Anatomic.us > Skeletal System Posterior

Occipital Bone

It is one of the bones of the skull and is present on posterior most aspect (back and lowermost part) of the skull. It is called occipital bone because it covers the occipital lobe of the brain. It is trapezoidal in its shape. This bone is shaped like a saucer. It has a large opening (foramen magnum) through which medulla oblongata (tail of the brain) passes and is continued downwards as spinal cord. Thus it allows the brain to communicate with the rest of the body to carry out the its activities.

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Posterior

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Occipital Bone


GENERAL BONY FEATURES

For the sake of study, it is divided into following three parts:

The squama occipitalis which is the portion of the occipital bone that is present behind and above the foramen magnum. It is a curved and expanded plate.

  • The external occipital protuberance is present on the external surface (which is convex) between the foramen magnum and the highest point of the bone.

  • The nuchal lines are the horizontal curved lines that are present on the external surface of the squama occipitalis. The superior most is mostly faint and is called highest nuchal line. From above downwards, below the highest nuchal line are superior nuchal line, median nuchal line and inferior nuchal line respectively.

  • Cruciform eminence is an elevation that divides the concave internal surface of squama of occipital bone into four fossae.

  • The internal occipital protuberance is an elevation that is present at the point where the four divisions of the cruciform eminence intersect.

  • The internal occipital crest is the prominent lower division of the cruciform eminence.

 

The basilar part is that part of the occipital bone that is present in front of the foramen magnum. It is thick and is quadrilateral in outline.

  • The pharyngeal tubercle is present on its lower surface, in front of the foramen magnum.

 

The lateral parts of occipital bone are present on both sides of the foramen magnum.

  • Condyles are present under its surface. These are for joint formation with the first cervical vertebra i.e. atlas vertebra.

  • The hypoglossal canal is present at the base of each condyle.

  • The condylar fossa is a depression which is present behind each condyle.

  • The jugular tubercle is an oval eminence that is present on the upper surface of this part.

 

SOME CLINICAL ASPECTS

 

The part of the occipital bone forming the base of the skull may be fractured (rarely) causing basilar skull fracture. The prominent sign of this fracture is flowing of CSF through the nose and ears.

The condyles of occipital bone may be fractured but it is also a rare injury.

Blow to the back of head, at occipital region causes sudden fainting due to the shock to the medulla of brain.

 



GENERAL BONY FEATURES

For the sake of study, it is divided into following three parts:

The squama occipitalis which is the portion of the occipital bone that is present behind and above the foramen magnum. It is a curved and expanded plate.

  • The external occipital protuberance is present on the external surface (which is convex) between the foramen magnum and the highest point of the bone.

  • The nuchal lines are the horizontal curved lines that are present on the external surface of the squama occipitalis. The superior most is mostly faint and is called highest nuchal line. From above downwards, below the highest nuchal line are superior nuchal line, median nuchal line and inferior nuchal line respectively.

  • Cruciform eminence is an elevation that divides the concave internal surface of squama of occipital bone into four fossae.

  • The internal occipital protuberance is an elevation that is present at the point where the four divisions of the cruciform eminence intersect.

  • The internal occipital crest is the prominent lower division of the cruciform eminence.

 

The basilar part is that part of the occipital bone that is present in front of the foramen magnum. It is thick and is quadrilateral in outline.

  • The pharyngeal tubercle is present on its lower surface, in front of the foramen magnum.

 

The lateral parts of occipital bone are present on both sides of the foramen magnum.

  • Condyles are present under its surface. These are for joint formation with the first cervical vertebra i.e. atlas vertebra.

  • The hypoglossal canal is present at the base of each condyle.

  • The condylar fossa is a depression which is present behind each condyle.

  • The jugular tubercle is an oval eminence that is present on the upper surface of this part.

 

SOME CLINICAL ASPECTS

 

The part of the occipital bone forming the base of the skull may be fractured (rarely) causing basilar skull fracture. The prominent sign of this fracture is flowing of CSF through the nose and ears.

The condyles of occipital bone may be fractured but it is also a rare injury.

Blow to the back of head, at occipital region causes sudden fainting due to the shock to the medulla of brain.

 

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