The occipital bone

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The cranium includes all the bones of the head. Altogether, there are 26 cranial bones plus the mandible. Except for the mandible, these bones mostly are fused together so that they do not move. The joints between most of the cranial bones are borders where the bones knit together, called sutures.

The bone of the rear and base of the skull is called the occipital bone. The occipital has a large hole in it, called the foramen magnum, which accommodates the passage of the spinal cord from the base of the brain. On either side of the foramen magnum are the occipital condyles, which are the articulation point of the skull and the atlas vertebra at the top of the vertebral column.

Like the frontal bone, the occipital bone also varies in its anatomy among humans and earlier hominins. Muscles of the neck and back attach to the occipital bone posterior to the foramen magnum, many of which leave traces or scars on the bone. The most external of these muscles, at the very back of the neck, sometimes leave a noticeable line across the occipital bone. In many primates, this line is elevated away from the skull in the form of a crest, called the nuchal crest. In some hominins, there is no elevated crest but instead a thickened bar of bone, called a nuchal torus. This is not unlike the thickened bar of bone at the front of the skull called the superorbital torus.

As you examine different kinds of hominins, look closely at their nuchal regions. Which kinds have a clear nuchal torus?

The occipital bone touches, or articulates with, 5 other cranial bones. Trace your way around the boundary of the occipital bone to find them.