DEVELOPMENT OF THE THORACIC DUCT 87 



trunk accompanying the internal mammary vessels (7) in front of 

 the jugulo-subclavian angle in a plexiform junction, which empties 

 into a ventral process prolonged down from both the jugular (18) 

 and subclavian (19) approaches of the jugular lymph sac. 



The preazygos segment of the thoracic duct, cephalad of its 

 connection with the broncho-mediastinal trunk, takes the usual 

 course upward under cover of the subclavian artery and behind 

 the vertebral vein (11) and opens into the dorsal aspect of the 

 jugular sac a little in front of the common jugular tap. 



The jugular lymph sac is well developed in its ventral division 

 and opens into the venous system at the two typical points, viz., 

 both at the common jugular and jugulo-subclavian taps. It 

 receives the internal and external jugular and the cephalic lym- 

 phatics in the usual way. The subclavian vein (5) is divided 

 close to the jugulo-subclavian confluence and turned forward to 

 expose the vertebral vein and the duct. 



The individual affords a good example of the full development 

 of the normal preazygos segment of the thoracic duct in combina- 

 tion with a well marked broncho-mediastinal trunk and plexus. 

 The former is still the main lymphatic channel, although the 

 broncho-mediastinal vessel is fully developed and capable of 

 draining the mediastinal lymphatics either cephalad into the 

 lymph sac or caudad into the thoracic duct. 



Fig. 96 (adult, no. 29) shows an instance in which the preazy- 

 gos segment of the thoracic duct (12) and the broncho-mediastinal 

 trunk (8) are of nearly equal size, the former receiving the inter- 

 nal mammary (6) and inferior thyroid (4) lymphatics and drain- 

 ing into the jugular sac through the ventral prolongation of its 

 subclavian approach. The preparation shows well the main 

 relations of the lymphatic complex to the vertical portion of the 

 left subclavian artery (13) and its branches, with the broncho- 

 mediastinal trunk (8) lying ventral to the artery, while the tho- 

 racic duct (2) ascends dorsal to the same, and further passes be- 

 neath the vertebral vessels (11) and the thyro-cervical artery (9) in 

 order to reach its connection with the lymph sac. The latter has 

 in this instance a single tap into the veins through the subclavian 

 approach at the jugulo-subclavian angle. The subclavian vein (5) 



