Anterior arch The anterior arch forms about one-fifth of the ring: its anterior surface is convex, and presents at its center the anterior tubercle for the attachment of the
longus colli muscles and the
anterior longitudinal ligament; posteriorly it is concave, and marked by a smooth, oval or circular facet (
fovea dentis), for articulation with the
odontoid process (dens) of the axis. The upper and lower borders respectively give attachment to the
anterior atlantooccipital membrane and the
anterior atlantoaxial ligament; the former connects it with the
occipital bone above, and the latter with the axis below.
Posterior arch The posterior arch forms about two-fifths of the circumference of the ring: it ends behind in the posterior tubercle, which is the
rudiment of a spinous process and gives origin to the
recti capitis posteriores minores and the
ligamentum nuchae. The diminutive size of this process prevents any interference with the movements between the atlas and the skull. The posterior part of the arch presents above and behind a rounded edge for the attachment of the
posterior atlantooccipital membrane, while immediately behind each superior articular process is the superior vertebral notch (
sulcus arteriae vertebralis). This is a groove that is sometimes converted into a foramen by ossification of the posterior atlantooccipital membrane to create a delicate bony spiculum which arches backward from the posterior end of the superior articular process. This
anatomical variant is known as an
arcuate foramen. This groove transmits the
vertebral artery, which, after ascending through the foramen in the transverse process, winds around the lateral mass in a direction backward and medially to enter the
vertebrobasilar circulation through the
foramen magnum; it also transmits the
suboccipital nerve (first spinal nerve). On the under surface of the posterior arch, behind the inferior articular facets, are two shallow grooves, the inferior vertebral notches. The lower border gives attachment to the
posterior atlantoaxial ligament, which connects it with the axis.
Lateral masses The lateral masses are the most bulky and solid parts of the atlas, in order to support the weight of the head. Each carries two articular facets, a superior and an inferior. • The
superior facets are of large size, oval, concave, and approach each other in front, but diverge behind: they are directed upward, medially, and a little backward, each forming a cup for the corresponding
condyle of the occipital bone, and are admirably adapted to the nodding movements of the head. Not infrequently they are partially subdivided by indentations which encroach upon their margins. • The
inferior articular facets are circular in form, flattened or slightly convex and directed downward and medially, articulating with the axis, and permitting the rotatory movements of the head.
Vertebral foramen Just below the medial margin of each superior facet is a small tubercle, for the attachment of the
transverse atlantal ligament which stretches across the ring of the atlas and divides the
vertebral foramen into two unequal parts: • the
anterior or
smaller receiving the
odontoid process of the axis • the
posterior transmitting the
spinal cord (
medulla spinalis) and its membranes This part of the vertebral canal is of considerable size, much greater than is required for the accommodation of the spinal cord.
Transverse processes The transverse processes are large; they project laterally and downward from the lateral masses, and serve for the attachment of
muscles which assist in rotating the head. They are long, and their anterior and posterior tubercles are fused into one mass; the foramen transversarium is directed from below, upward and backward.
Development The atlas is usually
ossified from three centers. Of these, one appears in each lateral mass about the seventh week of fetal life, and extends backward; at birth, these portions of bone are separated from one another behind by a narrow interval filled with
cartilage. Between the third 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 showing a posterior arch defect 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==