228 VASCULAE SYSTEM 



line through the pre-aortic subcardinal anastomosis, on the other hand, enlarge. 

 As the permanent kidneys assume their definitive position a series of changes takes 

 place, which need not here be entered on, until ultimately the renal veins are found 

 opening into the cardinals at the level of the cross-anastomosis, and close to them 

 the veins from the sex glands, spermatic or ovarian. The cardinals are at first 

 symmetrical, but the sections behind the anastomosis are now larger than the 

 sections in front of it. 



It will be convenient to trace the fate of these portions separately. A series 

 of post-aortic anastomoses is formed between the posterior and Wolffian sections 

 of the veins. One of these (transverse iliac vein) enlarges, and now all the blood 

 from the pelvis and limbs passes into the right cardinal, which accordingly increases 

 in size and becomes the posterior part of the vena cava inferior, while the left 



v. cardinalis anterior section 



v. cava inferior 



v. subcardinalis anterior section 



^_ __ v. cardinalis 



v. subcardinalis 7JL ^ ^f ^^^ posterior section 



posterior section 



FIG. '283. DEVELOPMENT OP THE INFEBIOB VENA CAVA IN THE BABBIT-EMBRYO (SECOND STAGE). 



(After Lewi?.) 



diminishes and ultimately forms the small ascending lumbar vein (fig. 284). 

 The transverse iliac vein becomes the terminal portion of the left common iliac 

 of the adult, and the other anastomoses carry the lumbar veins over the vertebral 

 column to the right cardinal, now the vena cava. The inferior vena cava is thus a 

 composite vessel formed from (1) the common hepatic vein ; (2) branches of the hepatic 

 in the liver ; (3) right sub-cardinal; (4) lower part of the right cardinal. 1 



The upper portions of the posterior cardinals become the azygos veins. The 

 right remains complete as the vena azygos major ; the left is interrupted by the 

 development of one or more post-aortic anastomoses. The lower portion becomes 



1 In a preliminary note, which has appeared since the above was written, Huntington and McClure 

 (Amer. Jour, of Anat. vi.) describe the development of the post-cava in the cat rather differently. 

 According to them, a pair of veins develops dorso-median to the primitive posterior cardinal by longi- 

 tudinal anastomosis between somatic post-cardinal tributaries. These form the post-renal parts of the 

 post-cava by a secondary median fusion by means of post-aortic anastomoses, and also become the azygos 

 veins, replacing the primitive cardinals. Only in its pre-renal section has the primitive cardinal any part 

 in forming the vena cava inferior. 



