Occipital Bone

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Published on June 24th 2022 by

What is the Occipital Bone

The occipital is an unpaired, trapezoid-shaped cranial bone covering the back of the head.

Where is the Occipital Bone Located

The bone is present at the back of the head.

Quick Facts

TypeFlat bone
How many are there in the human body1
Articulates withTotal 6 bones2 paired: Parietal and temporal bones2 unpaired: Sphenoid bone and atlas (C1)

Function

It protects the occipital lobe and cerebellum of the brain, along with the associated nerves and blood vessels.

Occipital Bone Anatomy

This trapezoidal-shaped bone is externally convex and internally concave. It has four parts: the basilar, two condylar or lateral, and the squamous. Each part has two surfaces: upper and lower, or external and internal.

Surfaces and Landmarks

Foramen Magnum

The bone features a large oval opening at its back, through which the brain stem (medulla oblongata) passes. The four parts of the occipital bone are present around this hole. Besides the brain stem, the spinal branch of the accessory nerve, anterior and posterior spinal arteries, vertebral arteries, tectorial membrane, and alar ligaments also pass through this foramen.

Basilar Part

This quadrilateral-shaped part is adjacent to the petrous part of the temporal bone and anterior to the foramen magnum. Its two surfaces are: upper and lower.

During adolescence, the upper surface of the basilar part articulates with the sphenoid bone to form the clivus. It has a broad, shallow groove that supports the medulla oblongata. The lower surface features the pharyngeal tubercle, where the superior pharyngeal constrictor muscle and fibrous pharyngeal raphe insert. The other muscles attached to the lower surface are rectus capitis anterior and longus capitis.

Condylar Parts

The condylar parts are commonly known as the lateral parts of the occipital bone, as they are found lateral to the foramen magnum. It also contains upper and lower surfaces, just like the previous. This part features two kidney-shaped prominences, called occipital condyles that form articulation with the first cervical vertebra (C1), thus giving rise to the atlanto-occipital joint.

Just behind the two condyles, condylar canals are present, through which the condylar emissary veins pass. The canals also connect the external vertebral venous plexuses with the sigmoid sinuses. The hypoglossal canal lies on the inferior surface of the condylar part through which the hypoglossal nerve leaves the cranium.

Squamous Part

It is the largest of all four parts and contains internal and external surfaces. This part features the external occipital protuberance, a bony prominence in the middle of the outer surface.

The trapezius muscle attaches here. The external surface also has three curved lines, called the nuchal lines. They are:

  1. Supreme nuchal line: Laterally extends from the external occipital protuberance. The epicranius muscle and epicranial aponeurosis originate from here.
  2. Superior nuchal line: Running inferior to the squamous part, it serves as a site of origin for the trapezius, splenius capitis, and sternocleidomastoid muscles.
  3. Inferior nuchal line: Found further inferior to the superior nuchal line, this is where the semispinalis capitis muscle gets inserted.

Several grooves mark the internal surface of this part by dural venous cranial sinuses, such as the superior sagittal sinus, the transverse sinuses, and the sigmoid sinus. The groove above the transverse sinus houses the occipital lobes and cerebellum of the brain.

Borders and Articulations

The bone articulates with total 6 bones. Among them, 2 are paired (parietal and temporal), while the other 2 are unpaired (sphenoid and first cervical vertebrae, C1). So, the occipital is the only cranial bone to form an articulation with a spine bone.

  1. Lambdoid Suture: The occipital and parietal bones connect at this suture.
  2. Occipitomastoid Suture: The bone connects with the mastoid part of the temporal bone through this suture.
  3. Petro-Occipital Suture: The occipital bone articulates with the petrous part of the temporal bone via this suture.
  4. Spheno-Occipital Suture: The occipital meets with the sphenoid bone at this suture, finally fusing during adolescence. As a result, the suture also disappears.
  5. Atlanto-Occipital Joint: This is the only joint formed by the occipital bone with the first cervical vertebrae (C1).

Development and Ossification

The ossification of this bone starts around the 9th week of fetal life. The squamous part of the bone undergoes membranous ossification, whereas its other parts have cartilaginous ossification.

The four parts remain separate at birth but fuse as time passes.

  • The squamous and condylar parts fuse around the 2nd year.
  • The condylar and basilar parts fuse around the 6th year.

References

    1. Occipital bone — Kenhub.com
    2. Anatomy, Head and Neck, Occipital Bone, Artery, Vein, and Nerve — Ncbi.nlm.nih.gov
    3. Occipital bone — Radiopaedia.org
    4.  Occipital Bone Anatomy — Getbodysmart.com
    5. Occipital Bone — Sciencedirect.com

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