

THE FRONTAL BONE 135 





frontal border is deeply serrated, and bevelled at the expense of the outer surface 

 above and of the inner below; it articulates with the frontal bone, forming one- 

 half of the coronal suture. The occipital border, deeply denticulated, articulates 

 with the occipital, forming one-half of the lambdoidal suture. 



Angles. The frontal angle is practically a right angle, and corresponds with 

 the point of meeting of the sagittal and coronal sutures; this point is named the 

 bregma; in the fetal skull and for about a year and a half after birth this region is 

 membranous, and is called the anterior fontanelle. The sphenoidal angle, thin 

 and acute, is received into the interval between the frontal bone and the great 

 wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes 

 a canal, for the anterior divisions of the middle meningeal artery. The occipital 

 angle is rounded and corresponds with the point of meeting of the sagittal and 

 lambdoidal sutures a point which is termed the lambda; in the fetus this part 

 of the skull is membranous, and is called the posterior fontanelle. The mastoid 

 angle is truncated; it articulates with the occipital bone and with the mastoid 

 portion of the temporal, and presents on its inner surface a broad, shallow groove 

 which lodges part of the transverse sinus. The point of meeting of this angle 



ith the occipital -and the mastoid part of the temporal is named the asterion. 



Ossification. The parietal bone is ossified in membrane from a single center, which appears 

 at the parietal eminence about the eighth week of fetal life. Ossification gradually extends in 

 a radial manner from the center toward the margins of the bone; the angles are consequently 

 r,he parts last formed, and it is here that the fontanelles exist. Occasionally the parietal bone 

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



Articulations. The parietal articulates with five bones: the opposite parietal, the occipital, 

 frontal, temporal, and sphenoid. 



The Frontal Bone (Os Frontale). 



The frontal bone resembles a cockle-shell in form, and consists of two portions 

 a vertical portion, the squama, corresponding with the region of the forehead; 

 and an orbital or horizontal portion, which enters into the formation of the roofs 

 of the orbital and nasal cavities. 



Squama (squama frontalis). Surfaces. The external surface (Fig. 134) of this 

 portion is convex and usually exhibits, in the lower part of the middle line, the 

 remains of the frontal or metopic suture ; in infancy this suture divides the bone into 



o, a condition which may persist throughout life. On either side of this suture, 

 bout 3 cm. above the supraorbital margin, is a rounded elevation, the frontal emi- 

 ,ce (tuber frontale) . These eminences vary in size in different individuals, are 

 ccasionally unsymmetrical,.and are especially prominent in young skulls; the sur- 

 'ace of the bone above them is smooth, and covered by the galea aponeurotica. 



elow the frontal eminences, and separated from them by a shallow groove, are 

 two arched elevations, the superciliary arches; these are prominent medially, and 

 are joined to one another by a smooth elevation named the glabella. They are 

 larger in the male than in the female, and their degree of prominence depends 

 to some extent on the size of the frontal air sinuses; 1 prominent ridges are, how- 

 ever, occasionally associated with small air sinuses. Beneath each superciliary 

 arch is a curved and prominent margin, the supraorbital margin, which forms the 

 upper boundary of the base of the orbit, and separates the squama from the orbital 

 portion of the bone. The lateral part of this margin is sharp and prominent, 

 affording to the eye, in that situation, considerable protection from injury; the 



edial part is rounded. At the junction of its medial and intermediate thirds is 



1 Some confusion is occasioned to students commencing the study of anatomy by the name "sinus" having been 

 iven to two different kinds of space connected with the skull. It may be as well, therefore, to state here that the 

 sinuses" in the interior of the cranium which produce the grooves on the inner surfaces of the bones are venous 

 channels which convey the blood from the brain, while the "sinuses" external to the cranial cavity (the frontal, 

 sphenoidal, ethmoidal, and maxillary) are hollow spaces in the bones themselves; they communicate with the nasal 

 savities and contain air. 





