We study the vibration modes of a short section in the middle turn of the gerbil 
cochlea including both longitudinal and radial interstitial fluid spaces between 
the pillar cells (PC) and the sensory hair cells to determine the role of the 
interstitial fluid flow within the organ of corti (OoC). Three detailed finite 
element (FE) models of the cochlear short section (CSS) are studied. In model 1, 
the CSS is without fluids; model 2 includes the OoC fluid, but not the exterior 
scalae fluids; and model 3 is the CSS with both scalae and OoC fluids. We find 
that: (1) the fundamental mode shape of models 1 or 3 is similar to the 
classical basilar membrane (BM) bending mode that includes pivoting of the arch 
of corti, and hence determines the low frequency vibrational mode shape of the 
cochlea in the presence of the cochlear wave. (2) The fundamental mode shape of 
model 2 is characterized by a cross-sectional shape change similar to the 
passive response of the cochlea. This mode shape includes a tilting motion of 
the inner hair cell (IHC) region, a fluid motion within the tunnel of corti 
(ToC) in the radial direction and along the OoC, and a bulging motion of the 
reticular lamina (RL) above the outer hair cell (OHC). Each of these motions 
provides a plausible mode of excitation of the sensory hair cells. (3) The 
higher vibrational modes of model 1 are similar to the electrically evoked 
response within the OoC and suggests that the higher vibrational modes are 
responsible for the active response of the cochlea. We also observed that the 
fluid flow through the OoC interstitial space is significant, and the model 
comparison suggests that the OoC fluid contributes to the biphasic BM motion 
seen in electrical stimulation experiments. The effect of fluid viscosity on 
cilium deflection was assessed by performing a transient analysis to calculate 
the cilium shearing gain. The gain values are found to be within the range of 
experimentally measured values reported by Dallos et al. (1996, The Cochlea, 
Springer-Verlag, New York).