Iliacus

Where is the iliacus?

It arises from the upper two-thirds of the iliac fossa and the side of the sacral wing. Its fibers pass under the inguinal ligament, in front of the hip joint, with those of psoas major. (StatPearls, Iliopsoas Muscle)

Inside the pelvis it lies behind the cecum on the right and the sigmoid colon on the left. (StatPearls, Psoas Major)

Where does the iliacus insert?

Its bundles merge into the tendon of psoas major and insert with it on the lesser trochanter of the femur. A few lateral and lower bundles attach to the femur directly. (StatPearls, Iliopsoas Muscle and Psoas Major)

The iliopectineal bursa lies under the shared tendon and separates it from the lesser trochanter and the upper femur. (StatPearls, Iliopsoas Muscle)

What does the iliacus do?

As part of the iliopsoas it flexes the thigh and adds to adduction and lateral rotation at the hip. With the lower limb fixed, the iliopsoas flexes the trunk and bends it to the same side. (StatPearls, Iliopsoas Muscle)

It can also act alone, steadying the pelvis and helping to flex the hip during running. (StatPearls, Iliopsoas Muscle)

What lies next to the iliacus?

The femoral nerve supplies it. On its way to the thigh the nerve runs between the iliacus and psoas major. (StatPearls, Iliopsoas Muscle and Thigh Femoral Nerve)

In the thigh, the iliopsoas forms part of the floor of the femoral triangle and rests on the fibrous capsule of the hip joint. Sartorius and rectus femoris run along its outer edge. (StatPearls, Adductor Canal and Psoas Major)

The external iliac artery supplies it through several muscular branches. (StatPearls, Iliopsoas Muscle)

Where the sources differ

Sources differ on the spinal roots behind its supply. The StatPearls anterior thigh article gives the femoral nerve to the iliacus as L1 to L4, and its thigh muscles article gives the muscular branch as L1 to L3. Its iliopsoas article gives the femoral nerve to the iliacus as L2 to L4, and its femoral nerve article has the nerve forming from L2 to L4. (StatPearls, Anterior Thigh Muscles, Thigh Muscles, Iliopsoas Muscle and Thigh Femoral Nerve)

Sources differ on whether the iliopsoas adducts or abducts the femur. The StatPearls iliopsoas article says the unit adds to adduction. Its psoas major article says in one passage that contraction abducts the femur, and in its function section that the psoas complex adducts it. (StatPearls, Iliopsoas Muscle and Psoas Major)