That part of the vertebral column which is directly connected with, or forms a part of, the pelvis. [
1913 Webster]
" It may consist of a single vertebra or of several more or less consolidated. In man it forms the dorsal, or posterior, wall of the pelvis, and consists of five united vertebræ, which diminish in size very rapidly to the posterior extremity, which bears the coccyx."
[1913 Webster]