MEDIASTINAL GEOWTHS. 333 



Accessory thyroids are rarely found in the superior mediastinum, 

 and dermoid cysts sometimes lie behind the mesosternum. Gum- 

 mata may also grow in mediastinal tissues. 



The mediastinum is easily invaded along the connective tissue 

 sheaths of the bronchi by new growths which originate in the 

 lung. The bronchial connective tissue sheaths contain lymphatic 

 vessels and blood-vessels, and are directly continuous with the 

 mediastinal connective tissue. Pulmonary new growths may 

 also give rise to detached secondary mediastinal growths, which 

 consist of infected lymphatic glands. 



A primary carcinomatous growth is theoretically most likely to 

 originate in the posterior part of the superior mediastinum or in 

 the posterior mediastinum. The presence of the epithelial 

 structures of the oesophagus, trachea, bronchi, and thoracic duct 

 in these situations accounts for this. Sarcomata arise chiefly 

 from the mediastinal lymphatic glands, but may grow also from 

 the periosteal boundaries of the space, that is from the vertebrae, 

 ribs, or sternum. 



The positions and relations of the glandular groups are of 

 great importance in the consideration of the pressure symptoms 

 induced by mediastinal tumours. 



The cardiac group of glands lies on the front of the arch of 

 the aorta, and is therefore in the superior mediastinum. Isolated 

 glands prolong this group upwards on each side to the termina- 

 tions of the right and left lymphatic ducts that is, to the 

 junction of the internal jugular and subclavian veins. The 

 main mass of glands lies in close relation with the innominate 

 veins and the three great branches of the aortic arch. The 

 trachea lies posterior to them. The upward prolongation of the 

 right side follows the superior vena cava. Enlargement of the 

 glands may cause venous obstruction, inequality of pulses, and 

 dyspnoea. 



The posterior mediastinal glands are grouped round the 

 oesophagus. Some are between the oesophagus and the peri- 

 cardium, and some lie close to the thoracic duct. These glands 

 are particularly likely to suffer in malignant disease of the gullet. 



