VEINS AND LYMPHATICS IN TRAGULUS 207 



between the aorta and pre-cardinal vein as far cephalad as the 

 arch of the aorta. The cephalic element begins as a plexus at 

 the bifurcation of the brachio-cephalic artery and continues cepha- 

 lad dorso-lateral of the jugular vein as far as the jugulo-sub- 

 clavian junction. At this point it is separated by an interval 

 of approximately 0.1 mm. from the blind end of the dorsal descend- 

 ing process of the lymph sac ( thoracic duct approach). The 

 pre-azygos segment on the left side is even more divided, for 

 although it presents a cephalic and caudal division, each of 

 which appears as a clear cut channel, it was impossible to detect 

 any connection between these two divisions on the one hand 

 and the azygos segment on the other. It ends blindly above 

 and appears to have no connection with the thoracic duct approach 

 of the sac, from which it is separated by an interval of about 

 0.1 mm. Its caudal end is also independent of any connection. 

 The caudal division begins as a distinct channel at the derivation 

 of the internal mammary artery and extends almost as far caudad 

 as the junction of the brachio-cephalic trunk with the aortic 

 arch. So that, above the azygos segment, the line along which 

 the thoracic duct is destined to develop its connection with the 

 duct approach of the lymph sac consists of two as yet indepen- 

 dent elements, the cephalic and caudal divisions of the left pre- 

 azygos segment. 



A summary of the systemic lymphatic organization shows 

 that the following elements must be recognized in the future 

 line of the thoracic duct: 



(a). The azygos segment (figs. 7, 28 and 9, 28). A spacious 

 sinus-like channel, in places entirely surrounding the aorta and 

 ending at either extremity in a plexus, in many regions resembling 

 the peri-aortic lymphatic sinus of reptiles. 



(b. The post-azygos segment, (fig. 7, 29), a series of alter- 

 nating plexiform and sinus-like channels, connected cephalad 

 with the caudal plexus of the azygos segment. 



(c). The pre-azygos segment, (fig. 7, 30 and 31), consisting 

 on the right side of two independent divisions, the cephalic and 

 caudal, of which the latter is connected with the cephalic plexus 

 of the azygos segment while the former has not yet established 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 2 



