58 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



cephalad of the pericardium, viz., the pericardial, tracheal, 

 bronchial, lateral oesophageal, and thymic areas. 



This lymphatic channel, associated with the embryonic venous 

 plexuses of the pretracheal region, the pulmonary vessels and 

 the vagi, develops through confluence of a large number of sep- 

 arate and independent extra-intimal lymphatic spaces following 

 and surrounding the embryonic venous plexuses of the ventral 

 mediastinum. The chain formed by these spaces eventually 

 unites with the similar chain forming the anlages of the preazygos 

 segment of the thoracic duct. 



B. The preazygos segment of the main thoracic duct (fig. 29, 35, 

 brown). This portion of the main duct extends from the point 

 of its entrance into the jugular lymph sac, through the thoracic 

 duct approach of the latter, caudad to its intersection with the 

 dorsal surface of the aortic arch. 



In the adult animal this segment forms the relatively long por- 

 tion which ascends cephalo-sinistrad from the point where the 

 duct parts company with the right azygos vein, to its junction 

 with the jugular lymph sac, under cover of the aortic arch, and 

 the vertical portion of the left subclavian artery, dorsal to the 

 vertebral vein and to the left innominate confluence. In this 

 part of its course the thoracic duct frequently receives the lym- 

 phatic return from the ventral mediastinum through channels 

 which unite it to the ventral mediastinal trunk as just defined. 

 The preazygos segment of the main duct is again formed in the 

 embryo by confluence of independent mesenchymal spaces 

 around and along the prevertebral and dorsal mediastinal venous 

 plexuses of the embryo. 



3. THE AZYGOS SEGMENT (fig. 29, III, 36, yellow) comprises 

 the portions of the thoracic ducts caudal to the level of the aortic 

 arch. It develops, again independently, as the result of fusion 

 of a number of extraintimal mesenchymal spaces closely applied 

 to the ventral surface of the azygos veins, or of their ventro- 

 medial tributaries, or surrounding the latter. 



4. THE POST- AZYGOS SEGMENT (fig. 29, IV, 52, green), through 

 which the thoracic ducts establish their connection with the 

 Receptaculum and the system of the abdominal lymphatics. 



