The first cervical vertebra, C1, is called the atlas. Its name is a reference to the Greek titan who supported the heavens, as the atlas supports the skull. This bone sits at the junction of the skull and the rest of the spinal column. The atlas is responsible for head movement and protection of the spinal cord and associated blood vessels.
Key Anatomical Features of the Atlas
The atlas is distinct from other vertebrae, as it lacks a vertebral body and a spinous process. Instead, it forms a ring-like structure composed of two arches and two lateral masses. This shape cradles the skull and facilitates a wide range of head motions. The open design provides space for the spinal cord, allowing movement without compression.
The atlas’s ring is formed by the anterior and posterior arches. The smaller anterior arch has a projection called the anterior tubercle. The posterior arch has a corresponding posterior tubercle and features grooves on its upper surface for the vertebral artery and the first spinal nerve.
The lateral masses are the thickest parts of the atlas, positioned between the arches to bear the cranium’s weight. Each mass has a superior and an inferior articular facet. The large, concave superior facets connect with the occipital condyles of the skull. The flatter inferior articular facets connect with the second cervical vertebra (C2), the axis.
Projecting from the sides of the lateral masses are the transverse processes. These processes extend more to the side than those of any other cervical vertebra. Each transverse process contains an opening called the transverse foramen. This foramen provides a protected channel for the vertebral artery and vein to pass through the neck.
Joints and Primary Movements
The atlas forms two specialized joints that permit distinct head movements. The atlanto-occipital joint is formed by the articulation of the superior articular facets of the atlas with the occipital condyles of the skull. The shape of these facets allows for the flexion and extension of the head. This movement is commonly recognized as nodding in a “yes” motion.
The articulation between the atlas and the axis (C2) is the atlanto-axial joint, which allows for rotational movement of the head. The axis has an upward-projecting feature called the dens that fits into the ring of the atlas. The atlas pivots around this dens. This motion allows the head to turn from side to side in a “no” gesture.
How the Atlas Differs from Other Vertebrae
A primary distinction of the atlas is its lack of a vertebral body, the large, weight-bearing portion found in other vertebrae. The space that would be occupied by a body is instead taken up by the dens of the axis. The dens is held in place by the transverse atlantal ligament.
Another difference is the absence of a spinous process. Most vertebrae have a prominent spinous process projecting from the back for muscle and ligament attachment. The atlas instead has a simple posterior tubercle on its posterior arch. This modification allows for an extensive range of motion at the top of the cervical spine.