Dorsocaudad of the internal auditory meatus is a deep fossa (o) for a small lobe, the so-called appendicular lobe, of the cerebellum. This may be called the appendicular fossa.
The dorsal surface is triangular and presents near its apex a foramen—the hiatus facialis (p), the opening of a canal which joins the canalis facialis and transmits the superficial petrosal branch of the nerve of the pterygoid canal (Vidian nerve). That part of the dorsal surface which lies caudad of the hiatus facialis is known as the tegmen tympani.
The base of the petrous is attached to the mastoid portion ([Fig. 23], e).
(For an account of the structures within the petrous bone and the tympanic cavity, see the description of [the internal and middle ear].)
The mastoid portion ([Figs. 22] and [23], c) is attached by its base to the pyramidal petrous portion, with which it forms an angle of about 120 degrees. It appears in the lateral wall of the skull between the parietal bone and the occipital ([Fig. 40], d). The lambdoidal ridge is continued on its outer surface to the caudal border of the external auditory meatus. Caudad of the stylomastoid foramen it forms a slight nipple-like eminence, the mastoid process ([Fig. 22], l). Its inner face looks into the cranial cavity.
Parietal Bone. Os parietale
([Figs. 39], [40], and [43], 3).—The parietal bones form the larger part of the lateral and dorsal boundary of the cranial cavity. Each is a thin rectangular bone, compact and curved and with a deeply notched shelf of bone, the tentorium ([Fig. 42], e, and [Fig. 43], f), projecting inward from near the caudal margin.
The outer surface is smooth and convex. The highest part of the convexity, a little caudad of the middle of the bone, is known as the parietal tubercle or eminence ([Fig. 39], d); it marks the point of beginning ossification. An obscure curved ridge ([Fig. 39], e), running from the caudodorsal angle or a point craniad of it craniolaterad, indicates the boundary of the origin of the temporal muscle. Near the ventral border the surface is roughened and is covered in the natural state by a part of the squamous portion of the temporal bone.
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.