60 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



process of the subclavian approach (14) of the jugular lymph sac, 

 in company with the main lymphatic channels returning from 

 the anterior extremity. 



1. THE THORACIC DUCT APPROACH OF THE JUGULAR LYMPH SACS 



(FIG. 29, 18) 



This structure is an integral part of the jugular lymph sacs 

 and forms in the adult animal the portal of entry of the thoracic 

 ducts into the lymph sacs, and through them into the systemic 

 veins. McClure, in the paper quoted Cl5) > has described its 

 general formation and relation to the jugular lymph sac and has 

 given an excellent illustration of its appearance in the dorsal 

 view of a reconstruction of a 14 mm. cat embryo. I have here 

 defined it as the "thoracic duct approach" of the sacs, in con- 

 formity with the nomenclature which McClure and I adopted 

 in describing (26> 27 ) the development of those portions of the 

 jugular sac which, forming the " jugular" and " subclavian" 

 "approaches," project as blind processes from the caudal end of 

 the sac, and are destined to effect subsequently the permanent 

 lymphatico-venous connections at either the common jugular, 

 or the jugulo-subclavian confluence, or at both of these venous an- 

 gles. The term, as here used in describing the similar process of 

 the sac which is to establish definite connections with the thor- 

 acic duct, is intended to emphasize the fact that the ' 'thoracic 

 approach" of the lymph sac is, like the rest of the structure from 

 which it proceeds, venous in origin, while the thoracic ducts proper, 

 with which it secondarily unites, are not derived from the veins, 

 but are, from their beginning, independent of the blood-vascular 

 channels, and develop by confluence of independent extra- or peri- 

 venous mesenchymal spaces. 



This fundamental difference in origin is shared, in my opinion, 

 by all the other main systemic lymphatics which, in addition to 

 the thoracic ducts, enter the jugular sacs as the subclavian, ex- 

 ternal jugular, cephalic and internal jugular lymphatics through 

 the processes which the sac sends to meet them. 



The thoracic duct approach of the earlier embryos arises as a 

 short, blunt, curved, conical process from the dorso-medial aspect 



