150 ORGANS OF RESPIRATION. 



side of this mediastinum,, and, after ascending, forms an arch 

 over the root of the right lung, and terminates by joining the 

 descending cava. The thoracic duct, after entering the thorax 

 between the crura of the diaphragm, ascends in front of the 

 dorsal vertebrae between the aorta and the vena azygos, and 

 behind the oesophagus, till it reaches the third dorsal vertebra; 

 it then inclines to the left side, and mounting into the root of 

 the neck near the vertebrae? it* finally, makes an arch, which, 

 by advancing forwards, terminates in the angle formed by the 

 junction of the left internal jugular and left subclavian vein. The 

 par vagum nerve, .of both sides, is also in the posterior medi-. 

 astinum. 



3. The Superior Mediastinum is- bounded in front by the up- 

 per part of the sternum, behind by the upper dorsal vertebra^ 

 and laterally by the first ribs. The cavity is conoidal, with the 

 base upwards, but is too peculiar to admit of a rigid comparison 

 with any thing else. The pleurae are reflected downwards from 

 the internal edge of the first ribs, not abruptly, but in a rounded 

 bulging manner, receiving there, as mentioned, the tip or apex; 

 of the lungs, and lining the inferior surface of. the scalenus an- 

 ticus muscle. In order to understand well the position of the 

 pleurae, it must be borne in mind that the upper rib is placed 

 very obliquely downwards and forwards, at an angle of about 

 forty-five degrees with the spine;, consequently, the pleura, on, 

 being reflected from its whole internal edge, is much higher at 

 the head of the rib than it is at the anterior extremity of the 

 s.ame. This cavity is continuous, of course, .with that of the 

 anterior, mediastinum in front, and also with that of the poste- 

 rior mediastinum behind.. 



The remains of the thymus gland are where this cavity joins 

 the anterior mediastinum:^ part of the gland is, indeed, in each 

 of these cavities just below the transverse vein. In contact 

 with the right pleura is the -descending. Vena Cava. The com- 

 mon trunk of the Left Subclavian, and -.Internal Jugular, called 

 the Transverse Vein, .or Vena Innominata, after crossing in an 

 oblique descent behind the upper portion of the sternum, joins 

 the descending cava an inch above the place where the latter 

 penetrates into the pericardium. Behind the transverse vein 

 are the top of the arch of the aorta, the arteria innominata, the 



