General Characteristics of the Internal Ear
The internal ear (auris interna) is a complex anatomical structure securely housed within the petrous part of the temporal bone (pars petrosa ossis temporalis). It consists of two key components: the osseous labyrinth and the membranous labyrinth enclosed within it.
The internal space of the osseous labyrinth is filled with perilymph (perilympha). The membranous labyrinth, located inside the osseous labyrinth, contains a different fluid called endolymph (endolympha).
Anatomically, the osseous labyrinth (labyrinthus osseus) includes three distinct yet interconnected parts:
- vestibule (vestibulum);
- osseous semicircular canals (canales semicirculares ossei);
- cochlea (cochlea).
Vestibule (Vestibulum)
The vestibule is the central part of the osseous labyrinth. It has an ovoid shape and is topographically located between the tympanic cavity laterally and the internal acoustic meatus medially.
The wall structure of the vestibule has several important features:
- On the lateral wall, there is the vestibular window (fenestra vestibuli), also known as the oval window. Normally, it is closed by the base (footplate) of the stapes, one of the auditory ossicles.
- On the medial wall, there are two distinct depressions: the elliptical recess (recessus ellipticus), intended for the utricle, and the spherical recess (recessus sphericus), where the saccule is located. These recesses are separated by a bony ridge known as the vestibular crest (crista vestibuli).
In the posterior part of the vestibule are the openings through which the cavity communicates with the osseous semicircular canals.
Osseous Semicircular Canals
The osseous semicircular canals (canales semicirculares ossei) are three arcuate tubes positioned in three mutually perpendicular planes. This spatial orientation enables the function of the vestibular system.
There are three canals:
- Anterior semicircular canal (canalis semicircularis anterior);
- Posterior semicircular canal (canalis semicircularis posterior);
- Lateral semicircular canal (canalis semicircularis lateralis).
Each semicircular canal features a characteristic dilation at one of its ends called the bony ampulla (ampulla ossea). Each canal attaches to the vestibule via two bony limbs (crura ossea). The adjacent limbs of neighboring canals may merge to form a common bony limb (crus osseum commune).
Cochlea
The cochlea is the anterior part of the osseous labyrinth and externally resembles a spirally wound cone. In humans, this cone makes 2½ to 2¾ turns around its axis.
The core of the cochlea is the modiolus, around which the osseous spiral lamina (lamina spiralis ossea) winds. The spiral canal of the cochlea (canalis spiralis cochleae) runs within it.
The cavity of the spiral canal is divided into two non-communicating compartments along its length:
- scala vestibuli (scala vestibuli);
- scala tympani (scala tympani).
These scalae are isolated from each other by the osseous spiral lamina and the structures of the membranous labyrinth, but they freely communicate at the apex of the cochlea through a specialized opening called the helicotrema (helicotrema).