Atlas (C1)

The atlas is the first cervical vertebra, a bony ring without a body that carries the skull and lets the head nod and turn.

At a glance

English: Atlas (C1)
Latin (TA2): Atlas (C1)
System: Bone
Region: Neck
Side: Single, on the midline
Series: Cervical vertebrae
Muscles on it: Splenius colli

About the atlas (c1)

The atlas is the first cervical vertebra, the top bone of the spine, on which the skull rests. It takes its name from the Titan of Greek myth who held up the heavens. With the axis beneath it, it forms a specialised joint complex between the head and the neck that allows far more movement than ordinary vertebrae do.

What sets it apart most is the lack of both a body and a true spinous process. Instead it is a ring made of a short anterior arch, a longer posterior arch, and two thick lateral masses at the sides. The anterior arch bears a tubercle in front for the longus colli and the anterior longitudinal ligament, and a small facet behind where the dens of the axis sits. The posterior arch ends in a small posterior tubercle, a remnant of a spine, for the rectus capitis posterior minor and the nuchal ligament. On its upper surface, just behind each lateral mass, runs a groove for the vertebral artery and the first cervical nerve; in some people a bony bridge roofs this groove to form an arcuate foramen.

The lateral masses carry the weight of the head. Their upper facets are long, hollow cups that receive the occipital condyles, while their lower facets are round and nearly flat, resting on the axis. A transverse ligament spans the ring between tubercles on the masses, splitting the opening into a small front part for the dens and a larger back part for the spinal cord and its coverings. The transverse processes are long and wide, each pierced by a foramen transversarium, and anchor muscles that turn and steady the head.

Nodding happens mainly at the atlanto-occipital joint above the atlas; turning the head side to side happens mainly at the atlanto-axial joint below it. A burst fracture of the ring, usually from a heavy load on the top of the head, is called a Jefferson fracture. Because the canal at this level is wide, the cord has some room to spare when the bone is injured.

Function and supply

Function: Carries the skull and allows nodding at the joint above and rotation of the head around the dens below.

Parts of the atlas (c1)

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Anterior arch of atlasArcus anterior atlantisThe short curved bar forming the front of the ring. Its outer face is convex and bears the anterior tubercle, while its inner face carries the facet where the dens of the axis sits.
Anterior tubercle of atlasTuberculum anterius atlantisA small knob in the middle of the front of the anterior arch. It anchors the anterior longitudinal ligament and the upper slips of longus colli.
Facet for densFovea dentisA smooth oval joint surface on the back of the anterior arch. The dens of the axis rotates against it at the median atlanto-axial joint, which allows the head to turn from side to side.
Groove for vertebral arterySulcus arteriae vertebralisA channel on the upper surface of the posterior arch, just behind each lateral mass. The vertebral artery bends backwards and inwards through it after leaving the transverse foramen, before passing up into the skull through the foramen magnum.
Inferior articular surface of atlasFacies articularis inferior atlantisA round, nearly flat facet on the underside of each lateral mass, facing down and inwards. It meets the axis at the lateral atlanto-axial joint and slides during rotation of the head.
Lateral massMassa lateralis atlantisThe thick bony block on each side of the ring, the strongest part of the atlas. Each carries a superior and an inferior joint surface and bears the weight of the head, passing it on to the axis.
Posterior arch of atlasArcus posterior atlantisThe longer curved bar forming the back of the ring. It ends in the posterior tubercle and carries the vertebral artery grooves on its upper surface near each lateral mass.
Posterior tubercle of atlasTuberculum posterius atlantisA small knob at the back of the posterior arch that stands in for a spinous process. The rectus capitis posterior minor muscles and the ligamentum nuchae are attached to it.
Superior articular surface of atlasFacies articularis superior atlantisA large, kidney-shaped hollow on top of each lateral mass, facing up and inwards. It cradles the occipital condyle of the skull at the atlanto-occipital joint, where nodding of the head takes place.
Transverse ligament tubercleTuberculum ligamenti transversiA small bump on the inner side of each lateral mass, just below the superior facet. The transverse ligament of the atlas stretches between the two tubercles and holds the dens against the anterior arch; if it fails, the dens can press on the spinal cord.

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At a glance

About the atlas (c1)

Function and supply

Related structures

See the atlas (c1) where it really sits, with every layer of the body around it.