Skull bone anatomy

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Here is a comprehensive overview of skull bone anatomy, based on Gray's Anatomy for Students, Rosen's Emergency Medicine, and Cummings Otolaryngology.

Skull Bone Anatomy

The skull has 22 bones (excluding the ossicles of the middle ear). Except for the mandible, all skull bones are connected by sutures — immovable fibrous joints. Collectively they form the cranium.

Overview: Divisions of the Cranium

Region	Function	Key Bones
Calvaria (cranial vault)	Encloses and protects the brain	Frontal, 2× parietal, 2× temporal, sphenoid, occipital
Cranial base	Floor of the cranial cavity	Sphenoid, temporal, occipital, frontal, ethmoid
Facial skeleton (viscerocranium)	Forms the face	Nasal, palatine, lacrimal, zygomatic, maxillae, inferior nasal conchae, vomer
Mandible	Lower jaw; only movable bone	Mandible

Anterior View

Frontal Bone

Forms the forehead and superior orbital rim
Bears the superciliary arches (more pronounced in males) and glabella (midline depression between them)
The supra-orbital foramen (or notch) transmits the supra-orbital nerve and vessels
Its zygomatic process projects laterally to form the upper lateral orbital rim, articulating with the frontal process of the zygomatic bone

Zygomatic Bone (Cheekbone)

Forms the lower lateral orbital rim
Its temporal process projects posteriorly to meet the zygomatic process of the temporal bone, forming the zygomatic arch
Bears the small zygomaticofacial foramen on its lateral surface

Nasal Bones

Paired small bones forming the bridge of the nose

Maxilla (paired)

Forms the upper jaw with its alveolar process (tooth-bearing)
Contributes to the inferior and medial orbital rim
Its frontal process articulates with the frontal bone; its zygomatic process articulates with the zygomatic bone

Mandible

The only freely movable skull bone
Has a body (anterior, tooth-bearing), ramus (vertical posterior projection), condylar process (articulates with temporal bone at the TMJ), and coronoid process (attachment for temporalis muscle)
The mental foramen on the lateral body surface transmits the mental nerve and vessels

Lateral View

Key Sutures (Lateral View)

Suture	Bones Joined
Coronal	Frontal ↔ Parietal
Squamous	Temporal (squamous) ↔ Parietal
Sphenoparietal	Sphenoid (greater wing) ↔ Parietal
Sphenosquamous	Sphenoid ↔ Temporal
Lambdoid	Parietal ↔ Occipital
Occipitomastoid	Occipital ↔ Mastoid part of temporal

Pterion ⚠️ Clinically Important

The pterion is the H-shaped junction where the frontal, parietal, sphenoid, and temporal bones meet. The bone here is particularly thin and overlies the anterior division of the middle meningeal artery. A skull fracture here can tear this artery, causing an extradural (epidural) hematoma.

Temporal Bone (Complex, Multi-part)

Squamous part — large flat plate forming the lateral cranial wall
Zygomatic process — projects anteriorly to form the zygomatic arch
Tympanic part — surrounds the external acoustic meatus (ear canal)
Petrous part — dense pyramidal part housing the inner ear structures; forms part of the cranial base
Mastoid part — posterior projection bearing the mastoid process (attachment for sternocleidomastoid); contains air cells
Styloid process — slender inferior projection

Posterior View

Occipital Bone

Dominant bone of the posterior cranium
Squamous part articulates with parietal bones at the lambdoid suture and with temporal bones at the occipitomastoid sutures
External occipital protuberance (inion) — palpable midline bump; origin of the superior nuchal line
Superior and inferior nuchal lines — bony ridges for posterior neck muscle attachment
Sutural (Wormian) bones — small accessory ossification centers occasionally found along the lambdoid suture

Inferior View (Skull Base)

The skull base is formed by five bones: frontal, ethmoid, temporal, sphenoid, and occipital.

Sphenoid Bone

A butterfly-shaped bone at the center of the skull base. Consists of:

Body — centrally placed, contains paired sphenoidal air sinuses
Greater wings — project laterally; contain foramen ovale (CN V₃, lesser petrosal n.) and foramen spinosum (middle meningeal a.)
Lesser wings — project anteriorly; not visible from below
Pterygoid processes — project inferiorly with medial and lateral plates enclosing the pterygoid fossa; medial plate ends in the pterygoid hamulus; contains the pterygoid canal
Foramen lacerum — filled with cartilage in life; at junction with temporal and occipital bones

Occipital Bone (Inferior Aspect)

Basilar part — articulates with sphenoid bone at the spheno-occipital synchondrosis (fuses ~age 25)
Occipital condyles — articulate with atlas (C1) at the atlanto-occipital joint
Foramen magnum — large opening transmitting the brainstem/spinal cord junction, vertebral arteries, anterior and posterior spinal arteries, accessory nerve (CN XI) roots
Hypoglossal canal — transmits CN XII
Jugular foramen — transmits the internal jugular vein (origin), CN IX, X, XI, and inferior petrosal sinus

Vomer

Small midline bone contributing to the bony nasal septum, resting on the sphenoid

Bone Structure

Each cranial bone consists of:

Outer cortical table (periosteum-covered)
Diploe (cancellous/spongy bone with red marrow)
Inner cortical table (lined by periosteal dura)

Adult skull bones average 2–6 mm in thickness; the temporal region is the thinnest.

Major Foramina Summary

Foramen	Transmits
Supra-orbital foramen	Supra-orbital n. & vessels
Infra-orbital foramen	Infra-orbital n. & vessels
Mental foramen	Mental n. & vessels
Foramen ovale	CN V₃ (mandibular n.), lesser petrosal n.
Foramen spinosum	Middle meningeal artery
Foramen lacerum	Filled with cartilage (ICA traverses superior)
Carotid canal	Internal carotid artery
Stylomastoid foramen	CN VII (facial n.)
Jugular foramen	IJV, CN IX, X, XI, inferior petrosal sinus
Hypoglossal canal	CN XII
Foramen magnum	Medulla/spinal cord, vertebral arteries, CN XI roots

Sutures and Fontanelles

In fetal and neonatal skulls, large gaps called fontanelles exist between the unfused bones:

Anterior fontanelle (bregma) — largest; junction of frontal, sagittal, and coronal sutures; closes by ~18 months
Posterior fontanelle (lambda) — junction of sagittal and lambdoid sutures; closes by ~2–3 months
Sphenoidal fontanelle — anterolateral
Mastoid fontanelle — posterolateral

These allow head molding during birth and accommodate postnatal brain growth. After the first few months of life the bones progressively fuse, ultimately forming the rigid, non-expandable cranial vault.

Sources: Gray's Anatomy for Students (9780323934237); Rosen's Emergency Medicine (9780323757898); Cummings Otolaryngology Head and Neck Surgery (9780323612173)

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