First cervical vertebra sits just below skull. It is called the atlas after the mythological hero that bore weight of shoulder on its shoulders. It can be identified by the following features.

• It is ring shaped. It has no body. It also has no spine.

• The atlas has a short anterior arch, a long posterior arch, right and left lateral masses, and transverse processes.

• The anterior arch is marked by a median anterior tubercle on its anterior aspect. Its posterior surface bears an oval facet which articulates with the dens.

• The posterior arch forms about two fifths of the ring and is much longer than the anterior arch. Its posterior surface is marked by a median posterior tubercle. The upper surface of the arch is marked (behind the lateral mass) by a groove.

• Each lateral mass shows the following important features.
  • Its upper surface bears the superior articular facet. This facet is elongated (forwards and medially), concave, and is directed upwards and medially. It articulates with the corresponding condyle to form an atlanto-occipital joint.
  • The lower surface is marked by the inferior articular facet. This facet is nearly circular, more or less flat, and is directed downwards, medially and backwards. It articulates with the corresponding facet on the axis vertebra to form an atlantoaxial joint.
  • The medial surface of the lateral mass is marked by a small roughened tubercle.

• The transverse process projects laterally from the lateral mass. It is unusually long and can be felt on the surface of the neck between the angle of the mandible and the mastoid process. Its long length allows it to act as an effective lever for rotatory movements the head. The transverse process is pierced by the foramen transversarium.

Attachments & Relations of Atlas

1. The anterior tubercle provides attachment (in the median plane) to the anterior longitudinal ligament, and provides insertion (on each side) to the upper oblique part of the longus colli muscle.
2. The upper border of the anterior arch gives attachment to the anterior atlanto-occipital membrane.
3. The lower border of the anterior arch gives attachment to the lateral fibres of the anterior longitudinal ligament.
4. The posterior tubercle provides attachment to the ligamentum nuchae (in the median plane) and gives origin to the rectus capitis posterior minor (on each side).
5. The groove on the upper surface of the posterior arch is occupied by the vertebral artery and by the first cervical nerve. Behind the groove the upper border of the posterior arch gives attachment to the posterior atlanto-occipital membrane.
6. The lower border of the posterior arch gives attachment to the highest pair of ligamenta flava.
7. The tubercle on the medial side of the lateral mass gives attachment to the transverse ligament of the atlas.
8. The anterior surface of the lateral mass gives origin to the rectus capitis anterior.
9. The transverse process gives origin to the rectus capitis lateralis (upper surface anteriorly), the superior oblique (upper surface posteriorly), the inferior oblique (lower surface of the tip), the levator scapulae (lateral margin and lower border), the splenius cervicis, and the scalenus medius.

Text adapted from: Human Anatomy by BD Chaurasia

Image Credit: Wikipedia

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