THE SKULL. 37 



The ventral border is concave, sharp, and bevelled at the 

 expense of the outer surface, for articulation with the squamous 

 portion of the temporal, except near the cranial end, where it 

 articulates with the wing of the sphenoid. 



The caudal border is thick and porous medially, but thin 

 laterally, and bevelled at the expense of the inner surface for 

 articulation with the interparietal and mastoid portion of the 

 temporal. 



Frontal Bone. Os frontale (Figs. 39, 40, and 41, 5; Fig. 

 43, 8; Fig. 26). The frontal bones meet one another in the 

 median dorsal line so as to form the roof of the skull between 

 the parietal and nasal bones. A part extends also ventrad, 

 forming a large part of the medial wall of the orbit and a part 

 of the temporal fossa. 



The bone may be divided into two portions, a plate forming 

 the cranial portion of the roof of the skull and a part of the 

 roof of the nasal cavity, the frontal plate (Fig. 40, 5), and a 

 part descending into the orbit, the orbital plate (Fig. 40, 5'). 



The frontal plate (Fig. 40, 5) is a right-angled triangle with 

 the hypothenuse lateral. Its dorsal surface is convex and 

 smooth. The cranial two-thirds 

 of its lateral border is separated 

 from the orbital fossa by a ridge, 

 the supraorbital arch or mar- 

 gin (Fig. 39, z; Fig. 40, o} ; 

 the caudal third passes gradually 

 into the temporal fossa. At its 

 cranial angle is a triangular pro- FIG. 26. FRONTAL" BONE, MEDIAL 

 lection, the frontal spine or SURFACE. 



/T-- s- 1-1 a > frontal spine; b, transverse ridge; 



nasal spine (big. 20, a), Which c , surface applied to the ethmoid; d, 

 fits into a space between the vertical plate of medial border. 



nasal and maxillary bones. 



The ventral surface is concave and smooth over its caudal 

 one-half and helps to form the cranial part of the brain-case. 

 It presents slight ridges and depressions for convolutions of the 

 cerebrum. At its narrowed middle region the ventral surface 

 is marked by a thick transverse ridge (Fig. 26, ). Caudal ly 

 the ridge descends by a gentle slope to the level of the ventral 



