Introduction

The occipital bone (os occipitale) is an unpaired, trapezoidal bone of the neurocranium that forms the posteroinferior aspect of the skull and a major part of the cranial base (basis cranii). Piercing its inferior aspect is the foramen magnum, a large oval aperture that transmits the brainstem (truncus encephalicus), accessory nerve (n. accessorius / CN XI), vertebral arteries (aa. vertebrales), anterior and posterior spinal arteries (a. spinalis anterior et posterior), alar ligaments (ligg. alaria), and the tectorial membrane (membrana tectoria).

The occipital bone is curved, featuring a concave internal surface that houses and protects the cerebellum and occipital lobes of the cerebrum, and a convex external surface. It is the only cranial bone that articulates with the vertebral column, forming the atlanto-occipital joint (articulatio atlantooccipitalis) with the atlas (C1).

Articulations and Borders:

The occipital bone articulates with five bones (two paired, one unpaired): the parietal bones (os parietale), temporal bones (ossa temporalia), and the sphenoid bone (os sphenoidale).

• Lambdoid border (margo lambdoideus): Articulates with the parietal bones to form the lambdoid suture (sutura lambdoidea).
• Mastoid border (margo mastoideus): Articulates with the mastoid part of the temporal bone to form the occipitomastoid suture (sutura occipitomastoidea).

Parts of the Occipital Bone

The occipital bone is divided into three distinct parts surrounding the foramen magnum, each presenting an internal (facies interna) and an external (facies externa) surface:

• Basilar part (pars basilaris): located anterior to the foramen magnum.
• Lateral parts (pars lateralis): paired portions flanking the foramen magnum.
• Squamous part (squama occipitalis): the large, flat plate situated posterior to the foramen magnum.

Basilar Part (Pars basilaris)

The basilar part (also known as the basiocciput) is a thick, quadrangular piece of bone projecting anterosuperiorly from the foramen magnum to articulate with the body of the sphenoid bone.

Internal Surface (Facies interna)

• Clivus: A broad, shallowly sloped bony surface extending from the dorsum sellae of the sphenoid bone down to the foramen magnum. It supports the pons and medulla oblongata (medulla oblongata). The lower portion serves as an attachment site for the tectorial membrane (membrana tectoria).
• Groove for inferior petrosal sinus (sulcus sinus petrosi inferioris): Paired grooves running along the lateral margins of the basilar part; combined with the petrous part of the temporal bone, they accommodate the inferior petrosal sinus.

External Surface (Facies externa)

• Pharyngeal tubercle (tuberculum pharyngeum): A median bony elevation giving attachment to the fibrous pharyngeal raphe and the superior pharyngeal constrictor muscle (m. constrictor pharyngis superior).
• Muscular Attachments: The longus capitis muscle (m. longus capitis) attaches lateral to the pharyngeal tubercle, while the rectus capitis anterior muscle (m. rectus capitis anterior) inserts into the area immediately anterior to the occipital condyles.

Lateral Parts (Pars lateralis)

The paired lateral parts bound the sides of the foramen magnum and articulate laterally with the petrous and mastoid portions of the temporal bones.

Internal Surface (Facies interna)

• Groove for sigmoid sinus (sulcus sinus sigmoidei): A shallow groove located posterior to the jugular process, lodging the terminal segment of the sigmoid sinus.
• Jugular process (processus jugularis): A prominent quadrilateral plate projecting laterally from the condyle; serves as the attachment point for the rectus capitis lateralis muscle (m. rectus capitis lateralis).
• Jugular tubercle (tuberculum jugulare): An oval bony elevation situated near the junction of the basilar and lateral parts, lying superior to the hypoglossal canal.
• Jugular notch (incisura jugularis): A smooth concavity that combines with the jugular notch of the temporal bone to form the jugular foramen (foramen jugulare). It transmits:
  • Glossopharyngeal nerve (CN IX)
  • Vagus nerve (CN X)
  • Accessory nerve (CN XI)
  • Internal jugular vein (v. jugularis interna)
  • Inferior petrosal sinus (sinus petrosus inferior)

• Intrajugular process (processus intrajugularis): An inconstant sharp spule of bone that, along with a similar process on the temporal bone, may bridge the jugular foramen to divide it into neural and venous compartments.

External Surface (Facies externa)

• Occipital condyle (condylus occipitalis): Paired kidney-shaped, convex articular processes flanking the anterior half of the foramen magnum. They articulate with the superior articular facets of the atlas (C1).
• Condylar fossa (fossa condylaris): A depression located directly behind each condyle that receives the posterior margin of the superior articular facet of the atlas during extension of the head.
• Condylar canal (canalis condylaris): An inconstant passage opening into the condylar fossa that transmits an occipital emissary vein (v. emissaria occipitalis).
• Hypoglossal canal (canalis hypoglossus): A bony canal running anterolaterally through the base of the condyle. It transmits the hypoglossal nerve (CN XII), a meningeal branch of the ascending pharyngeal artery (a. pharyngea ascendens), and an emissary vein from the basilar venous plexus (plexus venosus basilaris), connecting the posterior cranial fossa to the external skull base.

Squamous Part (Squama occipitalis)

The squamous part is the largest, most posterior portion of the bone. It borders the parietal and temporal bones superiorly and laterally, while its inferior border forms the posterior boundary of the foramen magnum.

Internal Surface of the squamous part (Facies interna)

• Cruciform eminence (eminentia cruciformis): A cross-shaped arrangement of bony ridges that divides the internal squama into four distinct fossae.
• Internal occipital protuberance (protuberantia occipitalis interna): The central, raised intersection of the cruciform eminence (often corresponding to the confluence of sinuses / confluens sinuum).
• Groove for superior sagittal sinus (sulcus sinus sagittalis superioris): Extends superiorly from the internal protuberance.
• Groove for transverse sinus (sulcus sinus transversi): Paired horizontal grooves running laterally from the internal protuberance toward the lateral angles of the bone.
• Internal occipital crest (crista occipitalis interna): A vertical ridge descending from the internal protuberance to the foramen magnum; gives attachment to the falx cerebelli.
• Cerebral fossa (fossa cerebralis): Paired superior triangular depressions above the transverse grooves that house the occipital lobes of the cerebrum.
• Cerebellar fossa (fossa cerebellaris): Paired inferior depressions below the transverse grooves that house the cerebellar hemispheres.
• Vermian fossa (fossa vermiana): A small, variable depression located along the lower portion of the internal occipital crest that lodges the inferior vermis of the cerebellum.
• Groove for marginal sinus (sulcus sinus marginalis): A shallow groove running along the margin of the foramen magnum accommodating the marginal sinus.

External Surface (Facies externa)

• External occipital protuberance (protuberantia occipitalis externa): A prominent midline landmark readily palpable through the skin (the midpoint of which is known as the inion).
• External occipital crest (crista occipitalis externa): A vertical ridge extending along the midline from the external protuberance to the foramen magnum; gives attachment to the nuchal ligament (ligamentum nuchae).
• Highest nuchal line (linea nuchalis suprema): A faint curved line above the external protuberance; serves as the attachment site for the epicranial aponeurosis (galea aponeurotica).
• Superior nuchal line (linea nuchalis superior): A prominent transverse ridge extending laterally from the external protuberance toward the lambdoid suture. It serves as the origin for the occipital belly of the occipitofrontalis muscle (m. occipitofrontalis), trapezius, and sternocleidomastoid.
• Occipital plane (planum occipitale): The smooth area of the squama lying superior to the superior nuchal lines.
• Nuchal plane (planum nuchale): The rough area below the superior nuchal lines that provides insertion for several neck muscles (including the splenius capitis, semispinalis capitis, and rectus capitis posterior major/minor).
• Inferior nuchal line (linea nuchalis inferior): A transverse ridge running parallel to the superior nuchal line midway between it and the foramen magnum, crossing the external occipital crest.