Bony labyrinth

The bony labyrinth refers to the outer bony covering of the labyrinth or the inner ear. It lies within the petrous part of the temporal bone and is lined by periosteum on the inside. The bony labyrinth hosts inside it, the membranous labyrinth. The latter is filled with endolymph. The space between the membranous and the bony labyrinth is filled up by a fluid called the perilymph.

The bony labyrinth comprises of three sub-spaces, including the cochlea, the bony vestibule and the semicircular canals.

Cochlea: The bony cochlea comprises of a long cochlear canal that winds around a central bony pillar–the modiolus–that renders it a conical snail’s shell-like appearance. A small bony ridge, called the spiral lamina, protrudes from the modiolus, and holds a cochlear duct within the bony cochlea. Cochlear duct is part of the membranous labyrinth. Consequently, the lumen of the bony cochlea gets subdivided into three channels, namely the scala vestibuli above and the scala tympani below, with the cochlear duct being in the middle. The two scalas communicate with each other through a small opening called the helicotrema, at the apex of the cochlea. At its base, the scala vestibuli opens into the bony vestibule, while the scala tympani is attached to the round window in the medial or labyrinthine wall of the middle ear cavity.

Vestibule: The vestibule is the central part of bony vestibule, which connects with the middle ear cavity through the oval and round windows. It communicates anteromedially with the cochlea, while posterolaterally with the semicircular canals. A narrow bony canal, called the vestibular aqueduct, protrudes out from the bony vestibule and opens up on the posterior surface of petrous part of temporal bone. Since, it is plugged in by an endolymphatic duct and sac–parts of membranous labyrinth–therefore, no perilymph leaks out of the vestibular aqueduct.

Semicircular canals: There are three semicircular canals, which are also parts of bony labyrinth, and are connected to the bony vestibule. These include the anterior, posterior and lateral semicircular canals and are all positioned at right angles to each other. Each semicircular canal forms two-thirds of a circle and is dilated at one end to form the ampulla. The three canals open up into the vestibule via five openings.

Similar to bony labyrinth, the membranous labyrinth also comprises of three sub-parts, namely the cochlear duct (within the bony cochlea), utricle and saccule (within the bony vestibule) and the semicircular ducts (within the bony semicircular canals). The cochlear duct is the organ of hearing, whereas the utricle, saccule and semicircular ducts are linked with balance regulation.