76 SPECIAL ANATOMY OF THE SKELETON 



Borders. The superior border (margo sagittalis), the longest and thickest, 

 is dentated to articulate with its fellow of the opposite side, forming the sagittal 

 suture. 



The inferior border (margo squamosus) is divided into three parts; of these, 

 the anterior is thin and pointed, bevelled at the expense of the outer surface, and 

 overlapped by the tip of the greater wing of the sphenoid; the middle portion is 

 arched, bevelled at the expense of the outer surface, and overlapped by the 

 squamous portion of the temporal; the posterior portion is thick and serrated for 

 articulation with the mastoid portion of the temporal. 



The anterior border (margo frontalis), deeply serrated, is bevelled at the expense 

 of the outer surface above and of the inner below; it articulates with the frontal 

 bone, forming the coronal suture. 



The posterior border (margo occipitalis), deeply denticulated, articulates with 

 the occipital, forming the lambdoid suture. 



Angles. The anterior superior angle (angulus frontalis), thin and pointed, 

 corresponds with that portion of the skull which in the fetus is membranous, and 

 is called the anterior fontanelle (bregma). 



The anterior inferior angle (angulus sphenoidalis) is thin and lengthened, being 

 received in the interval between the greater wing of the sphenoid and the frontal. 

 Its inner surface is marked by a deep groove, sometimes a canal, for the anterior 

 branch of the middle meningeal artery. At the anterior inferior angle the parietal 

 and frontal bones and the greater wing of the sphenoid bone meet. This spot is 

 called the pterion. 



The posterior superior angle (angulus occipitalis) corresponds with the junction 

 of the sagittal and lambdoid sutures. In the fetus this part of the skull is mem- 

 branous, and is called the posterior fontanelle (lambda). 



The posterior inferior angle (angulus mastoideus) articulates with the mastoid 

 portion of the temporal bone, and generally presents on its inner surface a broad, 

 shallow groove for the lodgement of part of the lateral sinus. 



Development. The parietal bone is formed in membrane, being developed from one centre, 

 which corresponds with the parietal eminence, and makes its first appearance about the seventh 

 or eighth week of fetal life. Ossification gradually extends from the centre to the circumference 

 of the bone; the angles are consequently the parts last formed, and it is in their situation that the 

 fontanelles exist previous to the completion of the growth of the bone. Occasionally the parietal 

 bone is divided into two parts, upper and lower, by an antero-posterior suture. 



Articulations. With five bones the opposite parietal, the occipital, frontal, temporal, and 

 sphenoid. 



Attachment of Muscles. One only, the Temporal. 



The Frontal Bone (Os Frontale). 



The frontal bone consists of two portions a vertical or frontal portion, situated 

 at the anterior part of the cranium proper, forming the forehead; and a horizontal 

 or orbital portion, which enters into the formation of the roof of the orbits and 

 nasal fossee. 



Vertical Portion. Surfaces. External Surface (fades frontalis} (Fig. 44). In 

 the median line, traversing the bone from the upper to the lower part, is occasionally 

 seen a slightly elevated ridge, and in young subjects a suture (frontal or metopic 

 suture) which represents the line of union of the two lateral halves of which the bone 

 consists at an early period of life; in the adult this suture is usually obliterated and 

 the bone forms one piece. On either side of this ridge, a little below the centre of 

 the bone, is a rounded prominence, the frontal eminence (tuber frontale). These 

 eminences vary in size in different individuals, and are occasionally unsymmetrical 



