Hjässbenen

Hjässbenen, Swedish for the occipital bone, is a single cranial bone located at the back and base of the skull. It forms the posterior portion of the skull and contributes to the base of the cranial cavity, housing and protecting the brainstem and cerebellum. It articulates with the parietal bones along the lambdoid suture, with the temporal bones laterally, and with the sphenoid anteriorly via the basioccipital region. The atlas (C1) articulates with the occipital bone at the occipital condyles, forming the atlanto-occipital joints.

Structure

The occipital bone consists of three parts: the squamous part (posterior portion of the skull vault behind the foramen magnum), the basilar part (anterior base of the skull), and the two lateral parts that form the occipital condyles. The foramen magnum is the large opening at the base through which the medulla oblongata and meninges pass; it is flanked by the occipital condyles that articulate with the atlas.

Surface features

Externally, notable landmarks include the external occipital protuberance, the superior and inferior nuchal lines, and the occipital condyles. Internally, the bone displays the internal occipital crest and the cruciform eminence that subdivides the posterior cranial fossa; grooves for dural venous sinuses, the hypoglossal canals (transmitting CN XII), and other openings are present near the foramen magnum. The internal surface also hosts the torcular herophili region where several sinuses meet.

Development and clinical notes

The occipital bone develops from multiple ossification centers (supraoccipital and exoccipital portions plus the basioccipital) that fuse during life. Clinically, basilar skull fractures can involve the foramen magnum, and occipital condyle fractures may affect the craniovertebral junction or surrounding nerves.