HEART, CIRCULATION, AND BLOOD 



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171 



Figure gj. Cross-sections through [A) the flukes [B) the pectoral fin of a Bottlenose Dolphin, 



showing position of arteries surrounded by a sheet of veins. One such 'sheet' is magnified in C. 



Small veins not accompanied by arteries are found close under the skin. {Modified after 



Scholander and Schevill, 1955-) 



predominantly of imperceptibly intertwined arterioles with many 

 anastomoses, i.e. of a real netwoi'k, the brachial artery splits up into a 

 great number of parallel vessels of uniform width with few interconnec- 

 tions (Fig. 95). Along and between these vessels there is a system of 

 similarly constructed brachial veins. As far as is known such special retia 

 occur in all Odontocetes. Mysticetes have not yet been examined in 

 sufficient detail, but Ommanney stated in 1932 that Fin Whales have a 

 single brachial artery surrounded with a net of veins. 



Similar retia are also found in Odontocete chevron canals in which the 

 artery carrying blood to the flukes is surrounded with a mixed arterial-cum- 

 venous vascular network. In the flukes themselves, the artery splits up 

 into a very large number of arterioles, all of which are surrounded with a 

 network of mainly longitudinal venules which link up with the retia in the 

 chevron canal (Fig. 97) . The flukes also have another multiple system of 

 venules just under the skin. These join up with the superficial caudal veins, 

 which return the blood to the abdominal retia. The chevron canals of 

 Rorquals have only one artery and one vein, but the arteries in the flukes 

 are surrounded with the same kind of venous network as are found in 

 Odontocetes. Arterioles surrounded with venules are also found in the 

 dorsal fin, the flippeis and, indeed, throughout the entire integument of 

 Rorquals as well as in the baleen (see Chapter 11). 



Their precise significance only became clear during the Second World 

 War, when the Norwegian physiologist P. F. Scholander who, as we have 

 seen, was working in the United States at the time, investigated the effect 

 of exposure on airmen who were forced to bale out over the sea. In this 



