Mediastinum

=**Objectives**=


 * 1. Delineate the superior, anterior, middle, and posterior mediastinum.**

The sternal angle cuts through the 2nd rib and the T4 vertebrae and divides the superior from the inferior mediastinum. The inferior mediastinum is subdivided into the anterior, middle, and posterior mediastinum.

The superior mediastinum is located above themanubriosternal angle. It is bound by T1-T4 posteriorly and the parietal pleura laterally. It is continuous with the neck and the anterior and posterior mediastinum.

Anterior mediastinum is bordered by the sternum anteriorlyl and the pericardial sac posteriorly.

Middle mediastinum is bordered ventrally by the anterior mediastinal compartment, and dorsally by the posterior mediastinum.

Posterior mediastinum is bordered anteriorly by the pericardial sac, and posteriorly by the anterior surface of the vertebral bodies.


 * 2. Describe the major structures associated with the four regions of the mediastinum.**

__Superior Mediastinum__ Thymus, aortic arch (brachiocephalic trunk, left common carotid artery, left subclavian artery), superior vena cava (left and right brachiocephalic veins), Trachea (left and right bronchi), esophagus, right vegas nerve, left vegas nerve (left recurrent laryngeal nerve),phrenic nerves, thoracic duct.

__Anterior Mediastinum__ Sternopericardial ligamnet, fat, and lymph nodes.

__Middle Mediastinum__ Pericardium, heart, phrenic nerves, pericardiacophrenic vessels, stems of great vessels.

__Posterior Mediastinum__ Descending aorta, esophagus, azygous system of veins, vagus nerve, thoracic duct, lymph nodes, and thoracic splanchnaic nerves.


 * 3. Identify the contents of the superior mediastinum and relationship to the heart and lungs.**

The superior mediastinum is located above the manubriosternal angle. It is bound by T1-T4 posteriorly and the parietal pleura laterally. It is continuous with the neck and the anterior and posterior mediastinum.

It basically contains all the great vessels (aortic arch, left subclavian, left coratid, bifercation of the superior vena cava) of the heart but not the heart itself, plus the superior portion of the esophagus, trachea at the bronchial bifercation point, thymus, left and right vegus nerves, phrenic nerves, left recurrent laryngeal nerve, and the thoracic duct.


 * 4. Identify the borders of the posterior mediastinum.**

Posterior mediastinum is bordered anteriorly by the pericardial sac, and posteriorly by the anterior surface of the vertebral bodies, and laterally by the parietal pleura.


 * 5. List the major vessels that occupy the posterior mediastinum.**

Descending aorta Esophagus Azygous system of veins (azygous, hemiazygous, and accessory hemiazygous) Vagus nerve Thoracic duct Lymph Nodes Thoracic splanchnic nerves


 * 6. Define the origin, course, and termination of the thoracic duct.**

The cisterna chyli is the dialation of origin of the thoracic duct in the abdomen. It ascends through the aortic hiatus of the diaphram and between teh azygous vein and aorta to the T4-5 level. There it crosses to teh left at the T4-5 level to ascend behind the esophagus into the neck where it joings conlfuence of internal jugular and subclavian veins to drain all of the body below the diaphragm and the left side of the thorax.

The right lymphatic duct has 3 branches: (1) right jugular which drains the right side of the head and neck (2) subclavian which drains the right upper limb (3) bronchomediastinal which drains the right side of the thorax


 * 7. Describe the path of the esophagus and trachea through the thoracic cavity.**

__Trachea__ Trachea courses from teh lyranx to the bifurcation at T4-5 in the superior mediastinum. It is flanked anteriorly by the great vessels and thymus, posteriorly by the esophagus, right by the azygous vein, and leftby the aortic arch.

The carina divides the bronchi; the left broncus is anterior to the aorta while the right bronchus is inferior to the azygous arch. The trachea is supplied by the bronchial arteries and inferior thyroid artery. It is innervated by the pulmonary plexus and branches of the vagus and sympathetic trunk.

__Esophagus__ Courses through the superior and posterior mediastinum and is continuous with the pharynx of the neck. It pierces the diaphragm at T10 to join the stomach. The thoracic portion is slightly to the left because of the trachea but pushed towards the midline by the aortic arch.

Bound anteriorly to the trachea,left bronchus, and pericardium of the left atrium. Bound posterior to the vertebral bodies, thoracic duct, azygous system, and intercostal arteries. Flanked on the right by the termianl azygous. Flanked on the left by the aortic arch as it crosses.

Constriced at the superior end in the neck, where the aorta and left bronchus compress it, and near the gastric end.

Bloiod supply by the esophageal branches of the thoracic artery anastomosing with the inferior thyroid artery and left gastric artery. Blood is drained by the submucosal plexus and surface plexus into the azygous system; it may drain superiorly to the inferior thyroid vein or inferiorly to the gastric vein.

The striated muscle is innervated by the regurrent branch of the vagal nerve. Smooth muscle is innervated by parasympathetic vagal. The esophageal plexus surroudns the lower thoracic esophagus and contains mostly vagal fibers and some sympathetic fibers.


 * 8. Describe the azygous system of veins**

The azygous system of veins drains intercostal veins,and some lumbar segmental veins. It contains the azygous, hemiazgyous (T9-T11), and accessory azygous (T4-T8) veins.

The azygous vein is formed by the union of the right ascending lumbar vein and right subcostal vein. It ascends along the right side of the vertebral column to T4 and is superficial to the intercostal artery and right of the thoracic duct. It arches over the broncus to join the superior vena vaca. Its tribulatires include the right posterior intercostal vein 5-11, right superior intercostal vein 2-4, hemiazygous vein, accessory hemiazygous vein, and esophageal, mediastinal, pericardial, and bronchial veins.

The hemiazygous vein is formed by union similar to the azygous, ascending left of the vertebral column and crosses over to join the azygous at T8-T9. It is posterior to the aorta and esophagus. Its tributaries include the left posterior intercostal veins 7-11, and the esophageal, mediastinal, and pericardial veins.

The acccessory hemiazygous vein is formed by posterior intercostals 4-8 and is to the left of the vertebral column. It crosses to join the azygous at T8.


 * 9. Describe and contrast the distribution of the vagus and phrenic nerves throughout the thoracic cavity.**

__Phrenic Nerve__ Supplies motor fibers to all of the diaphragm and sensory fibers to the parietal pleura, pericardium, and central diaphragm. The left phrenic descends along the right side of great veins (right brachiocephalic vein, superior vena cava, right atrium, inferior vena cava, and anterior root of the lung). The left phrenic nerve courses along the left side of th subclavian artery, crosses the left side of the aortic arch anterior to the root of the lung and along the left side of the pericardial sac.

__Vagus Nerve__ Preganglionic parasympathetic fibers for thoracic and abdominal viscera.

The right vagus enters thorax posterior-lateral to the barchiocephalic vein and anteriorto the right subclavian artery. The right recurrent branch hooks under the subclavina artery and ascends near the trachea and the esophagus. It descends to the lateral side of the trachea posterior to the root of the lung and descends posterior to the esophagus to form a plexus. It then passes through the diaphragm with the esophagus as the posterior vagal trunk.

The left vagus enters the thorax between carotid and subclavian arteries and descends to cross the left sideof the aortic arch. The recurrent branch recurves around the ligamentum arteriosum to descend in the neck between the trachea and esophagus. It courses behind the root of the lung and onto the anterior esophagus to form a plexus. It pierces the diaphragm with esophagus as anterior vagal trunk. The branches of the vagus in the thorax include the left recurrent lyrangeal nerve, the branch to the pulmonary plexus, esophagus, and cardiac plexus.


 * 10. Discuss the relationship of the heart, pericardium, lungs, and pleura to the structures in the posterior mediastinum.**

The heart and pericardium are anterior to the posterior mediastinum. The lungs and their pleura are not part of the mediastinum but are located to either side of the posterior mediastinum.