6. Examine the preaortic autonomic nerve plexuses of the abdomen and the branches of the abdominal aorta.
Turn your attention to the autonomic nerves in the abdomen. They form a large plexus, or network of nerve fibers on the anterior surface of the aorta called the pre-aortic plexus. Carefully remove the tissue covering the plexus. It extends from the aortic hiatus to the pelvis. The upper part of the plexus is associated with the celiac artery and is thus called the celiac plexus. Recall that the celiac ganglia are also associated with this plexus. Closely intermixed with the celiac plexus is the plexus associated with the superior mesenteric artery. Nerve fibers extend down the aorta from the superior mesenteric artery to the inferior mesenteric artery as the inter-mesenteric plexus. The inferior mesenteric artery has its own plexus: the inferior mesenteric plexus. You may be able to find a small inferior mesenteric ganglion associated with the artery. Sweeping down below the inferior mesenteric artery across the bifurcation of the aorta is the superior hypogastric plexus. This is the main sympathetic supply to the pelvis. Next observe the sympathetic trunk. It can be found along the vertebral column either behind the inferior vena cava on the right, as shown here or behind the aorta on the left. It lies anterior to the attachment of the psoas muscle to the vertebral column. Here is the sympathetic trunk with its chain of ganglia. The trunk has fibers lying both medially and laterally. The lateral fibers are rami communicantes that run to the spinal nerves. On the medial side of the trunk, the lumbar splanchnic nerves arise. Here the lumbar splanchnic nerves are passing medially to the pre-aortic plexus. Next, look at the abdominal aorta and its branches. The first branches off the aorta as it passes under the median arcuate ligament are the inferior phrenic arteries. Next, comes the celiac artery branching from the aorta at about T12 to L1 level. The superior mesenteric artery comes next, at the bottom of the L1 vertebra. The renal arteries arise at the top of the L2 level and pass laterally into the kidneys. The gonadal arteries branch off immediately below the renals at the lower L2 level. The inferior mesenteric artery arises at the L3 level. At L4 the aorta bifurcates into the common iliac arteries. At each lumbar level, except L5, a lumbar segmental artery arises from the posterior side of the aorta. These supply the abdominal wall. Lastly, the aorta gives off a branch posteriorly that runs down along the pelvic brand in the midline called the median sacral artery. Make a vertical incision in the anterior wall of the aorta. Take care to avoid as many nerves as possible. Reflect the sides of the incision laterally and observe the interior of the aorta. Try to locate the lumbar segmental vessels as they branch from the posterior wall. Many aortas will be diseased with atherosclerotic plaques.