The anterior mediastinum is narrow in the middle, where the edges of the lungs nearly meet, wider above, where the lungs diverge, and widest of all below, for the same reason. It is very shallow from before backward, and it is limited posteriorly by the anterior layer of the pericardium, in front by the sternum, with the fifth, sixth, and a small portion of the seventh costal cartilages, and by the triangularis sterni muscle. The region is occupied simply by connective tissue, save in its upper part, where lies, when it still persists, the shrivelled remnants of the thymus body. It also contains a few lymphatic glands and the left internal mammary artery and vein.
The superior mediastinum is bounded by a plane passing through the lower part of the body of the dorsal vertebra behind and the junction of the manubrium and the gladiolus in front. Its upper limit corresponds to the superior aperture of the thorax. The contents of this space are the transverse portion of the arch of the aorta and its three large branches, the trachea and oesophagus, the thoracic duct, the innominate veins, upper part of the superior vena cava, left recurrent laryngeal nerve, phrenic, pneumogastric, and cardiac nerves, with lymphatic glands and remains of the thymus body.
The posterior mediastinum is triangular in shape, placed in front of the lower border of the fourth dorsal vertebra downward, and bounded anteriorly by the pericardium and roots of the lungs. The lateral boundaries are formed by the pleuræ. The space contains the descending thoracic aorta: in front of the aorta the oesophagus with the pneumogastric nerves, the left in front, the right behind. On the right of the aorta is the vena azygos major; between this vein and the aorta is the thoracic duct; superiorly is the trachea; inferiorly are the splanchnic nerves and the posterior mediastinal lymphatic glands.
DEFINITION.—There are three principal forms of morbid growths in the mediastina—sarcoma, lymphoma or lymphadenoma, and carcinoma. Hyperplasia of the mediastinal glands also may arise, intertwined with various diseases, such as phthisis (especially the form known as pneumonic), pertussis, aneurism, rachitis, and syphilis. Enlargement of the lymphatic glands may occur in connection with the scrofulous diathesis, or similar enlargement associated with primary subacute or chronic bronchitis and the varieties of catarrhal fever and influenza.
Allusion in this place will only be made to the rare instances in which uncomplicated enlargement of the thoracic glands occurs in the mediastinal spaces. Aneurism, abscess, and pericardial effusions will be referred to only in so far as they affect differential diagnosis.
Mediastinal tumors, however, include certain forms which have the interest of pathological curiosities rather than possessing a clinical importance. Cysts in this region are rare, mostly of embryonic origin (dermoid), and contain epithelial structure, such as hair, sebaceous and sweat-glands, teeth, and occasionally bone, cartilage, and other tissues. These cysts often develop rapidly and may attain great size. Lipomata8 occur as the result of an undue increase of the mediastinal fat, and are associated with accumulation of the same in the pericardium and in the system at large. Such tumors are rare and of very gradual development. Kronlein9 has described a congenital lipoma of the anterior mediastinum in a child aged one year, which found its way through an intercostal space and then rapidly increased in size. Fibromata, osteomata, and enchondroma are also possible mediastinal and pulmonary tumors, but are seldom met with. Exostoses may form upon the internal surface, and gummata upon the anterior and posterior surfaces of the sternum.
8 Reigel, Virchow's Arch., vol. xlix.
9 Langenbeck, Klinic, p. 157.
PATHOLOGY AND MORBID ANATOMY.—Pulmonary processes associated with bronchial catarrh frequently lead to enlargement of the bronchial glands, because, owing to the impervious character of the basement membrane of the bronchial passages, the mucous and epithelial portion of the exudation is expectorated, and that portion of the exudate which occurs from the bronchial blood-vessels is absorbed and carried by means of the pulmonary lymphatics to the bronchial glands. Tubercular deposits frequently occur in the glands of the posterior, and much less frequently in those of the anterior, mediastinum.
Independently of the above conditions, caseating bronchial glands have been found as complications of scarlatina with nephritis or tubercular meningitis. An interesting case of this condition has been reported as following an abscess in the glands at the root of the neck as a sequel to measles nine months before.10 Riegel also mentions an instance in which some of the mediastinal glands were enlarged to the size of hen's eggs. The trachea was compressed at the point of bifurcation, so that its calibre was reduced to one-third its natural size. This case was free from other glandular enlargements. Coupland has described a case in a boy four years of age, in whom the cervical glands were enlarged and idiopathic hyperplasia of the bronchial glands was suspected. Autopsy: On raising the sternum a collection of indurated glands was found in the anterior mediastinum, and over the root of the right lung one of these glands had broken down into a cheesy mass. A chain of enlarged lymphatics accompanied the right bronchus. The largest caseous mass had ulcerated through the trachea just above the origin of the right bronchus by an aperture measuring half an inch along the axis of the tube, while for half an inch above its lumen was compressed. In this case the right lung was solidified and contained cheesy matter, with a cavity at the apex. The father of the child had also suffered from increase in the glandular tissues.