190 EXPERIMENTAL PHYSIOLOGY. 



Of the two membranes which form the walls of this triangular chamber 

 (Fig. 57) one is histologically quite simple. The other, the BASILAR MEMBRANE 

 supports a double row of pillars, standing with their bases apart and their tips 

 leaning together and projecting into the fluid of the middle column, the endolymph. 

 These are known as the PILLARS OF CORTI. Above these pillars on either side, 

 supported by the tent-like roof which they form, are several rows of cells, among 

 them a large number which have hair-like projections up into the endolymph, 

 The whole is known as the ORGAN OF CORTI. The fibres of the auditory nerve, 

 which runs within the inner pillar of the spiral, pass out by the bony projection 

 along its entire length and end in the hair cells, which must therefore be the cells 

 sensitive to the waves of sound. 



As we have already seen, the ear is capable of distinguishing not only pitch 

 and intensity but also quality, in any waves which it receives. Not only do 

 notes of the same pitch, one without and one with many overtones, not sound 

 alike but with training the hearer can distinguish what the various overtones in 

 the notes are, so that our theory of hearing must afford a physiological basis for 

 this analysis. We have no problem of simplification to deal with in the ear as we 

 had in the eye; in hearing the sensations are as numerous as the stimuli which 

 evoke them. The most generally accepted theory is one proposed by Helmholtz, 

 the resonance theory. If one places a number of tuning forks of different periods 

 near a vibrating fork or reed, giving out a simple note, among those forks which 

 have not been struck the one with the same natural period will begin to vibrate 

 and give out its note, while the others remain silent. If the vibrating source is a 



f. u-a, 1 1 a 



FIG. 58. Diagram of the relationship of the three fluid columns of the internal ear. 



violin-string, which gives a note with overtones, not only will the fork with a 

 period like the fundamental respond but those with periods equal to each over- 

 tone as well. Helmholtz suggested that some such apparatus must be contained 

 in the ear. He at first thought that it was the rods of Corti which, having each 

 a different natural period, acted as resonators to the composite vibrations of the 

 fluid of the outer columns. The ear however is capable of distinguishing more 

 notes than there are rods; besides, rods are absent from the ears of singing birds, 

 and this idea had to be abandoned. It was then suggested that the resonators 

 might be found in the basilar membrane itself. It contains a series of radial 

 fibres running from the bony ledge to the outer wall and the width of the mem- 

 brane, and therefore the length of these fibres, increases progressively from the 

 base of the spiral to its apex. If these fibres are the resonators the arrangement 



