THE PARIETAL BONE 57 



Ossification. — The occipital bone develops in four pieces. The squamous portion is ossi- 

 fied from four centres, arranged in two pairs, which appear about the eighth week. The 

 upper pair are deposited m membrane, and this part of the squamous portion represents the 

 interparietal bone of many animals. The lower oair, deposited in cartilage, form the true 

 supra-occipital element, and the four parts quickly coalesce near the situation of the future 

 occipital protuberance. For many weeks two deep lateral fissures separate the interparietal 

 and supraoccipital portions, and a membranous space extending from the centre of the squamous 

 portion to the foramen magnum partially separates the lateral portions of the supra-occipital. 

 This space is occupied later by a spicule of bone, and is of interest as being the opening through 

 which the form of hernia of the brain and its meninges, known as occipital meningocele or en- 

 cephalocele, occurs. The basi-occipital and the two ex-occipitals are ossified each from a single 

 nucleus which appears in cartilage from the eighth to the tenth week. 



At birth the bone consists of four parts united by strips of cartilage, and in the squamous 

 portion fissures running in from the upper and lateral angles are still noticeable. The osseous 

 union of the squamous and ex-occipital is completed in the fifth year, and that of the ex- 

 occipitals with the basi-occipital before the seventh year. Up to the twentieth year the basi- 

 occipital is united to the body of the sphenoid by an intervening piece of cartilage, but about 

 that date ossific union begins and is completed in the course of two or three years. Occasionally 

 the interparietal portion remains separate throughout life (fig. 70), forming what has been 

 termed the inca bone, or it may be represented by numerous detached ossicles or Wormian 

 bones. In some cases a large Wormian bone, named the pre-interparietal, is found, partly 

 replacing the interparietal bone (fig. 71). A pre-interparietal bone is found in some mammals, 

 and it has occasionally been observed in the human foetal skull. In fig. 71 the bone is seen in 

 an adult human skull — a distinctly rare condition. 



THE PARIETAL 



The two parietal bones (figs. 72, 73), interposed between the frontal before 

 and the occipital behind, form a large portion of the roof and sides of the cranium. 

 Each parietal bone [os parietale] is quadrilateral in form, convex e.xternally, 

 concave internally, and each presents for examination two surfaces, four borders, 

 and four angles. 



The parietal surface is smooth and is crossed, just below the middle, by two 

 curved lines known as the temporal lines. The superior line gives attachment 

 to the temporal fascia; the lower, frequently the better marked, limits the origin 

 of the temporal muscle; whilst the narrow part of the surface enclosed between 

 them is smooth and more polished than the rest. Immediately above the ridges 

 is the most convex part of the bone, termed the parietal eminence [tuber parietale], 

 best marked in young bones, and indicating the point where ossification com- 

 menced. Of the two divisions on the parietal surface marked off b}^ the temporal 

 lines, the upper is covered by the scalp, and the lower, somewhat striated, affords 

 attachment to the temporal muscle. Close to the upper border and near to the 

 occipital angle is a small opening — the parietal foramen — which transmits a vein 

 to the superior sagittal (longitudinal) sinus. 



The cerebral surface is marked wdth depressions corresponding to the cerebral 

 convolutions and by numerous deep furrows, running upward and backward 

 from the sphenoidal angle and the lower border, for the middle meningeal vessels 

 (sinus and artery). A shallow depression running close to the superior border 

 forms, with the one of the opposite side, a channel for the superior sagittal sinus, 

 at the side of which are small irregular pits for the Pacchionian bodies; the pits 

 are usually present in adult skulls, but are best marked in those of old persons. 

 The margins of the groove for the superior sagittal sinus give attachment to the 

 falx cerebri. 



Borders. — The sagittal or superior border, the longest and thickest, is deeply 

 serrated to articulate with the opposite parietal, with which it forms the sagittal 

 suture. The frontal or anterior border articulates with the frontal to form the 

 coronal suture. It is deeply serrated and bevelled, so that it is overlapped by the 

 frontal above, but overlaps the edge of that bone below. The occipital or 

 posterior border articulates with the occipital to form the lambdoid suture, and 

 resembles the superior and anterior in being markedly serrated. The squamosal 

 or inferior border is divided into three portions : — the anterior, thin and bevelled, 

 is overlapped by the tip of the great wing of the sphenoid; the middle portion, 

 arched and also bevelled, is overlapped by the squamous part of the temporal; 

 and the posterior portion, thick and serrated, articulates with the mastoid 

 portion of the temporal bone. 



Angles. — The frontal or anterior superior, almost a right angle, occupies that 

 part of the bone which at birth is membranous and forms part of the anterior 



