cauda equina

noun

cauda equi·​na -ek-ˈwē-nə  -ē-ˈkwī-nə 

plural caudae equinae -ek-ˈwē-ˌnī, -ē-ˈkwī-ˌnē 

: 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 spinal 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