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A morphometric study of the cochlea of the little brown bat (Myotis lucifugus)

A detailed morphometric study of the basilar membrane was made from serial sections and graphic reconstructions of the cochlea of three little brown bats. Four distinct morphometric changes were observed within the basilar membrane. First, between 0‐1.4 mm from the basal end of the cochlea, there is a rapid increase in width and cross‐sectional area of the basilar membrane. Secondly, between 1.4‐2.5 mm, there is little change in width of the basilar membrane (its cross‐sectional area is at its greatest in this region). Thirdly, between 2.7‐3.1 mm, there is a sudden decrease in cross‐sectional area concomitant with an increase in the width of the basilar membrane. Finally, between 3.1 mm and the apex, there is a gradual decrease in cross‐sectional area concomitant with an increase in the width of the basilar membrane. The magnitudes of the cross‐sectional areas of the scalae media and vestibuli decrease from base to apex, but this is not true for the scala tympani. The cross‐sectional area of the scala tympani appears to decrease from the base to 0.7 mm, then it increases up to 1.4 mm, and then it decreases to the apex. These morphometric changes in the basilar membrane of the little brown bat are compared to those in other echolocating and non‐echolocating mammals. The significance of these changes is discussed in relation to the range of hearing in the little brown bat.