174 



VETERINARY PHYSIOLOGY 



quality — the character of the sound given by the over-tones 

 to be distinguished. The perception of this last is essentially 

 a perception of pitch. 



Loudness. — It is easy to understand how the peripheral 

 neurons in the internal ear are more powerfully stimulated 

 by the greater variations in the degree of pressure which are 

 produced by more powerful aerial waves, and how the 

 greater stimulation of the receptive centre in the brain will 

 be accompanied by a sensation of greater loudness. 



Pitch. — The auditory mechanism has an extraordinarypower 



I.R.a 



O.R.C. 



H.G. 



Fig, 



87._To show the movements of the basilar membrane with the passage 

 of a sound wave, and the manner in which, by the displacement of 

 Cortis' arch I. B.C., O.R.C, the reticular membrane pulls upon the 

 cilia C, which are embedded in the tectorium T., and thus may 

 stimulate the nerve endings N. 



of enabling the appreciation of the pitch and quality of sound, 

 and of enabling complex sounds to be analysed. How it 

 does so is still somewhat problematical. Helmholtz main- 

 tained that it is by resonance, the fibres of the basilar 

 membrane acting like the strings of a piano, each one of 

 which, or each set of which, is made to vibrate by a particular 

 note and its overtones, and thus to stimulate the nerve endings 

 among the hair cells situated upon these parts of the mem- 

 brane. Some experiments on dogs, in which the apprecia- 

 tion of notes of lower pitch was apparently lost after the 

 upper turns of the cochlea were destroyed, and some few 

 cases of partial destruction of the cochlea in the human 



