434 GENEEATION. 



The mode of development of the veins is very simple. 

 Two venous trunks make their appearance by the sides of 

 the spinal column, which are called the cardinal veins, arid 

 run parallel with the superior vertebral arteries, or the two 

 aortse, emptying finally into the auricular portion of the heart 

 by two canals, which are called the canals of Cuvier. These 

 veins change their relations and connections as the first cir- 

 culation is replaced by the second. The omphalo-mesenteric 

 vein opens into the heart between the two canals of Cuvier. 

 As development advances, the liver is formed in the course 

 of this vessel, a short distance below the heart, and the vein 

 ramifies in its substance ; so that the blood of the omphalo- 

 mesenteric vein passes through the liver before it gets to the 

 heart. We have seen that the omphalo-mesenteric vein is 

 obliterated as the umbilical vein makes its appearance. The 

 blood from the umbilical vein is at first emptied directly into 

 the heart ; but this vessel soon establishes the same relations 

 with the liver as the omphalo-mesenteric vein, and its blood 

 passes through the liver before it reaches the central organ 

 of the circulation. As the omphalo-mesenteric vein atro- 

 phies, the mesenteric vein, bringing the blood from the in- 

 testinal canal, is developed, and this penetrates the liver, 

 becoming, finally, the portal vein. 



As the lower extremities are developed, the inferior vena 

 cava makes its appearance between the two inferior cardinal 

 veins. This vessel receives an anastomosing branch from 

 the umbilical vein, before it penetrates the liver, and this 

 branch is the ductus venosus. As the inferior vena cava in- 

 creases in size, it communicates below with the two inferior 

 cardinal veins ; and that portion of the two inferior cardinal 

 veins which remains constitutes the two iliac veins. The in- 

 ferior cardinal veins, between that portion which forms the 

 iliac veins and the heart, finally become the right and the left 

 azygos veins. 



The right canal of Cuvier, as the upper extremities are 

 developed, enlarges and becomes the vena cava descendens, re- 



