Now we’ll look at the muscles of the pelvic cavity. First we’ll look at two muscles which form part of the wall of the pelvic cavity, piriformis, and obturator internus. Then we’ll look at the complex sheet of muscles, collectively called the pelvic diaphragm, which form the floor of the pelvic cavity.
We'll look at these structures first in a male specimen. Piriformis and obturator internus are both hip rotator muscles, which arise within the pelvis, and pass outward through the sciatic foramina.
Here’s piriformis. Piriformis arises from here on the sacrum. It passes laterally, and leaves the pelvis through the greater sciatic foramen. We’ll see where it goes in a minute.
Next we’ll add obturator internus to the picture. Obturator internus arises from the obturator membrane, and from this wide area around it. Obturator internus leaves the pelvis through the lesser sciatic foramen. In doing so, it makes a 90° turn around the lower part of the ischium.
Piriformis and obturator internus pass laterally, to insert on the greater trochanter of the femur. Their actions as lateral hip rotators, are shown in Volume 2 of this atlas. In this section, we’re concerned to understand these two muscles simply as parts of the wall of the pelvic cavity.
The obturator internus muscle is covered on the inside by this layer of pelvic fascia. There’s an important line of thickening in the fascia, called the tendinous arch. The tendinous arch goes from the body of the pubis, to the ischial spine. We’ll see why the tendinous arch is important in a moment.