8(5 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



lymph paths, into the caudal end of the jugular sac, usually into 

 the ventral process of the subclavian approach caudal to the 

 jugulo-subclavian lymphatico-venous tap. In this course the 

 broncho-mediastinal trunk receives afferents from the medias- 

 tinal structures. Fig. 94 shows this general arrangement on the 

 left side in a typical adult (No. 56), in which the left jugular 

 lymph sac connects with the veins only through the single jugulo- 

 subclavian tap. A double ventral process from the subclavian 

 approach effects the cephalic connection of the broncho-medias- 

 tinal trunk with the jugular sac, while caudally two lymph vessels 

 unite it to the preazygos segment of the thoracic duct. 



It is evident in following the thoracic duct caudo-cephalad 

 that this arrangement enables the flow of lymph in the main 

 canal to take, near the level of the innominate bifurcation, one 

 of two equally available pathways in order to reach the jugular 

 sac and through it enter the venous stream: 



a. It may, in large part or entirely, continue cephalad in the 

 dorsal channel of the preazygos segment of the thoracic duct 

 proper, or 



b. It may ascend in the channel of the broncho-mediastinal 

 trunk, and empty into the venous system through the ventral 

 prolongation of the subclavian approach of the jugular lymph 

 sac. 



This arrangement, which furnishes the keynote for the main 

 variations of the thoracic duct in this region, and for the interpre- 

 tation of certain important embryological stages, is well illustrated 

 by the dissection of the left side of neck and thorax in the adult 

 cat shown in fig. 95 (adult, no. 22). The azygos segment of the 

 thoracic duct is seen caudad of the level of azygos arch (21) as a 

 plexiform channel whose meshes are perforated by the inter- 

 costal arteries. Cephalad of the azygos arch the preazygos seg- 

 ment of the thoracic duct continues as an undivided canal of 

 larger caliber to the level of division of the innominate artery 

 (20). At this point the broncho-mediastinal trunk (8) diverges 

 from the main canal and ascends ventral to the left subclavian 

 artery, receiving tributaries from the ventral mediastinal nodes. 

 It meets the subclavian lymphatic and a large ventral mediastinal 



