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both posterior cardinal veias, the aDterior sections of which may 

 renaain developed, as in Salamandra and Bombinator (Fig. 2E), or 

 partly or wholly degenerate (Fig. 2F); in any case the hepatic veins 

 open into the vena cava. 



While the renal section of the vena cava is now definitely known 

 to be derived from one or both posterior cardinal veins, the nature 

 of its anterior section is less certain, the prevailing opinion being that 

 it represents an hepatic vein wich has made a new connection with 

 the posterior cardinal vein. This view is based upon the manner of 

 its development in some of the Amphibia where the anterior section 

 of the cava appears to be originally one of the vitello-intestinal or 

 hepatic veins, although the accounts given leave room for reasonable 

 doubt as to the exactness of this description. The arrangement of 

 these veins and the method of their development in Ceratodus — the 

 most primitive form in which the cava develops — do not support this 

 view. It is especially significant that in Ceratodus the hepatic veins 

 open into the heart independently of the vena cava. Ceratodus is 

 the only known form where this arrangement exists. In Protopterus, 

 whose venous system is most nearly like that of Ceratodus, the hepatic 

 veins open into the vena cava (Fig. 2D) while in all Amphibia this 

 is the invariable arrangement. In the adult Ceratodus the vena cava 

 passes straight through the liver and receives from it only very small 

 twigs so that it has none of the characteristics of an hepatic vessel. 

 At the same time the two hepatic veins are developed typically in 

 addition to this hepatic section of the cava. The anterior portion of 

 the vena cava develops very late, long after the hepatic circulation i& 

 completely established. It is indicated first as a short but wide vessel 

 opening into the sinus venosus and develops centrifugally from the 

 heart lying at first just in the surface of the liver, not deeply em- 

 bedded like a typical hepatic vein ; later it extends above the liver 

 attached to the hepatic mesentery. At no time during its development 

 does it share in the hepatic circulation but always passes straight 

 through the liver without receiving any branches. There is nothing 

 in its anatomical relations or embryological history to indicate that it 

 is the derivative of an hepatic vessel. The word "hepatic" as applied 

 to this portion of the vena cava then would merely be descriptive 

 of its location and would not indicate the primary nature or derivation 

 of the vessel. 



I should offer the following physiological explanation of the origin 

 of the vena cava. It will be recalled that during the functional im- 



