Petrous part (pyramid)
The petrous part, or pyramid (Pars petrosa), is a massive bone element. Its apex (Apex partis petrosae) is directed forward and medially, while its base faces posteriorly and laterally, smoothly transitioning into the robust mastoid process (Processus mastoideus). This process is well-developed, deflected backward and outward, and is bounded by the occipital margin (Margo occipitalis).
On the mastoid process, one can find the mastoid notch (Incisura mastoidea), as well as the grooves for the occipital artery (Sulcus arteriae occipitalis) and the sigmoid sinus (Sulcus sinus sigmoidei). The groove for the sigmoid sinus communicates with the external surface of the cranium via the mastoid foramen (Foramen mastoideum). A crucial feature of the petrous part is that it encloses the tympanic cavity (Cavitas tympani), which belongs to the middle ear.
Surfaces of the pyramid
The pyramid has three distinct surfaces, each featuring complex relief:
- Anterior surface bears the trigeminal impression (Impressio trigeminalis) and the arcuate eminence (Eminentia arcuata). The tegmen tympani (Tegmen tympani) is also located here, along with the groove for the superior petrosal sinus.
- Posterior surface is notable for the internal acoustic opening (Porus acusticus internus), which leads into the internal acoustic meatus (Meatus acusticus internus). Nearby are the subarcuate fossa (Fossa subarcuata) and the aperture of the vestibular canaliculus (Apertura canaliculi vestibuli), which continues as the vestibular canaliculus (Canaliculus vestibuli). The groove for the inferior petrosal sinus (Sulcus sinus petrosi inferioris) is also localized on this surface.
- Inferior surface includes the jugular notch (Incisura jugularis) with the intrajugular process (Processus intrajugularis) and the adjacent jugular fossa (Fossa jugularis). The aperture of the cochlear canaliculus (Apertura canaliculi cochleae), leading into the cochlear canaliculus, opens here as well. The slender styloid process (Processus styloideus) projects in the anterior region. Between it and the mastoid process lies the stylomastoid foramen (Foramen stylomastoideum). The external aperture of the carotid canal (Apertura externa canalis carotici) is also located on the inferior surface.
Tympanic part
The tympanic part (Pars tympanica) forms the bony walls of the external acoustic meatus (Meatus acusticus externus). This meatus begins with the external acoustic opening (Porus acusticus externus), which is peripherally surrounded by the tympanic ring (Anulus tympanicus).
The ring itself features the major and minor tympanic spines (Spina tympanica major et minor). Along the border separating this part from the tympanic cavity runs the tympanic sulcus (Sulcus tympanicus), while the tympanic notch (Incisura tympanica) is localized at the upper margin.
Squamous part
The squamous part (Pars squamosa) consists of the temporal squama (Squama temporalis). It has two surfaces: the temporal surface (Facies temporalis) and the cerebral surface (Facies cerebralis), which are bounded by the sphenoidal and parietal margins (Margo sphenoidalis and Margo parietalis). The squama is separated from the mastoid process by the parietal notch (Incisura parietalis). The groove for the middle temporal artery (Sulcus arteriae temporalis mediae) is visible on the temporal surface.
The zygomatic process (Processus zygomaticus) projects from the squama and takes part in the formation of the zygomatic arch. In the region of this process are the supramastoid crest (Crista supramastoidea), the suprameatal foveola (Foveola suprameatica), and the suprameatal spine (Spina suprameatica). The inferior part of the process features the mandibular fossa (Fossa mandibularis), provided with an articular surface (Facies articularis), anterior to which projects the articular tubercle (Tuberculum articulare).
Briefly: answers to topical questions
What passes through the internal acoustic meatus?
The facial and intermediate nerves pass through the internal acoustic meatus. The canal for the facial nerve begins at the fundus of the meatus, where the facial nerve forms the sensory geniculate ganglion; the intermediate nerve accompanies the facial nerve.
Which canals run within the petrous part of the temporal bone?
Seven canals run within the petrous part of the temporal bone: three major and four minor (canaliculi). These include:
- Carotid canal (Canalis caroticus) — contains the internal carotid artery.
- Musculotubal canal (Canalis musculotubarius).
- Facial canal (Canalis nervi facialis).
- Caroticotympanic canaliculi (Canaliculi caroticotympanici).
- Tympanic canaliculus (Canaliculus tympanicus).
- Canaliculus for the chorda tympani (Canaliculus chordae tympani).
- Mastoid canaliculus (Canaliculus mastoideus).
Other passages (such as the vestibular and cochlear aqueducts) are not classified as canals.
Which nerve exits the cranial cavity through the stylomastoid foramen?
The facial nerve exits the cranial cavity through the stylomastoid foramen. This foramen is the terminal segment of the facial canal, located on the inferior surface of the pyramid of the temporal bone.
- Facial nerve (n. facialis) — leaves the skull and gives off branches before entering the parotid gland: the posterior auricular nerve, digastric, and stylohyoid branches.
Which cranial bones does the temporal bone articulate with?
The temporal bone articulates with the parietal bone via the squamous suture (sutura squamosa), with the occipital bone via the petro-occipital fissure (fissura petrooccipitalis), and forms the temporomandibular joint with the mandible. The zygomatic process of the temporal bone participates in forming the zygomatic arch.
What structures bound the mandibular fossa of the temporal bone?
The mandibular fossa is bounded by several bony structures of the temporal bone. Its anterior and posterior boundaries are formed by:
- The base of the zygomatic process and the articular tubercle (tuberculum articulare) — bound the fossa anteriorly.
- The external acoustic opening — bounds the fossa posteriorly.
The fossa itself is divided approximately in half by the petrotympanic fissure into anterior (intra-articular) and posterior (extra-articular) parts.
Class and exam questions
What parts does the temporal bone consist of?
It includes three elements: the petrous (Pars petrosa), tympanic (Pars tympanica), and squamous (Pars squamosa) parts.
Where is the tympanic cavity located?
The tympanic cavity of the middle ear is located inside the pyramid — the petrous part of the temporal bone.
Which foramen is located between the styloid and mastoid processes?
Between these two processes on the inferior surface of the pyramid lies the stylomastoid foramen (Foramen stylomastoideum).
What structures form the external acoustic meatus?
Its bony walls are formed by the tympanic part of the temporal bone, and it begins with the external acoustic opening.
What else to study on this topic
- Detailed structure of the inner ear within the substance of the pyramid
- Walls and contents of the tympanic cavity (Cavitas tympani)
- Complete course of the carotid canal and its internal apertures
- Topography of cranial nerves in the internal acoustic meatus
- Ligaments and articular capsule of the temporomandibular joint
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