274 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



The Posterior Cardinal Veins and the Origin of the Inferior Vena Cava.— 



The posterior cardinal veins course cephalad along the dorsal side of the meso- 

 nephroi and open into the common cardinal veins (Fig. 282 A). Each receives 

 an ischiadic vein from the posterior extremities, mesonephric branches from the 

 mid-kidney and dorsal intersegmental veins from the body wall (Fig. 282 B). 

 Median and ventral to the mesonephros are developed the subcardinal veins which 

 are connected at intervals with the posterior cardinal veins by mesonephric sinu- 

 soids, and with each other by anastomoses ventral to the aorta. Thus all the 

 blood from the mesonephroi, posterior extremities, and dorsal body wall is in early 

 stages drained by the posterior cardinal veins alone. 



The development of the unpaired vena cava inferior begins when communica- 

 tion is established between the right hepatic vein of the liver and the right sub- 

 cardinal vein of the mesonephros, primarily a tributary of the posterior cardinal 

 vein (Lewis, 1902). 



The liver on the right side becomes attached to the dorsal body wall and from 

 its point of union a ridge, the plica vence cavce (Fig. 199), extends caudalward. 

 According to Davis (1910), capillaries from the subcardinal vein invade the plica 

 venae cavae, and, growing cranially, meet and fuse with capillaries extending 

 caudad from the Hver sinusoids. 



Thus is formed the vein of the plica vence cava (Fig. 282 ^4), which is already- 

 present in human embryos of 2.6 mm. (Kollmann). This vein rapidly enlarges,, 

 as also do the sinusoidal connections between the subcardinals and posterior car- 

 dinals at one point. Thus the blood from both lower posterior cardinals is soon 

 carried to the heart, chiefly by way of the right subcardinal and right hepatic veins 

 (Fig. 282 B). Soon the posterior cardinals just cranial to their enlarged anasto- 

 moses with the subcardinals become small and are interrupted (Fig. 282 C). 

 Cranial to their interruption these veins communicate by a cross anastomosis 

 and were formerly believed to persist as the vv. azygos and hemiazygos of the 

 adult. 



In the pig the posterior cardinal veins develop along the sides of the large 

 mesonephroi and completely disappear with that organ. Sabin (Carnegie Publ. 

 No. 223, 1915) has thus been able to confirm and extend the conclusion of Parker 

 and Tozier that the m. azygos and hemiazygos are entirely new veins derived from a. 

 prevertebral capillary plexus. The more caudal portions of the posterior cardinal 

 veins likewise atrophy completely and are replaced by new prevertebral vessels 

 having essentially similar topographical relations. Caudal to their lowest trans- 

 verse connection (z, Fig. 274 C) , the new right vessel becomes the right common iliac 



