THE MEDIASTINUM. 



possible a collateral circulation in the posterior thoracic wall when this vessel is occluded, so that 

 the blood from the lower half of the body may reach the heart through the superior vena cava. 



The Vena Azygos Minor (Hemiazygos). This vein is subject to considerable variation 

 (Plates 8, a, and 14). Situated at the left side of the vertebral column, it receives blood from 

 the left intercostal veins and runs across the ninth dorsal vertebra to empty into the right azygos 

 vein. When there is a second communication with the right azygos vein, at a higher level, we 

 speak of a vena hemiazygos superior and of a vena hemiazygos inferior. This vein is frequently 

 connected with the superior vena cava by a vessel which passes transversely across the aortic 

 arch and is known as the superior intercostal vein (Plate 8, a, where this vein is very large, and 

 Plate 10). 



The Tlwrack Duct. The thoracic duct arises from the receptaculum chyli at the first or 

 second lumbar vertebra and is subject to many variations, in its method of division and plexus 

 formation (Plate 14). It is the main trunk of the lymphatic system and accompanies the aorta 

 through the aortic opening in the diaphragm. The duct runs in the areolar tissue between the 

 aorta and vena azygos major and then ascends behind the esophagus. Above the third dorsal 

 vertebra it gradually approaches the left side, is covered by the aortic arch, and at the level of the 

 seventh cervical vertebra passes anteriorly between the left common carotid and subclavian 

 arteries to empty into the left innominate vein. [According to some authors, the highest part 

 of the duct reaches to the level of the transverse process of the extra-cervical vertebra. According 

 tc Stiles, this is one inch vertically above the inner end of the clavicle. Its relation to the inner 

 border of the scalenus anticus should be kept in mind in all operations in this vicinity. ED.] 



The thoracic portion of the sympathetic nerve may be seen through the costal pleura to the 

 outer side of the corresponding azygos vein (Plate 8). It passes into the thorax, forming the 

 inferior cervical ganglion in front of the head of the first rib; the first thoracic ganglion is situated 

 anteriorly to the head of the second rib, and the remaining ten ganglia are in front of the heads 

 of the lower ribs. It runs in front of the intercostal nerves, with which it is connected by visceral 

 and communicating branches, and also in front of the intercostal arteries. The sympathetic 

 nerve, in addition to the branches to the cardiac plexus and to the lung, gives off the greater 

 and lesser splanchnic nerves, which run inward and downward, pass through the diaphragm, 

 and end in the celiac plexus. These nerves arise by roots from the sixth or seventh to the eleventh 

 thoracic ganglia and, in consequence of their white medullated fibers, may usually be seen through 

 the pleura as they lie in front of the vertebral column. 



REVIEW QUESTIONS. 



Why are penetrating wounds entering the intercostal spaces from behind more commonly associated 

 with injuries to the ribs than corresponding wounds through the anterior thoracic wall ? 



What large artery may be influenced in its course by the presence of a cervical rib ? What may 

 result from this anomalous relation? 



What is the practical result of the preponderance of spongy tissue in the sternum ? 



Into which three large cavities (and in which situations) may a hemorrhage take place after an 

 injury of the internal mammary artery ? 



What compression symptoms may follow a posterior dislocation of the sternal end of the clavicle ? 



