The internal cranial base is a complex, undulating surface divided into three cranial fossae: anterior, middle, and posterior. Through a system of foramina, fissures, and canals, these fossae communicate with the nasal cavity, orbits, inner ear, and external cranial base, transmitting critical nerves and blood vessels.
Three compartmentsThe internal cranial base is divided into the anterior, middle, and posterior cranial fossae.
Foramen lacerumIn a living person, it is closed by cartilage (synchondrosis) and is only open on a macerated skull.
Shared boundariesThe posterior border of any given fossa always serves as the anterior border for the next one.
Anterior Cranial Fossa
Anterior cranial fossa (Fossa cranii anterior) occupies the most anterior part of the internal cranial base.
Boundaries:
Anteriorly: bounded by the frontal squama (squama frontalis). The key landmark here is the origin of the frontal crest (crista frontalis).
Posteriorly: laterally bounded by the lesser wings of the sphenoid bone (Alae minores ossis sphenoidalis), and in the midline by the prechiasmatic sulcus (Sulcus prechiasmaticus).
Communications: This fossa communicates with the nasal cavity via the cribriform foramina (Foramina cribrosa) located on the ethmoid bone, and with the nose/face via the foramen caecum of the frontal bone.
Middle Cranial Fossa
Middle cranial fossa (Fossa cranii media) lies posterior to the anterior fossa and features the most complex system of apertures.
Boundaries:
Anteriorly: mirrors the posterior boundary of the anterior fossa (lesser wings of the sphenoid laterally and prechiasmatic sulcus centrally).
Posteriorly: laterally bounded by the superior border of the petrous part of the temporal bone (Margo superior partis petrosae) or the superior petrosal sulcus (Sulcus sinus petrosi superioris). In the midline, the boundary is the dorsum sellae (Dorsum sellae).
Communications with other spaces:
With the orbit: via the optic canal (Canalis opticus) and superior orbital fissure (Fissura orbitalis superior).
With the pterygopalatine fossa: via the foramen rotundum (Foramen rotundum).
With the external cranial base: via the foramen ovale (Foramen ovale), foramen spinosum (Foramen spinosum), carotid canal (Canalis caroticus), and foramen lacerum (Foramen lacerum). Note: in the living skull, the foramen lacerum is filled by the sphenopetrosal synchondrosis (Fissura sphenopetrosa) and appears as a true gap only on dried (macerated) bone preparations.
Additional pathways: via the hiatuses for the greater and lesser petrosal nerves (Hiatus canalis nervi petrosi majoris/minoris), the fossa connects to the facial canal and tympanic canaliculus, respectively.
Posterior Cranial Fossa
Posterior cranial fossa (Fossa cranii posterior) is the deepest and most posterior compartment, formed primarily by the occipital and temporal bones.
Boundaries:
Anteriorly: corresponds to the posterior boundary of the middle fossa (dorsum sellae centrally, superior border of the petrous temporal bone laterally).
Posteriorly: in the midline by the cruciform eminence (Eminentia cruciformis), corresponding to the internal occipital protuberance (Protuberantia occipitalis interna). Laterally, the boundary runs along the transverse sinus sulcus (Sulcus sinus transversi).
Communications with the external cranial base and inner ear:
Within the occipital bone: houses the foramen magnum (Foramen magnum), condylar canal (Canalis condylaris), and hypoglossal canal (Canalis nervi hypoglossi).
Within the temporal bone: contains the mastoid foramen (Foramen mastoideum) and internal acoustic meatus (Meatus acusticus internus), which leads into the facial canal.
Between the occipital and temporal bones: features the jugular foramen (Foramen jugulare) and the petro-occipital fissure (Fissura petrooccipitalis), which is prominent near the inferior petrosal sulcus (Sulcus sinus petrosi inferioris) on a macerated skull.
With the inner ear cavities: communicates via the aperture of the vestibular canaliculus (Apertura canaliculi vestibuli) and apertures of the cochlear canaliculus (Aperturae canaliculi cochleae).
Mnemonic
Think of the fossae boundaries like falling dominoes: the posterior border of the anterior fossa is precisely the anterior border of the middle fossa, and the posterior border of the middle fossa serves as the anterior border for the posterior fossa.
Frequently asked questions
What passes through the superior orbital fissure?
The superior orbital fissure transmits cranial nerves and venous structures from the middle cranial fossa into the orbit. These include the ophthalmic nerve (CN V1), oculomotor nerve (CN III), trochlear nerve (CN IV), and abducens nerve (CN VI). It also transmits the superior ophthalmic vein and sometimes anterior meningeal branches from the ophthalmic artery.
What passes through the foramen rotundum?
The foramen rotundum provides a communication pathway from the middle cranial fossa to the pterygopalatine fossa, transmitting the maxillary nerve (CN V2), the second division of the trigeminal nerve.
What passes through the jugular foramen?
Located in the posterior cranial fossa between the occipital and temporal bones, the jugular foramen transmits critical neurovascular structures. These include the glossopharyngeal nerve (CN IX), vagus nerve (CN X), and accessory nerve (CN XI), as well as the sigmoid sinus transitioning into the internal jugular vein. Additionally, the posterior meningeal artery enters the cranial cavity here.
What passes through the foramen spinosum?
The middle meningeal artery, a branch of the maxillary artery, passes through the foramen spinosum from the middle cranial fossa to the external cranial base. A meningeal branch of the mandibular nerve and middle meningeal veins also pass through.
What passes through the internal acoustic meatus?
The internal acoustic meatus of the posterior cranial fossa transmits cranial nerves and vessels of the inner ear, specifically the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), intermediate nerve, and the labyrinthine artery and veins.
What passes through the optic canal?
The optic canal connects the middle cranial fossa to the orbit, transmitting the optic nerve (CN II), the ophthalmic artery (a branch of the internal carotid artery), and the central retinal artery.
What structure forms the posterior boundary of the middle cranial fossa in the midline?
The dorsum sellae, which belongs to the body of the sphenoid bone.
How does the middle cranial fossa communicate with the pterygopalatine fossa?
Solely through a single opening — the foramen rotundum.
Why is the foramen lacerum not an open foramen in a living person?
Because in life this area is filled with a permanent synchondrosis (cartilaginous tissue) forming the sphenopetrosal fissure. The open hole is visible only on dried skeletal preparations.
Through which structures does the posterior cranial fossa connect with the inner ear?
Connection to the inner ear is established via the aperture of the vestibular canaliculus and the apertures of the cochlear canaliculus.
Go deeper
Detailed topography of the facial nerve canal and its branches
Skull base synchondroses and their age-related changes
Contents of the jugular foramen and foramen magnum
Topography of the optic canal and superior orbital fissure