DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 115 



the genetic opportunity of establishing, in later stages, a communi- 

 cation between the preazygos thoracic duct (35} and the broncho- 

 mediastinal channel (37), which would correspond to the distal 

 connection noted above (cf. p. (85)in the adult between these two 

 lymphatic channels (cf. figs. 94, 95, 96 and 97). 



The cephalic end of the preazygos segment of the thoracic duct 

 still ends blindly in this stage in two longer prolongations, the 

 connection with the thoracic duct approach of the jugular lymph 

 sac not having been as yet established. The latter rides in the 

 fork between the main jugular vein and the entrance of the left 

 superior intercostal vein (16-46). One process of the approach 

 (12) hooks caudad on the dorso-lateral side of this vein, between it 

 and the thyrocervical artery (24) and ends blindly lateral to the 

 sympathetic nerve (/). Another blunt terminal of the thoracic 

 duct approach turns ventro-mesad, between vagus (22) and sym- 

 pathetic nerve (1), and is exposed in the reconstruction by the 

 removal of a segment of the latter nerve which otherwise would 

 hide it in the dorsal view here given. A long slender process passes 

 from this portion of the thoracic duct approach ventro-mesad 

 between the vagus (22) and carotid (22). This process, which 

 seems to be constantly present, probably effects in later stages a 

 connection with the cephalic part of the ventral mediastinal lym- 

 phatic plexus. 



It is evident, in considering the detached and isolated lym- 

 phatic anlages intervening here between the thoracic duct approach 

 and the blind cephalic end if the preazygos segment of the thoracic 

 duct already formed (35) that in course of further development the 

 union between the latter and the jugular sac could have been es- 

 tablished either on the dorso-lateral or ventro-medial side of the 

 sympathetic nerve, thus leading to one or the other of the topo- 

 graphical conditions above discussed (cf. p. 75 and text figures). 



The fifth spinal nerve (54) still penetrates the caudo-lateral 

 part of the lymph sac. Below the same are seen the sixth, seventh 

 and eight segmental nerves of the brachial plexus (55). The thyro- 

 cervical artery (24) sends its ventral branch forward between the 

 jugular and subclavian approaches of the lymph sac (just caudal 

 to vein 16-46), and then continues cephalad on the doral aspect of 

 the sac. 



