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.