This is a complex region where the skull and upper cervical spine connect. The connection between the brain and the spinal cord is at the base of the brainstem in the region of the Craniocervical junction. The base of the skull forms joints with the Atlas (C1, First cervical vertebral body) and the Axis (C2, Second cervical vertebral body) at the Occipital-C1 articulation & the C1-C2 articulation.
The vertebral arteries take a complex course at the Craniocervical junction where they course laterally though the Second cervical vertebral body (also called the Axis) and then pass through the transverse foramina of the Atlas (First cervical vertebral body) which are widely spaced. The Vertebral arteries then course posteriorly then medially over the lateral edges of the C1 (Atlas, First cervical vertebral body) and pass through the dura mater into the subarachnoid space.