Henry Gray (18251861). Anatomy of the Human Body. 1918.
The roots of the lumbar, sacral, and coccygeal nerves run vertically downward to their respective exits, and as the medulla spinalis ends near the lower border of the first lumbar vertebra it follows that the length of the successive roots must rapidly increase. As already mentioned (page 750), the term cauda equina is applied to this collection of nerve roots.