150 HUMAN PHYSIOLOGY 



cochlea where it joins the rest of the labyrinth, we are 

 assumed to find each succeeding segment of the mem- 

 brane adapted to respond to a higher frequency. There 

 is some evidence from disease which is favorable to this 

 view. 



Let us trace what probably happens when three pipes 

 of an organ sound a chord. Three vibration rates cannot 

 strictly coexist in the air but they are represented by a 

 fusion just as they could be on the disc of the phono- 

 graph. The tympanic membrane moves back and 

 forth in a fashion faithful to the details of this com- 

 pound motion. It behaves as the diaphragm of the 

 phonograph or the telephone would do. The ossicles 

 transmit the peculiar type of movement to the fluid in 

 the labyrinth. Pulses run through the cochlea and 

 the basilar membrane is shaken. If the common con- 

 ception is correct there are three sharply limited regions 

 of the membrane which are thrown into energetic 

 vibration. They correspond with the three components 

 of the chord. The vibratory motion which was at first 

 compounded of three elements is now resolved again into 

 three. Three streams of impulses run simultaneously 

 to the brain and our sensation of harmonious sound is 

 the result. 



We contrast musical sounds with noises. The physical 

 difference lies in the fact that a musical sound has a 

 regular rate of vibration sustained long enough to 

 define its pitch. A noise has either too many com- 

 ponents to give the impression of precise pitch or its 

 pitch is too rapidly shifting. The chief difference be- 

 tween singing and speaking is that in the first case a suc- 

 cession of definite pitches can be recognized, while in the 

 second the vibration rate is changing every instant and 

 only a rough judgment of average pitch can be formed. 



