6o6 



STRUCTURE OF VERTEBRATES 



from the cardinal to the heart through the hepatic portal, the 

 anterior part of which is now called the posterior vena cava. 

 There are cross connections between the left and right posterior 

 cardinals near the kidney, and blood from the left also flows 

 into the vena cava. The anterior parts of the posterior cardinals 

 become much reduced or atrophy. One of the best known relics 

 is the azygos vein of the rabbit, which drains the intercostal 

 region from the fifth rib backwards and empties into the right 



3 

 5 



Fig, 468. — Diagrams of the venous system. 



A, oi a dogfish ; B, of an amphibian ; C, of a rabbit. 



The primary system in grey ; that of the lateral and anterior abdominal veins in black ; the inferior 

 vena cava in white. The hepatic portal system is omitted. 



I, Entry to heart ; 2, left superior vena cava or precaval or ductus Cuvieri ; 3, left internal jugular or 

 anterior cardinal ; 4, left external jugular or inferior jugular ; 5, left subscapular ; 6, left posterior 

 cardinal ; 6', position of same in a newt ; in the frog the posterior cardinals are absent ; in the rabbit 

 the portion shown by dots is wanting ; 6", right azygos vein representing right posterior cardinal in 

 a mammal ; 6', left azygos vein ; 7, left deep lateral vein ; 7', pelvic ; 7", anterior abdominal, repre- 

 senting both deep laterals fused ; 8, renal portal ; 9, caudal (wanting in frog) ; 10, external iliac or 

 femoral ; 11, internal iliac or hypogastric ; 12, inferior vena cava or postcaval ; 13, junction between 

 azygos veins ; 14, left subclavian. 



anterior vena cava. In mammals and birds the renal portal 

 system is lost, and only the right posterior cardinal is retained, 

 to form the posterior portion of the posterior vena cava. 



The anterior cardinals do not differ greatly in the vertebrate 

 classes, except that in mammals the original lateral veins of 

 the head are replaced by new intracranial internal jugulars. 

 Where the posterior cardinals have been replaced by the 

 posterior vena cava the common cardinal no longer receives 

 blood (except the little that comes from the azygos vein) from 

 behind the heart, and so it is associated only with the anterior 

 cardinals. Common cardinal and anterior cardinal of each side 



