the elements of the axial skeleton forming the spine of vertebrate animals and man.

A vertebra consists of a thickened ventral (in man, anterior) part, the vertebral body, and dorsal arch that fuses with the vertebral body and closes the cerebrospinal canal. In fishes, the lower vertebral arches form transversal processes in the truncal segment; the ribs are attached to these processes. In the caudal segment, the lower arches interlock in the hemal arch with the lower spinal process. The upper arch bears an unpaired upper spinal process. At the arch’s base in terrestrial vertebrates are paired anterior and posterior articular processes and a pair of transversal processes with facets for articulating with the tubercle of a rib. The facet for the head of a rib is usually located between two vertebrae, but it may be entirely displaced to the body of the second vertebra or even fuse with the facet for the tubercle of the rib. A vertebra may also have additional processes.

In fishes, some amphibians, and reptiles, the articular surfaces of the vertebrae may be biconcave, or amphicelous. In most amphibians and reptiles, the vertebral bodies become procelous (concave in front and convex in back) or opisthocelous (convex in front and concave in back) because of the need for mobility. The vertebral bodies are heterocelous, or saddle-shaped, in birds. Mammals have intervertebral cartilaginous disks, and consequently the vertebral bodies are platycelous, or flat in front and concave in back.

The vertebral bodies developed independently in different groups of animals. Their multiple origins may be clearly traced in ontogeny and especially in phylogeny. Up to four pairs of ossifications are formed in fishes: two correspond to the bases of the upper (epicentral) and lower (hypocentral) arches and two are pairs of intercalary (pleurocentral) elements. In most fishes the epicenters and hypocenters fused into a ring and the pleuro-centers were reduced. In crossopterygian fishes and many extinct amphibians, for example, the Stegocephalia, the hypocenters formed a crescent-shaped ventral element, paired in crossoptery-gians, with the pleurocenters remaining small and paired. The hypocenters are reduced in most reptiles as in all amniotes. The heads of the ribs, which originally articulated with the hypocenters, occupied a position between the vertebrae after the hypo-centers were lost. The hemal arches and the small lower spinal process fuse with the hypocenters in the caudal region.