DEVELOPMENT OF THE THORACIC DUCT 73 



encountered along the mesal and dorsal aspects of the main com- 

 mon jugular vein {26) are the following, proceeding meso-laterad : 



1, Thymus {21), with sub- and suprathymic venous tributaries 

 entering the main vein. 



2. Left common carotid artery {23). 

 »3. Vagus nerve {22). 



4. Sympathetic nerve {1). 



5. Thoracic duct approach {12). 



6. Dorsal somatic tributary of main vein {16). (Anlage of 

 left superior intercostal vein). 



7. Thyro-cervical artery {2 If). 



8. Subclavian approach of jugular lymph sac (74), on the 

 dorso-lateral aspect of the common jugular vein. 



This order and relationship is maintained uniformly and clearly 

 in the three succeeding sections shown in f gs. 80, 81 and 82. 



In the last of these the dorsal venous tributary {16) enters the 

 main common jugular channel. 



The sections can be easily followed and oriented by reference 

 to the dorsal view of the reconstruction shown in fig. 90. The 

 same is composed of the plates of all 44 sections of slide ix of series 

 245. In this embryo the thoracic duct approach has already 

 established its definite and permanent connection with the inde- 

 pendently developed channel of the preazygos segment of the 

 thoracic duct {35, in fig. 90), as described in detail below (cf. 

 p. 111). The point of this junction is indicated in the reconstruc- 

 tion by the narrowing of the two spindles 12 and 35 (thoracic 

 duct approach and preazygos segment of thoracic duct) to form 

 an isthmus situated just cephalad of the leader marked 16-46. 



The reconstruction includes the terminal of the jugular ap- 

 proach {13), with the origin from the same of the thoracic duct 

 approach {12), and the entire subclavian approach {14) of the 

 jugular lymph sac. The ventral division of the thyro cervical 

 artery {24) passes forward between the jugular and subclavian 

 approaches of the lymph sac, accompanied by a vein (not labelled 

 in the figure) which is constantly found in this situation in the 

 adult, draining from the prevertebral area into the common 

 jugular trunk. 



