Sacrum, or OS SACRUM, is a triangular bone situated at the lower part of the vertebral column (of which it is a natural continuation), and wedged between the two innominate bones so as to form the keystone to the pelvic arch. It is readily seen to consist of five vertebrae with their bodies and processes, all consolidated into a single bone. Its anterior surface (see illustrations at PELVIS) is concave, not only from above downwards, but also from side to side. The posterior surface is convex, and presents in the middle vertical line a crest, formed by the fusion of the spines of the vertebrae, of which the bone is composed. The last sacral vertebra has, however, no spine, and the termination of the vertebral canal is here very slightly protected. The sacrum of man differs from that of the lower animals by its greater breadth in comparison with its length. This proportion is expressed in the following way: . In the male European the average sacral index is 112, in the negro 106, in the Australian aboriginal 99, in the orang 87, in the gorilla 72. In the female the sacrum is broader than in the male, the sacral index of the European female being about 116 (Turner, Challenger Reports, Zoology, xvi.). The sacrum and its connections are illustrated at PELVIS.
Various reasons have been assigned for the name given from of old to this bone; Littre accepts the view that it was because it was a part that had special significance with the ancients in sacrifices. Another reason is based on the view maintained by the Jewish rabbins, who held that this part of the skeleton, which they called 'luz,' resisted decay, and became the germ from which the body would be raised.