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Now that we’ve looked at the dry bones, we’ll look at some major ligaments which are important in holding the sacrum and the hip bones together.

The weight of the body is transmitted from the vertebral column to the hip bone, at the sacro-iliac joint. The sacro-iliac joint is strengthened behind and in front by ligaments, the anterior sacro-iliac ligament in front and the massive posterior sacro-iliac ligament behind.

In addition, the sacro-iliac joint is strengthened by two major ligaments which pass from the sacrum to the ischium, the sacro-tuberous, and sacro-spinous ligaments.

Here’s the sacro-tuberous ligament. The sacro-tuberous ligament arises here on the back of the sacrum. It passes laterally, downward, and slightly forward. It’s inserted here on the ischial tuberosity.

Now we’ll add the sacro-spinous ligament to the picture. Here it is. The sacro-spinous ligament lies in front of the sacro-tuberous ligament, and medial to it. It goes from here on the edge of the sacrum, to here on the ischial spine.

These two ligaments divide the gap between the sacrum and the ischium into two openings: the greater sciatic foramen and the lesser sciatic foramen.

Let's take a look at a complete pelvic specimen, from behind, and from below. The sacro-tuberous ligaments behind, and the ischio-pubic rami in front, form the boundaries of an opening beneath the pelvis that’s called the inferior pelvic aperture.

Seen from beneath, the opening looks like an ellipse, but it’s not a flat ellipse. Because of the steep downward curve of the sacrotuberous ligaments behind, and the slight downward slope of the ischio-pubic rami in front, the ellipse has a marked bend in it.

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Here’s the inferior pelvic aperture, seen from above. When we look at it from up here, it’s not so easy to appreciate the three dimensional shape of the opening.