The mediastinum is the middle space of the chest between the spine behind, the sternum in front, and the pleurae to each side. It is subdivided into a superior mediastinum, which is the part above Ludwig's angle, between the first piece of the sternum in front and the vertebrae from the first thoracic to the upper portion of the fifth behind. The part below is divided into the anterior mediastinum, the middle mediastinum, and the posterior mediastinum.

Superior Mediastinum

The upper level of the superior mediastinum is oblique, as it runs from the upper edge of the sternum to the first thoracic vertebra. The lower level of the superior mediastinum runs from the junction of the first and second pieces of the sternum to the upper border of the fifth (or lower border of the fourth) thoracic vertebra. Laterally it is bounded by the pleurae and apices of the lungs.

The distance from the anterior surface of the spine to the posterior surface of the sternum is quite small, being only 5 to 6 cm. (2 to 2 1/4 in.). Through this pass most important structures. The trachea and ocsophagus are in the median line as well as the remains of the thymus gland. To each side are the great vessels, the innominate artery being on the right and the subclavian and carotid on the left. The left innominate vein crosses transversely just below the top of the sternum to meet the innominate vein of the right side and form the superior vena cava. Into the innominate veins empty the inferior thyroid, vertebral, superior intercostal, internal mammary, and pericardial veins; and into the descending vena cava empties the vena azygos major. On the posterior surface of the oesophagus and afterwards to its left side passes the thoracic duct. The trachea bifurcates opposite the junction of the first and second pieces of the sternum, and the transverse portion of the arch of the aorta rises as high as the middle of the manubrium. The phrenic nerves lie against the pleura, the right having the vena cava to its inner side.

Fig. 208.   The superior (red), anterior (blue), middle (yellow), and posterior (green), mediastina.

Fig. 208. - The superior (red), anterior (blue), middle (yellow), and posterior (green), mediastina.

(Modified from Piersol).

The right vagus (pneumogastric) nerve comes down between the innominate artery and vein and passes downward on the posterior surface of the oesophagus. It gives its recurrent laryngeal branch off at about the right sternoclavicular joint. The left vagus nerve comes down to the outer side of the left carotid artery and goes over the arch of the aorta, giving off its recurrent laryngeal branch, and thence proceeds to the anterior surface of the oesophagus. The presence of the trachea in the median line and the edges of the lungs which meet opposite the second rib give a resonant percussion note to the first piece of the sternum. With all these important structures crowded in the small space between the vertebrae and sternum it is easy to see why tumors in this region should cause serious symptoms.

Aneurism involving the arch of the aorta and the great vessels is common. Tumors, such as sarcoma, carcinoma, and glandular, though rare, do occur. Abscess from high dorsal Pott's disease has been known to cause serious effects.

The symptoms of all these affections resemble one another to a considerable extent. Interference with the blood-current, usually in the veins, almost never in the arteries, is marked. Alteration in the voice is produced by pressure on the recurrent laryngeal nerves. Dyspnoea from the pressure on the trachea and difficulty in swallowing also occur, as well as interference with the circulation and the action of the heart. The presence of growths in this region is indicated also by the presence of dulness over the region of the manubrium.

Fig. 209.   Contents of the mediastina viewed from the front.

Fig. 209. - Contents of the mediastina viewed from the front.