In the axial skeleton of vertebrates, one of a series of bony segments which replace the notochord, forming the vertebral column (or spinal column or backbone), which encases and so protects the spinal cord. Vertebrae differentiate into five types from anterior to posterior: cervical; thoracic; lumbar; sacral; and caudal. Cervical vertebrae facilitate the mobility of the head. The first two vertebrae of the vertebral column, the atlas and axis, are highly specialized cervical vertebrae, the former articulating with the occipital region of the cranium. The thoracic vertebrae articulate with the ribs that fuse with the sternum. Lumbar vertebrae are generally larger, with abbreviated ribs fused to the centrum and supporting the posterior coelomic musculature. Sacral vertebrae fuse with the pelvis, allowing the transfer of force to the appendicular skeleton. Caudal vertebrae are smaller and less specialized, forming the tail of the organism. Six anatomical features are usually recognizable in vertebrae: the centrum is a solid cylinder which surrounds and often replaces the notochord, forming the central body of the vertebra; the neural arch forms a dorsal ring surrounding the spinal cord; a hemal arch grows ventrally on post-anal vertebrae, enclosing blood vessels; neural and hemal spines are anterior–posterior-oriented blades of bone that project dorsally and ventrally respectively; apophyses are bilaterally paired projections to which musculature is usually attached, including prezygapophyses and postzygapophyses, which occur on the anterior and posterior ends of a vertebra respectively and articulate with zygapophyses of adjacent vertebrae; transverse processes are bilaterally paired lateral projections at each side of the neural arch with which the rib articulates.