286 



AUDITORY BIOPHYSICS 



2000 



supporting membrane is about 30 

 mm. Figure VII-18 shows the aver- 

 age dimensions and shape of the 

 human basilar membrane. 



The outer spiral ligament which 

 maintains the tension on these fibers 

 is stronger and heavier at the base 

 and thins away to a few strands at the 

 apex. The fibers of the basilar 

 membrane stretched between this liga- 

 ment and the inner edge of the spiral 

 bony shelf are straight, smooth and 

 without branches, forming a tough 

 elastic base upon which the arches of 

 the Corti organ rest. It is supposed 

 that when this membrane yields 

 the arches of Corti rock to and fro, 

 thus causing the hairlets to bend 

 with the torque applied by the 

 lightly touching tectorial membrane. 

 From this point of view, the arches of 

 Corti must act as levers magnifying 

 the motion of the tectorial hairs, which 

 in turn probably excite a mediator 

 to initiate the nerve impulses which are conveyed by the acoustic nerve 

 to the brain. 



Width at apex 0.50 mm 

 Average width 0.21 mm basal turn 

 0.34 mm middle tur.n 

 0.36 mm apical turn 

 Length 32 mm 



Base 0.04 mm 



Fig. VII-18. The basilar mem- 

 brane as it ascends the cochlea widens 

 from 0.04 mm at its base to about 

 0.50 mm at the apex. This scale dia- 

 gram shows the membrane projected 

 into the plane of the paper. The 

 numbers show the positions of maxi- 

 mum response at the frequencies indi- 

 cated. 



Mechanics of the Cochlea 



Broadly speaking, the incompressible fluids filling the membranous 

 labyrinth are bounded by the rigid bony walls of the cavity. A gradual 

 pressure applied by the footplate, at the base of the scala vestibuli, to 

 the cochlear fluid must displace the perilymph through the helicotrema 

 into the scala tympani. This fluid displacement causes the membrane 

 of the round window to bulge into the middle-ear cavity. The very small 

 diameter of the helicotrema prevents a volume displacement of the fluid 

 for all pressure changes except the slowest. 



An impulsive force impressed by the footplate upon the fluid at the 

 basal end of the scala vestibuli must travel across the membrane of 

 Reissner and the endolymph in the scala media to reach the aural mem- 

 brane. After crossing this membrane, and the basilar membrane which 

 supports this structure, the impulse must travel down the perilymph 



