DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 117 



view of the reconstruction, because it is largely under cover of the 

 extensive venous plexus encrusting the medial surface of the 

 left innominate vein on which it lies. It can, however, be clearly 

 traced in the corresponding sections of slides xi and x shown in 

 figs 172 to 183 (cf. infra). 



The preazygos segment of the thoracic duct then continues on 

 the mesal side of the sympathetic (1} cephalad, swings laterad on 

 the ventrolateral aspect of the nerve, between it and the common 

 jugular vein, mesal to the thyrocervical artery (24), and unites 

 in this situation with the thoracic duct approach of the jugular 

 lymph sac (12). The latter extends as a well defined prolongation 

 from the dorsal aspect of the jugular approach (13). 



The aortic arch (7) with the innominate trunk (45), the right 

 subclavian origin (57) and the two carotid arteries (23, 32'), 

 occupies the centre of the model. The left subclavian artery 

 cannot be followed in this view, but the left vertebral artery (41) 

 is shown. Further ventrad is the section of the left innominate 

 vein (40). The thymus (21} is nearly buried in the extensive 

 perithymic venous anastomosis (19, 20}, which also hides from 

 view the most of broncho-mediastinal lymphatic plexus. 



The thyrocervical artery (24) perforates between the two divi- 

 sions of the lymph sac, lateral to the jugular approach (13). The 

 fifth spinal nerve (54) and a companion vein still penetrate the 

 caudo-lateral part of the sac. Further caudad and laterad are the 

 sixth and seventh segmental nerves (55). The jugular approach 

 sends a caudal prolongation along the dorse-medial aspect of the 

 main vein, between it and the sympathetic nerve, which parallels 

 the thoracic duct approach, and may, together with a large de- 

 tached lymphatic island further caudad, represent an incomplete 

 form of double or bifid thoracic duct approach as previously 

 described (cf. supra, p. 63, text figure.) 



Union of broncho-mediastinal duct with preaortic portion of thor- 

 acic duct and junction of both with thoracic duct approach of the 

 jugular lymph sac. The achievement of this stage is excellently 

 illustrated in the microphotographs of this 15.5 mm. embryo 

 (series 143) shown in figs. 172 to 183. In fig. 172 (series 143, slide 

 xi, section 16), a section through the left side of the upper thoracic 



