Occipital bone

Os occipitale

The single bone at the back and base of the skull, ringing the foramen magnum and resting on the first neck vertebra.

At a glance

English: Occipital bone
Latin (TA2): Os occipitale
System: Bone
Region: Head
Side: Single, on the midline

About the occipital bone

The occipital bone closes the back of the cranium and forms much of its floor. It is a curved, roughly trapezoid plate, bowl-shaped on its inner side, lying behind the parietal and temporal bones and joined in front to the body of the sphenoid. Its central landmark is the foramen magnum, the oval opening through which the brainstem becomes the spinal cord. Around this opening the bone is described in parts: the basilar part in front, a lateral part on each side and the broad squamous part behind and above.

On the outside, the squama carries the external occipital protuberance, whose top point, the inion, can be felt at the back of the head. From it the external occipital crest runs down the midline to the foramen and holds the nuchal ligament. Curved nuchal lines cross the bone on either side and anchor the neck muscles, trapezius and semispinalis capitis among them. On the lateral parts the two occipital condyles rest on the atlas; the hypoglossal canal tunnels through the bone above each condyle, with the condylar fossa behind it. The basilar part slopes up and forward towards the sphenoid, and the pharyngeal tubercle on its underside anchors the raphe of the pharynx.

Inside, the bone forms most of the posterior cranial fossa. A cross of ridges, the cruciform eminence, divides it into upper hollows for the occipital lobes and lower hollows for the cerebellum, meeting at the internal occipital protuberance. Grooves for the transverse sinuses run outward from that point, and the internal occipital crest descends to the foramen and holds the falx cerebelli. In front of the foramen the smooth clivus rises towards the sphenoid. The foramen magnum carries the medulla oblongata and its meninges, the vertebral and spinal arteries, the spinal roots of the accessory nerves and the ligaments that steady the upper neck. The jugular notch, with the temporal bone, forms the jugular foramen.

The upper squama forms in membrane and the rest in cartilage; the parts unite in early childhood, and the basilar part fuses with the sphenoid between the late teens and the mid-twenties. A blow to the back of the head can produce a basilar skull fracture, and on radiographs the gap between the basion and the tip of the dens helps detect separation of the skull from the spine.

Function and supply

Function: Protects the occipital lobes, cerebellum and brainstem, passes the weight of the head to the atlas through its condyles (allowing nodding), and anchors the ligament and muscles at the back of the neck.
Nerve supply: Not supplied as a muscle is; the scalp over it is supplied by the greater and lesser occipital nerves, and the dura of the posterior cranial fossa by meningeal branches of the upper cervical nerves and the vagus nerve.
Blood supply: The occipital artery supplies the outer surface; meningeal branches of the vertebral, ascending pharyngeal and occipital arteries and the posterior branch of the middle meningeal artery supply the inner surface.

Parts of the occipital bone

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Basilar part of occipital bonePars basilaris ossis occipitalisThe thick, roughly four-sided bar ahead of the foramen magnum, slanting up and forward to join the body of the sphenoid. Its upper surface forms the lower clivus under the brainstem and its lower surface faces the roof of the pharynx.
Cerebellar fossaFossa cerebellarisEach of the two lower hollows of the inner surface, below the grooves for the transverse sinuses. They hold the cerebellar hemispheres, and the bone here is thin and translucent.
Cerebral fossaFossa cerebralisEach of the two upper, triangular hollows of the inner surface, above the transverse sinus grooves. The occipital lobes of the brain rest in them.
Condylar canalCanalis condylarisAn inconstant channel opening in the floor of the condylar fossa behind each condyle. It carries an emissary vein linking the sigmoid sinus with veins of the neck.
Condylar fossaFossa condylarisA hollow just behind each occipital condyle. It takes the back rim of the atlas's upper facet when the head is tipped backward.
Cruciform eminenceEminentia cruciformisThe cross-shaped set of ridges on the inner surface. Its arms divide the bowl into four hollows, two for the occipital lobes above and two for the cerebellum below, and they meet at the internal occipital protuberance.
External occipital protuberanceProtuberantia occipitalis externaA bump at about the centre of the outer squama, easily felt where the back of the neck meets the skull. Its highest point is the inion, and the nuchal ligament and the upper fibres of trapezius attach around it.
Foramen magnumThe large oval opening in the skull base, longer front to back and narrowed in front by the condyles. The medulla oblongata and its coverings, the vertebral and spinal arteries and the spinal roots of the accessory nerves pass through it.
Groove for marginal sinusSulcus sinus marginalisA faint, inconstant groove close to the back edge of the foramen magnum on its inner side. It holds the marginal sinus, a small venous channel around the opening.
Groove for occipital sinusSulcus sinus occipitalisA shallow groove along the lower arm of the cruciform eminence, between the two cerebellar hollows. It holds the occipital sinus within the attached edge of the falx cerebelli.
Groove for transverse sinusSulcus sinus transversiA broad groove running sideways from the internal occipital protuberance along each horizontal arm of the cruciform eminence, holding the transverse sinus; the tentorium cerebelli is fixed to its lips.
Hypoglossal canalCanalis hypoglossusA tunnel through the bone above the front part of each condyle. The hypoglossal nerve leaves the skull through it on its way to the muscles of the tongue.
Inferior nuchal lineLinea nuchalis inferiorA curved ridge crossing the squama between the superior nuchal line and the foramen magnum. Semispinalis capitis inserts in the area between the two lines, and the small suboccipital muscles attach below it.
Internal occipital protuberanceProtuberantia occipitalis internaThe raised point at the centre of the cruciform eminence on the inner surface. It lies at the confluence of the sinuses, where the superior sagittal, straight and transverse sinuses meet.
Intrajugular process of occipital boneProcessus intrajugularis ossis occipitalisA small spur jutting from the mid-point of the jugular notch. It partly divides the jugular foramen, helping separate the passage for the cranial nerves from that of the internal jugular vein.
Jugular notch of occipital boneIncisura jugularis ossis occipitalisA notch on the outer edge of each lateral part, in front of the jugular process. With a matching notch on the petrous temporal bone it forms the jugular foramen, through which the internal jugular vein and the glossopharyngeal, vagus and accessory nerves leave the skull.
Jugular processProcessus jugularisA plate of bone extending outward from the back half of each condyle. Its front edge forms the jugular notch, and rectus capitis lateralis attaches to its underside.
Lateral part of occipital bonePars lateralis ossis occipitalisThe thick part on each side of the foramen magnum, linking the basilar part in front to the squama behind. It carries the occipital condyle, the hypoglossal canal and the jugular process.
Mastoid border of occipital boneMargo mastoideus ossis occipitalisThe upper stretch of the lower side edge of the squama, running down from the lateral angle. It meets the mastoid part of the temporal bone at the occipitomastoid suture.
Occipital condyleCondylus occipitalisOne of two oval, convex knobs on the underside of the lateral parts, beside the front half of the foramen magnum. They sit in the upper facets of the atlas and form the atlanto-occipital joints, which allow nodding.
Occipital planePlanum occipitaleThe smooth upper part of the outer squama, above the protuberance and the nuchal lines. It lies under the scalp and the occipital belly of occipitofrontalis.
Pharyngeal tubercleTuberculum pharyngeumA small bump on the underside of the basilar part, roughly 1 cm forward of the foramen magnum. The fibrous raphe of the pharynx, with the superior constrictor, hangs from it.
Squamous part of occipital boneSquama occipitalisThe broad, curved plate behind and above the foramen magnum, the largest part of the bone. It bears the protuberance and nuchal lines on its outer face and the cruciform eminence on its inner face.
Superior nuchal lineLinea nuchalis superiorA curved ridge running out to each side from the external occipital protuberance. Trapezius attaches along its inner third, and sternocleidomastoid and splenius capitis along its outer part.
Supreme nuchal lineLinea nuchalis supremaA faint arched ridge above the superior nuchal line, starting from the top of the protuberance. The epicranial aponeurosis and the occipital belly of occipitofrontalis attach to it.

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At a glance

About the occipital bone

Function and supply

Related structures

See the occipital bone where it really sits, with every layer of the body around it.