398 



THE SPECIAL SENSES. 



eter, so that it might be conceived to respond to different periods 

 of vibrations in its different parts, its movements being com- 

 municated directly to the hair cells upon which it rests. The 

 fundamental assumption made by Helmholtz that the cochlea 

 must analyze musical sounds in accordance with the principle of 

 sympathetic resonance has met with many objections in recent 

 years. None of the structures suggested as acting the part of a 

 resonator seem to be adapted physically to play 

 such a role, and certainly none of them is so 

 connected with the hair cells and nerve fibers 

 as to make it probable that there is any near 

 approach to a one-to-one relation between the 

 audible vibrations and the nerve fibers. Other 

 theories have been suggested, but it must be 

 admitted that none of them possess the theo- 

 retical completeness of the view so ably pre- 

 sented by Hehnholtz. Two of these theories 

 may be mentioned, although it is not possible 

 in a brief notice to do justice to either one. 

 For satisfactory details reference must be made 

 to the originals. Ewald* believes that the 

 basilar membrane vibrates throughout its length 

 for each note. He has shown that a rubber 

 membrane of the dimensions of the basilar 

 membrane will be set into such vibrations 

 throughout its length, and when, examined 

 under the microscope presents such a picture 

 as is represented in Fig. 178 in which the crests 

 of the waves are at a fixed interval for each 

 tone. If at these intervals the corresponding 

 hair cells are stimulated, and through them 

 the nerve fibers, then our consciousness would 

 recognize each note in accordance with its special pattern. A 

 wholly different conception has been advanced by Wrightson 

 and Keith, t They abandon the idea of sympathetic reso- 

 nance entirely. The cochlear structures are interpreted in the 

 words of Keith as a delicate weighing machine, "a balance for 

 weighing the infinitesimal pressures of sound waves." The move- 

 ments of the stapes cause displacements of the perilymph and set 

 the basilar membrane into motion corresponding to the phases of 

 the sound wave. These movements of the membrane are vertical 

 and affect all the structures upon it, but by a rotation of the arch 



* Ewald, "Archiv f. d. ges. Physiologie," 76, 147, 1899. 



t Wrightson, "An Enquiry into the Analytical Mechanism of the In- 

 ternal Ear," London, 1918. Contains an interesting and valuable Appendix 

 by Keity on the anatomy of the internal ear. 



I'lK. ITS.— To il- 

 lustrate the idea of 

 a fixed sound wave. — 

 {Ewald.) The illus- 

 tration shows a fun- 

 damental note and its 

 first overtone. 



