Complex sentence: In anatomy, the Atlas (C1) is the topmost (first) cervical vertebra of the spine, which (along with the Axis) forms the joint connecting the skull and spine.
Split into simpler sentences: In anatomy, the Atlas (C1) is the topmost (first) cervical vertebra of the spine. The Atlas is the topmost vertebra, and--along with the Axis--forms the joint connecting the skull and spine.