It’s the ventral ramus of the 12th thoracic spinal nerve. It enters the abdomen behind the lateral precise ligament (lateral lumbocostal arch) and runs downward and laterally in front of the quadratus lumborum below the anterior layer of thoraco-lumbar fascia. At the lateral border of quadratus lumborum, it pierces the aponeurotic origin of transversus abdominis and enters the plane between the transversus abdominis and internal oblique muscles of the anterior abdominal wall.