Occipital bone anatomy: foramen magnum, condyles and external occipital protuberance

What it is

The bone forming the back and base of the skull.

Location

Lies at the back and base of the skull, articulating with the parietal, temporal and sphenoid bones and with the atlas.

Function

Protects the cerebellum and occipital lobes and surrounds the foramen magnum, where the brainstem continues as the spinal cord.

Body system

Skeletal · single, midline structure

FMA ID

FMA52735 (Foundational Model of Anatomy)

Key facts

• Its condyles articulate with the atlas.
• The external occipital protuberance is a palpable landmark.
• The hypoglossal canal above each condyle transmits CN XII, and it forms part of the jugular foramen with the temporal bone.

The occipital bone covers the back of the head (the occiput) and forms much of the cranial base behind the middle of the skull. It encloses the cerebellum and is the only cranial bone that articulates with the cervical spine. (Kenhub: Occipital bone.)

Radiopaedia describes it as a trapezoid bone forming the posteroinferior part of the cranial vault, pierced by the foramen magnum, which connects the cranial cavity with the vertebral canal. It is sometimes called C0 because it sits above C1. The word occiput refers to the back of the head and is not a synonym for the bone. (Radiopaedia: Occipital bone.)

The bone has three regions arranged around the foramen magnum. The squamous part is the largest and most posterior; it borders the parietal and temporal bones and forms the back of the foramen magnum. The two condylar (lateral) parts form the sides of the foramen and carry the occipital condyles. The basilar part forms the front of the foramen and joins the temporal and sphenoid bones. (TeachMeAnatomy: The Occipital Bone.)

The external occipital protuberance is the bump in the midline at the back of the skull that you can feel under the skin; its tip is called the inion. Running sideways from it is the superior nuchal line, along which trapezius, sternocleidomastoid, splenius capitis and the occipital belly of occipitofrontalis attach along it. (Radiopaedia: Occipital bone.)

Above the superior line, the faint supreme (highest) nuchal line anchors the epicranial aponeurosis. The median nuchal line runs down from the protuberance towards the foramen magnum and anchors the nuchal ligament. The inferior nuchal line, about halfway down, gives attachment to the small suboccipital muscles, rectus capitis posterior major and minor and obliquus capitis superior. Semispinalis capitis inserts between the superior and inferior lines. (TeachMeAnatomy: The Occipital Bone.)

The protuberance is often listed as a feature that differs between males and females, but a 2023 geometric study of 228 crania found it was not sexually dimorphic and varied mainly with overall skull size. The glabella, brow ridges, mastoid process and nasal region showed the strongest sex differences. (Del Bove A et al. Mapping sexual dimorphism signal in the human cranium. Sci Rep 2023.)

The foramen magnum (Latin for great opening) is the largest foramen of the skull. It is oval, longer from front to back, and lies at the lowest point of the posterior cranial fossa. Its front edge is formed by the basilar part, its sides by the occipital condyles, and its back by the squamous part. The midpoint of the front edge is called the basion and the midpoint of the back edge the opisthion. (Radiopaedia: Foramen magnum.)

The foramen magnum transmits the medulla oblongata with its meninges, the two vertebral arteries, the anterior and posterior spinal arteries, the spinal root of the accessory nerve (CN XI), and two ligaments that stabilise the head on the neck: the tectorial membrane and the alar ligaments. Dural veins also pass through it. (TeachMeAnatomy: Foramina of the Skull.)

The two occipital condyles lie on either side of the foramen magnum. They are egg-shaped, angled towards the midline at the front, and articulate with the atlas at the atlanto-occipital joints. These joints allow flexion and extension (nodding) with minimal side bending and no rotation at this joint. (Radiopaedia: Occipital bone.)

The hypoglossal canal pierces the bone at the base of each condyle and carries the hypoglossal nerve (CN XII) to the tongue. Behind each condyle is the condylar fossa, which may contain a condylar canal for an emissary vein. Lateral to the condyle, the jugular process and the jugular notch in front of it form the back of the jugular foramen, which the occipital bone shares with the petrous part of the temporal bone. (TeachMeAnatomy: The Occipital Bone.)

The jugular foramen transmits the glossopharyngeal (CN IX), vagus (CN X) and accessory (CN XI) nerves, the inferior petrosal sinus and the sigmoid sinus with the jugular bulb, the start of the internal jugular vein. (TeachMeAnatomy: Foramina of the Skull.)

Inside the skull, grooves for the dural venous sinuses make a cross-shaped ridge, the cruciform eminence, with the internal occipital protuberance at its centre. The groove for the superior sagittal sinus runs upwards from it, the grooves for the transverse sinuses run sideways, and the internal occipital crest runs down towards the foramen magnum, giving attachment to the falx cerebelli. The two upper hollows hold the occipital lobes and the two lower hollows the cerebellar hemispheres. (Radiopaedia: Occipital bone.)

The upper surface of the basilar part forms the clivus, a broad, sloping groove that supports the medulla and gives attachment to the tectorial membrane. On its under surface, the pharyngeal tubercle anchors the pharyngeal raphe, and longus capitis and rectus capitis anterior attach on either side of the midline. (Kenhub: Occipital bone.)

The occipital bone articulates with six bones: the two parietal bones (lambdoid suture), the two temporal bones (occipitomastoid and petro-occipital sutures), the sphenoid (spheno-occipital synchondrosis) and the atlas (atlanto-occipital joints). (Radiopaedia: Occipital bone.)

The basilar part fuses with the body of the sphenoid during adolescence, closing the spheno-occipital joint and completing the clivus. (Kenhub: Occipital bone.)

At birth the occipital bone is still in several pieces. Radiopaedia lists five centres present at birth: the interparietal part, the supraoccipital part, the Kerckring ossicle, the paired exoccipitals (lateral parts) and the basioccipital. The interparietal part, above the highest nuchal line, ossifies in membrane; the other parts ossify in cartilage. The joints between them fuse at variable times in childhood. (Radiopaedia: Occipital bone.)

On CT, cerebrospinal fluid normally surrounds the medulla at the level of the foramen magnum. When this space is reduced or lost, it suggests tonsillar herniation, in which the cerebellar tonsils are pushed down through the foramen. Chiari malformations and meningiomas also affect this region. (Radiopaedia: Foramen magnum.)

Raised intracranial pressure can compress the abducens nerve where it runs near the clivus, causing an abducens nerve palsy. The clivus is also used on cervical spine X-rays to check the alignment of the atlanto-occipital joint. Valveless emissary veins in the scalp, including the occipital emissary vein, can carry infection from the scalp to the venous sinuses inside the skull. (TeachMeAnatomy: The Occipital Bone.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Cerebellum · nervous
• Pineal body · endocrine
• Inferior colliculus · nervous
• Fourth ventricle · nervous
• Superior colliculus · nervous
• Brachium of inferior colliculus · nervous
• Occipital lobe · nervous
• Medial occipital artery · cardiovascular
• Tentorium cerebelli · nervous
• Hair of head · skin