288 



MAMMALIAN ANATOMY 



FIG. 208. 



The most obvious characters in the human frontal are the great 

 size and strong convexity of that portion of the horizontal plate situ- 

 ated behind the orbital rim, the marked 

 reduction of the parts of the horizontal 

 and vertical plates in front of the orbital 

 rim, and the almost transverse direction 

 of the orbital rim itself. 



The orbital portion of the vertical 

 plate is small, and its direction is so 

 changed that it faces downward and 

 slightly forward. 



The superior surface of the horizontal 

 plate (Fig. 208) is more strongly convex 

 EXTERNAL VIEW OF THE FRONTAL a t the frontal eminence, above the orbital 



BONE. 



1, position of the frontal eminence; 2, 

 superciliary ridge ; 3, supraorbital arch ; 4, 



5, external and internal angular processes; 



6, supraorbital notch or foramen ; 7, gla- 

 bella ; 8, temporal ridge ; 9, nasal spine. 



rim. Below the eminence is a transverse 

 depression, and beneath this an arching 

 elevation known as the superciliary 

 ridge. The union of the superciliary 



ridges of the two frontals over the nasal suture produces the nasal 

 eminence or glabella. Below the ridge is the sharp supraorbital 

 margin, which ends laterally at the external angular process and 

 medially at the internal angular process. At its inner part is a 

 notch, sometimes converted into a foramen, for the supraorbital vessels. 

 A well-marked temporal line curves from the external angular process 

 to the parietal border. 



The intracranial surface (Fig. 209) is strongly concave, and forms 

 an angle in front with the orbital plate. Its medial side is grooved 

 behind for the longitudinal sinus, but in front is elevated into a ridge 

 continuous with the crista galli on the superior surface of the eth- 

 moid ; at the extreme anterior end a vertical groove is converted by 

 the ethmoid into the foramen caecum for a vein. The intracranial 

 surface is marked by cerebral convolutions, by vessels, and by Pac- 

 chionian bodies. 



That part which corresponds to the median plate is so far reduced 

 that the opening to the frontal sinus is almost on the anterior surface 

 of the bone. 



The lower parts of the two rudimentary median plates are united 

 and produced forward and downward as the nasal spine, above and 



