Lying medial to quadratus lumborum is psoas major. Psoas major arises from the transverse processes, vertebral bodies, and intervertebral disks, from T12 to L5. It runs down, across the ala of the sacrum, across the sacro-iliac joint, and along the pelvic brim. Before seeing where psoas major goes, we’ll add its close neighbor, iliacus to the picture.
Here’s the iliacus muscle. It fills the iliac fossa. Iliacus arises from this wide area on the wing of the ilium. Down here, the medial fibers of iliacus and the lateral fibers of psoas major join, forming a single muscle belly known as the ilio-psoas. The ilio-psoas runs beneath the inguinal ligament, and passes backwards, to insert here on the lesser trochanter of the femur.
As they slope downward and forward toward the inguinal ligament, the iliacus and psoas muscles are covered by this layer of dense connective tissue, the ilio-psoas fascia, which we’ll meet when we look at the inguinal ligament. This is just the lower part of it. The ilio-psoas fascia is covered in turn by the membrane which lines the abdominal cavity, the peritoneum. This is a preview of the peritoneum: we’ll see it more fully later in this section.
When we’re looking at the abdomen we tend to see psoas and iliacus as static background structures, but they have an important function: they’re the principal muscles that produce flexion at the hip joint.
Now that we've looked at the individual muscles, let's take an overall look at the structures that form the posterior abodominal wall. To do that we’ll add the diaphragm to the picture. The steeply rising diaphragm forms the highest part of the abdominal wall, not only at the back, but all around.
The diaphragm makes a dome-shaped partition, separating the abdominal cavity below, from the thoracic cavity above. At the bottom, the middle part of the posterior abdominal wall, formed by the vertebral bodies, becomes continuous with the wall of the pelvic cavity here at the sacral promontory.
On each side the psoas muscle stands away from the vertebral bodies like a pillar. The iliacus muscles, with their investing fascia, form a continuation of the posterior abdominal wall, that slopes downward, forward and inward, ending here at the inguinal ligament.
This is the lowest part of the anterior abdominal wall. In this dissection all the abdominal blood vessels and nerves have been removed, together with most of the peritoneum. We’ll add those structures to the picture, later in this section.