Cranial Fossae: Internal Skull Base and Its Foramina

By Dr Richard Miller, MBChB FRCS · Reviewed 1 October 2026

The cranial fossae are the three stepped hollows of the internal skull base that hold the brain: anterior for the frontal lobes, middle for the temporal lobes and pituitary, and posterior for the cerebellum and brainstem. Their floors are pierced by the canals, fissures and foramina through which the cranial nerves and major vessels pass.

Cranial Fossae · key facts

Boundaries: Anterior ends at lesser wings of sphenoid; middle at petrous ridges and dorsum sellae; posterior at occipital squama
Roof: Open to the cranial cavity; tentorium cerebelli roofs the posterior fossa
Floor: Frontal, ethmoid, sphenoid, temporal and occipital bones
Contents: Frontal and temporal lobes, pituitary, cerebellum, pons, medulla; cranial nerves I to XII; dural venous sinuses
Clinical relevance: Basal skull fractures, CSF leak, pituitary tumours, vestibular schwannoma, coning at the foramen magnum

Boundaries and floor of each fossa

The three cranial fossae descend like steps from front to back, each lower than the one before.

Fossa	Floor	Holds	Posterior limit
Anterior	Orbital plates of frontal bone, cribriform plate of ethmoid, lesser wings and front of body of sphenoid	Frontal lobes, olfactory bulbs	Posterior borders of lesser wings, anterior clinoid processes, limbus of sphenoid
Middle	Body and greater wings of sphenoid, squamous and petrous temporal bone	Temporal lobes, pituitary gland	Superior border of petrous temporal bone, dorsum sellae
Posterior	Mainly occipital bone, with the petrous and mastoid parts of the temporal bone	Cerebellum, pons, medulla	Occipital squama; roofed by the tentorium cerebelli

Anterior and middle fossa landmarks

The anterior fossa lies over the orbits and nasal cavity, so its floor is thin.

Frontal crest: a midline ridge on the frontal bone that anchors the front of the falx cerebri.
Foramen caecum: a small pit at the lower end of the frontal crest, in front of the crista galli, sometimes carrying an emissary vein from the nose to the superior sagittal sinus.
Crista galli: the upward spur of the ethmoid, also attaching the falx cerebri.
Cribriform plate: the sieve-like plate either side of the crista galli. Olfactory nerve filaments pass up through it, and the anterior ethmoidal nerve and vessels also cross it.

The middle fossa is butterfly-shaped, with the sella turcica in the centre and a deep hollow on each side for a temporal lobe.

Sella turcica: the tuberculum sellae in front, the pituitary (hypophyseal) fossa in the middle and the dorsum sellae behind.
Anterior clinoid processes: the back ends of the lesser wings. The internal carotid artery lies just medial to each as it leaves the cavernous sinus.
Tegmen tympani: a thin plate of petrous temporal bone forming the roof of the middle ear and mastoid antrum.

Opening	Transmits
Optic canal	Optic nerve, ophthalmic artery
Superior orbital fissure	Oculomotor, trochlear and abducens nerves, branches of V1, superior ophthalmic vein
Foramen rotundum	Maxillary nerve (V2)
Foramen ovale	Mandibular nerve (V3), lesser petrosal nerve, accessory meningeal artery, emissary vein
Foramen spinosum	Middle meningeal artery and vein, meningeal branch of V3
Foramen lacerum	Closed by cartilage in life; the internal carotid artery crosses its upper end

Posterior fossa landmarks

The posterior fossa is the largest and deepest fossa, holding the hindbrain.

Landmark	Transmits or supports
Clivus	The sloping surface from the dorsum sellae to the foramen magnum, formed by the sphenoid and basilar occiput; supports the pons and medulla, with the basilar artery on it
Internal acoustic meatus	Facial and vestibulocochlear nerves, labyrinthine artery
Jugular foramen	Glossopharyngeal, vagus and accessory nerves; sigmoid sinus becoming the internal jugular vein; inferior petrosal sinus
Hypoglossal canal	Hypoglossal nerve
Foramen magnum	Junction of medulla and spinal cord with its meninges, vertebral arteries, spinal accessory nerves, spinal arteries

Grooves for the transverse and sigmoid sinuses curve round the fossa from the internal occipital protuberance to the jugular foramen.

Clinical relevance

Fractures and tumours of the skull base produce signs that point to the fossa involved.

Anterior fossa fracture: CSF rhinorrhoea, loss of smell and periorbital bruising (panda or raccoon eyes).
Middle fossa fracture: CSF otorrhoea, blood behind the eardrum, bruising over the mastoid (Battle's sign), and facial or vestibulocochlear nerve palsy.
Pituitary tumour: expands up from the sella to press on the optic chiasm, giving bitemporal hemianopia.
Vestibular schwannoma: grows from the internal acoustic meatus into the cerebellopontine angle, causing one-sided deafness and tinnitus.
Jugular foramen syndrome: a lesion here paralyses IX, X and XI together.
Coning: raised intracranial pressure forces the cerebellar tonsils through the foramen magnum and compresses the medulla.

On the specimen

The specimen is the inside of the skull base with the vault removed, viewed from above.

Foramen rotundum, ovale and spinosum: they run in a curve from front to back. Rotundum is forward-facing and round, below the superior orbital fissure; ovale is larger and oval; spinosum is the small round hole just posterolateral to ovale.
Foramen lacerum: a ragged gap medial to the ovale, at the tip of the petrous bone beside the sella.
Optic canal or superior orbital fissure: the optic canal is a round tunnel medial to the anterior clinoid process; the fissure is a slit below and lateral to it between the sphenoid wings.
Internal acoustic meatus or jugular foramen: the meatus opens on the back of the petrous bone, above; the jugular foramen is lower, at the end of the sigmoid groove.
Hypoglossal canal: just above the anterolateral margin of the foramen magnum.
Frontal crest or crista galli: the crest is on the frontal bone in front of the foramen caecum; the crista galli is behind it, on the ethmoid.

How it is examined

On a cadaveric spotter, in an OSPE and in MRCS Part B anatomy.

A pin in the round hole below the superior orbital fissure is the foramen rotundum (V2); the oval hole behind it is the foramen ovale (V3).
A pin in the opening on the back of the petrous bone is the internal acoustic meatus, not the jugular foramen, which lies lower at the end of the sigmoid groove.
Expect to be asked what passes through each foramen; the jugular foramen (IX, X, XI and the internal jugular vein) and the foramen magnum are favourites.
A pin on the thin plate over the middle ear is the tegmen tympani; the follow-up is how middle ear infection can spread to the temporal lobe.
Know which fossa each fracture pattern belongs to, and the signs of each.

Key points

The three cranial fossae step down from front to back: anterior, middle, posterior.
The cribriform plate carries the olfactory nerves; its fracture causes CSF rhinorrhoea and anosmia.
The middle fossa holds the pituitary in the sella turcica and the temporal lobes on each side.
V1, V2 and V3 leave through the superior orbital fissure, foramen rotundum and foramen ovale.
The internal acoustic meatus carries VII and VIII; the jugular foramen carries IX, X and XI.
The hypoglossal canal carries XII; the foramen magnum carries the medulla and vertebral arteries.

On the Dissectr specimen

Skull Vault: 24 labelled structures

Anterior cranial fossa
Middle cranial fossa
Posterior cranial fossa
Frontal crest
Foramen caecum
Crista galli
Cribriform plate
Optic canal
Anterior clinoid process
Superior orbital fissure
Foramen rotundum
Foramen ovale
Foramen spinosum
Foramen lacerum
Internal acoustic meatus
Jugular foramen
Foramen magnum
Hypoglossal canal
Tegmen tympani
Pituitary fossa
Dorsum sellae
Clivus
Lesser wing of sphenoid
Greater wing of sphenoid

Every one of these is a numbered marker in the timed spot test on this real prosection. Test yourself on it

Common questions

What passes through the jugular foramen?

The jugular foramen transmits the glossopharyngeal, vagus and accessory nerves, the sigmoid sinus as it becomes the internal jugular vein, and the inferior petrosal sinus. It lies between the petrous temporal bone and the occipital bone in the posterior cranial fossa. A tumour or fracture here can paralyse all three nerves together, a jugular foramen syndrome.

What are the three cranial fossae?

The anterior cranial fossa holds the frontal lobes and lies over the orbits and nasal cavity. The middle cranial fossa holds the temporal lobes and, centrally, the pituitary gland in the sella turcica. The posterior cranial fossa is the largest and deepest, holding the cerebellum, pons and medulla under the tentorium cerebelli. Each fossa lies lower than the one in front.

What passes through the foramen magnum?

The foramen magnum transmits the lower medulla as it becomes the spinal cord, together with its meninges, the two vertebral arteries, the anterior and posterior spinal arteries and the spinal roots of the accessory nerves. The ligaments joining the skull to the axis attach around its margin. Raised intracranial pressure can push the cerebellar tonsils through it.

What are the layers of the scalp?

The scalp over the skull vault has five layers, remembered by the word SCALP: skin, dense connective tissue carrying the vessels and nerves, the epicranial aponeurosis, loose areolar tissue, and pericranium. The loose areolar layer is the danger layer: blood and infection spread easily through it, and its emissary veins connect with the dural venous sinuses inside the skull.

References

Gray's Anatomy: The Anatomical Basis of Clinical Practice. Standring S (ed). Elsevier, 42nd edition, 2020.
Moore's Clinically Oriented Anatomy. Moore KL, Dalley AF, Agur AMR. Wolters Kluwer, 9th edition, 2022.
Last's Anatomy: Regional and Applied. Elsevier.