LYMPHATIC VESSELS 



thelial cells during the growth of the lymphatic vessel was demonstrated (Anat. Rec., 

 1909, vol. 3, pp. 183-198). 



DEVELOPMENT. The development of the mammalian lymphatic sys- 

 tem begins with the formation of a pair of very large sacs lined with 

 endothelium, situated at 

 the junction of the jugular 

 and subclavian veins (Fig. 

 176). These jugular lymph 

 sacs were first described 

 by Miss Sabin (loc. cit.}\ 

 they appear in human em- 

 bryos measuring about 10 

 mm. and are formed by 

 the union of several out- 

 growths from the veins. 

 In slightly older embryos, 

 another lymph sac is pro- 

 duced at the root of the 

 mesentery, below the place 

 where the renal veins 

 enter the vena cava in- 

 ferior (Lewis, Amer. Journ. 

 Anat., 1920, vol. i, pp. 

 220-244). The opinion 

 that this sac is a derivative 

 of the adjacent veins has 

 been confirmed by certain 

 later embryological studies, 

 and by finding permanent 

 communications between 

 the lymphatic and the 



venOUS SVStem at the level F IG - I7< *- LYMPHATIC VESSELS AND VEINS IN A RABBIT OF FOUR- 

 J TEEN DAYS, EIGHTEEN HOURS; 14.5 MM. X 11.5. 



of the renal Veins in adult The lymphatics are heavily shaded, x being a vessel along the left 



South American monkeys 

 (Silvester, Amer. Journ. 

 Anat., 1912, vol. 12, pp. 

 446-460). At other places, 

 which must be regarded as 

 secondary centers, lymphatic vessels appear to be derived from the veins 

 and to become detached from them. These vessels are seen in the 

 mesenchyma as isolated spaces, usually along the course of the veins, 

 at no great distance from the jugular and mesenteric lymphatics. Subse- 

 quently they become connected with one another by endothelial out- 



vagus nerve and y along the aorta. The large jugular lymph 

 sac is in contact with the internal jugular vein. In J.; it passes 

 to the junction of the external jugular (Ex. J.) and subclavian 

 veins, the latter being formed by the union of the primitive 

 ulnar, Pr. Ul., and external mammary veins, Ex. M. The 

 mesenteric sac is in front of the vena cava inferior (V. C. I.) 

 and below the renal anastomosis (R. A.). Other veins include 

 Az., azygos; V., vitelline; G., gastric; S. M., superior mesen- 

 teric; etc. The figures indicate the position of the correspond- 

 ing cervical nerves. 



