Axial Skeleton: The Skull and Cranial Bones

Introduction to the Axial Skeleton

The human skeleton is divided into the axial and appendicular skeletons. The axial skeleton forms the long axis of the body and is essential for protecting the brain, spinal cord, and thoracic organs.

• Axial Skeleton: Composed of 80 bones, including the skull, vertebral column, and bony thorax.

• Main Regions:

  • The Skull

  • The Vertebral Column

  • The Bony Thorax

The Skull

The skull is a complex bony structure that protects the brain and forms the framework of the face. It consists of cranial and facial bones, most of which are flat and joined by immovable joints called sutures.

• Cranium: Encloses and protects the brain. Divided into two major areas:

  • Cranial Vault (Calvaria): Forms the superior, lateral, and posterior aspects of the skull.

  • Cranial Base (Cranial Floor): Forms the skull's inferior aspect and is divided into anterior, middle, and posterior cranial fossae.

• Facial Bones: Form the framework of the face, anchor facial muscles, and secure teeth.

• Openings: The skull contains numerous foramina (singular: foramen), canals, and fissures for the passage of nerves and blood vessels.

• Total Bones: 22 bones (8 cranial, 14 facial).

Cranial Bones

The cranial bones form the protective case around the brain and provide attachment sites for head and neck muscles.

Frontal Bone

• "Cockle-shell" shaped bone forming the forehead and roofs of the orbits.

• Glabella: Smooth area between the superciliary arches (eyebrow ridges).

• Supraorbital Foramen (Notch): Opening above each orbit for nerves and vessels.

• Lacrimal Fossa: Marks the location of the lacrimal gland.

Parietal Bones (2)

• Paired, curved rectangular bones forming the sides and roof of the cranium.

• Located posterolateral to the frontal bone.

• Sutures: Four major sutures (coronal, sagittal, lambdoid, squamous) are associated with the parietal bones.

• Temporal Lines: Superior and inferior temporal lines mark muscle attachment sites.

• Parietal Eminence: Smooth, raised surface superior to the temporal lines.

Occipital Bone

• Most posterior cranial bone; forms the base and back wall of the skull.

• Foramen Magnum: Large opening for the spinal cord.

• Occipital Condyles: Articulate with the first cervical vertebra (atlas).

• Hypoglossal Canal: Passage for hypoglossal nerve (CN XII).

• External Occipital Protuberance: Prominent midline projection for muscle attachment.

• Inferior and Superior Nuchal Lines: Ridges for muscle attachment.

Temporal Bones (2)

The temporal bones form the sides and base of the cranium and house structures of the ears.

• Each temporal bone has four major regions:

  • Squamous Region: Flat, forms part of the cranial wall; includes the zygomatic process (cheekbone).

  • Tympanic Region: Surrounds the external acoustic meatus (ear canal); includes the styloid process for ligament/muscle attachment.

  • Mastoid Region: Contains the mastoid process, a site for neck muscle attachment.

  • Petrous Region: Houses the middle and inner ear; contains the carotid canal, jugular foramen, and internal acoustic meatus.

Sphenoid Bone

• "Bat-shaped" bone spanning the width of the cranial floor; forms part of the middle cranial fossa and orbits.

• Body: Central portion containing the sella turcica (houses the pituitary gland).

• Greater Wings: Extend laterally, forming part of the cranial floor and orbits.

• Lesser Wings: Smaller, superior projections.

• Pterygoid Processes: Vertical projections for muscle attachment.

• Key Openings:

  • Optic Canal: Passage for optic nerve (CN II).

  • Superior Orbital Fissure: Passage for cranial nerves III, IV, VI, and ophthalmic branch of V.

  • Foramen Rotundum, Ovale, Spinosum: Passage for branches of cranial nerve V and middle meningeal artery.

• Other Features:

  • Anterior/Posterior Clinoid Processes: Attachment points for dura mater.

  • Tuberculum Sellae and Dorsum Sellae: Boundaries of the sella turcica.

  • Optic Groove: Connects the optic canals.

Ethmoid Bone

• Most deeply situated cranial bone; located anterior to the sphenoid.

• Forms part of the nasal cavity, nasal septum, and medial wall of the orbits.

• Cribriform Plates: Form the roof of the nasal cavity and floor of the anterior cranial fossa; contain olfactory foramina for CN I.

• Crista Galli: Vertical projection for attachment of the brain's dura mater.

• Ethmoidal Labyrinth: Network of air cells (sinuses) within the bone.

• Superior and Middle Nasal Conchae: Thin, scroll-like bones that increase the surface area of the nasal cavity.

• Perpendicular Plate: Forms the superior part of the nasal septum.

• Lateral Masses: Contain ethmoid sinuses and contribute to the medial wall of the orbit.

Summary Table: Major Cranial Bones and Key Features

Bone	Key Features	Main Functions
Frontal	Glabella, supraorbital foramen, lacrimal fossa	Forehead, roof of orbits
Parietal (2)	Temporal lines, parietal eminence, sutures	Sides and roof of cranium
Occipital	Foramen magnum, occipital condyles, nuchal lines	Posterior skull, base of cranium
Temporal (2)	Squamous, tympanic, mastoid, petrous regions; zygomatic process, external acoustic meatus	Sides and base of skull, houses ear structures
Sphenoid	Sella turcica, greater/lesser wings, pterygoid processes, optic canal	Base of cranium, orbits, passage for nerves
Ethmoid	Cribriform plates, crista galli, perpendicular plate, nasal conchae	Nasal cavity, orbits, nasal septum

Key Terms and Definitions

• Suture: Immovable joint between skull bones.

• Foramen: Opening in bone for nerves and blood vessels.

• Process: Projection or outgrowth of bone for muscle/ligament attachment.

• Fossa: Shallow depression in bone.

• Canal: Tubular passage through bone.

Example: Clinical Application

• Damage to the foramen magnum can result in life-threatening injury due to its role in transmitting the spinal cord.

• Fractures of the ethmoid bone may lead to leakage of cerebrospinal fluid into the nasal cavity (CSF rhinorrhea).

Additional info: The notes above are expanded and clarified for academic completeness, with definitions and clinical examples added for context.