. Biology of the Heteromyidae. Heteromyidae. HETEROMYID EAR 271 (1984) present excellent discussions of this topic. All components of the middle ear are involved in impedance matching. Pressure at the small stapedial footplate is increased in two ways. The force impinging on the large area of the tympanum (=tympanic membrane or eardrum) is concentrated on the smaller area of the stapedial footplate and is increased essentially as the ratio of the two areas. Because the manubrium of the malleus is longer than the long process of the incus, the pressure is increased as the ratio of these two lev


. Biology of the Heteromyidae. Heteromyidae. HETEROMYID EAR 271 (1984) present excellent discussions of this topic. All components of the middle ear are involved in impedance matching. Pressure at the small stapedial footplate is increased in two ways. The force impinging on the large area of the tympanum (=tympanic membrane or eardrum) is concentrated on the smaller area of the stapedial footplate and is increased essentially as the ratio of the two areas. Because the manubrium of the malleus is longer than the long process of the incus, the pressure is increased as the ratio of these two lever arms. The total pres- sure increase is the product of these two factors and is called the transformer ratio. The air-filled middle ear cavities allow the tympanum to vibrate freely (Durrant and Lovrinic, 1984; Wever and Lawrence, 1954). The walls of the middle ear cavity are formed primarily by periotic and ectotym- panic elements which fuse to form the au- ditory bulla. Each bulla contains three com- partments: the hypotympanum, the epitympanum and the mastoid or antrum (Fig. 1). Webster and Webster (1975) use the term antrum in reference to the mastoid air cell(s) and have clarified the earlier ter- minology of Howell (1932). The hypotympanic cavity lies below the level of the horizontal semicircular canal. Its medial wall contributes to the formation of the braincase and supports the cochlea which extends into the hypotympanic cav- ity. The tympanum forms most of the lat- eral wall. The hypotympanic cavity com- municates with the mastoid air cells posteriorly and the epitympanic cavity su- periorly (Figs. 1-3). The epitympanic cavity lies above the level of the horizontal semicircular canal and its walls articulate with the parietal, in- terparietal and occipital bones. The head of the malleus and body of the incus project into this cavity (Figs. 1-3, 5B). The mastoid lies posterior to the hypo and epitympanic cavities and abuts the oc- cipital bone superiorly and mediall


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