The cauda equina (Latin for "horse's tail") is a bundle of spinal nerves and spinal nerve roots, consisting of the second through fifth lumbar nerve pairs, the first through fifth sacral nerve pairs, and the coccygeal nerve, all of which originate in the conus medullaris of the spinal cord. The nerves that compose the cauda equina innervate the pelvic organs and lower limbs to include motor innervation of the hips, knees, ankles, feet, internal anal sphincter and external anal sphincter. In addition, the cauda equina extends to sensory innervation of the perineum and, partially, parasympathetic innervation of the bladder.
The cauda equina was named after its resemblance to a horse's tail by the French anatomist Andreas Lazarius in the 17th century.
In humans, because the spinal cord stops growing in infancy while the bones of the vertebral column continue growing, the spinal cord in adults ends at about the level of the first or second lumbar vertebra (L1 or L2), and at birth at L3. However due to normal anatomical variation, the cord may end anywhere between the twelfth thoracic vertebra (T12) to L3 in adults. Individual spinal nerve roots arise from the spinal cord as they do closer to the head, but as the differential growth occurs the top end of the nerve stays attached to the spinal cord and the lower end of the nerve exits the spinal column at its proper level, this results in a "bundle"-like structure of nerve fibers that extends caudally from the end of the spinal cord, gradually declining in number further down as individual pairs leave the spinal column.
Clinical relevance 
The cauda equina exists within the lumbar cistern, which is the space formed from the surrounding dural sac. Cerebrospinal fluid is drawn from this space during a lumbar puncture.
See also 
Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function
External links