Fig. 2.—Diagram of the pelvic inlet, seen from above, with most important diameters.
As the pelvis occupies an oblique position in the body, the plane of this brim is not horizontal, but slopes up and back from the symphysis-pubis to the promontory of the sacrum. Being swung upon the heads of the femora, the relation of the pelvis to the entire body differs in the sitting and standing positions. When a woman stands upright, her pelvis is so markedly oblique in its position that she would tip backward but for strong tendons attached to the pelvis and running down the front of the thighs. Added strain upon these tendons during pregnancy may account for some of the apparently undue fatigue experienced by the expectant mother.
The shallow, expanded portion of the pelvis above the brim is the large, or false pelvis, its walls being formed by the sacrum behind, the fan-like flares of the ilia on each side, with the incompleteness of the bony wall in front made up by abdominal muscles.
The false pelvis ordinarily serves simply as a support for the abdominal viscera, which do not occupy the true pelvis unless forced down by some such pressure as that caused by tight, or poorly fitting corsets. The false pelvis is of little obstetrical importance, its function during pregnancy being to support the enlarged uterus, while at the time of labor it acts as a funnel to direct the child’s body into the true pelvis below.
Fig. 3.—Diagram of pelvic outlet, seen from below, with most important diameters.
The true pelvis, on the other hand, is of greatest possible obstetrical importance since the child must pass through its narrow passage during birth. It lies below and somewhat behind the inlet; is an irregularly shaped, bottomless basin, and contains the generative organs, rectum and bladder. Its bony walls are more complete than those of the false pelvis, and are formed by the sacrum, coccyx and innominate bones. Its lower margin constitutes the outlet, or inferior strait, and being longer in its antero-posterior dimension than in its transverse measurement, its long axis is at right angles to the long axis of the inlet. (Fig. [3].) A baby’s head, accordingly, must twist or rotate in making its descent through this bony canal, for the long diameter of the head must first conform to one of the long diameters of the inlet, either transverse or oblique, and then turn so that the length of the head is lying antero-posteriorly, in conformity to the long diameter of the outlet, through which it next passes.
The posterior wall of the pelvis, consisting of the sacrum and coccyx, forms a vertical curve and is about three times as deep as the anterior wall formed by the narrow symphysis pubis. The structure as a whole, therefore, curves upon itself, resembling a bent tube with its concavity directed forward. (Fig. [4].)
Fig. 4.—Diagram of sagittal section of the pelvis showing curve of the bony canal, with most important diameters.