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A TEXTBOOK OF PHYvSIOLOGY 



notes be sounded together, a cylinder will resonate with and reinforce 

 the note of corresponding wave-length. So, too, will tense strings. 

 This can be ascertained on the piano. If straws be attached to the 

 wires, it will be: seen that, when one note is struck with the loud pedal 

 raised to remove the action of the damper, other strings than the one 

 struck are vibrating at the same time. This is because their vibration 

 numbers correspond to the overtones. It is the overtones which, 

 when not excessive, give a pleasant fulness to the note. Uneven 

 overtones give a rough, penetrating note. 



When two different notes are sounded at the same time, they 

 interfere with each other, alternately strengthening and weakening 

 each other, and giving a succession of phases known as beats. The 

 number of beats per second depends upon, and is equal to, the differ- 

 ence of the rate of vibration of the two partial tones. A difference 

 of one vibration gives one beat per second, of two vibrations two 

 beats per second. When beats come very quickly, the alternate 

 strengthening and weakening is lost, and a whirring, dissonant sound 

 results. 



In harmony, or consonance, there is an absence of beats. The 

 greatest consonance is obtained from the same note with the same 

 overtones. After that come the octave notes, corresponding to the 

 note sounded. Then follow various chords, producing varying degrees 

 of consonance. 



Theories of Hearing. In the present state of knowledge it is not 

 possible to give any full and satisfactory explanation of the function 

 of the cochlea. The problem to be solved is whether sound-wavea 

 produce movement of the hair cells of the organ of Corti, and, if so, 

 the nature of the movement and the means by which it is produced. 

 It is generally held that some form of mass motion, the exact nature 

 of which is not clear, is normally produced in the perilymph through 

 the to-and-fro action of the footplate of the stapes upon the mem- 

 brane of the fenestra ovalis. This mass movement of the perilj-rnph 

 causes synchronous movements of the membrane of the fenestra 

 rotunda, and also affects the endolymph, producing therein waves of 

 compression and rarefaction. As the result of these waves, it is 

 believed that either the basilar membrane or the tectorial membrane 

 is caused to vibrate, and thus the hair cells are affectsd. Various 

 considerations arise as to the nature of the vibration of such mem- 

 branes. Is the vibration throughout the whole length, or only in 

 part, or in some particular part for a particular sound ? 



According to the view of Helmholtz, each portion of the basilar 

 membrane is set into reciprocal vibration by tones of a different 

 pitch, high tones being produced by vibration at the basal extremity, 

 where the membrane is narrowest, low tones in the apex of the 

 cochlea, where the membrane is widest and most lax. Except 

 for the fact that Helmholtz supposed that only musical tones were 

 perceived by the cochlea, and that noises were appreciated by the 

 vestibular apparatus, his view still meets with wide acceptance. It 



