THE FRONTAL BONE. 61 



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, 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, deeply denticulated, articulates with the 

 occipital, forming the lambdoid suture. 



Angles. The anterior superior angle, thin and pointed, corresponds with that 

 portion of the skull which in the foetus is membranous and is called the anterior 

 fontanelle. The anterior inferior angle is thin and lengthened, being received in 

 the interval between the great 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. The posterior superior angle corresponds with the 

 junction of the sagittal and lambdoid sutures. In the foetus this part of the skull 

 is membranous, and is called the posterior fontanelle. The posterior inferior angle 

 articulates with the mastoid portion of the temporal bone, and generally presents 

 on its inner surface a broad, shallow groove for lodging part of the lateral sinus. 



Development. The parietal bone is formed in membrane, being developed by 

 one centre, which corresponds with the parietal eminence, and makes its first 

 appearance about the seventh or eighth week of foetal life. Ossification gradually 

 extends from the centre to the circumference of the bone : the angles are conse- 

 quently 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. 



The Frontal Bone (frons, the forehead) resembles a cockle-shell in form, and 

 consists of two portions a vertical or frontal portion situated at the anterior part 

 of the cranium, forming the forehead ; and a horizontal or orbito-nasal portion 

 which enters into the formation of the roof of the orbits and nasal fossae. 



Vertical Portion. External Surface (Fig. 28). 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, 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 ; traces of the 

 obliterated suture are, however, generally perceptible at the lower part. On either 

 side of this ridge, a little below the centre of the bone, is a rounded eminence, the 

 frontal eminence. These eminences vary in size in different individuals, and are 

 occasionally unsymmetrical in the same subject. They are especially prominent 

 in cases of well-marked cerebral development. The whole surface of the bone 

 above this part is smooth, and covered by the aponeurosis of the Occipito-frontalis 

 muscle. Below the frontal eminence, and separated from it by a slight groove, is 

 the superciliary ridge, broad internally, where it is continuous with the nasal 

 eminence, but less distinct as it arches outward. These ridges are caused by the 

 projection outward othe frontal air sinuses, 1 and give attachment to the Orbicu- 

 laris palpebrarum and Corrugator supercilii. Between the two superciliary 

 ridges is a smooth surface, the alabella or nasal eminence. Beneath the super- 

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



1 Some confusion is occasioned to students commencing the study of anatomy by the name 

 "sinuses" having been given to two perfectly different kinds of spaces connected with the skull. 

 It may be as well, therefore, to state here, at the outset, that the "sinuses" in the interior of the 

 cranium which produce the grooves on the inner surface of the bones are venous channels along 

 which the blood runs in its passage back from the brain, while the "sinuses" external to the cranial 

 cavity (the frontal, sphenoidal, ethmoidal, and maxillary) arc hollow spaces in the bones themselves 

 which communicate with the nostrils, and contain air. 



