Posterior Mediastinum

The posterior mediastinum extends from the pericardium and roots of the lungs anteriorly to the vertebrae posteriorly. The pleura' are on each side. Behind the pericardium runs the oesophagus, lying in front of the aorta, which rests on the vertebrae. In the chink between the aorta and bodies of the vertebrae lies the thoracic duct and immediately to its right side is the vena azygos major. The vena azygos minor is on the left side of the vertebrae and crosses the sixth to join the vena azygos major. The descending thoracic aorta is not infrequently the seat of aneurism.

* The mediastina are arbitrary divisions, and it is a question as to whether the roots of the lungs should not be included in the posterior instead of the middle mediastinum.

Mediastinal Tumors

Cancer is the most frequent malignant new growth, then sarcoma and lymphoma. Great enlargement of the lymph-nodes occurs in Hodgkin's disease and is probably a factor in causing a fatal issue. Enlargements also result from tuberculosis and other diseases. They give rise to pressure symptoms. Dyspnoea may be due to pressure on the trachea or heart and great vessels. The circulation may be so much impeded that the enlargement of the collateral veins, especially those of the surface, may be very marked. There may also be difficulty of swallowing due to pressure on the oesophagus.

Pleural Effusions

Serous effusions into the pleurae are also known to accompany heart disease and have been attributed in some instances to obstruction of the circulation. They are apt to be unilateral and are most often found affecting the right pleural cavity. Baccelli attributed the effusion to obstruction of the blood current through the vena azygos major; the enlarged heart pulling the superior vena cava down drew the vena azygos majpr tightly over the right bronchus, as is well shown in Fig. 210. Steele ( Univ. Med. Mag., 1897; Journ. Am. Med. Asso., 1904) and Stengel ( Univ. Penna. Med. Bulletin, 1901) held that the dilated right heart by extension upward exerts pressure on the root of the right lung and indirectly pinches the azygos major vein as it curves over the right bronchus to enter the superior vena cava. Fetterolf and Landis (Am. Journ. Med. Sciences, 1909) believe that the fluid comes from the visceral pleura and not from the parietal pleura, and that the outpouring, so far as the pressure factor is concerned, is caused by dilated portions of the heart pressing on and partly occluding the pulmonary veins. They point out that Miller (Am. Journ. of Anat., vii) has shown that the veins draining the visceral pleura empty into the pulmonary veins; therefore, if these latter are obstructed, transudation may ensue; this may occur on either side. They point out that if the right atrium (auricle) dilates, it expands upward and backward and compresses the left auricle and root of the right lung; and of the parts composing the root the pulmonary veins are the most anterior, and, therefore, the ones most liable to be compressed. Left-sided effusions are accounted for by compression of the left pulmonary vein by the dilated left atrium (which is the most posterior of the four chambers) and its appendix. The greater frequency of right-sided effusions is due to the more common occurrence of dilatation of the right side of the heart.

(W. S. Handley (Brit. Med. Journ., Oct. 1, 1904) claims that the principal method of dissemination of carcinoma of the breast is not by the lymph stream or blood current but by spreading peripherally along the coarser meshes of the lymphatic channels which exist in the deep pectoral fascia. These are continuous downward with the surface of the recti muscles.

He therefore advises that the lower end of the usual skin incision be prolonged downward and inward so "that every particle of the origin of the great pectoral from the rectus sheath, and the surface of the latter, on both sides of the middle line, should be most carefully cleared" as far as two to three inches below the tip of the ensiform cartilage).