DEVELOPMENT OF THE VEINS 283 



ruption these veins were formerly believed to persist as the vv. azygos and hemi- 

 azygos of the adult (Fig. 274, C). 



Sabin {Anal. Record, vol. 8, p. 82, 1914) has confirmed the conclusions of Parker and 

 Tozier (Bull. Museum Comp. Z06L, Harvard, 1908) that the w. azygos and hemiazygos 

 are new veins. They are formed in pig embryos as longitudinal anastomoses ventral to the 

 vertebral and median to the posterior cardinal veins and open into the upper ends of the latter. 

 Except for this short upper region the posterior cardinals in the thoracic region drain into the 

 new veins and become tributary to them. The right upper posterior cardinal vein drains into 

 the 1). azygos, the left upper posterior cardinal vein into the v. hemiazygos. 



The portions of the posterior cardinal veins caudal to their interruption re- 

 main for a while symmetrical and connected by anastomoses (Fig. 274, C). Soon 

 the caudal anastomosis between them enlarges until the blood from both sides is 

 drained into the right posterior cardinal vein (Fig. 274, D) . A branch of the post- 

 cardinal vein encircles the ureter of the metanephros. This vein is known as the 

 supracardinal. Caudal to their transverse connection the right posterior cardinal 

 becomes the right common iliac vein. The corresponding portion of the left pos- 

 terior cardinal with the transverse anastomosis becomes the longer left common 

 iliac vein. The blood from these veins is now drained by the unpaired vena cava 

 inferior which is composed of the following veins: (1) the common hepatic and 

 right hepatic veins (primitive right vitelline); (2) the vein of the plica vena? 

 cava?; (3) a portion of the right subcardinal vein; (4) the supracardinal vein of 

 the right side; (5) a portion of the lower right posterior cardinal vein. 



The permanent kidneys take up their positions opposite the great anastomosis between 

 the posterior cardinals and the subcardinals and at this point the renal veins are developed. 

 On the left side, the anastomosis connecting the right subcardinal with the left posterior cardinal 

 persists as part of the left renal vein. A persisting portion of the lower left posterior cardinal, 

 according to Hochstetter, forms the proximal part of the left spermatic or ovarian veins. The 

 dorsal segmental veins of the lower posterior cardinals form the lumbar veins. Transverse 

 anastomoses connect those of the left side with the right posterior cardinal after the atrophy 

 of the left posterior cardinal. The cephalic portion of the left subcardinal vein persists as the 

 suprarenal vein, which thus opens into the renal vein instead of joining the vena cava inferior 

 as does the right suprarenal vein. 



The Veins of the Extremities. — The primitive capillary plexus of the upper and lower 

 limb buds gives rise to a border vein (Fig. 277), which courses about the periphery of the flat- 

 tened limb buds (Hochstetter). In the upper extremity, the ulnar portion of the border vein 

 persists, forming at different points the subclavian, axillary, brachial and basilic veins. The 

 border vein at first opens into the dorsal wall of the posterior cardinal vein (embryos of 10 mm.), 

 but, as the heart shifts its position caudalward it finally drains by a ventral connection into 

 the anterior cardinal or internal jugular vein (F. T. Lewis) . The cephalic vein develops second- 

 arily in connection with the ulnar border vein, later in embryos of 23 mm. anastomoses with the 

 external jugular and finally drains into the axillary vein as in the adult. With the development 



