The posterior atlantoöccipital membrane (posterior atlantoöccipital ligament), broad but thin, is connected above, to the posterior margin of the foramen magnum; below, to the upper border of the posterior arch of the atlas

Atlas (anatomy)

In anatomy, the atlas is the most superior cervical vertebra of the spine.It is named for the Atlas of Greek mythology, because it supports the globe of the head....

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On either side this membrane is defective below, over the groove for the vertebral artery

Vertebral artery

The vertebral arteries are major arteries of the neck. They branch from the subclavian arteries and merge to form the single midline basilar artery in a complex called the vertebrobasilar system, which supplies blood to the posterior part of the circle of Willis and thus significant portions of the...

, and forms with this groove an opening for the entrance of the artery and the exit of the suboccipital nerve

Suboccipital nerve

The first cervical nerve, the suboccipital nerve exits the spinal cord between the skull and the first cervical vertebra, the atlas.It supplies muscles around the suboccipital triangle including the rectus capitis posterior major, obliquus capitis superior, and obliquus capitis inferior. The...

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The free border of the membrane, arching over the artery and nerve, is sometimes ossified.

The membrane is in relation, behind, with the Recti capitis posteriores minores and Obliqui capitis superiores; in front, with the dura mater of the vertebral canal, to which it is intimately adherent.