88 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



has again been cut short and turned mesad to expose the deeper 

 structures. 



The azygos segment of the thoracic duct (16) shows the same 

 plexiform character and relation to the perforating intercostal 

 arteries as in the preceding dissection (fig. 95). 



Fig. 97 (adult no. 131) shows an instance in which the main 

 thoracic duct turns ventrad into the broncho-mediastinal trunk, 

 while a small channel follows the usual dorsal course of the normal 

 duct. 



The left jugular lymph sac has only a single tap into the venous 

 system at the jugulo-subclavian angle. 



The sac receives at its cephalic end two lymphatic trunks paral- 

 leling the internal jugular vein (17), which is larger than usual, 

 and the combined trunk formed by the external jugular and 

 cephalic lymphatics. 



The dorsal half of the thyro-cervical periarterial ring is smaller 

 than the ventral, but the foramen is complete. 



The jugulo-subclavian approach and tap is formed by a large 

 process of the sac which sends a ventral prolongation over the 

 jugulo-subclavian angle. This receives laterally the subciavian 

 lymphatic, ascending along the subciavian vein (5), and from 

 below the broncho-mediastinal trunk (8), which represents the 

 main cephalic continuation of the thoracic duct. The duct at the 

 aortic arch level (26) splits into a smaller dorsal (12) and larger 

 ventral division (8). The former ascends behind the vertical 

 part of the subciavian artery, crosses behind the vertebral vein, 

 and turning ventro-caudad around the lateral aspect of the thyro- 

 cervical artery (9), enters the dorsal aspect of the jugular lymph 

 sac, just in front of the jugulo-subclavian tap. This smaller 

 dorsal division represents the usual preazygos portion of the 

 thoracic duct of the adult. 



The second larger division of the duct takes the path of the 

 broncho-mediastinal channel (8). It ascends first on the outer 

 side and then in front of the subciavian artery, subdivides into 

 two branches, which reunite and receive a small lymphatic from 

 the left innominate plexus. The upper end of the duct crosses 

 in. front of the junction of the vertical and arched portions of the 



