A typical vertebra, superior view
A section of the human vertebral column, showing multiple vertebra in a left posterolateral view.
Each vertebra (pl. vertebrae) in a vertebrate’s spinal column is a complex structure composed of bone and some hyaline cartilage, which varies according to the area of backbone and species involved. There are several variations of the basic configuration of a vertebra. Vertebrates take their name from the presence of vertebrae in their vertebral column. A vertebra is classed as an irregular bone.
The large part of the vertebra is called its body and the central part of this is called the centrum. The upper and lower surfaces of the body give attachment to the intervertebral discs. The front part of the vertebral body gives attachment for the ligamenta flava. The posterior part of a vertebra forms a vertebral arch and this consists of two pedicles, two laminae and seven processes. There are vertebral notches formed from the shape of the pedicles, which form the intervertebral foramina when the vertebrae articulate. These foramina allow the entry and exit points for the spinal nerves. The body of the vertebra together with the vertebral arch forms the vertebral foramen, the larger central opening which accommodates the spinal canal, that houses and protects the spinal cord.
Vertebrae articulate with each other to give strength and flexibility to the spine, and the shape at their back and front aspects determines their range of movement. They are essentially alike across the vertebrate species with most difference seen between aquatic and other vertebrates.
The size of vertebrae varies according to placement, spinal loading, posture and pathology. Along the length of the vertebral column the vertebrae change to accommodate different needs related to stress and mobility.
Every vertebra has a body, which consists of a large anterior middle portion called the centrum and a posterior vertebral arch, also called a neural arch. The body is composed of cancellous bone, which is the spongy type of osseous tissue, covered by a thin coating of compact bone. The vertebral arch and processes have thick coverings of compact bone.
The vertebral arch is formed by pedicles and laminae. Two pedicles extend from the sides of the vertebral body to join the body to the arch. The pedicles are short thick processes that extend, one from each side, posteriorly, from the junctions of the posterior and lateral surfaces of the centrum, on its upper surface. From each pedicle a broad plate, a lamina, projects to join and form the roof of the arch which completes the triangle of the vertebral foramen. Above and below the pedicles are shallow depressions called vertebral notches. When the vertebrae articulate the notches align with those on adjacent vertebrae and these form the openings of the vertebral foramina. The foramina allow the entry and exit of the spinal nerves from each vertebra, together with associated blood vessels. The articulating vertebrae provide a strong pillar of support for the body. There are seven processes projecting from the vertebra; a spinous process, two transverse processes, and four articular processes. A major part of a vertebra is a backward extending spinous process which projects centrally. In humans this process points downwards but in those animals without an erect stance they are directed upwards. The spinous process serves to attach muscles and ligaments. There are two transverse processes one on each side of the vertebral body which project from either side at the point where the lamina joins the pedicle, between the superior and inferior articular processes. They also serve for the attachment of muscles and ligaments. There is a facet on each of the transverse processes of thoracic vertebrae which articulates with the tubercle of the rib. A facet on each side of the thoracic vertebral body articulates with the head of head of the rib. There are superior and inferior articular facets on each side of the vertebra, which serve to restrict the range of movement possible. These facets are joined by a thin portion of the neural arch called the pars interarticularis.
The thirty-three vertebrae in the human vertebral column are named after the regions they occupy. There are seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five fused sacral vertebrae and three to five coccygeal vertebrae.
The first and upper two cervical vertebrae of the neck differ from the others and have specific names of atlas (C1), and axis (C2). They form two joints which gives the head its greater range of movement and ability to twist round. These are the atlanto-occipital joint between the atlas and the occipital bone, and the atlanto-axial joint between the atlas and the axis.
The thoracic vertebrae attach to ribs and so have articular facets specific to them. As they progress down the spine they increase in size to match up with the adjoining lumbar section.
The last three to five coccygeal vertebrae form the tailbone or coccyx.
The vertebrae function in the skeletomuscular system by forming the vertebral column to support the body of an animal and to provide the opening, the vertebral foramen for the passage of the spinal canal and its enclosed spinal cord and covering meninges. They also afford sturdy protection for the spinal cord. The upper and lower surfaces of the centrum are flattened and rough in order to give attachment to the intervertebral discs. The vertebrae also provide the openings, the intervertebral foramina which allow the entry and exit of the spinal nerves. Similarly to the surfaces of the centrum, the upper and lower surfaces of the fronts of the laminae are flattened and rough to give attachment to the ligamenta flava. Working together in the vertebral column their sections provide controlled movement and flexibility.
Because of the different types of locomotion and support needed between the aquatic and other vertebrates the vertebrae between them show the most variation, though basic features are shared. The spinous processes which are backward extending are directed upwards in animals without an erect stance. These processes can be very large in the larger animals as they attach to the muscles and ligaments of the body.
Vertebrae with saddle-shaped articular surfaces on their bodies, are called "heterocoelous", which allows vertebrae to flex both vertically and horizontally, while preventing twisting motions. Such vertebrae are found in the necks of birds and some turtles.
In many species, though not in mammals, the cervical vertebrae bear ribs. In many groups, such as lizards and saurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. The transverse processes of mammals are homologous to the cervical ribs of other amniotes.
In all mammal species the thoracic vertebrae are connected to ribs and their bodies differ from the other regional vertebrae due to the presence of facets. Each vertebra has a facet on each side of the vertebral body which articulates with the head of a rib. There is also a facet on each of the transverse processes which articulates with the tubercle of a rib.
The number of thoracic vertebrae varies considerably across the species. Most marsupials have thirteen but koalas only have eleven. The norm is twelve to fifteen in mammals, (twelve in the human), though there are from eighteen to twenty in the horse, tapir, rhinoceros and elephant. In certain sloths there is an extreme number of twenty-five and at the other end only nine in the cetacean.
There are fewer lumbar vertebrae in the Pan genus species of chimpanzees and gorillas, which have three in contrast to the five in the Homo. This reduction in number gives an inability of the lumbar spine to lordose but gives an anatomy that favours vertical climbing, and hanging ability more suited to feeding locations in high-canopied regions. The bonobo differs by having four lumbar vertebrae.
Caudal vertebrae are the bones that make up the tails of vertebrates. They range in number from a few to fifty, depending on the length of the animal's tail. In humans, and other tailless primates they are called the coccygeal vertebrae, number from three to five and are fused into the coccyx.
A pinched nerve caused by pressure from a disc, vertebra or scar tissue might be remedied by a foraminotomy to broaden the intervertebral foramina and relieve pressure. It can also be caused by a foramina stenosis, a narrowing of the nerve opening, as a result of arthritis.
Another condition is spondylolisthesis when one vertebra slips forward onto another.
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- "Physical Characteristics of the Koala". Australian Koala Foundation. Retrieved 1 February 2012.
- Hyman (1922), p.124
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