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Occipital bone

The occipital bone is an unpaired bone which covers the back of the head (occiput). It makes up a large portion of the basilar part of the neurocranium and entirely houses the cerebellum

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.

Key facts about the occipital bone
Location Unpaired bone which covers the occiput
Borders Superolaterally: Lambdoid suture
Inferolaterally
: Occipitomastoid suture
Inferiorly
: Petrooccipital fissure
Parts Basilar part, condylar parts, squamous part
Landmarks Basilar 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
Lateral parts
:
Upper surface: Jugular tubercle
Under surface: Occipital condyles, hypoglossal canal, condylar fossa, jugular process 
Squamous part
:
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
Foramina 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)
Contents
  1. Borders
  2. Anatomy
    1. Basilar part
    2. Lateral parts
    3. Squamous part
    4. Foramen magnum
  3. Summary
  4. Sources
+ Show all

Borders

The occipital bone articulates with 6 bones, two paired bones (parietal and temporal bones) and two unpaired bones (sphenoid bone and atlas).

It is bordered superiorly and laterally by the lambdoid suture which separates it from the parietal bones.

It articulates with the mastoid process of the temporal bone through the occipitomastoid suture. While the petro-occipital suture joins 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.

Anatomy

The occipital bone is convex externally and concave internally. It is divided into four parts: the basilar part, two lateral parts and squamous part.

Basilar 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 longus capitis and rectus capitis anterior muscles attach to the lower surface of the basilar part of the occipital bone.

Lateral parts

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.

Squamous part

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).

Foramen magnum

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 anterior and posterior spinal arteries
  • The vertebral arteries
  • The tectorial membrane 
  • The alar ligaments

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