l68 WHALES 



gixater than that in the rest of the body. In that case, the blood from the 

 abdominal cavity will not be able to return to the heart, since normal flow 

 can only take place if the thorax is at a lower pressure. The blood stowed 

 in the abdomen would have injurious eflfects, were it not that it can be 

 stored in the venous retia and in the distended hepatic veins. The annular 

 muscle of the inferior vena cava will be closed, preventing more blood 

 being syphoned from the heart into the abdomen, while the blood from 

 the brain and the rest of the head, unable to return to the thorax, will 

 retiu'n to the abdomen via the spinal veins. No doubt the walls of the 

 inferior vena cava will be greatly strained by this tremendous influx of 

 blood, but their special structure is adequate to this task. This was shown 

 clearly by A. von Kügelgen, a German histologist, who found that the 

 wall of the vena cava of a Fin Whale is much more liberally endowed 

 with both elastic tissue and muscle tissue than it is in terrestrial mammals, 

 and is thus far more capable of withstanding great pressm^es than, say, 

 the inferior venae cavae of horses or men. 



With increased pressure in the thorax, little blood will be able to pass 

 through the lungs, and hence to the heart. The retia in the thorax are 

 'shut', and all the available blood will thus go to the brain by the most 

 direct route. 



Now, let us imagine the reverse case, i.e. the pressure in the thorax 

 being lower than in the rest of the body. Clearly blood will then be sucked 

 into the thorax from all directions, and the heart would become over- 

 loaded as it tries vainly to distribute blood to the rest of the body against 

 a high pressure gradient. This, too, is prevented by the retia, whose 

 ramified arterioles in the thorax and neck take up most of the blood, thus 

 reducing the pressure of the flow to normal. As we have seen, the structure 

 of the arterial system proper of Cetaceans shows no signs of constant or 

 intermittent high blood pressure, and it is obviously the function of the 

 retia to prevent high pressure from building up. When the pressure in the 

 thorax is low, the blood from the brain can easily return to the heart. 

 Moreover it is quite possible that blood stored in the abdominal cavity 

 returns not only by the inferior vena cava but also along the spinal veins, 

 whenever more blood is needed quickly by the heart. 



If we accept this explanation for the presence of the retia and other special 

 vascular characteristics, we naturally want to know under what conditions 

 pressure differences in the Cetacean body arise. We might be tempted to 

 think that they are associated with deep diving, in which the heart slows 

 down and the supply of blood to the muscles and intestines is reduced or 

 stopped. However, since the retia are more highly developed in porpoises 

 and dolphins than they are in Rorquals, Sperm Whales, Bottlenose Whales 

 and other champion divers, this hypothesis must be rejected. 



