RESPIRATION 



153 



Figure 88. The skull of an Indian 

 Porpoise, showing radiating muscles 

 for opening the blowhole. [Altered 

 after Howell, 1927.) 



the larynx is first expelled into the diverticula, and then released through 

 the blowhole. The exceptionally tough connective tissue surrounding the 

 diverticula and keeping the air pockets open is additional evidence in 

 favour of this hypothesis. This extra 'safety valve' is very useful indeed, 

 since, as we have seen, it regulates the escape of air under pressure, and 

 since whales are, in fact, known to release air while diving, possibly in 

 order to produce sounds (see Chapter 8). 



For the sake of completeness we must add that diverticulae apart, the 

 blowholes of Odontocetes and Mysticetes are opened and shut in identical 

 ways. True, Bottlenose Dolphins and Pigmy Sperm Whales are known to 

 have special muscles for closing the aperture, but nevertheless, the tensed 

 lips remain the main factor in shutting even their blowholes, which are 

 dilated in all Cetaceans by rays of muscle running down and attached 

 to the skull beneath (Fig. 88). Russian scientists have shown that in 

 dolphins the blowhole is opened by a reflex that is set off whenever the 

 animals surface, and that it is permanently shut underwater unless the 

 dolphin needs to open it for a special reason. This reflex may also be 

 linked with certain movements of the flukes which force up the blowhole 

 so that it is clear of the water. On the other hand, it must be stated that 

 when the blowhole of a porpoise is forcibly held above water, it opens 

 automatically at regular intervals, and the same phenomenon has been 

 observed in stranded whales and dolphins. 



