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The occipital bone is the cranial bone that forms the posterior part of the calvarium and also is the articulation point with the bones of the neck. Therefore, it has a pair of condyles, the occipital condyles found on the inferior surface that articulate with the atlas as discussed earlier.
Parts – The two main parts of the occipital bone are the squamous part which is the flat bone part that forms the posterior part of the calvarium and the basilar part which is the part that connects the bone to the sphenoid bone.
Markings – The markings on the external surface of the occipital bone are all involved with attachments of the ligaments and large muscles that are necessary to hold the head erect. If you look at a lateral view of the skull you can see that the greater mass of the skull is anterior to the occipital condyle. To counteract this weight difference the muscles at the back of the neck must be strong enough to resist the head falling forward. This is why when you read too much anatomy and start to nod off your head falls forward. These markings include the external occipital protuberance, the nuchal lines, and the external occipital crest.
The internal surface of the occipital bone forms much of the posterior cranial fossa which contains the cerebellum and brain stem. The depression to house the cerebellum is named appropriately the cerebellar fossa. There is also a groove along the border with the temporal bone to accommodate the large sigmoid sinus which drains venous blood from the brain into the jugular vein which begins at the aforementioned jugular foramen. There are also attachment points for the connective tissue coverings of the brain. These raised areas are the internal occipital crest and the internal occipital protuberance.
Openings – The largest foramen in the skull is located in the occipital bone. The foramen magnum is the opening for the spinal cord to attach to the brain stem. The basilar artery which supplies blood to the brain also passes through this opening. There are also foramina found in the base of the occipital condyles. These openings allow passage of the cranial nerve XII, the hypoglossal nerve and are like many things labeled in a logical manner as the hypoglossal canals. We have already noted that the jugular foramen is found at the junction of the occipital and temporal bones.Diagram Reference Guide