The gelatinous polysaccharide material that overlies the maculae is called the otolithic membrane (Fig. 25.12). Its outer surface contains 3-m to 5-m crystalline bodies of calcium carbonate and a protein (Fig. 25.13). Otoliths are heavier than the endolymph. The outer surface of the otolithic membrane
FIGURE 25.12 • Diagram of a macula within the utricle. A more detailed diagram of the cellular organization of the macula of the utricle is shown in the enlarged rectangle. Supporting cells can be seen lying between the two principal types of sensory hair cells (types I and II). The stereocilia and kinocilium of each  sensory hair cell are embedded in the otolithic membrane on which otoconia lie.
FIGURE 25.13 • Scanning electron micrograph of human otoconia. Each otoconium has a long cylindrical body with a three-headed facet on each end. 5,000.
lies opposite the surface in which the stereocilia of the hair cells are embedded. The otolithic membrane moves on the macula in a manner analogous to that by which the cupula moves on the crista. Stereocilia of the hair cells are deflected by gravity in the stationary individual when the otolithic membrane and its otoliths pull on the stereocilia. They are also displaced during linear movement when the individual is moving in a straight line and the otolithic membrane drags on the stereocilia because of inertia. In both cases, movement of the otolithic membrane activates MET channels and depolarizes hair cells.
The spiral organ of Corti is the sensor of sound vibrations.
The cochlear duct divides the cochlear canal into three par allel compartments or scalae:  Scala media, the middle compartment in the cochlear canal  Scala vestibuli  Scala tympani
The cochlear duct itself is the scala media (Figs. 25.14 and 25.15). The scala vestibuli and scala tympani are the spaces above and below, respectively, the scala media. The scala media is an endolymph-containing space that is continuous
FIGURE 25.14 • Schematic diagram of the cochlea.
a.
Schematic diagram of a midmodiolar section of the cochlea that illustrates the position of the cochlear duct within the 2.75 turns of the bony cochlea. Observe that the scala vestibuli and scala tympani are continuous apically (helicotrema). b. Cross section of the basal turn of the cochlear duct. The cochlear duct and the osseous spiral lamina divide the cochlea into the scala vestibuli and the scala tympani, which contain perilymph. The scala media (the space within the cochlear duct) is filled with endolymph and contains the organ of Corti. (Modified from Goodhill
V.
Ear, Diseases, Deafness, and Dizziness. Hagerstown, MD: Harper & Row, 1979.)