In this section we’ll look at the pelvic cavity. We’ll look first at the bones and ligaments that surround the cavity; then we’ll look at the muscles of the pelvic walls and the pelvic floor; then we’ll see the principal blood vessels and nerves of the region.
We’ll start with the bones. We’ve already seen the upper parts of the bony pelvis. Now we need to look at the parts of it that lie below the pelvic brim. Let’s get oriented. Here’s the bony pelvis, together with the fifth lumbar vertebra. Here’s the pelvic brim.
We’ll be looking at the pelvic cavity from four different viewpoints. We’ll look down into it from above; we’ll look at it from the side, with the opposite half of the pelvis removed; we’ll look at it from behind; and we’ll look at it from below.
We looked at the features of the upper part of the bony pelvis in the last section. The bones that contribute to the walls of the pelvic cavity are the sacrum and the coccyx behind, and the lower parts of the hip bone in front and at the side.
We’re looking at the bones in the position they occcupy, when we’re standing upright. In the upright position the surface of the upper part of the sacrum is angled at 30° to the horizontal. The tip of the coccyx points forward at about 40°.
So the pelvic surfaces of the sacrum and coccyx form a curve of a bit more than a quarter circle. The lower end of the sacrum is on a level with the top of the pubic symphysis.
This big gap between the sacrum and the hip bone is called the sciatic notch. It’s bridged by two major ligaments, as we’ll see shortly.
Now we’ll look at some details of the hip bone. This massively thick part of the hip bone is formed by the fusion of the ilium, the pubis, and the ischium.
It’s smooth on the inside, and on the back. It’s deeply indented on the outside, by the socket of the hip joint, the acetabulum. This is the body of the ischium, which ends below in this impressive projection, the ischial tuberosity, which is what we sit on
This sharp prominence is the ischial spine. The large hole in the lower part of the hip bone is the obturator foramen. In the living body it’s largely closed off by the obturator membrane.
This is the body of the pubis. The part of the hip bone below the obturator foramen is the ischio-pubic ramus. The two ischio-pubic rami, meeting in front at the pubic symphysis, form the pubic arch. When seen from the side, the ischio-pubic rami slope backwards and downwards, towards the ischial tuberosities.
There are important differences in shape between the male pelvis and the female pelvis, which is adapted to the important requirements of childbirth. The female pelvic cavity is wider from side to side, and deeper from front to back, than in the male.
In addition, the angle of the pubic arch is broader. When seen from below, the inferior pelvic aperture of the female is wider in all directions, than that of the male.