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 i ...

, the atlas (C1) is the most superior (first) cervical vertebra of the

spine Spine or spinal may refer to: Science Biology * Vertebral column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoolo ...

and is located in the

neck The neck is the part of the body on many vertebrates that connects the head with the torso. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In ...

. 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 geogra ...

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 o ...

because, just as Atlas supported the globe, it supports the entire

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals ...

. The atlas is the topmost vertebra and, with the axis (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 Range of motion (or ROM), is the linear or angular distance that a moving object may normally travel while properly attached to another. It is also called range of travel (or ROT), particularly when talking about mechanical devices and in mechanic ...

than normal vertebrae. They are responsible for the nodding and rotation movements of the head. The atlanto-occipital joint 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 The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

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''

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

s and the

anterior longitudinal ligament The anterior longitudinal ligament is a ligament that runs down the anterior surface of the spine. It traverses all of the vertebral bodies and intervertebral discs on their ventral side. It may be partially cut to treat certain abnormal curvature ...

; 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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and the anterior atlantoaxial ligament; the former connects it with the

occipital bone The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cer ...

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 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 ...

, 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 The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblon ...

; 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

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

s 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 Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...

. 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 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 The rectus capitis anterior (rectus capitis anticus minor) is a short, flat muscle, situated immediately behind the upper part of the Longus capitis. It arises from the anterior surface of the lateral mass of the atlas, and from the root of its t ...

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 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 tr ...

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 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 (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 A Jefferson fracture is a bone fracture of the anterior and posterior arches of the C1 vertebra, though it may also appear as a three- or two-part fracture. The fracture may result from an axial load on the back of the head or hyperextension of ...

. 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

Vertebral foramen

Transverse processes

Development

Variations

Function

Muscular attachments

Transverse processes

Posterior tubercle

Anterior arch

Clinical significance

Additional images

See also

References

External links