576 PHYSIOLOGY 



carried out in the cerebral cortex itself. This last hypothesis, the 

 relegation of the powers of analysis to the cerebral cortex, is, at the 

 present time at any rate, equivalent to giving up any attempt to 

 explain the power of analysis possessed by the organ of hearing. On 

 the other hand, the complex structure of the organ of Corti suggests 

 that here we have an actual battery of resonators, by means of 

 which sense waves are analysed into their components. This is 

 Helmholtz's theory of the function of the cochlea. It assumes that 

 in the organ of Corti there are vibrating structures tuned to 

 frequencies within the limits of hearing, viz. from 30 vibrations 

 to about 4000 vibrations per second. We can distinguish notes in 

 the middle of the musical scale which differ from one another only 

 by 0-3 to - 5 vibration per second. Within the limits of this scale 

 we should therefore require about 4200 resonators in the ear. In 

 order to account for the sensitiveness of the ear to sounds below 40 

 vibrations and above 4000 per second, we might allow another 300 

 vibrators, so that 4500 different resonators would be necessary alto- 

 gether. Helmholtz at first thought that these resonators were repre- 

 sented by the arches of Corti, but on Hensen pointing out that the 

 basilar membrane was composed of fibres varying from 0'041 to 0'495 

 mm. in length, he concluded that it was probably the breadth of the 

 basilar membrane which determined the tuning to any particular note. 

 This membrane from its structure behaves like a system of stretched 

 strings bound together by a semi-fluid substance. The parts of the 

 membrane near the fenestra rotunda would be adapted for the higher 

 notes, while those near the helicotrema would vibrate to deep tones. 

 Each fibre would, by its vibration, set the overlying part of the organ 

 of Corti into vibration, so that each nerve fibre composing the auditory 

 nerve would be stimulated by a different note. Miiller's law of specific 

 irritability is thus observed, each nerve fibre transmitting an impulse 

 which excites one quality, and only one quality, of sensation. When a 

 compound wave falls on the organ of Corti, it is actually resolved into 

 its simple component waves by the fibres of the basilar membrane, and 

 we therefore get stimulation of a number of nerve fibres and a sensa- 

 tion produced which is a true mixed sensation compounded of a number 

 of simple tone sensations. By an effort of attention therefore it is 

 possible to pick out from a mixed sensation its different components, 

 and in this way we may explain the analytic powers of the ear. 



Certain observations on fatigue of the auditory fibres support the 

 notion that each fibre reacts to notes of one pitch, and only to these 

 notes. If the vibrations of a tuning-fork be conducted by two tele- 

 phones to both ears the sound appears to come from somewhere in 

 front of the middle line of the body. If the sound be transmitted to 

 one ear for some time so as to produce a condition of slight fatigue, 



