cauda equina

:  the roots of the spinal nerves that extend beyond the termination of the spinal cord at the first lumbar vertebra in the form of a bundle of filaments within the vertebral canal resembling a horse's tail From its tip at the base of the spine, about 20 stringy nerves—called the cauda equina, or horse's tail—travel downward within a fluid-filled sac.—Bruce H. Dobkin, Discover, November 1996