Multiple vertebrae fuse at the lower end of the spinal column, to form a large triangular bone called the Sacrum. So called because it was considered, 'sacred' - the soul or the concentration of spiritual energy is supposed to reside here.

Incidentally, this also ties in with Hindu philosophy that believes that the life force is concentrated in the form of a tightly coiled knot of energy called, Kundalini, at the base of the spine. One of the purposes of Yoga and transcendental meditation is to attempt to elevate this energy through successive levels of the vertebral column; each level achieved thus, corresponding to a more exalted spiritual and mental well being of the individual.