SUPERIOR AND POSTERIOR Dr. Milton M. Sholley SELF­STUDY RESOURCES

Essential Clinical 3 rd ed. (ECA): pp. 80­82 and 101­115

Syllabus: 9 pages (Page 9 lists corresponding figures for Grant's 11 th & 12 th Eds.)

Head to Toe Questions in Gross Anatomy: Finish questions #216­253 and #465­541.

STRUCTURES TO BE OBSERVED:

Superior mediastinum: ­ remnant (may not be present), , tracheal bifurcation, ­Arch of , brachiocephalic , left , left , internal thoracic ­, brachiocephalic , right and left superior , arch of azygous , internal thoracic veins ­Thoracic duct ­Vagi, left recurrent laryngeal , phrenic ­Cardiac branches of vagi and , superficial and deep cardiac plexi (all of these structures are difficult and not mandatory to find) Posterior mediastinum ­Lower half of esophagus ­ (from vagi) ­Lower part of ­Azygous, hemiazygous, and accessory hemiazygous veins ­Transverse connecting veins of bilateral azygos system of vein ­Thoracic duct, sympathetic trunks, greater ­Right ­Left principal

LECTURE OUTLINE

I. GENERAL REMARKS

The mediastinum is the partition created by other organs that lie between the two pleural sacs. It extends from the in front to the behind, and from the above to the diaphragm below. For purposes of description it is divided into two parts, an upper part, which is named the superior mediastinum, and a lower part, which is subdivided into (a) the anterior mediastinum, in front of the , (b) the middle mediastinum, occupied by the pericardium and its enclosed , and (c) the posterior mediastinum, behind the pericardium. Subdivisions of the Mediastinum (Mid­ drawing from Textbook of Anatomy by W. Henry Hollinshead)

II. SUPERIOR MEDIASTINUM

A. The superior mediastinum is the area of mediastinum below the thoracic inlet and above a line drawn from the to the disk below T4. This small wedge­shaped space has boundaries composed of the anterior surfaces of the bodies of the first four posteriorly, the posterior surface of the manubrium sterni anteriorly, the pericardial sac inferiorly, and the two parietal (mediastinal) pleura laterally. Superiorly, it is continuous with the along a plane marked by the first pair of .

B. The structures found in the superior mediastinum are: the thymus, the trachea and esophagus, the aortic arch and its branches; brachiocephalic veins and superior vena cava; arch of the azygos vein; cervical cardiac branches of vagi and sympathetic ganglia; vagi and recurrent laryngeal nerves; phrenic nerves; right lymphatic trunk; and thoracic duct.

1. The thymus is the most anterior structure in the superior mediastinum. In the adult cadaver, it is largely replaced by fat and connective tissue, which maintain the shape of the organ.

2. The trachea and esophagus have a common embryological origin, with the trachea anterior to the esophagus. The posterior surface of the esophagus lies opposite the anterior surface of the vertebral bodies. The trachea bifurcates at its carina cartilage, into right and left principal or primary bronchi at, or slightly below the lower boundary of the superior mediastinum, whereas the esophagus continues into the posterior mediastinum. Above the root of the , both trachea and esophagus are crossed by the azygous vein on the right side and by the arch of the aorta on the left side. The aortic arch indents the esophagus slightly and shifts the trachea from its median position toward the right. The esophagus contacts the upper lobe of both with only the pleural sacs intervening, whereas the trachea becomes separated from the lung on the left side by the aortic arch.

3. The arch of the aorta emerges from the pericardial sac, in which the is located, and assumes a horizontal position in a near . It then curves backward in a near sagittal plane and at the 4th thoracic vertebral level it becomes vertical and enters the posterior mediastinum where it is called the descending thoracic aorta. The arch of the aorta accommodates the right pulmonary artery and left bronchus in its inferior concavity. The trachea and esophagus fit into a slight concavity that faces to the right. The profile of the left curve of the aorta creates the aortic knuckle, identifiable on a anteroposterior chest X­ray film as the upward continuation of the cardiac silhouette. Through the pleura, the arch indents the medial surface of the left lung just above the hilum. The ligamentum arteriosum connects the inferior surface of the arch of the aorta to the left pulmonary artery immediately after it is given off by the pulmonary trunk. The summit of the arch reaches more than half way up behind the manubrium and from it arise in a row three major vessels: the brachiocephalic, the left common carotid and the left subclavian artery.

4. The brachiocephalic trunk arises from the arch in the midline. It lies on the trachea and ascends to its right side. As the artery reaches the superior thoracic aperture, it divides into the right subclavian and right common carotid arteries. The left common carotid and left subclavian arteries arise independently from the aortic arch in close succession beyond the brachiocephalic artery. The left common carotid and left subclavian ascend more or less vertically along the left side of the trachea. Near their origin, all three arteries are crossed anteriorly by the left . The three major arteries do not give off any branches in the superior mediastinum. However, the internal thoracic artery, given off by the subclavian in the root of the neck, returns to the superior mediastinum and continues down through the anterior mediastinum.

5. The two brachiocephalic veins are formed by the confluence of the subclavian and internal jugular veins on each side of the root of the neck just above the superior thoracic aperture. The veins enter the superior mediastinum anterior to the major arteries and cervical pleura and become overlapped by pleura as they descend behind the manubrium sterni. The right brachiocephalic vein is vertical and is close to the right border of the manubrium. The left brachiocephalic vein is in a nearly horizontal position and its course is posterior to the manubrium. Behind the sternal end of the right first , the left vein unites with the right vein to form the superior vena cava, which is in a vertical position. The tributaries of the brachiocephalic veins include the vertebral and inferior veins from the neck, and the internal thoracic, supreme intercostal, left superior intercostal, and the thymic veins from the .

6. The superior vena cava continues the vertical course of the right brachiocephalic vein, and just before it enters the pericardial sac, it receives the azygos vein. All the venous from the upper half of the body, except that from the heart itself, is delivered to the right by the superior vena cava.

7. The deep cardiac plexus of nerves lies in front of the tracheal bifurcation and continues below the arch of the aorta as the superficial cardiac plexus. fibers from the superior, middle and inferior sympathetic ganglia in the neck, enter the superior mediastinum and end in the deep cardiac plexus. Also thoracic sympathetic ganglia T1 to T5 send cardiac branches to the deep cardiac plexus. The also contributes parasympathetic fibers to the deep plexus from the superior and inferior cervical branches in the neck, and from the left recurrent laryngeal nerve in the thorax. The superficial cardiac plexus usually receives sympathetic fibers from the left superior cervical sympathetic ganglion via a superior cervical branch and parasympathetic fibers from the vagus via an inferior vagal cervical branch. From these plexi autonomic fibers reach the lungs via the pulmonary arteries and bronchi, and the heart via the ascending aorta.

8. In the root of the neck, both right and left vagus nerves lie between and behind the internal and the common carotid artery on their respective sides, and cross the subclavian arteries anteriorly to enter the superior mediastinum. The two vagi then pass POSTERIOR TO the hilum of the lung and go directly to the esophagus, where they form an esophageal plexus from which anterior and posterior vagal branches enter the abdominal cavity through the esophageal hiatus in the diaphragm. The right vagus gives off a right recurrent laryngeal nerve, which loops around the subclavian artery and travels to the larynx in the neck. The left recurrent laryngeal nerve arises from the left vagus on the left side of the aortic arch and loops under the arch before ascending on the right side of the arch to go back up the neck to the larynx. The loop is just lateral to the ligamentum arteriosum.

9. The phrenic nerves originate in the neck from ventral rami of spinal nerves C3, C4, and C5, descend to the root of the neck, pass between the subclavian artery and vein, and enter the superior mediastinum. The right winds its way forward on the right brachiocephalic vein and superior vena cava and descends in the middle mediastinum between pleura and pericardium and ANTERIOR TO the root of the right lung to penetrate the diaphragm with the inferior vena cava. The left phrenic nerve crosses the left brachiocephalic vein and arch of the aorta, the left vagus and left superior intercostal vein in its descent ANTERIOR TO the hilum of the left lung and pierces the diaphragm near the apex of the heart. The phrenic nerves are motor to the diaphragm and sensory to the pericardium, mediastinal and diaphragmatic pleura and parietal peritoneum on the inferior surface of the diaphragm.

III. POSTERIOR MEDIASTINUM

A. The posterior mediastinum is continuous with the posterior part of the superior mediastinum, and this arbitrary division into two compartments is for descriptive purposes only. An imaginary line drawn from the sternal angle to the disk below the fourth thoracic vertebra serves to distinguish the superior from the inferior mediastinum, with its anterior, middle and posterior parts. The posterior part of the inferior mediastinum, or posterior mediastinum, is limited anteriorly and inferiorly by the diaphragm, posteriorly by thoracic vertebra 5 thru 12, and anteriorly in its superior aspect, by the pericardial sac.

B. The contents of the posterior mediastinum include the lower half of the esophagus with the esophageal plexus formed by the vagi, the descending aorta and its branches, the veins of the azygos system, the thoracic duct and associated nodes, and thoracic portions of the sympathetic trunks and branches.

1. The esophagus extends from the in the neck to the esophageal hiatus in the diaphragm. The esophagus lies opposite the anterior surfaces of the upper thoracic vertebral bodies 1 to 8; it then inclines forward and to the left of the midline to cross from the right side of the aorta to the front of this vessel where it penetrates the muscular part of the diaphragm in line with the 10th thoracic vertebra. The esophagus narrows at four places that can be seen in radiographs, but not grossly. These narrowings occur at its junction with the pharynx, at its contact point with the aortic arch, at its contact point with the left bronchus, and at the esophageal hiatus. Below the tracheal bifurcation, the following structures lie ANTERIOR TO the esophagus: the right pulmonary artery, the left principal bronchus, the oblique sinus of the pericardium, and thru it, the left atrium. The descending thoracic aorta is to its left superiorly, and posterior lower down. Its right side is in contact with mediastinal pleura. Abnormalities of neighboring structures may distort or displace the contours of the esophagus on X­ray films taken during a barium swallow. Such abnormalities include vertebral tumors, enlarged mediastinal, tracheal, and bronchial lymph nodes, aneurysms of the aorta and distention of the left atrium. The arteries to the esophagus include the inferior thyroid artery in the neck, esophageal arteries from the descending aorta and bronchial arteries, and left gastric artery and inferior phrenic arteries from the . Venous drainage is by inferior thyroid, azygous, hemiazygous to the superior vena cava, and left gastric veins to the portal vein through the to the inferior vena cava. Portal hypertension (due to cirrhosis of the liver) may cause esophageal varices with possible hemorrhage. Lymph from the esophagus drains into deep cervical nodes in the neck, posterior mediastinal nodes in the thorax and left gastric nodes in the abdomen.

Innervation of the esophagus: the cranial portion of the spinal accessory nerve (Cranial XI) travelling with the Vagus (X) supplies motor fibers to striated muscle (branchial efferents). The vagus supplies visceral efferents to neurons in the myenteric and submucosal plexuses located in the wall of the esophagus. Cervical and thoracic sympathetic ganglia and branches from the splanchnic nerves innervate the esophagus. Esophageal pain is mediated along visceral afferents from the vagus and sympathetic chain ganglia, T1 ­ T10. This pain is felt substernally and may be mistaken for angina pectoris. It may also radiate to the back (referred pain).

2. The descending thoracic aorta is the continuation of the aortic arch and begins where the latter comes in contact with the vertebral column on the left side of the vertebral bodies, at about the level of T5. From this point the aorta moves toward the midline and penetrates the diaphragm via the aortic hiatus at the level of the 12th thoracic vertebra to become the . The bronchial, esophageal, pericardial and mediastinal visceral branches arise from the anterior surface of the descending aorta. From the posterolateral aspects of the descending aorta, paired posterior enter the intercostal spaces 3 to 11 and the subcostal artery runs along the inferior margin of the 12th rib. The posterior intercostal arteries of the first two intercostal spaces arise from a common stem, the supreme intercostal artery, which arises from the , a branch of subclavian artery.

3. The azygous system of veins is composed of three longitudinal channels: one channel, the azygos vein, lies on right side of thoracic vertebral bodies in the posterior mediastinum; and two channels, the accessory hemizygous (located superiorly) and the hemizygous (located inferiorly) lie on the left side of the vertebral bodies. On the right side the azygos vein receives the subcostal and ascending lumbar veins, and posterior intercostal veins from intercostal spaces 5 thru 11. At the 4th vertebral body, the azygous vein lies posterior to the hilum of the right lung, then arches forward over the superior surface of the hilum and empties into the superior vena cava. The arch of the azygos receives the right superior intercostal vein, which drains the right posterior intercostal spaces 2, 3 and 4. The first on the right drains into the right brachiocephalic vein via the right supreme intercostal vein. On the left side, the hemiazygous vein receives the left ascending lumbar and left subcostal veins, and posterior intercostal veins from four or five lower intercostal spaces. The hemiazygous drains into the azygous by one or two transverse connecting veins. The superiorly located accessory hemiazygous vein receives the left posterior intercostal veins from intercostal spaces 5 thru 8 and it drains into the azygous vein by transverse connecting veins. The left intercostal spaces 2­4, may drain into the left brachiocephalic vein via a left superior intercostal vein, which also may have a communication with the accessory hemiazygous vein. The first intercostal space on the left often drains into the left brachiocephalic vein via a left supreme intercostal vein, but alternatively drains via the left superior intercostal vein if the former vein is absent. Tributaries of the azygous system from thoracic viscera include the bronchial, esophageal, pericardial and some mediastinal veins.

Venous anastomosis: Through the anastomosis of the posterior and anterior intercostal veins, the azygos system is linked to the internal thoracic veins, and through the ascending lumbar veins is linked to the inferior vena cava. The azygos system is also in communication with the vertebral venous plexus. The azygos system, devoid of valves, constitutes an important channel for collateral venous circulation when the inferior or superior vena cavae are obstructed.

4. The thoracic duct is the upward continuation of the cisterna chyli, a lymphatic reservoir which is located in the abdomen. It enters the thorax behind the aorta through the aortic hiatus of the diaphragm. In the posterior mediastinum, the duct ascends on the front of the vertebral bodies, running between the aorta and azygos vein, posterior to the esophagus. At the level of the sternal angle (T4 disk level), it crosses the midline to the left and leaves the thorax behind the left subclavian artery. In the base of the neck, the duct joins the venous system at the confluence of the left subclavian and left internal jugular veins.

The thoracic duct receives tributary vessels from several groups of lymph nodes located in the mediastinum, although most of the nodes drain first into the bronchomediastinal lymph trunks. The groups of lymph nodes are named according to location and include the parasternal and intercostal lymph nodes, the tracheobronchial and posterior mediastinal nodes, and the phrenic or diaphragmatic groups of lymph nodes. The posterior diaphragmatic, intercostal and the posterior mediastinal lymph nodes of the left side send their efferent vessels directly into the thoracic duct. The posterior mediastinal lymph nodes, situated behind the pericardial sac and esophagus, drain adjacent structures and the diaphragmatic surface of the liver. The efferents of the parasternal and tracheobronchial nodes form the right and left bronchomediastinal lymph trunks; the left one of these may join the thoracic duct in the base of the neck or may empty independently into the confluence of left subclavian and internal jugular veins. Before its termination, the thoracic duct usually receives the left internal jugular and subclavian lymph trunks. The corresponding lymph trunks on the right side form the short right lymphatic duct.

Unifying concept of thoracic drainage: The thoracic duct receives the majority of the lymph fluid from both sides of the body below the diaphragm and also from the left half of the body (including head and neck) above the diaphragm. Lymph from the right half of the body above the diaphragm (including the head and neck) is received by the right lymphatic duct.

5. The sympathetic trunks are paired autonomic nerve structures located in the medial portion of the posterior and covered with and costal pleura, and for the most part run along the front of the of the ribs. Their lower ends incline forward onto the sides of the vertebrae, so that by the time they penetrate the diaphragm, they are situated more anteriorly than laterally. Strictly speaking, therefore, only the inferior portions of the trunks are in the posterior mediastinum. The first thoracic sympathetic ganglion is usually fused with the inferior cervical ganglion to form the . The other 10 or 11 sympathetic chain ganglia are each located below the corresponding intercostal nerve, to which each is connected by two rami communicantes (white and gray). Medial branches from each ganglion go to viscera. In the case of ganglia T1 to T4, the medial branches go to the deep cardiac plexus. Ganglia T5 to T9 form the greater splanchnic nerve. Ganglia T10 and T11 form the lesser splanchnic and T12 forms the least splanchnic nerve. All three splanchnic nerves descend toward the diaphragm in front of the vertebral column lying medial to the sympathetic trunks. On each side, they pierce the muscular crus of the diaphragm and enter, respectively, the celiac, superior mesenteric, and renal ganglia or plexi in the abdomen. Figures from Grant's Atlas of Anatomy

(11 th ed.) (12 th ed.)

1.59, page 65 1.62, page 67 1.64A&B, page 68 1.66A&B, page 70 1.62A,B,C&D, page 66 1.61A,B,C,D&E, page 66 1.23, page 27 1.23, page 27 1.43, page 49 1.43, page 49 1.61, page 66 1.64A, page 68 1.42, page 48 1.42, page 48 1.37, page 42 1.37, page 42 1.25A,B&C, page 29 1.25A,B&C, page 29 1.69, page 72 1.71, page 74 1.44B, page 51 1.44B, page 51 1.57A, page 63 1.59, page 65 1.63, page 67 1.65, page 69 1.70C, page 73 1.72C, page 75 1.18, page 20 1.18, page 20 1.73, page 76 1.75, page 78 1.74, page 77 1.75, page 79 1.71, page 74 1.73, page 76 1.72, page 75 1.79C, page 85 1.15, page 17 1.15, page 17 1.75, page 78 1.76, page 80 1.76, page 79 1.77, page 81