Session 5: Superior and Inferior Mediastinum and Lungs

Remove the lungs

1.) Taking care not to damage the phrenic nn. and pericardiacophrenic vasculature, remove each lung by cutting through the primary bronchi and pulmonary vasculature (the structures comprising the bulk of the roots of the lungs) proximate to the hila of the lungs. Be careful not to cut the vagus nn., found just posterior to the roots of the lungs.

Photo 1. Incision of the root of lung

Photo 2. Superior and middle mediastina with lungs and heart removed

Examine the external features of each lung

2.) Examine the right and left lungs.

Find these structures:

• • Apex of lung

  • Base of lung

  • Lobes of lungs (Superior, Middle, Inferior)

  • Cardiac notch of left lung

  • Lingula of left lung

  • Fissures

    • Oblique fissure

    • Horizontal fissure of right lung

  • Hilum of lung

Photo 3. Lateral views of the lungs

Photo 4. Lobes of the lungs

Photo 5. Anterior view of the heart and lungs

3.) Examine structures comprising the roots of each lung. You may need to (bluntly) remove some parietal pleura and loose connective tissue in order to see the structures of the roots of the lungs.

Note: The root of the lung consists of all structures entering or exiting the lung at the hilum. These include pulmonary vasculature, bronchi, bronchial aa., and lymphatics (specifically hilar nodes). The relative position of these structures differs between the right and left lungs.

Photo 6. Medial views of the lungs

4.) On a prosection or plastinated specimen, examine the structure and relationships of the bronchial tree.

• • Bronchi (N202, N203)

    • Main (primary) bronchi (right & left)

    • Lobar (secondary)

      • Superior, middle and inferior (right lung)

        • Bronchus intermedius

      • Superior and inferior (left lung)

        • Lingular bronchus

    • Segmental (tertiary)

      • Superior segmental bronchus of right inferior lobe

Photo 7. Bronchial tree, right bronchi

Photo 8. Bronchial tree, left bronchi

Photo 9. The bronchial tree of the right lung

Remove the pericardium and locate the structures in the superior and middle mediastina.

4.) Using a combination of blunt and sharp dissection, remove the pericardium from the underlying and surrounding structures. Previously, the pulmonary vasculature was cut proximate to the heart and the hila of the lungs, thus short remnants of these vessels may persist with the pericardium, which you may also remove. Be very careful to not injure the pericardiacophrenic neurovasculature and the vagus nn. as you remove the pericardium.

Photo 10. Thorax with pericardium removed

5.) Examine the tracheal bifurcation. You may have to reflect the great vessels superiorly to best visualize the trachea and tracheal bifurcation. Using sharp dissection, make a midsagittal cut through the trachea just superior to the bifurcation.

Find these structures:

• • Tracheal bifurcation

  • Carina of trachea

  • Main (primary) bronchi (right & left)

Photo 12. Tracheal bifurcation

6.) Further remove the parietal pleurae and connective tissues from the great vessels and features of the superior and middle mediastina. As you clean the aortic arch and thoracic aorta, look for the cardiac plexus in the concavity of the aorta. The cardiac plexus is contiguous with the pulmonary plexuses found on the pulmonary vessels and primary bronchi in the roots of the lungs.

Photo 14. Thoracic wall

Photo 15. Removing the parietal pleurae

Photo 16. Cardiac plexus

As you continue along the aorta, you may find left bronchial aa. Typically, there are two left bronchial aa. and one right bronchial a., which may either be a branch of one of the left bronchial aa., or may originate from the right third posterior intercostal a.

Distal to the left bronchial aa. are (typically paired) esophageal aa. servicing the esophagus.

Find on a donor, a prosection, or a plastinated specimen:

• • Thoracic aorta

    • Visceral branches

      • Bronchial aa. (2 left, 1 right)

      • Esophageal aa. (multiple pairs)

    • Parietal branches

      •  Posterior intercostal aa. (intercostal spaces 3-11)

Photo 17. Posterior intercostal arteries

Examine the boundaries of the posterior mediastinum

7.) Explore the posterior mediastinum and adjacent regions.

Note: The posterior mediastinum is the space anterior to the T5-T12 vertebrae, posterior to the pericardium, superoposterior to the diaphragm, inferior to the transverse thoracic plane, and between the parietal pleurae.

The major features transmitted through the posterior mediastinum are the:

• • thoracic aorta

  • thoracic duct

  • azygos v., accessory hemi-azygos v., and hemi-azygos v.

  • esophagus

  • esophageal nerve plexus

  • vagus nn. (CN X)

Photo 19. Posterior mediastinum

Identify the vagus nn., and examine the esophagus and thoracic duct.

8.) Follow the courses of the vagus nn. through the posterior mediastinum. The left vagus n. assumes a more anterior position relative to the esophagus, whereas the right vagus n. assumes a more posterior position.

Note: A mnemonic to remember the ultimate relative positions of the vagus nn. with respect to the esophagus is LARP: Left Anterior, Right Posterior.

Photo 20. Vagus nerves and esophageal plexus

9.) Examine the esophagus throughout its length in the thorax. It enters the superior mediastinum between the trachea and vertebral column, anterior to the vertebral bodies of T1-T4. It passes through the esophageal hiatus of the diaphragm to enter into the stomach at the level of T10.

Photo 21. Esophagus

10.) Locate the thoracic duct. It is typically located adjacent to the esophagus, and anterior to the vertebral bodies of T6/T7-T12. In future dissections, we trace the thoracic duct to its drainage point, the left venous angle.

Note: The thoracic duct is typically to the right of the esophagus from the diaphragm until T4-6, where it crosses posterior to the esophagus to the left side. The thoracic duct often appears as a very clear vessel and may be overlooked or broken. Take great care in this region.

Photo 22. Thoracic duct

Identify the azygos system of veins.

11.) Locate the posterior intercostal vv. on the posterior thorax wall, in the costal grooves of the ribs. Trace these veins to their respective tributaries.

Photo 23. Azygos system of veins

Note: The azygos system of veins can exhibit a great deal of variation. The azygos v. typically runs along the right side of the vertebral column in this region, arching over the root of the R. lung to drain into the superior vena cava.

Photo 24. Azygos v.

Note: The hemi-azygos and accessory hemi-azygos are inconstant. Both vessels may be present; however, each vessel or both vessels may be absent. The accessory hemi-azygos v. is the superior partner of the azygos v. It typically runs along the left side of the vertebral column between T5-T8 and drains into the azygos v.

Note: The hemi-azygos v. is the main tributary of the azygos v. and is often referred to as the inferior partner of the azygos v. It typically ascends as far as T9 and drains into the azygos v.

Photo 25. Accessory hemi-azygos v. and hemi-azygos v.

Photo 27. Sympathetic trunk, ganglia, and rami communicantes

Follow the sympathetic trunk, and observe the thoracic ganglia. If present, locate the cervicothoracic (stellate) ganglion at approximately the C7-T1 level.

Note: The cervicothoracic (stellate) ganglion is noticeably larger than the other ganglia in the thoracic region. It is formed by the fusion of the inferior cervical ganglion (C7) and the first thoracic ganglion (T1). In some instances, the two ganglia do not fuse and can be observed as separate ganglia.

Photo 28. Cervicothoracic (stellate) ganglion

Photo 29. Separate C7 and T1 ganglia

Expose the branches from the sympathetic trunks that form the greater splanchnic nn.

Photo 30. Greater splanchnic n.

Note: The lesser and least splanchnic nn. also arise from thoracic sympathetic ganglia, but the origins and courses of these nerves are most likely obscured by the dome of the diaphragm.

Identify the anterior longitudinal ligament and subcostal mm.

13.) Gently move the esophagus, thoracic aorta, and the thoracic duct to the (donor’s) right to locate the vertebral column. Identify the anterior longitudinal ligament on the anteriormost aspect of the vertebral column.

Photo 31. Anterior longitudinal ligament

14.) Remove the costal parietal plurae and connective tissues lateral to the vertebral column to locate the subcostal mm.

Note: The subcostal mm. will be located about 2 fingers’ widths lateral from the vertebral column, along the ribs. The fibers of the subcostal mm. run obliquely, in the same orientation as the internal intercostal mm.

Photo 32. Subcostal mm.