A vertebra (plural: vertebrae) is an individual bone in the flexible column that defines vertebrate animals, including humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into the tail.
Vertebrae are defined by regions. Cervical vertebrae are those in the neck area, and can range from a single vertebra in amphibians, to seven in most mammals and reptiles, and 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 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 3 to 5 in mammals. When more than one 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 vertebra compose the tail, and the final few can be fused into the pygostyle in birds, or into the coccygeal or tail bone in chimpanzees or humans.
