Axis (anatomy)
Encyclopedia
In anatomy
, the second cervical vertebra (C2) of the spine
is named the axis (from Latin axis, "axle") or epistropheus.
It forms the pivot upon which the first cervical vertebra (the atlas
), which carries the head, rotates.
The most distinctive characteristic of this bone
is the strong odontoid process ("dens") which rises perpendicularly from the upper surface of the body. That peculiar feature gives to the vertebra a rarely used third name: vertebra dentata.
It presents in front a median longitudinal ridge, separating two lateral depressions for the attachment of the Longus colli muscle
s.
Its under surface is concave from before backward and convex from side to side.
, or odontoid process, exhibits a slight constriction or neck where it joins the body.
The pedicles
are broad and strong, especially in 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, and the vertebral foramen
large, but smaller than that of the atlas
.
The transverse processes are very small, and each ends in a single tubercle; each 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
.
The superior vertebral notches are very shallow, and lie behind the articular processes
; the inferior 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 bifid
, tuberculated extremity.
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, the second cervical vertebra (C2) of the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
is named the axis (from Latin axis, "axle") or epistropheus.
It forms the pivot upon which the first cervical vertebra (the atlas
Atlas (anatomy)
In anatomy, the atlas is the most superior cervical vertebra of the spine.It is named for the Atlas of Greek mythology, because it supports the globe of the head....
), which carries the head, rotates.
The most distinctive characteristic of this bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
is the strong odontoid process ("dens") which rises perpendicularly from the upper surface of the body. That peculiar feature gives to the vertebra a rarely used third name: vertebra dentata.
The body
The body is deeper in front than behind, and prolonged downward anteriorly so as to overlap the upper and front part of the third vertebra.It presents in front a median longitudinal ridge, separating two lateral depressions for the attachment of the Longus colli muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s.
Its under surface is concave from before backward and convex from side to side.
Other features
The densDens (anatomy)
The dens is a protuberance of the axis . It exhibits a slight constriction or neck, where it joins the main body of the vertebra...
, or odontoid process, exhibits a slight constriction or neck where it joins the body.
The pedicles
Pedicle of vertebral arch
The pedicles are two short, thick processes, which project dorsally, one on either side, from the superior part of the vertebral body at the junction of its posterior and lateral surfaces. They connect the body of the spinal vertebra to the arch...
are broad and strong, especially in 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
Lamina of the vertebral arch
The laminæ are two broad plates, extending dorsally and medially from the pedicles, fusing to complete the roof of the vertebral arch.Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava....
are thick and strong, and the vertebral foramen
Vertebral foramen
In a typical vertebra, the vertebral foramen is the foramen formed by the anterior segment , and the posterior part, the vertebral arch....
large, but smaller than that of the atlas
Atlas (anatomy)
In anatomy, the atlas is the most superior cervical vertebra of the spine.It is named for the Atlas of Greek mythology, because it supports the globe of the head....
.
The transverse processes are very small, and each ends in a single tubercle; each is perforated by the transverse foramen
Transverse foramen
The transverse foramen pierces the transverse processes of the seven cervical vertebrae. In the upper six vertebrae, the foramen gives passage to the vertebral artery, vertebral vein, and a plexus of sympathetic nerves. The seventh foramen lacks the artery, but contains the vein and sympathetic...
, 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
Cervical vertebrae
In vertebrates, cervical vertebrae are those vertebrae immediately inferior to the skull.Thoracic vertebrae in all mammalian species are defined as those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also...
.
The superior vertebral notches are very shallow, and lie behind the articular processes
Articular processes
The articular processes or zygapophyses of a vertebra, two superior and two inferior, spring from the junctions of the pedicles and laminæ...
; the inferior lie in front of the articular processes, as in the other cervical vertebrae.
The spinous process
Spinous process
The spinous process of a vertebra is directed backward and downward from the junction of the laminae , and serves for the attachment of muscles and ligaments. In animals without an erect stance, the process points upward and may slant forward or backward...
is large, very strong, deeply channelled on its under surface, and presents a bifid
Bifid rib
A bifid rib is a congenital abnormality of the rib cage and associated muscles and nerves which occurs in about 1.2% of humans. Bifid ribs occur in up to 8.4% of Samoans . The sternal end of the rib is cleaved into two...
, tuberculated extremity.
External links
- Netter, Frank. Atlas of Human Anatomy, "High Cervical Spine: C1-C2"