The lesser pelvis (or true pelvis) is that part of the pelvic cavity which is situated below and behind the pelvic brim.

Its bony walls are more complete than those of the greater pelvis.

For convenience of description, it is divided into an inlet bounded by the superior circumference, and outlet bounded by the inferior circumference, and a cavity.

Superior circumference/superior aperture/inlet

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The superior circumference forms the brim of the pelvis, the included space being called the superior aperture or inlet (apertura pelvis ^* superior).

It is formed laterally by the pectineal and arcuate lines, in front by the crests of the pubes, and behind by the anterior margin of the base of the sacrum and sacrovertebral angle.

The superior aperture is somewhat heart-shaped, obtusely pointed in front, diverging on either side, and encroached upon behind by the projection forward of the promontory of the sacrum.

It has three principal diameters: antero-posterior, transverse, and oblique.

• The anteroposterior or conjugate diameter extends from the sacrovertebral angle to the symphysis pubis; its average measurement is about 110 mm. in the female.

• The transverse diameter extends across the greatest width of the superior aperture, from the middle of the brim on one side to the same point on the opposite; its average measurement is about 135 mm. in the female.

• The oblique diameter extends from the iliopectineal eminence of one side to the sacroiliac articulation of the opposite side; its average measurement is about 125 mm. in the female.

Cavity

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The cavity of the lesser pelvis is bounded in front and below by the pubic symphysis and the superior rami of the pubes; above and behind, by the pelvic surfaces of the sacrum and coccyx, which, curving forward above and below, contract the superior and inferior apertures of the cavity; laterally, by a broad, smooth, quadrangular area of bone, corresponding to the inner surfaces of the body and superior ramus of the ischium and that part of the ilium which is below the arcuate line.

From this description it will be seen that the cavity of the lesser pelvis is a short, curved canal, considerably deeper on its posterior than on its anterior wall.

It contains, in the fresh subject, the pelvic colon, rectum, bladder, and some of the organs of generation.

The rectum is placed at the back of the pelvis, in the curve of the sacrum and coccyx; the bladder is in front, behind the pubic symphysis.

In the female the uterus and vagina occupy the interval between these viscera.

The Lower circumference/inferior aperture/outlet

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The lower circumference of the pelvis is very irregular; the space enclosed by it is named the inferior aperture or outlet (apertura pelvis ^* inferior), and is bounded behind by the point of the coccyx, and laterally by the ischial tuberosities.

These eminences are separated by three notches: one in front, the pubic arch, formed by the convergence of the inferior rami of the ischium and pubis on either side.

The other notches, one on either side, are formed by the sacrum and coccyx behind, the ischium in front, and the ilium above; they are called the sciatic notches; in the natural state they are converted into foramina by the sacrotuberous and sacrospinous ligaments.

When the ligaments are in situ, the inferior aperture of the pelvis is lozenge-shaped, bounded, in front, by the pubic arcuate ligament and the inferior rami of the pubes and ischia; laterally, by the ischial tuberosities; and behind, by the sacrotuberous ligaments and the tip of the coccyx.

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