CorpusHuman Anatomy
SkeletalBone

Atlas (C1)

Atlas

Also known as: C1, first cervical vertebra

The first cervical vertebra: a ring of bone without a body or spinous process that supports the skull on its lateral masses and rotates around the dens of the axis.

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Overview

The atlas consists of a short anterior arch (with an anterior tubercle in front and a facet behind for the dens), a longer posterior arch (with a small posterior tubercle in place of a spinous process) and two bulky lateral masses. Each lateral mass carries a concave, kidney-shaped superior articular facet that receives an occipital condyle (atlanto-occipital joint — the "yes" joint) and a flat, circular inferior articular facet for the axis (lateral atlanto-axial joint). A tubercle on the medial side of each lateral mass anchors the transverse ligament of the atlas, which holds the dens against the anterior arch.

The transverse processes are long and wide (palpable between the mastoid process and the angle of the mandible) and contain the transverse foramina. On the upper surface of the posterior arch, behind each lateral mass, a groove for the vertebral artery carries the artery and the suboccipital nerve (C1 dorsal ramus) after the artery leaves the transverse foramen and before it pierces the posterior atlanto-occipital membrane to enter the foramen magnum. Embryologically, the body of the atlas has become the dens of the axis.

Location

Uppermost vertebra, directly below the occipital bone.

Parts & subdivisions

  1. Anterior arch
    Facet for the dens on its posterior surface; anterior tubercle (longus colli).
  2. Posterior arch
    Groove for the vertebral artery; posterior tubercle.
  3. Lateral masses
    Superior facets for the occipital condyles; inferior facets for the axis.
  4. Transverse processes
    Long, with transverse foramina; levers for head rotation.

Anatomy

Articulations

Occipital condyles (atlanto-occipital joints); axis — dens (median atlanto-axial) and two lateral atlanto-axial joints.

Ligaments

Transverse ligament (part of cruciform ligament), anterior and posterior atlanto-occipital membranes, alar and apical ligaments (dens to occiput), tectorial membrane.

Ossification

Three centres — one for each lateral mass (≈7th week) and one for the anterior arch (≈1st year); the posterior arch unites at ≈3–4 years and the anterior arch at ≈6–8 years.

Function

  • Supports the skull ("Atlas holding up the world")
  • Permits nodding (flexion–extension) at the atlanto-occipital joints
  • Rotates with the skull around the dens

Clinical significance

Jefferson fracture

Axial loading (diving into shallow water, object on the head) drives the occipital condyles into the lateral masses, bursting the ring (anterior and posterior arches). Usually without cord injury because the canal widens, but instability if the transverse ligament ruptures.

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.