Skull, Cranial Fossae and Foramina

Neurocranium: The bones enclosing the brain, frontal, parietal (×2), temporal (×2), occipital, sphenoid, ethmoid.

• Viscerocranium: The facial skeleton, maxilla (×2), zygomatic (×2), nasal (×2), lacrimal (×2), palatine (×2), inferior nasal conchae (×2), vomer, mandible.
• Calvaria: The dome-shaped roof of the neurocranium (skull cap), formed by the frontal bone, paired parietal bones, and the upper occipital bone.
• Cranial base: The floor of the neurocranium, internally forming the three cranial fossae, externally forming the base upon which the facial skeleton and vertebral column articulate.

• Frontal bone: Forms the forehead, superior orbital margins/roofs, and part of the anterior cranial fossa floor; contains the frontal sinuses.
• Parietal bones (×2): Form the majority of the calvarial vault, meeting in the midline at the sagittal suture and articulating with the frontal bone (coronal suture) and occipital bone (lambdoid suture).
• Temporal bones (×2): Complex bones forming part of the lateral skull and middle/posterior cranial fossa floor, housing the middle and inner ear; divided into squamous, petrous, tympanic, mastoid, and styloid parts.
• Occipital bone: Forms the posterior skull and most of the posterior cranial fossa, containing the foramen magnum.
• Sphenoid: A complex, butterfly-shaped bone forming the central skull base, contributing to all three cranial fossae; body (containing the sphenoid sinus and sella turcica), greater and lesser wings, and pterygoid processes.
• Ethmoid: A delicate bone forming part of the anterior cranial fossa floor (cribriform plate), the nasal septum (perpendicular plate), and the lateral nasal wall (superior/middle conchae).
• Maxilla (×2): Forms the upper jaw, most of the hard palate, the orbital floor, and the lateral nasal wall; contains the maxillary sinus.
• Zygomatic bones (×2): Form the cheek prominence and part of the lateral orbital wall/floor.
• Nasal bones (×2): Form the bridge of the nose.
• Lacrimal bones (×2): Small bones forming part of the medial orbital wall, housing the lacrimal fossa/nasolacrimal canal.
• Palatine bones (×2): Form the posterior hard palate and part of the lateral nasal wall/orbital floor.
• Inferior nasal conchae (×2): Independent bones (not part of the ethmoid, unlike the superior/middle conchae) forming the lowest projection into each nasal cavity.
• Vomer: Forms the posteroinferior part of the nasal septum.
• Mandible: The lower jaw, the only mobile skull bone, articulating with the temporal bone at the temporomandibular joint.

Anterior Cranial Fossa

• Boundaries: Anteriorly/laterally, frontal bone; posteriorly, the sharp posterior border of the lesser wings of sphenoid (and the anterior margin of the chiasmatic groove).
• Floor formed by: Orbital plates of the frontal bone (laterally), cribriform plate of the ethmoid (centrally), and the lesser wings/body of the sphenoid (posteriorly).
• Contents: Frontal lobes of the brain, olfactory bulbs and tracts (resting on the cribriform plate).
• Important foramina: Cribriform plate foramina (olfactory nerve fibres, CN I).

Middle Cranial Fossa

• Boundaries: Anteriorly, posterior border of the lesser wings of sphenoid; posteriorly, the superior border of the petrous temporal bone and the dorsum sellae; laterally, the squamous temporal and parietal bones.
• Sphenoid landmarks: Sella turcica (containing the hypophyseal fossa, housing the pituitary gland), anterior and posterior clinoid processes (attachment points for the tentorium cerebelli's free edge), greater wings (forming the lateral fossa floor, containing foramen rotundum, ovale, spinosum).
• Temporal bone relationships: The petrous part of the temporal bone forms the posterolateral boundary, housing the internal ear structures and the carotid canal/internal acoustic meatus (the latter opening into the posterior fossa).
• Cavernous sinus relationships: The cavernous sinus lies on either side of the sella turcica/sphenoid body, a critical venous structure through which the internal carotid artery and several cranial nerves (III, IV, V1, V2, VI) pass or run in close proximity.
• Contents: Temporal lobes of the brain, pituitary gland (within the hypophyseal fossa).
• Important foramina: Superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, carotid canal (opens here after ascending through the petrous temporal bone), foramen lacerum (largely filled with cartilage in life, of limited functional significance as a true "opening").

Posterior Cranial Fossa

• Boundaries: Anteriorly, dorsum sellae and the superior border of the petrous temporal bones; posteriorly/laterally, the occipital bone.
• Brainstem relationships: Houses the brainstem (pons, medulla) and, via the foramen magnum, is continuous with the vertebral canal.
• Cerebellar relationships: Houses the cerebellum, separated from the overlying occipital/temporal lobes (in the middle fossa/supratentorial compartment) by the tentorium cerebelli.
• Major foramina: Internal acoustic meatus (CN VII, VIII, and the labyrinthine artery), jugular foramen (CN IX, X, XI, and the internal jugular vein's origin/sigmoid sinus continuation), hypoglossal canal (CN XII), foramen magnum (medulla/spinal cord continuation, vertebral arteries, spinal roots of CN XI, meninges).

• Sella turcica: A saddle-shaped depression in the sphenoid body, containing the hypophyseal fossa.
• Hypophyseal fossa: The specific depression within the sella turcica housing the pituitary gland, bounded by the anterior and posterior clinoid processes and the dorsum sellae.
• Clinoid processes (anterior and posterior): Bony projections flanking the sella turcica, attachment points for the diaphragma sellae/tentorial free edge.
• Petrous temporal bone: The dense, pyramidal part of the temporal bone forming the boundary between the middle and posterior cranial fossae, housing the inner ear structures.
• Jugular tubercle: A small elevation on the occipital bone, anteromedial to the jugular foramen, overlying the hypoglossal canal.
• Occipital condyles: Paired, oval articular surfaces on the occipital bone flanking the foramen magnum, articulating with the atlas (C1) at the atlanto-occipital joint.

Basilar skull fracture:

• Fractures involving the cranial base, often extending through the petrous temporal bone or the floor of the anterior/middle fossa.
• Classic clinical signs reflect the specific fascial/vascular disruption at the fracture site: Battle's sign (bruising over the mastoid process, from middle fossa fracture with blood tracking along the temporal bone), periorbital ecchymosis ("raccoon eyes") (anterior fossa fracture, blood tracking into the periorbital soft tissue), CSF rhinorrhoea (anterior fossa fracture through the cribriform plate, disrupting the dura and allowing CSF leak into the nasal cavity), CSF otorrhoea (petrous temporal bone fracture, disrupting the dura near the middle/inner ear).

Pterion fracture:

• The pterion is a clinically important, relatively thin region of the lateral skull where the frontal, parietal, temporal, and sphenoid (greater wing) bones meet in an "H" or "X"-shaped suture configuration.
• Directly overlies the middle meningeal artery (anterior division) on its internal surface, making this the single most vulnerable point of the skull for traumatic middle meningeal artery injury.

Middle meningeal artery injury:

• A blow to the pterion region can fracture the thin bone here and lacerate the underlying middle meningeal artery, causing an extradural (epidural) haematoma: a classic neurosurgical emergency, often presenting with a "lucid interval" (initial transient loss of consciousness from the impact, followed by a period of apparent recovery, then rapid deterioration as the arterial haematoma expands).

Cranial nerve involvement in skull base lesions:

• Pathology at specific skull base foramina produces predictable, foramen-specific cranial nerve deficit patterns, e.g., a jugular foramen syndrome (CN IX, X, XI deficits together, from a mass or thrombosis at this single foramen), or cavernous sinus syndrome (CN III, IV, V1, V2, VI deficits, from pathology within/around the cavernous sinus in the middle fossa), understanding which nerves share a given foramen or space is therefore directly predictive of the clinical deficit pattern seen with a lesion at that specific skull base location.