370 ADVANCED PHYSIOLOGY 



suit, the organ of Corti, which lies quite freely in the liquid, 

 will also be thrown into vibration. In this organ, as we 

 have learned, are the ends of the nerves of hearing, and it is 

 supposed that the slight shaking they thus receive is sufficient 

 to stimulate them so that they transmit impulses to the brain, 

 which are then interpreted as sound. 



Perception of Pitch. The method by which the ear recog- 

 nizes high and low tones is not fully understood, but it ap- 

 pears to be in part based upon a very simple fact. Sound is 

 the result of air waves, and the different pitches are due to 

 waves of different degrees of rapidity. In high sounds the 

 rate is very rapid, in low sounds it is slow; the longest strings 

 of a piano vibrate about thirty-three times per second, the 

 shortest strings about four thousand two hundred times. If 

 one stands close to a piano and plays a loud note upon a flute, 

 for example, and then stops, he will notice that he can hear 

 the same note sounding in the piano for several seconds. 

 The waves of air starting from the flute have passed into the 

 piano and strike upon the various strings. There is one 

 string in the instrument that naturally vibrates just as 

 rapidly as the waves which come from the flute, and these 

 wave motions from the flute set that particular wire into 

 vibration, so that even after the flute is silent, one can hear a 

 faint sound from the piano string. If two notes were played 

 near the piano at the same time, two wires would be set into 

 vibration, etc. This phenomenon takes place according to 

 a principle known as that of sympathetic vibration. 



By reference to Fig. 185 it is noted that the rods of Corti and 

 the "hair cells" seem to stand on a straight, basal membrane; 

 this is really a shelf -like curtain which is attached to the core 

 of the spiral cochlea along one edge, and to the outer curve of 

 the tube on the other, dividing it into two spaces lengthwise. 

 This basilar membrane is made up of about 24,000 threads 

 (Retzius) of practically as many lengths. These basilar mem- 

 brane threads, it is thought, vibrate at different rates, 



