DEVELOPMENT OF THE VEINS 273 



pelrosal sinus, but the inferior petrosal sinus is formed as a new channel median to the 

 internal ear. The anterior cerebral vein becomes the superficial middle cerebral of adult 

 anatomy. A more detailed account of these changes may be found in the original work 

 of Mall (Amer. Jour. Anat., vol. 4, 1905). 



The Posterior Cardinal Veins and the Origin of the Inferior Vena 

 Cava. The posterior cardinal veins course cephalad along the dorsal 

 sides of the mesonephroi 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 inter segmental 

 veins from the body wall (Pig. 282 B). Median and ventral to the meso- 

 nephros are developed the subcardinal veins, which are connected at 

 intervals with the posterior cardinal veins by mesonephric sinusoids, 

 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 com- 

 munication is established between the right hepatic vein of the liver and the 

 right subcardinal 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 vena caves (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 liver sinusoids. Thus is 

 formed the vein of the plica venae cavos (x, Fig. 282 A), 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 pos- 

 terior cardinals at one point. Thus the blood from both lower posterior 

 cardinals is now 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 atrophy and are replaced 

 by the thoracic portions of new vessels, the supracardinal veins, which 

 lie dorso-mesial to the posterior cardinals and extend on each side from 

 the common cardinals veins to the union of the primitive iliac vessels. 

 The new veins communicate in the thoracic region by a cross anastomosis 

 and become the azygos and hemiazygos of the adult. The more caudal 

 portions of the posterior cardinal veins likewise atrophy and are also re- 

 placed by the supracardinal vessels of the lumbar region (Fig. 282 C). 



The blood from the lower trunk and leg region is now drained by the 

 unpaired inferior vena cava, which is composed of the following veins: (i) 



18 



