en. Lin.] ACTION OF THE COCHLEA. 7 2 5 



stapes is attached ; the scala tympani leads to the fenestra 

 rotunda ; the two scalae communicate at the helicotrema, and are 

 separated from the canal of the cochlea by the basilar membrane, 

 and the membrane of Reissner. C.R. is the canalis reuniens 

 leading to the saccule. The two scalse contain perilymph ; the 

 canal of the cochlea contains endolymph which is set in vibration 

 by the perilymph through the membranes. Every time the 

 membrane of the oval window is bulged in by the stirrup, the 

 membrane of the round window is bulged out, and vice versd. 

 If there were no vent in this way the propagation of vibrations 

 through the fluid would be impossible. 



The theories in connection with the cochlea are two in number: 



F.Ovalis 

 Stapes 



fl 



Scala Vestibuli (Perilymph) 



~Cn<il vf~Cui.niva (SncJoiitmohl 



Scala Tympani (Perilymph) 



Helicotrema 



F. Rotunda 



Fig. 548. Diagram to illustrate the use of the fenestra rotunda. 



one is Helmholtz' piano theory ; the other is the telephone theory 

 of Rutherford and Waller. 



The Piano Theory. If one sings a note in front of a piano, 

 the string of the piano that emits that note will take up the 

 vibration and answer ; another note will elicit an answer from 

 another string. It was supposed by Helmholtz that there is an 

 analogous arrangement in the cochlea. Different parts of the 

 organ of Corti will respond to different notes as do the strings 

 of a piano. At first he thought it was the rods of Cord which 

 acted in this way, but when it was shown that in birds there are 

 no rods, he referred it to the different fibres of the basilar 

 membrane. This is supported by the fact tha! this membrane 

 increases in breadth from below upwards ; low notes will set in 

 sympathetic vibration the long fibres of the upper part, and high 

 notes the short fibres of the lower part of the organ. 



These responsive vibrations extend to the hair-cells resting 

 on these particular portions of the membrane, and give rise to 

 excitations which, conducted along the nerve-fibres to the brain, 

 produce, different auditory sensations. This theory therefore 

 localises the analysis of sounds in the cochlea. The memfsrana 

 tectoria acts as a damping mechanism. 



The Telephone Theory. Just as in a telephone one membrane 



