382 The Development of the Lymphatic System 



studying any area in the lymphatic system, one must inject close to the 

 area. This is due to two facts: first, the ducts are very irregular and 

 many exceedingly narrow ducts here and there check the flow of 

 the fluid. These minute channels represent simply the process of 

 growth and are not valves, for at this stage fluid always runs in both 

 directions from the point of injection. Secondly, the lymph clumps 

 the granules of the injecting mass and so closes the ducts. India 

 ink will always run farther than Prussian blue. The study of the 

 development of the thoracic duct depends on being able to inject it 

 centrally. 



The thoracic duct in these early stages lies actually in the edge of 

 the wall of the aorta. The side of the thorax is cut away from the 

 embryo, cutting the ribs close to their origin. The lower tip of the 

 lung is freed and pushed aside, thus exposing the aorta and the pos- 

 terior cardinal or azagos vein lying close beside it. The needle must 

 now be introduced between the aorta and the vein and it must actually 

 pick up the edge of the wall of the aorta. It is inserted just behind 

 the intercostal branches of the aorta. In pigs 4 cm. long and upwards 

 a fine hypodermic needle can be used, but in pigs smaller than -i cm. a 

 needle is too large and glass tubes drawn out to a fine point are neces- 

 sary. At these stages, if one simply enters the loose connective tissue 

 between the aorta and the vein without picking up the wall of the 

 aorta, the injected fiuid will extravasate. In older pigs, however, for 

 example, over 20 cm. in length, the thoracic duct is separate from the 

 aorta and can be injected by inserting the needle between the aorta 

 and the vein. At this stage there is a plexus of lymphatic ducts in the 

 aorta wall from which the thoracic duct can also be injected. The 

 right lymphatic duct can be injected by dissecting the neck and intro- 

 ducing the needle behind the junction of the subclavian and cardinal 

 veins. 



Figure 9, from a pig 3 cm. long, shows the completion of the thoracic 

 duct. In the thorax the two branches of the thoracic duct, lie either 

 side of the aorta, between it and the azagos vein, but between the two 

 kidneys they widen out to make a double receptaculum chyli and lie 

 close together just behind, or dorsal to the aorta. The thoracic ducts 

 on either side have now joined with the posterior lymph hearts and so 

 the lower connection of the lymphatic system with the veins has been 

 given up. 



It will be noted that there has been a change in the venous system 

 at the same time, for the femoral and sciatic veins now pass to the 

 vena cava instead of following the old course into the Wolffian body as 



