260 OUTPOSTS OF THE INTELLIGENCE SERVICE 



mass, by variations in which a range of 16 to 20,000 complete 

 vibrations per second could be detected. 



Length. It is clear from Fig. 66 that the fibres progressively 

 increase in length from base to apex. Various estimates have been 

 made of the length of the fibres and values ranging from 0-041 — 

 0-495 mm. (Hensen) to 0-176 — 0-4 mm. (Keith), i.e. according to 

 Hensen there is a twelve-fold increase in size, while Keith and other 

 modern workers give only a fourfold variation. If the fibres 

 varied in length alone, a thousandfold variation would be necessary 

 to provide resonators tuned to the highest and the lowest note 

 perceptible. 



Tension. Reference to the figure (66) will show the presence in 

 the spiral ligament of radiating fibres attached at one end to the 

 outer wall of the cochlear galleries, and at the other end running 

 in a direction continuous with the basilar fibres to which they are 

 attached. At the basal end the ligament is about 0-5 mm. thick, 

 and is full of closely-packed fibres, while at the apical turn it is 

 only about 0-1 mm. thick and contains only a few fibres. Gray 

 and Wilkinson consider that the function of these fibres is to exert 

 a progressively increasing tension on the basilar fibres — applying 

 the greatest tension where the fibres are shortest and thus raising 

 the pitch of the resonators responding to the high notes, and least 

 on the longer fibres, i.e. making them capable of resonating to 

 notes lower than they would accept if stretched. (Cf. vibration of 

 stretched string.) 



Mass. Most investigators consider that the progressive varia- 

 tion in the length and in the tension of the fibres of the basilar 

 membrane is sufficient to account for the 10-| octaves of the audible 

 range, but Helmholtz vaguely suggested a third physical factor, 

 the load on the fibres. The load is made up of two components, 

 viz. (i.) the organ of Corti and appendages, and (ii.) the endolymph 

 or fluid load. The main reason for postidating load as well as 

 tension and length is to explain why we can hear very low notes, 

 e.g. 15-30 vibrations per second. The variation in tension seems 

 adequate for the higher frequencies, but when we come to the lower 

 ones we would need to have very slack strings as resonators on 

 account of their shortness, (i.) The weight of the arches of Corti, 

 the hair cells, sustentacular cells, etc., loads the fibres of the 

 basilar membrane, and so increases their period of vibration. 

 (ii.) Wilkinson considers the fluid load as of fundamental import- 

 ance. He has constructed models to demonstrate his case, and, 

 if we can accept models as evidence, then he has gone far to prove 

 the correctness of his assumption. The essential feature of his 

 modification of the resonance theory is that the fluid filling the 



