The unpaired occipital bone (os occipitale) forms the posterior part f the base and posterior wall of the skull. It is made up of four parts, which fuse to form a single bone forming the borders of the foramen magnum as follows, the:
- Basilar part (pars basilaris) – anteriorly.
- Squamous part (squama occipitalis) – posteriorly.
- Lateral or condylar parts (partes laterales) –laterally.
The basilar partfuses with the sphenoid bone after the 18th year of life. The basilar part has the:
Clivus (clivus) which is the smooth superior surface of the basilar part. Together with the body of sphenoid bone, it forms the anterior boundry of foramen magnum. It is site where the medulla oblongata, pons and vessels (vertebral and basilar artery) are going on.
Pharyngeal tubercle (tuberculum pharyngeum) is situated on the inferior surfaces and is an attatchment area for the anterior longitudinal ligament.
Groove for inferior petrosal sinus (sulcus sinus petrosi inferioris).
The squamous part of the occipital bone (squama occipitalis)is shaped like a plate, with the:
External occipital protuberance (protuberantia occipitalis externa) which is a fine palpable eminence located in the middline of the occipital bone.
External occipital crest (crista occipitalis externa) which is a ridge extending inferiorly from the external occipital protuberance.
Highest or supreme nuchal line (linea nuchae suprema)for the attachment of epicranial aponeurosis.
Superior nuchal line (linea nuchae superioris) which is a transverse ridge that runs laterally from the external occipital protuberance on each side.
Inferior nuchal line (linea nuchae inferioris) which is a transverse ridge that runs laterally from the midpoint of the external occipital crest. It lies between the superior nuchal line and foramen magnum.
Occipital plane (planum occipitale) which is the surface above the external occipital protuberance.
Internal surface is determined by the shape of the brain and the attachment of its meninges, it is divided by two crests that form the:
Cruciform eminence (eminentia cruciformis) which is a cross-shaped elevation with the internal occipital protuberance (protuberantia occipitalis interna)at the center (midpoint).
Internal occipital crest (crista occipitalis interna) which is a bony ridge extending inferiorly from the internal occipital protuberance.
Groove for superior sagittal sinus (sulcus sinus sagittalis superioris) which is for the attachment of falx cerebri to its margins.
Groove for transverse sinus (sulcus sinus transversi) that extends laterally from the internal occipital protuberance. The tentorium cerebelli is attached to its margins.
Groove for the occipital sinus (sulcus sinus occipitalis) which is found beneath the internal occipital protuberance appears as a continuation of the superior sagittal sinus.
Groove for the sigmoid sinus (sulcus sinus sigmoidei).
Paramastoid process (processus paramastoideus) that sometimes projects inferiorly from the jugular processs in the direction of the transverse process of atlas which may even articulate.
Cerebral fossa (fossa cerebralis) which is a depression for the occipital lobes of the cerebrum.
Lateral part (pars lateralis)contains the:
Occipital condyle (condylus occipitalis) which is a large process for articulation with the atlas.
Condylar fossa (fossa condylaris) which is behind each occipital condyle sometimes perforated by the condylar canal (canalis condylaris) along the long axis of the condyle and it transmits an emissary vein from the sigmoid sinus.
Hypoglossal canal (canalis hypoglossi) which lies anterolateraly to the foramen magnum for the hypoglossal nerve (CN XII) and posterior meningeal artery.
Jugular tubercle (tuberculum jugulare) which is an eminence on the superior (internal) surface of the condyle, above the hypoglossal canal.
Jugular notch (incisura iugularis) which is usually divided by the intrajugular process (processus intraiugularis)which projects anterolaterally.
Jugular process (processus iugularis) that projects laterally from the posterior part of each condyle.
Groove for sigmoid sinus (sulcus sinus sigmoidei) which is inferomedial prolongation of the groove for transverse sin.