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The lumbar plexus (Fig. 21.1)
• Sympathetic supply: the lumbar sympathetic chain is a continuation
• The lumbar plexus is formed from the anterior primary rami of L1–4.
of the thoracic sympathetic chain as it passes under the medial arcuate
The trunks of the plexus lie within the substance of psoas major and,
with the exceptions of the obturator and genitofemoral nerves, emerge Lumbar sympathetic chain (Fig. 21.2)
ligament of the diaphragm. The chain passes anterior to the lumbar ver-
at its lateral border. tebral bodies and usually carries four ganglia which send grey rami
• The 12th intercostal nerve is also termed the subcostal nerve as it has communicans to the lumbar spinal nerves. The upper two ganglia
no intercostal space but, instead, runs below the rib in the neurovascu- receive white rami from L1 and L2.
lar plane to supply the abdominal wall. The lumbar sympathetic chain, the splanchnic nerves and the vagus
• The iliohypogastric nerve is the main trunk of the 1st lumbar nerve. It contribute sympathetic and parasympathetic branches to plexuses
supplies the skin of the upper buttock, by way of a lateral cutaneous (coeliac, superior mesenteric, renal and inferior mesenteric) around the
branch, and terminates by piercing the external oblique above the abdominal aorta. In addition, other branches continue inferiorly to form
superficial inguinal ring where it supplies the overlying skin of the the superior hypogastric plexus (presacral nerves) from where they
mons pubis. The ilioinguinal nerve is the collateral branch of the iliohy- branch into right and left inferior hypogastric plexuses. The latter also
pogastric. The ilioinguinal runs in the neurovascular plane of the receive a parasympathetic supply from the pelvic splanchnic nerves. The
abdominal wall to emerge through the superficial inguinal ring to pro- branches from the inferior hypogastric plexuses are distributed to the
vide a cutaneous supply to the skin of the medial thigh, the root of the pelvic viscera along the course and branches of the internal iliac artery.
penis and anterior one third of the scrotum (or labium majus in the The coeliac ganglia are prominent and lie around the origins of the
female). coeliac and superior mesenteric arteries.
• The genitofemoral nerve (L1,2) emerges from the anterior surface of • Parasympathetic supply: to the pelvic viscera arises from the anter-
psoas major. It courses inferiorly and divides into: a genital component ior primary rami of S2,3,4athe pelvic splanchnic nerves. The latter
that enters the spermatic cord and supplies the cremaster (in the male), parasympathetic supply reaches proximally as far as the junction
and a femoral component that supplies the skin of the thigh overlying between the hindgut and midgut on the transverse colon.
the femoral triangle.
• The lateral cutaneous nerve of the thigh (L2,3), having emerged Lumbar sympathectomy
from the lateral border of psoas major, encircles the iliac fossa to pass This procedure is performed in cases of severe peripheral vascular dis-
under the inguinal ligament (p. 99). ease of the lower limbs where vascular reconstructive surgery is not
• The femoral nerve (L2–4, posterior division): see p. 99. possible and skin necrosis is imminent. The operation involves excision
• The obturator nerve (L2–4, anterior division): see p. 99. of the 2nd to 4th lumbar ganglia with the intermediate chain.
• A large part of L4 joins with L5 to contribute to the sacral plexus as
the lumbosacral trunk.

The nerves of the abdomen 51