The Cochlea Is Located in the Inner Ear
The inner ear (labyrinth) contains the receptor organs of two sensory systems: the vestibular system» which detects acceleration and static tilt of the head (see Chapter 11), and the auditory system which delects and analyzes sound.
The inner ear consists of the bony labyrinth and» within the bony labyrinth, the membranous labyrinth. The bony labyrinth is a series of tunnels within the petrous temporal bone. Inside these tunnels, surrounded by a fluid called perilymph, is the membranous labyrinth. The membranous labyrinth follows the contour of the bony labyrinth and contains endolymph. The vestibular and auditory portions of the inner ear are contiguous, and the “membranous tunnel within a bony tunnel” design is an anatomical feature of both parts. The auditory portion of this inner ear complex is called the cochlea (see Figure 11-1).The cochlear portion of the labyrinth is coiled like a snail shell. If we could mentally uncoil this arrangement to a linear form and then take a transverse section through it, perpendicular to the long axis (like cutting a salami and then looking at the cut end), we would see two membranes, the basilar and Reissners, divide the cochlea into three chambers, or scalae (Figure 17-3). The dorsally located scala vestibuli and ventrally located scala tympani contain perilymph. The flexible middle scala, or scala media (cochlear duct), is formed by the membranous portion of the labyrinth and contains endolymph. The basilar membrane is the floor of the scala media, and atop this membrane lies the hair cell receptor organ for hearing called the organ of Corti. An anchored, gel-coated ridge, called the tectorial membrane, lies just atop the hair cells of the organ of
Corti. The morphological organization just described is virtually the same all along the length of the cochlea, except that the scala vestibuli and scala tympani connect with each other at the distal end (farthest from the oval window).