HEARING 205 



hearing with that of fish. It has long been known that fish, too, respond 

 to sounds, and that, for instance, sharks can be frightened off by shouting. 

 However, the real facts only come to light in the course of the past fifty 

 years through ingenious investigations in which Prof. S. Dijkgraaf of 

 Utrecht played an important part. Now, hearing does not apparently play 

 a major role in the life offish since, though they can aurally detect under- 

 water vibrations, their ears alone cannot tell them the direction from 

 which the sound comes. As far as we know, fish lack both a middle and an 

 external ear, and sounds must therefore reach their inner ear by bone 

 conduction, i.e. by vibrations of the skull. When this happens, it is impos- 

 sible to tell the direction of the sound, which can only be detected if the 

 right and left ears are acoustically isolated from each other. In other 

 words, only if one ear receives the sound a fraction later than the other, 

 i.e. if there is a slight phase lag and a consequent difference in intensity, 

 can the source of the sound be located Avith any certainty. The greater the 

 distance between its two ears, the more accurate is a given animal's 

 'directional hearing'. 



The fact that bone conduction makes it impossible to locate a sub- 

 marine source of sound, was proved by Dr Reysenbach de Haan by 

 experiments on himself and a few collaborators. No special apparatus was 

 needed, since man's eardrums and auditory ossicles, which are designed 

 for receiving vibrations of the air, do not function under water, where 

 bone conduction takes over. The experiments showed clearly that men 

 cannot tell the source of a sound under water. The fact that fish seem to 

 be able to do so despite this handicap must probably be attributed to 

 their having a special organ, the so-called lateral organ which, until 

 recently, was erroneously believed to have a purely tactile function. 

 J. W. Kuiper of Groningen has shown that the lateral organ can also 

 respond to sounds, thus providing a measure of acoustic isolation, the 

 details of which are not yet fully understood. From Dr Reysenbach de 

 Haan's experiments, it further appeared that human sound reception 

 deteriorates under water in other respects also, and that the intensity of 

 normal sounds has to be increased by sixty decibels before we can hear 

 them. This corresponds precisely to the loss of hearing we should expect 

 in a man with impaired external and middle ears. 



Since bone conduction is, as we have seen, a method of hearing with 

 serious limitations, the famous Dutch anatomist, Petrus Camper, the first 

 scientist ( 1 765) to write a treatise specifically on the hearing of whales 

 (and of Sperm Whales in particular) thought that these mammals might 

 not hear as well below water as at the surface, on the assumption that their 

 ears were similar to those offish. Since then Schevill and Lawrence's experi- 

 ments in the Marineland Aquarium have shown clearly that Bottlenose 



