Final answer:
The ring-shaped vertebra that supports and balances the skull is known as the atlas, or C1 vertebra, which allows for the nodding movement of the head.
Step-by-step explanation:
The ring-shaped vertebra supporting and balancing the skull is known as the atlas. This vertebra is also designated as the first cervical (C1) vertebra, uniquely characterized by the absence of a body and a spinous process. Unlike other cervical vertebrae, the atlas is a bony ring consisting of an anterior and posterior arch. The superior articular processes of the atlas are deeply curved and articulate with the occipital condyles of the skull, allowing for the nodding movement of the head.
The second cervical vertebra, called the axis, is located just below the atlas and is recognizable by the dens, which is a bony projection that allows the head to rotate on the vertebral column. Together, the atlas and axis form the atlanto-axial joint, which enables the rotation of the head.