External acoustic meatus The temporal bone contains the bony (osseous) part of the external acoustic meatus. Ossification The four temporal components ossify independently (Fig. 37.2). The squamous part is ossified in a sheet of condensed mesenchyme from a single centre near the zygomatic roots, which appears in the seventh or eighth week in utero. The petromastoid part has several centres that appear in the cartilaginous otic capsule during the fifth month; as many as 14 have been described. These centres vary in order of appearance. Several are small and inconstant, soon fusing with others. The otic capsule is almost fully ossified by the end of the sixth month. The tympanic part is also ossified in mesenchyme from a centre identifiable about the third month; at birth, it is an incomplete tympanic ring, deficient above, its concavity grooved by a tympanic sulcus for the tympanic membrane. The malleolar sulcus for the anterior malleolar process, chorda tympani and anterior tympanic artery inclines obliquely downwards and forwards across the medial aspect of the anterior part The superior border of the mastoid part is thick and serrated for articulation with the mastoid angle of the parietal bone. The posterior border is also serrated and articulates with the inferior border of the occipital bone between its lateral angle and jugular process. The mastoid element is fused with the descending process of the squamous part; below, it appears in the posterior wall of the tympanic cavity. Petrous part The petrous part is a mass of bone that is wedged between the sphenoid and occipital bones in the cranial base; it contains the labyrinth. It is inclined superiorly and anteromedially, and has a base, apex, three surfaces (anterior, posterior and inferior) and three borders (superior, posterior and anterior). The base would correspond to the part that lies on the base of the skull and is separated from the squamous part by a suture. However, this suture disappears soon after birth. The subsequent development of the mastoid processes means that the precise boundaries of the base are no longer identifiable. The apex, blunt and irregular, is angled between the posterior border of the greater wing of the sphenoid and the basilar part of the occipital bone. It contains the anterior opening of the carotid canal and limits the foramen lacerum posterolaterally. The anterior surface contributes to the floor of the middle cranial fossa (Ch. 28) and is continuous with the cerebral surface of the squamous part (although the petrosquamosal suture often persists late in life). The whole surface is adapted to the inferior temporal gyrus. Behind the apex is a trigeminal impression for the trigeminal ganglion. Bone anterolateral to this impression roofs the anterior part of the carotid canal but is often deficient. A ridge separates the trigeminal impression from another hollow behind, which partly roofs the internal acoustic meatus and cochlea. This, in turn, is limited behind by the arcuate eminence, which is raised by the superior (anterior) semicircular canal but is not necessarily directly over it. Laterally, the anterior surface roofs the vestibule and, partly, the facial canal. Between the squamous part laterally and the arcuate eminence and the hollows just described medially, the anterior surface is formed by the tegmen tympani, a thin plate of bone that forms the roof of the mastoid antrum, and extends forwards above the tympanic cavity and the canal for tensor tympani. The lateral margin of the tegmen tympani meets the squamous part at the petrosquamosal suture, turning down in front as the lateral wall of the canal for tensor tympani and the osseous part of the pharyngotympanic tube; its lower edge is in the squamotympanic fissure. Anteriorly, the tegmen bears a narrow groove related to the greater petrosal nerve (which passes pos.