Mechanical leverage in the middle ear of the American bullfrog, Rana catesbeiana.
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.
Werner
Yehudah L
YL
Department of Psychology, Princeton University, Princeton, NJ 08544, USA. yehudah_w@yahoo.com
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