=The Petrous Portion= (Fig. 23, _b_, and Fig. 25).--This consists of two parts, a very dense part (the petrous portion proper, Fig. 25), which has the form of a triangular pyramid and encloses the auditory labyrinth, and a less dense part, the =mastoid= portion (Figs. 22 and 23, _e_), which is flattened and triangular and is attached by its base to the base of the pyramid. The petrous portion may be described as having a base and three sides, lateral, dorsal, and medial. It completes the medial wall of the tympanic bulla, so that it is not possible to see it from the exterior of a skull except through the auditory meatus (Fig. 22, _i_). When the bones of the skull are articulated its dorsal surface is covered by the tentorium and alisphenoid. Its lateral face looks into the tympanic cavity, while the medial face looks into the cranial cavity. [Illustration: FIG. 25.--PETROUS BONE OF RIGHT SIDE, LATERAL SURFACE, ENLARGED. _a_, fenestra cochleæ; _b_, promontory; _c_, fenestra vestibuli; _d_, fossa for the tensor tympani muscle; _e_, fossa for incus and malleus; _f_, fossa continuous with stylomastoid foramen; _g_, foramen leading to facial canal.] Its lateral face (Fig. 25) (medial wall of the tympanic cavity) presents just ventrad of the middle of its base a large circular foramen, the =fenestra cochleæ= (_a_) (or fenestra rotunda), which looks caudolaterad; it leads into the cochlea. The fenestra cochleæ lies at the summit of a nipple-like elevation, the =promontory= (_b_), which is continued toward the apex of the bone as a gradually diminishing semicylindrical ridge, due to the presence within it of the bony cochlea. Dorsad of the fenestra cochleæ is the much smaller =fenestra vestibuli= (_c_) (or ovalis) which leads into the vestibule. It is occupied in the natural condition by the base of the stapes. Dorsocraniad of the fenestra vestibuli is a large fossa (_d_) which contains the tensor tympani muscle. Dorsocaudad of this, partly bounded by the squamous portion of the temporal, is another large fossa (_e_), the cranial end of which is occupied by the incus, while its caudal end is occupied by the head of the malleus. Nearly caudad of this fossa and separated from it by an oblique bony septum is a third fossa (_f_) which is narrow and curved. It is continuous with a notch in the mastoid portion of the bone. When the tympanic is articulated the notch is converted into a foramen (=stylomastoid foramen=, Fig. 22, _j_) for the exit of the seventh nerve. The fossa gives passage to the seventh nerve and also lodges the stapedius muscle. A groove may be traced from the stylomastoid foramen to the caudal border of the fossa for the tensor tympani muscle, where it passes into a canal (_g_). The groove and canal are parts of the =canalis facialis= or =facial canal= (aqueductus Fallopii) for the passage of the seventh nerve through the petrous bone.