AIR-BREATHING IN FISHES 



31 



and the venous return is collected into the hepatic portal vein 

 before entering the liver. This efferent blood vessel, of course, 

 will contain more highly oxygenated blood than that which 

 comes to it from the dorsal aorta after its passage through the 

 less well-developed gills. In conditions where the gills are able 

 to oxygenate the blood sufficiently, the accessory organ is 

 scarcely functional and at most acts as a booster station. 



Table 4 

 Blood supply to accessory respiratory organs 



