Axial skeleton · Vertebral column
Atlas (C1)
The atlas is the first cervical vertebra. It is a ring of bone with no body and no spinous process; it carries the skull on its two lateral masses and turns around the dens of the axis below it.
- Latin (TA)
- Atlas
- Type
- Irregular bone: an atypical cervical vertebra shaped as a ring, with no body and no spinous process
- Side
- Midline — single
- Model size
- 8.6 cm longest
Rotate the bone to inspect every surface. Blue dots are landmarks — tap one.
3D model: Z-Anatomy (CC BY-SA 4.0), derived from BodyParts3D © DBCLS (CC BY-SA 2.1 JP). Adapted by PharmaWallah. Credits & licence
Landmarks
10 of 12 marked on the modelTap a landmark to find it on the bone and read what it is.
The short, curved front part of the atlas ring.
Where: Front of the ring, joining the two lateral masses.
Why it matters: Lies in front of the dens, which takes the place of the atlas's body; its back surface articulates with the dens of the axis.
Marked on the model at the point Z-Anatomy places this label.
A small midline projection on the front of the anterior arch.
Where: Anterior surface of the anterior arch, in the midline.
Why it matters: Attachment for the anterior longitudinal ligament and longus colli.
Related: Longus colli; anterior longitudinal ligament.
Marked on the model at the point Z-Anatomy places this label.
A small, oval, smooth surface on the inner aspect of the anterior arch.
Where: Posterior surface of the anterior arch, in the midline.
Why it matters: Articulates with the anterior facet of the dens of the axis at the median atlanto-axial (pivot) joint.
Related: Median atlanto-axial joint.
Viva: What holds the dens against the facet on the anterior arch?
Marked on the model at the point Z-Anatomy places this label.
The longer curved back part of the atlas ring.
Where: Back of the ring, joining the two lateral masses.
Why it matters: Corresponds to the pedicles and laminae of a typical vertebra; its upper surface is grooved by the vertebral artery.
Marked on the model at the point Z-Anatomy places this label.
A small roughened projection in the midline of the posterior arch.
Where: Posterior surface of the posterior arch, in the midline.
Why it matters: A rudimentary spinous process; gives attachment to rectus capitis posterior minor and the ligamentum nuchae.
Related: Rectus capitis posterior minor.
Marked on the model at the point Z-Anatomy places this label.
A wide groove crossing the upper surface of the posterior arch.
Where: Superior surface of the posterior arch, just behind each lateral mass.
Why it matters: Lodges the vertebral artery as it turns medially to enter the skull through the foramen magnum; the first cervical (suboccipital) nerve also lies here.
Viva: What lies in the groove on the upper surface of the posterior arch of the atlas?
Marked on the model at the point Z-Anatomy places this label.
The thick block of bone on each side of the ring.
Where: Each side of the atlas, between the anterior and posterior arches.
Why it matters: Carries the weight of the head from the occipital condyles down to the axis.
Marked on the model at the point Z-Anatomy places this label.
A large, concave, kidney-shaped articular surface on top of each lateral mass.
Where: Upper surface of the lateral mass, facing upwards and medially.
Why it matters: Receives the occipital condyle at the atlanto-occipital joint, which allows nodding (flexion and extension of the head).
Related: Atlanto-occipital joint.
Viva: Which movement occurs mainly at the atlanto-occipital joints?
Marked on the model at the point Z-Anatomy places this label.
A flatter, roughly circular articular surface under each lateral mass.
Where: Lower surface of the lateral mass, facing downwards and medially.
Why it matters: Articulates with the superior articular facet of the axis at the lateral atlanto-axial joint.
Related: Lateral atlanto-axial joint.
Marked on the model at the point Z-Anatomy places this label.
A small tubercle on the inner side of each lateral mass.
Where: Medial surface of the lateral mass.
Why it matters: Attachment of the transverse ligament of the atlas, which holds the dens in place.
Related: Transverse ligament of atlas.
Marked on the model at the point Z-Anatomy places this label.
A projection from the side of each lateral mass, long and easy to feel.
Where: Lateral to each lateral mass.
Why it matters: Lever for muscles that rotate the head; its tip can be felt between the mastoid process and the angle of the mandible.
Related: Levator scapulae; obliquus capitis superior and inferior.
A foramen in each transverse process.
Where: Within the transverse process, lateral to the lateral mass.
Why it matters: Transmits the vertebral artery (and accompanying veins) upwards towards the groove on the posterior arch.
Classification & region
Irregular bone: an atypical cervical vertebra shaped as a ring, with no body and no spinous process.
- Anterior arch
- The short curved bar in front, with the anterior tubercle on its front and the facet for the dens on its back.
- Posterior arch
- The longer curved bar behind, with the posterior tubercle and, on its upper surface, the groove for the vertebral artery.
- Lateral masses
- The two thick blocks at the sides that carry the superior and inferior articular surfaces and take the weight of the head.
- Transverse processes
- Projections from the lateral masses, each pierced by a foramen transversarium.
Hold it in the anatomical position
- The shorter anterior arch faces forwards; the longer posterior arch faces backwards.
- The large, concave, elongated superior articular surfaces face upwards to cradle the occipital condyles.
- The flatter, roughly circular inferior articular surfaces face downwards to rest on the axis.
- The groove for the vertebral artery runs across the upper surface of the posterior arch, just behind each lateral mass.
Orientation (a midline bone has no side)
- Anterior vs posterior: the anterior arch is short and has a small smooth facet on its inner (posterior) surface for the dens; the posterior arch is longer.
- Superior vs inferior: the superior articular surfaces are larger, deeply concave and kidney-shaped; the inferior surfaces are flatter and circular.
- Confirm with the grooves for the vertebral artery, which are on the upper surface of the posterior arch.
The atlas is a single midline bone with symmetrical halves, so it has no side; you orient it instead.
Articulations
- With the occipital bone (occipital condyles) at the Atlanto-occipital joint, synovial condylar (ellipsoid) joint — Superior articular surfaces of the lateral masses..
- With the axis (superior articular facets) at the Lateral atlanto-axial joint, synovial plane joint — Inferior articular surfaces of the lateral masses..
- With the axis (dens) at the Median atlanto-axial joint, synovial pivot joint — Facet for the dens on the posterior surface of the anterior arch..
Muscle and ligament attachments
| Structure | Type | Where on the bone |
|---|---|---|
| Anterior longitudinal ligament | ligament | Anterior tubercle and anterior arch. |
| Longus colli | insertion | Anterior tubercle (its upper oblique part). |
| Anterior atlanto-occipital membrane | ligament | Upper border of the anterior arch. |
| Posterior atlanto-occipital membrane | ligament | Upper border of the posterior arch. |
| Transverse ligament of the atlas | ligament | Tubercles on the medial sides of the lateral masses. |
| Rectus capitis posterior minor | origin | Posterior tubercle. |
| Obliquus capitis superior | origin | Transverse process. |
| Obliquus capitis inferior | insertion | Transverse process. |
| Levator scapulae | origin | Transverse process (with those of the next cervical vertebrae). |
Functions
- Supports the skull on its lateral masses.
- Allows nodding of the head at the atlanto-occipital joints.
- Rotates around the dens of the axis, carrying the skull with it (shaking the head 'no').
- Protects the upper spinal cord and transmits the vertebral arteries.
Clinical & practical relevance
- Jefferson fracture: a heavy blow on top of the head can drive the occipital condyles down and burst the ring of the atlas, often at both arches.
- Atlanto-axial instability: if the transverse ligament is weakened (for example in rheumatoid arthritis) the dens can move backwards and threaten the spinal cord.
Identification tips
- Ring with no body and no spine: that is the atlas.
- 'Yes' at the atlanto-occipital joints, 'no' at the atlanto-axial joints.
- The facet for the dens is on the back of the short anterior arch.
Common student mistakes
- Putting the groove for the vertebral artery on the anterior arch: it is on the upper surface of the posterior arch.
- Thinking there is an intervertebral disc between atlas and skull or atlas and axis: there is none at either level.
- Calling the posterior tubercle a spinous process: it is only a rudiment of one.
Common exam questions
How does the atlas differ from a typical cervical vertebra?Show answer
It has no body and no spinous process; instead it has anterior and posterior arches, two lateral masses, and a posterior tubercle.
Where does the body of the atlas go?Show answer
Developmentally it fuses with the axis to form the dens.
Name the joints formed by the atlas.Show answer
Two atlanto-occipital joints with the skull, and the median and two lateral atlanto-axial joints with the axis.
What lies in the groove on the posterior arch?Show answer
The vertebral artery and the first cervical nerve.
How do you tell the superior from the inferior articular surfaces?Show answer
Superior surfaces are larger, concave and kidney-shaped; inferior ones are flatter and circular.
What is attached to the tubercle on the medial side of the lateral mass?Show answer
The transverse ligament of the atlas.
Revision summary
- C1 has no body and no spinous process.
- Anterior arch: anterior tubercle in front, facet for dens behind.
- Posterior arch: posterior tubercle and grooves for the vertebral arteries.
- Lateral masses carry concave superior surfaces (for the skull) and flatter inferior surfaces (for C2).
- Transverse ligament attaches to tubercles on the medial sides of the lateral masses.
- Atlanto-occipital joints: nodding. Atlanto-axial joints: rotation.
Test yourself on the atlas (c1)
Spotting questions on bones, sides and landmarks — untimed, with explanations.
References
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. 5th ed. Philadelphia: Elsevier; 2023.
- Moore KL, Dalley AF, Agur AMR. Moore's Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2023.
- Federative International Programme for Anatomical Terminology (FIPAT). Terminologia Anatomica. 2nd ed. FIPAT.library.dal.ca; 2019. fipat.library.dal.ca/TA2
- Kervyn G, Zielinski M, Vinent L, et al. Z-Anatomy: the libre 3D atlas of anatomy. Models derived from BodyParts3D, The Database Center for Life Science (CC BY-SA 2.1 JP). CC BY-SA 4.0. www.z-anatomy.com/
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