom of certain illnesses and hereditary diseases. Something interrupts the ossification process in these situations.
Large parietal foramina have been misinterpreted by archaeologists and palaeontologists for indications of trephination (trepanation), an early surgical technique.
Granular foveolae, tiny crevices where the arachnoid mater membrane protrudes into the bone, are another characteristic of the interior surface (of all cranial bones).
Sulci arteriosi, which are smaller counterparts of parietal grooves, offer room and protection for smaller arteries.
Parietal Bone Function
The major role of the parietal bone is structural. This set of bones aids in the formation of a sturdy, spherical shell around the brain. The skull protects the brain’s soft tissue from damage while also preventing enlargement in the event of infection, increased cerebrospinal fluid production, or haemorrhage.
Increased pressure inside the skull can result from these conditions. Cases that are not treated surgically might rapidly turn deadly. The interior surface of the skull also houses the meningeal blood supply, which supplies oxygen and nutrients to the three membranes that surround the brain.
Parietal Bone Lumps and Bumps
A bump on the parietal bone is not a normal symptom. It might be a benign skull osteoma, which is a slow-growing disease in which the two compact bony plates (but not the core cancellous bone) grow larger.
The majority of cranial osteomas develop in the parietal plates and only rarely arise on their own. The contour of the skull might be affected as an external parietal bone bump or osteoma grows in size.
Surgically, these enlarged regions of bone can be removed. Bone growth on the interior surface of the skull, where pressure on the brain might occur, is uncommon; most skull lesions are detected by chance.
Parietal Bone Fractures
A fracture of the parietal bone can have significant implications. The bones cover the brain’s parietal lobes, but they also encroach on the frontal, occipital, and temporal lobes.
Several senses, as well as cognition, may be impaired if the parietal lobe is damaged by a parietal bone fracture. Gerstmann’s syndrome is characterised by agraphia (difficulty writing), acalculia (difficulty doing arithmetic), aphasia (speech abnormalities), and agnosia (lack of knowledge) (recognizing objects).
Balint syndrome can occur if both parietal lobes are injured. Memory and personality problems can be caused by damage to the right parietal lobe.
Fractures in the parietal lobe aren’t usually evident. It might take a long time for symptoms to appear. It may take a few days for intracranial pressure to produce neurological symptoms after a skull fracture causes bleeding into the brain.
These signs and symptoms vary depending on which part of the brain is damaged. Although a stroke is not caused by a fracture, it can elicit comparable symptoms. A stroke is bleeding inside the brain caused by clogged or damaged blood arteries.
There is no need to treat a parietal fracture that is not displaced, such as a hairline fracture or a minor fracture. An open fracture can allow germs to infect the brain membranes (meningitis) and spread to the brain, whereas a depressed fracture will push into the brain tissue below (encephalitis).
Because the parietal plates are so robust, severe assault (kicks to the head and blunt force trauma) and side-on car crashes are the most common causes of this type of fracture. The abuse of very young children is a darker aspect of parietal bone fractures.
The skull is more vulnerable to damage before the sutures are fused. Skull fractures are also more common in children who fall down the stairs, off playground equipment, or out of a stroller.
Parietal Bone Citations
- Benign fibrous histiocytoma of parietal bone: case report and review of the literature. World J Surg Oncol . 2015 May 8;13:177.
- Gorham-Stout disease with parietal bone osteolysis: a case series and review of literature. Br J Neurosurg . 2021 Feb;35(1):27-31.
- Enlarged Parietal Foramina. PMID: 20301307
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