242 HENRY K. DAVIS 



The thoracic duct may be single and pass cephalad into the 

 thorax on the right side of the aorta and at about the level of 

 the 5th thoracic vertebra cross over to the left side and open 

 into the venous system of the left side. This type of duct 

 occurred in 14 instances in my series of 22, or in 63.63 per cent 

 (figs. 16-29). This is the most predominant type of thoracic duct 

 and is described as normal by all anatomists. 



The thoracic duct may lie to the right of the aorta in its entire 

 extent and open into the venous system of the right side. This 

 type of duct occurred in 1 instance in my series of 22, or in 

 4.545 per cent (fig. 30). Ducts of this type have been described 

 by five authors. 



The thoracic duct may lie to the left of the aorta in its entire 

 extent and open into the venous system of the left side. This 

 type of duct occurred in 1 instance in my series of 22, or in 

 4.545 per cent (fig. 31). A similar thoracic duct has been de- 

 scribed by one author. 



Assuming that the thoracic duct is developmentally bilaterally 

 symmetrical, one might expect to find in the adult some cases 

 in which a single duct was situated on the left side of the aorta 

 and divided in the thorax into two branches, one of which would 

 open into the venous system of the left side and the other into 

 the venous system of the right side. This type of duct would 

 be similar to the diagram, figure 5. T found no duct of this type 

 in my own series, nor could I find any described in the literature. 



Again, assuming that the thoracic duct develops with bilater- 

 al symmetry, it ma^^ start in the abdominal cavity as a single 

 duct and pass cephalad into the thorax on the left side of the 

 aorta and at about the level : f the 5th thoracic vertebra cross 

 over to the right side and open into the venous system of the 

 right side. This type of thoracic duct would be similar to the 

 diagram, figure 7. I found no ducts of this type among my own 

 cases nor could I find any described in the literature. It seems 

 strange that no ducts of these last two types have been reported 

 inasmuch as ducts of all the other types have been found and 

 reported. 



