VERTEBRA, in comparative anatomy, one of the bony segments of which the spine, or backbone, consists. Theoreti cally, a typical vertebra consists of a central piece or body, from which two arches are given off, one (the neural), protecting the nervous system, the other (the hamal) protecting the organs of circulation, and thus corresponding to the doubly tubular structure of the body 'if the Vertebra. In practice the second arch is only recognizable with difficulty, the parts being either absent or much modified, but a good example may be seen in the human thorax. The fundamental element of each vertebra is the body or centrum, from the surface of which spring two bony arches, called the neural arches, or neurapophyses, because they form with the body the neural canal, which incloses the spinal cord. From the point of junction there is usually developed a spine, called the spinous process, or neural spine, rudimentary in the atlas or first cervical vertebra. From the neural arches are also developed the articular process or zygapophyses, which aid the centra in uniting the vertebra to each other. From the sides of the body proceed the transverse processes.
The number of vertebra varies greatly in different animals. The vertebral col umn is divisible into distinct regions, of which the following are recognizable in the higher Vertebrata: The cervical vertebra (seven in man), composing the neck; the dorsal (twelve in man), usually carrying well-developed ribs; the lumbar (five in man). These form the cervical, dorsal and lumbar regions respectively, and are sometimes called true vertebra, to distinguish them from the false ver tebra, which consists of those in the sa cral region usually anchylosed to form a single bone, the os sacrum, and a variable number of vertebra forming the caudal region or tail. The spaces between the vertebra are filled with an elastic substance, admitting of an amount of motion which, though slight between each pair, is in the aggregate sufficient to give the spinal column con siderable flexibility. The vertebra and their projections or processes afford at tachments for a number of muscles and ligaments, and passages of blood ves sels and for the nerves passing out of the spinal cord.