The internal ear consists of a series of bony cavities (the bony labyrinth) and membranous ducts and sacs (the membranous labyrinth) within these cavities. All these structures are in the petrous part of the temporal bone between the middle ear laterally and the internal acoustic meatus medially (Figs. 8.127 and 8.128).
The bony labyrinth consists of the vestibule, three semicircular canals, and the cochlea (Fig. 8.128). These bony cavities are lined with periosteum and contain a clear fluid (the perilymph).
Suspended within the perilymph but not filling all spaces of the bony labyrinth is the membranous labyrinth, which consists of the semicircular ducts, the cochlear duct, and two sacs (the utricle and the saccule). These membranous spaces are filled with endolymph.
The structures in the internal ear convey information to the brain about balance and hearing:
The cochlear duct is the organ of hearing.
The semicircular ducts, utricle, and saccule are the organs of balance.
The nerve responsible for these functions is the vestibulocochlear nerve [VIII], which divides into vestibular (balance) and cochlear (hearing) parts after entering the internal acoustic meatus (Fig. 8.128).
The vestibule, which contains the oval window in its lateral wall, is the central part of the bony labyrinth (Fig. 8.129). It communicates anteriorly with the cochlea and posterosuperiorly with the semicircular canals.
A narrow canal (the vestibular aqueduct) leaves the vestibule, and passes through the temporal bone to open on the posterior surface of the petrous part of the temporal bone.
Projecting in a posterosuperior direction from the vestibule are the anterior, posterior, and lateral semicircular canals (Fig. 8.129). Each of these canals forms two-thirds of a circle connected at both ends to the vestibule and with one end dilated to form the ampulla. The canals are oriented so that each canal is at right angles to the other two.
Projecting in an anterior direction from the vestibule is the cochlea, which is a bony structure that twists on itself two and one-half to two and three-quarter times around a central column of bone (the modiolus). This arrangement produces a cone-shaped structure with a base of the cochlea that faces posteromedially and an apex that faces anterolaterally (Fig. 8.130). This positions the wide base of the modiolus near the internal acoustic meatus, where it is entered by branches of the cochlear part of the vestibulocochlear nerve [VIII].
Extending laterally throughout the length of the modiolus is a thin lamina of bone (the lamina of the modiolus, or spiral lamina). Circling around the modiolus, and held in a central position by its attachment to the lamina of the modiolus, is the cochlear duct, which is a component of the membranous labyrinth.
Attached peripherally to the outer wall of the cochlea, the cochlear duct creates two canals (the scala vestibuli and the scala tympani), which extend throughout the cochlea and are continuous with each other at the apex through a narrow slit (the helicotrema):
The scala vestibuli is continuous with the vestibule.
The scala tympani is separated from the middle ear by the secondary tympanic membrane covering the round window (Fig. 8.131).
Finally, near the round window is a small channel (the cochlear canaliculus), which passes through the temporal bone and opens on its inferior surface into the posterior cranial fossa. This provides a connection between the perilymph-containing cochlea and the subarachnoid space (Fig. 8.131).