OCCIPITAL BONE ANATOMY

GENERAL CHARACTERISTICS

• Trapezoidal cranial bone positioned at the posteroinferior aspect of the skull
• Forms the posterior and inferior walls of the cranial cavity
• Articulates with parietal, temporal, and sphenoid bones

STRUCTURAL COMPONENTS

• Composed of four parts surrounding the foramen magnum
  • Squamous part
  • Basilar part
  • Two lateral parts

FORAMEN MAGNUM

• Large central aperture of the occipital bone
• Transmits the brainstem at the junction of medulla oblongata and spinal cord
• Serves as a conduit for nerves and vessels passing between cranial cavity and vertebral canal

BASILAR PART

• Located anterior to the foramen magnum
• Fuses with the sphenoid bone anteriorly

LATERAL PARTS

• Positioned on both sides of the foramen magnum
• Occipital Condyles — oval prominences on inferior surface; articulate with atlas (C1) forming the atlanto-occipital joint for head nodding
• Condylar Fossa — depression posterior to each condyle; may contain opening of the condylar canal (transmits emissary veins)
• Hypoglossal Canal — located superior to foramen magnum, courses anterolaterally; transmits hypoglossal nerve (CN XII)
• Jugular Tubercle — oval elevation on superior surface; contributes to jugular foramen formation
• Jugular Notch — forms part of jugular foramen; transmits internal jugular vein and cranial nerves
• Jugular Process — extends laterally; provides attachment for muscles and ligaments
• Intrajugular Process — bony projection that may divide jugular notch; can form septum within jugular foramen

SQUAMOUS PART

• Largest and most posterior portion of the occipital bone
• Located posterior to the foramen magnum
• Provides attachment sites for nuchal muscles and ligaments

External Surface

• External Occipital Protuberance — midline prominence palpable through the scalp
• External Occipital Crest (Median Nuchal Line) — descends from protuberance toward foramen magnum; attachment for nuchal ligament
• Nuchal Lines:
  • Supreme Nuchal Line — faint ridge above external occipital protuberance
  • Superior Nuchal Line — horizontal ridge at level of protuberance; muscle attachment
  • Inferior Nuchal Line — horizontal ridge below superior line; additional muscle attachments
• Occipital Plane — region above supreme nuchal line; contributes to posterior cranial curvature

Internal Surface

• Cruciform (Cruciate) Eminence — cross-shaped elevation dividing surface into four fossae; Internal Occipital Protuberance at center
• Internal Occipital Crest — descends from protuberance; forms inferior limb of cruciform eminence
• Venous Sinus Grooves:
  • Groove for Transverse Sinuses — horizontal grooves accommodating transverse dural venous sinuses
  • Groove for Occipital Sinus — follows internal occipital crest; houses occipital sinus
  • Groove for Marginal Sinus — traces inferior margin of foramen magnum
• Cranial Fossae:
  • Cerebral Fossae — superior fossae accommodating occipital lobes of cerebrum
  • Cerebellar Fossae — inferior fossae housing cerebellar hemispheres
  • Vermian Fossa — small depression accommodating cerebellar vermis