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) isa 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, 2; Fig. 40, 0); 
the caudal third passes gradually 
into the temporal fossa. At its 
cranial angle isa triangular pro- Fic. 26,—FrontaL Bons, MEDIAL 
jection, the frontal spine or ates Se ree ae 
nasal spine (Fig. 26, a), which c, ates ariied to the sthineide a 
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, 4). Caudally 
the ridge descends by a gentle slope to the level of the ventral 
