1. Mediastinum

2. The mediastinum extends superiorly to the thoracic inlet and the root of the neck and inferiorly to the diaphragm. It extends anteriorly to the sternum and posteriorly to the 12 thoracic vertebrae of the vertebral column.

3. It contains the remains of the thymus, the heart and large blood vessels, the trachea and esophagus, the thoracic duct and lymph nodes, the vagus and phrenic nerves, and the sympathetic trunks.

5. The mediastinum may be divided up, for descriptive purposes, into superior and inferior mediastina by an imaginary plane passing from the sternal angle anteriorly to the lower border of the body of the fourth thoracic vertebra posteriorly.

6. The inferior mediastinum is further subdivided into the middle mediastinum, which consists of the pericardium and heart; the anterior mediastinum, which is a space between the pericardium and the sternum; and the posterior mediastinum, which lies between the pericardium and the vertebral column.

7. Dissection of Superior Mediastinum

8. Identify again if possible the remains of the thymus embeded in fatty connective tissue immediately posterior to the manubrium sterni.

9. This organ reaches its greatest size relative to the remainder of the body at birth. It continues to enlarge until puberty, when it gradually starts to atrophy, and very little is present in old age. It receives its arterial supply from the internal thoracic arteries.

11. Reflect the thymus and its surrounding connective tissue superiorly and expose and clean the two brachiocephalic veins. Note that the two brachiocephalic veins unite to form the superior vena cava. Note also that the azygos vein arches forward over the superior margin of the root of the right lung to enter the superior vena cava.

12. Transect the left brachiocephalic vein at its junction with the right brachiocephalic vein and turn it superiorly. Identify the aortic arch and its branches and the trachea. Clean the aortic arch and identify the three large arteries that arise from its superior margin, the brachiocephalic, the left common carotid, and the left subclavian arteries. The arch of the aorta is a continuation of the ascending aorta.

13. The brachiocephalic artery passes upward and to the right of the trachea and divides into the right subclavian and common carotid arteries behind the right sternoclavicular joint.

14. The left common carotid artery arises from the aortic arch on the left side of the brachiocephalic artery. It runs upward and to the left of the trachea and enters the neck posterior to the left sternoclavicular joint.

15. The left subclavian artery arises from the aortic arch behind the left common carotid artery. It runs upward along the right side of the trachea and the esophagus to enter the root of the neck.

16. Identify and examine the right and left phrenic nerves. Follow the right phrenic nerve as it descends in the thorax along the right side of the right brachiocephalic vein and the superior vena cava.

17. Note that it passes in front of the root of the right lung and runs along the right side of the pericardium. It then descends on the right side of the inferior vena cava to the diaphragm. Its terminal branches supply the musculature of the right half of the diaphragm and pass through the caval opening of the diaphragm to supply the central part of the peritoneum on its under aspect.

18. Identify and examine the right and left vagus nerves.

19. Follow the right vagus nerve as it descends in the thorax, first lying posterolateral to the brachiocephalic artery, then lateral to the trachea and medial to the terminal part of the azygos vein. Note that it passes behind the root of the right lung and assists in the formation of the pulmonary plexus. On leaving the plexus, the vagus passes onto the posterior surface of the esophagus and will be followed later in the posterior mediastinum.

20. Now follow the left vegus nerve as it descends to the thorax between the left common carotid and left subclavian arteries. Note that it then crosses the left side of the aortic arch and is itself crossed by the left phrenic nerve. The vagus then turns posteriorly behind the root of the left lung and assists in the formation of the pulmonary plexus. On leaving the plexus, the vagus passes onto the anterior surface of the esophagus and will be followed later in the posterior mediastinum.

21. Identify the ligamentum arteriosum, which is a fibrous band that connects the bifurcation of the pulmonary trunk to the lower concave surface of the aortic arch.

22. Pick away the connective tissue around the ligamentum arteriosum and identify the left recurrent laryngeal nerve, which is an important branch of the left vagus. The left recurrent laryngeal nerve hooks around the lower border of the ligamentum arteriosum and eventually ascends in the interval between the trachea and the esophagus to reach the larynx.

23. The trachea divides into the right and left bronchi at the carina, at about the level of the sternal angle. Identify the tracheobronchial lymph nodes along the walls of the bifurcation of the trachea.

24. The esophagus lies immediately posterior to the trachea. Carefully transect the lower end of the trachea and remove it to expose the esophagus. Note that the esophagus lies anterior to the vertebral column and the left margin projects slightly to the left of the trachea.

25. Identify the thin-walled thoracic duct as it emerges from behind the left margin of the esophagus. It ascends into the root of the neck, where its termination will be dissected with the head and neck.

26. Dissection of Anterior Mediastinum

27. The anterior mediastinum is small; it lies between the body of the sternum and the pericardium. Expose the remains of the thymus in the loose connective tissue in the anterior and superior mediastina.

28. Dissection of Middle Mediastinum

29. Pericardium. Open the pericardium with an inverted-T incision and clean the pericardial cavity. Note the outer dull fibrous pericardium and the inner shiny serous pericardium. Note also that the visceral layer of serous pericardium is reflected onto the heart and closely covers it.

30. Insert your finger into the transverse pericardial sinus, which is a serous pericardial-lined tunnel situated between the superior vena cava and the ascending aorta.

31. Now lift up the apex of the heart and push a finger posterior to the heart up into the oblique pericardial sinus. This closed-ended, serous- lined sinus is bounded on the right by the inferior vena cava and two right pulmonary veins and on the left by the two left pulmonary veins.

32. Remove the heart by cutting across the great vessels that enter and leave the heart. Leave the pericardium intact.

33. Dissection of Posterior Mediastinum

35. Remove the remains of the pericardium and that it lies immediately anterior to the esophagus and descending thoracic aorta. Note also the fusion of the fibrous pericardium with the central tendon of the diaphragm.

36. Examine and clean the esophagus. Using the scalpel handle, gently elevate the esophagus from the vertebral column and the descending thoracic aorta.

37. Clean the descending thoracic aorta and identify the paired posterior intercostal arteries to the lower nine intercostal spaces. Arising from the anterior surface of the aorta arte the bronchial arteries and a number of small esophageal and mediastinal branches.

38. The thoracic duct is a very important lymphatic channel. It is grayish white in color and is thin-walled. Identify the duct as it ascends through the posterior mediastinum behind the esophagus. It enters the thorax through the aortic opening in the diaphragm.

39. Expose and clean the azygos veins and the posterior intercostal arteries arising from the aorta.

40. Although strictly speaking the sympathetic trunks are not placed within the mediastinum, they should be cleaned and examined now. The sympathetic trunk has segmentally arranged ganglia, each with a white and gray ramus communicans passing to the corresponding spinal nerve.

41. Identify the greater and lesser and least splanchnic nerves as they arised as branches of each sympathetic trunk.