DEVELOPMENT OF THE THORACIC DUCT 107 



B. The development of the proximal portion of the thoracic duct 

 proper, between the termination of thoracic duct approach of the 

 jugular lymph sac and the beginning of the azygos segment of the 

 thoracic ducts, caudal to the level of the aortic arch, viz. the preazy- 

 gos segment of the thoracic duct. 



In the earlier purely venous stages a venous plexus between 

 oesophagus and vertebral column {17, fig. 101), drains caudo- 

 laterad into the mesal susface of the main jugular and innomi- 

 nate trunks. This plexus continues the supracardinal venous 

 line cephalad beyond the level of the azygos-Cuvierian junc- 

 tion. The terminals of this plexus are frequently joined by 

 dorsal somatic venous tributaries (16, fig. 101), near their entrance 

 into the main vein. These vessels have been previously discussed 

 in their relation to the general tributary system of thejugulo- 

 innominate line (cf. supra p. (92), and their disposition is sche- 

 matically shown in fig. 101. 



In a 14 mm. embiyo (series 210, slide viii, section 41, X 225) 

 the dorso-medial or prevertebral tributary (17) and the dorsal 

 somatic branch (16) are shown in fig. 102, just prior to their 

 union into the common large trunk emptying into the dorso- 

 medial angle of the internal jugular vein (25), dorsal to the sym- 

 pathetic nerve (1). 



The same plexus elements {16 and 17) are seen in another 14 

 mm. embryo (series 212, slide vii, section 23, X 150) in fig. 159. 

 In this section their point of confluence and the entrance of the 

 resulting combined trunk into the internal jugular vein takes 

 place ventral to the sympathetic nerve {1), between the same 

 and the left subclavian artery {33), by utilizing the pathway 

 afforded by the peritracheal plexus {18, in fig. 102). 



Figs. 160 and 161 show these tributaries {16 and 17) in a 13.5 

 mm. cat embryo (series 189, shde vii, sections 20 and 21) in a 

 magnification of 225 diameters, as they are approaching their 

 confluence and point of entrance into the internal jugular vein 

 {25). In addition these sections afford admirable examples of 

 two other conditions, previously noted (vide supra pp. 49) : 



a. The development of independent mesenchj^mal intercellular 

 lymphatic spaces, with well defined endothelial limiting walls. 



