Cauda equina

The cauda equina is a structure within the lower end of the spinal column of most vertebrates, that consists of nerve roots and rootlets from above. Cauda equina is sometimes considered as an extension of piamater of spinal cord. The space in which the cerebro spinal fluid is present is actually an extension of the sub arachnoid space.

In humans, because the spinal cord stops growing in infancy while the bones of the spine continue growing, the spinal cord in adults ends at about the level of the vertebra L1/L2, and at birth at L3. Though in adults there is some variation and the cord may end anywhere between vertebrae T12 to L3. 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 fibres that extends caudally from the end of the spinal cord, gradually declining in number further down as individual pairs leave the spinal column. At the base of the Cauda Equina, there are approximately 10 fibre pairs, 3-5 lumbar, 5 sacral, and the single coccygeal nerve.

Etymology
All these roots and rootlets down the vertebral column give the appearance of a horse's tail, which is the meaning of the Latin name cauda equina.

Clinical relevance
Cauda equina is the part of the spinal cord where a spinal tap is performed in order to get a sample of cerebral spinal fluid (CSF) for diagnostic purposes.