Superior and Posterior Mediastinum

1. Re-examine the large vessels arising from the arch of the aorta and trace them towards the root of the neck. Note the division of the brachiocephalic trunk into the right subclavian and right common carotid arteries lying behind the right sternoclavicular joint.

2. Replace the heart and note that the oesophagus lies immediately behind the left atrium.

3. Now, remove the pulmonary arteries and the remains of the pericardium and clean the trachea which lies partly above and partly behind the arch of the aorta. Note that it bifurcates into the right and left main bronchi opposite the lower border of the fourth thoracic vertebral body. Observe the presence of tracheobronchial lymph nodes which lie in front of the lower part of the trachea. Examine the two main bronchi and note that the right main bronchus is almost vertical, i.e., in line with the direction of the trachea, whereas the left one is more horizontal.

4. Identify the deep part of the cardiac plexus which lies on the anterior aspect of the tracheal bifurcation. The cardiac plexus receives branches from the right and left vagus nerves, and all the cervical sympathetic and the upper five thoracic ganglia and supplies the heart. It also sends branches to the bronchi.

5. Trace the thoracic part of the oesophagus which lies posterior to the trachea and note the constrictions where the arch of aorta and left bronchus cross it. Follow the oesophagus down to the oesophageal hiatus in the diaphragm which is opposite the level of the tenth thoracic vertebra. Secure the left recurrent laryngeal nerve at the aortic arch and trace it upwards as it lies in the groove between the trachea and oesophagus.

6. Follow the right and left vagus nerves distally. Observe that the main part of each vagus nerve passes behind the root of the lungs to form the posterior pulmonary plexus. Note the contributions from the second, third, fourth and fifth thoracic sympathetic ganglia to this plexus. Trace each vagus nerve from the lower end of this plexus towards the oesophagus where the left and right nerves form the oesophageal plexus.

7. Trace the oesophageal plexus downwards towards the diaphragm where it gives rise to the anterior and posterior vagal trunks which follow the oesophagus through the oesophageal hiatus. Each trunk carries both right and left vagus nerve fibres.

8. Clean the sympathetic trunks and their ganglia which lie on either side of the thoracic vertebral bodies. There are usually eleven ganglia. Locate the cervicothoracic ganglion opposite the neck of the first rib. Note that grey and white rami communicantes connect the ganglia with the intercostal nerves. Secure the greater splanchnic nerve arising from the fifth to the ninth ganglia and the lesser splanchnic nerve from the ninth and tenth ganglia. These ganglia are found close to the sides of the respective thoracic vertebral bodies. Trace these splanchnic nerves downwards towards the crura of the diaphragm which they pierce before entering the abdomen. Note that the greater splanchnic nerves provide branches to the oesophageal plexus.

9. Clean the descending thoracic aorta and trace it towards the diaphragm where it passes through the aortic hiatus of the diaphragm opposite the twelfth thoracic vertebra. Trace the posterior intercostal arteries from the aorta to the lower nine intercostal spaces. These arteries anastomose with the anterior intercostal arteries. Note that the aorta also has bronchial, oesophageal, phrenic and mediastinal branches, all of which are small arteries.

10. Identify the superior intercostal artery, a branch of the costocervical trunk which is a branch of the subclavian artery in the root of the neck. Note that it supplies the first two intercostal spaces. Observe the following structures which lie opposite the neck of the first rib: the cervicothoracic sympathetic ganglion, superior intercostal artery and the ventral ramus of the first thoracic nerve. Trace the ventral ramus of the first thoracic nerve which ascends over the first rib to join the brachial plexus.

1. Observe that the first intercostal space of both sides is drained by the first posterior intercostal vein which drains into the corresponding brachiocephalic vein. Note that the left superior intercostal vein drains the second and third intercostal spaces. Its termination into the left brachiocephalic vein has already been seen.

2. Identify the thoracic duct and azygos vein passing through the aortic hiatus of the diaphragm. Trace the azygos vein upwards where it arches over the root of the right lung to enter the superior vena cava just outside the pericardium. Secure the following tributaries of the azygos vein:

(a)right superior intercostal vein draining the second and third intercostal spaces;

(b) right posterior intercostal veins of the lower eight spaces; and

(c) accessory hemiazygos vein receiving the fourth to eighth posterior intercostal veins and the hemiazygos vein draining the lower posterior intercostal veins of the left side of the thorax. They pass behind the aorta and thoracic duct to join the azygos vein opposite the seventh and eighth thoracic vertebral bodies. Note that the oesophageal veins drain into the azygos veins.

3. Trace the thoracic duct upwards in front of the thoracic vertebral bodies up to the fifth thoracic vertebra where it inclines towards the left. It then ascends upwards into the neck on the left side of the oesophagus.

4. Medial to the angle of the lower ribs identify the subcostales muscles, part of the innermost intercostals.

Joints of Thorax

1. These joints should be studied on your cadaver and on a skeleton.

2. In the portion of sternum and costal cartilages that has been preserved, study the following joints:

(a)manubriosternal joint: note that a fibrocartilaginous disc intervenes between the manubrium and the body of the sternum. This is an example of a secondary cartilaginous joint; and

(b) sternocostal joints: note that the first sternocostal joint is a primary cartilaginous joint and that the union between the second to seventh costal cartilages and the sternum is by synovial joints.

3. Clean one or two joints between the vertebral bodies and the heads of the corresponding ribs. These are the costovertebral joints. Note that the capsule is strengthened anteriorly by radiate ligaments which pass to the two vertebrae related to the rib and to the intervertebral disc. The head of most ribs has two demifacets which articulate with the bodies of two adjacent vertebrae by synovial joints. In these cases, an intervening intraarticular ligament extends from the rib to the intervertebral disc, thus dividing the synovial cavity into two.

4. Next examine a costotransverse joint. This is a synovial joint between the articular part of the tubercle of the rib and the corresponding transverse process of the vertebra. The capsule of the joint is strengthened by the superior costotransverse, costotransverse and lateral costotransverse ligaments.

5. Note that there are synovial joints between the sixth, seventh, eighth and ninth costal cartilages (interchondral joints). Summary The oesophagus extends from the lower end of the pharynx to the cardial orifice of the stomach. It is constricted at its commencement, where the left bronchus and the arch of the aorta cross in front of it and where it pierces the diaphragm. These have to be borne in mind during oesophagoscopy. The oesophagus runs behind the left atrium. Thus, any enlargement of the left atrium, as for example in mitral stenosis, can cause symptoms of dysphagia. The oesophagus deviates a little to the right of the midline as it lies behind the heart and consequently produces a visible impression on the mediastinal surface of the right lung in front of the groove for the azygos vein. The other structures of interest in this region are the trachea and thoracic duct. The fact that the right main bronchus which is one of the terminal divisions of the trachea runs vertically, carries with it the danger of foreign bodies entering it more frequently than into the left main bronchus. The tracheobronchial lymph nodes are closely related to the left recurrent laryngeal nerve which may sometimes be compressed by these nodes, leading to hoarseness of the voice. The thoracic duct commences from the upper end of a sac known as the cisterna chyli which is situated in front of the upper two lumbar vertebrae in the abdomen. The thoracic duct is a thin walled vessel draining lymph from almost the entire body except the right upper limb, right half of the thorax and right side of the head and neck. It is provided with a number of valves. It terminates in the neck by opening into the junction of the left subclavian and internal jugular veins.