DEVELOPMENT OF THE THORACIC DUCT 65 



can be followed accurately in the successive sections shown in figs. 

 43 to 55 inclusive (series 189, slide vii, sections 10 to 22 inclusive). 



In section 11 (fig. 44) the thoracic duct approach is separated 

 by a thin endothelial partition from the lumen of the main jugular 

 lymph sac, which it rejoins in the following section (section 12, 

 fig. 45), only to again separate in the next section (section 13, 

 fig. 46). In the meanwhile, the thoracic duct approach has 

 arched from the lateral to the medial side of the internal jugular 

 vein (25), closely applied to its dorsal surface, between the same 

 and the thyro-cervical artery (24), and its blind terminal is 

 beginning to bend ventro-mesad into the neuro-venous recess 

 between the vagus-sympathetic strand (1, 22) and the internal 

 jugular vein (25). In the next three sections (sections 14, 15, 

 16, figs. 47, 48 and 49) the thoracic duct approach becomes 

 slightly dilated and diverticular (sections 15 and 16, figs. 48 and 

 49), but remains detached from the jugular sac. In section 15 

 (fig. 48) it appears subdivided, by endothelial partition, into two, 

 and in section 16 (fig. 49) into three components. The protru- 

 sion which the common jugular confluence sends towards the 

 jugular approach of the lymph sac and which forms the point 

 of the subsequent invagination of the venous angle by the lym- 

 phatic tap, begins to appear in section 16 (fig. 49), and can be 

 traced through the succeeding sections as a derivative of the lateral 

 aspect of the main internal jugular vein, between it and the 

 median surface of the jugular lymphatic sac. 



In the six following sections, 17 to 22 inclusive, (figs. 50 to 

 55 inclusive) the proximal end of the process, connected with the 

 sac, gradually recedes, while the blind distal end (12') continues 

 to develop in the ventro-medial direction between the main vein 

 (25, in fig. 55) and the dorsal and dorso-medial venous tributaries 

 (16 and 17, fig. 55), dipping ventrad and approaching the interval 

 between the internal jugular vein (25) and the sympathetic 

 nerve (1), the site of its subsequent junction with the independ- 

 ently developed perivenous lymphatics of the ventral medias- 

 tinal and dorsal prevertebral regions, which together constitute 

 the anlages of this cephalic portion of the future thoracic duct, 

 (cf. series 143, slides x and xi, figs. 172 to 183). The thoracic 

 duct approach ends blindly in section 26 of slide vii. The 



