anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

, the atlas (C1) is the most superior (first)

cervical vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sa ...

of the spine and is located in the neck. It is named for

Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities ...

because, just as Atlas supported the globe, it supports the entire head. The atlas is the topmost vertebra and, with the

axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis * Axis (mathematics), a designator for a Cartesian-coordinat ...

(the vertebra below it), forms the joint connecting the

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

and spine. The atlas and axis are specialized to allow a greater range of motion than normal vertebrae. They are responsible for the nodding and rotation movements of the head. The

atlanto-occipital joint The atlanto-occipital joint (''Capsula articularis atlantooccipitalis'') is an articulation between the atlas bone and the occipital bone. It consists of a pair of condyloid joints. It is a synovial joint. Structure The atlanto-occipital join ...

allows the head to nod up and down on the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

. The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side. The atlas's chief peculiarity is that it has no body. It is ring-like and consists of an anterior and a posterior arch and two lateral masses. The atlas and axis are important neurologically because the brainstem extends down to the axis.

Structure

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 The longus colli muscle (Latin for ''long muscle of the neck'') is a muscle of the human body. The longus colli is situated on the anterior surface of the vertebral column, between the atlas and the third thoracic vertebra. It is broad in the mid ...

'' 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 In anatomy, the axis (from Latin ''axis'', "axle") or epistropheus is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests. The axis' defining feature is its strong odontoid process (bon ...

(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 In ''rudimental drumming'', a form of percussion music, a drum rudiment is one of a number of relatively small patterns which form the foundation for more extended and complex drumming patterns. The term "drum rudiment" is most closely associa ...

of a spinous process and gives origin to the ''

Recti capitis posteriores minores The rectus capitis posterior minor (or rectus capitis posticus minor, both being Latin for ''lesser posterior straight muscle of the head'') arises by a narrow pointed tendon from the tubercle on the posterior arch of the atlas, and, widening as i ...

'' and the

ligamentum nuchae The nuchal ligament is a ligament at the back of the neck that is continuous with the supraspinous ligament. Structure The nuchal ligament extends from the external occipital protuberance on the skull and median nuchal line to the spinous proc ...

. 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 The posterior atlantooccipital membrane (posterior atlantooccipital ligament) is a broad but thin membrane. It is connected above to the posterior margin of the foramen magnum and below to the upper border of the posterior arch of the atlas. On ea ...

, while immediately behind each superior articular process is the superior vertebral notch (''

sulcus arteriae vertebralis ''Sulcus'' (plural ''sulci'') may refer to: * Gingival sulcus, the space between a tooth and surrounding tissue * Sulcus (morphology), a groove, crevice or furrow in medicine, botany, and zoology * Sulcus (neuroanatomy), a crevice on the surface ...

''). 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 In human anatomy, arcuate foramen, also known as ponticulus posticus (Latin for "little posterior bridge") or Kimmerle's anomaly, refers to a bony bridge on the atlas (C1 vertebra) that covers the groove for the vertebral artery. It is a common a ...

. This groove transmits the

vertebral artery The vertebral arteries are major arteries of the neck. Typically, the vertebral arteries originate from the subclavian arteries. Each vessel courses superiorly along each side of the neck, merging within the skull to form the single, midline ba ...

, 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 The suboccipital nerve (first cervical dorsal ramus) is the dorsal primary ramus of the first cervical nerve (C1). It exits the spinal cord between the skull and the first cervical vertebra, the atlas. It lies within the suboccipital triangle ...

(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 The posterior atlantoaxial ligament is a broad, thin membrane attached, above, to the lower border of the posterior arch of the Atlas (anatomy), atlas; below, to the upper edges of the laminæ of the Axis (anatomy), axis. It supplies the place of ...

, 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 A condyle (;Entry "condyle"
in
which stretches across the ring of the atlas and divides the

vertebral foramen In a typical vertebra, the vertebral foramen is the foramen (opening) formed by the anterior segment (the body), and the posterior part, the vertebral arch. The vertebral foramen begins at cervical vertebra #1 (C1 or atlas) and continues inferio ...

into two unequal parts: * the ''anterior'' or ''smaller'' receiving the

of the axis * the ''posterior'' transmitting the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...

(''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 Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

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

Accessory transverse foramen of the atlas is present in 1.4%-12.5% across the population.

Foramen arcuale In anatomy and osteology, a foramen (;Entry "foramen"
in

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 An arch is a vertical curved structure that spans an elevated space and may or may not support the weight above it, or in case of a horizontal arch like an arch dam, the hydrostatic pressure against it. Arches may be synonymous with vault ...

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 The rectus capitis lateralis, a short, flat muscle, arises from the upper surface of the transverse process of the atlas, and is inserted into the under surface of the jugular process of the occipital bone. Additional images File:Rectus capitis ...

occipital bone (beneath the jugular process) *

obliquus capitis superior The obliquus capitis superior muscle () is a small muscle in the upper back part of the neck and is one of the suboccipital muscles and part of the suboccipital triangle. It arises from the lateral mass of the atlas bone. It passes superiorly a ...

occipital bone (between the superior and inferior nuchal lines) Interior and dorsal part: *

obliquus capitis inferior The obliquus capitis inferior muscle () is the larger of the two oblique muscles of the neck. It arises from the apex of the spinous process of the axis and passes laterally and slightly upward, to be inserted into the lower and back part of the t ...

spinous process of the axis Lower surface: * splenius cervicis (part) spinous processes of T02–T05 *

levator scapulae The levator scapulae is a slender skeletal muscle situated at the back and side of the neck. As the Latin name suggests, its main function is to lift the scapula. Anatomy Attachments The muscle descends diagonally from its origin to its inserti ...

(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 The rectus capitis posterior minor (or rectus capitis posticus minor, both being Latin for ''lesser posterior straight muscle of the head'') arises by a narrow pointed tendon from the tubercle on the posterior arch of the atlas, and, widening as i ...

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 The longus colli muscle (Latin for ''long muscle of the neck'') is a muscle of the human body. The longus colli is situated on the anterior surface of the vertebral column, between the atlas and the third thoracic vertebra. It is broad in the mid ...

(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 in the first vertebra is referred to as a Jefferson fracture. Craniocervical junction misalignment is also suspected as a factor in neurodegenerative diseases where altered CSF flow plays a part in the pathological process. Hyperextension (Whiplash) Injury A rear-end traffic collision or a poorly performed rugby tackle can both result in the head being whipped back on the shoulders, causing whiplash. In minor cases, the anterior longitudinal ligament of the spine is damaged which is acutely painful for the patient. In more severe cases, fractures can occur to any of the cervical vertebrae as they are suddenly compressed by rapid deceleration. Again, since the vertebral foramen is large there is less chance of spinal cord involvement. The worst-case scenario for these injuries is that dislocation or subluxation of the cervical vertebrae occurs. This often happens at the C2 level, where the body of C2 moves anteriorly with respect to C3. Such an injury may well lead to spinal cord involvement, and as a consequence quadriplegia or death may occur. More commonly, subluxation occurs at the C6/C7 level (50% of cases).

Additional images

File:Atlas (C1) from top animation small.gif, Shape and position of atlas (shown in red), from above. The skull is shown in semi-transparent. File:Cervical vertebra 1 animation top.gif, Atlas from above File:Human atlas bone.stl, 3D image File:Gray305.png, Posterior atlantoöccipital membrane and atlantoaxial ligament (atlas visible at center) File:Atlas vertebrae.jpg, Atlas from above File:Atlante (C1).JPG, Atlas from above File:Atlante (C1) inf.JPG, Atlas, inferior surface File:Atlas The First Cervical Vertebra.gif, Computer generated 3d model of atlas

References

External links

* Netter, Frank
''Atlas of Human Anatomy''
, "High Cervical Spine: C1–C2"
Atlas
- BlueLink Anatomy - University of Michigan Medical School {{DEFAULTSORT:Atlas (Anatomy) Bones of the thorax Bones of the vertebral column Atlas (mythology)

Structure

Anterior arch

Posterior arch

Lateral masses

Transverse processes

Development

Variations

Function

Muscular attachments

Transverse processes

Posterior tubercle

Anterior arch

Clinical significance

Additional images

See also

References

External links