THE BONES OF THE HEAD 



65 



recognised by the large anterior meningeal groove on its inner 

 surface. 



The bone receives its chief blood-supply from the middle 

 meningeal artery internall}^ and the occipital externally. 



Articulations. — Posteriorly with the occipital, superiorly with its 

 fellow, anteriorly with the frontal, antero-inferiorly with the sphenoid, 

 and inferiorly with the temporal. 



Structure.— It is a characteristic tabular bone. 



Varieties. — (i) The bone may persist in two parts, upper and lower, con- 

 nected by an antero-posterior suture. (2) The parietal foramen may be absent 

 on one or both sides. 



Ossification. — The parietal ossifies in membrane from two centres, which 

 apgear about the seventh week in the region of the future parietal eminence, 

 one above and the other below it, and soon coalesce. The ossification radiates 



- . Appear about the 

 ^ 7th Week 



Jj (intra-uterine) 



Fig. 44. — Ossification of the Parietal Bone. 



from this point in such a manner as to leave a notch on the upper border a 

 little in front of the postero-superior angle, which forms one-half of the sagittal 

 fontanelle of the earlier half of foetal life. 



The Frontal Bone. 



The frontal bone forms the forehead and greater part of the 

 roof of each orbit, and it lies in front of the parietals. It is divisible 

 into a frontal portion and two orbital plates, the latter being 

 situated inferiorly, where they are separated by the ethmoidal notch. 



The frontal portion presents two surfaces, external and internal. 

 The external surface, which is smooth and convex, presents a little 

 below its centre, on either side, an elevation, called the frontal 

 eminence. Below this, and separated from it by a shallow groove, 

 there is the curved superciliary ridge which supports the eyebrow. 

 This ridge is prominent internally, but it subsides externally. It 

 supports the upper half of the orbicularis palpebrarum, and 

 internally it gives origin to the comigator supercilii, whilst the 

 surface above each ridge supports the frontalis and part of the 

 epicranial aponeurosis. Between the two superciliary ridges is an 

 elevation, called the nasal eminence or glabella. Below each ridge 

 is the curved supra-orbital arch, which is most prominent over its 



