=The sacrum and coccyx= behind are really the termination of the spinal column, the sacrum consisting, usually, of five rudimentary vertebrae which have fused into one bone. It sometimes consists of four bones, sometimes six, but more often of five. The sacrum completes the pelvic girdle behind by uniting on each side with the ossa innominata by means of strong cartilages, thus forming the _sacro-iliac joints_. The spinal column rests upon the upper surface of the sacrum. The coccyx, a little wedge-shaped, tail-like appendage, which ordinarily has but slight obstetrical importance, extends in a downward curve from the lower margin of the sacrum, to which it has a cartilaginous attachment, the _sacro-coccygeal joint_. This joint between the sacrum and coccyx is much more movable in the female than in the male pelvis. We find, therefore, that although the pelvis constitutes a rigid, bony, ringlike structure, there are four joints: the symphysis pubis, the sacro-coccygeal, and the two sacro-iliac articulations. As the cartilages in these joints become somewhat softened and thickened during pregnancy, because of the increased blood supply, they all permit of a certain, though limited amount of motion at the time of labor. This provision is of considerable obstetrical importance, since the sacro-coccygeal joint allows the child’s head to push back the forward-protruding coccyx, as it passes down the birth canal, thus removing what otherwise might be a serious obstruction. And when, as is sometimes necessary, because of a constricted inlet, the pubic bone is cut through (the operation known as pubiotomy), the hingelike motion of the sacro-iliac joint permits of an appreciable spreading of the two hip bones and a consequent widening of the birth canal. [Illustration: A. Normal female Pelvis. ] [Illustration: B. Normal male Pelvis. FIG. 1.—Normal Pelves. Note the broad, shallow, light construction of the female pelvis, A, as compared with the more massive male pelvis, B. ] =The pelvic cavity= as a whole is divided into the _true_ and _false pelves_ by a constriction of the entire structure known as the _brim_ or _inlet_. The inlet is not round, its antero-posterior diameter being shortened by the _sacro-vertebral joint_ which protrudes forward and gives the opening something of a blunt, heart-shaped outline. (Fig. 2.) [Illustration: FIG. 2.—Diagram of the pelvic inlet, seen from above, with most important diameters. ]