PROBABLE ACTION OF THE COCHLEA. 1185 



4. Nerve Fibres in Cochlear Division of Auditory Nerve, 14,000. 

 Range of audition, 11 octaves. 



— = 1273 nerve fibres for each octave. 



Assuming that the number of auditory filaments is the same for each of the 

 eleven octaves — ■ 



1273 x 6 = 7638 for 6 octaves used in music. 



1273 



^106 nerve fibres for each semitone. 



12 

 — = less than 2 nerve fibres for fa of a semitone. 



5. Fibres in Membrana Basilaris (Retzius)— 

 24,000 



11 



2182 basilar fibres for each octave. 



2182 x 6 = 13,092 for 6 octaves used in music. 

 2182 



12 



180 

 "64 



= 180 basilar fibres for each semitone. 



= less than 3 basilar fibres for each fa semitone. 



6. Hair Cells— Inner, 3487 ; Outer, 11,700 = 15,237— 



— =1385 cells for each octave. 



11 



1385 x 6 = 8310 hair cells for 6 octaves used in music. 

 = 115 cells for each semitone. 



12 

 115 



64 



= lcss than 2 for each fa semitone. 



7. Corti's Rods. Inner, 5590 ; Outer, 3848 = 9438. 

 9438 



11 



: 858 rods for each octave. 



858 x 6 = 5148 rods for 6 octaves. 



— = 71 for each semitone. 

 12 



71 



— — at least one rod for each fa semitone. As each rod forms half of a 



61 Corti's arch, there will be nearly one arch for each fa. 



There are thus in the cochlea a sufficient number of possible vibratHe 

 masses to satisfy theory. 



Careful measurements of parts of the cochlea have also shown remark- 

 able differentiation, indicating that we have an organ which can inn be 

 supposed to act only as a whole. These measurements show, for 

 example, that the breadth of the basilar membrane increases towards 

 the apex; that the osseous lamina spiralis diminishes in breadth as 

 the basilar membrane increases ; that from base to apex, both the 



vol. it. — 75 



