Parietal Bone

PARIETAL BONE (os parietale), a paired, quadrilateral, flat bone of the cerebral part of the skull, located on either side of the midline (sagittal suture) between the frontal, occipital, temporal, and sphenoid bones. Together with the neighboring bones, the parietal bone participates in the formation of the skull vault. The parietal bone has four margins: 1) margo sagittalis ossis pariet., s. superior, s. parietalis—the margin connecting the parietal bone of one side with the corresponding margin of the parietal bone of the other side (by the suture—sutura sagittalis) (Figs. 1 and 2); 2) margo frontalis, s. coronalis—for connection with the squama of the frontal bone (sutura coronalis) (Figs. 1 and 2); 3) margo occipitalis—for connection with the squama of the occipital bone (sut. lambdoidea); and 4) margo squamosus, s. sphenotemporalis, s. temporalis—for connection with the squama of the temporal bone (sut. squamosa). As a quadrilateral bone, the os parietale has four angles: 1) the posterior-inferior (angulus mastoideus, s. margo mastoideus), inserted into the incisura parietalis of the temporal bone; 2) the anterior-inferior (ang. sphenoidalis, s. margo sphenoidalis), connecting with the superior margin of the greater wing of the sphenoid bone; 3) the anterior-superior (ang. frontalis) (equal to 106°); and 4) the posterior-superior (ang. occipitalis). Two surfaces of the parietal bone are distinguished: the external, convex (facies superficialis) and the internal, concave—cerebral (facies cerebralis). On the external surface, which is relatively smooth, there are visible, to varying degrees of prominence, two parallel arcs running as the superior and inferior temporal lines (ridges) (lineae temporales sup. et inf.).

Figure 1. Skull vault—external view: 1—squama occipitalis; 2—sutura sagittalis; 3—sutura lambdoidea; 4—foramen parietale; 5—os parietale; 6—sutura coronalis; 7—squama frontalis.

The first of these is the site of attachment for the temporal fascia (fascia temporalis); the second, which is more sharply defined and constitutes an intercalated part of the common ridge that extends from the zygomatic process of the frontal bone to the squama of the temporal bone, serves as the site of attachment for the temporal muscle (m. temporalis) and as the superior boundary of the temporal fossa (fossa temporalis). The most prominent part of the external surface is designated by the term parietal eminence (tuber parietale); the degree of prominence of the parietal eminences varies depending on age, sex, individuality, etc. The internal surface bears a series of impressions of anatomical structures adjacent to it from the side of the cranial cavity: 1) sulcus sagittalis—a groove running along the margo parietalis of the parietal bone (Fig. 2). From the two semi-grooves (right and left) along the line of the sagittal suture, a groove is formed in which the greater part of the sinus sagitt. superior is housed, and to the margins of which the large falciform process (proc. falciformis major) of the dura mater is attached. 2) Part of the sulci transversi in the region of the posterior-inferior angle. 3) Sulci meningei, s. arteriosi

(Fig. 2)—a dendritically branching groove—the trace of the adjacent branches of the art. meningeae mediae. This groove begins at the anterior-inferior angle, from where it extends backward and upward, giving off smaller grooves along the way. Sometimes the initial part of the groove is represented by a canal (from 0.2 mm to 3.6 mm) (Yatsuta). 4) Foveolae granulares (Pacchioni)—for the accommodation of small outgrowths of the arachnoid membrane, the so-called Pacchionian granulations. Larger ones are called Trolardian pits; they are the receptacles for expansions of the venous sinuses (parasinusoids). 5) Impressiones digitatae—depressions, as if they were imprints of the convolutions of the brain, and juga cerebralia—ridges between them corresponding to the sulci of the brain. Among the latter, a ridge corresponding to the fiss. Sylvii is often distinguished, which is called the crista Sylvii. The most concave part of the internal surface is marked as the fossa parietalis (corresponding to the tuber pariet.). The parietal bone, in its posterior-superior quadrant (2–3 cm from the sut. lambdoidea), has a small opening (foramen parietale) (Fig. 1) for the passage of an emissary vein (emissarium pariet.). This opening is sometimes absent, rarely strongly enlarged in its diameter; finally, the right and left parietal foramina may merge into one unpaired one. The foramen parietale is a remnant of a small fontanelle (fonticulus obelicus), which separates from the posterior fontanelle in the seventh month of intrauterine life. The parietal bone belongs to the category of covering (investing) bones. In the primordial skull, it is represented by a connective tissue primordium and does not pass through a cartilaginous stage. At the 10th week of intrauterine life (according to Toldt) or at the end of the 2nd month (according to Mall and others), two ossification centers appear in the central part in the shape of an hourglass. The process of ossification of the parietal bone spreads from the center to the periphery (to the margins and angles of the bone). In the newborn, ossification is not complete, so that connective tissue areas—fontanelles—remain between the parietal bones and the neighboring bones (Fig. 3). Investing paired parietal bones are already found in bony fish. In lungfish, the parietal bones fuse with the frontal bones, forming the skull roof—an unpaired fronto-parietale. In the parietal region of the amphibian skull, the parietal bones fuse with the frontal bones into a pair of bones (fronto-parietale). Fossil Stegocephala have an unpaired foramen parietale in the middle of the parietal bone for the passage of the nerve of the parietal eye.

Figure 2. Skull vault—internal view: 1—squama occipitalis; 2—sutura sagittalis; 3—sutura lambdoidea; 4—os parietale; 5—sulci arteriosi; 6—squama frontalis; 7—sulcus sagittalis.

Figure 3. Diagram (after Scammon) of the arrangement of fontanelles (permanent and accessory): a—os frontale; b—os parietale; c—os occipitale; 1—fonticulus glabellaris; 2—fonticulus metopicus; 3—fonticulus frontalis; 4—fonticulus sphenoidalis; 5—fonticulus parietalis; 6—fonticulus mastoideus; 7—fonticulus occipitalis; 8—fonticulus cerebellaris.

fused into one bone.

Reptiles (except turtles). The fused parietal bones of lizards have a median for. parietale. In birds, the parietal bones remain paired. The parietal bones of various representatives of mammals differ only in size, external and internal relief of the surfaces, and the presence, as for example in primates, carnivores, etc., of an os interparietale that fuses with the occipital bone, or in other cases remains free. A similar bone in human skulls is called the os Incae. Splitting of the ossification nucleus of the parietal bone can lead to the appearance of a sutura interparietalis, dividing the bone into separate parts in a longitudinal or vertical direction. Hence the presence of variations of the parietal bone: e.g., os parietale bipartitum (Fig. 4), extremely rarely tripartitum, and with greater division. A double parietal bone is not found in lower vertebrates and is a progressive formation (H. Braus). Among the variants of the parietal bone, one should include

Figure 4. Double parietal bone (os parietale bipartitum): 1—frontal bone; 2—temporal bone; 3—lower parietal bone; 4—upper parietal bone; 5—greater wing of the sphenoid bone. A defect in the ossification of the posterior part of the superior margin of the right and left parietal bones (near the for. pariet.), as a result of which fibrous tissue remains in the form of the Gerdy parietal fontanelle (font. parietal. Gerdy) (Fig. 3); at the posterior-inferior angle, near the connection with the temporal bone, the presence of a tubercle (tub. asteriacum) is sometimes observed. As a flat bone, the parietal bone consists of external and internal plates of compact substance (the internal one is called the lamina vitrea due to its fragility upon impact from the outside) and a layer of spongy substance between them, the so-called diploe. The latter is interspersed with channels for veins (vv. diploicae). For the pathology of the parietal bone, see Skull. V. Uskov.