500 PREPARATION OF THE THORAX. 



consider that embracing the pericardium as a middle medias- 

 tinum. 



The oesophagus is in contact with the right lamen ; in its 

 progress downwards, it inchnes to the left side and is advanced 

 before the aorta. 



The vena azygos appears posterior to the oesophagus ; it 

 proceeds upwards until it is as high as the right branch of the 

 windpipe : here it bends forward, round that branch, and opens 

 into the superior cava, before that vein opens into the right 

 auricle. 



The thoracic duct proceeds upwards from below, lying in the 

 space between the aorta and the vena azygos, until the begin- 

 ning of the curve of the aorta, when it inclines to the left, pro- 

 ceeding towards the place of its termination. 



— The anterior and posterior mediastina are formed as is 

 shown above, by the layers of the pleura, between the sternum 

 and pericardium, and between the pericardium and spine. 

 But the pericardium does not extend the whole length of the 

 thoracic cavity ; it terminates about two inches short of the top 

 of the sternum, and at this part, there being nothing interposed 

 to divide the layers into an anterior and posterior portion, they 

 pass directly from the vertebrae to the sternum, and constitute 

 what is called the Superior Mediastinum. The two layers con- 

 stituting this, continuous below with the anterior and superior 

 mediastina, and each lining the upper margin of the first rib, so 

 as to form a conical pouch projecting a slight distance above 

 the middle of the clavicle, constitute a triangular cavity, the 

 base of which is upwards, and corresponds to the root of the 

 neck. This cavity contains the thymus gland, the arteria 

 innominata, the primitive carotid and subclavian of the left side, 

 the superior vena cava, the trachea, oesophagus, and par vagum 

 nerves. 



— The sympathetic nerve is not contained in this mediastinum ; 

 it passes a little to the outside of the posterior external angle 

 of it. — 



The formation of the mediastinum, and the arrangement of the pleura, as well 

 as the connexion of these membranes with the parts contained in the thorax, 



