HEART, CIRCULATION, AND BLOOD 165 



well developed vascular network towards the outer side of the brain case, 

 especially near the joint of the jaws, the bulla tympani, and the foramen 

 magnum. Sinaller networks occur particularly in the pelvic region and 

 in the sexual organs, but by far the largest mass of retia is found in the 

 head, the neck, and the thorax. Another remarkable network at the base 

 of the pectoi'al fin will be discussed later, since it probably has quite a 

 different function from the other retia. 



Siinilar networks are also cominon in sea-cows, but are not found in 

 sea-lions, seals, or other Pinniped Carnivores. 



Before we discuss the function of the retia in greater detail, we must 

 look at some other characteristics of the vascular system, and particularly 

 at those retia which consist entirely of veins. Unlike the mixed arterial- 

 cum-venous retia we have discussed, they are found almost exclusively in 

 the abdominal cavity. 



These retia, too, are generally most highly developed in porpoises and 

 less so in other Odontocetes. In Rorquals, they are restricted to the pelvic 

 region and the sexual organs, but in porpoises, tremendous networks 

 which, during dissection, are shown to be greatly distended with blood, 

 stretch right round the dorsal and lateral sides of the abdominal and pelvic 

 cavities. They obtain their blood mainly from a large vein running close 

 under the skin on the side of the tail and carrying venous blood from tail 

 and flukes. The retia give up their blood chiefly to the inferior vena cava 

 which runs close to the aorta on the dorsal side of the abdominal cavity 

 and which returns the blood to the heart. Hence it looks as if all the blood 

 from the abdomen and tail returning to the heart by way of the inferior 

 vena cava must pass through these retia on its way (Fig. 95) . However, 

 in all mammals, man included, the blood returning from the stomach, 

 the intestines and the spleen must first go to the liver by the portal vein, 

 usually to return by the hepatic veins to the inferior vena cava, after first 

 passing through the capillaries of the liver. In Cetaceans, this blood 

 therefore by-passes the abdominal retia. In addition, the portal system 

 itself has certain peculiarities, at least in all Odontocetes investigated 

 so far, i.e. porpoises and diflferent species of dolphins. 



At the turn of this century, French scientists, in particular, were 

 responsible for showing that the two main hepatic veins and also that part 

 of the inferior vena cava into which they run are very inuch distended in 

 these animals (Fig. 96). Moreover, the hepatic veins are sometimes, and 

 the inferior vena cava in the region of the diaphragm is always, equipped 

 with a muscular sphincter by which their diameter can be decreased or 

 by which they may be closed. I found that such distensions and annular 

 muscles were very pronounced in a 6i-foot female Bottlenose Dolphin. 

 While other Odontocetes still need to be investigated I have personally 



