The cranial border is bevelled at the expense of the inner surface and articulates with the frontal. Just ventrad of the middle of the border projects a sharp spine which fits into a corresponding notch in the caudal border of the frontal.

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 margin ([Fig. 39], i; [Fig. 40], o); the caudal third passes gradually into the temporal fossa. At its cranial angle is a triangular projection, the frontal spine or nasal spine ([Fig. 26], a), which fits into a space between the nasal and maxillary bones.

[Fig. 26].—Frontal Bone, Medial Surface.

a, frontal spine; b, transverse ridge; c, surface applied to the ethmoid; d, vertical plate of medial border.