The petrous part of the temporal bone is pyramid-shaped and is wedged in at the base of the skull between the sphenoid and occipital bones. Directed medially, forward, and a little upward, it presents a base, an apex, three surfaces, and three angles, and houses in its interior the components of the inner ear. The petrous portion is among the most basal elements of the skull and forms part of the endocranium. Petrous comes from the Latin word petrosus, meaning "stone-like, hard". It is one of the densest bones in the body. In other mammals, it is a separate bone, the petrosal bone, itself a fusion of the prootic, and opisthotic, and possibly epiotic. The petrous bone is important for studies of ancient DNA from skeletal remains, as it tends to contain extremely well-preserved DNA.
Base The base is fused with the internal surfaces of the squamous, tympanic, and mastoid parts.
Apex The apex, which is rough and uneven, is received into the angular interval between the posterior border of the great wing of the sphenoid bone and the basilar part of the occipital bone; it presents the anterior or internal opening of the carotid canal, and forms the postero-lateral boundary of the foramen lacerum.
Surfaces
Anterior surface The anterior surface forms the posterior part of the middle cranial fossa of the base of the skull, and is continuous with the inner surface of the squamous portion, to which it is united by the petrosquamous suture, remains of which are distinct even at a late period of life. It is marked by depressions for the convolutions of the brain, and presents six notable points:
near the center, the arcuate eminence (eminentia arcuata), which indicates the location of the superior semicircular canal. in front of and a little lateral to this eminence, a depression indicating the position of the tympanic cavity: Here, the layer of bone that separates the tympanic from the cranial cavity is extremely thin, and is known as the tegmen tympani a shallow groove, sometimes double, leading lateralward and backward to an oblique opening, the hiatus for greater petrosal nerve, for the passage of the greater petrosal nerve and for the petrosal branch of the middle meningeal artery lateral to the hiatus, a smaller opening, occasionally seen, for the passage of the lesser superficial petrosal nerve near the apex of the bone, the termination of the carotid canal, the wall of which in this situation is deficient in front above this canal the shallow trigeminal impression for the reception of the trigeminal ganglion.
Posterior surface
The posterior surface forms the anterior part of the posterior cranial fossa of the base of the skull, and is continuous with the inner surface of the mastoid portion. Near the center is a large orifice, the internal acoustic opening, the size of which varies considerably; its margins are smooth and rounded, and it leads into the internal auditory meatus a short canal, about 1 cm. in length, which runs lateralward. It transmits the facial and acoustic nerves and the internal auditory branch of the basilar artery. The lateral end of the canal is closed by a vertical plate, which is divided by a horizontal crest, the falciform crest, into two unequal portions. Each portion is further subdivided by a vertical ridge into an anterior and a posterior part.
In the portion beneath the falciform crest are three sets of foramina; these openings together with this central canal transmit the nerves to the cochlea. one group, just below the posterior part of the crest, situated in the area cribrosa media, consists of several small openings for the nerves to the saccule; below and behind this area is the foramen singulare, or opening for the nerve to the posterior semicircular duct; in front of and below the first is the tractus spiralis foraminosus, consisting of a number of small spirally arranged openings, which encircle the canalis centralis cochleæ. The portion above the crista falciformis presents behind, the area cribrosa superior, pierced by a series of small openings, for the passage of the nerves to the utricle and the superior and lateral semicircular ducts, and, in front, the area facialis, with one large opening, the commencement of the canal for the facial nerve (aquæductus Fallopii). Behind the internal acoustic meatus is a small slit almost hidden by a thin plate of bone, leading to a canal, the aquæductus vestibuli, which transmits the ductus endolymphaticus together with a small artery and vein. Above and between these two openings is an irregular depression that lodges a process of the dura mater and transmits a small vein; in the infant, this depression is represented by a large fossa, the subarcuate fossa, which extends backward as a blind tunnel under the superior semicircular canal.
Falciform crest Area facialis, with (2') internal opening of the facial canal Ridge separating the area facialis from the area cribrosa superior Area cribrosa superior, with (4') openings for nerve filaments Anterior inferior cribriform area, with (5') the tractus spiralis foraminosus, and (5") the canalis centralis of the cochlea. Ridge separating the tractus spiralis foraminosus from the area cribrosa media Area cribrosa media, with (7') orifices for nerves to saccule Foramen singulare.
Inferior surface The inferior surface is rough and irregular, and forms part of the exterior of the base of the skull. It presents eleven points for examination:
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