The skull, or cranium, provides a protective casing for the brain, and the parietal bones form a significant portion of this structure. These bones are a pair of large, flat plates that contribute to the roof and sides of the cranial vault. The term “parietal” comes from the Latin word paries, meaning wall, aptly describing their function as the walls of the upper skull.
Where the Parietal Bone Sits
There are two parietal bones, a left and a right, which together form the largest part of the top and sides of the human head. Each bone is roughly quadrilateral in shape, presenting a broad surface area for protection. The external surface of the bone is convex, creating the outward curve of the skull. Conversely, the inner surface is concave, conforming to the shape of the brain beneath it.
This pair of bones sits high on the head, extending from the midline down toward the ears. Their primary function is to provide broad, protective coverage over the cerebral hemispheres. Near the center is a slight outward bulge called the parietal eminence, which marks the point where the bone began to form during development. The parietal bones are composed of two layers of compact bone with a spongy layer in between, which acts as a shock absorber.
How the Parietal Bone Connects
The parietal bones are united with their neighbors through immovable fibrous joints known as cranial sutures. These connections fit together like intricate puzzle pieces, providing strength and rigidity to the skull. The two parietal bones meet along the midline, forming the sagittal suture. This long, straight joint runs from the front to the back of the skull.
Moving forward, the parietal bones articulate with the frontal bone at the forehead to create the coronal suture. At the back of the skull, the parietal bones join the occipital bone, which is located at the base and back of the head. This junction is called the lambdoid suture, named because its shape often resembles the Greek letter lambda.
Along the lower sides, each parietal bone meets the temporal bone. This connection is known as the squamous suture, which has a beveled, overlapping edge, giving it a scale-like appearance. The parietal bone also connects briefly with the sphenoid bone at the side of the skull, an area known as the pterion.
Importance in Development and Injury
The connections of the parietal bones are significant during development and birth, as they define the location of the fontanelles, or “soft spots,” in infants. These areas are membrane-covered gaps where several bones meet, allowing the skull to temporarily compress during passage through the birth canal. The largest is the diamond-shaped anterior fontanelle, located where the two parietal bones meet the frontal bone at the junction of the coronal and sagittal sutures.
This anterior fontanelle remains open until a child is approximately 18 to 24 months old, accommodating the rapid growth of the brain. The smaller posterior fontanelle, found where the parietal bones meet the occipital bone at the lambdoid suture, typically closes within the first few months of life. Because of their exposed position, the parietal bones are commonly involved in skull fractures resulting from direct impact.
The internal surface of the parietal bone contains grooves that accommodate the middle meningeal artery. Fractures in this area can damage the artery, leading to serious internal bleeding. A small opening, the parietal foramen, is sometimes present near the sagittal suture and allows for the passage of an emissary vein, which connects veins outside the skull to the large venous sinuses within.