52 



AN ANOMALY OF THE THORACIC DUCT. 



the deep abdominal lymphatics, and, passing through the inguinal lymphatics to the super- 

 ficial body wall, eventually reach the subclavian vein by way of the axilla. In the present 

 case such an obstruction may well have been afforded by the anomalous condition of the 

 azygos vein, for the line of division between the rudimentary thoracic duct and the lym- 

 phatics draining posteriorly occurs at exactly the place where the large connecting branch 

 from the veins draining the left thoracic cavity crosses in front of the aorta. While this 

 vessel appears relatively small in the adult, much too small, perhaps, to be considered 

 sufficient to cause an obstruction to the flow of lymph, it should be remembered that in 

 young embryos the veins are relatively enormous. Moreover, since the left renal vein 

 runs posteriorly to the junction of the two iliacs, and since there is still a connecting vein of 

 considerable size from the left renal to the lower branch of the left azygos, it is highly 

 probable that for a considerable embryonic period this vein carried much of the blood 

 from the left leg, left half of the pelvis, left kidney, and left lumbar region. 



Fig. 1. 



Transverse section through the region of the seventh 

 thoracic vertebra of an injected human embryo 

 (Carnegie Institution of Washington, Embryo- 

 logical Research, Collection No. 460, 21 mm. 

 long, slide 26, row 1, section 4). The section 

 passes through one of the anastomoses between 

 the hemiazygos and azygos veins, passing, as it 

 does normally, dorsal to the aorta. There is a 

 lymphatic vessel on each side of the aorta, ven- 

 tral to the azygos and hemiazygos veins respect- 

 ively. The lymphatic on the right becomes 

 normally the thoracic duet. Enlarged 38X- 

 Drawn with camera lueida. 



t. A., Intercostal artery. 



7 t. v., Seventh thoracic vertebra. 



a. v., Azygos vein. 



T. D., Thoracic duct. 



a., Aorta. 



K. L., Right lung. 



o., Esophagus. 



i. v. c, Inferior vena cava. 



H. v., Hemiazygos vein. 



l., Lymphatic. 



There is another possibility which should be mentioned: While the exact mode of 

 development of the thoracic duct is not entirely settled, it is probable, as Miss Sabin 

 (1, 1913) has suggested, that the anterior part is formed by the growth (in a posterior 

 direction) of rudiments which have budded off from the veins in the neck, and that the 

 posterior part is formed by the growth (in an anterior direction) of rudiments which bud 

 off from the renal veins; that the two sets meet and anastomose, and eventually form the 

 continuous thoracic duct. If such is the case, it is possible that the anomalous azygos 

 branch has prevented the union of the two sets. 



