Temporal bone

  • The temporal bone (latin: os temporale) is a paired bone situated at the lateral side and base of the skull. Each temporal bone consists of four parts: petrous, mastoid, tympanic, and squamous parts. The temporal bone features important structures of the vestibulocochlear apparatus, including the external acoustic meatus, the tympanic cavity and the structures of the inner ear.

    Petrous part of temporal bone

    The petrous part (also called the pyramid) is the part of the temporal bone which houses the inner ear. It is located in the base of the skull between the sphenoid and occipital bones. The petrous part of the temporal bone is pyramid shaped, it has an apex and three surfaces (anterior, posterior, inferior) and three margins (anterior, superior, posterior).

    Apex of petrous part of temporal bone

    The apex of the petrous part of the temporal bone features the anterior or internal opening of the carotid canal, and forms the posterolateral margin of the foramen lacerum.

    The carotid canal is a bony passage for the internal carotid artery located in the temporal bone through which the artery reaches the middle cranial fossa from the neck region.

    The foramen lacerum is a triangular-shaped hole in the base of the skull, located between the sphenoid bone, the apex of the petrous part of the temporal bone, and the basilar part of the occipital bone. The foramen lacerum fills with cartilage after birth.

    Surfaces of petrous part of temporal bone

    The anterior surface of the petrous part of the temporal bone presents the following features:

    • tegmen tympani,
    • arcuate eminence,
    • hiatus for greater petrosal nerve,
    • hiatus for lesser petrosal nerve,
    • trigeminal impression

    The tegmen tympani is the roof of the tympanic cavity, a thin bony plate found on the pyramid of the temporal bone anterolateral to the arcuate eminence.

    The arcuate eminence is an elevation on the anterior surface of the petrous part of the temporal bone produced by the underlying anterior semicircular canal.

    The hiatus for the greater petrosal nerve is a shallow groove in the petrous part of the temporal bone leading to an oblique opening for the passage of the greater petrosal nerve and the petrosal branch of the middle meningeal artery. It connects the facial canal to the middle cranial fossa.

    The hiatus for the lesser petrosal nerve is an opening in the petrous part of the temporal bone which serves as a passage for the lesser petrosal nerve as it separates from the glossopharyngeal nerve (CN IX). It connects the posterior cranial fossa to the external surface of the cranium.

    The trigeminal impression is a shallow, oval depression on the anterior surface of the petrous part of the temporal bone produced by pressure from the trigeminal ganglion located here.

    The posterior surface of the petrous part of the temporal bone presents two openings:

    • internal acoustic opening (leads into the internal acoustic meatus),
    • external opening of vestibular aqueduct.

    The internal acoustic opening is a small opening leading to the internal acoustic meatus, located inside the posterior cranial fossa of the skull and runs laterally into the temporal bone.

    The internal acoustic meatus (or internal auditory canal) is a bony canal located in the petrous part of the temporal bone. The contents of the internal acoustic meatus include the facial nerve (CN VII), the vestibulocochlear nerve (CN VIII), the vestibular ganglion and the labyrinthine artery.

    There are five nerves that run through the internal auditory canal: the intermediate nerve (sensory component of CN VII), facial motor root (motor component of CN VII), cochlear nerve (component of CN VIII), inferior vestibular nerve (component of CN VIII) and the superior vestibular nerve (component of CN VIII). A horizontal ridge called the falciform crescent, divides the internal auditory canal into superior and inferior portions. The facial nerve and the superior vestibular nerve go in the superior portion of the canal. The cochlear nerve and inferior vestibular nerve run inferior to the falciform crescent with the cochlear nerve situated more anteriorly.

    The vestibular aqueduct is a narrow canal extending from the endolymphatic space of the internal ear to the posterior wall of the petrous part of the temporal bone, presenting with an external opening on the posterior surface of the petrous part.

    The inferior surface of the petrous part of the temporal bone features:

    • styloid process,
    • stylomastoid foramen (external opening of the facial canal),
    • jugular fossa,
    • petrosal fossula, featuring
      • tympanic canaliculus,
    • carotid canal,
    • musculotubal canal, divided into
      • semicanal for tensor tympani muscle
      • semicanal for auditory tube

    The styloid process is a long process extending from the inferior surface of the petrous part of the temporal bone located in front of the jugular fossa.

    The stylomastoid foramen is a round opening located on the external surface of the skull on the inferior surface of the petrous part of the temporal bone between the styloid and the mastoid processes. It is the external opening of the facial canal and transmits the facial nerve (CN VII) and the stylomastoid artery.

    The facial canal is a bony canal running through the temporal bone containing the facial nerve (CN VII), and is the longest canal (around 3 cm) for a nerve in the human body. The inner opening of the canal is located in the internal acoustic meatus, while the outer opening is the stylomastoid foramen presenting on the external base of the skull. There is also a small canal arising from the facial canal within the temporal bone called the canaliculus for chorda tympani.

    The canaliculus for chorda tympani is a small canal in the temporal bone, which starts at the descending part of the facial canal and ends at the tympanic cavity, serving as a passage for chorda tympani into the tympanic cavity.

    The jugular fossa is a deep depression on the external base of the skull, specifically, on the inferior surface of the temporal bone posterior to the carotid canal. The jugular fossa houses the superior bulb of the internal jugular vein.

    The petrosal fossula is a slight depression in the bony ridge between the carotid canal and the jugular fossa (on the inferior surface of the petrous part of the temporal bone), and it is occupied by the otic ganglion. The petrosal fossula presents the opening of the tympanic canaliculus.

    The tympanic canaliculus is presented as a small opening located on the inferior surface of the petrous part of the temporal bone near the fossula petrosa serving as a passage of the tympanic nerve and the inferior tympanic artery.

    The musculotubal canal is a double canal within the petrous part of the temporal bone for the auditory tube and the tensor tympani muscle located in front of the carotid canal and leading into the tympanic cavity. It is divided into two divisions: semicanal for the tensor tympani muscle and semicanal for the auditory tube.

    The semicanal for tensor tympani muscle is the superior division of the musculotubal canal for the tensor tympani muscle.

    The semicanal for auditory tube is the inferior division of the musculotubal canal that forms the bony part of the auditory (pharyngotympanic) tube.

    Margins of petrous part of temporal bone

    The anterior margin (or anterior angle) of the petrous part of the temporal bone can be divided into two parts: the lateral part joins the squamous part of the temporal bone with a suture - the petrosquamous suture, while the medial part is free and articulates with the spinous process of the sphenoid bone.

    The superior margin (or superior angle) of the petrous part presents the groove for the superior petrosal sinus, and is the attachment site for the tentorium cerebelli.

    The groove for superior petrosal sinus is grooved into the superior border of the petrous part of the temporal bone accommodating the superior petrosal sinus.

    The posterior margin (or posterior angle) of the petrous part of the temporal bone presents the following features:

    • groove for inferior petrosal sinus,
    • jugular notch, with
      • external opening of cochlear canaliculus

    The groove for inferior petrosal sinus is formed by the junction of the petrous part of the temporal bone with the basilar part of the occipital bone.

    The jugular notch is an indentation on the posterior margin of the petrous part of the temporal bone forming the anterior margin of the jugular foramen. The jugular notch also features the external opening of the cochlear canaliculus.

    The cochlear canaliculus is a small bony canal in the temporal bone for the cochlear aqueduct.

    Other structures of petrous part

    The petrous part of the temporal bone also houses the tympanic cavity and the internal ear.

    The tympanic cavity is a narrow, irregular space located in the petrosal part of the temporal bone and situated between the external and internal ear.

    The internal ear is an essential part of the auditory and vestibular system, where sound waves are converted into electrical impulses. It is located in the temporal bone and consists of the bony labyrinth (vestibule, semicircular canals and the cochlea) and the membranous labyrinth, which lies within the bony labyrinth and forms small sacs and tubules.

    Note that there are several important canals located within the petrous part of the temporal bone described earlier, and these are:

    • Carotid canal
    • Facial canal
    • Canaliculus for chorda tympani
    • Tympanic canaliculus
    • Musculotubal canal
    • Cochlear canaliculus
    • Vestibular aqueduct
    • Internal acoustic meatus

    Squamous part of temporal bone

    The squamous part of the temporal bone is the largest and most superiorly situated part of the temporal bone. The squamous part has two surfaces: temporal (outer) and cerebral (inner) surface.

    The temporal surface of the squamous part of the temporal bone contributes to forming the temporal fossa, and features the following structures:

    • zygomatic process,
    • mandibular fossa,
    • articular tubercle.

    The zygomatic process of the temporal bone is an extension of the temporal bone that contributes to the formation of the zygomatic arch. It extends from the temporal surface of the squamous part of the temporal bone. Note, that any zygomatic process is the process of a bone which articulates with the zygomatic bone.

    The mandibular fossa is a depression for the head of the mandible located on the temporal surface of the squamous part of the temporal bone.

    The articular tubercle is a cylindrical elevation on the temporal surface of the squamous part of the temporal bone located in front of the mandibular fossa.

    The cerebral surface of the squamous part of the temporal bone features the typical landmarks of the inner surface of the skull:

    • impressions of cerebral gyri,
    • arterial grooves.

    The impressions of cerebral gyri (also called the digital impressions) are flat indentations on the inner surface of the skull corresponding to the cerebral gyri which produce them.

    The arterial grooves are located on the inner surface of the skull produced by pressure from arteries, primarily the middle meningeal artery and its branches.

    Mastoid part of temporal bone

    The mastoid part of the temporal bone is situated posteroinferior to the squamous part, and presents the following landmarks:

    • mastoid process, containing
      • mastoid cells,
      • mastoid antrum,
    • mastoid notch,
    • groove for occipital artery,
    • mastoid foramen,
    • groove for sigmoid sinus.

    The mastoid process is an extension of the temporal bone located just posterior to the external acoustic opening.

    The mastoid cells are a number of air-filled spaces within the mastoid part of the temporal bone, variable in size and number. They become pneumatized during the first year of life.

    The mastoid antrum is a closed space within the mastoid part of the temporal bone, with its posterosuperior part continuous with the tympanic cavity, and the inferior part communicating with the mastoid cells.

    The mastoid notch is a medial notch on the inferior surface of the mastoid process. It is the site of origin to the posterior belly of the digastric muscle.

    The groove for the occipital artery is a sulcus located medial to the mastoid notch on the mastoid part of the temporal bone.

    The mastoid foramen is an opening behind the mastoid process on the mastoid part of the temporal bone for additional venous drainage from the cranial cavity.

    The groove for the sigmoid sinus is located in the posterior cranial fossa, found on the lateral part of the occipital bone, then curving around the jugular process on the mastoid part of the temporal bone, finally, turning sharply on the inner surface of the parietal bone, continuing as the transverse groove.

    Tympanic part of temporal bone

    The tympanic part of the temporal bone is a relatively small part situated inferior to the squamous part, anterior to the mastoid part, and superior to the styloid process. The tympanic part presents a gap called the petrotympanic fissure.

    The petrotympanic fissure is a gap in the tympanic part of the temporal bone which stretches from the temporomandibular joint to the tympanic cavity, serving as passage for the facial nerve (CN VII) to the infratemporal fossa, as well as chorda tympani running through and joining the lingual nerve.

    External acoustic meatus

    There is also an important opening presented on the outer surface of the temporal bone between its parts called the external acoustic opening (also external auditory opening), that leads into the external acoustic meatus.

    External acoustic meatus (or external auditory meatus) is an air-filled tubular space in the temporal bone extending from the auricle of the external ear. It conducts sounds to the tympanic membrane.