Vertebrae

A diagram of a human thoracic vertebra. Notice the articulations for the ribs

A vertebra (plural: vertebrae) is an individual bone in the flexible column that defines vertebrate animals, via 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.

Structure

Individual vertebra are composed of a centrum (body), arches protruding from the top of 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 hemal arch or chevron is found underneath the centrum in the caudal (tail) vertebrae of fish, most reptiles, some birds, 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 tranverse process, diapophyses, parapophyses, and zygapophyses (both the cranial zygapophyses and the caudal zygapophyses).

Amphicelous refers to a centrum that is concave at both ends, similar to those found in most fish. Opisthocoelous centra are convex in the front and concave in the back, similar to those of most salamanders. In contrast, procelous centra are concave in the front and convex in the back, as found in most frogs and modern reptiles. Centra with flat ends are acelous, like those in mammals. Birds have heterocelous centra, shaped like saddles at both ends.