The inner surface (Fig. 43, 3 and 3′) is smooth and marked by ridges and grooves for the convolutions of the cerebrum. Near the medial border is a ridge which, when the bone is articulated with that of the opposite side, forms a shallow groove for the =superior sagittal sinus=. Beginning near the middle of the ventral margin and passing dorsad is a groove for the middle meningeal artery. The =tentorium= (Fig. 43, _f_) arises from the inner surface near its caudal margin and projects mediad as a thin curved or notched shelf of bone which separates the cerebellar fossa (Fig. 43, _I_) of the cranium from the cerebral fossa (Fig. 43, _II_). When the parietals are articulated there is left between the tentoria a large foramen by means of which the two fossæ communicate. The foramen is bounded laterally and dorsally by the free margins of the tentoria, while the ventral end of each tentorium articulates with the alisphenoid, and its dorsal end with the opposite tentorium. The medial border is straight and is united by suture to the opposite bone. 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. [Illustration: FIG. 26.--FRONTAL BONE, MEDIAL SURFACE.