Parietal Bone

What is the Occipital Bone

The occipital is a substantial, flat, paired cranial bone that constitutes a significant portion of the skull.

Where is the Occipital Bone Located

The occipital bones are positioned on either side of the skull, forming most of the top and sides of the head. They are part of the neurocranium, along with frontal, parietal, sphenoid, temporal, and ethmoid bones, sharing borders with the first four of these five bones. The name ‘occipital’ derives from their location at the back of the brain.

Quick Facts

Functions

Its main function as a skull bone is structural. The paired bones have the following functions:

• Forms the sides and roof of the skull towards the back, hence shaping the head.
• The left and right occipital bones protect the brain’s left and right occipital lobes, respectively. 
• Restricts any swelling in case of a brain condition involving an infection, bleeding, or an increase in cerebrospinal fluid levels.

Occipital Bone Anatomy and Landmarks

The curved bone is somewhat quadrilateral, with 2 rather broad surfaces and 4 borders. 

Borders and Sutures

The borders articulate with the other bones around the occipital, forming 5 sutures. Sutures are immovable joints in the skull.

1. Sagittal Border: It is the upper medial border, where the thickest and longest suture forms. This is where the left and right occipital bones join with each other, forming the sagittal suture.
2. Squamosal Border: The lower border, where the occipital and the greater wing of the sphenoid bones join, giving rise to the sphenoccipital suture. It also joins with the temporal bone at the occipitomastoid suture. The border is thin at its starting point, arching slightly along the middle, then thickening towards the back. 
3. Frontal Border: The most serrated of all the occipital bone’s margins, this is where the bone articulates with the posterior part of the frontal bone. The associated suture is called the lambdoidal suture.
4. Parietal Border: This is where the bone articulates with the parietal bone towards the top. Also highly serrated, it forms the lambdoidal suture.

Angles

Since there are 4 borders, there are 4 angles where the borders meet:

1. Frontal Angle: Where the sagittal border meets the frontal border. 
2. Sphenoidal Angle: The thin, pointed angle where the frontal border intersects the squamosal. 
3. Parietal Angle: The angle at the upper back side of the bone; it is formed where the sagittal and parietal borders meet. It is the most rounded of all the angles in the occipital bone.
4. Mastoid Angle: The angle at the lower back side, it forms where the parietal border intersects the squamosal border.

Surfaces

The External Surface

As the name suggests, this is the convex outer surface of the bone that protects the brain. This smooth curved surface also shapes the back of the head. It has several important landmarks for muscular attachments:

• The superior temporal line is the arch between the parietal and frontal borders; it is where the superficial temporal fascia attaches to the occipital. 
• The inferior temporal line is where the temporalis muscle originates. This line forms a similar arch to the superior temporal line but is located lower in the skull.
• Occipital foramina are present at the upper back side of each of the occipital bones, near the sagittal border. These foramina drain into the superior sagittal sinus and allow passage to the occipital artery’s branches.
• The occipital eminence is the central part of the occipital bones, where the bones start to ossify.

The Internal Surface

It is the concave inner surface of the bone on the side of the brain. The internal surface is highly irregular to accommodate different blood vessels. The most important of them include the following: 

• The arterial grooves, or grooves for the middle meningeal artery
• The grooves for superior sagittal sinus
• The grooves for sigmoid sinus

The granular foveolae surrounding the groove for superior sagittal sinus and containing arachnoid granulations

References

1. Occipital Bone: KenHub.com
2. Occipital Bone: RadioPaedia.org 
3. Occipital Bone Anatomy & Function: Study.com
4. Occipital Bone: Anatomy.app
5. Occipital Bone Anatomy: GetBodySmart.com