Vertebra

Collier's New Encyclopedia (1921)

in comparative anatomy, one of the bony segments of which the spine, or backbone, consists. Theoretically, 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 ha hæmal) protecting the organs of circulation, and thus corresponding to the doubly tubular structure of the body of 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 vertebræ to each other. From the sides of the body proceed the transverse processes. The number of vertebræ varies greatly in different animals. The vertebral column is divisible into distinct regions, of which the following are recognizable in the higher Vertebrata: The cervical vertebræ (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, rue vertebre, and are sometimes called true vertebræ, to distinguish them from the false vertebræ, which consists of those in the sacral region usually anchylosed to form a single bone, the os sacrum, and a variable number of vertebræ forming the caudal region or tail. The spaces between the vertebræ 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 considerable flexibility. The vertebræ and their projections or processes afford attachments for a number of muscles and ligaments, and passages of blood vessels and for the nerves passing out of the spinal cord.