General structural plan
Anatomically, the occipital bone consists of four main parts that encircle the foramen magnum like a ring:
- Basilar (anterior) part (pars basilaris) — lies anterior to the foramen.
- Lateral parts (partes laterales) — paired structures on either side.
- Occipital squama (squama occipitalis) — the largest, dorsal part of the bone.
Basilar part (*Pars basilaris*)
This structure is directed forward and fuses with the sphenoid bone. Two surfaces are distinguished on it:
- Intracranial (superior) surface. Here lies the so-called clivus (clivus), against which the ventral surface of the brainstem rests. On either side of the clivus runs the groove for the inferior petrosal sinus (sulcus sinus petrosi inferioris), which continues onto the petrous parts of the temporal bones.
- External (inferior) surface. Along the midline is the pharyngeal tubercle (tuberculum pharyngeum) — an important site of attachment for the fibrous layer of the pharynx.
Lateral parts (*Partes laterales*)
These bear the articular surfaces for connection with the vertebral column and contain important canals.
- External surface: the occipital condyle (condylus occipitalis) projects here, participating in the formation of the atlanto-occipital joint. Behind it may lie the condylar fossa (fossa condylaris), at the base of which an inconstant condylar canal (canalis condylaris) sometimes opens, transmitting an emissary vein.
- Intracranial surface: laterally, the groove for the sigmoid sinus (sulcus sinus sigmoidei) is visible. Closer to the foramen magnum, the jugular tubercle (tuberculum jugulare) projects, and within the bone beneath it runs the hypoglossal canal (canalis nervi hypoglossi).
- External border: here lies the jugular notch (incisura jugularis). Together with the temporal bone, it forms the jugular foramen. The notch is bounded posteriorly by the jugular process (processus jugularis) and anteriorly by the intrajugular process (processus intrajugularis). An accessory paramastoid process (processus paramastoideus) can occasionally be found on the external surface.
Occipital squama (*Squama occipitalis*)
The most massive part of the bone, shaped like a broad plate. Its superior border is called the lambdoid border (margo lambdoideus), and its lateral border the mastoid border (margo mastoideus). Accessory interparietal bones (ossa interparietalia) are occasionally found along the suture line.
External surface (facies externa): At the center projects the external occipital protuberance (protuberantia occipitalis externa), from which the external occipital crest descends inferiorly. Nuchal lines (muscle attachment sites) radiate laterally from the protuberance and crest:
- Highest nuchal line (linea nuchalis suprema) — inconstant.
- Superior nuchal line (linea nuchalis superior).
- Inferior nuchal line (linea nuchalis inferior).
Below these lines lies the occipital plane (planum occipitale).
Internal surface (facies interna): The relief of this surface reflects the contact with the brain and venous sinuses. At the center is the cruciform eminence (eminentia cruciformis), with the internal occipital protuberance at its midpoint. Grooves radiate from it:
- Superiorly — the groove for the superior sagittal sinus.
- Laterally — the grooves for the transverse sinus.
- Inferiorly, the internal occipital crest descends, often continuing into the groove for the occipital sinus (and, at the edge of the foramen magnum, into the groove for the marginal sinus).
The cruciform eminence divides the squama into four fossae: two superior — the cerebral fossae (fossae cerebrales), and two inferior — the cerebellar fossae (fossae cerebellares).