The Human Skull — 22 Bones, Every Foramen & Their Clinical Relevance

Introduction: The Skull at a Glance

The adult human skull comprises 22 bones: 8 cranial bones (forming the neurocranium — the braincase) and 14 facial bones (forming the viscerocranium — the facial skeleton). The only movable joint in the skull is the temporomandibular joint (TMJ) — a synovial condylar joint between the mandibular condyle and the mandibular fossa of the temporal bone, separated by an articular disc and capable of depression, elevation, protrusion, retraction, and lateral excursion.

In infants, the cranial bones are separated by fibrous fontanelles — the anterior (bregmatic) fontanelle closes between 12–18 months; the posterior (lambdoid) fontanelle closes by 3 months. The fontanelles allow moulding during passage through the birth canal and permit the brain's rapid postnatal growth.

The 8 Cranial Bones (Neurocranium)

• Frontal Bone (1): Forms the forehead, roof of the orbits, and the anterior cranial fossa. Contains the frontal sinuses (pneumatised air cells). The supraorbital notch (or foramen) in its margin transmits the supraorbital nerve and vessels (V1 branch).
• Parietal Bones (2): Paired; form the superior and lateral cranial vault. The middle meningeal artery runs in a groove on the internal surface — rupture here (typically from a pterion fracture — see below) causes an extradural (epidural) haematoma. The sagittal suture articulates the two parietal bones at the midline.
• Temporal Bones (2): Complex, multi-regional bone containing the external auditory meatus, middle ear cavity, mastoid air cells, internal auditory meatus (transmits CN VII and VIII), and the carotid canal (transmits internal carotid artery). The petrous part of the temporal bone is the densest bone in the body.
• Occipital Bone (1): Forms the posterior and inferior cranial vault. Contains the foramen magnum — the largest foramen in the skull, transmitting the medulla oblongata, vertebral arteries, anterior and posterior spinal arteries, and the spinal roots of CN XI. The occipital condyles articulate with the C1 vertebra (atlas).
• Sphenoid Bone (1): The "keystone" bone of the skull — articulates with all other cranial bones. Bat-shaped. Contains the sella turcica (housing the pituitary gland), greater and lesser wings, pterygoid plates (processus pterygoideus — attachments for medial and lateral pterygoid muscles of mastication), and multiple foramina. One of the clinically richest bones in anatomy.
• Ethmoid Bone (1): A delicate sieve-like bone forming the roof of the nasal cavity and part of the orbital medial wall. The cribriform plate contains fine holes (foramina) transmitting the olfactory nerve (CN I) fibres. Base of skull fractures crossing the cribriform plate → CSF rhinorrhoea (salty nasal discharge), anosmia, and meningitis risk.

The 14 Facial Bones (Viscerocranium)

Bone	Quantity	Key Notes
Mandible	1	Only movable skull bone; lower jaw; articulates with temporal bone at TMJ
Maxillae	2	Upper jaw, hard palate (anterior), floor and medial wall of orbit, lateral nasal walls. Contain maxillary sinuses (largest paranasal sinuses).
Palatine Bones	2	Form posterior hard palate and part of orbital floor
Zygomatic Bones	2	Cheek bones; each forms the zygomatic arch with temporal process
Nasal Bones	2	Bridge of nose; the most commonly fractured facial bones
Lacrimal Bones	2	Smallest facial bones; medial orbital wall; contain lacrimal groove (nasolacrimal duct passes here)
Inferior Nasal Conchae	2	Scroll-like projections from lateral nasal wall; increase surface area for air warming and humidification
Vomer	1	Forms inferior part of nasal septum; named for its plough-like shape

The Cranial Sutures

• Coronal Suture: Separates frontal from parietal bones (coronally orientated).
• Sagittal Suture: Joins two parietal bones along the midline.
• Lambdoid Suture: Separates parietal bones (posteriorly) from the occipital bone; shaped like the Greek letter lambda (Λ).
• Squamosal Suture: Joins temporal bone to parietal bone laterally.
• Metopic Suture: A midline frontal suture present in the foetal frontal bone, normally fused by age 6. Persistence in adults (metopism) is a normal variant (~10%).

Craniosynostosis: Premature fusion of one or more cranial sutures in infancy → restricted brain growth in the direction perpendicular to the fused suture → characteristic skull shape deformations (scaphocephaly with sagittal synostosis, plagiocephaly with coronal synostosis, trigonocephaly with metopic synostosis). Surgical correction is required when craniosynostosis restricts brain growth.

The Essential Cranial Foramina and Their Contents

Foramen	Location (Bone)	Contents
Foramen magnum	Occipital	Medulla oblongata, vertebral arteries, CN XI spinal roots, anterior/posterior spinal arteries
Cribriform plate	Ethmoid	CN I (olfactory nerve fibres)
Optic canal	Sphenoid (lesser wing)	CN II (optic nerve), ophthalmic artery
Superior orbital fissure	Sphenoid	CN III, IV, V1, VI, ophthalmic veins. SOF syndrome: complete ophthalmoplegia + V1 sensory loss
Foramen rotundum	Sphenoid	CN V2 (maxillary nerve)
Foramen ovale	Sphenoid	CN V3 (mandibular nerve)
Foramen spinosum	Sphenoid	Middle meningeal artery and vein
Foramen lacerum	Sphenoid/temporal/occipital	Greater petrosal nerve (small artery of pterygoid canal); ICA passes above but not through
Internal auditory meatus	Temporal (petrous)	CN VII (facial), CN VIII (vestibulocochlear), labyrinthine artery
Jugular foramen	Temporal + occipital	CN IX, X, XI; internal jugular vein origin
Hypoglossal canal	Occipital	CN XII (hypoglossal nerve)

The Pterion — A Critical Danger Zone

The pterion is an H-shaped sutural junction on the lateral skull where four bones meet: frontal, parietal, temporal, and sphenoid. It is located approximately 3 cm behind the zygomatic arch and 3.5 cm above the midpoint of the zygomatic arch — roughly at the temple.

The pterion is the thinnest part of the lateral skull wall (as little as 2–3 mm in some individuals). Immediately deep to the pterion runs the anterior branch of the middle meningeal artery in its groove on the inner surface of the temporal bone.

Clinical relevance: A direct blow to the temple (e.g., a fall, assault, sporting injury) can fracture the thin pterional bone and rupture the middle meningeal artery → extradural (epidural) haematoma. Classic presentation: loss of consciousness → "lucid interval" (arterial haematoma takes time to compress the brain) → secondary deterioration with contralateral limb weakness, ipsilateral dilated fixed pupil (CN III compression by uncal herniation), and declining consciousness. A neurosurgical emergency — craniotomy and evacuation within hours to prevent death.

The Three Cranial Fossae

The cranial cavity is divided into three fossae (steps), each housing a specific part of the brain:

• Anterior Cranial Fossa: Formed by frontal bone, cribriform plate of ethmoid, and lesser wings of sphenoid. Houses the frontal lobes and olfactory bulbs. Fractures → anosmia, CSF rhinorrhoea, "raccoon eyes" (periorbital ecchymosis).
• Middle Cranial Fossa: Formed by sphenoid and temporal bones. Houses the temporal lobes and pituitary gland (in sella turcica). Contains foramina rotundum, ovale, spinosum, and the cavernous sinus.
• Posterior Cranial Fossa: Formed by occipital and temporal bones. The largest and deepest fossa — houses the cerebellum, pons, medulla oblongata. Contains the foramen magnum, internal auditory meatus, jugular foramen, and hypoglossal canal.

For educational purposes only. Medical disclaimer →