General characteristics of the inner ear
The inner ear (auris interna) is a complex anatomical structure securely housed within the petrous part of the temporal bone. It consists of two key components: the bony labyrinth and the membranous labyrinth enclosed within it.
The interior space of the bony labyrinth is filled with perilymph (perilympha). The membranous labyrinth, located inside the bony labyrinth, contains another fluid called endolymph (endolympha).
Anatomically, the bony labyrinth (labyrinthus osseus) includes three separate yet interconnected parts:
- vestibule (vestibulum);
- bony semicircular canals (canales semicirculares ossei);
- cochlea (cochlea).
Vestibule (vestibulum)
The vestibule is the central part of the bony labyrinth. It has a characteristic oval shape and is topographically located between the tympanic cavity on one side and the internal acoustic meatus on the other.
The structure of the vestibular walls has several important features:
- On the lateral wall is the vestibular window (fenestra vestibuli). Normally, it is closed by the base 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 elliptical sac (utricle), and the spherical recess (recessus sphericus), where the spherical sac (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, there are openings through which the cavity communicates with the bony semicircular canals.
Bony semicircular canals
The bony semicircular canals (canales semicirculares ossei) are three arched tubes located in three mutually perpendicular planes. This spatial orientation ensures the function of the vestibular system.
There are three canals:
- Anterior (canalis semicircularis anterior);
- Posterior (canalis semicircularis posterior);
- Lateral (canalis semicircularis lateralis).
Each semicircular canal has a characteristic dilatation at one of its ends—the osseous ampulla (ampulla ossea). The canal attaches to the vestibule by two bony limbs (crura ossea). Additionally, some limbs of adjacent canals may merge into a single common bony limb (crus osseum commune).
Cochlea
The cochlea is the anterior part of the bony labyrinth and externally resembles a spirally coiled cone. In humans, this cone makes 2½ to 2¾ turns around its axis.
The base of the cochlea is formed by the modiolus (modiolus), around which the osseous spiral lamina (lamina spiralis ossea) winds. Inside runs the spiral canal of the cochlea (canalis spiralis cochleae).
The cavity of the spiral canal is divided into two non-communicating parts throughout 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 with each other at the apex of the cochlea through a specialized opening—the helicotrema (helicotrema).
Briefly: answers to topical questions
What structures open into the posterior part of the vestibule of the bony labyrinth?
The bony semicircular canals open into the posterior part of the vestibule of the bony labyrinth. They communicate with the vestibular cavity via five openings.
Located in three mutually perpendicular planes are:
- Anterior semicircular canal (canalis semicircularis anterior) — has an osseous ampulla and two bony limbs;
- Posterior semicircular canal (canalis semicircularis posterior) — has an osseous ampulla and two bony limbs;
- Lateral semicircular canal (canalis semicircularis lateralis) — has an osseous ampulla and two bony limbs.
Some of the bony limbs of these canals combine into a common bony limb (crus osseum commune).
To remember the parts of the bony labyrinth, use the mnemonic: Vestibule, Cochlea, Semicircular canals.
Class and exam questions
Where is the inner ear located?
The inner ear is located within the petrous part of the temporal bone and consists of the bony and membranous labyrinths.
What fluids fill the labyrinth?
The bony labyrinth is filled with perilymph, while the membranous labyrinth inside it contains endolymph.
What closes the vestibular window on the lateral wall?
The vestibular window (fenestra vestibuli) is closed by the base of the stapes.
How do the scala vestibuli and scala tympani communicate with each other?
The cochlear scalae communicate with each other only at the apex of the cochlea through a narrow opening—the helicotrema.
What else to study on this topic
- Structure and functions of the membranous labyrinth
- Mechanism of sound vibration transmission via the base of the stapes
- Anatomy of the utricle and saccule
- Formation of the common osseous limb of the semicircular canals
- Topography of the internal acoustic meatus
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