nucleus pulposus and the fibers of the anulus fibrosus make up the intervertebral disc. the primary curves (thoracic and sacral curvatures) form during fetal development. the secondary curves develop after birth. the cervical curvature forms as a result of lifting the head and the lumbar curvature forms as a result of walking. function spinal cord the spinal cord nested in the vertebral column. main article: spinal cord the vertebral column surrounds the spinal cord which travels within the spinal canal, formed from a central hole within each vertebra. the spinal cord is part of the central nervous system that supplies nerves and receives information from the peripheral nervous system within the body. the spinal cord consists of grey and white matter and a central cavity, the central canal. adjacent to each vertebra emerge spinal nerves. the spinal nerves provide sympathetic nervous supply to the body, with nerves emerging forming the sympathetic trunk and the splanchnic nerves. the spinal canal follows the different curves of the column; it is large and triangular in those parts of the column that enjoy the greatest freedom of movement, such as the cervical and lumbar regions, and is small and rounded in the thoracic region, where motion is more limited. the spinal cord terminates in the conus medullaris and cauda equina. clinical significance 3d medical animation still shot of spina bifida disease spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch. sometimes the spinal meninges and also the spinal cord can protrude through this, and this is called spina bifida cystica. where the condition does not involve this protrusion it is known as spina bifida occulta. sometimes all of the vertebral arches may remain incomplete. another, though rare, congenital disease is klippel–feil syndrome, which is the fusion of any two of the cervical vertebrae. spondylolisthesis is the forward displacement of a vertebra and retrolisthesis is a posterior displacement of one vertebral body with respect to the adjacent vertebra to a degree less than a dislocation. spondylolysis, also known as a pars defect, is a defect or fracture at the pars interarticularis of the vertebral arch. spinal disc herniation, more commonly called a "slipped disc", is the result of a tear in the outer ring (anulus fibrosus) of the intervertebral disc, which lets some of the soft gel-like material, the nucleus pulposus, bulge out in a hernia. spinal stenosis is a narrowing of the spinal canal which can occur in any region of the spine though less commonly in the thoracic region. the stenosis can constrict the spinal canal giving rise to a neurological deficit. pain at the coccyx (tailbone) is known as coccydynia. spinal cord injury is damage to the spinal cord that causes changes in its function, either temporary or permanent. spinal cord injuries can be divided into categories: complete transection, hemisection, central spinal cord lesions, posterior spinal cord lesions, and anterior spinal cord lesions. scalloping vertebrae is the increase in the concavity of the posterior vertebral body. it can be seen on lateral x-ray and sagittal views of ct and mri scans. its concavity is due to the increased pressure exerting on the vertebrae due to a mass. internal spinal mass such as spinal astrocytoma, ependymoma, schwannoma, neurofibroma, and achondroplasia causes vertebrae scalloping. curvature diagram showing normal curvature of the vertebrae from childhood to teenage excessive or abnormal spinal curvature is classed as a spinal disease or dorsopathy and includes the following abnormal curvatures: kyphosis is an exaggerated kyphotic (convex) curvature of the thoracic region in the sagittal plane, also called hyperkyphosis. this produces the so-called "humpback" or "dowager's hump", a condition commonly resulting from osteoporosis. lordosis is an exaggerated lordotic (concave) curvature of the lumbar region in the sagittal plane, is known as lumbar hyperlordosis and also as "swayback". temporary lordosis is common during pregnancy. scoliosis, lateral curvature, is the most common abnormal curvature, occurring in 0.5% of the population. it is more common among females and may result from unequal growth of the two sides of one or more vertebrae, so that they do not fuse properly. it can also be caused by pulmonary atelectasis (partial or complete deflation of one or more lobes of the lungs) as observed in asthma or pneumothorax. kyphoscoliosis, a combination of kyphosis and scoliosis. anatomical landmarks surface projections of organs of the torso. the transpyloric line is seen at l1 individual vertebrae of the human vertebral column can be felt and used as surface anatomy, with reference points are taken from the middle of the vertebral body. this provides anatomical landmarks that can be used to guide procedures such as a lumbar puncture and also as vertical reference points to describe the locations of other parts of human anatomy, such as the positions of organs. other animals variations in vertebrae the general structure of vertebrae in other animals is largely the same as in humans. individual vertebrae are composed of a centrum (body), arches protruding from the top and bottom of the centrum, and various processes projecting from the centrum and/or arches. an arch extending from the top of the centrum is called a neural arch, while the haemal arch or chevron is found underneath the centrum in the caudal (tail) vertebrae of fish, most reptiles, some birds, some dinosaurs and some mammals with long tails. the vertebral processes can either give the structure rigidity, help them articulate with ribs, or serve as muscle attachment points. common types are transverse process, diapophyses, parapophyses, and zygapophyses (both the cranial zygapophyses and the caudal zygapophyses). the centrum of the vertebra can be classified based on the fusion of its elements. in temnospondyls, bones such as the spinous process, the pleurocentrum and the intercentrum are separate ossifications. fused elements, however, classify a vertebra as having holospondyly. a vertebra can also be described in terms of the shape of the ends of the centrum. centra with flat ends are acoelous, like those in mammals. these flat ends of the centra are especially good at supporting and distributing compressive forces. amphicoelous vertebra have centra with both ends concave. this shape is common in fish, where most motion is limited. amphicoelous centra often are integrated with a full notochord. procoelous vertebrae are anteriorly concave and posteriorly convex. they are found in frogs and modern reptiles. opisthocoelous vertebrae are the opposite, possessing anterior convexity and posterior concavity. they are found in salamanders, and in some non-avian dinosaurs. heterocoelous vertebrae have saddle-shaped articular surfaces. this type of configuration is seen in turtles that retract their necks, and birds, because it permits extensive lateral and vertical flexion motion without stretching the nerve cord too extensively or wringing it about its long axis. in horses, the arabian (breed) can have one less vertebrae and pair of ribs. this anomaly disappears in foals that are the product of an arabian and another breed of horse. regional vertebrae vertebrae are defined by the regions of the vertebral column that they occur in, as in humans. cervical vertebrae are those in the neck area. with the exception of the two sloth genera (choloepus and bradypus) and the manatee genus, (trichechus), all mammals have seven cervical vertebrae. in other vertebrates, the number of cervical vertebrae can range from a single vertebra in amphibians to as many as 25 in swans or 76 in the extinct plesiosaur elasmosaurus. the dorsal vertebrae range from the bottom of the neck to the top of the pelvis. dorsal vertebrae attached to the ribs are called thoracic vertebrae, while those without ribs are called lumbar vertebrae. the sacral vertebrae are those in the pelvic region, and range from one in amphibians, to two in most birds and modern reptiles, or up to three to five in mammals. when multiple sacral vertebrae are fused into a single structure, it is called the sacrum. the synsacrum is a similar fused structure found in birds that is composed of the sacral, lumbar, and some of the thoracic and caudal vertebra, as well as the pelvic girdle. caudal vertebrae compose the tail, and the final few can be fused into the pygostyle in birds, or into the coccygeal or tail bone in chimpanzees (and humans). fish and amphibians a vertebra (diameter 5 mm) of a small ray-finned fish the vertebrae of lobe-finned fishes consist of three discrete bony elements. the vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. just beneath the arch lies a small plate-like pleurocentrum, which protects the upper surface of the notochord, and below that, a larger arch-shaped intercentrum to protect the lower border. both of these structures are embedded within a single cylindrical mass of cartilage. a similar arrangement was found in the primitive labyrinthodonts, but in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body. in most ray-finned fishes, including all teleosts, these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. in living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods. in cartilaginous fish, such as sharks, the vertebrae consist of two cartilaginous tubes. the upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. the lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification. lampreys have vertebral arches, but nothing resembling the vertebral bodies found in all higher vertebrates. even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail. other vertebrates the general structure of human vertebrae is fairly typical of that found in mammals, reptiles, and birds. the shape of the vertebral body does, however, vary somewhat between different groups. in mammals, such as humans, it typically has flat upper and lower surfaces, while in reptiles the anterior surface commonly has a concave socket into which the expanded convex face of the next vertebral body fits. even these patterns are only generalisations, however, and there may be variation in form of the vertebrae along the length of the spine even within a single species. some unusual variations include the saddle-shaped sockets between the cervical vertebrae of birds and the presence of a narrow hollow canal running down the centre of the vertebral bodies of geckos and tuataras, containing a remnant of the notochord. reptiles often retain the primitive intercentra, which are present as small crescent-shaped bony elements lying between the bodies of adjacent vertebrae; similar structures are often found in the caudal vertebrae of mammals. in the tail, these are attached to chevron-shaped bones called haemal arches, which attach below the base of the spine, and help to support the musculature. these latter bones are probably homologous with the ventral ribs of fish. the number of vertebrae in the spines of reptiles is highly variable, and may be several hundred in some species of snake. in birds, there is a variable number of cervical vertebrae, which often form the only truly flexible part of the spine. the thoracic vertebrae are partially fused, providing a solid brace for the wings during flight. the sacral vertebrae are fused with the lumbar vertebrae, and some thoracic and caudal vertebrae, to form a single structure, the synsacrum, which is thus of greater relative length than the sacrum of mammals. in living birds, the remaining caudal vertebrae are fused into a further bone, the pygostyle, for attachment of the tail feathers. aside from the tail, the number of vertebrae in mammals is generally fairly constant. there are almost always seven cervical vertebrae (sloths and manatees are among the few exceptions), followed by around twenty or so further vertebrae, divided between the thoracic and lumbar