Sparing of sensation in sacral dermatomes and of sphincter control was found in eight out of fourteen cases of severe cauda equina compression from massive central lumbar disc prolapse. Although the triangular shape of the lumbar spinal canal may be one factor for this it was found from a necropsy model that the increase in linear strain on the stretched roots of the cauda equina is least in the more centrally placed lower sacral roots. It is argued that the lower tension in these roots is determined by Young's Modulus.