72 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



in the figure) is seen in the deep recess between this entrance, 

 the main vein (26) and the thyro-cervical artery {24-) - The mesal 

 end of the thoracic duct approach terminates bhndly in this 

 section just dorsal to the sympathetic nerve (1). In section 13 

 (fig. 77) the lateral terminal of the thoracic duct approach (12) 

 can still be clearly seen in the interval between the wide dorsal 

 opening of the combined somatic and prevertebral (17) tributary 

 stem into the main vein (26) and the thyro-cervical artery {24-) . 

 It appears very much reduced in the same situation in section 

 14 (fig. 78), and ends blindly in the following section. The last 

 three sections of this series (figs. 76, 77 and 78) show the begin- 

 ning of the jugulo-subclavian confluence. 



In the later stages the thoracic duct approach has extended 

 further ventro-mesad into the recess between the common jugular 

 vein and the sympathetic nerve, and in this situation it eventually 

 makes its connection with the two independently developed lym- 

 phatic channels of the anterior mediastinal region, viz., the 

 preazygos segment of the thoracic duct dorsally, and the broncho- 

 mediastinal trunk ventrally. The mode of this union is dis- 

 cussed below under a separate heading (cf. infra, p. 111). Figs. 

 79 to 82 show transverse sections of the left side of the lower 

 cervical region in a 15 mm. cat embryo (series 245, slide xi, 

 sections 24 to 27, X 225), and fig. 90 gives the dorsal view of the 

 reconstruction on the same scale of the thoracic duct approach 

 and the related structures in the same embryo (slide xi, sections 

 1-44). In fig. 79 (slide xi, section 24) the terminal extremity 

 of the thoracic duct approach (12) is seen between the common 

 jugular vein (26) and the sympathetic nerve (1), having reached 

 this situation by extending ventro-mesad across the dorsal tribu- 

 tary 16 (left superior intercostal vein), between this vessel and 

 the main venous trunk. 



The section is taken below the common jugular confluence, 

 consequently the jugular approach is no longer seen, and the 

 blind end of the thoracic duct approach appears isolated. The 

 only other portion of the jugular lymph sac carried caudad to this 

 level is the subclavian approach {14), seen on the lateral aspect 

 of the main vein. The sequence of structures now successively 



