1909.] 



VISCERA OF A AVALRUS. 



73S 



In Cetacea and Pinnipedes the vena cava and hepatic veins 

 combine to form an enormous reservoir between the liver and the 

 diaphragm (Barkow, "Die Blutgefasse ") in which an immense 

 quantity of blood can be stored. The exit from this reservoir is 

 comparatively narrow and can obviously be still further decreased 

 or possibly completely closed by sucli a sphincter band as that 

 described above. 



Text-fig. 231. 



Sph. 



The tliovacic vena cava inferior of a Walrus, showing the diaphragmatic 

 sphincter band, Spli. 



There seems little reason to doubt that the venous reservoir 

 and the sphincter of the vena cava form parts of one mechanism, 

 the use of which is possibly to restrict the flow of venous blood 

 to the heart and so to keep up the average ]3urity of the blood 

 when the animal is immersed. For it is clear that the more the 

 aeration of the blood is confined to that necessary for the action 

 of the central nervous system and the voluntary parts of the 

 animal, the further the oxygen stored in the lungs will go in 

 carrying on the absolutely necessary activities of the body and 

 the longer the animal will be able to stay immersed. It would 

 thus seem to be of decided advantage to a diving mammal to be 

 able to prevent, temporarily, the blood returning from the alimen- 

 tary canal and liver (which forms a very great part of the blood 

 carried by the vena cava inferior) from reaching the heart and 

 lungs, for if this mass of impure blood were poured into the 

 lungs, as in the ordinary course of circulation, it would tend to 

 very greatly hasten the general fouling of the blood. 



But during immersion the action of the vegetative as opposed 



