ANTERTOR ABDOMIXAL VETX IN THE INDIAN TOAD. 



447 



I have also concluded that the resistance offered to the blood 

 traversing the hepatic portal system is very little more than that 

 experienced by the blood when the anterior abdominal vein opens 

 directly into a pre-caval vein, and this is supported by the fact 

 that in such abnormalities (in which the anterior abdominal opens 

 directly into a pre-caval) the renal afferent veins are of about the 

 normal size. The renal venous meshwork in both kidneys then 

 offering considerable resistance to the passage of blood from the 

 hind limits and tail, I again suggest that this blood has sought an 

 additiontd path by means of which it can reach the heart wdthout 

 traversing the " renal portal " systems, and this suggestion is in 

 complete agreement with the view of morpliologists that the 

 anterior abdominal vein of Amphibia and lleptilia is a " new " 

 structure, and has nothing in common with the vein which it so 

 often resembles — viz., the nmbilical vein. 



Since we have provisionally concluded that the blood in the 

 anterior abdominal vein is of no use to the liver, and that the 

 opening of this vein into the hepatic portal vein offers practically 

 no more resistance to the passage of the blood than if it opened 

 directly into a pre-caval, tlie answer to the second question must 

 evidently be to the effect that the anterior abdominal vein opens 

 into the hepatic })ortal vein merely because it is more convenient, 

 the hepatic portal vein being more accessible than the pre-caval, 

 and offering, as we have seen, but little jnore resistance to the 

 tiovvof the blood. Occasionally, however (in " abnormal" frogs), 

 the ancestral connection of the anterior aJ)domina.l vein with a 

 pre-caval vein is retained. It may be added that in some animals 

 (e.g., in most Eiasmobranclis) the ])osterior cardinal sinuses are 

 so little broken up by the relatively small kidneys (Vialleton*) 

 that a bypath for the blood (anterior abdominal vein) is 

 unnecessary. 



* Vialleton, M. L., " Caracteres; lynijiliatiqiu's de cevhiines veiiies clicz quelqut,". 

 Squales." C. R. Hebdoiu. des Seances de la Soc. Biol. Paris, Touie liv. 1902. 



