and fourth years they unite either directly or through the medium of a separate center developed in the cartilage. At birth, the anterior arch consists of cartilage; in this a separate center appears about the end of the first year after birth, and joins the lateral masses from the sixth to the eighth year. The lines of union extend across the anterior portions of the superior articular facets. Occasionally there is no separate center, the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses; sometimes this arch is ossified from two centers, one on either side of the middle line. Variations Accessory transverse foramen of the atlas is present in 1.4–12.5% across the population. Foramen arcuale or a bony bridge above the vertebral artery on the posterior arch of the atlas may be present. This foramen has an overall prevalence of 9.1%. Arch defects refer to the condition where a gap or cleft exists at the anterior arch or posterior arch of the atlas. The prevalence of the posterior arch defect and anterior arch defect was 0.95% and 0.087%, respectively. The anterior arch defect may be presented along with posterior arch defect, a condition known as combined arch defect or bipartite atlas. Function Muscular attachments Transverse processes Upper surface: rectus capitis anterior occipital bone (inferior surface of the base) rectus capitis lateralis occipital bone (beneath the jugular process) obliquus capitis superior occipital bone (between the superior and inferior nuchal lines) Interior and dorsal part: obliquus capitis inferior spinous process of the axis Lower surface: splenius cervicis (part) spinous processes of T02–T05 levator scapulae (part) superior part of medial border of the scapula intertransversarius posterior cervicis transverse process of the axis (posterior tubercle) intertransversarius anterior cervicis transverse process of the axis (anterior tubercle) Posterior tubercle Upper surface: rectus capitis posterior minor occipital bone (medial part of the interior nuchal line, and the surface between it and the foramen magnum) Lower surface: interspinalis cervicis spinous process of the axis Anterior arch longus colli (superior oblique) transverse processes of C03–C05. Clinical significance There are 5 types of C1 fractures referred to as the Levine Classification of Atlas Fractures Type 1: Isolated bony apophysis (transverse process fracture Type 2: Isolated posterior arch fractures Type 3: Isolated anterior arch fracture Type 4: Comminuted fracture of the lateral mass Type 5: Bilateral burst fracture (AKA Jefferson Fracture) A break