The C1 vertebra, known as the atlas, is the superior-most vertebra in the spinal column. It plays vital roles in the support of the skull, spinal cord, and vertebral arteries and provides attachment points for several muscles of the neck.
The C1, or first cervical vertebra, is commonly called the atlas due to its unique position in the spine. In Greek mythology, Atlas was the titan who held the Earth on his shoulders, just like the atlas holds the skull on top of the neck. The atlas is located at the top of the neck, just inferior to the condyles of the occipital bone of the skull and superior to the C2 vertebra.
The atlas is the thinnest and most delicate of the cervical vertebrae, consisting of a thin ring of bone with a few small projections. Unlike most vertebrae, there is no body or centrum on the anterior end of the atlas. Instead, there is a thin band of bone known as the anterior arch and a small mass on its anterior surface known as the anterior tubercle. The anterior arch curves posteriorly and laterally from the anterior tubercle and has a smooth joint-forming process on its posterior side. The posterior of the anterior arch meets the odontoid process of the C2 vertebra (axis) to form the median atlantoaxial joint.
On either side of the anterior arch are the lateral masses, which are the widest regions of the atlas. Each lateral mass has a smooth, oval, cup-like depression known as the superior articular facet on its superior surface. The superior articular facets form the atlanto-occipital joint with the condyles of the occipital bone of the skull, allowing the head to flex and extend at the neck. On the inferior side of each lateral mass is a short, flat, cylindrical projection of bone known as the inferior articular facet. The inferior articular facets are extremely smooth on their inferior surfaces and form the lateral atlanto-axial joint between the atlas and the axis.
Extending laterally from each lateral mass is an irregular ring of bone known as the transverse process. Each transverse process surrounds a tiny transverse foramen, a hole that provides room for the vertebral artery and vein to travel through the neck. The transverse process protects the blood vessels in the transverse foramen and serves as an attachment point for muscles that move the neck.
The posterior arch extends posteriorly from the lateral masses and completes the ring of the atlas around the hollow vertebral foramen. While it is thin throughout its length, the posterior arch widens slightly at its posterior-most point to form the posterior tubercle. The posterior tubercle is similar in structure and function to the much larger spinous process found in most vertebrae. Slight depressions on each side of the posterior arch provide room for the C1 spinal nerve to exit the vertebral foramen and allow the vertebral artery to pass into the vertebral foramen before entering the skull at the foramen magnum of the occipital bone.
The atlas plays a vital role in the support and movement of the head and neck. Several muscles in the neck pivot the skull at the atlanto-occipital joint to make the head flex and extend in a nodding motion. Of these muscles, the longus colli muscle inserts at the anterior tubercle of the atlas and the rectus capitis posterior muscle arises from the posterior tubercle. Lateral flexion, or rotating the head from side to side, requires pivoting of the skull and atlas at the atlanto-axial joint. Many neck muscles work together to rotate the head, but several of these muscles — including the rectus capitis lateralis and obliquus capitis superior — arise from the transverse processes of the atlas. In addition, several intertransversarii muscles connect the transverse processes of the atlas to those of the axis, allowing lateral flexion of the entire neck to pull the head laterally toward one of the shoulders.
The atlas also plays a vital role in the protection of the delicate structures of the neck. The spinal cord and spinal nerves that pass through the vertebral foramen of the neck are shielded from physical injury by the bony ring of the atlas. Compared to the other vertebrae of the spine, the vertebral foramen is much larger in the atlas, providing significant space for movement of the soft nervous tissue during flexion and rotation of the head and neck. The transverse foramina also play an important protective role for the vertebral arteries and veins that provide vital blood flow to and from the brain. The bony tissue of the transverse processes of the atlas prevents these blood vessels from being compressed or damaged on their way through the neck.