1 66 



VV HALES 



Figure g6. Highly diagrammatic rear view of the 

 liver ofRisso's Dolphin, showing position of distended 

 hepatic veins and their connexion with the posterior 

 vena cava. {Richards and Xeuville, i8cj6.) 



confirmed that Rorquals show no traces of such special hepatic distensions 

 or annular muscles, though it is known that they are present in sea-lions, 

 seals, walruses, and probably also in beavers, so that they are clearly 

 connected with diving habits. (Such annular muscles occur in some 

 terrestrial mammals as well, although in a more rudimentary form.) In 

 Common Dolphins, the branches of the portal vein are, moreover, pro- 

 vided with a special valve system similar to that found in their bronchioles 

 and pulmonary veins (see Chapter 4) . However, nothing is known about 

 this phenomenon in other Cetaceans. 



As the factory ship's powerful bone saws rip their way through the 

 vertebral column, we often get an excellent view of it in cross-section. 

 Below the strikingly small section of the spinal cord and the predominantly 

 arterial vascular network there appear two wide veins which, in Blue 

 Whales, can have a diameter of up to 4 inches. Such spinal veins are 

 found in all Cetaceans, where they are the principal vessels for returning 

 the blood from the brain, the jugular vein being very narrow. Moreover, 

 the spinal veins receive blood from all the thoracic intercostal veins, and 

 are connected at each lumbar vertebra with the posterior vena cava by 

 special vessels. Only in the caudal region do they thin out, for the caudal 

 blood returns either through the lateral veins found close under the skin 

 of the tail, or through the vein in the chevron canal. Two wide vessels, 

 which lie behind the second and third ribs (in Odontocetes on the right 

 side only) join the spinal veins (which also communicate with each other) 

 to the anterior vena cava, through which the blood is returned to the 

 heart. 



The spinal veins are by and large of uniform diameter throughout, and 

 have no valves. These two characteristics make it seem probable that the 

 blood in them can flow in both directions, depending on the animal's 

 needs. After all, the veins supply the anterior vena cava in the thorax, 



