Atlas (anatomy)
First cervical vertebra, or Atlas
Posterior atlantoöccipital membrane and atlantoaxial ligament. (Atlas visible at center.)
Gray's	subject #21 99
Dorlands/Elsevier	a_70/12167134

In anatomy, the atlas (C1) is the topmost (first) cervical vertebra of the spine. Image File history File links Gray86. ... A diagram of a thoracic vertebra. ... Image File history File links Gray305. ... Elseviers logo. ... Anatomical drawing of the human muscles from the Encyclopédie. ... A diagram of a thoracic vertebra. ... The vertebral column seen from the side The vertebral column (backbone or spine) is a column of vertebrae situated in the dorsal aspect of the abdomen. ...

It is named for the Atlas of mythology, because it supports the globe of the head. In Greek mythology, Atlas was one of the primordial Titans. ... // For the Derek Sherinian album, see Mythology (Derek Sherinian album). ... For other uses of the word head, see head (disambiguation). ...

The atlas is the topmost vertebra, and – along with the Axis – forms the joint connecting the skull and spine. The atlas and axis are specialized to allow a greater range of motion than normal vertebrae. In anatomy, the second cervical vertebra (C2) of the spine is named the axis or epistropheus. ... It has been suggested that temporal fenestra be merged into this article or section. ...

The Atlas' chief peculiarity is that it has no body, and this is due to the fact that the body of the atlas has fused with that of the next vertebra (the Axis). Its other peculiarities are that it has no spinous process, is ring-like, and consists of an anterior and a posterior arch and two lateral masses. A diagram of a thoracic vertebra. ...

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; 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 Longus colli muscle is a muscle of the human body. ... It has been suggested that this article or section be merged into Muscular system. ... In geometry, concavity is a property of certain geometric figures, and in calculus, a property of certain graphs of functions. ... The dens or odontoid process of the axis exhibits a slight constriction or neck, where it joins the body. ...

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. The anterior atlantoöccipital membrane (anterior atlantoöccipital ligament) is broad and composed of densely woven fibers, which pass between the anterior margin of the foramen magnum above, and the upper border of the anterior arch of the atlas below. ... The anterior atlantoaxial ligament is a strong membrane, fixed, above, to the lower border of the anterior arch of the atlas; below, to the front of the body of the axis. ... The occipital bone [Fig. ...

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. The human vermiform appendix is a vestigial structure: it no longer retains its original function. ... The Rectus capitis posterior minor (Rectus capitis posticus minor) arises by a narrow pointed tendon from the tubercle on the posterior arch of the atlas, and, widening as it ascends, is inserted into the medial part of the inferior nuchal line of the occipital bone and the surface between it...

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 a groove (sulcus arteriae vertebralis), sometimes converted into a foramen by a delicate bony spiculum which arches backward from the posterior end of the superior articular process. The posterior atlantoöccipital membrane (posterior atlantoöccipital ligament), broad but thin, is connected above, to the posterior margin of the foramen magnum; below, to the upper border of the posterior arch of the atlas. ...

This groove represents the superior vertebral notch, and serves for the transmission of the vertebral artery, which, after ascending through the foramen in the transverse process, winds around the lateral mass in a direction backward and medially; it also transmits the suboccipital nerve (first spinal nerve). In a common anatomic variant the vertebral artery passes through an arcuate foramen. The vertebral arteries are branches of the subclavian arteries. ... The first spinal nerve, the suboccipital nerve exits the spinal cord between the skull and the first cervical vertebra, the atlas. ... The vertebral arteries are branches of the subclavian arteries. ... In human anatomy, arcuate foramen, also known as ponticulus posticus (Latin for little posterior bridge), refers to a bony bridge on the atlas (C1 vertebra) that covers the groove for the vertebral artery. ...

On the under surface of the posterior arch, behind the 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. The posterior atlantoaxial ligament is a broad, thin membrane attached, above, to the lower border of the posterior arch of the atlas; below, to the upper edges of the laminæ of 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.

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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 transverse ligament of the atlas is a thick, strong band, which arches across the ring of the atlas, and retains the odontoid process in contact with the anterior arch. ... In a typical vertebra, the vertebral foramen is the foramen formed by the anterior segment (the body), and the posterior part, the vertebral arch. ...

• 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, and hence lateral displacement of the atlas may occur without compression of this structure. The dens or odontoid process of the axis exhibits a slight constriction or neck, where it joins the body. ... The Spinal cord nested in the vertebral column. ...

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. The transverse processes of a vertebra, two in number, project one at either side from the point where the lamina joins the pedicle, between the superior and inferior articular processes. ... It has been suggested that this article or section be merged into Muscular system. ...

Development

The atlas ossifies from 3 centers.

The atlas is usually ossified from three centers. Image File history File links Gray104. ... Image File history File links Gray104. ... Ossification is the process of bone formation, in which connective tissues, such as cartilage are turned to bone or bone-like tissue. ...

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. Cartilage is a type of dense connective tissue. ...

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.

Additional images

Vertebral column. Image File history File links Download high-resolution version (233x800, 113 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Atlas (anatomy) Coccyx Sacrum Lumbar vertebrae Spinal cord injury Cervical vertebrae Thoracic vertebrae Vertebra prominens Sacrovertebral angle ...	Anterior atlantoöccipital membrane and atlantoaxial ligament. Image File history File links Gray304. ...	Articulation between odontoid process and atlas. Image File history File links Gray306. ...	Membrana tectoria, transverse, and alar ligaments. Image File history File links Gray307. ...
Median sagittal section through the occipital bone and first three cervical vertebræ. Image File history File links Gray308. ...	The anterior vertebral muscles. Image File history File links Gray387. ...	Posterior primary divisions of the upper three cervical nerves. Image File history File links Gray800. ...

External links

• Dictionary at eMedicine Atlas

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant. eMedicine is an online clinical medical knowledge base that was founded in 1996. ... The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ... An illustration from the 1918 edition Henry Grays Anatomy of the Human Body, commonly known as Grays Anatomy, is an anatomy textbook widely regarded as a classic work on human anatomy. ...