As we know, the mediastinum is divided into the superior mediastinum and the inferior mediastinum.The superior mediastinum is separated from the inferior mediastinum by the transverse thoracic plane, which is a horizontal plane that extends from the sternal angle anteriorly and the junction of T4 and T5 posteriorly, an important anatomical landmark representing the bifurcation of the trachea and the beginning and end of the arch of the aorta.The inferior mediastinum lies between the transverse thoracic plane superiorly, and the diaphragm inferiorly, and is further subdivided by the pericardium into anterior, middle and posterior parts.So first, let’s look at the posterior mediastinum.It is located anterior to the T5 through T12 vertebrae, posterior to the pericardium and diaphragm, inferior to the transverse thoracic plane and between the parietal pleura of the lungs.The posterior mediastinum contains the thoracic aorta, thoracic duct and lymphatic trunks, posterior mediastinal lymph nodes, azygos and hemiazygos veins, the esophagus and the esophageal nerve plexus.The thoracic aorta continues the aortic arch and it begins on the left side of the inferior border of the body of T4, moving downwards in the posterior mediastinum on the left side of T5 through T12 vertebrae.On its way down, the thoracic aorta is close to the median plane and moves the esophagus towards the right.The thoracic aorta lies posterior to the root of the left lung, pericardium and esophagus and is surrounded by the thoracic aortic plexus.Anterior to the inferior border of the T12 vertebra it continues on as the abdominal aorta and enters the abdomen through the aortic hiatus in the diaphragm.The thoracic duct and azygos vein climb on the right side of the thoracic aorta and accompany it through the aortic hiatus.The vascular branches of the thoracic aorta are: the esophageal arteries, which supply the esophagus, the bronchial arteries which supply the lungs, the 9 pairs of posterior intercostal arteries that supply all the intercostal spaces except for the upper two, and the subcostal arteries which supply the muscles and skin below the 12th rib.Other branches include the superior phrenic arteries, which supply part of the diaphragm and pericardial branches, which help supply the pericardium.Now, the posterior mediastinum also contains the inferior portion of the esophagus as it continues its course from the superior mediastinum.In the posterior mediastinum, the esophagus passes posterior and to the right of the aortic arch and posterior to the pericardium and left atrium, so it’s the main structure posterior to the base of the heart.Then, it deviates to the left and passes through the esophageal hiatus in the diaphragm at the level of T10, and at this point it’s anterior to the aorta.Remember that the thoracic aorta passes through the diaphragm at the level of T12, and the esophagus passes at level T10.It’s also important that the inferior vena cava ascends through the diaphragm at level T8.To remember these anatomical landmarks, think of the phrase “I ate 10 eggs at 12”.The ‘I’ represent the IVC at ’ate’, the eggs represent the esophagus at T10, and the at 12 is aorta at T12!Also remember, the phrenic nerve pierces level T8 along with the IVC, the vagus nerve pierces T10 along with the esophagus, and the thoracic duct pierces at level T12 along with the aorta.Now, the esophagus may have 3 anatomic impressions or constrictions in the posterior mediastinum caused by three structures it meets on its way down: the aortic arch, the left main bronchus and the diaphragm.These narrowings can be seen in oblique chest radiographs if a person were to undergo a barium swallow study.Also know that there is another potential constriction of the proximal esophagus prior to it entering the posterior mediastinum at the level of the cricoid cartilage.Remember that the surface esophagus is covered by the esophageal plexus, which is derived from the left and right vagus nerves as they descend through the superior and posterior mediastinum.The esophageal plexus goes on to form the anterior and posterior vagal trunk found on the inferior esophagus just before the esophagus passes through the diaphragm.These vagal trunks pass through the diaphragm with the esophagus and go on to innervate a large part of the GI tract.Let’s now tackle the veins of the azygos venous system.It’s located on each side of the vertebral column and it drains the back, the thoracoabdominal walls, and the mediastinal viscera.Now, this system contains the azygos vein, the hemiazygos vein and accessory hemiazygos veins.So, the azygos vein travels proximally up the posterior mediastinum and forms a collateral pathway from the posterior aspect of the inferior vena cava at the level of the renal veins.In the posterior mediastinum, the azygos vein passes close to the right side of the bodies of the inferior 8 thoracic vertebrae posterior to the root of the right lung.Then it arches over the superior aspect of the root of the right lung to join the superior vena cava.The azygos vein also receives the mediastinal, esophageal and bronchial veins.Now, the hemiazygos vein originates on the left side of the vertebral column, posterior to the thoracic aorta as far as the T9 vertebra.At this point, it crosses to the right, posterior to the aorta, thoracic duct and esophagus and joins the azygos vein.It receives the inferior three posterior intercostal veins, the inferior esophageal veins and many small mediastinal veins.The accessory hemiazygos vein originates at the medial end of the 4th or 5th intercostal space and goes down on the left side of the vertebral column from T5 through T8.It receives blood from veins in the 4th through 8th intercostal spaces and sometimes even from left bronchial veins.It crosses over the T7 or T8 vertebra, posterior to the thoracic aorta and thoracic duct, at which point it joins the azygos vein.Okay, let’s now talk about the thoracic duct, which is the largest lymphatic channel in the body and conveys most of the lymph in the body to the venous system, specifically all lymph except from the right superior quadrant of the body.In the posterior mediastinum, it lies on the anterior part of the bodies of T12 to T5.The thoracic duct originates from the cisterna chyli in the abdomen and enters the mediastinum through the aortic hiatus in the diaphragm at the level of T12.It then goes up in the posterior mediastinum among the thoracic aorta on its left, the azygos vein on its right, the esophagus anteriorly and the vertebral bodies posteriorly.At the level of T4, T5 or T6, the thoracic duct crosses to the left, becoming posterior to the esophagus and enters the superior mediastinum.The thoracic duct typically empties into the venous system close to the joining of the left internal jugular and subclavian veins at a point called the left venous angle, or origin of the brachiocephalic vein.Now, the lymph nodes of the posterior mediastinum lie posterior to the pericardium, close to the esophagus and thoracic aorta.Some nodes lie posterior to the inferior part of the esophagus, but most of them lie anterior and lateral to it.The posterior mediastinal lymph nodes receive lymph from the esophagus, the posterior aspect of the pericardium and diaphragm and the middle posterior intercostal spaces.Lymph eventually collects into the thoracic duct from these nodes on their way to the left venous angle, or depending on location lymph may also drain into the right lymphatic duct to drain into the right venous angle.Next up, there are the nerves of the posterior mediastinum; namely, the thoracic sympathetic trunks and their ganglia, which form a major portion of the autonomic nervous system.In the superior part of the thorax, the thoracic trunks lie against the heads of the ribs.In the mid thorax, they lie against the costovertebral joints and in the inferior part of the thorax, they lie on the sides of the vertebral bodies.The sympathetic trunk has one sympathetic ganglion for each thoracic vertebral level and each ganglion is connected to their respective intercostal nerves via two rami communicantes.These two rami are the afferent white rami and efferent gray rami.White rami carry preganglionic sympathetic fibers from the spinal nerves to the sympathetic ganglia, while gray rami carry postganglionic sympathetic fibers from the sympathetic ganglia to the spinal nerves.Preganglionic sympathetic fibers only originate from the T1 to L2 spinal levels, so there are no white rami above T1 or below L2/L3, so sympathetic fibers ascend or descend the sympathetic trunk in order to provide sympathetic innervation from our heads down to our toes.This is possible because the thoracic sympathetic trunk is continuous with the cervical trunk above and lumbar sympathetic trunk below, both which don't have white rami communicantes.Now, distally the sympathetic trunks give rise to the lower thoracic splanchnic nerves, specifically the greater from T5 to T9, lesser from T10 and T11, and least splanchnic nerves coming from T12, which descend into the abdomen and supply the abdominal viscera.They have presynaptic fibers from the 5th through the 12th sympathetic ganglia that pass through the diaphragm and synapse in prevertebral ganglia in the abdomen, which go on to supply abdominal viscera below the diaphragm.Okay, let’s take a quick break and see if you can remember what are the limits of the posterior mediastinum and the structures it contains.Next, let’s focus on the middle mediastinum, which includes the pericardium, heart and roots of its great vessels, such as the ascending aorta, pulmonary trunk and superior vena cava.Let’s take a look at the pericardium, which is a fibroserous membrane that covers the heart and the beginning of its great vessels.It’s a closed sac with 2 layers, an external one called the fibrous pericardium and an internal one called the serous pericardium.The external layer is continuous superiorly with the tunica adventitia of the great vessels and the pretracheal layer of the deep cervical fascia.

It’s attached anteriorly to the posterior surface of the sternum through the sternopericardial ligaments, and it is continuous inferiorly with the central tendon of the diaphragm.The fibrous pericardium is lined with a serous membrane called the parietal layer of the serous pericardium.This layer is reflected onto the heart and its great vessels as the visceral layer of the serous pericardium.Now, the visceral layer of the serous pericardium forms the epicardium, which is the external layer of the heart.It extends onto the beginning of the great vessels and becomes continuous with the parietal layer of the serous pericardium.Within the pericardial cavity, which is a potential space formed between the parietal and visceral pericardial layers, the transverse pericardial sinus is a running passage between the vessels that leave the heart, specifically the aorta and pulmonary trunk, and the vessels that enter the heart, specifically the superior vena cava, the inferior vena cava and pulmonary veins.The oblique pericardial sinus is the reflection of the serous pericardium around the vessels that enter the heart, bounded laterally by the IVC and pulmonary veins, forming a pouch that can be accessed inferiorly.The arterial supply of the pericardium comes from the pericardiophrenic artery, which is a branch of the internal thoracic artery.This branch usually accompanies or parallels the phrenic nerve to the diaphragm.Other arteries include the musculophrenic artery, which is a terminal branch of the internal thoracic artery, the bronchial, esophageal and superior phrenic arteries, which are branches of the thoracic aorta and finally, the coronary arteries which are the first branches of the aorta.The venous drainage of the pericardium is handled by pericardiophrenic veins, which are tributaries of the brachiocephalic veins, and various tributaries of the azygous venous system.The nerve supply of the pericardium mainly comes from the phrenic nerves, which are primary sources of sensory fibers and pain sensations which are referred to the skin on the C3 through C5 dermatomes of the ipsilateral supraclavicular region.Other fibers come from the vagus nerves, and the sympathetic trunks.Can you remember the course of the phrenic nerves?Correct, anterior to the lung hilum and over the pericardium towards the diaphragm.How about the vagus nerve?Correct!Behind the lung hilum to the esophagus.Finally, the anterior mediastinum, which is the smallest subdivision of the mediastinum, lies between the body of the sternum and the transversus thoracis muscles anteriorly and the pericardium posteriorly.The anterior mediastinum consists of loose connective tissue, fat, lymphatic vessels, few lymph nodes and branches of the internal thoracic vessels.In children, it also contains the inferior part of the thymus.All right, as a quick recap, the mediastinum is divided into the superior mediastinum and an inferior mediastinum which are separated from each other by the transverse thoracic plane.The inferior mediastinum is bounded inferiorly by the diaphragm, and is further subdivided into the anterior, middle, and posterior mediastinum.The posterior mediastinum is located anterior to T5 through T12 vertebrae, posterior to the pericardium and diaphragm, inferior to the transverse thoracic plane and between the parietal pleura of the lungs.It contains the following structures: thoracic aorta, thoracic duct and lymphatic trunks, posterior mediastinal lymph nodes, azygos and hemiazygos veins, esophagus and the esophageal nerve plexus.The middle mediastinum includes the pericardium, heart and roots of its great vessels, such as the ascending aorta, pulmonary trunk and superior vena cava.The anterior mediastinum lies between the body of the sternum and the transversus thoracis muscles anteriorly and the pericardium posteriorly.The anterior mediastinum consists of loose connective tissue, fat, lymphatic vessels, few lymph nodes and branches