74 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



The dorsal and dorso-medial venous tributary plexus has 

 united into the stem of the left superior intercostal vein (16-4-6}. 

 The reconstruction shows well .the caudo-mesal turn of the 

 thoracic duct approach (12) in the fork between the main vein (26) 

 and the superior intercostal (16-46), accompanied by the mesal 

 branch of the thyro-cervical artery. The terminal of the thor- 

 acic duct approach (12) meets and unites with the preazygos 

 segment of the thoracic duct (35) on the mesal shoulder of the 

 superior intercostal vein terminal, between it and the sympathetic 

 nerve (1). This is one of the two types of topographical relation- 

 ship encountered (cf. infra pp. 75, 76). Further laterad follow 

 the vagus (22) and the sympathetic strand (1), intersecting at an 

 acute angle. Then the left common carotid artery (23) and the 

 thymus (21) with the perithymic venous plexuses. 



Figs. 83 to 8,8 show the caudal end of the left jugular lymph 

 sac and the thoracic duct approach in a selected series of sections 

 from a 17 mm. cat embryo (series 142), all magnified 225 diame- 

 ters, and fig. 91 gives the dorsal view of the reconstruction of 

 this region from the same embryo. 



These preparations are introduced here for the purpose of 

 illustrating, in comparison with the preceding series (245) the 

 second type of relationship which the thoracic duct approach 

 may bear to the combined vagus-sympathetic strand. 



Fig. 83 (series 142, slide x, section 13) shows the transection 

 of the caudal end of the left jugular lymph sac in the interval 

 between the internal jugular (25) and jugulo-cephalic (29) veins 

 which are approaching their confluence to form the common 

 jugular trunk. 



The form of the jugular lymph sac already indicates its coming 

 division into the three terminal " approaches," the jugular ap- 

 proach (14), the subclavian approach (13) and the thoracic duct 

 approach (12). The latter covers the entire dorsal circumference 

 of the internal jugular vein (25) and its termination points to the 

 angular recess between the sympathetic (1) and vagus (22). 

 Ventrad to the latter the field includes the thymus (21). The 

 thyro-cervical artery is cut in several of its divisions, two of which 

 are indicated by the forked leader 24- 



