THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

216 10. Physiology of Hearing and PsychoacousticsFigure 10.3. Cross section of the organ of Corti in the cochlear canal. (From H. Gulick.1971. Hearing. Physiology and Psychophysics. New York: Oxford University Press.Reproduced with permission of the publisher.)shown in enlarged detail in Figure 10.2, is the sensory system that converts thevibratory energy of sound into electrical signals to the brain for the detection andinterpretation of that sound. The cochlea can be described as a 3.5-cm long tubeof roughly circular cross section, wound about 2 1 / 2 times in a snail-like coil. Thistube’s cross-sectional area decreases in a somewhat uneven manner from its baseto its apex. Its total volume is about 5 (10) –2 cm 3 .The coils of the cochlea surrounds an area called the modiolus; and the membranouslabyrinth of the cochlear sector of the inner ear divides into three ducts orgalleries (scalae). The cochlear duct (ductus cochlearis) runs the length of the spiralingcochlea, and because it occupies the central portion of the cochlea’s interior,it has been termed the scala medi (i.e., the middle gallery), whose walls effectivelypartition the cochlea into two longitudinal channels, the scala vestibuli (or uppergallery) and the scala tympani (lower gallery). The only communication betweenthe two galleries occurs through the helicotrema, a small opening at the apex ofthe cochlea. The other ends of the upper and lower galleries terminate in the ovaland round windows, respectively.Figure 10.3 shows an enlarged view of the cochlear duct. This duct is bounded byReissner’s membrane, the basilar membrane, and the stria vascularis. The basilarmembrane extends from the bony spiral lamina, a ledge extending from the centralcore of the cochlea, to the spiral ligament. The length of the basilar membraneis about 32 mm long, from the base to the apex of the cochlea; the width varies