The temporal bone contains the external ear, middle ear (tympanic membrane, tympanic cavity, three ossicles, Eustachian tube, and antrum), internal ear (vestibule, semicircular canals, and cochlea), and facial canal. The facial, cochlear, and superior and inferior vestibular nerves course inside the bone, especially within IAC (Fig. 18.1a).

The temporal bone, which is joined anteriorly to the sphenoid bone, superiorly to the parietal bone, and posteriorly to the occipital bone, is comprised by three parts: squamous, petrous, and tympanic. The junction of these parts on the lateral side forms the external auditory canal (Fig. 18.1b) [21]. The squamous part—a round, bony plate standing erect sagittally—has a convex external surface, and its zygomatic process projects forward from the anterosuperior part of the porus acusticus externus to join the zygomatic bone, forming the zygomatic arch. The petrous part contains the mastoid process, which projects downward, and the petrous bone, which projects anteromedially and wedges between the sphenoid and occipital bones (Fig. 18.1c, d). The digastric groove below the mastoid process is the site of attachment of the sternocleidomastoid muscle and can be palpated behind the lower part of the auricle (Fig. 18.1b). The occipital artery and occipital nerve pass superomedially through this groove. The inner surface of the mastoid process also has a groove, called the sigmoid sulcus, in which the sigmoid sinus courses (Fig. 18.1d). The interior of the mastoid process in adults is filled with mastoid air cells, which communicate with the mastoid antrum. Therefore, the mastoid bone ultimately communicates with the external environment.

The petrous bone contains IAC, which opens as the oval porus acusticus internus (PAI) in the center of the posterior surface of the petrous bone (Fig. 18.1d). The cochlea lies anterolateral to IAC, the semicircular canals lie lateral to it, the carotid canal courses anteroinferior to it, and the jugular foramen lies posteroinferior to it.

IAC is surrounded by the cochlea and three semicircular canals. After coursing in IAC, CN VII runs in the facial canal between the cochlea and semicircular canals from the posterior to middle cranial fossa, where it forms the geniculate ganglion. The nerve then runs downward in the mastoid bone toward the stylomastoid foramen. The dry bone shown in Fig. 18.2a, b demonstrates the cochlea (highlighted in red) anterolateral to IAC and the semicircular canals (highlighted in green) posterolateral to it. CN VII, highlighted in yellow, which enters the temporal bone through PAI, is also visible coursing around the mastoid antrum after passing the middle cranial fossa.

Surrounding PAI on the surface of the posterior cranial fossa are several structures relevant to neurosurgeons, including the subarcuate fossa, which contains the subarcuate artery, and the vestibular aqueduct, which communicates with the aquaeductus cochleae (Fig. 18.2c). In the dry bone shown in Fig. 18.2d, the bone covering IAC and semicircular canals have been removed, exposing the interior of IAC, superior semicircular canal, and posterior semicircular canal. The junction between the superior and posterior semicircular canals is termed the common crus. The lateral end of IAC is divided into four parts. The transverse crest divides it into the upper and lower parts, and the vertical crest further subdivides the upper portion into the anterior and posterior parts. The anterior part is the facial canal, in which CN VII passes, and the posterior part is the superior vestibular part, through which the superior vestibular nerve passes. The area below the transverse crest is also divided approximately into two areas, and the cochlear nerve and inferior vestibular nerve traverse them, respectively.

Osseous structures in the temporal bone are visible with computed tomography (CT). Axial-view CT scans shown in Fig. 18.3a, b demonstrate the structures present at the levels of the external auditory canal and IAC. Air cells, including the mastoid air cells, occupy most of the temporal bone. Malleus and incus are visible in the middle ear, and the cochlea, which is connected with them, is visible in the internal ear (Fig. 18.3a). The compact bone, which does not contain any air cells, lies posterolateral to IAC. It can be seen as a white, high-density mass on CT (Fig. 18.3b). The semicircular canals are situated inside this compact bone (Fig. 18.3b).

Regarding the relationships between the external, middle, and internal ears, the external auditory canal and middle ear are bounded by the tympanic membrane, and a small foramina called the round window and the oval window exist in the boundary between the middle and internal ears. The base of the stapes, an ear ossicle, fits into the oval window, and it propagates vibration from the tympanic membrane to this window (Fig. 18.3c). Vibration transmitted from the middle ear excites the hair cells of the organ of Corti (or spiral organ) in the cochlea, which transmits the stimulus to the cochlear nerve. The semicircular canals monitor equilibrioception: the semicircular canals, which contain endolymph liquid, are located at right angles to one another, thus permitting differentiation of rotational movement in all directions. The nerves in the semicircular canals and saccule combine, forming the superior and inferior vestibular nerves, and enter the medulla oblongata after merging with the cochlear nerve within IAC. The cochlear nerve from the cochlea runs in the anteroinferior portion of IAC, and the superior and inferior vestibular nerves run in the posterior half of IAC (Fig. 18.3c).

Figure 18.4 shows the dry temporal bones after mastoidectomy or unroofing of IAC. CN VII is visible running from the posterior to the middle cranial fossa, then descending toward the stylomastoid foramen inside the petrous bone, running around the external auditory canal (Fig. 18.4). CN VII, which originates from the lower pons, first runs in the anterosuperior part of IAC, coursing in the facial canal toward the middle cranial fossa to form the geniculate ganglion (Fig. 18.4a, b). The parasympathetic branch, originating from the medulla oblongata and coursing together with the nerve bundle of CN VII, divides from the geniculate ganglion and joins the greater petrosal nerve. The nerve courses in the great petrosal sulcus to the facial hiatus (Fig. 18.4b). Gustatory nerve fibers, which originate from nerve cells in the geniculate ganglion, combine to form the nervus intermedius. Its central branch runs in IAC with CN VII to enter the pons. While CN VII descends in the mastoid bone, the peripheral branch divides from CN VII, joins the parasympathetic secreting fibers to form the chorda tympani, and innervates the tongue.