342 HISTORY OF THE HUMAN BODY 



As has been shown above, the Class of fishes comprises 

 forms which have remained at the stage last described, the 

 one in which the cardinal system holds the supremacy; but 

 by the time the amphibians are reached there has been another 

 usurpation in that part of the body posterior to the heart, 

 and the posterior cardinal system has, in its turn, become 

 subordinated to a third system, that of the vena cava posterior, 

 or, more briefly, the postcava. How this appears in full func- 

 tional activity is seen in the diagram representing the main 

 venous channels of the urodele (Fig. 96, pc), where it has 

 secured nine-tenths of the traffic between the kidneys and the 

 heart, and allows but a small part to be conveyed by the pos- 

 terior cardinals, formerly completely in charge of this territory. 

 Still another rival of the cardinal system has appeared in the 

 abdominal vein (abd), which begins as two lateral veins that 

 issue from the iliacs, run along the ventral abdominal wall 

 until they meet in the median line, and continue as a single 

 vessel until opposite the liver, when the vessel leaves the 

 body wall, and enters this latter organ, forming a part of the 

 hepatic portal system. 



The origin of the vena cava historically cannot be now 

 learned from adult anatomy, since it undoubtedly took place 

 in those forms which successfully achieved the transition from 

 an aquatic to a terrestrial life, or to, at least, a paludic one, 

 and, having left for their descendants this new world with 

 its opportunities, perished and left no trace save in the per- 

 fected parts which render a terrestrial life possible. 



Here again, however, embryology furnishes us with some 

 information concerning at least the place and mode of origin 

 of this new vein, as may be seen by a comparison of the 

 diagrams given in Fig 97, d and e, where is shown the develop- 

 ment of the postcava in the lizard. During the early stages in 

 the development of the liver and its extensive system of capilla- 

 ries, developed in association with the portal system to be con- 

 sidered later, the postcava appears as a partially distinct element 

 in this capillary system, and becomes gradually more definite. 

 This vein grows posteriorly and ultimately reaches the renal 



