CHAPTER LXX 

 HEARING 



The Receptor Mechanism. The receptor mechanism for hearing 

 in mammals consists of a specialized nerve-epithelium, contained 

 deep in the skull within the cochlea of the internal ear, in what 

 is known as the organ of Corti. To this the vibrations of the 

 air are conducted by means of the elaborate accessory apparatus of 

 the external, middle, and internal ears. The fully developed spiral 

 cochlea exists only in mammals. In birds its homolegue, the lagena, 

 is a simple, slightly curved tube; in reptiles it is rudimentary, and in 

 amphibians and fishes does not exist. 



For a long time it was thought that the whole of the apparatus 

 of the ear was connected with the sensation of hearing. This is now 

 known not to be the case, part of the internal apparatus being con- 

 nected with the sense of position and equilibration. The rudimentary 

 forms of ears described in lower forms, both vertebrate and invertebrate, 

 are connected with this mechanism rather than with hearing (see 

 p. 654). It is improbable that molluscs, fishes, etc., have any sensation 

 of hearing, although the notion that they have is one which dies hard. 

 Recent experiments have shown that snails are supremely indifferent 

 to sounds of any description, but are exceedingly sensitive to touch. 

 The so-called hearing of fishes is probably due to tactile response 

 to the vibrations of the water surrounding them, rather than to 

 hearing. The fish in a pond which came to be fed when a bell was 

 rung did not hear the bell, but felt the vibrations of the water pro- 

 duced by the approaching of the steps of the bell-ringer. The power 

 of hearing in amphibians and in reptiles is also very doubtful, and 

 has been assumed rather than proved. It is more than probable 

 that such apparent hearing is in reality due to the vibrations of 

 the ground on which they lie, or, as in the case of fish, of the 

 surrounding water. 



The hearing powers of birds and mammals are undisputed. From 

 the point of view of comparative anatomy, it is interesting that the 

 bird, with its lovely range of sound production, and therefore probable 

 wide range of hearing, has only the simple, slightly curved lagena; 

 whereas the animal with the most complex cochlea, a shy rodent of 

 South America, has a very limited range of sound production with 

 which to charm the five whorls of the cochlea of his fellow-kinsmen. 

 There is therefore reason to suppose that the complexity of the 

 cochlea has to do with sensitivity rather than with the range of 



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