The parietal bones form, by their union, the sides and the roof of the cranium. Each bone is irregularly quadrilateral in shape, and has two surfaces, four borders, and four angles.

The external surface (fig. 360) is convex, smooth, and marked near the centre by a slight elevation, termed the parietal eminence. Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines ; the former gives attachment to the temporal fascia; the latter marks the upper limit of the origin of the temporal muscle. The part of the bone above these lines is covered, in the recent condition, with the epicranial aponeurosis (galea aponeurotica) ; that below the lines forms a part of the temporal fossa. At the posterior part and close to the upper or sagittal border the parietal foramen transmits a vein from the superior sagittal sinus and, sometimes, a small branch of the occipital artery; the foramen is not constantly present, and its size varies considerably.

The internal surface (fig. 361) is concave; it presents impressions for the cerebral gyri, and numerous furrows for the ramifications of the middle meningeal vessels; these furrows run upwards and backwards from the anterior inferior angle, and from the central and posterior part of the inferior border. Along the upper or sagittal border there is a shallow groove, which, with that on the opposite parietal bone, forms the sagittal sulcus, for the superior sagittal sinus; to the edges of this sulcus the falx cerebri is attached. A number of granular pits, which lodge arachnoid granulations, mark the bone in the neighborhood of the sagittal sulcus ; they are best marked in the skulls of old persons.

The upper border, the longest and thickest, is dentated ; it articulates with the corresponding border of the opposite parietal bone to form the sagittal suture. The lower border is divided into three parts : of these, the anterior, short, thin and pointed, is bevelled at the expense of the external surface, and overlapped by the tip of the greater wing of the sphenoid; the middle portion is arched, bevelled at the expense of the external surface, and overlapped by the squamous part of the temporal bone; the posterior part, short, thick and serrated, articulates with the mastoid portion of the temporal bone. The anterior border is deeply serrated, and bevelled at the expense of the external surface above and of the internal below ; it articulates with the frontal bone, forming one-half of the corona] suture. The posterior border, deeply dentated, articulates with the occipital bone, forming one-half of the lambdoid suture.

Figure 360
Parietal bone external view - Figure 360
The anterior superior angle is almost a right angle, and corresponds with the bregma or point of meeting of the sagittal and eoronal sutures. The anterior inferior angle, thin and acute, is received into the interval between the frontal bone and the greater wing of the sphenoid. Its internal surface is marked by a deep groove, sometimes a canal, for the anterior divisions of the middle meningeal vessels. In some skulls the frontal bone articulates with the squama of the temporal bone, and the parietal bone then fails to reach the greater wing of the sphenoid. The region where these four bones approach closely to one another is termed the pterion (p. 249). The posterior superior angle is rounded and corresponds with the lambda or point of meeting of the sagittal and lambdoid sutures. The posterior inferior angle is blunt and articulates with the occipital bone and with the mastoid portion of the temporal bone, the meeting point of the three bones being named the asterion. On the internal surface of this angle there is a broad, shallow groove, which lodges the end of the transverse sinus and the commencement of the sigmoid sinus.

At birth there are unossified or membranous intervals in the skull at the angles of the parietal bones; they are named fontanelles (fonticuli)

Ossification – The parietal bone is ossified in membrane from two centres, Which appear one above the other at the parietal eminence about the seventh week of fetal life. These centres unite earls, and ossification gradually extends in a radial manner towards the margins of the bone ; the angles are consequently the parts last formed, and it is here that the fontanelles are found. At birth the temporal lines are situated low down; they reach their permanent position only after the eruption of the molar teeth. Occasionally the parietal bone is divided into two parts, upper and lower, by an anteroposterior suture.

Figure 361
Parietal bone internal view - Figure 361

 


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