AUDITORY SENSATIONS 517 



together so as to form a reticulate membrane over the hair-crlk tin- hairs 

 themselves projecting through orifices in the membrane. Resting on tin- 

 upper surface of the membrana reticularis is the membrana tectoria. To this 

 membrane is often ascribed a damping effect on the vibrations of the struc- 

 tures below. Any movement of the basilar membrane would be transmitted 

 to the rods of Corti, and by these to the overlying hair-cells. With every 

 vibration these would move in the line of their, long axis so that their hairs 

 would move up and down in the membrana reticularis and possibly strike 

 against the under surface of the membrana tectoria. The fibres of the 

 auditory nerve pass up through the column of the cochlea, through the 

 bipolar ganglion-cells which form the spiral ganglion, and then out along 

 grooves in the spiral lamina to end in arborisations, partly in the inner hair- 

 cells and partly among the outer hair-cells. 



The complexity of the structure above described suggests that a large 

 amount of discriminating and analysing power possessed by the ear for 

 sounds of different qualities is determined by the differentiation of the end- 

 organ itself. Not only are we able to appreciate differences in amplitude and 

 pitch of the sound waves which arrive at the ear, but we are also capable of 

 analysing the compound sounds and determining the simple tones out of 

 which they have been composed. This power of analysis must be due either 

 to the presence of some battery of resonators in the end- organs of the 

 auditory nerve, or to the existence of a large number of different nerve 

 fibres, each of which is excited only by a distinct number of vibrations per 

 second, or, finally, we must assume that the end- organ of hearing is affected 

 as a whole and that the nerve fibres transmit to the brain the different forms 

 of wave caused by various complex sounds, the analysis being 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 resona- 

 tors, 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 O3 to O5 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 altogether. Helmholtz 

 at first thought that these resonators were represented by the arches of Corti, 

 but on Hensen pointing out that the basilar membrane was composed of 

 fibres varying froniVo41 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 



