Outer and Middle Ears

The function of the mammalian external and middle ears (at least in terrestrial mammals) appears qualitatively similar. The external ear collects sound power and couples the collected power to the middle ear, and the middle ear transmits the power to the inner ear via motion of the tympanic membrane and ossicles. However, there are large differences in the scale and form of mammalian middle and external ears (Fig. 6.1), e.g., the African elephant (Loxodonta africana) has an external ear flap or pinna with an area of about 106 (mm)2 and a tympanic membrane area of almost 103 (mm)2, whereas the dwarf shrew (Suncus etruscus) has a pinna flap of only 10 (mm)2 and a tympanic membrane area of only 1 (mm)2 (Fleischer 1973; Heffner, Heffner, and Stichman 1982). There are also differences in the orientation and relative size of the ossicles (Fig. 6.1). In Loxodonta, the linear dimensions of the malleus are about twice those of the incus and the long arm of the malleus (the manubrium) is nearly vertical (perpendicular to the horizontal plane). In Suncus, the linear dimensions of the malleus are three to four times those of the incus and the manubrium of the malleus runs nearly parallel to the horizontal plane.