The lumbar plexus is formed by the anterior rami of nerves L1 to L3 and most of the anterior ramus of L4 (Fig. 4.173 and Table 4.6). It also receives a contribution from the T12 (subcostal) nerve.
Branches of the lumbar plexus include the iliohypogastric, ilio-inguinal, and genitofemoral nerves, the lateral cutaneous nerve of the thigh (lateral femoral cutaneous), and femoral and obturator nerves. The lumbar plexus forms in the substance of the psoas major muscle anterior to its attachment to the transverse processes of the lumbar vertebrae (Fig. 4.174). Therefore, relative to the psoas major muscle, the various branches emerge either: anterior—genitofemoral nerve, medial—obturator nerve, or lateral—iliohypogastric, ilio-inguinal, and femoral nerves and the lateral cutaneous nerve of the thigh.
The iliohypogastric and ilio-inguinal nerves arise as a single trunk from the anterior ramus of nerve L1 (Fig. 4.173). Either before or soon after emerging from the lateral border of the psoas major muscle, this single trunk divides into the iliohypogastric and the ilio-inguinal nerves.
The iliohypogastric nerve passes across the anterior surface of the quadratus lumborum muscle, posterior to the kidney. It pierces the transversus abdominis muscle and continues anteriorly around the body between the transversus abdominis and internal oblique muscles. Above the iliac crest, a lateral cutaneous branch pierces the internal and external oblique muscles to supply the posterolateral gluteal skin (Fig. 4.175).
The remaining part of the iliohypogastric nerve (the anterior cutaneous branch) continues in an anterior direction, piercing the internal oblique just medial to the anterior superior iliac spine as it continues in an obliquely downward and medial direction. Becoming cutaneous, just above the superficial inguinal ring, after piercing the aponeurosis of the external oblique, it distributes to the skin in the pubic region (Fig. 4.175). Throughout its course, it also supplies branches to the abdominal musculature.
The ilio-inguinal nerve is smaller than, and inferior to, the iliohypogastric nerve as it crosses the quadratus lumborum muscle. Its course is more oblique than that of the iliohypogastric nerve, and it usually crosses part of the iliacus muscle on its way to the iliac crest. Near the anterior end of the iliac crest, it pierces the transversus abdominis muscle, and then pierces the internal oblique muscle and enters the inguinal canal.
The ilio-inguinal nerve emerges through the superficial inguinal ring, along with the spermatic cord, and provides cutaneous innervation to the upper medial thigh, the root of the penis, and the anterior surface of the scrotum in men, or the mons pubis and labium majus in women (Fig. 4.175). Throughout its course, it also supplies branches to the abdominal musculature.
The genitofemoral nerve arises from the anterior rami of nerves L1 and L2 (Fig. 4.173). It passes downward in the substance of the psoas major muscle until it emerges on the anterior surface of the psoas major. It then descends on the surface of the muscle, in a retroperitoneal position, passing posterior to the ureter. It eventually divides into genital and femoral branches.
The genital branch continues downward and enters the inguinal canal through the deep inguinal ring. It continues through the canal and: in men, innervates the cremasteric muscle and terminates on the skin in the upper anterior part of the scrotum, and in women, accompanies the round ligament of the uterus and terminates on the skin of the mons pubis and labium majus.