Figure 14-3
The lumbrosacral plexus as it appears in the posterior abdominal wall with the psoas removed.

We have already seen many of the nerves of the lumbosacral plexus such as the sciatic, superior and inferior gluteal. All of these nerves we found in the gluteal region. If the abdomen has been dissected, identify some of these same nerves as well as others within the lower abdominal and pelvic cavities. Your instructor may not wish to go into the detail of the nerves in the lumbar plexus, however these nerves found on the posterior abdominal wall often give anatomists and anatomy students alike a great deal of trouble. It may benefit our anatomical knowledge to investigate these nerves. (Figure 14-3)

In the posterior abdominal wall area, we can see the first three nerves that run something like ribs. The most superior of these nerves is the subcostal nerve (T12).

The next most inferior nerve is the ilioinguinal nerve (L1). This nerve is easily identified as it emerges through the superficial inguinal ring.

Trace this nerve from the superficial inguinal ring to its origin near the vertebral column. This nerve serves the medial thigh and genitalia.

The next nerve is the lateral cutaneous of the thigh (L2-3) and is named for its function: sensory to the skin.

Running through from the psoas muscle, the genitofemoral (L1-2) runs through the inguinal canal to the genitals.

The femoral nerve (L2, 3, 4) can be seen deep and in between the psoas and iliacus. This nerve runs from under the inguinal ligament out to the anterior thigh region.

The obturator nerve (L2, 3, 4) can be found by identifying the obturator foramen. The obturator nerve exits the pelvis through this foramen on its way to the groin or adductor region of the thigh. You can then follow this nerve superiorly as it runs on the medial side of the psoas muscle.

The lumbosacral trunk (part of L4 and all of L5) can be seen on the medial side of the superior gluteal artery. Usually the superior gluteal artery runs in between the lumbosacral trunk and S1.

Can you find S2 and S3 as they emerge from the musculature of the piriformis? The inferior gluteal artery usually separates S1 from S2.

As these spinal nerve ventral rami (L4-S3) merge inferiorly and disappear from sight, they are forming the substance of the sciatic nerve.