In
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 axis (from Latin ''axis'', "axle") or
epistropheus is the second
cervical vertebra (C2) 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 ...
, immediately inferior to the
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 ...
, upon which the head rests.
The axis' defining feature is its strong odontoid process (bony protrusion) known as the dens, which rises dorsally from the rest 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 m ...
s.
Odontoid Process of Axis (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. 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 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 ...
.
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 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 ( 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 ...
. 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
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...
.
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 is large, but smaller than the
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 ...
.
The
transverse processes
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
are very small, and each ends in a single tubercle. Each process is perforated by the
transverse foramen, which is directed obliquely upward and laterally.
The superior
articular surfaces 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 (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 ...
.
The superior
vertebral notches are very shallow, and lie behind the
articular processes
The articular processes or zygapophyses ( Greek ζυγον = " yoke" (because it links two vertebrae) + απο = "away" + φυσις = " process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vert ...
. The inferior vertebral notches lie in front of the articular processes, as in the other cervical vertebrae.
The
spinous process is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity.
Variation
Contact sport
Contact sports are sports that emphasize or require physical contact between players. Some sports, such as mixed martial arts, are scored on impacting an opponent, while others, including rugby football, gridiron football and Australian rules f ...
s are
contraindicated for individuals with anomalous dens, as any violent impact may result in a
catastrophic injury.
This is because a malformed odontoid process may lead to instability between the
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 ...
and axis (the C1 and C2
cervical vertebrae
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 ...
).
Development
The axis is
ossified 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
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
, viz., one centre for the body, and two for the
vertebral arch
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
.
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
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 ...
, consist 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 lamella of the atlas and the upper
epiphyseal lamella of the axis may sometimes be found.
The apex of the odontoid process has a separate centre that appears in the second and joins about the twelfth year; this is the upper epiphyseal lamella of the atlas.
In addition to these, there is a secondary centre for a thin
epiphyseal plate
The epiphyseal plate (or 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, wit ...
on the undersurface of the body of the bone.
Clinical significance
Fracture of dens
Fractures of the dens, not to be confused with
Hangman's fracture
Hangman's fracture is the colloquial name given to a fracture of both pedicles, or '' partes interarticulares'', of the ''axis vertebra'' ( C2).
Causes
The injury mainly occurs from falls, usually in elderly adults, and motor accidents mainly d ...
s, 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. (Axis visible at center.)
File:Gray308.png, Median sagittal section through the occipital bone and first three cervical vertebræ
File:Gray994.png, Sagittal section of nose mouth, pharynx, and larynx
File:Dens axis.jpg, Axis on X-ray
File:Atlasbogenspalt.jpg, Unfused arch of atlas at CT
See also
References
External links
*Netter, Frank
''Atlas of Human Anatomy'', "High Cervical Spine: C1-C2"
{{Authority control
Bones of the thorax