Lab Manual - Posterior Mediastinum

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Define the boundaries of the posterior mediastinum. (explanation)
  2. Describe the major contents of the posterior mediastinum and their relationships: (explanation)
  3. Describe the organization of the thoracic sympathetic trunk, in addition to its visceral and splanchnic branches. (explanation)

Readings and Modules:

Procedure:

Define the boundaries of the posterior mediastinum. For continuity and completeness, your area of dissection will range beyond the limits of the posterior mediastinum and will include superior mediastinum, thoracic wall, and a portion of the neck. Some structures will not be entirely visible because of the position of the diaphragm and the abdominal viscera.

Familiarize yourself with the arrangement of the vertebrae and ribs as components of the thoracic wall.

1. Identify the vagus nerves as they pass posterior to the roots of the lungs; examine the esophagus throughout its length. (Play movie; View images: N 63, 75, 80, 194, 195, 206, 209, 212, 232, 234, 235, 236, 237,238, 240, TG 4-29, 4-36, 4-37, 4-38, 4-40, 4-45, 4-46, 5-33, 7-10, 7-92)

Vagus nerves. Pick up each vagus nerve as it enters the thorax and trace its course. Note relations of both to bronchi and pulmonary arteries and their continuation onto the esophagus as the esophageal plexus. Note branches of both to the anterior and posterior sides of the tracheal bifurcation (cardiac and pulmonary plexus). Trace branches along bronchi to lung. Are there sympathetic branches to the lung? Along what do they distribute? On the left side trace the vagus and note its relation to the aorta. Review the course of the left recurrent laryngeal nerve around the arch, noting its relationship to the ligamentum arteriosum.

Esophagus. Examine the esophagus throughout its length in the superior and posterior mediastina. Where does it begin? Where does it pass into the abdomen? Where does it terminate? Consider the course, relations, constrictions (why and where? ), and the formation of the esophageal plexus. What is the blood supply of esophagus? Venous drainage? Venous collaterals to stomach?

2. Identify the branches of the descending thoracic aorta. (Play movie; View images: N 169, 171, 172, 191, 191, 192, 194, 195, 207, 212, 232, 237, 258, 260, TG 1-21, 4-09, 4-10, 5-33, 4-37, 4-38, 4-39, 4-39)

Aorta. Carefully strip away the remaining mediastinal and costal pleura. As you do so, note structures that contact the pleura behind the pericardium. Do the right and left mediastinal pleurae come together? Review branches of the ascending aorta and arch. Observe the descending thoracic aorta throughout its course. Note relations of aorta to pulmonary trunk, tracheal bifurcation, left pulmonary artery, esophagus, and vertebral bodies. How does the aorta enter the abdominal cavity? At what level? Now locate visceral branches (bronchial, esophageal arteries) and parietal branches to chest wall (posterior intercostal arteries). Using blunt dissection, expose the whole sequence of posterior intercostal arteries and determine the manner of distribution on both the right and left sides. What is the subcostal artery (Note: this structure cannot be seen yet)? Completely review the blood supply to an intercostal space. Collateral supply? What is the posterior branch of the posterior intercostal (segmental) artery? What does it supply? In the intercostal space note relationships of the posterior intercostal artery, vein, and intercostal nerve to one another, to the internal intercostal membrane and to the innermost intercostal muscle. Look for subcostal muscles.

3. Identify the azygos system of veins. (Play movie; View images: N 238, TG 4-10, 5-28, 4-40)

Azygos system. Clean the posterior intercostal veins (segmental veins) of both sides and trace to the azygos vein and right superior intercostal vein on the right side, and to the hemiazygos, accessory hemiazygos and left superior intercostal veins on the left side. Do they communicate with one another? How? Where? What is the pattern of venous drainage in your specimen? Are all the veins present? If not, where does the drainage go? Locate the major venous trunks. What vein drains the first intercostal space? To what? What other veins drain to the azygos system? (bronchial, esophageal).

4. Identify the thoracic duct. (Play movie; View images: N 208, 238, 239, 266, TG 5-37 4-42, 4-44,)

Thoracic duct. Identify the thoracic duct between the aorta and azygos vein. Trace it through the thorax, noting course and relations. How does it get into the thorax? Where does it deviate to the left side? Do you find posterior mediastinal lymph nodes? Bring together the lymph drainage of the thorax, considering parietal and visceral drainages. What are bronchomediastinal lymph trunks?

5. Clean and identify the sympathetic trunks and their branches. (Play movie; View images: N 130, 166, 209, 240, 254, TG 4-45, 4-46, 7-15, 8-01)

Sympathetic trunks. At this time read about and organize the autonomic nervous system. Observe the sympathetic trunks and their ganglia along the heads of the ribs and consider their continuity with the cervical sympathetic trunks. Is the sympathetic trunk located within the posterior mediastinum? Does it change positions in different regions of the chest? Define the cervicothoracic (stellate) ganglion. How many thoracic ganglia do you find? Identify white and gray rami communicantes. What is their significance? Distribution? What do they contain? Do you see thoracic visceral nerves to the aorta, esophagus, and trachea, and to cardiac and pulmonary plexuses? Expose the branches that form the greater (thoracic) splanchnic nerve. Where do they come from? Level? What types of fibers does it contain and to what does it distribute? The lesser and least splanchnic nerves are hidden by the diaphragm and will be seen later.