The innominate bones (ossa innominata), symmetrically placed on each side, are broad, flaring and scoop-shaped. Each bone consists of three main parts, which are separate bones in early life, but firmly welded together in adults: the ilium, ischium and pubis. The ilia are the broad, thin, plate-like sections above, their upper, anterior prominences, which may be felt as the hips, are the anterior superior spinous processes used in making pelvic measurements. The margins extending backward from these points are termed the iliac crests.
The ischii are below and it is upon their projections, known as the tuberosities, that the body rests when in the sitting position, and which also serve as landmarks in pelvimetry. The pubes form the front of the pelvic wall, the anterior rami uniting in the median line by means of heavy cartilage and forming the symphysis pubis.
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.
A. Normal female Pelvis.
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].)
