lumbar vertebra, one of the five largest segments of the movable part of the vertebral column, distinguished by the absence of a foramen in the transverse process and by vertebral bodies without facets between the sacrum and thoracic vertebrae. The body of each lumbar vertebra is flattened or slightly concave superiorly and inferiorly and is deeply constricted ventrally at the sides. The spinous process of each is thick, broad, and somewhat quadrilateral. The body of the fifth lumbar vertebra is much deeper ventrally than dorsally and in some individuals is defective, tending to weaken the spinal column. Compare cervical vertebra, coccygeal vertebra, sacral vertebra, thoracic vertebra.
