Description
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.