PROBABLE ACTION OF THE COCHLEA. 1179 



Probable action of the cochlea. — We have already seen that the im- 

 pact of sound-waves must produce a variable pressure in the fluids in the 

 internal ear. Each movement inwards of the base of the stapes, acting 

 like a piston, must cause a pressure in the perilymph, which must pass 

 up the scala vestibuli. This pressure must act on the ductus cochlearis, 

 pressing it from the membrane of Eeissner down on the membrana 

 basilaris to the fluid in the scala tympani ; and, finally, it must cause a 

 movement outwards of the membrane covering the fenestra rotunda. 

 It is not necessary to consider that a wave will pass through the heli- 

 cotrema from one scala to the other, as it is clear that any pressure in 

 the scala vestibuli must act in the manner indicated. In all probability, 

 the helicotrema serves the office of equalising pressures between the two 

 scalae, just as the aqueductus vestibuli does for the fluids in the saccule 

 and utricle, and the canalis reuniens for the fluids in the saccule 

 and the ductus cochlearis. These variations of pressure will be made 

 with each movement of the stapes corresponding to a vibration, and it is 

 evident that the pressures may vary in number, in intensity, and in 

 character in a way exactly corresponding to the pitch, intensity, and 

 quality or character of tones. Such being the case, there are only three 

 ways in which the ductus cochlearis, which contains the nerve-endings, 

 may be affected. Either (1) small vibratile bodies may exist between 

 the pressures sent into the organ and the filaments of the auditory nerve, 

 each vibratile body having a frequency period of its own; or (2) indi- 

 vidual nerve fibres may be directly excited by waves of a definite period ; 

 that is to say, there may be differences in the nerve fibres, so that they 

 have a selective action ; or (3) the organ may be affected as a whole, all 

 the nerve fibres being affected by any variations of pressure, and thus 

 the power of analysis, which is admitted, is relegated from the peripheral 

 to the cerebral organs. 



The first hypothesis seems a priori to be probable, for the following 

 reasons : — (1) The existence of such bodies would give a natural explana- 

 tion of many if not all of the phenomena ; (2) the evidence of com- 

 parative physiology points to a gradually increasing complexity in the 

 structure of all the terminal organs of special sense, as if there arose a 

 necessity for differentiation and discrimination in the effects of various 

 kinds of stimuli ; and (3) investigations into the action of all the sense 

 organs, such as those of touch and temperature in the skin, of light and 

 colour in the retina, of taste in the tongue, and of smell in the olfactory 

 region — all indicate specialisation of function in the peripheral apparatus. 

 It is not necessary to enlarge on the general evidence indicated under 

 (2) and (3). 



The theory that vibrators exist in the cochlea is associated with the 

 name of Helmholtz, because, although it was a conception that flitted 

 before the minds of Thomas Young, Johannes Miiller, and others, it was 

 first clearly enunciated and discussed by him. 1 It may be shortly stated 

 as follows : — (1) In the cochlea there are vibrators, tuned to frequencies 

 within the limits of hearing, say from 30 to 40,000 or 50,000 vibs. per 

 second. (2) Each vibrator is capable of exciting its appropriate nerve 

 filament or filaments, so that a nervous impulse corresponding to the 

 frequency of the vibrator is transmitted to the brain, not corresponding, 

 necessarily, as regards the number of nervous impulses, but in such a way 

 that when the impulses along a particular nerve fibre reach the brain, a 



1 Helmholtz, op. cit., pp. 217-226. 



