DEVELOPMENT OF THE THORACIC DUCT 89 



subclavian artery and in front of the subclavian vein to terminate, 

 in common with the subclavian lymphatic, in the ventral process 

 of the subclavian approach of the lymph sac. 



More rarely the broncho-mediastinal channel forms the only 

 terminal path of the thoracic duct. In these instances the 

 preaortic segment of the thoracic duct abandons its normal dorsal 

 position and turns ventrad into the path of the broncho-medias- 

 tinal channel. Thus in fig. 98 (adult, no. 168), in which the 

 common jugular tap (18) is the only lymphatico-venous con- 

 nection of the left jugular lymph sac, the ventral process of the 

 jugular approach is free and prolonged caudo-ventrad over the 

 jugulo-subclavian angle as a large trunk which receives the entire 

 drainage of the thoracic duct. 'The latter (8), after passing the 

 aortic arch, swings ventrad of the subclavian artery into the 

 pathway of the normal broncho-mediastinal trunk and ascends 

 to meet the ventral prolongation of the jugular approach of the 

 lymph sac. 



It thus becomes apparent that in dealing with the ontogenesis 

 of the cephalic portion of the cat's thoracic duct it is necessary 

 to reckon with the ventral mediastinal and broncho-mediastinal 

 channels as well as with the preazygos segment of the thoracic 

 duct proper, since the former, as just seen, may take over, par- 

 tially or entirely, the drainage of the duct into the jugular lymph 

 sac. 



A composite schema of the adult preazygos portion of the tho- 

 racic duct in relation to the broncho-mediastinal and ventral 

 mediastinal trunks is given in fig. 99 based on the individual con- 

 ditions encountered in the adult series examined. The plan rep- 

 resents every recorded development of the lymphatic pathways 

 in this region, but is rarely seen in its full development in the 

 adult (cf. adult, no. 29, fig. 96). Usually one or more of these po- 

 tential segments and connections default and thus a great variety 

 of distinct adult conditions is produced. These lymphatic path- 

 ways are, however, all present in the embryo and account for 

 the observed adult variants. 



