POSTERIOR MEDIASTINUM. 123 



second gives off the intercosto-humeral branch (p. 108), and 

 the twelfth a branch to the iirst lumbar nerve, to assist in 

 forming the lumbar plexus. 



By removing a portion of the oesophagus, the az}-gos 

 veins will be brought into view ; they are two in number, 

 and are named major and minor. 



The AZYGOS MAJOR VEIN, commencing by branches com- 

 municating with the right lumbar and renal veins, and 

 sometimes also with the inferior vena cava, passes through 

 the aortic opening of the diaphragm, and ascends upon the 

 bodies of the vertebrae at the right side of the thoracic 

 aorta, receiving the right intercostal veins in its course. 

 Opposite the third intercostal space it arches forward 

 above the root of the right lung, and enters the superior 

 cava, just before that vessel penetrates the pericardium. 

 The AZYGOS MINOR VEIN commences on the left side 

 >m the lumbar, or renal veins, passes into the thorax with 

 ie aorta, or beneath the border of the diaphragm, ascends 

 >n the left side of the vertebral Column, and at about the 

 >venth or eighth dorsal vertebra, crosses beneath the aorta 

 id thoracic duct, to enter the vena azygos major. It re- 

 iives the lower intercostal veins of the left side. The 

 iperior intercostal veins enter a trunk which joins with 

 ie left vena innominata, or with the azygos minor. 



The thoracic duct is difficult to demonstrate or isolate from the sur- 

 rounding tissues, which it resembles in color ; by making a snip with 

 the scissors into the duct, then inserting the blow-pipe, and inflating 

 it, it will become distended, and its course made apparent. 



The THORACIC DUCT commences in an enlargement, to 

 which the lymphatics of the abdomen converge, called the 

 receptaculum chyli ; this lies beside the right crus of the 

 diaphragm, between the aorta and vena cava. The duct, 

 about the size of a wheat straw, ascends between the aorta 

 and vena az}'-gos major, crosses the vertebral column at 

 the second dorsal vertebra, and continuing upward along 

 the left side of the ojsophagus, enters the left subclavian 

 vein near its junction with the internal jugular. A small 

 duct, called the ductus lymphaticus dexter, being the termi- 

 nal duct of the lymphatics of the head, neck, and portions 

 of the right side of the upper part of the bod}-, enters the 

 right subclavian at its junction with the right internal 

 jugular vein (p. 55). The thoracic duct is sometimes 

 double, either in the whole or part of its course. It oc- 



