The Occipital Bone
It is the only cranial bone to articulate with the cervical spine, and besides this articulation, it articulates with many other bones of the skull. Because of that, the occipital bone is described in terms of separate parts which are: the basilar part, squamous part, lateral parts (placed laterally to the foramen magnum).
Superolaterally: lambdoid suture that separates it from parietal bones
Inferolaterally: occipitomastoid suture that separates it from the mastoid process
Inferiorly: petrooccipital suture that separates it from the petrous part of the temporal bone
- external occipital protuberance - prominence at the middle of the outer surface
- inion - highest point of the external occipital protuberance
- external occipital crest (medial nuchal line) - a ridge that runs from the inion along the midline to the foramen magnum
- highest, superior and inferior nuchal lines - lines that run horizontally across the outer surface
- condyles - articulation surfaces for the atlanto-occipital joint, placed laterally to foramen magnum
- internal occipital protuberance - inner prominence analogue to the external occ. prominence
- internal occipital crest - a ridge that runs from the internal occipital protuberance to the foramen magnum
- clivus - smooth stripe travelling in front and upwards from the foramen magnum
- cruciform eminence - cross-shaped lines that cross at the internal occipital protuberance, divide the surface into 4 fossae
Foramen magnum - large oval opening at the center of the bone through which pass: medulla oblongata, accessory nerves, vertebral arteries, anterior and posterior spinal arteries, tectorial membrane, alar ligaments
Jugular foramen - located between the petrous part of the temporal bone and corresponding margin of the occipital bone; contains inferior petrosal and sigmoid sinuses; glossopharyngeal, vagus, accessory nerves
Hypoglossal canal - pierces condylar parts of the bone, contains hypoglossal nerve
This article will deal with detailed anatomy of the occipital bone.
The bone is convex externally and concave internally. It is divided into four parts: the basilar part, two condylar parts and the squamous part. All four are arranged around a large opening, the foramen magnum. Structures that pass through this foramen magnum include:
- the brainstem (medulla oblongata)
- spinal branch of accessory nerve
- anterior and posterior spinal arteries
- vertebral artery
- spinal vein
The basilar part sits anterior to the foramen magnum and adjacent to the petrous part of the temporal bone. Anteriorly it fuses with the sphenoid bone to form the clivus during adolescence (tribasilar bone). Both the superior pharyngeal constrictor muscle and the pharyngeal raphe insert onto the pharyngeal tubercle found on the inferior surface of the basilar part.
The condylar parts are located lateral to the foramen magnum. They comprise two kidney-shaped prominences (occipital condyles) that articulate with the first cervical vertebra (atlanto-occipital joint).
Posterior to the them are the condylar canals where the condylar emissary veins pass through and connect the external vertebral venous plexuses with the sigmoid sinuses. The hypoglossal nerve exits the neurocranium through the hypoglossal canal which pierces through the condylar part of the occipital bone.
The squamous part is the largest of all four.
A palpable prominence known as the external occipital protuberance lies on the midline of the external surface which serves as an attachment for the trapezius muscle.
Furthermore the external surface features three curved lines referred to as nuchal lines:
- The highest nuchal line extends laterally from the external occipital protuberance and is the site of origin of the epicranius muscle and epicranial aponeurosis.
- The superior nuchal line runs slightly inferior. It provides origin to the trapezius, sternocleidomastoid and splenius capitis muscles.
- The inferior nuchal line runs further inferiorly. The semispinal capitis muscle inserts above it.
The squamous part of the occipital bone is marked by grooves on its internal surface due to dural venous cranial sinuses: the superior sagittal sinus, the transverse sinuses and the sigmoid sinus. Superior to the groove for the transverse sinus is a depression that accommodates the occipital brain lobes (cerebral fossa) and the corresponding inferior depression houses the cerebellum (cerebellar fossa).
The occipital bone is bordered superiorly and laterally by the lambdoid suture which separates it from the parietal bones. It articulates with the mastoid process through the occipitomastoid suture. The petro-occipital suture joints the petrous part of the temporal bone with the occipital bone. The spheno-occipital suture between the sphenoid and occipital bones disappears as they fuse during adolescence.
The basilar and condylar parts develop through endochondral ossification. Most of the squamous part undergoes endochondral ossification as well, except its superior part which develops through the intramembranous pathway. In newborns all four parts are separated by connective tissue. By the age of four the squamous and condylar parts will usually have ossified together whereas the basilar portion joins them at age six.
The occipital bone is an unpaired bone which covers the back of the head (occiput). It is divided into four parts arranged around the foramen magnum, as follows:
- basilar part - sits anterior to the foramen magnum and adjacent to the petrous part of the temporal bone. There is also a pharyngeal tubercle on the inferionr surface of the basilar part.
- condylar parts - are located lateral to the foramen magnum and comprise two occipital condyles. Posterior to them are the condylar canals, while the hypoglossal canal also pierces through the condylar part of the occipital bone.
- squamous part - it contains a palpable prominence named the external occipital protuberance, together with three nuchal lines and grooves for the dural venous sinuses,
The occipital bone is bordered by the lamboid, occipitomastoid, and the petro-occipital sutures. In addition, it develops through both endochondral and intramembranous ossifications.