THE EAR 293 



the cupola to the base of the cochlea (see Fig. 44). 

 membrane is composed of transverse fibres, 

 its tension in this direction is greater than a 

 that in the longitudinal direction, and there- 

 fore as a resonator it acts like the strings of 

 a piano. If one sings a certain note near 

 an open piano, the string which has the 

 same number of vibrations as the note sung 

 is set into sympathetic vibration, the other 

 strings remaining quiet. In the same man- 

 ner if a note strikes the membrana basilaris, 

 that segment of the membrane whose num- 

 ber of vibrations correspond to that of the 

 note will be made to vibrate. Each segment 

 which can vibrate by itself stimulates the 

 cells of Corti found on it, and therefore 



only certain fibres of the auditory nerve FlG DIAGRAM OF 



are stimulated. The corresponding cerebral THE MEMBRANA BASI- 

 cells, because of their specific energy, per- LARIS, UNROLLED. 

 ceive tones of certain pitch. a'd', width of membrane 



at the cupola; ad, width 



The quality or timbre of tones, at the base of the coch- 

 Most tones are not simple tones but are ^th^f .^piuL'of 

 accompanied by overtones which, as a Cord, 

 rule, are higher than the fundamental tone. Each tone, in 

 a mixture of tones, gives rise to a sensation, hence several 

 sensations are produced which we call the quality, or timbre. 

 The timbre of one and the same fundamental tone varies 

 with the number and strength of the accompanying over- 

 tones. 



If two tones whose number of vibrations have a simple 

 ratio (i : 2, 2 : 3, 3 : 4, 4 : 5) are sounded simultaneously, 

 the resulting sound is agreeable consonance. The simul- 

 taneous sounding of two tones whose number of vibrations 

 are not in a simple ratio produces a disagreeable sound dis- 

 sonance. 



Frequently we are able to analyze a mixed sound into its 

 components; we are able, for example, to distinguish the 

 parts played by the different instruments of an orchestra. 



If two tones differing but little in their number of vibra- 

 tions are sounded simultaneously in such a way that at one 

 time the crests of both waves correspond and at another the 



