EAR AS AN ORGAN FOR SOUND SENSATIONS. 



401 



that the tectorial membrane, especially its free end, serves as the 

 mechanism for sympathetic vibration. This membrane increases 

 in width from the base to the apex of the cochlea and varies in 

 thickness in its radial diameter, so that it might be conceived to 

 respond to different periods of vibrations in its different parts, 

 its movements being communicated directly to the hair cells upon 

 which it rests. Unfortunately we have no direct experimental 

 evidence in favor of any of these views. Several observers, how- 

 ever, have demonstrated apparently that, whatever may be the 

 mechanism for sympathetic vibration, it is so arranged that at the 

 base of the cochlea the higher notes are received and at the apex 

 the notes of the lowest pitch. Thus, Munk, in experiments upon 

 dogs, in which by an operation through the fenestra rotunda he had 

 destroyed the basal portion of the cochlea, found that the animals, 

 after a temporary deafness of some days, could hear apparently 

 only low tones and noises. Baginsky,* in a later series of experi- 

 ments, opened the bulla ossea on each side, destroyed the cochlea 

 on one side entirely so as to render that ear deaf, while on the other 

 he injured it in certain areas only. He found 

 that when the apex of the cochlea was des- 

 troyed the animal appeared to perceive only 

 the high tones, c'" ', c"", c'"". 



The fundamental principle of the theory of the 

 function of the cochlea as developed by Helmholtz 

 has been subjected to some criticism. The theory of a 

 series of resonators each responding to a definite note 

 does not explain with entire satisfaction some of the 

 known acoustic phenomena. Thus, it is known that 

 when two notes are sounded together combinational 

 tones may be heard, either a low difference tone whose 

 pitch is equal to that of the difference between the 

 rates of the two notes, or a summation tone whose 

 pitch is equal to the sum of the vibrations of the two 

 notes. It is difficult to conceive that these combina- 

 tional tones have an objective existence, as vibrations, 

 and the means by which they are perceived by the 

 cochlea is not explained satisfactorily by the theory 

 of resonators. Other theories of the function of the 

 cochlea have been proposed to avoid such difficulties. 

 Thus, Ewald f suggests a view according to which 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 pic- 

 ture 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 and 

 nerve fibers are supposed to be stimulated, then our 

 consciousness would recognize each note by its appro- 

 priate interval. For the application of this theory to musical harmony- 

 combinational tones and beats reference must be made to the original. 



* Baginsky, "Virchow's Archiv f. pathol. Anat.," 94, 65, 1883. 

 t Ewald, "Archiv f. d. gesammte Physiologic," 76, 147, 1899. 

 26 



Fig. 178. To il- 

 lustrate the idea of 

 a fixed sound wave. 

 (Ewald.) The illus- 

 tration shows a fun- 

 damental note and its 

 first overtone. 



