2 Anatomy of the Temporal Bone

■ Surface Anatomy

The temporal bone is fused to the sphenoid, parietal, occipital, and zygomatic bones, and therefore contributes to the cranial, skull base, and facial structure (Fig. 2.1). The temporal bone has a pyramidal shape, the sides of which form the middle fossa floor (superior face), the anterior limit of the posterior fossa (posterior face), muscle attachments of the neck and infratemporal fossa (anterior-inferior face), and the muscular-cutaneous-covered side of the head (lateral), which forms the base of the pyramid. The temporal bone consists of four embryologically distinct components: the squamous, mastoid, petrous, and tympanic parts.1

The petrous part has the shape of a pyramid whose base is united with the mastoid laterally; the apex is oriented anteromedially between the occipital and sphenoid bones (Fig. 2.1). The jugular foramen is formed at the junction between the petrous and occipital bones (at the clivus) and is partitioned into the pars nervosa (posterior) and pars venosa (anterior) by the jugular spine.2

The squamous part (temporal squama) forms the lateral wall of the middle fossa (Fig. 2.1, Fig. 2.2, and Fig. 2.3). It consists of inner and outer cortical plates with intervening diploë and an anterior extension known as the zygomatic process, which forms the bony roof of the glenoid fossa. A horizontal ridge, the temporal line, is formed along the most inferior insertion by the temporalis muscle and is aligned with the zygomatic process. The temporal line provides a first approximation for the location of the middle fossa floor,3,4 which on average is positioned ~ 4.7 mm inferiorly.

The mastoid part is a bulbous bony structure shaped by the expansion of air-filled spaces within (Fig. 2.2). The constant pull by the sternocleidomastoid muscle and posterior belly of the digastric muscle elongates the mastoid inferiorly to form the mastoid tip or process. The mastoid cortex is perforated by multiple small emissary vessels that drain from the central air cell or antrum, forming a triangular (Macewen’s triangle) and depressed cribriform area at the anterior junction of the mastoid process with the tympanic bone. The foramen of a single emissary vein is evident near the posterior limit of the outer mastoid cortex and communicates with the sulcus of the sigmoid sinus.

The tympanic part of the temporal bone forms the anterior wall and floor and part of the posterior wall and roof of the bony external auditory canal (EAC), and the anterior wall and floor of the middle ear (Fig. 2.2). The anterior edge of this open ring forms the tympanosquamous suture line within the EAC, and the petrotympanic suture line within the middle ear, through which the chorda tympani exits the ear. The posterior edge of the tympanic ring forms the tympanomastoid suture line, which curves from the posterior EAC inferiorly to within millimeters of the stylomastoid foramen, serving as a landmark for the main trunk of the facial nerve as it exits the temporal bone (Fig. 2.4).

The posterior surface of the petrous part of the temporal bone is oriented in the vertical plane forming the anterior bony limit of the posterior fossa (Fig. 2.3). This surface is framed by sulci for the sigmoid, superior petrosal, and inferior petrosal sinuses (along the petroclival junction). At the center of the posterior face is the porus acusticus or internal auditory meatus. At the most lateral extent or fundus of the internal auditory canal (IAC) can be seen the falciform (horizontal) crest, Bill’s bar (vertical crest), and foraminae of the cochlear nerve fibers (a), the inferior vestibular nerve (b), the superior vestibular nerve (c), and the facial nerve (d). The subarcuate artery emerges from a fossa of the same name located superior and lateral to the acoustic meatus, whereas the endolymphatic sac and duct occupy the depression and opening located inferolaterally, known as the operculum.

The inferior surface of the temporal bone is irregular due to the presence of multiple muscle attachments (Fig. 2.4). Medial to the mastoid tip, the posterior belly of the digastric muscle is inserted in a sulcus that terminates anteriorly at the stylomas toid foramen. The styloid process is located anterior to the stylomastoid foramen, and both are located at the anterior limit in line with the digastric groove. Medial and almost parallel to the digastric sulcus is the sulcus for the occipital artery. The jugular bulb occupies a dome-shaped fossa located lateral to the jugular foramen directly under the middle ear space. The inferior foramen of the carotid canal is located directly anterior to the jugular bulb depression, from which it is separated by a wedge-shaped bone called the keel. The tympanic canaliculus penetrates the keel to transmit sensory and preganglionic parasympathetic fibers from the inferior ganglion of the glossopharyngeal nerve into the middle ear as Jacobson’s nerve.1,5,6 The external aperture to the cochlear aqueduct is located just medial and anterior to the jugular spine within the pars nervosa and marks the most superior limit of the jugular foramen and the site of entry of the glossopharyngeal nerve. The pars venosa located posterior to the jugular spine is occupied by the sigmoid sinus. Along with the inferior petrosal sinus, cranial nerves IX to XI enter the anterior pars nervosa and project inferolaterally along the anterior wall of the jugular bulb within a dense fibrous sheath anchored to the jugular spine.6 The cochlear aqueduct eventually opens into the scala tympani at the cochlear base.5,7,8 In the translabyrinthine approach to the IAC, the cochlear aqueduct is an important inferior limit of dissection used to avoid injury of the lower cranial nerves.9

The anterior surface forms the posteromedial margin of the middle fossa floor (Fig. 2.5). Prominent surface features include medially, the arcuate eminence formed by the prominence of the superior semicircular canal (SCC), and the sulcus of the superior petrosal sinus; anteriorly at the junction with the greater sphenoid wing, the musculotubal canal containing the more superficial semicanal of the tensor tympani and a deeper semicanal of the eustachian tube; and at the apex, a smooth depression occupied by the trigeminal ganglion, just posterior to which are located the foramina and sulci of the greater and lesser superficial petrosal nerves, running parallel to the sphenoid suture line. The roof of the middle ear and mastoid extends lateral to the arcuate eminence.

■ Internal Anatomy

A review of temporal bone sections promotes a better understanding of the complex anatomical relationships within the ear. Nine horizontal (axial) sections from the same ear are presented in Fig. 2.6, Fig. 2.7, and Fig. 2.8. Although not perfectly orthogonal to the horizontal series, three vertical sections from a different donor provide additional perspectives to these three-dimensional relationships. For additional details on these anatomical relationships as demonstrated in temporal bone sections, publications by Nager,7 Schuknecht,5,8 and Merchant and Nadol10 are recommended.

Fig. 2.6 shows three horizontal sections (A1–3) through the superior aspects of the labyrinth, IAC, middle ear, and mastoid regions. The facial nerve is located within the anterior half of the IAC in panel A3 and then occupies the narrow labyrinthine segment of the fallopian canal (k, A2 and C382) that terminates at the geniculate ganglion (h, A1 and C382). The greater superficial petrosal nerve (i, A1) is projected anteriorly, whereas the facial nerve makes an acute turn posteriorly and inferior at the first genu, becoming the tympanic or horizontal segment (j, A2 and A3).

The superior SCC forms the arcuate eminence on the middle fossa floor (ae, C562), under which the subarcuate artery runs (saa, A2 and C562) from the mastoid to the subarcuate fossa (Fig. 2.3). The ampulla of the superior SCC is an important surgical landmark for the labyrinthine segment of the facial nerve, occurring just posterior to its exit from the IAC at the meatal foramen (see A2 and compare C382/562). The geniculate ganglion is located immediately superior to the cochlea. Ampulae of the superior and lateral SCCs (l, m in A2) are adjacent to the macula of the utricle (A3), and all are...