102 Proceedings of the Royal Physical Society. 



XIII. — Note on the Posterior Vena Cava in Polypterus. 

 By J. Graham Kerr. 



(Read 28th March 1910. Received 7th April 1910.) 



One of the interesting phylogenetic puzzles connected with the morphology 

 of the higher vertebrates is that associated with the evolution of the inferior 

 or posterior vena cava — the great vein which, in the higher vertebrate, serves 

 to drain the blood from the hinder region of the body to the heart. The 

 special element of puzzle about the inferior vena cava is afforded by its 

 developmental history, for, as is well known, the vessel in question has a 

 double origin. Its hinder portion is simply a persisting portion of the 

 posterior cardinal or interrenal vein, while its front portion develops quite 

 independently in relation with the veins of the liver, and only secondarily 

 becomes joined up to the posterior cardinals, forming a direct channel by 

 which their blood passes straight to the heart, and the establishment of 

 which speedily brings about the reduction of the anterior parts of these 

 vessels. 



How has this short circuiting of the renal blood stream come about in 

 phylogeny ? A probably satisfactory answer to this question is afforded 

 by the conditions in the lung-fishes. We see how, in these forms, the 

 kidneys extend relatively far forward, while the liver extends relatively 

 far back along the right side of the splanchuocoele. As a result of this 

 arrangement we see, in Lepidosiren and Protopterus, that the tip of the 

 liver is in contact, and fused, with the tip of the right kidney. 



A natural result of the contact between the liver, with its perfect blood 

 drainage towards the heart via the hepatic vein, on the one hand, and the 

 highly vascular kidney on the other, has been the establishment of a venous 

 anastomosis between the two organs leading to the passage of renal blood 

 to the heart through the substance of the liver. With the establishment 

 of this anastomosis the hepatic vein has become a large direct channel 

 leading from the tip of the right kidney straight through the liver, receiving 

 numerous branches from the liver substance as it does so, to the heart. It 

 is this main trunk of the hepatic vein which forms the anterior section of 

 the posterior vena cava. In other words, the condition in the two lung- 

 fishes in question supports the view that what we call the anterior section 

 of the posterior vena cava is really the primitive hepatic vein, and that 

 what we commonly call hepatic veins in the higher vertebrates are lateral 

 branches of this primitive hepatic vein. 



