Anterior Mediastinum

This is the space below the second costal cartilages, between the sternum in front, the pericardium behind, and the two pleurae on the sides. It is only a narrow slit in the median line above from the second to the fourth costal cartilage; from here the right pleura is prolonged obliquely down and outward to the seventh costal cartilage, which it follows. On the left side the pleura leaves the median line about the fourth cartilage and passes out about 2 cm. to the left of the sternum and then down to the seventh costal cartilage, which it follows. The triangularis sterni muscle arises from the under surface of the lower third of the sternum and from the xiphoid cartilage and passes upward and outward to insert into the costal cartilages of the second to the sixth ribs inclusive. The muscle lies in front of the anterior mediastinum and the internal mammary artery runs down between it and the bone about 1 cm. distant from the edge of the sternum. There are a few lymphatic nodes in the anterior mediastinum on the diaphragm below and in the superior mediastinum on the arch of the aorta and left innominate vein above. A chain of nodes also accompanies the internal mammary artery along the edge of the sternum between the pleura and chest wall.

Abscess of the anterior mediastinum may result from infection due to injury or punctured wounds. It may break into the pleurae on the sides, into the pericardium posteriorly, work its way down toward the abdomen, or point in the intercostal spaces at the edge of the sternum.

Paracentesis pericardii is performed in the sixth interspace close to the sternum; also, the fifth and sixth cartilages may be resected, the internal mammary artery ligated, and the pericardium opened and even drained.

If one attempts to pass a trochar into the pericardium by a puncture through the fifth or sixth interspace sufficiently far out to avoid wounding the internal mammary artery the pleura is apt to be wounded, as it passes farther toward the median line than does the lung.

The Middle Mediastinum

The middle mediastinum is limited in front by the anterior wall of the pericardium and behind by the posterior wall of the pericardium and roots of the lungs. It contains the heart with the lower half of the descending vena cava and the vena azygos major emptying into it, and the ascending aorta;

Fig. 210.   Contents of the mediastina viewed from the rear.

Fig. 210. - Contents of the mediastina viewed from the rear.

also the structures forming the roots of the lungs, viz., the right and left bronchi, and the pulmonary arteries and veins.* The phrenic nerves lie between the pericardium and pleurae anteriorly.

The bronchial lymphatic nodes are numerous between the structures forming the roots of the lungs. It is these nodes that are so often enlarged in diseases of the lungs. They are affected in malignant disease as well as in tuberculosis, etc. Enlargements of the heart pressing on the vessels, particularly the vena azygos major, are sometimes thought to cause pleural effusions, especially if one-sided.

When the pericardium is distended with fluid it enlarges more in an up and down direction, but when the heart is enlarged its size increases mainly laterally - from side to side.