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Other Terms: Os parietale, Os pariétal, Hueso parietal
The parietal bones are large quadrilateral bones forming the greater part of the roof and sides of the cranium. The external surface is slightly convex while the internal surface is concave and marked with impressions from meningeal vessels. The inferior border forms a beveled articular surface, while the superior, anterior, and posterior borders form deeply denticulate articular surfaces. The bone consists of inner and outer laminae of compact bone sandwiching a layer of spongy bone or diploe.
This word derives from the Latin term paries meaning a wall. The two large lateral walls forming the greater part of the cranial vault were then called parietal bones. The term is traceable back to the first century when it was used by Galen, the Roman physician.
The parietal bone articulates with five bones: the frontal, the opposite parietal, the occipital, the temporal, and the sphenoid. Its anterior border, forming the coronal or frontoparietal suture, is denticulate laterally and becomes more serrate medially. The interparietal or sagittal suture formed at the junction of the medial borders of the two parietal bones is deeply denticulate. The sagittal suture becomes more butt-like, sometimes fusing, toward the internal surface while it remains denticulate toward the external surface. This reflects the mechanical design to resist the greater compressional forces near the internal surface and the corresponding tensile forces near the external surface. The posterior border, also deeply denticulate, forms the lambdoidal or parieto-occipital suture. The lateral border forms a small denticulate suture with the mastoid region of the temporal bone. The remaining greater portion of the lateral border forms the squamous suture with the squamous temporal bone and the greater wing of the sphenoid bone.
The parietal bone forms intramembranously within mesenchyme from two centers. The centers first appear in the eighth week of embryonic life near the parietal tuberosity. One center is superior to the other in the same anteroposterior line. The two centers typically unite within a few weeks and ossification spreads radially toward the four margins. The four angles of the bone are the last regions to ossify, thus forming at these sites the fontanelles of the postnatal skull. Occasionally the two primary centers of ossification do not join and an anterior to posterior suture separates the bone into two.