66 DE\'ELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



course and the relations to surrounding structures of the thoracic 

 duct approach of this embryo are shown in the dorsal view of 

 the reconstruction (fig, 89), containing the 50 sections of shde 

 vii in a magnification of 225 diameters. 



This view of the reconstruction presents from the lateral to 

 the mesal border the following structures. 



1. Caudal end of. left jugular lymph sac, with the jugular {13) 

 and subclavian (14) approaches, between which passes the ventral 

 branch of the thyro-cervical artery {24)- 



The fifth spinal nerve (54) penetrates the lymph sac, while 

 the sixth nerve lies on its lateral aspect with the primitive ulnar 

 veno-lymphatic arching over it to enter the lymph sac. 



2. The dorsal aspect of the internal jugular vein (25). 



3. The main trunk and branches of the thryo-cervical artery (24) . 



4. The left superior intercostal vein (46), with the dorsal (16) 

 and dorso-medial or prevertebral (17) tributaries, enters thedorso- 

 medial angle of the internal jugular vein between the thyro- 

 cervical artery (24) and 



5. the double strand of the vagus (22) and sympathetic {l) 

 nerves. 



6. The thymus {21). 



7. The aortic arch (7), with the left carotid artery {23) and the 

 origin of the innominate artery (45). 



The thoracic duct approach {12) arises by two branches from 

 the dorso-medial aspect of the sac, turns caudo-mesad between 

 the internal jugular vein and the ascending branch of the thyro- 

 cervical arterj'^, in front of the left superior intercostal vein, and 

 approaches the recess formed between the main vein and the 

 bundle of the nerves. 



The reduplication of the origin of the thoracic duct approach 

 from the jugular lymph sac present in this embryo is not unusual, 

 and leads to the type of divided termination of the thoracic duct 

 in the adult shown in fig. 56. The left jugular lymph sac {3) 

 connects with the venous system at the common jugular angle 

 (2) by a larger ventral and smaller dorsal jugular approach {18), 

 It also taps at the jugulo-subclavian angle {19). The thoracic 

 duct enters the dorsal aspect of the sac by a divided terminal, the 

 lower branch connecting with the subclavian approach. 



