THE LUNGS. 151 



left carotid, and the left subclavian. The trachea, with the 

 oesophagus behind it, descends along the middle line in front of 

 the spinal column. The arteria innominata crosses the front 

 of the trachea from left to right in ascending; it is in contact 

 with the transverse vein, and more superficial than either of 

 the other arteries.. The phrenic nerve, passing at the internal 

 edge of the scalenus anticus, between the subclavian artery 

 and vein, descends vertically in contact with the pleura. The 

 par vagum passes along the side of the trachea, and afterwards 

 behind the corresponding bronchus, having got into the superior 

 mediastinum between the subclavian vein and artery: its infe- 

 rior laryngeal branch encircles the subclavian artery on : the 

 right side, and the arch of the aorta on the left.. 



The internal surface of the pleura* is smooth and polished, and 

 is moistened and kept lubricated by an unctuous serum, the na- 

 tural quantity of which is merely sufficient to allow the parts to 

 slide freely upon each other. In dropsy of the chest, it is aug- 

 mented frequently to such an amount as to cause the collapse 

 of the lung by pressing upon it. 



In the cellular tissue, between the pleura and pericardium, as 

 well as on the diaphragm, .adipose matter, in considerable abun- 

 dance, is found in corpulent persons advanced in age. 



The blood vessels of the pleura costalis are derived from those 

 which supply the parietes of the thorax, as the intercostals and 

 phrenics. They ramify in the subjacent cellular substance, and 

 end by exhalent orifices on the internal face of the pleura, from 

 which a, minute injection Js, poured, out very copiously. 



