LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 433 



DISCUSSION 



In connection with these observations of the great suscepti- 

 bility of the developing lymphatics to various mechanical factors, 

 an anomaly of the adult human lymphatic system, recently 

 described by Dr. Clark,^^ is of interest. In a specimen from the 

 dissecting room he found a diminutive thoracic duct which was 

 only present as far as the ninth interspace, while all the lym- 

 phatics of the rest of the body below the ninth vertebra, drained 

 into a large main duct which emerged below the ligamentum 

 inguinale and ran superficially in the body wall, into the axilla 

 and emptied at the angle formed by the jugular and subclavian 

 veins. A significant anomaly of the hemiazygos vein was 

 present: all the veins draining the left thoracic wall joined with 

 a fairly large branch from the left renal vein and crossed the mid- 

 line ventrally instead of dorsally to the aorta, at the exact place 

 where the miniature thoracic duct ended and the lymphatics 

 draining posteriorly commenced. 



Dr. Clark has shown a section through this region in a human 

 embryo of 21 mm which demonstrates the relatively enormous 

 size of the embryonic veins and shows clearly that in a case such 

 as the one described, where the connection during embryonic 

 life between the two azygos veins must have been ventral instead 

 of dorsal to the aorta, an obstacle sufficient to block the flow of 

 lymph had been present. Since, as Miss Sabin has shown, valves 

 in the lymphatics do not develop until a relatively late stage 

 (7 cm. in pig embryos), such an obstacle might have prevented 

 the flow of lymph in the thoracic duct and caused it to follow an 

 entirely different course. Dr. Clark has pointed out that this 

 case shows strikingly the response of the lymphatic vessels to 

 mechanical forces ''since the passage of a large amount of lymph 

 through an unusual route had led to the formation of a duct many 

 times larger than the vessels normally present, while the dimi- 

 nution of lymph-flow through the thoracic duct has been accom- 

 panied by a corresponding diminution in size." 



13 E. R. Clark, An anomaly of the thoracic duct with a bearing on the embry- 

 ology of the lymphatic system. Contributions to Embryology, no. 3, Carnegie 

 Inst. Washington, 1915. 



