The petrous portion is an elongated, pyra midal mass, of which two of the surfaces enter into the formation of the cavity of the cranium, and the third is underneath. It is situated on a line which, if prolonged, would extend from behind the ear to the opposite external angular process of the frontal bone; but it is limited by the body of the sphenoid. It occupies the space between the posterior border of the ala major of the sphenoid and the basilar process of the occipital bone, in the angle of which its free extremity is impacted. In its substance is contained the labyrinth of the ear.
Of the two surfaces which are in the cra nium, one is superior, the cerebral; the other is posterior, the cerebellar.
On the cerebral surface near its middle, is a smooth, convex, and transverse elevation (the processus senzicircularis), produced by the su perior semicircular canal of the labyrinth ; immediately in front of this is a depression on which the Gasserian ganglion lies ; more out wardly and running lengthwise, is a faint sulcus (the sukus Vidianus), which terminates at a small opening (the hiatus Fallopii ) for the entrance of the Vidian nerve into the ague ductus Fallopii.
On the cerebellar surface is seen theforamen auditorium internum, the superior and posterior part of the margin of which is more prominent than the anterior, which, in fact, degenerates into a sulcus. It is the commencement of a canal (the meatus auditorius internus) into which pass the acoustic and facial nerves, and the bottom of which is divided by a ridge into two unequal depressions; the upper one being the fbssula parva, in which is the orifice of the aqueduct of Fallopius for the exit of the facial nerve; the lower one being the fossula magna, in which are several minute perforations for the acoustic nerve. Behind the foramen audi torium is an indistinct slit, which is the ter mination of the aqueductus vestibuli; above and rather anterior to this slit is a triangular orifice for the entrance of vessels; and below it, extending to the foramen lacer= posticum, is a slight groove.
Between the cerebral and cerebellar surfaces there is a sharp ridge on which there is a groove (the sukus petrosus), more evident pos teriorly than anteriorly; to the ridge is attached the tentorium ; the groove lodges the petrosal sinus.
The Ifncler surface is divided into two parts by a sharp, prominent ridge, which has on either side of it a considerable fossa. That on its outer side is the finsa parotidea for the upper part of the parotid gland ; that on its inner side is a thimble-like depression (the fossa jugularis), which forms with the occipital bone the foramen lacerum posterius. In this bone, however, it is not so wide as it is in the occipital; from which it results that the fora men is imperfectly divided into two parts— the anterior for the nerves, the posterior for the vein; and it is the latter organ which is lodged in the fossa jugularis of the temporal bone. The fossa parotidea is limited, above
and in front, by a fissure (the fissura Glasseri), which penetrates to the tympanum and gives exit to the chorda tympani and entrance to the laxator tympani muscle; behind, by the external auditory process, The margin of the fbramen auditorium externunz, which is ellip tical, has its long diameter vertical, and .is the commencement of the nzeatus auditortus externus ; a tube which is curved a little downwards, is more expanded at its extre mities than in its middle, and terminates at the membmna tympani, in front, by a sulcus which is situated on the border between the cerebral and under surfaces, and passes back wards, between the petrous and squamous portions as a canal (the canalis Eustachianus), which is divided by a lamina of bone, called the processus cochleariformis, into two parts, the inferior of which contains the Eustachian tube, and the superior the tensor membranm tympani muscle. Immediately behind the fossa jugularis there is a rough surface, for the articulation of the jugular process of the occi pital bone; and to the outer side of this sur face is the foramen stylo-mastoideum for the exit of the facial nerve. In front of and close to this foramen, and between it and the jugular fossa, is the long pointed process (the styloid process) for the attachment of the stylo-maxil lary and stylo-hyoid ligaments, and the stylo pharyngeus, stylo-glossus and stylo-hyoideus muscles ; this process is embraced on the outer side at its root by a portion of the ridge separating the parotid and jugular fossm ; that portion is called the vaginal process. In front of the fossa jugularis are two foramina ; one very large, the foramen caroticum ; the other 1"rery small, to the inner side of the former and nearly on the margin between this and the cerebellic surfaces, being the termination of the aqueduct of the cochlea. The foramen caro ticum is the inferior opening of the canalis caroticus, a canal which exists in the bone, and consists of two parts that are at right angles with each other—the inferior, short, vertical, and extending upwards from the fo ramen caroticum into the substance of the bone; the superior, horizontal, running length vvise, and extending to the end of the petrous process : in this canal there pass the carotid artery to the cavity of the cranium, and a filament of the nervus abducens, as well as one of the Vidian, to the neck. A rough sur face is observed anterior to the foramen caro ticum for the attachment of the levator palati and the tensor tympani muscles.