14 PRACTICAL ANATOMY. 



sphenoid ; the ethmoidal border, thickened and cellular, 

 articulates with the ethmoid. Two shallow grooves are 

 observed traversing this border, one anterior and one 

 posterior, and which, when articulated with the ethmoid, 

 form the anterior and posterior ethmoidal foramina. 

 Along the anterior part of the ethmoidal border large 

 openings are observed, which lead into almond-shaped 

 cavities behind the supra-orbital ridges. These cavities 

 are the frontal air sinuses, and are separated from each 

 other in the median line by a plate of bone. They are 

 formed by the divergence of the tables of the frontal 

 bone ; on a vertical section they appear triangular ; their 

 largest diameter is horizontal. The superior border of 

 the frontal articulates with the two parietal bones and 

 the greater wing of the sphenoid ; the external angular 

 process with the malar ; the internal angular process 

 with the nasal and superior maxillary. The frontal 

 bone develops from two centres, which appear near the 

 centre of the frontal eminences. These centres first ap- 

 pear about the third month of fetal life. 



THE PARIETAL BONES. 



The parietal bones, two in number, are located at 

 the sides and vault of the skull behind the frontal bone. 

 They articulate with each other at the vertex of the 

 skull, and form the interparietal or sagittal suture. 

 Each bone has four borders, four angles, and two sur- 

 faces, and is traversed from the anterior to the posterior 

 border by a curved line, the temporal ridge, continuous 

 with the temporal ridge on the frontal bone. The tem- 

 poral ridge divides the external surface into two portions, 

 the upper and lower, the upper surface being the larger 

 and smooth, the lower surface the smaller and marked by 

 faint irregularities and minute foramina ; it assists in form- 



