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MEDIASTINUM Dr SUPERIOR AND POSTERIOR MEDIASTINUM Dr. Milton M. Sholley SELF­STUDY RESOURCES Essential Clinical Anatomy 3 rd ed. (ECA): pp. 80­82 and 101­115 Syllabus: 9 pages (Page 9 lists corresponding figures for Grant's Atlas 11 th & 12 th Eds.) Head to Toe Questions in Gross Anatomy: Finish questions #216­253 and #465­541. STRUCTURES TO BE OBSERVED: Superior mediastinum: ­Thymus remnant (may not be present), trachea, tracheal bifurcation, esophagus ­Arch of aorta, brachiocephalic artery, left common carotid artery, left subclavian artery, internal thoracic arteries ­Superior vena cava, brachiocephalic veins, right and left superior intercostal veins, arch of azygous vein, internal thoracic veins ­Thoracic duct ­Vagi, left recurrent laryngeal nerve, phrenic nerves ­Cardiac branches of vagi and sympathetic ganglia, superficial and deep cardiac plexi (all of these structures are difficult and not mandatory to find) Posterior mediastinum ­Lower half of esophagus ­Esophageal plexus (from vagi) ­Lower part of descending aorta ­Azygous, hemiazygous, and accessory hemiazygous veins ­Transverse connecting veins of bilateral azygos system of vein ­Thoracic duct, sympathetic trunks, greater splanchnic nerves ­Right pulmonary artery ­Left principal bronchus 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 sternum in front to the vertebral column behind, and from the thoracic inlet 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 pericardium, (b) the middle mediastinum, occupied by the pericardium and its enclosed heart, and (c) the posterior mediastinum, behind the pericardium. Subdivisions of the Mediastinum (Mid­sagittal plane 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 sternal angle to the disk below vertebra T4. This small wedge­shaped space has boundaries composed of the anterior surfaces of the bodies of the first four thoracic vertebrae 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 neck along a plane marked by the first pair of ribs. 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 lung, 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 lungs 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 ascending aorta is located, and assumes a horizontal position in a near coronal plane. 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 brachiocephalic vein. 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 rib, 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 thyroid veins from the neck, and the internal thoracic, supreme intercostal, left superior intercostal, and the thymic veins from the thorax. 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 blood from the upper half of the body, except that from the heart itself, is delivered to the right atrium 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. Autonomic nerve 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 vagus nerve 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 jugular vein 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 phrenic nerve 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.
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