sacral vertebra, one of the five segments of the vertebral column that fuse in the adult to form the sacrum. The ventral border of the first sacral vertebra projects into the pelvis. The bodies of the other sacral vertebrae are smaller than that of the first and are flattened and curved ventrally, forming the convex, anterior surface of the sacrum. The rudimentary spinous processes of the first several sacral vertebrae surmount the middle sacral crest, and the transverse processes of the sacral vertebrae form the lateral sacral crests. The sacral hiatus at the caudal end of the sacral canal develops from the incomplete growth of the spinous processes of the last two sacral vertebrae. The resultant widened aperture is used by anesthesiologists for the insertion of a needle to administer caudal analgesia. Compare cervical vertebra, coccygeal vertebra, lumbar vertebra, thoracic vertebra. See also sacrum, vertebra.
