1 70 WHALES 



Some authors hold that the duct [ductus arteriosus; see Fig. go) connecting 

 the aorta and the puhnonary artery in foetuses, which closes up after 

 birth, remains open in adult Cetaceans. They claim that this has some 

 connexion with changes in blood pressure when the animal dives, but I 

 have failed to find any evidence for this hypothesis. I hav-e, in fact, 

 examined the hearts of many adult porpoises and dolphins of different 

 species, and ha\^e even managed to dissect the ductus arteriosus of twenty 

 Blue and Fin Whales aboard the JVillem Barendsz, but all the ducts were 

 found to be entirely closed or so constricted that practically no blood could 

 have passed through them. The ducts of adult animals could therefore not 

 have played any part in regulating the pressure of the vascular system. 

 However, while after birth the ductus arteriosus of terrestrial mammals very 

 quickly closes up into a band of hard connective tissue, this closure is 

 significantly retarded in Cetaceans as well as in the Common Seal. A very 

 narrow passage may be left open until the age of 8-12 weeks in the Com- 

 mon Seal, 4-14 months in porpoises and dolphins and 5-13 years in Blue 

 and Fin Whales. The explanation of this phenomenon may be found in 

 the fact that the closure of the duct is highly influenced by the oxygen 

 saturation of the blood. Shortage of oxygen in the blood, and perhaps 

 also a rise of the blood pressure in the pulmonary artery, may cause a 

 temporary reopening of the duct. Consequently, respiratory difficulties 

 in the first weeks after birth may prevent or retard its closure. Such 

 difficulties may be expected to occur in all Cetaceans, because they 

 are born in the water and swim and dive immediately after birth. The 

 same difficulties may occur in the Common Seal because the pups of this 

 species are obliged to enter the water a few hours after birth (they are 

 born on sand banks which are flooded at high tide). In most other 

 Pinnipeds the pups generally do not enter the water before they are 3-4 

 weeks old or even older. All data available at the moment point to the 

 fact that in these animals the duct closes as quickly as in terrestrial 

 mammals. 



Since we are discussing embryonic blood vessels, the reader may be 

 interested to know that the ductus venosus, i.e. the duct joining the portal 

 and umbilical veins to the inferior vena cava, though present in the early 

 stages of the Cetacean embryo, disappears half-way through the period of 

 gestation. In Blue and Fin Whales, it is found in foetuses less than about 

 yi feet in length, but not in others. This peculiarity of the Cetaceans also 

 needs to be investigated further. 



Not all the Cetacean retia have an exclusively or predominantly blood- 

 pressure-regulating function. If, for instance, we examine the retia of the 

 brachial artery, i.e. the artery running to the flippers, we are struck by 

 characteristic differences from the other retia. Where the latter consist 



