anatomy Anatomy () is the branch of morphology concerned with the study of the internal 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 scien ...

, the axis (from Latin ''axis'', "axle") is the second

cervical vertebra In tetrapods, cervical vertebrae (: 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 sauropsid s ...

(C2) of the

spine Spine or spinal may refer to: Science Biology * Spinal 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 (zoology), ...

, immediately inferior to the

atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...

, upon which the

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 ani ...

rests. The

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

passes through the axis. The defining feature of the axis is its strong bony protrusion known as the dens, which rises from the superior aspect of the bone.

Structure

The body is deeper in front or in the back and is prolonged downward anteriorly to overlap the upper and front part of the third vertebra. It presents a median longitudinal ridge in front, separating two lateral depressions for the attachment of the

longus colli muscle 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 mi ...

Dens

The dens, also called the odontoid process, or the peg, is the most pronounced projecting feature of the axis. The dens exhibits a slight constriction where it joins the main body of the vertebra. The condition where the dens is separated from the body of the axis is called ''os odontoideum'' and may cause nerve and circulation compression syndrome. On its anterior surface is an oval or nearly circular facet for articulation with that on the

anterior arch of the atlas In anatomy, the atlas (C1) is the most superior (first) cervical vertebra of the spine and is located in the neck. The bone is named for Atlas of Greek mythology, just as Atlas bore the weight of the heavens, the first cervical vertebra suppor ...

. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed and gives attachment to the apical odontoid ligament. Below the apex, the process is somewhat enlarged and presents on either side a rough impression for the attachment of the alar ligament; these ligaments connect the process to the

occipital bone The occipital bone () is a neurocranium, 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 lies over the occipital lob ...

. The internal structure of the odontoid process is more compact than that of the body. The odontoid peg is the ascension of the atlas fused to the ascension of the axis. The peg has an articular facet at its front and forms part of a joint with the anterior arch of the atlas. It is a non-

weight bearing In orthopedics, weight-bearing is the amount of weight a patient puts on an injured body part. Generally, it refers to a leg, ankle or foot that has been fractured or upon which surgery Surgery is a medical specialty that uses manual and ins ...

joint. The alar ligaments, together with the apical ligaments, are attached from the sloping upper edge of the odontoid peg to the margins of the

foramen magnum The foramen magnum () 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 oblongata, passes thro ...

. The inner ligaments limit rotation of the head and are very strong. The weak apical ligament lies in front of the upper longitudinal bone of the cruciform ligament and joins the apex of the deltoid peg to the anterior margin of the foramen magnum. It is the fibrous remnant of the

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

Other features

The pedicles are broad and strong, especially in the front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces. The laminae are thick and strong. They play a large role in the stability of the cervical spine alongside the laminae of C7. The

vertebral foramen In a typical vertebra, the vertebral foramen is the foramen (opening) of a vertebra bounded ventrally/anteriorly by the body of the vertebra, and the dorsally/posteriorly by the vertebral arch. In the articulated spine, the successive vertebral ...

is large, but smaller than the

. The

transverse processes Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

are very small, and each ends in a single tubercle. Each process is perforated by the

transverse foramen In tetrapods, cervical vertebrae (: 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 sauropsid s ...

, which is directed obliquely upward and laterally. The superior

articular surface A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

s are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes. The inferior articular surfaces have the same direction as those of the other

cervical vertebrae In tetrapods, cervical vertebrae (: 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 saurop ...

. The superior

vertebral notches Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

are very shallow, and lie behind the

articular processes The articular process or zygapophysis ( + apophysis) of a vertebra is a projection of the vertebra that serves the purpose of fitting with an adjacent vertebra. The actual region of contact is called the ''articular facet''.Moore, Keith L. et al. ...

. The inferior vertebral notches lie in front of the articular processes, as in the other cervical vertebrae. The

spinous process Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity.

Variation

Contact sport A contact sport is any sport where physical contact between competitors, or their environment, is an integral part of the game. For example, gridiron football. Contact may come about as the result of intentional or incidental actions by the playe ...

s are

contraindicated In medicine, a contraindication is a condition (a situation or factor) that serves as a reason not to take a certain medical treatment due to the harm that it would cause the patient. Contraindication is the opposite of indication, which is a rea ...

for individuals with anomalous dens, as any violent impact may result in a

catastrophic injury A catastrophic injury is a severe injury to the spine, spinal cord, or brain. It may also include skull or spinal fractures. This is a subset of the definition for the legal term ''catastrophic injury'', which is based on the definition used by ...

. This is because a malformed odontoid process may lead to instability between the

and axis (the C1 and C2

Development

The axis is

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 five primary and two secondary centres. The body and vertebral arch are ossified in the same manner as the corresponding parts in the other

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

, viz., one centre for the body, and two for the

vertebral arch Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

. The centres for the arch appear about the seventh or eighth week of fetal life, while the centres for the body appear in about the fourth or fifth month. The dens, or odontoid process, consists originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed. During about the sixth month of fetal life, two centres make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage. The base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age. In this cartilage, rudiments of the lower

epiphyseal plate The epiphyseal plate, epiphysial plate, physis, or growth plate is a hyaline cartilage plate in the metaphysis at each end of a long bone. It is the part of a long bone where new bone growth takes place; that is, the whole bone is alive, with ma ...

of the atlas, and the upper epiphyseal plate of the axis may sometimes be found. The apex of the dens has a separate centre that appears in the second and joins about the twelfth year; this is the upper epiphyseal plate of the atlas. In addition to these, there is a secondary centre for a thin epiphyseal plate on the undersurface of the body of the bone.

Clinical significance

Fractures

A fracture of both pedicles of the axis is termed a

Hangman's fracture Hangman's fracture is the colloquial name given to a fracture of both Pedicle of vertebral arch, pedicles, or ''pars interarticularis, partes interarticulares'', of the ''axis vertebra'' (C2 vertebra, C2). Causes The injury mainly occurs from fal ...

. Fractures of the dens are classified into three categories according to the Anderson Alonso system: * Type I fracture - Extends through the tip of the dens. This type is usually stable. * Type II fracture - Extends through the base of the dens. It is the most commonly encountered fracture for this region of the axis. This type is unstable and has a high rate of non-union. * Type III fracture - Extends through the vertebral body of the axis. This type can be stable or unstable and may require surger

File:PdensfracX.png, A fracture of the base of the dens as seen on plain X-ray File:PdensfracCT.png, A fracture of the base of the dens as seen on CT File:OdontoidType3Mark.png, Type 3 odontoid fracture File:Type2DensFracMark.png, Type 2 dens fracture

Additional images

File:C2 from top animation small.gif, Its shape and position (shown in red) from above. The skull is shown in semi-transparent. File:Cervical vertebra 2 close-up top animation.gif, Axis seen from above File:Human axis bone.stl, 3D image File:Gray305.png, Posterior atlantooccipital membrane and atlantoaxial ligament; the axis is visible at center. File:Gray308.png, Median sagittal section through the occipital bone and first three cervical vertebra File:Gray994.png, Sagittal section of nose mouth, pharynx, and larynx File:Dens axis.jpg, Axis on X-ray taken through an open mouth, teeth visible File:Atlasbogenspalt.jpg, Unfused arch of atlas at CT

References

External links

*Netter, Frank
''Atlas of Human Anatomy''
, "High Cervical Spine: C1-C2" {{Authority control Bones of the thorax