THE INTERNAL EAR IN VERTEBRATES 593 



certainly a formidable array of difficulties can be raised on the 

 anatomical side. 



It has, for instance, been said that the basilar membrane in 

 all its parts is too thick and too narrow ! to be set in sympathetic 

 vibration by sound, although as a matter of fact a model of the 

 basilar membrane, an indiarubber sheet o - 5 mm. broad, has 

 been made to vibrate in sympathy with a tuning fork. There 

 seems, therefore, to be no physical reason why the basilar 

 membrane should not be thrown into sympathetic vibrations 

 but recent histological research 2 raises doubts whether the 

 fibres of the membrane are sufficiently free to vibrate inde- 

 pendently. Instead of lying more or less isolated and free in a 

 homogeneous semi-fluid bed, they are now shown to be bundles 

 of fibrous tissue loosely felted together at all points and thus 

 quite incapable of the individual movement generally assumed 

 to be necessary to satisfy the demands of the Helmholtz 

 theory. 



But supposing the fibres are capable of sufficient individual 

 movement, it is maintained that their vibration would im- 

 mediately be damped by the soft tissues that cover both 

 surfaces 3 of the basilar membrane. 



Yet further objections may be urged with regard to the 

 number of the cords. 



For the theory to hold good, it is necessary that there should 

 be fibres in sufficient quantity and of sufficient variation in 

 length to resonate to every distinguishable note. Now we can 

 fairly gauge the hearing limits of certain birds by their powers 

 of mimicry. The parrot 4 in particular has obviously an 

 extremely critical and discriminative ear with great appreciation 

 of the quality of sound. But in its cochlea there are only 

 some 1,200 cords in the basilar membrane 5 with little or no 

 variation in length except towards the extreme base. Here 

 undoubtedly is a very formidable difficulty to the Helmholtz 

 theory, at least among birds. 



Supposing, however, that the basilar membrane in mammals 

 is capable of doing all that is required of it under the theory, it 



1 Shambaugh, Am. Jour. Atiat. 7, 1907, p. 247. 

 * Hardesty, Am. Jour. Anat. 8, 1908, p. 156. 



3 Kishi, Arch. ges. Physiol. 116, 1907, p. 121. 



4 Denker, Biol. Col. 26, 1906, p. 600. 



5 In man there are some 24,000. 



