Parietal bone

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The parietal bones are two bones that form the sides and the roof of the skull part that protects the brain. Each bone is roughly four-sided and has two surfaces, four edges, and four corners. They get their name from a Latin word meaning “wall.”

On the outside, the bone is curved and smooth. In the middle there is a raised area called the parietal eminence, where bone growth starts. Across the bone run two curved lines—the superior and inferior temporal lines. The upper line marks where the tough tissue over the skull is attached, and the lower line marks the upper edge of where the temple muscles attach. The outside is partly covered by a fibrous sheet called the epicranial aponeurosis.

Towards the back, near the top edge, is the parietal foramen. This small hole, when present, can transmit a vein to the top of the brain’s drainage system and sometimes an artery branch; it doesn’t always exist and varies in size.

On the inside, the surface is concave and has grooves for the branches of the middle meningeal artery. At the upper edge there is a groove, the sagittal sulcus, which forms a channel with the same groove on the opposite parietal bone. This channel holds the superior sagittal sinus and provides attachment for a sturdy fold of brain-covering tissue called the falx cerebri. There can also be depressions for arachnoid granulations, small structures involved in cerebrospinal fluid pressure regulation.

Developmentally, the parietal bone starts from a single center of ossification near the parietal eminence around eight weeks in the fetus and grows outward. The corners are the last parts to form, and fontanelles (soft spots) appear there. Sometimes, the bone is split into an upper and a lower part along a line.

In other animals, the parietal bones may make up the rear part of the skull roof. Some reptiles have a parietal foramen for a tiny pineal “third eye.” The parietal bone also touches other skull bones: in front with the frontal and postorbital bones, and behind with the squamosal (and sometimes the supraoccipital).

In dinosaurs, extensions of the parietal bone helped form neck frills in ceratopsians, and in pachycephalosaurs the parietal and frontal bones fused to make domed skulls.