Nerve · Lumbar spine

Cauda equina

Cauda equina

Not a nerve at all, but the bundle of nerve roots that fills the spinal canal after the spinal cord itself has ended. The cord tapers to a point at about the level of the first lumbar vertebra, yet the vertebral column continues for another ten segments — so every root below that level has to travel down inside the dural sac to reach its own exit. The result is a long sheaf of roots hanging almost vertically in cerebrospinal fluid, named for its resemblance to a horse's tail. A root destined to leave the spine at the sacrum has therefore already run the length of the lumbar spine before it gets there, so the vertebral levels a root passes and the level at which it finally leaves the canal are not the same.