Where is the atlas (C1)? Ring shape, dens, joints with the skull and axis

Where is the atlas?

Put a hand on the back of your neck and nod, then shake your head. Most of that movement happens where the skull meets the neck, and the bone at that junction is the atlas.

The atlas is the top of the seven cervical vertebrae, also written C1. It lies at about the level of the earlobe, just under the base of the skull. With the pharynx in front and several layers of neck muscle behind, it sits deep and is hard to feel clearly through the skin.

A ring: arches and lateral masses

Other cervical vertebrae have a cylindrical body and a spinous process pointing backwards. The atlas has neither. Seen from above it is almost a ring.

This view keeps only the atlas and the axis (C2) below it. A peg rising from the axis pokes up into the ring: the dens, the pivot the atlas turns around.

Two joints: occipital bone and axis

The upper joint is the atlanto-occipital joint. The upper facets of the lateral masses are oval hollows that hold the occipital condyles of the skull, and this is where most of the nodding movement happens.

The lower joint is the atlanto-axial joint, with three contacts: the two pairs of lateral facets and one between the anterior arch and the dens. The transverse ligament of the atlas holds the dens from behind, and the atlas and skull rotate around it together. Roughly half of side-to-side head rotation happens here; textbooks give a range of figures depending on how it is measured.

The gaps between the skull and the atlas are closed by the anterior atlanto-occipital membrane in front and the posterior atlanto-occipital membrane behind. The vertebral artery passes through the posterior membrane after running along the groove on the posterior arch.

Muscles attached to the atlas

Four small suboccipital muscles lie on each side just below the skull, and three of them attach to the atlas:

The fourth, the rectus capitis posterior major, runs from the axis past the atlas to the skull. In front, the rectus capitis anterior and lateralis run from the atlas to the occipital bone, and the levator scapulae arises from its transverse process.

Name and terms

The name comes from Atlas, the Titan of Greek myth who holds up the sky: the bone holds up the globe of the skull. The axis below is named for the pivot it provides. Related terms: dens, atlanto-occipital joint, atlanto-axial joint. The Latin is also atlas.

See it in the app

With the skull above and the axis below, the ring of the atlas is hard to see on a full skeleton. In the 3D anatomy app teamLabBody you can hide the other bones, keep just the atlas and axis, and rotate them to see where the dens enters the ring and how far the transverse processes reach out. Turn the muscles back on to see where the suboccipital muscles attach. Cross sections are available with the PREMIUM plan.

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