The correct option is Atlas.
Explanation
The vertebral column in humans and most mammals is divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal. The cervical vertebrae are located in the neck region, and the first two possess specialized structures to support the movement of the head.
- Atlas is correct: The first cervical vertebra (C1) is called the Atlas. It is ring-shaped and lacks a vertebral body. It articulates superiorly with the occipital condyles of the skull to form the atlanto-occipital joint. This articulation facilitates the nodding movement of the head (signifying "Yes").
- Axis is incorrect: The Axis is the second cervical vertebra (C2). It is characterized by a peg-like projection called the odontoid process (or dens), which acts as a pivot for the Atlas to rotate around. This allows the side-to-side shaking movement of the head (signifying "No").
- Sacrum is incorrect: The Sacrum is a large, triangular bone located at the base of the spine and at the upper and back part of the pelvic cavity. It is formed by the fusion of five sacral vertebrae.
- Coccyx is incorrect: The Coccyx, commonly known as the tailbone, is the final segment of the vertebral column. It is formed by the fusion of four rudimentary coccygeal vertebrae.
Key Takeaway: The Atlas (C1) supports the globe of the head and articulates with the occipital condyles, whereas the Axis (C2) provides the pivot for rotation.