HEART, CIRCULATION, AND BLOOD I57 



This is due to the fact that the heart contracts roughly once every second, 

 forcing large quantities of blood into the aorta and the pulmonary artery 

 as it does so. In order to temper the effects of these sudden spurts and to 

 avoid excessive pressure changes in the smaller blood vessels, the main 

 arteries leaving the heart have thick, strong walls containing a large 

 proportion of elastic tissue which make them look yellow. In the medium- 

 sized and smallest arterial branches, smooth muscle is more abundant, the 

 elastic tissue being correspondingly reduced. As we grow older, the general 

 elasticity of the arteries decreases, and more force is needed to push the 

 blood through them. As the blood pressure rises, a greater strain is put 

 on the heart which now has to force the blood through a more and more 

 rigid pipe. Elasticity is, therefore, a sine qua non of sound circulation. 



The more blood is squeezed out with every heartbeat, and the greater the 

 force of the contraction, the longer the elastic region of the arteries needs 

 to be. Now from my own comparative study of the arteries of a hoise and 

 a Bottlenose Dolphin (see Fig. 8g), it appeared that in both animals the 

 elastic region had the same length, and that consequently, in this respect, 

 too. Cetaceans do not differ greatly from other mammals. 



'Le coeur est énorme,'' the well-known French physiologist, Paul Portier, 

 wrote in 1938, in his excellent little book on aquatic animals. How right 

 he was emerges clearly when we realize that, while the hearts of the 

 rhinoceros and the elephant can be handled on a dissection table, the 

 heart of the whale has to be dragged over the slippery deck of a whaling 

 ship by seven strong men - and then with some difficulty. The poor 

 biologist who wishes to cut up this huge mass, almost 6 feet wide and 

 lo-ii cwt in weight, may well shy from such an Augean labour. Still, 

 with the occasional help of a derrick or steam winch, he has managed to 

 solve this problem remarkably well. 



Since inere weight tells us very little about the comparative size of the 

 heart, biologists are much more concerned with the weight of the organ 

 expressed as a percentage of the total body weight. Now, while earlier 

 workers, e.g. Zenkovich (1937), found the heart of the Sperm Whale and 

 some larger Rorquals to represent 2 -6-3 -9 per cent of the animal's total 

 weight, more detailed Japanese investigations made since the Second 

 World War have revealed that the average percentages are 0-5 per cent 

 in Blue Whales, Fin Whales and Humpback Whales and o -4 per cent in 

 Sei Whales. I myself measured averages of o -5 per cent in Little Piked 

 Whales, o -85 per cent in porpoises, o -93 per cent in Bottlenose Dolphins, 

 and 0-6 per cent in all dolphins taken together, the Beluga included. 

 Figures of 0-34 per cent and o -7-0 -8 per cent are reported for Sperm 

 Whales and Biscayan Right Whales respectively. 



We have already seen that in mammals there exists a close correlation 



