cranial bone anatomy

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Cranial Bone Anatomy

The skull is divided into two functional parts: the neurocranium (braincase) and the viscerocranium (facial skeleton). The neurocranium itself has two components: the calvaria (vault/roof) and the cranial base (floor).

1. The 8 Bones of the Neurocranium

Bone	Count	Location
Frontal	1	Forehead, roof of orbits
Parietal	2 (paired)	Lateral walls and roof
Temporal	2 (paired)	Lateral walls, contains ear structures
Occipital	1	Posterior and floor
Sphenoid	1	Central base, "keystone" bone
Ethmoid	1	Anterior base, nasal septum

2. The Calvaria (Skullcap)

The calvaria is the dome-shaped roof formed by parts of the frontal, parietal, and occipital bones. Its internal surface shows characteristic grooves and features visible in the inferior view image below.

Calvaria - Inferior View showing frontal bone, parietal bones, occipital bone, coronal suture, sagittal suture, lambdoid suture, diploe, and meningeal vessel grooves

Key features of the calvaria:

Frontal crest - midline ridge on internal surface of frontal bone; attachment point for the falx cerebri
Groove for the superior sagittal sinus - midline groove running front-to-back, deepening posteriorly
Granular foveolae - small pits for arachnoid granulations (CSF reabsorption)
Grooves for middle meningeal vessels - arborizing grooves on parietal bone inner surface
Coronal suture - between frontal and parietal bones
Sagittal suture - between the two parietal bones (midline)
Lambdoid suture - between parietal bones and occipital bone

Bone structure: The skull bones have two compact tables (outer table ~twice as thick as inner) separated by diploe (cancellous bone with marrow). Diploe begins forming at ~4 years of age. The thinnest region is the temporal bone (~4 mm); the occipital bone at midline can reach ≥15 mm. Frontal and parietal bones average 6-10 mm. - P.C. Dikshit Forensic Medicine, p. 193

3. Anterior View of the Skull

Anterior view of skull showing all cranial and facial bones color-coded: frontal bone (blue), parietal bone, temporal bone, sphenoid (greater and lesser wings), ethmoid, nasal bone, lacrimal, zygomatic, maxilla, mandible

Notable anterior landmarks:

Glabella - smooth midline prominence of frontal bone between the superciliary arches
Supraorbital notch/foramen - transmits supraorbital nerve and vessels
Infraorbital foramen - transmits infraorbital nerve (branch of V2)
Mental foramen - transmits mental nerve (branch of V3), on mandible

4. Cranial Base - Superior (Internal) View

The cranial base is divided into three cranial fossae:

Anterior Cranial Fossa

Formed by: orbital part of frontal bone, cribriform plate of ethmoid, lesser wing + jugum of sphenoid
Key features: crista galli (ethmoid), cribriform plate (olfactory nerve bundles CN I pass through), foramen cecum, anterior clinoid process

Middle Cranial Fossa

Formed by: greater wing of sphenoid + squamous and petrous parts of temporal bone
Key features: hypophyseal fossa (sella turcica), optic canal (CN II, ophthalmic artery), superior orbital fissure, foramen rotundum (V2), foramen ovale (V3), foramen spinosum (middle meningeal artery), foramen lacerum, carotid sulcus

Posterior Cranial Fossa

Formed by: occipital bone (basilar, squamous, lateral parts), petrous part of temporal bone
Key features: foramen magnum (medulla, vertebral arteries, CN XI spinal roots), internal acoustic meatus (CN VII, VIII), jugular foramen (CN IX, X, XI; sigmoid + inferior petrosal sinus), hypoglossal canal (CN XII), internal occipital protuberance

5. Foramina and Their Transmissions

Cranial base - foramina labeled with all cranial nerves and major vessels passing through each foramen

Foramen	Bone	Contents
Foramen cecum	Frontal/ethmoid	Emissary vein to superior sagittal sinus
Cribriform plate foramina	Ethmoid	CN I (olfactory nerve bundles)
Optic canal	Sphenoid	CN II, ophthalmic artery
Superior orbital fissure	Sphenoid	CN III, IV, V1, VI; superior ophthalmic vein
Foramen rotundum	Sphenoid	CN V2 (maxillary nerve)
Foramen ovale	Sphenoid	CN V3 (mandibular nerve), accessory meningeal artery
Foramen spinosum	Sphenoid	Middle meningeal artery and vein
Foramen lacerum	Sphenoid/temporal	Filled with cartilage in life; internal carotid artery passes over it
Internal acoustic meatus	Temporal	CN VII, CN VIII, labyrinthine artery
Jugular foramen	Temporal/occipital	CN IX, X, XI; sigmoid sinus → internal jugular vein; inferior petrosal sinus
Hypoglossal canal	Occipital	CN XII
Foramen magnum	Occipital	Medulla oblongata, vertebral arteries, CN XI spinal roots, meninges
Condylar canal	Occipital	Emissary vein (inconstant)
Mastoid foramen	Temporal	Emissary vein (inconstant)

6. Individual Bone Highlights

Frontal Bone

Forms the forehead, roof of orbits, and part of anterior cranial floor
Contains the frontal sinuses (paired, separated by intersinus septum)
Glabella = smooth area between superciliary arches

Parietal Bones (paired)

Form the upper sides and roof of the skull
Strong areas: sagittal ridge; thin areas: parieto-temporal zone
Inner surface: superior sagittal sinus groove and meningeal vessel grooves

Temporal Bone

Most complex skull bone; has 4 parts: squamous, petrous, tympanic, mastoid
Petrous part = hardest bone in body; contains inner ear structures
Mastoid process = posterior projection (attachment for sternocleidomastoid)
Styloid process = slender projection below external acoustic meatus
The thinnest bone of the calvaria (~4 mm)

Occipital Bone

Has 4 parts: squamous, 2 condylar (lateral), basilar
Foramen magnum is the largest foramen
External occipital protuberance (inion) = posterior midline palpable landmark
Occipital condyles articulate with atlas (C1) - atlanto-occipital joint
Thickest skull bone at midline (≥15 mm)

Sphenoid Bone

Often called the "keystone" - articulates with all other cranial bones
Has a body (containing sphenoidal sinuses), 2 lesser wings, 2 greater wings, 2 pterygoid processes
Sella turcica (Turkish saddle) = houses the pituitary gland
Pterygoid processes project inferiorly - lateral and medial plates

Ethmoid Bone

Lightest cranial bone; highly aerated
Cribriform plate = horizontal plate in anterior cranial floor (CN I passes through)
Crista galli = superior midline projection (falx cerebri attachment)
Perpendicular plate = upper part of nasal septum
Contains the ethmoid air cells (ethmoidal sinuses)

7. Cranial Sutures

Suture	Bones Joined	Clinical Note
Coronal	Frontal + Parietals	Lies in coronal plane
Sagittal	Parietal + Parietal	Midline, AP direction
Lambdoid	Parietals + Occipital	Lambda-shaped
Squamosal	Temporal + Parietal	Weakest area of lateral skull
Metopic	Frontal halves	Usually fuses by age 2

The pterion is where the frontal, parietal, temporal, and sphenoid bones meet - the thinnest and most vulnerable area of the lateral skull. The anterior branch of the middle meningeal artery runs deep to it; fractures here can cause epidural hematoma.

The bregma is the junction of the coronal and sagittal sutures (site of the anterior fontanelle in infants). The lambda is the junction of the sagittal and lambdoid sutures (posterior fontanelle site).

Sources: Netter's Atlas of Human Anatomy (ISBN 9780323547048); P.C. Dikshit Textbook of Forensic Medicine and Toxicology (ISBN 9788188867967)

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