EAR AS AN ORGAN FOR SOUND SENSATIONS. 395 



we recognize the variations in quality of different compound tones. 

 The principle upon which the cochlea acts in thus separating the 

 compound tones into their elements is not explained with entire 

 satisfaction. According to the view so admirably presented by 

 Helmholtz,* the analysis depends upon the existence in the ear of 

 a mechanism for sympathetic vibrations or resonance. 



Sympathetic Vibrations or Resonance. — By sympathetic 

 vibration is meant the fact that an elastic body is easily set into 

 vibration by movements of the surrounding medium when these 

 movements correspond with its own period of vibration. A string 

 whose period of vibration is 128 per second will be little affected 

 by vibrations of the surrounding air unless they have the same 

 periodicity. If, however, a note of this period is sounded by the 

 voice, for instance, the string will be set into vibration with rela- 

 tive ease. By means of this jjrinciple the untrained ear can readily 

 pick out the more prominent of the upper harmonics of any given 

 note of a musical instrument. It is only necessary to select a series of 

 resonators corresponding to the series of overtones. Each reso- 

 nator is set into vibration by its corresponding overtone and so 

 emphasizes this particular tone that it may be easily recognized. 

 If one stands in front of a piano with the strings exposed and 

 sings a given note it may be shown that a series of the piano strings 

 is set into vibration corresponding, in the first place, to the rate 

 of vibration of the fundamental tone, and secondly to the more 

 prominent of the harmonic overtones. In this case the com- 

 pound wave strikes upon the collection of strings of the piano, 

 and is analyzed into its component simple tones by the sympa- 

 thetic vibrations of the corresponding strings. Helmholtz assumes 

 that the cochlea analyzes compound musical waves by an essentially 

 similar method. 



The Functions of the Cochlea. — The vibratory movement, 

 whatever may be its form, in the air of the external meatus im- 

 parts to the tympanic membrane a similar form of movement, and 

 this, in turn, through the ear bones and the membrane of the fenes- 

 tra ovalis sets the perilymph into vibrations of the same form. That 

 the perilymph can swing or vibrate under the influence of the move- 

 ments of the stapes is explained by the existence of the second 

 opening, the fenestra rotunda, between the middle and the inter- 

 nal ear (see Fig. 167). As the membrane of the fenestra ovalis is 

 pushed in, that of the fenestra rotunda is pushed out, and vice 

 versa, and the wave movement is transmitted through the peri- 

 lymph of the cochlea in a manner illustrated by the schema repre- 

 sented in Fig. 177. These vibratory movements of the perilymph 

 affect the membranous cochlea, which may be regarded as being 

 suspended in the perilymph, and, according to the resonance theory, 

 * Helmholtz, loc. cit. 



