Download Unit 29: Posterior Abdominal Wall

Unit 29: Posterior Abdominal Wall
Dissection Instructions:
Any peritoneum or fat which remains on the diaphragm which obscures the direction of muscle
fibers should be removed (Plates 189, 255; 2.74). Clean the crura, noting that the right crus attaches as
low as the third lumbar vertebra and the left crus as low as the second lumbar vertebra. The tendinous
fibers of origin blend with the anterior longitudinal ligament. Clean the fleshy part of the right crus and
verify that its fibers split to form the esophageal hiatus. The left crus occasionally has fibers related to
this hiatus, but its fibers usually pass to the left of it. Passing through the esophageal hiatus with the
esophagus are the vagus nerves and esophageal branches of the left gastric vessels. The left and right
crura are connected anterior to the aorta by the median arcuate ligament, forming the aortic hiatus,
which transmits the aorta, thoracic duct (Plates 246, 249, 300, 325; 1.78, 1.80) and azygos veins (Plates
331; 1.81). The crura are pierced by the greater and lesser splanchnic nerves (Plates 189, 229, 236,
257, 330, 334; 2.72-2.74, 2.76). Laterally fibers from the crura arch over the psoas muscles and attach to
the transverse process of the first lumbar vertebra, forming the medial lumbocostal arch(Plates 255;
2.72, 2.74). The sympathetic trunk appears in the abdominal region by passing under the medial
lumbocostal arch (Plates 259, 330; 2.72, 2.73). The lateral lumbocostal arch extends from the end of the
medial lumbocostal arch to the tip of the twelfth rib (Plates 255; 2.72, 2.74). It arches over the
quadratus lumborum muscle and the subcostal vessels and nerve (Plates 259; 2.72, 2.73). The
diaphragm is frequently deficient of fleshy fibers above the lateral lumbocostal arch, so that peritoneum
and parietal pleura are separated by only areolar connective tissue.
The fleshy fibers arising from the median arcuate ligament and the lumbocostal arches ascend
vertically, then arch toward the central tendon of the diaphragm (Plates 246; 2.100). Laterally from
the tip of the twelfth rib, the diaphragm takes origin from the costal margin, interdigitating with the
transversus abdominis muscle. The intercostal nerves and branches of the musculophrenic vessels pass
deep to the costal cartilages of the ribs and enter the abdominal wall. Anteriorly, a small gap appears
between the costal origin of the diaphragm and its sternal origin from the back of the xiphoid process.
The superior epigastric vessels pass through the gap and into the rectus sheath to reach the rectus
abdominis muscle (Plates 176, 179, 200; 2.6).
The central tendon of the diaphragm is shaped something like a clover leaf (Plate 255; 2.74). It
has a large opening for the inferior vena cava, which also transmits branches of the right phrenic nerve
and accompanying vessels. On the left side of the pericardial attachment, the left phrenic nerve and
vessels pierce the central tendon. The phrenic nerves spread out on the abdominal side of the diaphragm.
Besides the phrenic nerves, the diaphragm is innervated by branches from the intercostal nerves.
Clean the psoas muscles (Plates 189, 255, 478; 2.71B , 2.72, Table 2.5-p. 164). Do not destroy
the genitofemoral nerve which lies on the anterior surface of these muscles. The psoas minor muscle is
not always present, but when it is, it lies anterior to the psoas major. The psoas minor takes origin from
TV12 and LV1 and intervening disk to become tendinous before it reaches the pelvic brim where it
inserts. It also strengthens the psoas fascia before the psoas major exits the abdominal region to enter the
thigh. It is a weak flexor of the trunk. The psoas major muscles take origin from the intervertebral disks
and adjacent parts of the bodies of the vertebrae. However, the muscle does not attach to the intermediate
part of the vertebral bodies, where there is a fibrous arch to attach to, allowing the lumbar vessels and
communicating rami of the sympathetic trunks to pass between muscle and bone. The psoas muscles also
attach to the transverse processes of the lumbar vertebra. The psoas major is the primary flexor of the hip
joint. Laterally, most of the branches of the lumbar plexus exit the psoas muscles. The obturator nerve
exits the medial surface of the muscle to enter the pelvic cavity.
Unit 29 - 1
Clean the iliacus muscle without destroying the nerves and vessels on its surface (Plates 189,
255, 478; 2.71B, 2.72, Table 2.5-p. 164). This muscle fills the iliac fossa and inserts with the psoas major
on the lesser trochanter of the femur. The iliopsoas muscle is the name of the two muscles considered as
one.
The quadratus lumborum muscle takes origin from the crest of the ilium and iliolumbar
ligament and ascends, inserting on the transverse processes of the lumbar vertebrae and the inferior border
of the twelfth rib (Plates 189, 255, 478; 2.71B , 2.72, Table 2.5-p. 164). The quadratus lumborum and
psoas muscles are innervated by branches from the lumbar nerves. The lumbar arteries pass posterior to
the quadratus lumborum, whereas the lumbar nerves pass anterior to this muscle. The action of this
muscle is to laterally flex the trunk and stabilize the twelfth rib.
Clean the common and external iliac vessels and the origin of the internal iliac vessels, but do
not follow the latter vessel into the pelvic cavity. Note the relationship of the arteries and veins (Plates
256, 257; 2.75). The left common iliac vein crosses under the right common iliac artery. The right
common iliac vein also crosses deep to the right common iliac or external iliac artery, as the right
external iliac vein is medial to the artery as they leave the abdominal cavity.
Locate and clean the branches of the lumbar plexus (Plates 259, 478, 479; 2.72, 2.73, 2.82B,
Table 2.6-p. 165). Like all of the somatic nerve plexuses, it is formed from anterior primary rami. The
genitofemoral nerve has already been seen on the anterior surface of the psoas muscles. This nerve
divides into the genital branch, which enters the deep inguinal ring to innervate the cremaster muscle,
and the femoral branch, which passes under the inguinal ligament near the external iliac artery to supply
skin over the femoral triangle. This nerve receives fibers from L1 and L2. L1 also forms the
iliohypogastric and ilioinguinal nerves, which leave the lateral surface of the psoas major and cross the
abdominal muscles to reach the iliac crest. The iliohypogastric nerve is superior to the ilioinguinal nerve,
but they may exchange fibers during their course. They pierce the transversus abdominis, travel
anteriorly to the region of the anterior superior iliac spine, then pierce the internal oblique to lie deep to
the aponeurosis of the external oblique. The ilioinguinal nerve exits the superficial inguinal ring to
innervate the external genitalia and medial aspect of the upper thigh. The iliohypogastric nerve pierces
the aponeurosis of the external oblique superior to the superficial inguinal ring to supply skin over the
lower abdomen.
The lateral femoral cutaneous nerve leaves the lateral surface of the psoas muscles, crosses the
iliacus muscle in the iliac fossa and leaves the abdominal cavity by passing under the inguinal ligament as
far laterally as it can. It is purely sensory to the lateral aspect of the thigh down to the knee. This nerve
arises from the posterior division of L2 and 3. The largest nerve of the lumbar plexus is the femoral
nerve, which can be found just lateral to the psoas major muscle before they pass under the inguinal
ligament. The femoral nerve supplies all the muscles in the anterior osteofascial compartment of the
thigh, and the pectineus muscle and is cutaneous to the skin of the anterior thigh and medial aspect of the
leg down to the foot. It is formed from the posterior divisions of L2, 3 and 4.
Dissect between the psoas muscles and sacral promontory and locate the obturator nerve (Plates
259, 478; 2.72). It is formed from the anterior divisions of L2,3 and 4 and passes through the obturator
canal to reach the medial osteofascial compartment. It innervates all of the adductor muscles except the
pectineus. Deeper in the space where the obturator nerve is found will be the lumbosacral trunk. It is
formed from fibers from L4 and all of the anterior primary ramus of L5. It is the superior portion of the
sacral plexus.
Unit 29 - 2
On one side only, remove the psoas muscle piece by piece, preserving the lumbar plexus (Plates
259, 478, 479; 2.72, 2.73, 2.82B). Carefully clean each nerve back to their origin and verify the
anterior/ventral primary ramus which contributes to each peripheral. Locate and clean the sympathetic
trunk on the side from which the psoas muscle was removed, preserving lumbar splanchnic nerves and
communicating rami. Locate both gray and white communicating rami at the level of the first and
second lumbar nerves, but only gray rami below that level. The sympathetic trunk continues into the
pelvis, but should not be followed there at this time.
Be sure to identify all of the following in this unit:
diaphragm
central tendon
right & left crura
median arcuate ligament
aortic hiatus
aorta
thoracic duct
azygos veins
esophageal hiatus
esophagus
vagus nerve
esophageal branches of left gastric a.
greater splanchnic nerve
lesser splanchnic nerve
psoas muscle
medial lumbocostal arch
sympathetic trunk
lateral lumbocostal arch
quadratus lumborum muscle
subcostal vessels & nerve
intercostal nerves
brs of musculophrenic vessels
superior epigastric vessels
phrenic nerves
psoas major muscle
obturator nerve
iliacus muscle
common iliac vessels
external iliac vessels
genitofemoral nerve
iliohypogastric nerve
ilioinguinal nerve
lateral femoral cutaneous nerve
femoral nerve
lumbosacral trunk
sympathetic trunk
lumbar splanchnic nerves
gray & white communicating rami
Unit 29 - 3