The occipital bone (also known as C0) is a trapezoid skull bone that contributes to the posteroinferior part of the cranial vault. It is pierced by the foramen magnum, permitting communication from the cranial cavity to the vertebral canal.
Occiput is a noun referring to the back of the head, it is not a synonym for the occipital bone. The occipital bone is also known as "C0" because it joins the skull to the first cervical vertebra or C1, forming the atlanto-occipital joint.
The occipital bone is composed of four parts:
- squamous part: external/internal surfaces
- basilar part (basiocciput): lower/upper surfaces
- lateral (jugular) parts (two): under/upper surfaces
The squamous part is the curved, expanded plate located behind the foramen magnum.
The external surface of the squamous part features:
- external occipital protuberance, the tip is known as the inion
- highest nuchal line
- superior nuchal line: occipitalis, trapezius, sternocleidomastoid, splenius capitis
- inferior nuchal line: rectus capitis posterior major and minor
- median nuchal line: attachment of ligamentum nuchae
- attachment of posterior atlanto-occipital membrane
The internal surface of the squamous part features:
- cruciate eminence divide region into four fossa; the upper two contain the occipital lobes while the lower two contain the cerebellar hemispheres
- internal occipital tuberosity at intersection
- sagittal sulcus for superior sagittal sinus, falx cerebri attaches to margins
- internal occipital crest gives attachment to falx cerebelli
- transverse grooves hold transverse sinuses
The basilar part is the quadrilateral piece in front of the foramen magnum.
The lower surface of the basilar part features:
- pharyngeal tubercle: attachment to fibrous raphe of pharynx
- either side of middle line: longus capitis, and rectus capitis anterior
- anterior atlanto-occipital membrane
The upper surface of the basilar part features:
- the clivus has a broad, shallow groove, for medulla oblongata
- lateral grooves for the inferior petrosal sinus
The under surface of the lateral part features:
- occipital condyles: articulate with the atlas (C1), permitting flexion and extension (nodding) with minimal lateral flexion. No rotation occurs at the atlanto-occipital joint.
- the condyles are egg shaped and sloped inferomedially from anterior to posterior which helps limits mobility
- a third condyle may be a variant
- hypoglossal canal: at base of occipital condyles
- condylar fossa: behind condyle, receives posterior margin of superior facet of atlas, can be perforated by condyloid canal which contains an emissary vein from the transverse sinus
- jugular process, excavated in front by jugular notch, forming posterior part of jugular foramen
The upper surface of the lateral part features
The occipital bone articulates with 6 bones:
- paired bones
The apical portion of the squamous part (above highest nuchal line): ossifies in membrane. Otherwise, the four parts ossify in cartilage.
Four parts are present at birth. Fusion occurs:
- 2year: squamous and jugulars
- 6 year: jugulars with basilar
- 25 year: basilar with basisphenoid