Which vertebra has a structure that fits into the ring of C1 and allows head rotation?

The vertebra that has a structure fitting into the ring of C1 and allowing for head rotation is C2, also known as the Axis. This vertebra features a unique bony structure called the odontoid process or dens, which protrudes upward and fits into the atlas (C1). This anatomical relationship is crucial for enabling the pivot mechanism that allows the head to turn side to side.

When the head rotates, it is primarily the rotation of the atlas around the odontoid process of the axis that facilitates this movement. This specific arrangement is a key feature of the cervical spine and is what distinguishes C2 from other vertebrae, emphasizing its role in the mobility of the head and neck. Understanding the relationship between the atlas and axis is fundamental in recognizing the biomechanics of cervical movement.