<P> 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 . </P> <P> 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 . </P> <P> 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). </P> <P> 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 . </P>

Division of the back contains the neck vertebrae