234 The DevelojDinent of the Vena Cava Inferior 



the cardinal in the caudad region, emptying again into the cardinal 

 anteriorly, and connected with the cardinal all along the line by cross 

 branches running between the mesonephric arteries. This new vessel 

 I would designate as the suhcardinal vein, Vena subcardinalis. The bi- 

 lateral symmetry of the veins at this stage is complete. The small ves- 

 sels from the mesentery, and mesenchyma ventral to the aorta are rep- 

 resented in the drawings, one of them being labelled V. m., and two 

 points of anastomosis with the veins of the opposite side are designated 

 by the letters V. m. x. Thus the suhcardinal veins are connected with 

 one another by vessels of small calibre. 



In Fig. 1, Plate I, a portion of the liver has been outlined. The 

 portal vein, V. 0. M., is shown cut across in the median plane. It 

 passes behind the intestine toward the right and forward through the 

 liver connecting with the ductus venosus, D. V. A., and ending in a 

 vessel known as the vena liepatica communis. This comprehensive 

 name was applied by Hochstetter (93, p. 552) to that trunk passing from 

 the liver to the heart, and formed by the union of hepatic, umbilical, and 

 omphalo-mesenteric veins, to which the inferior vena cava is later added. 

 In the figure a black, partly dotted line marks the limit of the dorsal 

 hepatic lobe, which is filled with sinusoids. Some of these channels, 

 Y. c. m., have extended over into the caval mesentery, into which the 

 hepatic cylinders are to follow them. 



A large stream of blood traverses the liver through the broad venous 

 spaces unimpeded, whereas the current through the cardinal veins is 

 clogged by the AYolffian tubules. The development of the posterior 

 limbs demands a freer passage, and the formation of the subcardinals 

 may be regarded as the attempt of the cardinal veins to become disen- 

 tangled from the Wolffian body. Probably the recuri'ent bend of the 

 duct of the Cuvier is another obstruction to the posterior cardinal system. 

 At all events the right suhcardinal and the hepatic sinusoids approach 

 one another and unite, thus forming a new access to the heart. All 

 the component parts of the adult vena cava inferior have now become 

 connected. The new passage is so favorable that it enlarges rapidly. 

 On the left side, the suhcardinal can make no connection with the liver, 

 since the stomach has cut off any approach to that organ. There has 

 been no reversal of blood currents on either side, but blood from the 

 lower left area crosses to the right through the anastomoses between 

 the subcardinals. The cardinal system has been tapped by the hepatic. 



Figs. 3 and 4, PI. I, represent a rabbit of 8.8 mm. The suljcardinal 

 veins, V. Sc, are now large, and that on the right has connected with 

 the vena hepatica communis. See also the cross section, Fig. 4, p. 231. 



