Theparietal bones (/pəˈraɪ.ətəl/pə-RY-ə-təl) are two bones in theskull which, when joined at afibrous joint known as acranial suture, form the sides and roof of theneurocranium. Inhumans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latinparies (-ietis), wall.
The external surface [Fig. 1] is convex, smooth, and marked near the center by an eminence, theparietal eminence (tuber parietale), which indicates the point whereossification commenced.
Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferiortemporal lines; the former gives attachment to thetemporal fascia, and the latter indicates the upper limit of the muscular origin of thetemporal muscle.
Above these lines the bone is covered by a tough layer of fibrous tissue – theepicranial aponeurosis; below them it forms part of thetemporal fossa, and affords attachment to the temporal muscle.
At the back part and close to the upper or sagittal border is theparietal foramen which transmits a vein to thesuperior sagittal sinus, and sometimes a small branch of theoccipital artery; it is not constantly present, and its size varies considerably.
The internal surface [Fig. 2] is concave; it presents depressions corresponding to the cerebral convolutions, and numerous furrows (grooves) for the ramifications of themiddle meningeal artery; the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border.
Along the upper margin is a shallow groove, which, together with that on the opposite parietal, forms a channel, thesagittal sulcus, for thesuperior sagittal sinus; the edges of the sulcus afford attachment to thefalx cerebri.
Near the groove are several depressions, best marked in the skulls of old persons, for thearachnoid granulations (Pacchionian bodies).
In the groove is the internal opening of the parietal foramen when that aperture exists.
Figure 1 : Left parietal bone. Outer surface.
Figure 2 : Left parietal bone. Inner surface.
Left parietal bone (shown in green). Animation stops for a few seconds at inner and outer surface.
Thesagittal border, the longest and thickest, is dentated (has toothlike projections) and articulates with its fellow of the opposite side, forming thesagittal suture.
Thefrontal border is deeply serrated, and bevelled at the expense of the outer surface above and of the inner below; it articulates with thefrontal bone, forming half of thecoronal suture. The point where the coronal suture intersects with the sagittal suture forms a T-shape and is called thebregma.
Thesquamous border is divided into three parts: of these:
theanterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid;
themiddle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squama of the temporal;
theposterior part is thick and serrated for articulation with themastoid portion of thetemporal.
Theoccipital border, deeply denticulated (finely toothed), articulates with theoccipital bone, forming half of thelambdoid suture. That point where the sagittal suture intersects the lambdoid suture is called thelambda, because of its resemblance to the Greek letter.
Skull seen from top.Sagittal suture separates left and right parietal bone.
Thefrontal angle is practically a right angle, and corresponds with the point of meeting of the sagittal and coronal sutures; this point is named thebregma; in the fetal skull and for about a year and a half after birth this region is membranous, and is called theanterior fontanelle.
Thesphenoidal angle (alsoWelcker's angle), thin and acute, is received into the interval between the frontal bone and the great wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes a canal, for the anterior divisions of themiddle meningeal artery.
Theoccipital angle is rounded and corresponds with the point of meeting of the sagittal and lambdoidal sutures—a point which is termed the lambda; in the fetus this part of the skull is membranous, and is called theposterior fontanelle.
Themastoid angle is truncated; it articulates with the occipital bone and with the mastoid portion of the temporal, and presents on its inner surface a broad, shallow groove which lodges part of the transverse sinus. The point of meeting of this angle with the occipital and the mastoid part of the temporal is named theasterion.
The parietal bone is ossified in membrane from a single center, which appears at theparietal eminence about the eighth week of fetal development.
Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist.
Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture.
Skull of thetuatara (Sphenodon punctatus), with the pineal foramen enclosed by the parietal bones
In non-human vertebrates, the parietal bones typically form the rear or central part of theskull roof, lying behind the frontal bones. In many non-mammaliantetrapods, they are bordered to the rear by a pair ofpostparietal bones that may be solely in the roof of the skull, or slope downwards to contribute to the back of the skull, depending on the species. In the livingtuatara and some lizards, as well as in many fossil tetrapods, a small opening, theparietal foramen (also called thepineal foramen), is present between the two parietal bones at the midline of the skull. This opening is the location of theparietal eye (also called the pineal or third eye), which is much smaller than the two main eyes.[1][2][3]
The parietal bone is usually present in the posterior end of the skull and is near the midline. This bone is part of the skull roof, which is a set of bones that cover the brain, eyes and nostrils. The parietal bones make contact with several other bones in the skull. The anterior part of the bone articulates with thefrontal bone and thepostorbital bone. The posterior part of the bone articulates with thesquamosal bone, and less commonly thesupraoccipital bone. The bone-supported neck frills of ceratopsians were formed by extensions of the parietal bone. These frills, which overhang the neck and extend past the rest of the skull is a diagnostic trait of ceratopsians. The recognizable skull domes present in pachycephalosaurs were formed by the fusion of the frontal and parietal bones and the addition of thick deposits of bone to that unit.[4]
