Anatomy of the Skull

Anatomy of the Skull

The skull is divided into the neurocranium or calvaria (contains the brain and its meningeal coverings) and the viscerocranium (facial skeleton). The skull is composed of 22 bones (excluding the middle ear ossicles), with 8 forming the cranium and 14 forming the face. The orbits (eye sockets) lie between the calvaria (skull cap) and the facial skeleton and are formed by contributions from 7 different bones.

 

Neurocranium		Viscerocranium
Ethmoid	1	Zygomatic	2
Frontal	1	Vomer	1
Occipital	1	Inferior nasal concha	2
Sphenoid	1	Maxilla	2
Parietal	2	Nasal	2
Temporal	2	Palatine	2
		Lacrimal	2
		(Mandible)	1
N=22	8	+	14

• Function of skull
  • Encloses, supports and protects brain and meninges
  • Contains foramina for the transmission of nerves and vessels
  • Forms foundation for the face
  • Contains specialized cavities and openings for sense organs (e.g., nasal, oral)
• Neurocranium
  • Cranial vault and base of skull
  • Encloses and protects brain
  • Composed of 8 bones
  • Bones united by interlocking sutures
  • Can be divided
    • Calvaria-dome-like roof
    • Cranial base
  • Calvaria composed of 4 bones
    • Frontal bone anteriorly
    • Occipital bone posteriorly
    • Two parietal bones laterally
  • Cranial base formed from
    • Ethmoid bone
    • Parts of occipital and temporal bones
• Viscerocranium
  • = facial skeleton
  • Composed of 14 bones
  • Encloses orbits, nose, paranasal sinuses, mouth, and pharynx
  • Maxillae and mandible form upper and lower jaw, respectively, and house the teeth
• There are also three auditory ossicles
  • Malleus, incus, and stapes
  • Found spanning tympanic cavity
  • First bones to be completely ossified during development

Major sutures of the skull (Anatomy of the Skull)

Most bones of the skull are bound by sutures, a type of fibrous joint that fuses with age and becomes immobile.

• Coronal suture separates frontal and parietal bones
• Sagittal suture separates two parietal bones
• Lambdoid suture separates parietal and temporal bones from occipital bones
• Squamous suture separates squamous part of temporal bone from parietal bone
• Sphenosquamous suture separates squamous part of temporal bone from greater wing of the sphenoid
• Metopic suture between two frontal bones is largely obliterated with fusion of frontal bones

Internal Features of Base of Skull – (Anatomy of the Skull)

• Divided into anterior, middle, and posterior cranial fossae
• Anterior cranial fossa
  • Contains frontal lobe of brain
  • Formed by frontal bone anteriorly, ethmoid bone medially, and lesser wing of sphenoid posteriorly
  • Features
    • Frontal crest-midline bony extension of frontal bone
    • Foramen cecum-foramen at base of frontal crest
    • Crista galli-Midline ridge of bone from ethmoid posterior to foramen cecum
    • Cribriform plate-Thin, sieve-like plate of bone on either side of crista galli, which transmits olfactory nerves from nasal cavity to olfactory bulbs
• Middle cranial fossa
  • Contains temporal lobe, hypothalamus, and pituitary gland
  • Formed by greater wing and body of sphenoid, petrous temporal bone, lesser wing sphenoid
  • Features
    • Sella turcica-central depression in body of sphenoid for pituitary gland
    • Tuberculum sellae-Swelling anterior to sella turcica
    • Dorsum sellae-crest on body of sphenoid posterior to sella turcica
    • Anterior clinoid processes-medial projections of lesser wings of sphenoid bones
    • Posterior clinoid processes-swelling at either end of dorsum sellae
    • Foramen lacerum (one on each side)-jagged opening closed by plate of cartilage in life, transmits nothing
  • Contains four foramina in a crescent on either side in the body of the sphenoid
    • Superior orbital fissure
    • Foramen rotundum
    • Foramen ovale
    • Foramen spinosum
• Posterior cranial fossa:
  • Contains cerebellum, pons, and medulla oblongata
  • Composed largely of occipital bone, body of sphenoid, petrous, and mastoid parts of temporal bone
  • Features
    • Foramen magnum-transmits spinal cord
    • Internal occipital crest-divides posterior fossa into two lateral cerebellar fossae
    • Grooves for transverse and sigmoid dural venous sinuses
    • Jugular foramen-transmits sigmoid sinus (internal jugular vein) and several cranial nerves
    • Internal acoustic meatus-anterior and superior to jugular foramen, transmits facial and vestibulocochlear nerves (CN VII and CN VIII)
    • Hypoglossal canal-anterolateral and superior to foramen magnum, transmits hypoglossal nerve (CN XII)

Foramina of Skull – (Anatomy of the Skull)

Numerous holes appear in the cranial floor and they are called foramina. Important structures, especially cranial nerves arising from the brain, pass through the foramen to access the exterior.

Foramen/Opening	Bone	Structures Transmitted
Optic canal	Lesser wing sphenoid	Optic nerve Ophthalmic artery Sympathetic plexus
Superior orbital fissure	Greater and lesser wings sphenoid	Lacrimal nerve (V1) Frontal nerve (V1) Trochlear nerve (IV) Oculomotor nerve (III) Abducent nerve (VI) Nasociliary nerve (V1) Superior ophthalmic vein
Inferior orbital fissure	Between greater wing of sphenoid and zygomatic	Infraorbital vein Infraorbital artery Infraorbital nerve
Foramen spinosum	Greater wing of sphenoid	Middle meningeal artery and vein
Foramen rotundum	Greater wing of sphenoid	Maxillary division trigeminal nerve (V3)
Foramen ovale	Greater wing of sphenoid	Mandibular division trigeminal nerve Lesser petrosal nerve
Foramen lacerum	Between temporal bone (petrous area) and sphenoid bone	Internal carotid artery
Foramen magnum	Occipital bone	Medulla oblongata Vertebral artery Meninges Spinal roots of accessory nerve
Hypoglossal canal	Occipital bone	Hypoglossal nerve (XII)
Jugular foramen	Between temporal bone (petrous area) and occipital bone	Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Inferior petrosal sinus Sigmoid sinus Posterior meningeal artery