The mediastinum is the central partition that lies between the two pleura cavity. Therefore it is covered with the mediastinal pleura. It is bounded anteriorly by the sternum, posteriorly by the body of the second thoracic vertebra, on each side (laterally) by the mediastinal pleura, superiorly by the plane of the thoracic inlet and inferiorly by the plane of the thoracic outlet which is flood in by the diaphragm. The mediastinum contains only small amount of loose connective tissue with space in between the mediastinal structures. It is a highly mobile region.
DIVISION
The mediastinum is divided into two parts by a plane passing behind the manubruosternal joint to the lower border of T4 (in between the intervertebral disc between T4 and T5). The part that lies above is known as the superior mediastinum while the lower part is the inferior mediastinum.
The inferior mediastinum is further divided into three compartments by the fibrous pericardium which includes the anterior, middle and posterior mediastinum by the serous pericardium.
SUPERIOR MEDIASTNUM
It is bounded anteriorly by the manubrum of the sternum and posteriorly by the upper four thoracic vertebrae, on each side is the mediastinal pleura, superiorly by the plane of the thoracic inlet and inferior by the plane passing between the manubrosternal joint and lower border of the fourth thoracic vertebra.
The posterior wall is longer and more concave than the anterior wall therefore it is wedged-shaped.
CONTENTS OF THE SUPERIOR MEDIASTINUM INCLUDE:
1. Lower end of the sternohyoid, sternothyroid and longus coli muscles.
2. Some of the great veins of the thorax which include; the right and left brachiocephalic vein which are formed by the union of the internal jugular vein and the subclavian vein of the corresponding side, superior vena cava formed by the union of the right and left brachiocephalic vein.
3. The arch of the aorta and its three great branches. The left subclavian artery, the right brachiocephalic trunk, and the left common carotid artery.
4. Left superior intercostal vein.
5. The remnants of the thymus
6. The trachea
7. Oesophagus
8. Thoracic duct
9. Tracheobronchial, bronchopulmonary and brachiocephalic lymph nodes
10. The superficial part of cardiac plexus.
11. The vagus nerve and left recurrent laryngeal nerve.
12. The third phrenic nerve.
13. The right lymphatic trunk.
ANTERIOR MEDIASTINUM
It is bounded between the body of sternum and the anterior wall of the serous pericardium. It narrows above the level of the 4th costal cartilage, where the two pleura come close to each other at the median plane, below this level the left pleura deviates from the right pleura.
CONTENTS OF ANTERIOR MEDIASTINUM INCLUDE:
1. Some loose areolar tissues.
2. Few lymphatic vessels.
3. Mediastinal branches of the internal thoracic artery.
4. The sternopericardial ligament
5. Sometimes the thymus especially during the pubertal ages.
THE MIDDLE MEDIASTINUM
It is bounded anteriorly and posteriorly by the fibrous pericardium.
CONTENTS OF THE MIDDLE MEDIASTINUM:
Structures lying within the pericardium include:
1. The ascending aorta.
2. The pulmonary trunk.
3. The superior vena cava.
4. Azygous vein as it empties into the superior vena cava.
5. Right and left pulmonary veins.
6. The heart.
7. Thoracic part of the inferior vena cava.
Other structures include:
1. The right and left phrenic nerve, which is accompanied by pericardiophrenic vessels.
2. The bifurcated end of the trachea and main bronchus.
3. The tracheobronchial lymph nodes.
4. The deep part of the cardiac plexus.
THE POSTERIOR MEDIASTINUM
The posterior mediastinum and the anterior mediastinum are continues superiorly with the superior mediastinum. Its division is more or less descriptive that anatomical. The posterior mediastinum is bounded in front, from above downward, by the bifurcate end of the trachea, the posterior surface of the pericardium and the upper surface of the posterior part of the diaphragm. Posteriorly it is bounded by the bodies of the fifth to twelfth thoracic vertebrae.
CONTENTS OF THE POSTERIOR MEDIASTINUM
1. The descending thoracic aorta and its intercostal aortic branches.
2. The oesophagus and the anterior and posterior vagus nerve that accompanies it.
3. The thoracic duct.
4. Azygous system of veins.
5. Splanchnic nerve.
6. Posterior intercostal lymph nodes.
APPLIED ANATOMY
Mediastinum
The loose space of the neck is in continuity with the mediastinum and it is limited by fascia, the pre-vertebral and pre-tracheal fascia which extends from the neck into the superior mediastinum. The pre-tracheal fascia blends with the arch of the aorta while the pre-vertebral fascia blends with the 4th thoracic vertebra. Therefore infection occurring anterior to the pre-tracheal fascia spreads downward into the anterior mediastinum while infection occurring posterior to the pre-vertebral fascia is limited to the superior mediastinum between the fascia and the posterior wall. Infection occurring in between the fascia spreads down into the posterior mediastinum.
2. MEDIASTINAL RADIOLOGY
The anterior posterior radiograph of the mediastinum presents a mediastinal shadow which is created by the heart and the structure passing to and fro it. The structures that from the boundary of the left shadow include the left subclavian artery, the arch of the aorta which is referred to as aortic knuckle by the radiologist, the left auricle and the left pulmonary vein while on the right side are the terminal end of the right brachiocephalic vein, superior vena cava, the right atrium and the thoracic part of the inferior vena cava.
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