The Origin and Development of the Lymphatic Syst 



em. 



43 



veins around the aorta arc very abundant, so thai the non-vascular 

 space is small. .Inst opposite the adrenal anlage, however, there is a 



transition zone which is less vascular. It marks the end of the azygos 

 veins. Below this level the segmental veins drain through the Wolffian 

 bodies, and here the lymphatics form a wider plexus, winch becomes 

 the cisterna ehyli, fig. 10. As in the adult there is a gradual tran- 



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Fig. 13. — Section through the upper thoracic region of an embryo pig 

 measuring 25 mm. in which the blood vessels are injected, to show the 

 vessels of the right lymphatic duct approaching the heart. Magnified 40 

 times. The injection of the blood vessels was made through the umbilical 

 artery. A. = aorta; D. C. — duct of Cuvier; D. t. = ductus thoracicus; E. 

 = esophagus; L. = lymph vessels to the heart from the right lymphatic 

 duct; N. v. = nervus vagus; P. = pericardium ; T. = trachea; V. a. = 

 vena azygos. 



sition between the cisterna ehyli and the rest of the aortic plexus, 

 both the part which extends caudalward and the part which extends 

 cerebralward. Figure 10 shows the cisterna ehyli while it is still a 

 plexus and not a sat-. The series shows that in many places this dorsal 

 plexus of lymphatics is sending sprouts around to the ventral pre- 

 aortic sac. All of the abdominal sacs, the retroperitoneal, ventral to 

 the aorta, the two lateral iliac sacs and the dorsal plexus connect with 



