THE FRONTAL BONE 



59 



inferior angle is thick and articulates with the mastoid portion of the temporal 

 bone. Its inner surface presents a shallow groove which lodges a part of the 

 transverse (lateral) sinus. 



Blood-supply. — The parietal bone receives its blood-supply from the middle meningeal, 

 occipital, and supra-orbital arteries. 



Articulations. — The parietal articulates with the occipital, frontal, sphenoid, temporal, 

 its fellow of the opposite side, and the epipteric bone when present. Occasionally the temporal 

 and epipteric bones exclude the parietal from articulation with the great wing of the sphenoid. 



Ossification. — The parietal ossifies from a single nucleus which appears in the outer layer 

 of the membranous wall of the skull about the seventh week. The ossification radiates in 

 such a way as to leave a cleft at the upper part of the bone in front of the occipital angle, the 



Fig. 74. 



-Unusual Form of Pariet.^l Exhibiting a Horizontal Suture Separating the 

 Bone into Two Pieces, Upper and Lower. 



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cleft of the two side forming a lozenge-shaped space across the sagittal suture known as the sag- 

 ittal fontanelle. This is usually closed about the fifth month of intra-uterine life, but traces maj- 

 sometimes be recognised at the time of birth, and the parietal foramina are to be regarded as 

 remains of the cleft. According to Dr. A. W. W. Lea, a well-developed sagittal fontanelle is 

 present in 4.4 per cent, of infants at birth. In such cases it closes within the first two months 

 of life, but at times it may remain open for at least eight months after birth and possibly longer. 

 Rarely the parietal bone is composed of two pieces (fig. 74), one above the other, and 

 separated by an antero-posterior suture (sub-sagittal suture), more or less parallel with the 

 sagittal suture. In such cases the parietal is ossified from two centres of ossification. 



THE FRONTAL 



The frontal bone [os frontale] closes the cranium in front and is situated above 

 the skeleton of the face. It consists of two portions — a frontal (vertical) portion 

 [squama frontalis], forming the convexity of the forehead, and an orbital [hori- 

 zontal) portion, which enters into formation of the roof of each orbit. 



Frontal (vertical) portion. — The frontal surface is smooth and convex, antl 

 usually presents in the middle line above the root of the nose some traces of the 

 suture which in young subjects traverses the bone from the upper to the lower 

 part. This suture, known as the frontal or metopic suture, indicates the line of 

 junction of the two lateral halves of which the bone consists at the time of birth; 

 in the adult the suture is usually obliterated except at its lowest part. On each 

 side is a rounded elevation, the frontal eminence [tuber frontale], very prominent 

 in young bones, below which is a shallow groove, the sulcus transversus, separat- 

 ing the frontal eminence from the superciliary arch. The latter forms an arched 

 priojection above the margin of the orbit and corresponds to an air-cavity within 

 the bone kno^vn as the frontal sinus; it gives attachment to the orbicularis oculi 

 and the corrugator muscles. The ridges of the two sides converge toward the 



