A pain in the sacrum or the "os sacrum" which is the "sacred bone"; so called because it was thought never to disintegrate or break up: Dr. Smith described Mark's suffering as a condition of hieralgia that affected the bone which formed the posterior section of the pelvis.

The "os sacrum" is a large triangular bone formed by the fusion of the five sacral vertebrae at the base of the vertebral column and lying between the hip bones at the back part of the pelvic cavity.