CHAPTER 127 Anatomy of the Temporal Bone, External Ear, and Middle Ear
Osteology of the Temporal Bone
The temporal bone articulates with the sphenoid, parietal, occipital, and zygomatic bones, contributing to the cranial, skull base, and facial structure. 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 neck and infratemporal fossa (anteroinferior 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 squamous part forms the lateral wall of the middle fossa (Fig. 127-1). It consists of a plate of bone with an anterior extension known as the zygomatic process, which forms the bony roof of the glenoid fossa. The temporalis muscle inserts on the outer cortex, whereas the masseter muscle inserts on the zygomatic process. A horizontal ridge known as the temporal line is formed along the most inferior insertion by the temporalis muscle, is aligned with the zygomatic process, and is used as a surface landmark that estimates the location of the middle fossa floor,2 with an average offset of about 4.7 mm.
Figure 127-1. Lateral view of left temporal bone surface, showing squamous, tympanic, and mastoid portions.
(Redrawn from art by Sarah Williamson, provided courtesy of H. W. Francis and J. K. Niparko.)
The mastoid part is a bulbous bony structure shaped by the expansion of air-filled spaces within (see Fig. 127-1). 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 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, which is evident on the posterior medial aspect of the temporal bone. The site of the sternocleidomastoid muscle insertion is indicated by a rough, irregular surface at the mastoid tip. Medial to the mastoid tip, the posterior belly of the digastric muscle is inserted in a sulcus that terminates anteriorly at the stylomastoid foramen. Medial and almost parallel to the digastric sulcus is the sulcus for the occipital artery.
The petrous part has the shape of a pyramid whose base is united with the mastoid laterally; the apex is oriented anteromedially between occipital and sphenoid bones. The anterior surface forms the posteromedial margin of the middle fossa floor (Fig. 127-2). Significant surface features include (1) medially, the arcuate eminence formed by the prominence of the superior semicircular canal, and the sulcus of the superior petrosal sinus; (2) 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 (3) at the apex, a smooth depression occupied by the trigeminal ganglion, just posterior to which is located 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 is located lateral to the arcuate eminence.
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. 127-3). This surface is framed by sulci for the sigmoid sinus, and the superior petrosal and inferior petrosal sinuses. At the center of the posterior face is the porus acusticus at the fundus of which can be seen the falciform (horizontal) crest, Bill’s bar (vertical crest), and foramina of cranial nerves VII and VIII. The subarcuate artery emerges from a depression 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 jugular foramen is formed at the junction between the petrous and occipital bones (Fig. 127-4), and is partitioned into the pars nervosa (posterior) and pars venosa (anterior) by the jugular spine or process.3
The inferior surface of the temporal bone is irregular because of the presence of multiple muscle attachments (Fig. 127-5). The external aperture to the cochlear aqueduct is located just medial and anterior to the jugular spine within the pars nervosa, and marks the superiormost limit of the jugular foramen. The glossopharyngeal nerve enters the jugular foramen adjacent to the opening of the cochlear aqueduct. In the translabyrinthine approach to the internal auditory canal, the cochlear aqueduct is an important inferior limit of dissection used to protect the lower cranial nerves. The cochlear aqueduct eventually opens into the scala tympani at the cochlear base.4–6 The jugular bulb occupies a dome-shaped compartment located lateral to the pars venosa of 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 The styloid process is located anterior to the stylomastoid foramen, and both are located at the anterior limit of the digastric groove.
The tympanic part of the temporal bone forms the anterior wall, 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 (see Fig. 127-1). 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 (see Fig. 127-5).
External Ear
The auricle is a funnel-shaped cartilaginous structure that is continuous with the meatus and EAC (Fig. 127-6). Intricate ridges and depressions formed by the auricular cartilage and cutaneous envelope are shown and labeled in Figure 127-5. The blood supply of the external ear originates from the external division of the carotid artery via the posterior auricular and superficial temporal vessels. The EAC (Fig. 127-7) is about 2.5 cm long and is composed of a lateral cartilaginous (membranous) portion and a medial bony portion.1,7 The membranous portion accounts for the lateral third of the EAC, whereas the bony portion forms the medial two thirds. The skin lining the membranous canal is thicker, more mobile, and endowed with sebaceous and apocrine (ceruminous) glands and hair follicles. Sebaceous and apocrine ducts empty into a follicular canal surrounding each hair follicle.5,7
Figure 127-6. Surface anatomy of the auricle.
(Adapted from Adamson PA, Tropper GJ, McGraw BL. Otoplasty. In: Krause CJ, Mangat DS, Pastorek N, eds. Aesthetic Facial Surgery. Philadelphia: Lippincott; 1991:709.)
The external ear is developed from ectodermal and mesodermal components of the first and second branchial arches and the intervening first branchial groove.8,9 Distinct condensations of tissue, known as the hillocks of His, give rise to the tragus and most of the helix from the first branchial arch, and to the antihelix, antitragus, lobule, and inferior helix from the second branchial arch. Sensory innervation is provided by the corresponding first branchial nerve: the auriculotemporal branch of the trigeminal nerve and a cutaneous branch of the facial nerve. The EAC develops from the dorsal portion of the first branchial cleft, which extends toward and eventually makes contact with endoderm of the expanding tubotympanic recess. There is a transient obstruction of the medial canal by proliferating epithelial cells to form a meatal plug that eventually dissolves, leaving a patent canal.
Errors of embryogenesis may result in impaired hearing or render the ear vulnerable to other conditions. The failed or incomplete involution of the meatal plug results in canal atresia or severe stenosis with conductive hearing loss and the risk of canal cholesteatoma. Auricular anomalies may also result from imperfect development of the hillocks of His with consequences ranging from anotia to the development of preauricular cysts and sinuses by duplication of the first branchial groove and arch.10
