62 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



(^4) lies at a little distance directly dorsad of the internal jugular 

 vein. The dorso-medial angle of the main vein is related to a 

 large compound medial venous tributary, draining the dorso- 

 medial (prevertebral), {17) and the dorsal somatic areas {16). 

 This compound vessel is the anlage of the future superior inter- 

 costal vein. In fig. 33 (section 42) the common trunk opens 

 into the main vein just dorsal to the sympathetic nerve {1). 

 In this section, the blunt medial extremity of the thoracic duct 

 approach {12) is separated from the portion still connected with 

 the jugular lymph sac, thus expressing the caudo-mesal curve 

 of the entire process. It ends blindly in the following section, and 

 does not reappear in the succeeding sections. The common jugu- 

 lar confluence occurs in section 44 of this slide. 



We are dealing, therefore, with a short, blunt conical process 

 of the left jugular lymph sac, which curves meso-caudad for two 

 sections, and terminates blindly dorsal to the main vein. 



In another 14 mm. embryo (series 212) the thoracic duct ap- 

 proach is much more extensively developed and occupies sections 

 11 to 26 of slide ix. Selected sections of this series are shown 

 in figs. 34 to 42 inclusive. 



Fig. 34 (series 212, slide ix, sec. 14) shows part of the caudal 

 end of the left jugular lymph sac {11) with the thoracic duct 

 approach {12) arising from its medial aspect and extending mesad 

 across the dorsal surface of the internal jugular vein {25). On 

 the mesal aspect of the vein are seen the sympathetic {1) and 

 the vagus {22). 



The large common trunk formed by the union of the dorsal 

 somatic and prevertebral venous tributaries {16, 17) lies dorso- 

 medial to the main jugular channel. It joins the latter, as left 

 superior intercostal vein, {16, 46,) in section 26 of this slide 

 ffig. 41). On the lateral aspect of this vessel is the ascending 

 dorsal branch of the thyro-cervical artery {24-) - 



The six succeeding figures (35 to 40 inclusive) are cut down so 

 as to only show the dorsal circumference of the internal jugular 

 vein and the superincumbent thoracic duct approach of the 

 jugular lymph sac. 



In fig. 35 (section 15) the approach has extended further mesad 



