THORACIC DUCT IN MAN 213 



of the aorta. Sabin ('09) reports the first appearance of the 

 cisterna chyU in 23 mm. human embryos. Here it is a definite 

 sac opposite the 3rd and 4th lumbar vertebrae. The thoracic 

 duct is first found in human embryos of 24 mm. In human 

 embryos of 30 mm. the thoracic duct is complete. 



Sabin ('02) in her study of the thoracic duct in pig embryos 

 found it to be essentially the same as she found in human embryos. 



Lewis ('06) found the thoracic duct in the rabbit embryo to 

 be practically the same as Sabin found in the human and pig- 

 embryos. 



Huntington (11) in his study of the lymphatic vessels of the cat 

 states that the thoracic duct is ''potentially bilaterally symmetri- 

 cal" and he pictures a bilaterally symmetrical duct in figure 29, 

 plate 22. 



It is interesting also to note that Sala ('99-'00) and Pensa 

 ('08-'09) picture bilaterally symmetrical thoracic ducts in birds 



DIVISION OF THE THORACIC DUCTS INTO GROUPS 



Assuming that the embryonic thoracic duct is bilaterally 

 symmetrical and that the duct in the adult is produced by the 

 persistence and growth of a part of the embryonic duct and the 

 disappearance of other parts, one might expect to find variations 

 in the adult thoracic duct. These variations depend then upon 

 which portions of the embryonic thoracic ducts atrophy and dis- 

 appear and which continue to develop. These possible varieties 

 of the thoracic duct may be divided into the following types. 



Type 1. To this type of thoracic duct belong those ducts 

 which would retain more or less the early embryological conditions 

 and would consist of a completely bilaterally symmetrical duct 

 connected by numerous cross anastomoses (fig. 1). 



Type 2. In this type of thoracic duct we would have caudad 

 the persistence of the original double thoracic duct of the embryo. 

 There would be a right and left duct, which starting in the ab- 

 dominal cavity would pass cephalad through the thorax and at 

 about the level of the 4th thoracic vertebra, the right duct would 

 cross by persistence of one of the embryonic cross anastomosing 



