Posterior atlantooccipital membrane
The posterior atlantooccipital membrane (posterior atlantooccipital ligament) is a broad but thin membrane. It is connected above to the posterior margin of the foramen magnum and below to the upper border of the posterior arch of the atlas. On each side of this membrane there is a defect above the groove for the vertebral artery which serves as an opening for the entrance of the artery. The suboccipital nerve also passes through this defect. The free border of the membrane arches over the artery and nerve and is sometimes ossified. The membrane is deep to the Recti  capitis posteriores minores and Obliqui capitis superiores and is superficial to the dura mater of the vertebral canal to which it is closely associated. In 2015, Scali et al. revisited the anatomy of the posterior atlantooccipital membrane via plastination. Their findings revealed that the PAO membrane superiorly consisted of periosteum of the occiput, whereas inferiorly it formed part of the dura at the cerebrospinal junction, terminating at the level of the third cervical vertebra (rather than attaching to the posterior arch of the atlas). It is believed that this anatomical arrangement permits a superiorly located anchor point for epidural bridging  structures and allows dural tensile forces to act in a summated synchronized manner. The author's hypothesize that this complex area assists with outflow of cerebrospinal fluid.