The lumbosacral spines of two fetal and twelve adult cadavers have been studied by dissection. Evidence shows that the fifth lumbar intervertebral foramen is crossed on its external aspect by a strong, cord-like corporotransverse ligament passing obliquely downwards, forwards and medially from the inferior aspect of the accessory process of the fifth lumbar vertebra to the lateral surface of the intervertebral disc and the adjacent parts of the bodies of the fifth and first sacral vertebrae. Superficially, the ligament is related to another flat band--the lumbosacral hood. Together these ligaments separate and provide openings for the sympathetic ramus, the ventral ramus and blood vessels related to the intervertebral foramen. On the dorsal aspect, a tripartite ligament, the mamillo-transverso-accessory ligament, bears important relationships to the subdivisions of the dorsal ramus and also the zygapophyseal joint. The significance of these findings is discussed.