title: Mechanical leverage in the middle ear of the American bullfrog, Rana catesbeiana.
abstract: Textbooks lump the middle ears of 'submammalian Tetrapoda' as being 'one-ossicle ears'. Conventionally the anuran middle ear is depicted with a shaft-like skeletal unit connecting the tympanic membrane to the inner ear. This shaft comprises mediad a long bony columella and laterad a short cartilaginous extracolumella. But dissection of Rana catesbeiana ears showed: the extracolumella, as long as the columella, is proximally expanded in the vertical plane, forming dorsal and ventral heads. The medio-dorsal head is movably jointed to the columella, between these two there is an obtuse angle ventrad; the extracolumellar medio-ventral head is anchored by a ligament to the middle-ear cavity ceiling. When the tympanic membrane moves outwards, pulling the extracolumella, the medio-dorsal head of the extracolumella must be forced inwards, rotating on the ventral anchorage, pushing the columella towards the inner ear. The ossicular chain thus includes a mechanical lever, possessing the magnitude of the ratio length:width of the extracolumella; this is additional to the lever known from the columellar footplate, which rotates on its firm ventral attachment. These levers are confirmed physiologically, by the difference between the inner-ear sensitivity (shown by isopotential audiograms of microphonic potentials) when stimulated by a vibrator first at the tympanic membrane, then at the proximal stump of the amputated columella. Perusal of the primary literature showed that this morphology is widespread among anuran ears.
authors.affiliation: Department of Psychology, Princeton University, Princeton, NJ 08544, USA. yehudah_w@yahoo.com
authors.forename: Yehudah L
authors.initials: YL
authors.lastname: Werner
mesh_terms: D000161:Acoustic Stimulation; D000818:Animals; D001299:Audiometry; D003055:Cochlear Microphonic Potentials; D004429:Ear Ossicles / Q000033:anatomy & histology* / Q000502:physiology*; D004432:Ear, Middle / Q000033:anatomy & histology / Q000502:physiology; D004594:Electrophysiology; D005260:Female; D016014:Linear Models; D008297:Male; D011892:Rana catesbeiana / Q000033:anatomy & histology* / Q000502:physiology*; D013696:Temperature; D014732:Vibration