Abstract
The tympano-ossicular system has been anatomically analyzed in 26 species of heteromyid rodents. Based on anatomical measurements, 24 of the 26 species should have a transmission efficiency of 94%–100% from tympanic membrane to cochlear, at the resonant frequency. The areal ratio of stapes footplate to 2/3 tympanic membrane is remarkably constant among the species, varying only from 0.04 to 0.07: in Dipodomys and Microdipodops this small ratio is due to the very large tympanic membrane; in Perognathus and Liomys it is due to the extremely small stapes footplate. The lever ratio of incus to malleus varies from 0.28 to 0.33 in Dipodomys and Microdipodops, from 0.37 to 0.46 in Perognathus, and from 0.55 to 0.60 in Liomys. In addition, the middle-ear volumes and the morphology of tympanic membrane, ossicles, ligaments, and muscles all combine to minimize both mass and stiffness. All these data suggest middle-ear mechanisms which are very efficient over a broad frequency range. We have cochlear microphonic data consistent with this conclusion. [Supported by NIH Grant 11459.]
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