SUPERIOR AND POSTERIOR MEDIASTINUM; ANTEROLATERAL ABDOMINAL WALL AND INGUINAL CANAL (Grant's Dissector (16th Ed.) pp. 93-98; 99-112) TODAY’S GOALS (Superior and Posterior Mediastinum): 1. Access the posterior mediastinum. 2. Identify the major structures of the superior and posterior mediastinum. 3. Dissect and identify the components of the sympathetic trunk.
DISSECTION NOTES: Remove the posterior wall of the pericardial sac (may already be gone from removing the heart) and examine the posterior relations of the heart (Dissector p. 96, Fig. 3.26).
In the posterior mediastinum observe the following: Esophagus: collapsed muscular tube posterior to the trachea.
Right and left vagal trunks, and the esophageal plexus (parasympathetics are from CN X and sympathetics from the thoracic sympathetic trunk).
Left recurrent laryngeal nerve (Dissector Fig. 3.24) as it passes around the ligamentum arteriosum (formerly the embryonic ductus arteriosus), which connects the left pulmonary artery to the aortic arch.
The left vagus nerve contributes parasympathetic fibers to the esophageal plexus and then regroups as the anterior vagal trunk. The right vagus becomes the posterior vagal trunk. The positions of the vagal trunks are due to the rotation of the gut during development.
Q. What structure does the right recurrent laryngeal nerve loop around and pass posterior to on its course to the larynx?
Azygos system of veins (Dissector p. 97, Fig. 3.27): The azygos vein courses on the right side of vertebral column, along the posterior body wall. It is formed from the union of the ascending lumbar veins and right subcostal vein. It may also arise as a branch of the IVC. It receives the right posterior intercostal veins and the right superior intercostal vein (to the arch of the azygos). The azygos vein drains into the superior vena cava.
The accessory hemiazygos and hemiazygos vein course in intercostal spaces on the left side.
Descending aorta: Is the thoracic aorta in the thorax and the abdominal aorta in the abdomen. It gives off several branches: • posterior intercostal arteries (usually 9 pairs that are larger than the anterior intercostal arteries) • esophageal branches (minor/branches may be difficult to see) • pericardial and mediastinal branches • bronchial arteries (left usually 2; right usually 1)
8 Thoracic duct (Dissector Fig. 3.27): The largest lymphatic duct in the body; ascends on the anterior surfaces of thoracic vertebral bodies posterior to the esophagus and between the azygos vein and the descending aorta. Its termination in the venous angle of the neck will be studied in the next unit.
Q. What areas of the body does the thoracic duct normally drain?
Sympathetic trunk (Dissector Fig. 3.27): Remove the parietal pleura from the posterior thoracic cage and dissect out the thoracic sympathetic chain/trunk. Note that three pairs of splanchnic nerves arise from each sympathetic trunk. They are listed below. Realistically, only portions of the greater splanchnic nerve will be visible in our dissections. Greater splanchnic nerve (T5-T9) Lesser splanchnic nerve (T10 and T11) Least splanchnic nerve (T12)
At the intersection of the sympathetic trunk with an intercostal nerve, attempt to find rami communicantes (white and gray ramus communicans) connecting the intercostal nerve with a thoracic sympathetic ganglion.
Q. Which rami communicans carry preganglionic fibers? Q. Which rami may be found over the entire length of the sympathetic trunk? Q. Where do the greater, lesser, and least splanchnic nerves synapse?
PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. Demonstration of the structures and relationships of the posterior mediastinum including the esophagus, descending aorta, thoracic duct, and azygos system of veins. 2. Demonstration of the sympathetic chain, rami communicantes, and splanchnic nerves. 3. Demonstration of the recurrent laryngeal nerve and discussion of its clinical significance. 4. Demonstration of the posterior intercostal VAN (vein, artery, nerve) relationship.
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