332 CLINICAL APPLIED ANATOMY. 



occur from the adjacent vertebrae, ribs, or sternum. Occasionally 

 dermoids and hydatids of the mediastinum suppurate. Pus may 

 reach the spaces from above by burrowing down in the retro- 

 pharyngeal space or around the trachea. When the mediastinitis 

 is secondary to perforations of the oesophagus or of cesophageal 

 pouches, it is especially likely to become gangrenous. 



The course of the pus is determined by anatomical relations. 

 It may invade the subpleural tissue or the interstitial tissue of 

 the lung by continuity. It may burst into the oesophagus or air 

 passages, or into the pleural or pericardial sacs. Lying around 

 the large blood-vessels it may soften and erode their walls. The 

 connective tissue planes may direct it to the root of the neck, or 

 it may come forwards in the interchondral spaces by tracking 

 along the investing sheaths of the anterior intercostal vessels. 

 The continuity of the mediastinal tissue with the subperitoneal 

 tissue around the aorta may direct the abscess through the 

 aortic opening of the diaphragm, or anteriorly it may pass 

 through the interval between the sternal and costal portions of 

 the same muscle. 



Should an attempt be made to open the mediastinum from the 

 back it is necessary to bear in mind the relations the pleural 

 sacs bear to it. Below the eighth dorsal vertebra on the right 

 side a small cul-de-sac of pleura intervenes between the 

 oesophagus and the spinal column. On the left side the posterior 

 mediastinal pleura passes direct from the left side of the aorta 

 to the bodies of the vertebrae. Hence there is less risk of 

 opening the pleural sac if the space be approached from the left 

 side of the vertebral column in this region. 



MEDIASTINAL GROWTHS. 



There are many structures in or near the mediastina from 

 which new growths may originate. The most important are 

 the groups of lymphatic glands, the oesophagus, the trachea, and 

 bronchi. Tumours may arise in the residue of the thymus or 

 spring from the periosteum of the vertebras, ribs, or sternum. 



