The human vertebral column, commonly known as the spine, is a complex and flexible structure that provides support for the body. Composed of stacked bones called vertebrae, it forms the central axis from the skull to the pelvis. While each vertebra contributes to the column’s overall function, the topmost bone of the spine has a unique structure important for head movement.
Named After a Titan
The first vertebra in the spine is called the Atlas, or C1. Named after the Greek Titan Atlas, who bore the weight of the heavens, it supports the skull on top of the neck.
Distinctive Anatomy
The Atlas (C1) is an atypical vertebra with a unique structure. Unlike most vertebrae, it lacks a vertebral body and a spinous process. Instead, the Atlas is ring-shaped, composed of an anterior and posterior arch joined by two lateral masses. Its robust lateral masses support the head’s weight.
The Atlas features large, concave superior articular facets that articulate with the occipital condyles at the base of the skull. Its inferior articular facets are flatter and articulate with the second cervical vertebra, the Axis (C2). Each transverse process contains a transverse foramen, providing a passageway for the vertebral artery and vein. A large central foramen accommodates the spinal cord.
Important Role in Head Movement
The unique structure of the Atlas enables significant head movements through its articulations with the skull and the Axis (C2). The articulation between the occipital condyles of the skull and the superior articular facets of the Atlas forms the atlanto-occipital joint. This paired joint allows for nodding movements, such as flexing and extending the head in a “yes” motion. This movement involves the occipital condyles rolling and gliding on the concave facets of the Atlas.
Below the Atlas, its articulation with the Axis (C2) forms the atlanto-axial joint, which is a complex of three synovial joints. The central median atlanto-axial joint is a pivot joint where the Atlas rotates around the dens, a tooth-like projection extending upward from the Axis. This specialized arrangement allows for approximately 40-70% of the head’s axial rotation, facilitating the “no” movement. Beyond movement, the Atlas also protects the delicate spinal cord and provides passage for vertebral arteries, which supply blood to the brain.