114 DEVELOPMENT OF THE THORACIC DUCT 



the cephalic portion of the azygos system. The left (6') and 

 the right (3') azygos veins are seen arching caudo-cephalad to 

 empty into the left (6) and right (3) precavae. They are joined 

 by the broad plexiform plate of the interazygos anastomosis 

 whose meshes are perforated by the intersegmental arteries. The 

 sympathetic nerve strands (1) lie between the arterial branches 

 and the azygos trunks, in the a gle between them and their 

 dorsal somatic tributaries. Ventro-mesad of the right precava 

 (S) is seen the right vagus (22). The distal end of the right pul- 

 monary artery (10) is seen below the right azygos-caval arch 

 (3' -3), surrounded by the channel of the broncho-mediastinal 

 trunk (37). In the azygos-caval arch, and along the right pre- 

 cava (3) are the precaval lymphatic anlages (53) previously 

 referred to (cf. supra pp. (30) as being the first extraintimal 

 lymphatic spaces to develop in this region, as early as the 12 

 mm. stage (cf. figs. TO and 11. 



The cephalic portion of the azygos segment of the thoracic 

 duct, well developed in this stage, is hidden in this view of the 

 reconstruction by the dorsal interazygos venous plexus, on the 

 ventral face of which the lymphatic network is situated. Only 

 a few scattered and isolated lymphatic anlages are seen projecting 

 cephalad of this anastomosis, and not yet joined to the preazygos 

 segment of the thoracic duct (35). 



Cephalad of the area just described are the large arterial ves- 

 sels, viz., the Ductus arteriosus Botalli (56), the aortic arch (7), 

 the right subclavian artery (57), the innominate continuation of 

 the carotid trunk (45), and the left subclavian artery (33), with 

 the left vertebral (41) and thyro-cervical (24) arteries arising 

 from it. The ventrolateral continuation of the left subclavian 

 cannot be seen in this view of the reconstruction. Ventrad of 

 the thoracic aorta is seen the ventral mediastinal venous plexus 

 with the accompanying cephalic part of the broncho-mediastinal 

 lymphatic complex (upper 37 in fig. 170). 



The preazygos segment of the thoracic duct (35) appears as a 

 well developed channel on the dorso-medial aspect of the aortic 

 arch, ascending behind the left subclavian artery (33). A number 

 of scattered lymphatic anlages, at the level of leader 35, offer 



