HEART, CIRCULATION, AND BLOOD 169 



This unexpected state of affairs points to the conckision that great 

 internal pressure difi'eixnces occur not so much when the animal swims 

 at great depths as when it makes sudden vertical movements. After all, 

 when a lOO-foot whale dives down perpendicularly, the difference in 

 water pressure between its snout and tail is 3 atmospheres - quite sufficient 

 to affect the circulation. Moreover, it is quite possible that, as a result of 

 changes in the rhythm of the heartbeat which occur on surfacing and 

 sounding, further pressui"e differences arise in the vascular system, and 

 that such diflferences may also be due to the whale's characteristically 

 violent respiration, which causes the entire lung to contract or expand 

 and to expel or suck in large quantities of air within a very short space 

 of time. 



Experiments with terrestrial animals and man have long ago shown 

 that sudden pressure differences in the lung can produce very appreciable 

 pressure fluctuations in the vascular system. This influence of respiratory 

 movement on man's system is demonstrated very clearly by \'alsalva's 

 experiment. In this, the mouth and nose are closed and the subject tries 

 to exhale. The pulse rate is accelerated and the arterial pressure raised 

 owing to the great rise in thoracic pressvue. Blood pressure changes are 

 more marked still, when air from the lungs is allowed to escape suddenly 

 through an open mouth. Now, if we bear in mind that porpoises and 

 dolphins not only breathe and dive much faster and more frequently than 

 any of the big whales, but that their relative lung capacity is far greater, 

 we may appreciate not only why their retia are so very well developed 

 but also that retia must be adaptations to quick and frequent rather than 

 to deep or long dives. However, our knowledge of the entire subject is still 

 so incomplete that it would be wisest to await experimental proof before 

 we jump to hasty conclusions. 



Apart from the retia and their related structures, the vascular system 

 of Cetaceans shows other peculiarities that are only partly associated 

 with pressure differences. Thus, some animals were found to have marked 

 distensions of the aortic arch and of the pulmonary artery. However, the 

 literature is so full of contradictory explanations of this phenomenon, and 

 my own observations are so inconclusive, that a more detailed discussion 

 would be pointless. 



Though it is generally held that the internal carotid artery of Cetaceans 

 closes up before birth, just as that of Even-Toed Ungulates, de Kock has 

 shown recently that, at least in porpoises and Pilot Whales, the vessel is 

 still open in adult animals, although its lumen is very narrow. The thick 

 layer of circular muscle in its wall, and the fact that it is innervated, 

 suggest that the vessel plays a part in controlling the flow of the blood. 



