The mediastinum is the central partition of tissue in the thoracic cavity separating the lungs and their enclosing pleurae in the right and left pulmonary cavities (Fig. 1; Gr 1.25; Ne 210 [no image]).  The mediastinum consists both of organs intrinsic to the thorax (e.g., the heart) and structures that traverse the thorax between other body regions (e.g., the esophagus and thoracic duct).  It is subdivided into the superior mediastinum and inferior mediastinum by a horizontal plane connecting the sternal angle with the intervertebral disc joining vertebrae Tv4/Tv5.  The inferior mediastinum is further subdivided with reference to the heart and pericardium.  The heart and its enclosing pericardium comprise the middle mediastinum.  The anterior mediastinum lies between the pericardium and the sternum, and the posterior mediastinum is posterior to the pericardium and the downwardly sloping posterior portion of the diaphragm, in front of thoracic vertebrae Tv5-Tv12.  During this period we will be studying structures of the posterior mediastinum and the superior mediastinum. 

The posterior mediastinum is posterior to the pericardium and posterior portion of the diaphragm and is anterior to the bodies of the lower 8 thoracic vertebrae (Gr 1.25A, 1.75, 1.76; Ne 210 [no image], 224, 225).  Laterally it is bounded by mediastinal parietal pleura on each side.   

The esophagus is a muscular tube that extends from the pharynx in the neck to the stomach in the abdomen.  It transports food and drink to the stomach through peristaltic contractions of its muscular walls aided by gravity.  Contractions of the esophagus are controlled mainly by the esophageal plexus of nerves.  The esophageal plexus is formed by branches of the right and left vagus nerves, which are joined by thoracic sympathetic branches to form a network on the external surface of the esophagus (Gr 1.72; Ne 203, 226).  Distally the esophageal plexus coalesces into anterior and posterior vagal trunks just proximal to the esophageal hiatus, where the esophagus traverses the diaphragm at the level of Tv10 to enter the abdominal cavity (Gr 1.72; Ne 226, 227).  Due to the counterclockwise rotation of the gut during embryonic development, the anterior vagal trunk contains mainly fibers from the left vagus nerve, and the posterior vagal trunk contains fibers from the right vagus nerve.  Small bilateral branches to the esophageal plexus from thoracic sympathetic chain ganglia include accompanying general visceral (GVA) afferent nerve fibers, which carry pain from the esophagus. 

The azygos system of veins receives blood from the posterior thoracic wall via posterior intercostal veins and from the posterior abdominal wall via ascending lumbar veins (Gr 1.73, 1.74, 1.75; Ne 186 [no image], 224, 232).  The vertically oriented azygos vein is formed by the union of the right ascending lumbar vein and the right subcostal vein, which is located below the 12th rib.  The azygos ascends along the right side of thoracic vertebral bodies, receiving multiple tributaries, including the lower 7 right posterior intercostal veins and right superior intercostal vein. The right superior intercostal vein is formed by the union of the 2nd, 3rd, and 4th posterior intercostal veins. The azygos vein terminates as the arch of the azygos vein, which passes above the root of the right lung to empty into the superior vena cava (Gr 1.75; Ne 224). 

On the left side, the left ascending lumbar vein and left subcostal vein unite to form the vertically oriented hemiazygos vein (Gr 1.73, 1.74; Ne 186 [no image], 232), which ascends along the left side of thoracic vertebral bodies.  The hemiazygos vein receives left posterior intercostal veins 9-11 as tributaries before turning to the right and crossing over the vertebral column at approximately Tv9 to drain into the azygos vein. 

Superior to the hemiazygos vein and in line with it may be an accessory hemiazygos vein, which receives left posterior intercostal veins 5-8 (Gr 1.73, 1.74; Ne 186 [no image], 225, 232).  The lower end of the accessory hemiazygos may cross to the right side to enter the azygos vein or may empty into the hemiazygos vein.  The left superior intercostal vein drains the posterior portion of intercostal spaces 2-4 into the left brachiocephalic vein but may communicate with the accessory hemiazygos vein (Fig. 1). 

On both the right and left sides of the body the first intercostal space is drained by a supreme intercostal vein into its respective brachiocephalic vein.

Expect to find variations.  For example, the azygos vein may be a large median vein receiving posterior intercostal veins from both sides, and the hemiazygos and/or accessory hemiazygos veins may be absent.  Although the connections are difficult to demonstrate, it is important to be aware that the azygos system communicates with the vertebral venous plexus, providing a pathway for the metastasis of cancer cells and the spread of infection from the thorax to the vertebral column, spinal cord, and brain(Gr 4.26, 4.44; Ne 166).  The connections of the azygos system are also one alternative route of venous return to the heart from the lower bodyif the inferior vena cava becomes obstructed (e.g., by a tumor) (Gr 1.74; Ne 232). 

The thoracic duct drains lymph from the entire body below the diaphragm and from the left half of the body above the diaphragm.  Thus, it usually receives the lymphatic channels from the left upper extremity (left subclavian lymphatic trunk), left half of the head and neck (left jugular lymphatic trunk), and left lung (left bronchomediastinal lymphatic trunk) before draining into the junction of the left subclavian and internal jugular veins, which is known as the left venous angle (Gr 1.38, 1.71; Ne 202, 259, 295). 

The thoracic aorta consists of the ascending aorta leaving the left ventricle in the middle mediastinum, the arch of the aorta in the superior mediastinum, and the descending thoracic aorta in the posterior mediastinum (Fig. 1; Gr 1.43, 1.64; Ne 206, 209, 226, 231).  The descending thoracic aorta has visceral and parietal branches.  Visceral branches include the esophageal arteries and the bronchial arteries.  Parietal branches include the nine lower pairs of posterior intercostal arteries, the subcostal arteries (below the 12th ribs), and the superior phrenic arteries to the diaphragm (Gr 1.57, 1.63, 1.70; Ne 201, 231). 

The right and left sympathetic trunks (sympathetic chains) descend from anterior to the necks of the upper ribs anteromedially to lie along the lateral surfaces of lower thoracic vertebral bodies (Gr 1.72, 1.75, 1.76; Ne 203, 224, 225).  The thoracic portion of each sympathetic trunk typically features 11-12 swellings, the sympathetic chain (paravertebral) ganglia.  Each ganglion is a collection of nerve cell bodies of postganglionic sympathetic neurons, where the axons of preganglionic sympathetic neurons synapse (Fig. 3). 

Like the subcostal and first two lumbar chain ganglia, thoracic sympathetic chain ganglia T1-11 are connected to the anterior rami of spinal nerves by both white and gray rami communicantes (Figs. 1, 3).  In other words, sympathetic chain ganglia T1-L2 have both white and gray communicating rami, but the sympathetic chain ganglia above and below these levels have only gray communicating rami. 

The white ramus communicans carries preganglionic sympathetic fibers (Fig. 3) and general visceral afferent nerve fibers.  The preganglionic sympathetic fibers are traveling from the spinal nerve to the sympathetic ganglion, and the general visceral afferent fibers are going the opposite direction, from the sympathetic ganglion to the spinal nerve.  (Remember that the cell bodies of the general visceral afferent nerve fibers are located either in posterior root ganglia or in cranial nerve ganglia.)  A gray ramus communicans carries postganglionic sympathetic fibers from the ganglion to the spinal nerve for distribution with its anterior and posterior primary rami.  Gray rami communicantes do not carry afferent fibers.  The white and gray rami communicantes are grossly indistinguishable, but the white ramus communicans usually is located lateral to the gray. 

Other visceral nerve branches emerge ventrally from the lower thoracic sympathetic ganglia en route to prevertebral ganglia located in the abdomen.  There are three of these thoracic splanchnic nerves on each side in the thorax —greater, lesser, and least.  The greater thoracic splanchnic nerve usually receives contributions from the 5th to 9th thoracic chain ganglia (Fig. 1; Gr 1.72, 1.75-1.77; Ne 160, 203, 224, 225, 234).  The lesser thoracic splanchnic nerve is formed from the 10th and 11th ganglia.  The least thoracic splanchnic nerve comes from the 12th thoracic ganglion.  The thoracic splanchnic nerves supply sympathetic innervation for most of the abdominal viscera.

The superior mediastinum is located between the manubrium of the sternum anteriorly and the bodies of vertebrae Tv1-Tv4 posteriorly.  It extends from the superior thoracic aperture above to a transverse plane connecting the sternal angle with the intervertebral disc joining vertebrae Tv4/Tv5 below.  Contents of the superior mediastinum include the remnants of the thymus; right and left brachiocephalic veins and superior vena cava; arch of the aorta and its three branches; vagus, phrenic, and left recurrent laryngeal nerves; trachea and its division into right and left primary bronchi; esophagus; and thoracic duct. 

The thymus is a bi-lobed lymphoid organ that begins to undergo fatty degeneration (involution) at puberty (Gr 1.58; Ne 205).  In the adult it is represented by a mass of fatty tissue, although it is still functional.   

Each brachiocephalic (innominate) vein is formed posterior to the sternoclavicular joint by the union of the subclavian vein and internal jugular vein of its side (Fig. 2; Gr 1.42, 8.5, 8.16E-F; Ne 200, 205, 206).  The left brachiocephalic vein crosses to the right side, and the right and left brachiocephalic veins join posterior to the first right costal cartilage to form the superior vena cava.  The superior vena cava returns to the right atrium all of the venous blood from structures superior to the diaphragm except that from the lungs and the tissues of the heart itself.   

Turn your attention to the arch of the aorta (aortic arch).

The arch of the aorta passes posterolaterally to the left from the ascending aorta to become continuous with the descending thoracic aorta (Fig. 1; Gr 1.76, 1.80B; Ne 225-227, 231).  The first and largest branch of the arch of the aorta is the brachiocephalic trunk (brachiocephalic artery) (Gr 1.64, 1.69, 1.77; Ne 187, 226, 231).  It ascends toward the right to divide into the right common carotid artery and right subclavian artery behind the right sternoclavicular joint. 

The second branch of the arch of the aorta, located to the left of the brachiocephalic trunk, is the left common carotid artery (Fig. 1; Gr 1.69, 1.76, 1.77; Ne 187, 226, 231).  It ascends into the neck. 

The third branch is the left subclavian artery, whichalso ascends into the root of the neck (Fig. 1).  It passes above the first rib between the anterior and middle scalene muscles to enter the axilla, where it was seen in an earlier dissection. 

Variations in the branching pattern from the arch of the aorta may occur, including one or more additional arteries arising from the arch of the aorta (Gr 1.65, 8.16D).  These include a thyroid ima artery and a left vertebral artery. 

The origin and course of the left recurrent laryngeal nerve in the thorax—the right recurrent laryngeal nerve arises in the root of the neck—means that the left recurrent laryngeal may be injured by intrathoracic lesions(e.g., metastatic bronchogenic carcinoma, aortic aneurysms).  The resulting persistent hoarseness is sometimes the first sign of disease. 

The phrenic nerves descend through the superior mediastinum anterior to the roots of the lungs to enter the middle mediastinum (Fig. 1; Gr 1.21, 1.42, 1.75, 1.76; Ne 187, 206, 224, 225).  Each phrenic nerve is derived from the anterior rami of spinal nerves 3-5 (“C3,4,5 keep the diaphragm alive”).  Each nerve innervates the musculature in its half of the diaphragm (hemidiaphragm) and provides sensory innervation to all but the periphery of that half. 

A lesion of a phrenic nerve results in paradoxical movement of the paralyzed hemidiaphragm during inspiration.  For example, if the left half of the diaphragm is paralyzed, then it is pushed upward by abdominal viscera during inspiration while contraction of the intact right half of the diaphragm pulls it downward.  This paradoxical movement can be detected on x-rays.  Irritation of the diaphragmatic pleura or diaphragmatic peritoneum (i.e., pleura or peritoneum covering the diaphragm) may cause referred pain to the ipsilateral shoulder.  That is because the skin of the shoulder receives innervation from the suprascapular nerves, which also contains nerve fibers from C3,4. 

The trachea is the portion of the airway that begins in the neck at the inferior border of the cricoid cartilage, which is the lowest of the laryngeal cartilages (Gr 1.24, 1.33, 8.16E, 8.41; Ne 63, 68, 190, 196, 200, 228).  The trachea has a wall that is reinforced anteriorly and laterally by C-shaped cartilaginous rings.  The trachea descends through the lower neck and superior thoracic aperture and then behind the left brachiocephalic vein, brachiocephalic trunk, and arch of the aorta.  The esophagus lies posterior to the trachea, separating it from the bodies of vertebrae (Gr 1.69, 1.75; Ne 226-228). 

The trachea bifurcates into two primary (main) bronchi—right and left—at the level of the intervertebral disc between vertebrae Tv4/Tv5 (Gr 1.33, 1.63, 1.69; Ne 196, 226).  Note that the right primary bronchus is wider and more vertically oriented than the left, making an inhaled foreign object more likely to enter the right primary bronchus.  At the bifurcation of the trachea, the last tracheal cartilage sends a process, the carina, posteriorly between the primary bronchi.  On an internal view of the bifurcation through a bronchoscope, the carina forms a ridge between the openings of the primary bronchi. 

During the clinical evaluation of the trachea and bronchi, a bronchoscope is passed through the nose, pharynx, and larynx into the trachea.  This allows examination of the internal structure of the trachea and bronchi and the extraction of small tissue samples.  Distortion or deviation of the carina to one side during a bronchoscopic evaluation suggests the presence of a space-occupying lesion in the thorax (e.g., bronchogenic carcinoma that has metastasized to tracheobronchial lymph nodes [Gr 1.38, 1.69; Ne 202, 233]).  The carina is covered by sensitive mucous membrane, and contact with an aspirated foreign object stimulates the cough reflex in an attempt to expel it from the airways.  The general visceral afferent fibers of the cough reflex are part of the vagus nerves.