B6 THE ANATOMY OF YERTEBIIATED ANIMALS. 



Of tlie three veins wliicli open into the venous sac — viz., 

 the inferior cava, and the right and left ductus Cuvieri — all 

 may persist, the latter receiving the title of right and left sur 

 perior cavce. Or, as very often happens in the higher Verte- 

 hrata^ the left ductus Cuvieri becomes more or less obliterated ; 

 the veins which properly open into it acquiring a connection 

 v^'ith the right ductus^ which then remains as the sole superior 

 cava. The posterior cardinal veins give off anastomosing 

 branches, which are converted into the veiice azygos j the an- 

 terior cardinal veins become metamorphosed into the external 

 jugular veins and venae innominatce. 



In Fishes, the sinus venosus and the cardinal veins persist 

 throuo'hout life ; but the anterior cardinal veins, which brins* 

 back the blood from the head and from the anterior extremi- 

 ties, are called ve7ia3 jugulares. 



The caudal veins are either directly continued into the 

 cardinal veins, as in Marsipohrancliii and Elasmohranchii, or 

 branch out into the kidneys, as in many Teleostei, In either 

 case the efferent renal veins open into the cardinal veins. 



The portal veins, conveying the blood of the chylopoietic 

 viscera, and sometimes that of other organs and of the abdomi- 

 nal walls, may be one or many. In A.mp)hioxus and 3Iyxine 

 the vein is rhythmically contractile, and forms a portal heart. 



In most Amphibia and Heptilia the sinus venosus persists, 

 and is rhythmically contractile, valves being placed at its 

 opening into the right auricle. 



The anterior cardinal veins are represented by jugular 

 veins, the posterior cardinal by vertebral veins ; these, and the 

 veins of the anterior extremities, when they are present, pour 

 their blood into the ductus Cuvieri, which are now termed an- 

 terior vencB cavce. 



The veJia cava inferior takes its origin chiefly by the coa- 

 lescence of the efferent veins of the kidneys and reproductive 

 organs, and does not always receive the whole of the hepatic 

 veins — more or fewer of the latter opening independently into 

 the sinus venosus. 



The blood which leaves the kidneys b}- its efferent veins 

 is supplied, not only by the renal arteries, but by the veins of 

 the caudal region, and of the hinder extremities, which branch 

 out like a vena portce in the substance of the kidneys. This 

 renal portal system is less developed in Heptilia than in Am.- 

 p>hihia. All the blood of the posterior extremities and caudal 

 region does not traverse the kidneys, however, more or less 

 of it being led away by great branches of the iliac veins, which 



