Superior Mediastinum

1. In the superior mediastinum look for remnants of the thymus gland (142/N207). Note and then remove them.


2. With removal of thymic tissue and fat in the anterior mediastinum the right and left brachiocephalic veins (143/N226) will be exposed. If the manubrium sternum prevents exposure of these veins, it may be removed. The following tributaries of the brachiocephalics should be identified:
   1. Internal thoracic (N208).
   2. Superior intercostal (left side only) (N227) - lies on arch of aorta and passes between the phrenic and vagus nerves. May have continuity with accessory hemiazygos.
   3. Inferior thyroid (N208).

      
      Thymic tributaries may also be seen. Note the formation of the superior vena cava and its major tributary, the azygos vein. Cut the left brachiocephalic in the midline and reflect the ends.



3. Locate the phrenic nerves (149) and follow them to their destination. The pericardiacophrenic arteries (120/N208), branches of internal thoracic arteries, and veins will be found accompanying the phrenic nerves. These are very small vessels.


4. Locate the left and right vagus nerves (149/N208) in the superior mediastinum and follow them until they course posterior to the roots of the lungs. Locate and expose the following nerves and branches:
   1. Left vagus - crosses the aortic arch between the phrenic and left superior intercostal vein.
   1. Recurrent laryngeal (N222) - found hooking around the ligamentum arteriosum (N227).
   2. Cardiac branch of recurrent laryngeal. Difficult to find.
   2. Right vagus - lies in contact with the right side of the trachea medial to the azygos vein.
      1. Recurrent laryngeal (N222)-loops around right subclavian.
      2. Thoracic cardiac branch. Difficult to find.
         
5. The arch of the aorta can now be inspected (147/N233). Follow its branches into the root of the neck. The normal sequence of branches is the brachiocephalic, left common carotid (N233) and left subclavian (N233), but there may be variation.


6. The trachea (N233) and the esophagus (N233) lie behind the arch of the aorta.

To expose the structures in the posterior mediastinum the remainder of the pericardial sac must be removed. Detach the phrenic nerves from the pericardium before doing so.

1. When the arch of the aorta is moved laterally the bifurcation of the trachea (150/N228) into two main stem bronchi may be seen. The bifurcation of the trachea lies in the superior mediastinum. Adjacent lymph nodes are tracheo-bronchial nodes (110/N235). Small nerve trunks on the anterior surface of the bifurcation represent the deep cardiac plexus (139/N236) with contributions from the vagus nerves in the neck as well as thorax and the sympathetic chain in both neck and thorax. The superficial cardiac plexus (N222) is a small plexus on the anterior surface of the ligamentum arteriosum. This has probably been removed.


2. Follow the vagus nerves posterior to the bronchi and onto the esophagus (149). The vagi often break up into a plexus on the esophagus and cannot be identified as distinct nerves. When they do remain intact, however, they do not remain lateral to the esophagus but course anteriorly and posteriorly. Because of rotation of the gut during development the left tends to lie on the anterior surface. Follow the left recurrent laryngeal superiorly in the groove between the trachea and esophagus.


3. Free the esophagus and locate the thoracic duct (153/N235) that lies posterior to it. As the duct ascends, it passes to the left side of the esophagus. Follow the duct superiorly and establish continuity with it in the root of the neck emptying into the junction of internal jugular and subclavian veins.


4. Note the azygos system of veins (155/N234). Considerable variation is to be expected, because of its developmental history. Locate the vertical channels of the azygos, hemiazygos, and accessory hemiazygos vessels (N234) and determine the territory drained by each. In some specimens all the vessels will be present, in others there may only be an azygos vein with posterior intercostal tributaries. The azygos vein will be located in the midline if there is absence of hemiazygos and accessory hemiazygos.


5. Expose the descending aorta and look for the following branches (151):
   1. Intercostal (N233) - paired.
   2. Bronchial (N233) - unpaired, may arise from a posterior intercostal on the right.
   3. Esophageal (N233) - unpaired direct, tiny ventral branches.


6. Remove the costal parietal pleura and fat to expose the sympathetic chain (N205). Follow it superiorly and inferiorly. Locate white and gray rami (N205). The white ramus joining the ganglion with the spinal nerve is lateral to the gray. At times the posterior intercostal artery may lie between the rami. Locate and study the greater splanchnic nerves (156/N205) which arise from ganglia T5-T9. The lesser splanchnic nerves arising from T10 & T11 will be difficult to display because of the upward bulge of the diaphragm. The least planchnic nerve arising from T12 will not be found.


7. Follow an intercostal nerve, artery, and vein to a point previously dissected. Note they course external to the innermost intercostal muscle.


8. Note that in the posterior mediastinum vertically oriented structures lie anterior to those which are horizontal.