It is the only cranial bone to articulate with the cervical spine. Besides this joint, 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).
This article will deal with detailed anatomy of the occipital bone.
|Location||Unpaired bone which covers the occiput|
Superolaterally: Lambdoid suture
Inferolaterally: Occipitomastoid suture
Inferiorly: Petrooccipital fissure
|Parts||Basilar part, condylar parts, squamous part|
Upper surface: Clivus, lateral grooves for inferior petrosal sinus
Lower surface: Pharyngeal tubercle, attachment sites for longus capitis, rectus capitis anterior and anterior atlanto-occipital membrane
Upper surface: Jugular tubercle
Under surface: Occipital condyles, hypoglossal canal, condylar fossa, jugular process
External surface: External occipital protuberance, supreme nuchal line, superior nuchal line, inferior nuchal line, attachment point for posterior atlanto-occipital membrane
Internal surface: Cruciform eminence, internal occipital protuberance, sagittal sulcus, internal occipital crest, groove for transverse sinus, groove for occipital sinus, groove for marginal sinus
Foramen magnum: Contents include: Medulla oblongata, spinal root of accessory nerve (CN XI), vertebral arteries, anterior and posterior spinal arteries, tectorial membrane, alar ligaments
Jugular foramen: Contents include: Inferior petrosal sinus, sigmoid sinus, glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI)
Hypoglossal canal: Contents include: Hypoglossal nerve (CN XII)
The occipital bone articulates with 6 bones, two paired bones (parietal and temporal bones) and two unpaired bones (sphenoid bone and atlas).
The spheno-occipital suture between the sphenoid and occipital bones disappears as they fuse during adolescence.
The occipital bone is convex externally and concave internally. It is divided into four parts: the basilar part, two lateral parts and squamous part.
The basilar part is the quadrilateral piece of bone that sits anterior to the foramen magnum and adjacent to the petrous part of the temporal bone. It contains two surfaces: lower surface and upper surface.
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 lower surface of the basilar part of the occipital bone.
The lateral parts of the occipital bone are located lateral to the foramen magnum and contain both under and upper surfaces.
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 under surface of the lateral part of the occipital bone.
The squamous part is the largest of all four and contains both internal and external surfaces.
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 supreme 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).
All four parts of the occipital bone are arranged around a large opening, the foramen magnum.
Structures that pass through this foramen magnum include:
- The spinal branch of the accessory nerve
- The anterior and posterior spinal arteries
- The vertebral arteries
- The tectorial membrane
- The alar ligaments
The occipital bone is an unpaired bone which covers the back of the head (occiput). It is bordered by the lamboid, occipitomastoid, and the petro-occipital sutures.
The occipital bone is divided into four parts arranged around the foramen magnum, as follows:
- The basilar part: Located anterior to the foramen magnum and adjacent to the petrous part of the temporal bone. The pharyngeal tubercle can be identified from the inferior surface of the basilar part of the occipital bone.
- The lateral parts: Located lateral to the foramen magnum and are composed of two occipital condyles. Posterior to the occipital condyles are the condylar canals. The hypoglossal canal, containing the hypoglossal nerve is located at the base of the occipital condyles.
- The squamous part: Located posterior to the foramen magnum and forms the largest part of the occipital bone. It contains a palpable prominence named the external occipital protuberance and is marked by three nuchal lines and a number of grooves which allow for the passage of the dural venous sinuses.
Occipital bone: want to learn more about it?
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