The ventral rami of the L1, L2, L3 spinal nerves and a large part of L4, which constitute the lumbar plexus, are located at the exit of their respective foramina in the lumbar paravertebral region. Often the involvement of the anterior branch of spinal nerve T12 in the formation of this plexus is observed (pre-fixed plexus)

In a cross-section, the lumbar plexus is located in the angle between bodies and transverse processes of the lumbar vertebrae, between the superficial and deep origin of the psoas muscle. In this segment, the lumbar plexus is related to the lumbar arteries and the ascending lumbar vein and is connected to the lumbar sympathetic trunk by rami communicantes

The spinal nerves that form the lumbar plexus follow variable combination patterns, but the arising branches are kept constant. These comprise the iliohypogastric nerve, ilioinguinal nerve, lateral femoral cutaneous nerve, femoral nerve (not shown), and obturator nerve

The iliohypogastric, ilioinguinal, lateral femoral cutaneous, and femoral nerves emerging caudally from the lateral side of the psoas major muscle. The iliohypogastric nerve runs behind the kidney and ventral to the quadratus lumborum. It pierces the posterior part of the transversus abdominis above the iliac crest and divides into lateral and anterior cutaneous branches. The ilioinguinal nerve follows a similar course extending towards the iliac crest, where it pierces the transversus and internal oblique muscles. The lateral femoral cutaneous nerve runs obliquely ventral to the iliacus muscle towards the anterior superior iliac spine. The femoral nerve (L4, together with L2 and L3) is the largest branch of the lumbar plexus and is the lowest emerging branch, appearing along the lateral aspect of the psoas major. It descends between the psoas and iliacus muscles. The lumbar plexus also gives direct branches to the quadratus lumborum, psoas major, and psoas minor muscles

The genitofemoral nerve descends within the psoas major muscle and emerges from its anterior surface. It descends ventrally to the psoas, dividing into genital and femoral branches. Occasionally (as shown in this picture), this division occurs within the psoas muscle, so the branches appear separately at the anterior surface of the muscle

The obturator nerve emerges from the medial aspect of the psoas at the inlet of the pelvis. It runs downwards and forwards along the lateral wall of the lesser pelvis. It is placed caudal to the external iliac artery in the obturator internus muscle fascia. The nerve enters the thigh through the obturator foramen

Although it is difficult to demonstrate the formation of anterior and posterior divisions in the structure of the lumbar plexus, some authors state that the ventral rami of the L2, L3, and L4 spinal nerves give an anterior and posterior division for each one. This division represents an essential functional organization of the plexus in its flexor and extensor components, as occurs in the trunks of the brachial plexus. The anterior divisions merge to form the obturator nerve, whereas the posterior divisions give rise to the formation of the femoral nerve