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WHALES 



Figure loj. Alan's ear. X = external 

 auditory meatus; Tm ^= tympanic 

 membrane; To = tympanic cavity ; A = 

 auditory ossicles [malleus, incus, and 

 stapes) ; 5 = semi-circular canals 

 (organ of equilibrium) ; P = petrosal; 

 C = cochlea; E = Eustachian tube 

 {connecting middle ear with the posterior 

 part of the nose). {IJsseling and Schey- 

 grond, 1 95 1.) 



Dolphins not only pick up sounds below the surface at distances of 

 eighty feet, but that they can locate the source of the sound, and Dudok 

 van Heel's porpoise experiments at Texel have shown just how accurately 

 they can do so. His observations were made in an echo-free basin after 

 the animal had lost its natural curiosity about the tank and reacted only 

 to sounds representing food signals. By gradually bringing two sources of 

 sound nearer to each other, it was found that the animals could distinguish 

 food signals of 6,000 cycles down to an angle of 16° between the sources. 

 Now, man can distinguish sounds of 1,500 cycles coming from two sources 

 which make an angle of only 8°, but when we consider that sound travels 

 four times as fast in water as it does in air, and that the distance between 

 a porpoise's eardrums is half the distance it is in man, the porpoise's 

 degree of directional hearing can be said to be comparable to man's. 



The fact that it took so many centuries before the auditory apparatus 

 of Cetaceans was properly understood may possibly be explained by the 

 persistent fallacy on the part of biologists that there was some connexion 

 between the hearing offish and Cetaceans and that the latter, too, relied 

 on bone conduction. From Claudius in 1858 to Guggenheim in 1948 and 

 Yamada in 1953, scientists have time and again fallen into the error of 

 thinking that the role of the middle ear and of acoustic insulation, which 

 play such important roles in the hearing of terrestrial mammals, was 

 negligible in Cetaceans. All the greater is Fraser and Purves's achievement 

 in being the first to show that whales hear exactly like other mammals, 

 even though acoustic isolation is produced in a very special way. 



In terrestrial mammals the bony skull does not respond to airborne 

 vibrations and the two eardrums are set into independent vibration by 



